seminar of biomechanics of hip and total hip replacement.

1. MODERATOR:- DR PAVAN CHEBBI (ASSISTANT PROFESSOR
SGITO)
PRESENTER:-DR ABHISHEK CHUADHARY (DNB RESIODENT
AT SGITO)

2.  It can be defined as the science concerned with
the internal and external forces acting on the
human body and the effects produced by these
forces.
 Julius Wolff addressed the relation between the
inner architecture of the bone and the functional
loading already in the nineteenth century.
 Friedrich Pauwels built the foundation for a
mechanical approach to understand joint loading
65 years later.

3. ACETABULAM
•innominate bone with
contributions from the
ilium (approximately
40%
of the acetabulum),
ischium (40%) and the
pubis (20%) .
•fusion of this starts to
occur around the age of
14 – 16 years and is
complete usually by the
age
of 23.

17.  Force
◦ definition
 a push or pull on an object resulting from the object's
interaction with another object
◦ equation
 force = mass x acceleration, F=ma
 1 Newton = force required to give 1 kg mass an
acceleration of 1 m/s2

18.  Vector
◦ definition
 a quantity that contains both direction and magnitude
 scalar quantities do not have direction
◦ forces and velocity can be broken down into vectors

19.  Moment (torque)
◦ definition
 the tendency of a force to rotate a body around an axis
◦ Equation
◦ moment (torque) = force(perpendicular) X
distance

20.  Work
◦ definition
 when a force acts upon an object to create
displacement
◦ equation
 work = force (vector parallel to displacement) x
distance

21.  Newtons laws
◦ first law
 if there is no net force on an object, its velocity
remains constant
◦ second law
 force equals mass multiplied by acceleration
 F=ma
◦ third law
 when a first body exerts a force on a second body, the
second body exerts a force that is equal in magnitude
and opposite in direction on the first body
 F2=-F1

22.  The hip is a true ball-and-socket joint
surrounded by powerful and well-balanced
muscles, enabling a wide range of motion in
several physical planes while also exhibiting
remarkable stability.

23.  Joint reaction force
◦ defined as force generated within a joint in response to forces acting on the
joint
◦ in the hip, it is the result of the need to balance the moment arms of the body
weight and abductor tension (see diagram to right)
◦ maintains a level pelvis
 Coupled forces
◦ when two movements and associated forces are coupled
 Joint congruence
◦ relates to fit of two articular surfaces
◦ high congruence increases joint contact area
 Instant center of rotation
◦ point about which a joint rotates
◦ often changes during rotation due to joint translation
◦ center of gravity of human is just anterior to S2
 Friction and lubrication
◦ not a function of contact area
◦ lubrication decreases friction
◦ examples
 coefficient of friction of human joints is .002 to .04
 TJA (metal on PE) is .05 to .15

26.  Actions that decrease joint reaction force include
◦ increase in ratio of A/B (shift center of rotation medially)
 moving the acetabular component as far medial, inferior, and anterior
 shifting body weight over affected hip
 this results in Trendelenburg gait .
 increasing offset of femoral component
 long stem prosthesis
 lateralization of greater trochanter
 varus neck-shaft angulation
 increases shear across joint
◦ cane in contralateral hand
◦ reduces abductor muscle pull and decreases the moment arm
between the center of gravity and the femoral head
 carrying load in ipsilateral hand
produces additional downward moment on same side of rotational
point
 Actions that increase joint reaction force include
◦ valgus neck-shaft angulation
 decreases shear across joint

32. The location of
center of rotation of
femoral head is
determined by
1. Vertical offset
2. Horizontal(medial) offset
3. Anterior offset
(Anteversion)
So restoration of these
dimentions is must

33.  MEDIAL RESTORATION IS SIMPLY CORRECTED
BY MAKING NECK ADJUSTMENT BUT……
LIMB LENGTH INCREASES

35.  NORMAL FEMUR IS 10 TO 15
DEGREE ANTEVERTED.
 USUALLY ACCOMPLISHED BY
ROTATING THE COMPONENT IN
FEMORAL CANAL.
 IN PRESS FIT FIXATION IS USED –
MODULAR FEMORAL
COMPONENT IS USED.

37.  AFFECTS ROM ,IMPINGEMENT,STABILITY OF
ARTICULATION.

38.  Its complicated …..
 Because all joint parameters are influenced by
the operation: joint center, neck angle, offset,
lever arms, and the range of motion until
impingement.
 Our goal is best possible range of motions
with good stability and prolong implant life .

39.  design and implantation procedure, which
directly or indirectly influence the occurrence
of loosening, lysis, or dislocation. These
aspects are:
1. • Range of motion
2. • Impingement implant fixation
3. • Tissue damage during implantation and
tissue tension after THA
4. • Component orientation (stem, cup)
5. • Bearing material

41.  Biomechanically most important is the position of the
femur with respect to the pelvis, in which the end of
the RoM is reached and the prosthesis neck
“impinges” on the cup or impingement occurs
somewhere else between femur and pelvis.
 Impingement can lead to subluxation or even
dislocation of the hip joint.
 If impingement occurs repeatedly in positions inside
the RoM required by the patient for either daily or
athletic activities, dislocation is rather probable.
 The “jumping distance,” which is the distance the
head has to “jump” before leaving the cup, amounts in
hemispherical cups to 50% of the head diameter.

42.  Advantages
1.Increased ROM
2.Increased jumping distance.
Disadvantages
1. higher friction moments(especially MOM
bearings).
2. Requires better fixation of bearing other
componenets .
3. Cup loosening and Corrosion
4. More dissection required –more soft tissue
damage during primary surgery or during
relocating a large head dislocation.

43. Head size Dislocation rates
22mm Upto 18 %
28mm 0.6-3%
32mm 0.5%
38mm 0.0%
Size is not the only parameter for dislocation tho…
Implant position and soft tissue tension is equally
important

44.  Practically head diameter is limited to about
36 mm for primary hip arthroplasty; in the
case of Polyethylene possibly even to 32 mm,
since for hard-soft bearings wear increases
with head diameter.
 Geometry of Taper..
 A further design aspect important for the
technical RoM is the location of the entrance
plane of the cup and the cup profile

45.  Presently, cemented fixation still shows
statistically the best results in terms of the
whole THA population (Table 2.1).
 This changes when young and active patients
are involved; in this patient collective,
cemented prostheses do not perform as well as
in the older population

46.  The success of uncemented fixation depends
on the ingrowth of bone. The ingrowth of
bone is only possible, if the patient’s activity-
induced relative interface motion
(micromotion) remains below a critical
threshold in the early postoperative period
(primary stability).
 So good quality reaming and press fitting
followed by non/partial weight bearing for
few weeks results in good bone ingrowth.

47.  The amount of soft tissue damage during
surgery and the tension in the remaining soft
tissue after THA implantation are also
important factors for the stability of the joint.
 Hard tissue damage can also occur during
implantation. This can either result in direct
complete fractures or fissures of the bone, or
in micro-fractures of trabecular bone, which
can develop to a complete fracture later on

48.  Component orientation and position is
probably the most important biomechanical
aspect for the tribological and functional
success of a THA procedure.

49.  If the combined offset decreases, abductor
muscle tension drops and hip instability can
occur. Opposite, an increased combined
offset can produce excessive tension within
the gluteal muscles and the iliotibial band
and cause trochanteric pain.

51. Without leg length
discrepancy, the
interteardrop line is
perpendicular to the
longitudinal axis of the
patient, and the cup
inclination angle equals
the lateral opening
angle.

52.  The radiographic cup anteversion is defined
as the angle between the acetabular axis and
the coronal plane.
 Because the projection of the acetabulum
depends on the pelvic tilt (i.e. the angle
between the anterior pelvic plane and the
coronal plane of the body), it is important to
measure cup anteversion on standardized
radiographs, i.e. with the radiographic film
parallel to the anterior pelvic plane

53.  Cup and stem anteversion have opposite effects
on impingement. Increasing cup anteversion and
decreasing stem anteversion will favour
impingement of the posterior aspect of the neck
against the posteroinferior rim of the acetabulum
in extension and exorotation . Opposite,
decreasing cup anteversion and increasing stem
anteversion will favour anterosuperior
impingement in flexion and adduction.
 Stem anteversion is difficult to control specially
in uncemented ones.

54.  Cup inclination influences the degree of
anterosuperior and posteroinferior coverage, and
so the impingementfree range of motion.
 A horizontal cup position increases
anterosuperior coverage, but uncovers the
posteroinferior aspect of the acetabulum.
 Opposite (more vertical cup), increasing the cup
abduction angle will expose the anterosuperior
aspect of the acetabulum but will increase the
posteroinferior coverage.

55.  To avoid impingement, a cup inclination of 45° to
55° has been recommended, the optimal
combined anteversion depending on the
acetabular abduction angle.
 According to Yoshimine et al , a cup position
avoiding impingement can be estimated with the
following formula :
 (Cup inclination) + (Cup anteversion) + 0.77×
(Stem anteversion)) = 84.4.
 (Stem anteversion) = (37° - Cup anteversion)/0.7.
 From a clinical point of view, a “safe zone to avoid
hip dislocations” has been defined between 40° ±
10° of cup abduction and 15° ± 10° or 30° ± 10°
of anteversion.

56.  To avoid excessive wear, the cup abduction
angle should be 45° or less .Steep cups, with
an abduction angle of 50°-55° or more, are
prone to excessive wear and edge loading
especially with metal-on-metal articulations
and when microlaterization or
microseparation do occur..not much of an
issue with ceramic heads..

58. HORIZONATAL CUP VERTICAL CUP
Increases ant sup coverage
Preffered to reduce wear while
walking.
Uncovers post-inf acetabulam
Reduce cup loading area in deep
flexion(rising from chair)
Increase risk of dislocation in
flexion,adduction,internal
roationespecially when a posterior
approach has weakened the soft
tissues.
As a more vertical cup position
tends to decrease the
anterosuperior contact area of the
cup,
Edge loading
Increase postero-inferior coverage
As a more vertical cup position tends to decrease the
anterosuperior contact area of the cup, this should
be compensated for by decreasing cup anteversion.
Opposite, a horizontal cup should be implanted with
more anteversion.

60.  So, lowering the cup coverage to limit
impingement decreases the boundaries of an
acceptable cup position in terms of wear.
 Additionally, lowering cup coverage decreases
the press fit of the cup in the acetabular bone
and the implant/bone contact area, making these
cups more vulnerable to early loosening.
 Accepting such compromises to allow a better
ROM only makes sense for hip resurfacings
because of the unfavourable head/neck ratio.

61.  Acta Orthop. Belgium., 2014, 80, 336-347
 The Open Sports Medicine Journal,2010, 4,
51-57 Damien P. Byrne, Kevin J. Mulhall and
Joseph F. Baker Orthopaedic Research and
Innovation Foundation, Sports Surgery Clinic,
Santry, Dublin, Ireland.
 Atlas of Human Anatomy, Sixth Edition-
Frank H. Netter, M.D
 Apley’s System of Orthopaedics and Fractures
9th Ed
 Campbell's Operative Orthopaedics 12th