1. Review Article
Mechanical aetiology,
recognition, and treatment of
spondylolisthesis
Jennifer E. Earl
Objectives: To provide the reader with information about the aetiology of spondylolysis and
spondylolisthesis, and describe a new treatment approach that has been successful in treating
these conditions. Data sources: A MEDLINE search was performed using the keywords
`spondylolysis', `spondylolisthesis', and `low back pain' for the years 1980±2000. Data synthesis:
Spondylolysis is a very common cause of low back pain, and is characterized by the presence of a
fatigue fracture of the pars interarticularis region of the vertebra. The mechanical design of the
lumbar spine allows it to withstand the high forces that are placed on it. When these stresses are
above the threshold that can be tolerated by the bone, a fatigue fracture results. Poor healing
leads to instability of the pars interarticularis and eventually spondylolisthesis. Although this
condition often develops in early adolescence, it is frequently undetected until adulthood. A
thorough assessment of adolescents with low back pain may allow these fatigue fractures to heal
properly, and prevent problems later in life. Recent emphasis on training the deep abdominal
muscles has shown to be a more effective rehabilitation technique than traditional measures.
Conclusions: Early detection and treatment is the key to successful rehabilitation of spondylolysis.
Decreasing the offending activity while increasing the neuromuscular control of the deep
abdominal muscles provides the best atmosphere for bony healing, and decreases the chance of
future complications. * 2002 Published by Elsevier Science Ltd.
c
Introduction result in spondylolisthesis, which is de®ned as
Jennifer E. Earl MEd, Low back pain is one of the most common a subluxation or `slippage' of two adjacent
ATC, Doctoral
musculoskeletal complaints, and can be vertebrae (Wiltse et al. 1975; Stinson 1993;
Candidate, Athletic
Training Research attributed to many different causes. Children Whiting & Zernicke 1998).
Laboratory, and adolescents often endure low back pain for Epidemiological studies have shown that the
Department of
several years before being evaluated by a incidence of spondylolysis is related to age,
Kinesiology,
Pennsylvania State clinician (King 1999). The differential diagnosis heredity, gender, race, and activity level (Wiltse
University, should include, but not be limited to, tumour, et al. 1975; Johnson 1993; Comstock et al. 1994;
University Park,
Philadelphia, USA. herniated disc, spondylolysis, Hickey et al. 1997). Spondylolysis most
spondylolisthesis, infection, and in¯ammation frequently manifests during adolescence (8±20
Correspondence to:
Jennifer E. Earl, 266 (Renshaw 1995). Spondylolysis and years) (Johnson 1993; Comstock et al. 1994),
Recreation Hall, spondylolisthesis are two conditions that particularly during the teenage growth spurt
Pennsylvania State directly involve changes in the vertebra. (Comstock et al. 1994). Of patients less than 19
University,
University Park, PA Spondylolysis is de®ned as a defect in the pars years old, 32% of those whose chief complaint
16802, USA. Tel: ‡1 interarticularis ( pars), the region of the lamina was back pain had at least one pars defect
814 865 7936; Fax: between the superior and inferior articular (Morita et al. 1995). The risk declines through
‡1 814 865 1275;
E-mail: jee128@psu. facets (Wiltse et al. 1975; Stinson 1993; Whiting middle age, then increases slightly from 60±80
edu & Zernicke 1998). Progression of the defect can years (Johnson 1993). There appears to be a
* 2002 Published by Elsevier Science Ltd.
c Physical Therapy In Sport (2002) 3, 79±87 79
1466-853X/02/$ - see front matter doi : 10.1054/ptsp.2001.0084, available online at http://www.idealibrary.com on

2. Physical Therapy in Sport
genetic predisposition in that 33±50% of in deformation and incongruence of the
relatives may also have spondylolysis (Johnson vertebrae (Junghanns 1990). Although the
1993; Stinson 1993). The genetic contribution is defect may appear radiographically before age
higher in boys than in girls, but the overall 8, symptoms may not develop until later in life
incidence is highest in girls, possibly because (Johnson 1993; Smith & Hu 1999).
girls participate in more at risk activities such as
gymnastics, ®gure skating, and dance
(Comstock et al. 1994; Omey et al. 2000). In the Isthmic
USA spondylolysis occurs in 2% of the African
American population and 6% of the white Isthmic spondylolysis is the most common type,
population, while up to 60% of native Alaskans and occurs at the L5±S1 level in adolescents
may develop this condition (Wiltse et al. 1975). and young adults (Wiltse et al. 1976; Whiting &
As will become clear in the following sections, Zernicke 1998), It results from fatigue failure of
activities that require repetitive hyperextension the pars due to repetitive stress. While some
or hyper¯exion of the lumbar spine increases cases may indeed be dysplastic, current thought
the risk for spondylolysis/listhesis (Comstock is that very few defects are noted before the age
et al. 1994). Sports such as gymnastics, football, of 5±61 (Smith & Hu 1999). Perhaps the
2
diving, wrestling, weight-lifting, cricket, and incidence increases during this time because
rowing have exceptionally high incidences long periods of sitting are introduced as
(Johnson 1993; Hickey et al. 1997; Whiting & schooling begins. Spondylolisthesis results
Zernicke 1998; Leary & White 2000). when the pars elongates or separates and
Spondylolysis is classi®ed into ®ve categories allows the superior vertebra to slide forward on
based on the suspected aetiology: dysplastic, the inferior one (Wiltse et al. 1976).
isthmic, degenerative, traumatic and pathologic
(Wiltse et al. 1976). Although these conditions
have been studied for many years, there is Degenerative
much debate on possible aetiologies. The This type of spondylolisthesis occurs more
purpose of this review is to de®ne each type of frequently in adults over 40, and more
spondylolysis, describe the aetiology of the two frequently in women than men (Wiltse 1976).
most common types, and present a brief over Sometimes called pseudospondylolisthesis, it
view of clinical ®ndings and treatments. usually occurs at the L4±L5 level (Whiting &
Zernicke 1998). The slippage is a result of
degeneration of the disc and/or inter-segmental
Classi®cation of spondylolysis instability, rather than a defect in the pars
Dysplastic interarticularis. Hypomobility of the L5±S1 joint
causes the L4±L5 joint to become hypermobile
Dysplastic spondylolysis was ®rst described by and degenerative changes are likely to occur at
Wiltse (1975) and results from abnormal tissue this site as well as anterior sliding (Wiltse et al.
development of the neural arch. The neural arch 1976; Whiting & Zernicke 1998).
develops from each side of the vertebral body
and joins together via sutures in the region of
the pars interarticularis (Junghanns 1990).
Traumatic and pathologic
Failure or delay of suture closure during
development results in abnormal formation of These types of spondylolysis are much less
the neural arch. Examples of this would be common, and therefore will receive little
spina bi®da and an abnormal isthmus angle. A attention in this review. Traumatic fracture to
defective pars is less able to withstand the the pars usually occurs in conjunction with
forces that are applied to it. In a `normal' other fractures, and heals well with
vertebra, the hyaline cartilage growth plates immobilization. Other pathologies, such as a
remain active until about 20 years of age, so tumour, can weaken the tissue of the vertebra
intense activity during childhood that places and make it susceptible to damage (Wiltse et al.
high stresses on the immature spine can result 1975).
80 Physical Therapy in Sport (2002) 3, 79±87 * 2002 Published by Elsevier Science Ltd.
c

3. Mechanical aetiology, recognition, and treatment of spondylolisthesis
Aetiology of isthmic produced by the muscles. The pars interarti-
spondylolysis cularis is the thickest part of the lamina because
it is subjected to the high force being transferred
Children and adolescents most commonly suffer between superior and inferior facets. The
from isthmic spondylolysis, therefore the posterior position of the ligaments is suited to
aetiology of this type will be the focus of this withstand the high tensile forces that are applied
review. There are two theories about which to the posterior elements. The viscoelastic
movements cause increased stress at the pars. properties of the ligaments indicate that their
The ®rst and most common is that direct loading ability to withstand forces is dependent on the
of the facets during hyperextension causes high loading conditions (Simons 1994). A posterior
stress concentrations at the pars. The second is shift in the bodies' centre of mass causes the
that unbalanced shear forces at the pars during posterior elements to become compressed and
¯exion causes high stress concentrations. their structure is poorly suited for this (Schulitz
To understand these mechanisms, it is & Niethard 1980; Haher et al. 1993). As the spine
necessary to examine the structure and normal moves into extension, the force between the
mechanics of the lumbar vertebrae. The body of facets increases, thus directly increasing the
the vertebra is designed like a cardboard box strain on the pars interarticularis (Schulitz &
with walls made out of cortical bone. The Niethard 1980; Simons 1994). Introducing
mechanical properties of the vertebra are highly rotation to a spine that is already extended
dependent of the trabecular structure within the
vertebra (Whiting & Zernicke 1998). Vertically
COMPRESSION
oriented trabeculae are supported by horizontal
ones to prevent them from buckling and further
strengthen the structure (Fig. 1). This design is
well adapted to withstand the high compressive
forces on the anterior spine during standing
(Junghanns 1990; Pope 1991; Simons 1994). Age +
or repetitive stress causes a decrease in the COMPRESSION
Tension
number of horizontal trabeculae, therefore the
rest of the structure is susceptible to collapse
(Haher et al. 1993). Marrow and water in the
vertebral body act as a hydraulic cushion to
absorb some of the compressive force (Haher
+
et al. 1993; Simons 1994; Whiting & Zernicke
1998). The discs and cartilage end plates assist in Tension
this shock absorptive capacity (Junghanns 1990).
Intense cyclic loading of the spine decreases the
water content of the discs and vertebral bodies,
therefore decreasing the hydraulic effect (Haher COMPRESSION
Tension
et al. 1993). Tension
=
The posterior elements of the spine are the
pedicles, laminae, and associated processes. All Segment of
Relative
of the muscles attached to the posterior spine Weakness
exert a downward force on the posterior
elements. This means the pedicle is subject to a
bending moment where the inferior surface is Fig. 1 Bony trabeculae within the vertebra are oriented
compressed and the superior surface is under according to the type of load that occurs at that region.
In the vertebral body, they are vertical and suited for
tension. The pedicles are thick-walled cylinders compressive forces. In the posterior elements, they are
of cortical bone that are suited to withstand aligned according to the line of tensile stress.
these bending forces (Simons 1994). The cortical (Reproduced with kind permission from Pope MH et al.
Structure and function of the lumbar spine. In:
layer in the lamina is very thin. The trabeculae Occupational Low Back Pain: Assessment, Treatment, and
are oriented in a way to resist the tensile stress Prevention. St Louis, MO: C V Mosby).
* 2002 Published by Elsevier Science Ltd.
c Physical Therapy In Sport (2002) 3, 79±87 81

4. Physical Therapy in Sport
further increases the stress experienced at the vertebrae is increased during ¯exion. The
pars (Schulitz & Niethard 1980). This is believed anterior shear causes increased pressure on the
to be the mechanism by which gymnasts, facet joints, which in turn increases the stress on
American football linemen, divers, and the pars (Farfan et al. 1976; Johnson 1993).
wrestlers develop or exacerbate spondylolysis. Testing has shown that when these forces are
The second theory is that repetitive ¯exion applied the pars is the ®rst structure to
movements produce unbalanced shear forces experience stress concentrations (Farfan et al.
that results in stress at the pars. During quiet 1976). This mechanism has been postulated to
stance, the vertebral bodies, the spinal lead to micro fracture of the pars that progresses
ligaments, and the psoas muscle support the with repetitive ¯exion. Sports such as
weight of the trunk. Compressive force is gymnastics, rowing, weight-lifting, diving,
sustained by the bony structure of the vertebral cricket and wrestling require repetitive or
bodies (Junghanns 1990; Simons 1994). Because constant ¯exion movements that exposes the
of the incline of S1, an anterior shear force acts pars to high stress (Johnson 1993; Hickey et al.
upon the L5±S1 joint (Farfan et al. 1976). As the 1997; Motley et al. 1998 ; Whiting & Zernicke
trunk ¯exes, higher shear forces are developed 1998; Leary & White 2000). Spondylolysis in
(Schulitz & Niethard 1980). In addition to rowers has been especially attributed to psoas
compression, gravity causes a ¯exion moment at hypertrophy (Hickey et al. 1997). The psoas
L5±S1. The forces acting upon a lumbar vertebra originates from the anterior lumbar vertebrae
in a forward ¯exed position are illustrated in and acts to ¯ex the hip. Contraction of the psoas
Fig. 2. The shear stress that acts on the pars can increases the anterior shear force on the lumbar
be mostly attributed to the forces between the vertebrae (Kreighbaum & Barthels 1996).
superior and inferior facets (Farfan et al. 1976). Fatigue fractures result when a sub-maximal
The erector spinae muscles must exert an load is repeatedly applied to a region of bone. A
extension moment to equal the bending moment fatigue fracture at the pars interarticularis
caused by the weight of the trunk. Because most occurs because increased stress at that region
of the erector spinae muscles have a downward causes bone resorption to occur faster than
line of action, compression between the bone formation (Martin et al. 1998). Fatigue
F
Fc M
Fm
F’
Fc
Fig. 2 Forces acting upon a lumbar vertebra when the trunk is in a ¯exed position. F,FH ˆ forces on articular facets,
causes shearing across pars region. M ˆ ¯exion moment caused by gravity acting upon the trunk, Fc ˆ compressive
disc force, FM ˆ force produced by posterior ligaments and muscles.
82 Physical Therapy in Sport (2002) 3, 79±87 * 2002 Published by Elsevier Science Ltd.
c

5. Mechanical aetiology, recognition, and treatment of spondylolisthesis
fractures in this region do not heal like they do (Fig. 3). Therefore, anything that increased the
in other locations. Less periosteal callus is seen, compressive force also increased the facet joint
and the defect often is ®lled with ®brocartilage force (Dietrich & Kurowski 1985). The total
without bony healing (Junghanns 1990; Johnson reaction force was greatest for all lumbar
1993). Poor blood supply to the neural arch and vertebrae in ¯exion, as compared to neutral and
excessive motion at the healing site could be the extension, and the L5±S1 segment had the
cause of the non-union fractures (Junghanns highest force of all. The highest loads were
1990). If the same loads continue to be applied, found in the pars when the trunk was ¯exed to
the defect can become unstable and 808. Tests to failure resulted in fracture of the
spondylolisthesis occurs (Farfan et al. 1976). pars in all vertebrae that were tested. An
Fatigue tests have been performed on cadaver interesting ®nding was that, when intra-
vertebrae to determine where and when a abdominal pressure was increased, the reaction
fatigue fracture might occur (Cyron & Hutton force at all levels was decreased by as much as
1978). A model was designed to simulate an 20% (Fig. 4). This supports the theory that
average person walking with a 50-kg back-pack contraction of the abdominal muscles lessens
with 408 of trunk ¯exion. Out of 74 vertebrae the compressive load on the vertebra by acting
tested, 53 fractured across the pars, the as a hydraulic lift (Dietrich & Kurowski 1985;
remainder fractured in the pedicles or did not Junghanns 1990; Simons 1994).
fracture. This indicates that the posterior In addition to intra-abdominal pressure,
elements are the most susceptible to fatigue passive structures have been hypothesized as
fracture under sub-maximal cyclic loading. In playing a role in reducing anterior shear.
the 14±30 year age group, all vertebrae According to one group of researchers, once 608
fractured within ten hours of cyclic loading. of ¯exion is reached, the posterior ligaments and
The 40±60 year group had a very wide range of fascia become taut and resist the bending
failure times, from 2 minutes to 100 hours when moment. Beyond 608, the posterior ligament
the cyclic loading was stopped. This indicated system becomes tighter, producing more
that the neural arch was stronger during this posterior resistance, thereby decreasing the
stage of life than during the adolescent period. anterior shear forces imposed on the facets
The oldest age group (60±80) fractured within 8 (Farfan et al. 1976). If the posterior connective
hours, suggesting that osteoporosis or other
disease had weakened the bone (Cyron &
Hutton 1978). The results of this study are
supported by Schulitz & Niethard (1980), who
also demonstrated that during axial loading of
the spine even small movements greatly
increase the strain at the pars.
One group of researchers examined the loads Fa
and stresses placed on the lumbar spine during
different conditions with the use of a model. An
epoxy resin vertebral model was created from
information gained through radiographs of a
human spine and cadaver dissection. The Fc-N
model was created with relatively accurate Fc-H
geometric and physical properties. Forces
exerted by the erector spinae and abdominal
Fc
muscles were altered, as well as the amount of
trunk ¯exion. Reaction forces of interest were Fig. 3 The position of the intervertebral disc relative to
the inferior facet causes the horizontal component of
the compressive force between vertebral bodies, the intervertebral disc compressive force to be additive
and the facet joint force. Due to the relationship with the reaction force of the inferior facet. This
of the inferior facet to the intervertebral disc, it increases the anterior shear at the pars interarticularis.
Fa ˆ articular shear force, Fc ˆ Compressive force,
was assumed that the shear component of the Fc-H ˆ horizontal component of compressive force,
intervertebral force added to the facet joint force Fc-N ˆ normal component of compressive force.
* 2002 Published by Elsevier Science Ltd.
c Physical Therapy In Sport (2002) 3, 79±87 83

6. Physical Therapy in Sport
Fdn kN
Pabd = 0 5
Pabd = Popt
4
3
2
1
-80o -60o -40o -20o 0o 20o 40o 60o 80o 100o 120o 140o 180o
Fig. 4 The relieving effect of the intra-abdominal pressure (load in hands 400 N) to the normal force (force that
compresses the vertebrae) in the motion segment L5±S1. Positions of the body out of the range ˆ À 20 to ˆ ‡ 100
were not recorded in vivo. (Reproduced with kind permission from Dietrich M, Kurowski P. The importance of
mechanical factors in the aetiology of spondylolysis. Spine 1985; 10(6) 532±542).
tissues are lengthened, more anterior shear can prior to the age of six, but then increases to 5%
occur during ¯exion before the passive restraints at that age. This incidence is equal to that in the
are activated. This increases the demand on the general population (Smith & Hu 1999). The
muscles to balance the force (Farfan et al. 1976). increase may be attributed to two causes:
The combination of increased shear and Firstly, it is around this age that formal
increased posterior muscle force puts additional schooling begins. Children are likely to be
stress on the pars. When these muscles become sitting for longer periods of time than they have
fatigued there is even less restraint to the before. While this is not likely to cause enough
damaging anterior shear forces (Farfan et al. pain or disability to send the child to the
1976; Johnson 1993; Motley et al. 1998). doctor, it may be starting the spondylolithic
The ¯exion mechanism seems to explain why reaction that will become a problem later in life.
factors such as prolonged sitting, bending, and The second cause is the introduction of
activities that demand repetitive ¯exion organized sport and practices. Society is
exacerbate symptoms of spondylolysis demanding much more out of young athletes
(Junghanns 1990). Two common mechanisms when their skeletons are not physically mature.
by which worker's compensation injuries occur What may have previously been a benign
is prolonged sitting and incorrect lifting deformity of the pars develops into a painful
techniques. Sitting with poor posture places the condition that limits activity and can lead to
lumbar spine in a functionally ¯exed position, long-term problems.
and the anterior shear force will be constant. If
the muscles do not balance this, the facets and
Clinical examination
pars are subjected to continued high stress.
While lifting, anterior shear is affected by how Patients with spondylyolsis/-listhesis present
much weight is being carried, the distance of complaining of pain that began as incidental
the weight from the body, and the amount of and worsened with activity. Young athletes
trunk ¯exion (Johnson 1993). often endure the pain for a long time before they
Another interesting observation is that the are seen by a physician (Stinson 1993). The pain
incidence of spondylolysis is virtually zero is exacerbated by extension, rotation, and
84 Physical Therapy in Sport (2002) 3, 79±87 * 2002 Published by Elsevier Science Ltd.
c

7. Mechanical aetiology, recognition, and treatment of spondylolisthesis
extending from a ¯exed position (Stinson 1993; of the injury can be determined by radiographic
Comstock et al. 1994; Motley et al. 1998). Weak ®ndings. An old injury is detectable on
abdominal muscles increase lordosis, which radiographs, but no active healing is evident
increases the anterior shear on the vertebrae with a bone scan. In this case, the defect has
(Motley et al. 1998). Hamstring tightness is a probably ®lled with ®brocartilage and scar
frequent occurrence, and is thought to be an tissue, and bony healing is no longer possible
attempt to tilt the pelvis anteriorly to move the (Johnson 1993; Renshaw 1995). If the patient is
center of mass forward on the vertebral bodies symptomatic, pain control modalities such as
(Stinson 1993; Comstock et al. 1994; Motley et al. NSAIDs, thermal agents, and electrotherapy
1998). If spondylolisthesis has occurred, should be used until symptoms resolve.
palpation of the lumbar spine will reveal a Conservative rehabilitation can include
`step-off' of one spinous process to the next. The abdominal strengthening, postural and
step-off is a result of the vertebrae superior to the movement mechanics training, hamstring and
damaged one slipping forward (Motley et al. psoas stretching, pelvic stabilization training
1998). A `pelvic waddle' gait is described as a (Comstock et al. 1994). A soft lumbosacral brace
shortened stride length, with increased pelvic is often used in conjunction with rehabilitation
rotation and decreased knee extension. This is (Morita et al. 1995; Smith & Hu 1999). High-risk
believed to be a result of the hamstring tightness activities should be limited, and follow-up
(Comstock et al. 1994). evaluation is necessary to monitor progression
Spondylolisthesis will appear on lateral and (Johnson 1993; Comstock et al. 1994).
oblique radiographs. The severity is most often If plain radiographs do not show a fracture
measured by the Talliard method that divides line, and a bone scan reveals increased uptake,
the slip distance by the total sacral thickness and bony healing is possible with proper
this is interpreted as percent slip. Spondylolysis immobilization (Comstock et al. 1994; Renshaw
in its early stages may not be visible on plain 1995). Bony healing is possible if symptoms
radiographs. A bone scan will reveal increased have been present for less than two months
uptake in the pars indicating a stress reaction (Renshaw 1995). Morita et al. (1995) determined
(Letts et al. 1986; Stinson 1993; Comstock et al. that conservative treatment was successful and
1994; Renshaw 1995; Omey 2000). bony healing occurred in 73% of those with
Radiography can be used to determine early stage spondylolysis (hairline fracture),
whether a pars defect occurring in a young child while this number declines to 0% for those with
will progress to spondylolisthesis. The extent of terminal stage spondylolisthesis (separation
the listhesis has been correlated to the amount of with sclerotic changes). This supports the
damage to the cartilage end plate of the necessity of early detection to obtain optimal
vertebrae (Ikata et al. 1996). Radiographs results. Lumbar movement is limited by a rigid
demonstrating a listhesis also revealed brace similar to that used in scoliosis treatment.
signi®cant damage to the end plate of S1 or L5. The brace should be worn 23 hours a day for
Increased damage to the endplate increases 12 weeks, with follow up X-rays every 4 weeks
instability, and therefore increases the likelihood (Letts et al. 1986; Comstock et al. 1994; Renshaw
of listhesis. It was once thought that the shape of 1995). If, at the end of 12 weeks, symptoms
L5 and S1 were factors in how much a segment have resolved, bracing is continued for another
slipped, but it is now believed that the shape of 6 months (Comstock et al. 1994). If symptoms
L5±S1 is a result of endplate damage. The have not resolved with bracing and decreased
amount of endplate damage present can be used activity, surgical intervention should be
as a predictive factor of how much the slip will considered. This treatment is most successful in
progress (Ikata et al. 1996). young children whose growth plates are still
open. Displacements that progress to 50% or
more are treated surgically. Bilateral fusion of
Treatment
the transverse processes of the involved
Treatment is based on the age of the patient, the segments is a common procedure (Comstock
age of the fracture, the progression of the et al. 1994; Renshaw 1995). Any case of
listhesis, and the patient's activity level. The age adolescent spondylolysis should be treated
* 2002 Published by Elsevier Science Ltd.
c Physical Therapy In Sport (2002) 3, 79±87 85

8. Physical Therapy in Sport
conservatively to prevent progression and 1975). It is now known that defects in the pars
problems in later life. interarticularis are not present at birth in the
Some researchers are suggesting that majority of the cases, and instead are often the
traditional treatment of rest and lumbar ¯exion result of fatigue fractures. The high incidence of
exercises may not be appropriate for spondylolysis may be due to increased
spondylolysis (Panjanbi 1992; Renshaw 1995; compression of the posterior elements due to
Richardson & Jull 1995) Patients with lumbar extension, or increased shear caused by ¯exion
dysfunction often lack voluntary control of the on the immature spine. The incidence of
deep stabilization muscles and therefore use spondylolysis in young people may be on an
substitution patterns to stabilize the spine increase as a result of more children being
(Panjabi 1992; Richardson & Jull 1995). Exercises involved in highly competitive training at an
such as the pelvic tilt and abdominal crunch early age.
require strong contractions of the large torque Testing has indicated that the pars
producing muscles which may enforce interarticularis is subjected to very high forces
substitution patterns. The abdominal hollowing and indeed is the mechanically weak link of the
technique has been described by Richardson vertebra. There are several mechanisms
and Jull (1995) as targeting the transverse available to decrease the forces imposed on the
abdominus and internal oblique. To perform facet joints, such as increasing intra-abdominal
this activity, subjects are instructed to lift their pressure, strengthening the erector spinae, and
ribcage, and attempt to draw the navel closer to tightening the posterior ligamentous structures.
the spine. Biofeedback would be an effective tool However, when repetitive movements that stress
to assist patients in performing this task without the pars are performed, normal protective
substitution patterns (Allison et al. 1998). mechanisms sometimes fail, subjecting the bone
Contraction of these muscles increases intra- to fatigue failure. Repeated stress after pars
abdominal pressure which has been shown to failure can cause adjoining vertebrae to slide
decrease the stress on the pars interarticularis apart, resulting in spondylolisthesis. If the initial
(Dietrich & Kurowski 1985; Junghanns 1990; defect is detected early, bony healing can occur,
Simons 1994). with conservative treatment preventing further
Recent research indicates that targeting the separation. Most often, the defect is ®lled with
deep stabilizing muscles of the abdomen and ®brocartilage and therefore will always be
lower back may be more bene®cial than weaker than the rest of the bone. Much debate
traditional rehabilitation exercises in the still exists on which type of motion is more likely
treatment of spondylolysis (O'Sullivan to cause the problem, but it seems that
et al. 1997). A group of patients with unbalanced shear forces are a likely culprit.
spondylolysis who performed speci®c exercises Given the substantial effect of the deep
to train the deep abdominal muscles reported abdominal muscles on spinal mechanics, it is
decreased pain, increased function, and less important to include speci®c exercises to train
medication use as compared to a group of these muscles. Traditional exercise routines may
similar patients who performed traditional enforce substitution patterns and, while the
exercises (O'Sullivan et al. 1997). Once patient recovers temporarily, future recurrences
contraction of the deep abdominal muscles was are likely due to continued improper
mastered, functional positions and activities mechanics. Early detection of spondylolysis in
were performed while maintaining the children is crucial to insure bony healing and
contraction of the deep abdominals. This minimize the chance of future instability. Any
allowed patients to learn how to safely perform child or adolescent being evaluated for low
activities that had been painful. back pain should be carefully screened for
spondylytic defects.
Conclusion
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c

9. Mechanical aetiology, recognition, and treatment of spondylolisthesis
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