Jaw relation /certified fixed orthodontic courses by Indian dental academy

JAW RELATIONS
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

Biological and clinical considerations
in making maxillo mandibular
relation records:


Introduction
Jaw relations are defined as any one of the many
relations of the mandible to the maxillae (Boucher -3)
Maxillomandibular relationship is defined as
any spatial relationship of the maxillae to the
mandible; any one of the infinite relationships of the
mandible to the maxilla. (Glossary of prosthodontic terms, 1999-1)
These relations may be of orientation, vertical
and horizontal relations. They are grouped as such
because the relationship of the mandible to the
maxillae is in the three dimensions of space i.e.,
sagittal, vertical and horizontal planes. (Gunnar E Carlson-2)

The occlusal surfaces of the teeth determine the relation
of the mandible to the maxillae when the natural
teeth are present, thereby aiding in mastication,
phonetics and the general appearance of the patient.
With the turn of events the natural teeth are lost due
to trauma or disease, thus oral rehabilitation is at a
standstill and has to be achieved by the process of
jaw relations by restoring the lost orofacial balance
and comfort of the patient.


When the mandible goes through functional
and
parafunctional
movements,
the
relationship it assumes defy description
because of their complexity. when the mandible
is at rest, definite relationship to the cranium
or the maxilla can be established. Thus one
needs to study certain static relationships to
understand the motions made by the mandible
in function.




Biologic consideration:

A good prosthodontic treatment bears a direct relation
to the structures of the temporomandibular articulation, since
occlusion is one of prime concern to the prosthodontist during
the treatment of patients with complete denture prosthesis
prosthesis. The temporomandibular joints affect the complete
denture prosthesis prosthesis prosthesis and likewise the
complete denture prosthesis prosthesis prosthesis affect the
health and function of the joints. Therefore a knowledge of
the interrelationship of the bony structures, tissue resislency,
muscle function, movements of the lips, facial muscles,
muscles of mastication, occlusions of the teeth,
temporomandibular joints and overriding mental attitudes
seem indispensable for treatment of edentulous patients.

Review of literature
John D. Rugh, carl J. Drago in 1981 suggested that in an
upright position, certain jaw muscle must be in slight
contraction to maintain the jaw in clinical rest position ,what
has been reffered to as “Clinical Rest Position” may be more
approximately called an upright posturel position.

Manns, Miralles & Guerraro in 1981 suggested that there
is a decrease of electrical activity in the three muscles as VD
increases. This may be explained by the passive elastic force of
muscles carrying larger part of load on muscle as it s length
increases. Further more, the action of opening the mouth
implies a mechanism of reciprocal innervation with nervous
impulses that excite the motor neurons of mandible depressor
muscles & inhibit those of elevator muscles.

. Ito et al suggested that a wide range of condylar loading
could occur during unilateral biting & chewing at second
molar. If the ratio of force of masseter muscle on working
side to force of masseter muscle on the balancing side is
large, condylar loading on the working side condyle will be
greater than on the balancing side condyle. If this ratio is
low, condylar loading will be greater on the balancing side.

Franco Mongini in 1986– suggested that
a. Extensive remodeling of TMJ takes place
thoroughout adult life, leading the marked typical
changes in www.indiandentalacademy.com
shape.

b. The degree of remodeling & a new shape imposed on the
condyles are closely related to changes in the dentition. The
influence of the latter is both direct, as in the close relations
between the edentulismand remodeling indeces and between the
index of abrasion & condyle shape, & indirect as the cause of
defective occlusal contacts. Similar changes in shape may in fact,
be observed in patients with complete dentitions & varying
degrees of edentulism.
C . Characteristic alterations in the shape of the condyles may be
brought about as the result of condylar displacement in centric
occlusion. Symmentric posterior displacement appears to occur
more frequently in older subjects with fewer teeth. Other forms
of displacement are caused by the loss of one or a few teeth,
malocclusion of various kinds & eruption of wisdom teeth

d. The accepted definition of “Centric Relation” does
not appear applicable to posterior displacement of one or
both condyles in centric occlusion.
e. Remodeling of condyles can, to a certain extent, be
considered as a functional adaptation of the joint to a new
occlusion situation and may be a distance prescursor of
symptoms of a pain-dysfunction syndrome in some
subjects. It may reasonably be supposed that in other
subjects satisfactory readjustment is achieved, and no
disterbances appear.


Temporomandibular Joint(TMJ)
(Gray’s Anatomy-11)

(okeson-7)

The area where craniomandibular articulation
occurs is called temporomandibular joint. The TMJ is
by far the most complex joint in the body. It provides for
hinging movement in one plane and therefore can be
considered as ginglymoid joint. At the same time it also
provides for a gliding movements, which classifies it as
arthroidal joint. Thus it has been technically considered
a ginglymoarthroidal joint.
the TMJ is formed by the mandibular condyle
fitting into the mandibular fossa of the temporal bone.
Separating these two bones from direct articulation is
the articular disc. The TMJ is classified as compound
joint.

By definition a compound joint requires the
presence of atleast three bones, yet the TMJ is made
up only two bones. Functionally the articular disc
serves as a nonossified bone that permits the complex
movement of the joint. Since the articular disc
functions as a third bone the cranionmandibular
articulation is considered as a compound joint.
The articular disc is composed of dense fibrous
connective tissue devoid of any blood vessels or nerve
fibres. In saggital plane it can be divided into three
regions according to thickness. The central area is
thinnest and is called the intermediate zone. Both
anterior and posterior to the intermediate zone the
disc becomes considerably thicker.

In the normal joint the articular surface of the condyle
is located on the intermediate zone of the disc. The
precise shape of the disc is determined by the
morphology of the condyle and mandibular fossa


The articular disc is attached posteriorly to an area of
loose connective tissue that is highly visualized and
innervated. This is known as retrodiscal tissue.
Superiorly it is bordered by a lamina of connective
tissue that contains many elastic fibres, the superior
retrodiscal lamina. Since this region consists of two
areas it has been referred to as Bilaminary Zone.

The superior retrodiscal lamina attaches the
articular disc posteriorly to the tympanic plate. At the
lower border of the retrodiscal tissues is the inferior
retrodiscal lamina, which attaches the inferior border
of the posterior edge of the disc to the posterior margin
of the articular surface of the condyle. The inferior
retrodiscal lamina is composed chiefly of collagenous
fibres. The remaining body of the retrodiscal tissue is
attached posteriorly to a large ligament that surrounds
the entire joint, the Capsular Ligament. The superior
and inferior attachments of the anterior region of the
disc are also by the capsular ligament.
(Sahler L.G, Morris T.W – 69)

Like the articular disc, the articular surfaces of
the mandibular fossa and condyle are lined with dense
fibrous connective tissue rather than hyaline cartilage
as in most other joints. The fibrous connective tissue in
the joints affords several advantages over hyaline
cartilage. Its is generally less susceptible than hyaline
cartilage to the effects of aging and therefore less likely
to break down over time. Also is has a much greater
ability to repair than does hyaline cartilage.
The articular disc is attached to the capsular
ligament, not only anteriorly and posteriorly but also
medially and laterally. This divides the joint into two
distinct cavities.

The upper or superior cavity is bordered by the
mandibular fossa and the superior surface of the disc. The
lower or inferior cavity is bordered by the mandibular
condyle and the inferior surface of the disc. The internal
surface of the cavities are surrounded by specialized
endothelial cells that form a synovial lining. This lining
produces synovial fluid which fills both joint cavities. Thus
the TMJ is referred to as synovial joint. The synovial fluid
serves two purposes.
1. Since the articular surfaces of the joint are non
vascular, the synovial fluid acts as a medium for providing
metabolic requirements to these tissues.
2. It lubricates the articular surfaces by two
mechanisms; boundary lubrication and weeping lubrication.
(Shengyi. T, Yinghuax – 70)


Ligaments (Jeffrey P. Okeson)
As in any joint system, ligaments play an important
role in protecting the structures. The ligaments of the
joint are made up of collagenous connective tissues, which
do not stretch. They do not enter actively in joint function
bit instead act as passive restraining devices to limit and
restrict joint movement. There are three functional
ligaments that support the TMJ: (1) the collateral
ligaments, (2) the capsular ligament and (3) the
temporomandibular ligament. There are also two
accessory ligaments: (4) the sphenomandibular and (5)
the stylomandibular.

Collateral (discal) ligaments
The collateral ligaments attach the medial and lateral
borders of the articular disc to the poles of the condyle. They are
commonly called the discal ligaments, and there are two. The
medial discal ligament attachees the medial edge of the disc to
the medial pole of the condyle. The lateral discal ligament
attaches the lateral edge of the disc to the lateral pole of the
condyle. They cause the disc to move passively witht eh condyle
as it glides anteriorly and posteriorly. These ligaments are
responsible for hinging movement of the TMJ, which occurs
between the condyle and the articular disc.


Capsular ligament
The entire TMJ is surrounded and encompassed by the
capsular ligament. The fibers of the capsular ligament are attached
superiorly to the temporal bone along the borders of the articular
surfaces of the mandibular fossa and articular eminence. Inferiorly
the fibers of the capsular ligament attached to the neck of the
condyle. It acts to resist any medial, lateral or inferior forces that
tend to separate or dislocate the aricular surfaces. A significant
function of the ligament is to encompass the joint, thus retaining
the synovial fluid. It is well innervated and provides proprioceptive
feedback regarding the positional movement of the joint.


Temporomandibular ligament
The lateral aspect of the capsular ligament is
reinforced by strong tight fibers that make up the lateral
ligament or temporomandibular ligament. The TM ligament
is composed of two parts. An outer oblique portion and an
inner horizontal portion. The outer portion extends from the
outer surface of the articular tubercle and zygomatic process
posteroinferiorly to the outer surface of the condylar neck.
The inner horizontal portion extends from the outer surface
of the articular tubercle and zygomatic process posteriorly
and horizontally to the lateral pole of the condyle and
posterior part of the articular disc. The oblique portion of the
TM ligament resists excessive dropping of the condyle and
therefore acts to limit the extent of mouth opening. The inner
horizontal portion of TM ligament limits posterior movement
of the condyle and disc.

Sphenomandibular ligament
It is one of the accessory ligaments of the TMJ. It
arises from the spine of sphenoid bone and extends
downward and laterally to a small bony prominence on the
medial surface of the ramus of the mandible called the
lingual. It does not have any significant effects on the
mandibular movements.
Stylomandibular ligament
It arises from the styloid
process and extends downwards
and forwards to the angle of the
posterior border of the ramus of
the mandible. It limits excessive
protrusive movements of the
mandible.

In understanding the function of this structure it is
important to recognize that the mandibular fossa does not
normally participate in joint activities except for its
anterior wall, which forms the posterior slope of the
articular eminence. The functional bony element of this
joint, should be perceived as two convex structures, namely
the condyle and articular eminence. The superior and
posterior areas of the fossa do not participate in bearing
functional loads. Such loads are normally borne by the
posterior slope of the articular eminence, where the fibrous
connective tissue is thickest on the posterior slope and crest
of the articular eminence. It has been hypothesized that
the natural dentition carries most of the compressive load
so that the joint is not ordinarily required to withstand
such forces.

The loss of natural dentition may therefore place additional
compressive forces on the temporomandibular joint, which is then
required to adapt to these new functional demands. Continued stress
beyond the adaptive capabilities of the articular tissues may lead to
degenerative joint diseases. The collagen fibres become “unmasked”
under the compressive loads and uncontrolled and aberrant
remodeling ensues and portions of the articular tissues may break
down leading to a subluxation of the mandible.
Thus recording of the centric relation position becomes
difficult.
The edentulous patients are more susceptible to
degenerative joint diseases, particularly those individuals whose
tissues cannot adapt adequately to the functional changes. Although
there is no evidence to suggest that properly constructed complete
denture prosthesis prosthesis can reverse the course of this disease,
there is an empirical possibility that its progression may be prevented
or slowed by reestablishment of more normal types of functional
relationships and activities.

The articular disc or meniscus plays a prominent part in the
movement of the mandible. Though it has very little movement in
the first opening movements when the condyle merely rotates, it
undergoes extensive movements when the mandible makes wider
opening movements or protrusive movements. The disk can move
forward and back over the condyle but cannot move from side to
side.
Unhealthy temporomandibular joints complicate the
registration of jaw relation records and sometimes even preclude
them completely. Centric relation depends on both structural and
functional harmony of osseous structures, the intraarticular tissue
and the capsular ligaments if it is to be a function position. If these
specifications cannot be fulfilled, the patient will not have a centric
relation or for that matter provide the prosthodontist with a
recordable one.
The auricolotemporal, the posterior deep temporal nerves
and the mesenteric nerves innervate the temporomandibular joints.
(Gray’s Anatomy -11)


Muscles of Mastication (Gray’s Anatomy-11)
The energy that moves the manndible and allows function
of masticatory system is provided by muscles. There are four pairs
of muscles making up a group called “muscles of mastication”
1. Masster
2. Temporalis
3. Medial Pterygoid

}

4. Lateral Pterygoid. -

Elevators of
mandible

Depressor of Mandible


The accessory muscles of mastication are:
1. Suprahyoid muscles (Myelohyoid,
Stylohyoid, Geniohyoid, Hyoglossus)

Digastric,

2. Infra Hyoid Muscles (Sternothyroid, Sternohyoid,
Thyrohyoid, Omohyoid)
3. Facial Muscles (Buccinator, Orbicularis oris,
Zygomaticus major, Zygomaticus minor, Mentalis, Levator
anguli oris)
4. Muscles of back of neck (Scalenus anterior, Scalenus
medius, Scalenus posterior, Splenius capitus, Levator scapulae,
suboccipital muscles)
5. Muscles of side of neck (Splenius capitus, Semispinalis
capitus)

There are three groups of muscles that act to depress
the mandible. (Guyton A. C)
1.
The suprahyoid muscles (Digastrics, mylohyoid,
geniohyoid and stylohyoid) and platysma act as a group and
are primarily responsible for opening the mandible.
2.
The infrahyoid muscles (Sternothyroid, Sternohyoid,
Thyrohyoid, Omohyoid) act to stabilize the hyoid bone so that
the suprahyoid muscles can act.
3.
The lateral pterygoid muscles pull the condyles forward
or medially as the other group of muscles act.


The Masseter
The masster is a rectangular muscle which originates
from the zygomatic arch and extends downwards to the lateral
aspect of the lower border of the ramus of the mandible. Its
insertion on the mandible extends from the region of second
molar at the inferior border posteriorlt to include the angle.
The muscle is made up of two portions or heads:
superficial portion and deep portion. As fibres of the
masseter contract mandible is elevated and the the teeth are
brought into contact. Masseter is a powerful muscle which
provides necessary force to chew effeciently. Its superfical
portion also aids in protruding the mandible. When the
mandible is protruded and biting force is applied the
fibres of the deep portion stabilizes the condyle against the
articular eminence. www.indiandentalacademy.com

Masseter


TEMPORALIS MUSCLE
The temporal muscle(temporalis) is a large, fanshaped muscle that originates from the temporal fossa and
the lateral surface of the skull. Its fibres come together as
they extend downward between the zygomatic arch and
the lateral surface of the skull to to form a tendon that
inserts on the coronoid process and anterior border of the
ascending ramus.
Fibres of temporalis are classified into three types
according to their direction and their distinct function.
Anterior vertical fibres
Middle oblique fibres
Posterior horizontal fibres.

When the entire temporalis contracts, it elevates
the mandible and the teeth are brought into contact. If
only anterior portions contract the mandible

is

elevated. Contraction of the middle portion will
elevate and retruded the mandible. Function of the
posterior portion is controversial. Although it would
appear that contraction of this portion retrudes the
mandible, DuBrul suggest that the fibers below the
root of the zygomatic process are the only significant
ones and therefore contraction causes elevation and
only slight retrusion.

Temporalis


MEDIAL PTERYGOID
The medial (internal) pterygoid muscle originates
from the pterygoid fossa and extends downward, backward
and outward to insert along the medial surface of the
mandibular angle.
Along with masseter forms a muscle that supports
the mandible at the mandibular angle. When its fibres
contract, the mandible is elevated and the teeth are brought
into contact.
Unilateral contraction along with lateral pterygoid
will bring about a mediotrusive movement of the mandible.


LATERAL PTERYGOID
Lateral pterygoid is described as two distinct portions.
1 Inferior portion (or) belly
2.Superior portion(or) belly
The inferior lateral pterygoid muscle originates at the
outer surface of the lateral pterygoid plate and extends
backward, upward and outward to its insertion primarily on
the neck of the condyle. When the right and left inferior
lateral pterygoids contact simultaneously, the condyles are
pulled down the articular eminences and the mandible is
protruded. Unilateral contraction creates a mediotrusive
movement of the condyle and causes a lateral movement of
the mandible to the opposite side.

The superior lateral pterygoid muscle is considerably
smaller than the inferior and originates at the infratemporal
surface of the greater sphenoid wing, extending almost
horizontally, backward and outward to insert on the articular
capsule the disc and the neck of the condyle.
The functions of these two portions are different and
nearly opposite . and hence described as inferior lateral
pterygoid and superior lateral pterygoid.

Superior lateral

peterygoid is considerably smaller than the inferior. This is
responsible for keeping the disc properly aligned with the
condyle during function.


Lateral and medial
pterygoids


Functional neuroanatomy and physiology of the
masticatory system.(Jeffery P. Okeson-7)
1. Muscle
Motor Unit: The basic component of the neuromuscular
system is the motor unit, which consists of a number
muscle fibers that are innervated by one motor neuron.
Each neuron joins with a muscle fiber at a motor end plate.
When the neuron is activated, the motor end plate is
stimulated to release small amounts of acetylcholine, which
initiates the depolarization of muscle fibers. Depolarization
causes the muscle fibers to shorten or contract. Fewer the
muscle fibers per motor neuron, more precise is the
movement. Hundreds to thousands of motor units along
with blood vessels and nerves are bundled together by
connective tissue and fascia to make up the muscle.

2. Neurologic structures
The neurons: Each skeletal muscle has both sensory and motor
innervations. The sensory or afferent neurons carry
information from the muscle to the central nervous system at
both the spinal cord and higher center levels. The type of
information carried by the afferent nerve fibers most often
depends on the sensory nerve endings. Some nerve endings
relay sensation of discomfort and pain, as when the muscle is
fatigued or damaged. Others provide information regarding the
state of contraction of relaxation of the muscle. Still others
provide information regarding joint and bone positions
(proprioception)
Once the sensory information has been received and
processed by the central nervous system, regulatory
information is returned to the muscles by way of the motor or
efferent nerve fibers. www.indiandentalacademy.com

The information from the tissues outside the CNS needs
to be transferred into the CNS and onto the higher centers in
the brainstem and the cortex for interpretation and evaluation.
Once this information is evaluated, appropriate action must be
taken. The higher centers then send information down the
spinal cord and back out to the periphery to an efferent organ
for the desired action. The primary afferent neuron (first
order neuron) receives stimulus from the sensory receptor.
This impulse is carried by the primary afferent neuron into
the CNS by way of dorsal root to synapse in the dorsal horn of
spinal cord with a secondary neuron (second order neuron).
The impulse is then carried by the second order neuron across
the spinal cord to the anterolateral spinothalamic pathway
that ascends to the higher centers. Multiple interneurons
(third and fourth order, etc) are involved with the transfer of
this impulse to the thalamus and cortex.

3. Brainstem and Brain (Guyton A.C-10 and okeson-7)
Once the impulse has been passed to the second order
neurons, these neurons carry them to the higher centers for
interpretation and evaluation. Numerous centers in the
brainstem and brain help to give meaning to the impulses.
The Prosthodontist should remember that numerous
interneurons may be involved in transmitting the impulses
onto higher centers. The important areas that will be
reviewed are spinal tract nucleus, the hypothalamus, the
limbic structures and the cortex. They are discussed in the
order by which neural impulses pass on to the higher centers.
(Okeson J.P – Bell’s orofacial pain)


a. Spinal tract nucleus
Throughout the body, primary afferent neurons synapse with the
second order neurons in the dorsal horn of the spinal column. Afferent
input from the face and oral structures, does not enter the spinal cord
by way of spinal nerves. Instead, sensory input from the face and
mouth are carried by way of fifth cranial nerve (Trigeminal nerve).
The cell bodies of the trigeminal afferent neurons are located in the
large gasserian ganglion. Impulses carried by trigeminal nerve enter
directly into the brainstem in the region of Pons to synapse in the
trigeminal spinal nucleus. The brainstem-trigeminal nucleus complex
consists of two main parts.
i) Main sensory trigeminal nucleus (receives periodontal and some pulpal
afferents)
ii) The spinal tract of trigeminal nucleus (Delaat A)
•

Subnucleus oralis

•

Subnucleus interpolaris

•


Subnucleus caudalis

The subnucleus caudalis has been implicated in
trigeminal nociceptive mechanisms based on electrophyiological
observations of nociceptive neurons. (Sessle B.J, Dostrovsky J.O)
The subnucleus oralis appears to be a significant area of
this trigeminal-brainstem complex for oral pain mechanisms.
(Lund J.P, Donga R., Widmer C.G, Stohler C.H)


b. Reticular formation

(Guyton A C-10)

After the primary afferent neurons synapse in the
spinal tract nucleus, the interneurons transmit the impulses
up to the higher centers. The interneurons ascend by way of
several tracts passing through an area of the brainstem
called the reticular formation. Within the reticular
formation are concentrations of cells or nuclei that
represent centers for various functions. The reticular
formation plays an extremely important role in monitoring
impulses that enter the brainstem. The reticular formation
controls the overall activity of the brain by either enhancing
the impulses on to the brain or by inhibiting the impulses.
This portion of the brainstem has an extremely important
influence on pain and other sensory input.

c. Thalamus (Jeffery p okeson-7)
The thalamus is located in the very centre of the brain,
with the cerebrum surrounding it from the top and sides and the
mid-brain below. It is made up of numerous nuclei that function
together to interrupt impulses. Almost all impulses from the
lower regions of the brain, as well as from the spinal cord, are
relayed through synapses in the thalamus before proceeding to
the cerebral cortex. The thalamus acts as a relay station for most
of the communication between the brainstem, cerebellum, and
cerebrum. While impulse arise to the thalamus, the thalamus
makes assessments and directs the impulses to appropriate
regions in the higher centers for interpretation and response.


d. Hypothalamus
The hypothalamus is a small structure in the middle of the
base of the brain. Although it is small, its function is great, the
hypothalamus is the major center of the brain for controlling
internal body functions, such as body temperature, hunger, and
thirst. Stimulation of the hypothalamus excites the sympathetic
nervous system throughout the body, increasing the overall level
of activity of many internal parts of the body, especially
increasing heart rate and causing blood vessel construction. An
increased level of emotional stress can stimulate the
hypothgalamus to up regulate the sympathetic nervous system
and greatly influence nonciceptive impulses entering the brain.
This simple statement should have extreme meaning to the
clinician managing pain10 .

e. Limbic structures
The word limbic means border. The limbic system
comprises the border structures of the cerebrum and the
diencephalons. The limbic structures function to control our
emotional and behavioral activities. Within the limbic structures
are centers, or nuclei, that are responsible for specific behaviors,
such as anger, rage etc. The limbic structures also control
emotions, such as depression, anxiety, fear or paranoia.
Impulses from the limbic system leading into the
hypothalamus can modify any or all of the many internal bodily
functions controlled by the hypothalamus. Impulses from the
limbic system feeding into the midbrainm and medulla can
control such behavior as wakefulness, sleep, excitement and
attentiveness.

f. Cortex (okeson-7)
This cerebral cortex represents the outer region of the
cerebrum and is made up predominantly of gray matter. The
cerebral cortex is the portion of the brain most frequently
associated with the thinking process, even though it cannot
provide thinking without simultaneous action of deep structures
of the brain. The cerebral cortex is the portion of the brain in
which essentially all of our memories are stored, and it is also
the area most responsible for our ability to acquire our many
muscle skills. The basic psychologic mechanisms by which the
cerebral cortex stores either memories or knowledge of muscle
skills are not known.
In most areas the cerebral cortex is about 6mm thick and
contains an estimated 50 to 80 billion nerve cell bodies. Perhaps
1 billion nerve fibers lead away from the cortex, and comparable
numbers lead into it. These nerve fibers pass to other areas of
the cortex, to and from deeper structures of the brain; some
travel all the way to thewww.indiandentalacademy.com
spinal cord.

Different regions of the cerebral cortex have been identified
to have different functions. A motor area is primarily
involved with coordinating motor function; (precentral
gyrus) a sensory area receives somatosensory (post central
gyrus) input for evaluation. Areas for specials senses, such
as visual and auditory areas, also are found. (Guyton-10)


THE SENSORY RECEPTORS (William F. Ganong-71)
Sensory receptors are neurologic structures or organs located
in the tissues that provide information to the central nervous system
regarding the status of these tissues. As in other areas of the body,
various types of sensory receptors are located throughout the tissues
that make up the masticatory system. There are specialized sensory
receptors that provide specific information to the afferent neurons
and thus back to the central nervous system. Some receptors are
specific for discomfort and pain. Others provide information
regarding the position and movement of the mandible and associated
oral structures. These movement and positioning receptors are called
proprioceptors.
The masticatory system utilizes four major types of sensory
receptors to monitor the status of its structures: (1) the muscle
spindles, which are specialized receptor organs found in the muscle
tissue; (2) the Golgi tendon organs, located in the tendons; (3) the
pacinian corpuscles, located in tendons, joints, periosteum, fascia and
subcutaneous tissues, and (4) the nociceptors, found generally
throughout all the tissues www.indiandentalacademy.com system
of the masticatory

a. Muscle spindles (Jeffery P Okeson-7)
Skeletal muscles consist of two types of muscle fiber: the first is
the extrafusal fibers, which are contractible and make up the bulk of
the muscle, the other is the intrafusal fibers, which are only minutely
contractile. A bundle of intrafusal muscle fibers bound by a connective
tissue sheath is called a muscle spindle. The muscle spindles are
interspersed throughout the skeletal muscles and aligned parallel to
the extrafusal fibers. Within each muscle spindle the nuclei of the
intrafusal fibers are arranged in two distinct fashions. Chainlike
(nuclear chain type) or clumped (nuclear bag type)
There are two types of afferent nerves that supply the
intrafusal fibers. They are classified according to their diameters. The
larger fibers conduct impulses at a higher speed and have lower
thresholds. Those that end in the central region of the intrafusal fibers
are the larger group (la) and are said to be the primary endings (socalled annulospiral endings.) Those that end in the poles of the spindle
(away from the central region) are the smaller group (II) and are the
secondary endings (so-called flower spray endings)

The afferent neurons originating in the muscle spindles of
the muscles of mastication have their cell bodies in the trigeminal
mesencephalic nucleus.
The intrafusal fibers receive efferent innervation by way of
fusimotor nerve fibers, alpha nerve fibers, which supply the
extrafusal. There are two manners in which the afferent fibers of
the muscle spindles can be stimulated: generalized stretching or
elongation of the entire muscle (extrafusal fibers) and contraction
of the intrafusal fibers by way of the gamma efferents. The muscle
spindles can only register the stretch and cannot differentiate
between these two activities. Therefore the activities are recorded
as the same activity by the central nervous system.
The extrafusal muscle fibers receive innervation by way of
the alpha efferent motor neurons. Most of these have their cell
bodies in the trigeminal motor nucleus.

From a functional standpoint the muscle spindle acts as a length
monitoring system. It constantly feeds back information to the
central nervous system regarding the state of elongation or
contraction of the muscle.

AFFERENT
FIBERS II

AFFERENT
FIBERS IA

EFFERENT

FIBERS (γ )

EFFERENT

FIBERS (α )

EXTRAFUS
AL FIBERS

CAPSULE OF
MUSCLE FIBER

NUCLEAR CHAIN
INTRAFUSAL FIBER

NUCLEAR BAG
INTRAFUSAL FIBER


INTRAFUSAL
FIBER

b. Golgi tendon organs
The golgi tendon organs are located in the muscle tendon
between the muscle fibers and their attachment to the bone. They
occur in series with the extrafusal muscle fibers and not in parallel as
with the muscle spindles. Each of these sensory organs consists of
tendinous fibers surrounded by lymph spaces enclosed within a
fibrous capsule. Afferent fibers enter near the middle of the organ
and spread out over the extent of the fibers. Tension on the tendon
stimulates the receptors in the Golgi tendon organ. Therefore
contraction of the muscle also stimulates the organ. Likewise, an
overall stretching of the muscle creates tension in the tendon and
stimulates the organ.
At one time it was thought that the Golgi tendon organs had a
much higher threshold than the muscle spindles and therefore
functioned solely to protect the muscle from excessive or damaging
tension. It now appears that they are more sensitive and are active in
reflex regulation during normal function. The Golgi tendon organs
primarily monitor tension whereas the muscle spindles primarily
monitor muscle length. www.indiandentalacademy.com

c. Pacinian Corpuscles
The pacinian corpuscles are large oval organs made up of
concentric lamellae of connective tissue. At the center of each
corpuscle is a core containing the termination of a nerve fibre.
These corpuscles are found the tendons, joints, periosteum,
tendinous insertions, fascia, and subcutaneous tissue. There is a
wide distribution of these organs, and because of their frequent
location in the joint structure they are considered to serve
prinicp0ally for the perception of movmement and firm pressure
(not light touch).


d. Nociceptors
Generally nociceptors are sensory receptors that are
stimulated by injury and transmit this information to the central
nervous system by way of the afferent nerve fibres. Nocieceptors
are located throughout most of the tissue s in masticatory system.
There are several general types; some respond exclusively to
noxious mechanical and thermal stimuli; other respond to a wide
range of stimuli, from tactile sensation to noxious injury; still
others are low threshold receptors specific for light touch,
pressure, or facial hair movement. The last type is some times
called mechanoreceptors.


NEUROMUSCULAR FUNCTION
Function of the sensory receptors:
The dynamic balance of the head and neck muscles
previously described is possible through feedback provided by the
various sensory receptors. When a muscle is passively stretched, the
spindles infor the central nervous system of this activity. Active
muscle contraction is montitored by both the Golgitendon organs
and the muscle spindles. Movement of the joints and tendons
stimulates the pacinian corpuscles, which relay this information to
the central nervous system. Pain as well as fine movement and tactile
sensations are monitored through the nociceptors. All these sensory
organs provide constant feedback to the central nervous system.
This input is continually monitored and evaluated both day and
night, during both activity and relaced periods. The central nervous
system evaluates and organizes the sensory input and initiates
appropriate efferent input to create a desired motor function. Most
of the efferent pathways running from the higher centrers to the
muscles of mastication pass through the trigeminal motor nucleus.

Neuromuscular regulation of mandibular motion:
(Boucher-3)

The muscles that move, hold, or stabilize the
mandible do so because they receive impulses from
the central nervous system. The impulses that
regulate mandibular motion may arise at the
conscious level and result in voluntary mandibular
activity. They also may arise from subconscious
levels as a result of the stimulation of oral or muscle
receptors or of activity in other parts of the central
nervous system. When a closing movement occurs,
the neurons to the closing muscles are being excited
and those to the opening muscles are being inhibited.
Impulses from the subconscious level, including the
reticular activating system, also regulate muscle tone,
which plays a primary role in the physiological rest
position of the mandible.

Certain receptors in mucous membranes of the oral
cavity can be stimulated by touch, thermal changes, pain
or pressure. Other receptors located principally in the
periodontal ligaments, mandibular muscles, and
mandibular ligaments provide information as to the
location of the mandible in space and are called
proprioceptors. The impulses generated by stimulation of
these oral receptors travel to the sensory nuclei of the
trigeminal nerve or, in the case of proprioceptors, to the
mesencephalic nuclei.


From there they are transmitted –
(1) By way of the thalamus to the sensorimotor cortex
(conscious level) to produce a voluntary change in the
position of the mandible;
(2) By way of the reflex arc to the motor nuclei of the
trigeminal nerve and directly back to the mandibular
muscles to cause an involuntary movement of the
mandible or
(3) By a combination of these two under the influence of
subcortical areas such as the hypothalamus, basal
ganglia, or reticular formation.


REFLEX ACTION
A reflex action is the response resulting from a stimulus
that passes as an impulse along an afferent neuron to posterior
nerve root or its cranial equivalent, where it is transmitted to the
efferent neuron leading back to the skeletal muscle. Although
the information is sent to the higher center influence. A reflex
action may be monosynaptic or polysynaptic. A monosynaptic
reflex occurs when the afferent fiber directly stimulates the
efferent fiber in the central nervous system. A polysynaptic
reflex is present when the afferent neuron stimulates one or more
interneurons in the central nervous system, which in turn
stimulate the efferent nerve fibers.
Two general reflex actions are important in masticatory
system
1.
2.

The myotatic reflex
The nociceptive reflex.

Myotatic (stretch) reflex: (Dale. R.A-22) the myotatic or
stretch reflex is the only monosynaptic jaw reflex is the only
monosynaptic jaw reflex. When skeletal muscle is quickly
stretched, this protective reflex is elicited and brings about a
contraction of the stretched muscle.
The myotatic reflex can be demonstrated by observing the
masseter as a sudden downward force is applied with a small
rubber hammer. As the muscle spindles within the masseter
suddenly stretch, afferent nerve activity is generated from the
spindles. These afferent impulses pass into the brainstem is the
trigeminal motor nucleus by way of the trigeminal mesencephalic
nucleus, where the primary afferent cell bodies are located. These
same afferent fibers synapse with the alpha efferent motor
neurons leading directly back to the extrafusal fibers of the
masseter.

Clinically this reflex can be demonstrated by relaxing the
jaw muscles, allowing the teeth separate slightly. A sudden
downward tap of the chin will cause the jaw to be reflexly
elevated. The masseter contracts, resulting in tooth contract.
The myotatic reflex occurs without specific response from
the brain and is very important in determining the resting postion
of the jaw. If there were complete relatxation of the muscles that
support the jaw, the forces of gravity would act to lower the jaw
and separate and articular surfaces of the TMJ. To prevent this
dislocation, the elevator muscles (and other muscles) are
maintained in a mild state of contraction (called muscle tonus).
The myotatic reflex is a principal determinant of mucle tonus in
the elevator muscles. As gravity pulls down on the mandible, the
elevator muscles are passively stretched, which also creates
stretching of the muscle spindles. Thus passive stretching causes
a reactive contraction that relieves the stretch on the muscle
spindle.(Hellsing and klineberg -23)

Nociceptive (flexor) reflex: (Stohler C S –20) The nociceptive or
flexor reflex is a polysynaptic reflex to noxious stimuli and therefore is
considered to be protective. In masticatory stystem this reflex becomes
active when a hard object is suddently encountered during masticatory.
As the tooth is forced down on the hard object, a noxious
stimulus is received by the tooth and surrounding periodontal structures.
The associated sensory receptors trigger afferent nerve fibers, which
carry the information to the interneurons in the trigeminal motor
nucleus. Not only must the elevatory muscles be inhibited to prevent
jruther jaw closure on the hard object, but the jaw opening muscles must
be activated to bring the teeth away from potential damage. As the
afferent information from the sensory receptors reaches the
interneurons, two distinct actions are taken excitatory interneurons
leading to the efferent fibers of the jaw opening muscles are stimulated.
This action causes therse muscles to contract. At the same time the
afferent fibers stimulate inhibitory interneurons, which have their effect
on the jaw elevating muscles and cause them to relax. The overall lresult
is that the jaw quickly drops and the teeth are pulled away from the
object causing the noxious stimulus.

The myotatic reflex protects the masticatory system
from sudden stretching of a muscle the nocieceptive reflex
protects the teeth and supportive structures from dameage
created by sudden and unusually heavy functional forces.

Influence of opposing tooth contacts: (Gunner E Carlsson - 2)
An important aspect of many jaw movements includes the
contacts of opposing teeth. The manner in which the teeth
occlude is related not only to the occlusal surfaces of the teeth
themselves but also to the muscles, TMJs, and
neurophysiological components including the patient’s mental
well being. When patients wearing complete denture prosthesis
prosthesis bring their teeth together in centric or eccentric
positions within the functional range of mandibular movements,
the occlusal surfaces of the teeth should meet evenly on both
sides. In this manner, the mandible is not deflected from its
normal path of closure, nor are the dentures displaced from the
residual ridges. In addition, when mandibular movements are
made with the opposing teeth of complete denture prosthesis
prosthesis in contact, the inclined planes of the teeth should pass
over one another smoothly and not disrupt the influences of the
condylar guidance posteriorly and the incisal guidance

Mandibular movements . (Jeffery .P Okeson -7)
Mandibular movements occur as a complex series of
interrelated three-dimensional rotational and translational
activities.
It is determined by the combined and
simultaneous activities of both temporomandibular joints.

Types of movements
Two types of movement occur in the temporomandibular
joint;
1.

Rotation or hinge movement

2.

Translatory movement

Rotational Movement

(Lindaver. S J –72)

Rotational movement occurs as movement
within the inferior cavity of the joint. It is thus
movement between the superior surface of the
condyle and the inferior surface of the articular
disc. Rotational movement of the mandible can
occur in all three reference planes; horizontal,
frontal (vertical), and sagittal. In each plane it
occurs around a point, called the axis.


Translational Movement
Translation can be defined as a movement in
which every point of the moving object has
simultaneously the same velocity and direction. In the
masticatory system it occurs when the mandible moves
forward, as in protrusion. The teeth, condyles, and rami
all move in the same direction and to the same degree.
Translation occurs within the superior cavity of the joint
between the superior surface of the articular disc and the
inferior surface of the articular fossa.


Sagittal Plane Border and Functional movements
Mandibular motion viewed in the sagittal plane can be
seen to have four distinct movement components
1.

Posterior opening border

2.

Anterior opening border

3.

Superior contact border

4.

Functional


Posselt’s Curve

Horizontal Plane Border
Functional Movements:

And

When mandibular movements are
viewed in the horizontal plane, a
rhomboid shaped pattern can be
seen that has four distinct
movement components plus a
functional component:
1.

Left lateral border

2. Continued left lateral border
with protrusion
3.

Right lateral border

4. Continued right lateral border
with protrusion.

CR

3

1
CO

4

2

Frontal (Vertical) Border and Functional Movements:
When mandibular motion is viewed in the frontal plane, a
shield-shaped pattern can be seen that has four distinct
movement components along with the functional
component:
1.

Left lateral superior border

2.

Left lateral opening border

3.

Right lateral superior border

4.

right lateral opening border


The biologic factors which include the anatomy and
physiology of the temporomandibular joints, the axes
around which the mandible rotates, the actions of
muscles and ligaments, contacts of opposing teeth and
the neuromuscular integration must be well
understood by the prosthodontist during the
treatment of edentulous patients.


A. Roy MacGregor described the following
procedure of adjusting the upper and lower record
blocks during jaw relation.


TRIMMING THE UPPER RECORD BLOCK
When trimming the rim there are four
considerations and they must be taken in the order given.

main

• Labial fullness: The lip is normally supported by the alveolar
process and teeth which, at this stage, are represented by the
base and rim of the record block. Therefore, the labial surface
must be cut back or added to until a natural and pleasing
position of the upper lip is obtained.


2. The height of occlusal rim: It should be trimmed vertically until it
represents the amount of anterior teeth intended to show below the
lip at rest. The average adult shows approximately 3mm of upper
central incisors when the lips are just parted, but there are many
variations from this amount which should be accepted as a guide
rather than a rule
A greater length of tooth than normal may be shown if the patient
has:
a. A short upper lip
b. Superior protrusion
c. An Angle’s Class II malocclusion of natural teeth
And less will be shown:
a.
With a long upper lip
b. In most old people, owing to attrition of natural teeth and
some loss of tone of www.indiandentalacademy.com muscle
the orbicularis oris

3. Anterior plane: Generally the plane to which the anterior teeth
should be set, and to which the rim must be trimmed, is parallel to
an imaginary line joining the pupils of the eyes or a line at right
angles to the midsagittal plane of the face.


4. The anteroposterior plane: This plane indicates the
position of occlusal surfaces of the posterior teeth and is
obtained in conjunction with the anterior plane. The rim is
trimmed parallel to Ala-tragus line (an imaginary line
running from the external auditary meatus or tragus of the
ear to the lower border of ala of the nose). It has been found
from the study of many cases that the occlusal plane of
natural teeth is usually parallel to this line

Thus when the rim has been trimmed to these planes
it indicates the place of orientation for setting the artificial
teeth.

GUIDELINES
1. The centre line or midline
In the normal natural dentition, the upper central
incisors have their mesial surfaces in contact with an imaginary
vertical line which bisects the face and, for esthetic reasons, it is
desirable that the artificial substitutes should occupy the same
position. Few human faces are symmetrical. Therefore there can
be no hard and fast rule for determining the centre line, which
thus depends on the artistic judgement of the prosthodontist.


The following aids are suggested as a help in deciding
where to mark a vertical line on the labial surface of the upper
rim
• Where it is crossed by an imaginary line from the centre of the
brows to the centre of the chin.
• Immediately below the centre of the philtrum
• Immediately below the centre of the labial tubercle
• At the bisection of the line from one corner to the other corner
of the mouth, when the lips are relaxed.
• Where it is crossed by a line at right angles to the
interpupillary line from a point midway between the pupils
when the patient is looking directly forwards.
• Midway between the angles of the mouth when the patient is
smiling.

2. High lip line
This is a line just in contact with the lower border of the
upper lip when it is raised as high as possible unaided, as in
smiling or laughing. It is marked on the labial surface of the rim
and indicates the amount of denture which may be seen under
normal conditions, and thus assists in determining the length of
tooth needed.
3. Canine lines
These mark the corners of the mouth when the lips are
relaxed and are supposed to coincide with the tips of the upper
canine teeth but are only accurate to within 3 or 4 mm. These
lines give some indication of the width to be taken up by the six
anterior teeth from tip to tip of the canines.

TRIMMING THE LOWER RECORD BLOCK
Having trimmed and marked the upper block, all that now
requires to be done is to trim the lower block so that when it
occludes evenly with the upper, the mandible will be separated
from the maxilla by the same distance that it was when the natural
teeth were in occlusion. The location of the occlusal plane
posteriorly will ultimately be determined by the height of the
mandibular anterior teeth and anterior 2/3 rd of retromolar pads.
After recording the tentative occlusal vertical relation and the
centric relation position, the maxillary occlusion rims are oriented
to the opening axis of the jaws with the help of the face bow.

Orientation Relations
Orientation relations are those that orient the
mandible to the cranium in such a way that when the
mandible is kept in its most posterior unstrained
position, the mandible can rotate in the sagittal plane
around an imaginary transverse axis passing through
or near the condyles
Transverse horizontal axis or Hinge Axis is defined as
an imaginary line around which the mandible may
rotate within the saggital plane.


The ‘Terminal Hinge position or retruded
contact position, is defined as the guided occlusal
relationship occurring at the most retruded
position of the condyles in the joint cavities. A
position that may be more retruded that the
centric relation position.


The Face bow
1.
The face bow is an instrument used to record the
spatial relationship of the maxillae to some anatomic
reference and transfer this relationship to an articulator.
Customarily this reference is a plane established by a
transverse horizontal axis and a selected anterior point.
- Glossary of prosthodontic terms, 1987
2.
A caliper like instrument used to record the spatial
relationship of the maxillary arch to some anatomic
reference point or points and then transfer this relationship
to an articulator; it orients the dental cast in the same
relationship to the opening axis of the articulator.
Customarily, the anatomic references are the mandibular
condyles transverse horizontal axis and one other selected
anterior point; called also as hingebow
- (Glossary of www.indiandentalacademy.com
prosthodontic terms, January 1999 –1)

The face bow is a caliper like device that is used to record
the relationship of the jaws to the temporomandibular joints or
the opening axis of the jaws and to orient the casts in this
relationship to the opening axis of the articulator. (Boucher. 10th ed)
A face bow is used to record the three dimensional
relation of the maxillae to the cranium. The face bow record is
used to orient the maxillary cast to the articulator this
procedure is called the face bow transfer. Mandibular opening
and closing movement are reproduced when the transverse
horizontal axis is coincident with the articulator hinge axis. In
order to create precise occlusion, the casts would be oriented
correctly which depends on an accurate face bow transfer.
(Lucia 1960)

Types of Face bow:
There are two types of face bows.
1.

Kinematic face bow

2.

Arbitrary face bow – Facial type
- Earpiece type


Review Literature:

The study of hinge axis opening of the mandible and the need to
accurately locate it has occupied many distinguished workers over
the years.
Locating the transverse hinge axis was first discussed by
Campion (1902), who felt that the axis of the articulator should
coincide with that of the patients.

Gysi (1910), in his treatise stated “the mandible in opening and
closing rotates around another center, which, however has no
influence in the setting up of teeth for articulators, and therefore
need not be considered in construction of an articulation”

Other important workers in this field were Bennet (1908,
1924), Needles (1923, 1927), and Wardsworth.
Stansberry (1928), was dubious about the value of face
bows and adjustable articulators. He thought that since an
opening movement about the hinge axis took the teeth out of
contact, the use of these instruments was ineffective except
for the arrangement of the teeth in centric occlusion. In his
opinion, the plain line hinge type of articulator was just as
effective.
Mclean (1937) stated; “the hinge functions of the lower
portion of the temporomandibular joints are still disputed
and little understood”. The hinge portion of the jaw has two
function of great importance to Prosthodontists

First, the hinge portion of the joint is the great equalizer
for disharmonies between the gnathodynamic factors of
occlusion when occlusions are synthesized on articulator
without accurate hinge axis orientation, there may be
minor cuspal conflicts, which must be removed by
selective spot grinding.
The second function of the hinge portion of the joint is
inherent in the fact that in it takes place all changes of the
level of biting closure, commonly called opening or closing
the bite.”


Regarding the satisfactory construction of full dentures, he said
that opening or closing the bite on a articulator with an incorrect
hinge axis location would result in unsatisfactory occlusion of a
dentures when they were placed in the mouth. When the hinge
axis on the articulator was too far forward compared with its
location on a patient, closing the interocclusal distance would
result in the dentures meeting prematurely posteriorly. If the axis
was too far posteriorly, premature contact would occur anteriorly.
If the axis was too low, the lower denture would be forward of
centric relation. If too high, the lower denture would be posterior
to centric occlusion. The conclusion was that any alteration in the
interocclussal distance must be made in the mouth or by the use of
a hinge articulator. If the latter were to be use, then the hinge axis
must be determined as a stationary point (i.e. rotatory but not
translatory) over the head of the condyle during hinge axis
movements and not by palpation or anatomical location.

McCollum (1939), was one of the leading advocated of the
hinge axis theory and published a very important series of
articles concerning restorative remedies. He stated: “In
1921 I became convinced that the opening and closing center
of the mandible was a most important factor in dental
articulation and that its determination was preliminary to
the transferring to an articulating instrument a record of
jaw relations..


In his articles he lauded Snow for his discovery of the face
bow and its use and at the same time he critici1zed Gysi on his
views of the hinge axis and for saying that changing vertical
dimension is a chair side operation. McCollum also described
how be came to demonstrate conclusively the existence of the
definite opening and closing axis by using a face-bow rigidly
attached to the lower teeth with an orthodontic appliance. He
found wide variation in anatomic location of the points and
between sides of the same individual. He said that the hinge
axis point remained constant throughout life.
Other important workers in this field were Higley (1940),
Stuart (1947), Logan (1941), McLean (1944), and Branstad
(1950).


Robert. G. Schallhorn (1947), studying the arbitrary
center and kinematic center of the mandibular condyle
for face bow mountings concluded that using the
arbitrary axis for face bow mountings on a semi
adjustable articulator is justified. He says that since, in
over 95% of there subjects, the kinematic center lies
within a radius of 5 mm. from the arbitrary center.

Craddock and Symmons (1952), considered that the
accurate determination of the hinge axis was only of
academic interest since it would never be found to be
move that a few millimeters distant from the assumed
center in condyle itself.


Posselt (1952), conducted extensive studies on the hinge
axis. He found that the extent of hinge opening between
the upper and lower incisor teeth was 19.2 mm. 1.9mm.

Page (1952), described the ‘hinge bow’ developed by Mc
Collum in 1936 as one of the most important contributions
made to dental science.

Lucia (1953) stated “the practical importance of the
hinge axis and hinge axis transfer to an articulator is of
tremendous importance. “ without a hinge axis transfer he
thought it impossible to diagnose an occlusal problem.

Bandrup – Wognesen (1953), discussed the theory and
history of face bows. He quoted the work of Beyron who
had demonstrated that the axis of movement of the
mandible did not always pass through the centers of the
condyle. They concluded that complicated forms of
registration were rarely necessary for practical work.

Other very important workers in this field were
Laurizten (1951), Clapp (1952), Sloane (1951), Granger
(1952), Lucia (1953), Sicher (1954), Thompson (1954),
page (1955), Collet (1955), Kornfield, (1955), Trapozzano
(1955), and Beck and Morrision (1956)

Teteruck and Lundeen (1966), evaluated the
accuracy of the ear face bow and concluded that
only 33% of the conventional axis locations were
within 6 mm of true hinge axis as compared to
56.4% located by ear face – bow. They also
recommended the use of ear bow for its accuracy,
speed of handling, and simplicity of orienting the
maxillary cast.
Thorp, Smith, & Nicholos ( 1978), evaluated
the use of face bow in complete denture
prosthesis occlusion. Their study revealed very
small differences between a hinge axis face bow
Hanau 132-SM face bow, and Whipmix ear-bow.


Neol D. Wilkie 1979, analyzed and discussed five commonly used
anterior points of reference for a facebow transfer.
He said that not utilizing a third point of reference may result in
additional and unnecessary record making, an unnatural
appearance in the final prosthesis and even damage to the
supporting tissues. He suggest the use of the axis-orbital plane
because of the ease of marking and locating orbital and therefore
the concept is easy to teach and understand.

Bailey J.O.J.R.. and Nowlin T.P in 1981 in their study concluded
that face-bow transfer utilization orbital as the third point of
reference does not accurately establish the relationship of the
Frankfurt horizontal to the occlusal plane on the articulator.

Elwood. H.
Staele et al 1982, evaluated esthetic
considerations in the use of face-bow.

Goska and Christensen (1988), investigated cast positions
using different face-bows. They concluded that it was not
possible to establish clinical superiority between one type of
face bow and another because the casts are mounted in
relation to anatomic land marks that vary from subject to
subject.


Parts of a Face Bow

(Winkler –5, Whipmix manufacturers

manuel –25)

It consists of a “U” shaped frame or assembly that is large
enough to extend from the region of the temporomandibular
joints to a position 2-3 inches in front of the face and wide
enough to avoid contact with the sides of the face. The facia
type of face bow has condyle rods that contact the skin over
the temporomandibular joints. Whereas in the ear piece type
it is known as a condylar compensator since their location on
the articulator approximately compensates for the distances
the external auditory meatuses are posterior to the transverse
opening axis of the mandible. The part that attaches to the
occlusion rims is the fork. The fork is attached to the face
bow by means of a locking device, which also serves to
support the face bow, the occlusion rims and the cast while
they are being attached to the articulator.

Kinematic Face bow (Rosensteil –26)
The Kinematic face bow is initially used to accurately
locate the hinge axis. It is attached to a clutch, which in
turn attaches to the mandibular teeth. As the mandible
makes opening and closing movements the condylar styli
move in an arc. Their position is adjusted until they
exhibit pure rotation and not translation, when the
mandible is opened and closed. The points of rotation are
marked on the skin and this determines the true hinge
axis. The mandibular clutch is removed and the face bow
is attached to the maxillary arch. The true rotation points
are again used to orient the tips of the condylar styli .

Kinematic location of the hinge axis works well
when natural mandibular teeth remain to stabilize the
clutch mechanism. However, they are generally not used
for complete denture prosthesis prosthesis fabrication
because the resiliency of the soft tissues and the resultant
instability of the mandibular record base make precision
location of the rotational centers almost impossible.


Arbitrary face bow: (Rosensteil –26)
The arbitrary type of face bow is so called because it uses
arbitrarily located marks on the skin at the condyle points as
the hinge axis position.

1. Facia type:

In the facia type the
condyle rods are positioned on a line extending from the outer
canthus of the eye to the superior inferior center of the tragus
and approximately 13mm. anterior to the distal edge of the
tragus of the ear.
(winkler-5,

McCollum -28)

This locates the condyle rods within 5mm. of the true
center of the opening axis of the jaws. The presence of an
assistant is required to hold the bow while the prosthodontist
without clamping the condyle rods centers the device so that
equal readings are obtained on both sides. The wing nut of
the clamp is tightened to hold the face bow in place on the
occlusal fork attached to the maxillary occlusion rim.

2. Ear piece type: the earpiece face bow is designed to
fit into the external auditory meatuses. Here also the fork
is attached to the maxillary occlusion rim. The whip mix,
Hanau earpiece and Denar slide matic face bow are
equipped with plastic earpieces at the condylar ends of the
bow. When an earpiece face bow is removed, it is
attached to the articulator by orienting “centering holes”
in the earpieces on the side of the condylar housings of the
articulator. With the denar slidematic face bow, the
anterior portion of the apparatus is removed from the
bow proper and supported in the articulator by a special
jig, which replaces the incisal guide table.


All articulators require either an arbitrary or specific
third point of reference for articulating the maxillary cast.
This is done with an orbitate pointer or a nasion relator .
(Neol D Wilkie –32)

It is important to remember that the critical relationship
being transferred is between the maxillae and the hinge axis,
to raising or lowering the anterior part of the face bow does
not alter this relationship. Varying the position of the
anterior part of the face bow will create a change in the
absolute values for the condylar guidance settings.
However, as long as eccentric records are used to determine
condylar guidance’s after the casts are mounted the values
for condylar guidance will be equivalent relative to the
mounting of the casts.(Ulf Posselt –30, Cristensen R L-31)


#

Description

1

Screw

2

T- Screw

3

T- Screw

4

Horizontal clamp

5

Toggle clamp

6

Lock washer

7

Toggle clamp

8

Retaining ring

9

Bite fork

10 Cross bar assembly
11 HEX nut
12 Face bow (Right)
13 Center locking nob
14 Face bow (Left)
15 Upright post
16 Nose piece shaft
17 Face bow nob

Whip Mix
Model 9600
Face bow

18 Nose piece
19 Washer


The Plane of orientation
The maxillary cast in the articulator is the baseline from
which all occlusal relationships start and it should be
positioned in space by identifying three points, which cannot
be on the same line. The plane is formed by two points located
posterior to the maxillae and one point located anterior to it.
The posterior points are referred to as the posterior points of
reference and the anterior one is known as the anterior point
of reference.


Posterior points of reference: (Neol D. Wilkie –32)
The position of the terminal hinge axis on either side of the
face is generally taken as the posterior reference points.
Terminal Hinge position is the most retruded hinge position.
The limits of opening at this position have been determined to
be around 12 to 15 degrees or 19 to 20mm at incisal edges.

Location of the Posterior References Points:
Prior to aligning the face bow on the face, the posterior
reference points must be located and marked.
The posterior points are located by
•

Arbitrary method

•

Kinematic method.


The Anterior points of reference

(Neol D Wilkie-32,Baily

JoJr-33)

It was important to ascertain at what level in the articulator
the occlusal plane should be placed. The selection of the
anterior point of the triangular spatial plane determines
which plane in the head will become the plane of reference
when the prosthesis is being fabricated. The prosthodontist
can ignore but cannot avoid the selection of the anterior point.
The act of affixing a maxillary cast to an articulator relates
the cast to the articulaaror’s hinge axis, to the vertical axes, to
the condylar determinants to the anterior guidance, and to the
mean plane of the articulator.

Reasons for selecting an anterior point of reference
(

Neol D Wilkie –32)

1.
When three points are used the position can be
repeated, so that different maxillary casts of the same patient
can be positioned in the articulator in the same relative
position to the end controlling guidance’s. For this reason it
is important to identify the mark permanently or be able to
repetitively measure an anterior point of reference as well as
the posterior points of reference.
2.
A planned choice of an anterior reference point will
allow the prosthodontist and the auxiliaries to visualize the
anterior teeth and the occlusion in the articulator in same
frame of reference that would be used when looking at the
patient. For example, when using the Frankfort horizontal
plane as the plane of reference, the teeth will be viewed as
though the patient were standing in a normal postural
position with the eyes looking straight ahead.

3. An occlusal plane not parallel to the horizontal in the
beginning steps of denture fabrication may be
unknowingly located incorrectly because of a tendency
for the eye to subconsciously make planes and line
parallel. Therefore the prosthodontist may wish to
initially establish the restored occlusal plane parallel to
the horizontal in order to better control the occlusal
plane in its final position.
4. The prosthodontist may wish to establish a baseline for
comparison between patients, or for the same patient at
different periods of time.


Selection of an anterior reference point (Neol D Wilkie-32)
The various anterior points that may be used are as follows.
1.
Orbitale: In the skull, orbitale is the lowest point of the
infraorbital rim. On a patient it can be palpated through the
overlying tissue and the skin. One orbitale and the two posterior
points that determine the horizontal axis of rotation will define
the axis orbital plane. The orbitale is transferred from the
patient to the articulator with the help of an orbital pointer on
the face bow or by raising the face bow itself to the level of the
orbitale.


Advantage:
A) It is easy to locate and mark the orbitale.
B) The concept is easy to teach and understand.

Disadvantage:
Relating the maxillae to the axis orbital plane will slightly
lower the maxillary cast anteriorly from the position that
would be established if the Frankfort horizontal plane
were used.


2.      Nasion minus 23mm: According to Sicher
another skull landmark, the nasion, can be
approximately located in the head as the deepest part
of the midline depression just below the level of the
eyebrows.  The nasion guide, or positioner, of the face
bow, which is designed to be used with the whipmix
Articulator, fits into this depression. This guide can
be moved in and out, but not up and down, from its
attachment to the face bow crossbar.  The crossbar is
located 23mm. below the midpoint of the nasion
positioner. When the nasion guide of face bow is
positioned anteriorly on the nasion the crossbar will
be in the approximate region of orbitale. The facebow crossbar and not the nasion guide is the actual
anterior reference point locator.

3. Obitale minus 7 mm

4. Alae of the nose: this method uses the
Campers line as the plane of orientation – the right or
left ala is marked on the patient and the anterior
reference pointer of the face-bow is set. This relation
is then transferred to the articulator


A.

Whip Mix face bow

B.

Hanau articulator with arbitrary face bow

C.

Dentatus articulator with arbitrary face bow

D.

Modified Whip Mix face bow


Face bow transfer (Sloane R B –34)
An arbitrary mounting of the maxillary cast without a
face-bow transfer can introduce errors in the occlusion
of the finished denture. A faulty or careless mounting,
with or without a face bow, will obviously lead to errors
in cast inclination that can seriously affect the condylar
inclination. A face bow transfer is essential when cusp
teeth are used allows minor changes in the occlusal
vertical dimension without having to make new maxillo
mandibular records, and is also most helpful in
supporting the maxillary cast while it is being mounted
on the articulator.


Arbitrary Axis for various Face bows  (winkler-5,Thorp-35)
When using a Hanau face-bow, a Rechey condylar marker is used to
scribe an arc about 13mm. anterior to the external auditory meatus.
Using a ruler, held so that it runs from the corner of the eye to the
top of the tragus of the ear, place a mark where this line intersects
the arc made by the condyle marker.  This locates the arbitrary axis
for the Hanau face bow condyle rods, which is within 2 mm of the
true center of the opening axis of the jaws.  If desired, a plane of
orientation can be determined by utilizing the infraorbital notch as a
third point of reference with the infraorbital pointer of the Hanau
face-bow, Whereas for whip mix face bow locating an arbitary axis
is not necessary when using the Whip Mix articulator, since it was
designed and constructed after much research with a built in
locator.  The inserting of plastic earpieces into the external auditory
meatus automatically locates the face bow in the proper

Arbitrary axis for denar slidematic face bow:
The Slidematic face bow uses the external auditory
meatus for determining the arbitrary hinge axis location.
A built in reference pointer aligns the face bow with the
horizontal reference plane. The anterior reference point
is marked on the patient’s right side using the Denar
reference plane locator. The point is 43 mm. above the
incisal edge of the right central or lateral incisor for a
dentulous patient. For an edentulous patient this distance
is measured up from the lower border of the upper lip
when the lips are relaxed.


Face Bow Transfer - Whip-Mix Face Bow
(Winkler –7)

Attach the maxillary stabilized base to the bite fork.
Insert in the mouth and have the patient hold it in place
with both thumbs using light pressure, or place the
lower base in the mouth and close against the bite fork.
The face bow is carried to patient’s face, and the face
bow fork toggle assembly is slipped onto the stem of the
bow fork; the plastic earpieces are inserted into the
external auditory meatus and brought slightly forward.
The nasion relator assembly is attached to the face bow;
the plastic nosepiece should rest on the nasion, and the
face bow is tightened.  The face bow is locked to the bite
fork.  The positioning of the face bow and locking of the
bite fork to the face bow must be done carefully or the
purpose of the face bow transfer is defeated.  The entire
assembly is then carried to the articulator

The upper cast is attached to the articulator. The
proper use of the face bow prevents errors of
occlusion in the finished dentures during eccentric
movement of the lower jaw within the functional
range.


Indications for Face Bow Use. ( Heartwell –4, bandrup-morgsen36)

When the disharmonies in occlusion resulting from failure to use
the face bow are analyzed, it can be concluded that the face bow
should be used when.
1.  Cusp form teeth are used
2.  Balanced occlusion in the centric positions is desired
3   A definite cusp fossa or cusp tip to cusp incline relation is
desired.
4.  When interocclusal check records are used for verification of
jaw positions.
5   When the occlusal vertical dimension is subject of change, and
alterations of tooth occlusal surfaces are necessary to
accommodate the change

6    To diagnose existing occlusion in-patient’s mouth.

Vertical Jaw relations
Introduction:
A vertical jaw relation is defined as the distance
between two selected points, one on the maxillae and
one on the mandible. That is, they are established
by the amount of separation of the two jaws in a
vertical direction under specified conditions.
The physiologic rest position of the mandible as
related to the maxillae and the relations of the
mandible to the maxillae when the teeth are in
occlusion are the two dimensions of jaw separation
of primary concern in complete denture prosthesis
prosthesis constructions.

Thus vertical jaw relations are classified as (Boucher –3)
1.      The vertical relation of rest position
2.      The vertical relation of occlusion
3.      The differences between the vertical relation of rest
and the occluding vertical relation, also known as
“freeway space.”
The “rest vertical relation” is the distance measured
when the mandible is in the rest position.
In infants and in edentulous adults the vertical relation of
rest position is established by muscles and gravity and is
assumed only when the muscles that open and close the
jaws are in a state of minimal contraction to maintain the
posture of the mandible.

REVIEW OF LITERATURE
Wallisch (1906) was the first to define physiologic rest
position.
In the late 1920’s, Sicher and Jandler described the
role of the musculature in controlling the posture of the
mandible and stated that the rest position of the
articulation is that in which the mandible is at a slight
distance from the maxilla and in this position the
mandible is kept against gravity by the forces of the
closing muscles.


Niswonger (1934) was perhaps the first
investigator to study extensively the rest position of
the mandible. He established that the interocclusal
distance measured 4/32 inch i.e. 3 mm. in majority
of the cases and that the patients whose vertical
dimension of occlusion was excessive complained of
soreness of the residual ridges due to mastication,
and once this space was developed after tissue
changes, the patient was able to masticate with
satisfaction and comfort.


Many observers pointed out the role of muscles
physiology in limiting the extent to which vertical
dimension of occlusion could be increased.
Mershon (1938 ) contended that muscles cannot
lenghthen to accommodate an increase in bony size, but
rather bone adapts itself to the length of the muscles.
Tench ( 1939 ) felt that the functional length of the
muscles could not be increased after observing failures of
restorations constructed at an excessive vertical dimension
of occlusion.
Gillis ( 1941 ) stated the mandibular rest position is
not artificially but naturally established and that the
interocclusal distance did not vary greatly between
different individuals, with average of 3mm.

Schlosser (1941) conducted a series of phonetic
experiments indicating that the movements of the
mandible during speech were subject to habitual
fixation. He concluded that edentulous patients were
repeatedly able to bring the mandible to and identical
rest position by sounding the letter ‘m’
. Thompson (1946) reported on the cephalometric
analysis of the rest position in edentulous and semi
edentulous adults and concluded that if the mandible is
carried to a greater than normal rest position by dental
restorations. The mandible will return to its
preordained position at the expense of the alveolar
process or by the intrusion of occluding teeth.

Sicher (1954) felt that the mandibular rest position was
completely dependent on the tonicity of the musculature
and that only in disturbed muscle forms as in disease, over
work and nervous tension could the rest position vary
from normal. He also pointed out that since the muscle
tonus is fairly constant for each individual, the
mandibular rest position is fairly position


Another school felt that rest position was variable.
In the 1930’s when the ground work of the
concept of constancy was being developed, Harris and
Hight (1936 ) reasoned that the vertical dimension of
occlusion was dependent on the occlusal contacts in
the closing movements of the mandible. They felt that
reduction of the vertical dimension occlusion was
caused by abrasion of teeth, loss of posterior teeth,
resorption of ridges under dentures and faulty dental
work. Hence, the correct vertical opening in
edentulous patients was debatable.


Leof (1950) stressed that muscles tone rather than
muscle length controls the rest position and that muscle
tone can and does vary by exercise or excessive rest.
Hypertonicity of mandibular muscles through grinding
habits may interfere with the maintenance of a constant
rest position and result in a reduction of the normal
interocclusal distance.


Atwood (1956) conducted a longitudinal
radiographic analysis of face height using a
combination of swallowing and phonetics before and
after extraction and demonstrated variability within a
sitting; between sittings and between readings, with
and without dentures. He concluded that rest position
is a dynamic rather than a static concept and that it
varied from person to person and within the same
person. A cine fluoroscopy technique coupled with
electronics was suggested to provide a better insight
into the variability of rest position.


Tallgren ( 1957 ) studied the changes in adult face height by means
of cephalometrics and her findings were similar to that of Olsen
and Atwood in showing a certain instability of the rest position
after removal of teeth.
Swerdlow ( 1964 ) studied a group of immediate denture patients
over a period of 6 months. He recorded cephalometrically, changes
in occlusal vertical dimension, rest vertical dimension and
interocclusal distance during the transition from natural teeth to
immediate dentures. He concluded that (a) the phonetic method of
recording rest position gave consistent values for interocclusal
distance than did the swallowing method. (b) The occlusal vertical
dimension and rest vertical dimension increased initially and then
decreased markedly in the 6 months of wearing dentures. (c) The
interocclusal distance adjusted itself to accommodate to the
variations in facial vertical dimension. (d) and a change in
mandibular load appeared to influence the rest position of the
mandible.

“Physiologic rest position” is the postural position of the
mandible when an individual is resting comfortably in an
upright position and the associated muscles are in a state of
minimal contractual activity.
This position is controlled by the muscles that open, close,
protrude and retrude the mandible and further is controlled
by the position of the head, which modifies the effect of
gravity. The force applied by the Jaw opening muscles is
added to the force of gravity, when the head is upright. In a
reclining patient, gravity does not pull the mandible down and
so one may find the distance between the jaws to be less than it
is when the head is upright. When observations of physiologic
rest position are being made, the patients’ head should be
upright and unsupported.

The second thing that establishes the vertical relation of the
mandible to the maxillae is the occlusal stop provided by
teeth or occlusion rims.  The “occlusal vertical relation” is the
distance measured when the occluding members are in
contact.
    In the course of a lifetime, many things happen to natural
teeth.  Some are lost, some are abraded so that they lose their
clinical crown length, dental caries attacks some of them, and
restorations fail to maintain their full clinical crown length.
Even dentulous patients may have a reduced occlusal vertical
relation. The pre-extraction occlusal vertical relation may
not be a reliable indication of the vertical relation to be
incorporated in complete denture prosthesis prosthesis. But
any information available about the occlusal vertical relation
with natural teeth should not be ignored and modifications
from it should be made as indicated.

The health of the periodontal membranes that support
the natural teeth and the health of the mucosa of the
basal seat for dentures depend on rest from occlusal
forces to maintain their health. For this reason, an
interocclusal rest space between the maxillary and
mandibular teeth is essential for the opening and closing
muscles and gravity to be in balance when the muscles
are in a state of minimum tonic contraction. The
interocclusal rest space is the difference between the rest
vertical relation and the occlusal vertical relation and
amounts to 2-4 mm. in a vertical direction if observed at
the position of the first pre molars.


Once the vertical relation of rest position has been
determined it is easy to adjust the vertical relation of the
occlusion rims sufficiently to provide for the necessary
interocclusal distance

   Other vertical relation such as the vertical relations of the
two jaws when the mouth is half open or wide-open are of no
significance in the construction of dentures.

Methods   ( Boucher-3)
   Many methods have been proposed for determination of the
correct vertical relation of the mandible to the maxillae.
Some of them have been offered as ‘Scientific”, but as yet
none is accurate.  Others have been offered as helpful aids to
good clinical judgment. All those currently in use will be
discussed

Other classifications:
The methods for determining the vertical maxillomandibular
relations can be grouped roughly into two categories.
1. The mechanical methods
2. The Physiologic methods
The use of esthetics as a guide combines both the mechanical
and physiologic approaches to the problem.


Mechanical Method:
1.      Ridge relations
a)      Distance of incisive papilla from mandibular incisors:
The incisive papilla is used to measure the patients’ vertical
relation since it is a stable landmark and is changed little
by resorption of the residual alveolar ridge.   The distance
of the incisive papilla from the incisal edge of the
mandibular incisors is about 4 mm. in the natural
dentition.  The incisal edge of the maxillary central incisor
is an average of 6mm. below the incisive papilla. So the
average vertical overlap of the opposing central incisor is
about 2 mm. the disadvantage of this method is the absence
of lower teeth and so is only useful in the treatment of
single dentures.

b)  Parallelism of the ridges:  Paralleling of the ridges, plus
a 5 degree opening in the posterior region as suggested by
sears, often gives a clue to the correct amount of jaw
separation. This theory if used alone, is not reliable;
because many patients present such marked resorption
that the use of this rule would generally close the vertical
relation.  But when considered with other observations, it
may be of value.  However, in most patients the teeth are
lost at irregular intervals and the residual ridges are no
longer parallel


2. Measurement of former dentures:(Majid Bissasu-39)
Measurements are made between the borders of the maxillary
and mandibular dentures by means of a boley gauge and
corresponding alterations can be made in the new denture to
compensate the occlusal wear.

3. Pre-extraction records  (Ricketts –40, Crabtree 41 ) -:  It is
frequently possible to see the patient before he or she becomes
edentulous.  In such cases one can usually establish the
occlusal position, record it in some manner and transfer this
record to the edentulous situation.  This is a relatively easy
procedure and can be accomplished in several ways

a) Profile radiograph: the exposure of a full
lateral radiograph is made with the teeth in
occlusion, and after extraction trial plates are
made to an apparently correct vertical relation.
They are inserted, the patient closes on them and
another radiograph is taken. The two films are
compared and necessary adjustment is made to
bring the mandible in correct position as in the
initial film. The image should have approximately
1:1 ratio to the patient. Disadvantages include
inaccuracy due to enlargement of the image, it is
time consuming and it may result in too frequent
exposure to radiation.


b) Profile Photographs (Alexander Morton –42): Profile
photographs are made and enlarged to life size. The
photographs should be made with the teeth in
maximum occlusion. Measurements of anatomic
landmarks on the photograph are compared with
measurements of the face, using the same landmarks.
These measurements can be compared when the
records are made and again when the artificial teeth
are tried in. Disadvantage of this method is that the
angulation of the photograph might differ with the
patients. Posture.


c) Lead wire adaptation

(Crabtree Ballard): Lead

wires may be adapted carefully to pre extraction
profiles, and this contour is transferred to a
cardboard. The resultant cutout is stored until after
extraction. When the prosthodontist estimates the
vertical relation using the trial plates, the cardboard
cutout is placed against the profile in order to see
whether the facial contour has been maintained or
reestablished. It is not in common use today.


d) Swenson’s method (Swenson-70): Swenson
suggested that acrylic resin face marks made before the
extraction, and later when the patients is rendered
edentulous, it is fitted on the face to see whether the
vertical relation has been restored properly. Drawbacks
of this method is that, it is time consuming requires lot of
skill and experience with the use of facial impressions
and casts for the fabrication of artificial facial parts and
lastly the face assumes a different topography in the erect
posture from that in the recumbent or semirecumbant
position.


Jaw relation /certified fixed orthodontic courses by Indian dental academy

Jaw relation /certified fixed orthodontic courses by Indian dental academy

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