Pediatric Spondylolysis

Paediatric spondylolysis is a stress fracture of the pars interarticularis, the isthmus of the neural arch. It is a fatigue failure of bone under repetitive extension and rotation.

Who. A teenager, with incidence peaking in the growth spurt and a peak age of 14-16. The sports that load the spine in repeated extension carry the risk: gymnastics, cricket fast bowling, diving and football, where linebackers and linemen are the classic patients. Hyperlordotic posture adds to it.

Associations. Prevalence is extremely high in the Inuit population, at about 50%. There is an association with spina bifida occulta at S1.

Scheuermann's disease. Lysis is more frequent in Scheuermann's, because the compensatory lumbar hyperlordosis below a rigid thoracic kyphosis loads the pars. The figure usually quoted is 50%, from Ogilvie and Sherman (Spine 1987, PMID 3589821), but that was 18 patients, all with increased lordosis, and the lysis was asymptomatic in every case.

Treat that figure as a reason to look rather than a rate to quote: their practical conclusion was that a Scheuermann patient who develops low back pain should have the lumbar spine imaged. The landmark 32-year natural-history cohort of 67 Scheuermann patients (Murray, JBJS 1993) observed no spondylolisthesis at all, so lysis without slip is the expected pattern.

The terminology changes with the pathology:

• Spondylolysis: a defect in the pars interarticularis, with no slip
• Spondylolisthesis: translation of one vertebra on another
• Isthmic spondylolisthesis: a slip caused by a lysis; the L5 body slips forward and leaves the posterior elements behind
• Dysplastic spondylolisthesis: a slip caused by congenital facet insufficiency, in which the pars is often intact
• Spondyloptosis: complete dislocation of L5 in front of the sacrum, a slip of over 100%

In children the slips are mostly isthmic or dysplastic. Degenerative slips belong to adults.

The weak link. The pars interarticularis is the bridge of bone between the superior and inferior articular facets, and the weak link in the neural arch. In extension the inferior facet of the vertebra above impacts the pars of the vertebra below, the nutcracker mechanism. A defect here is spondylolysis; when it is bilateral, the vertebral body can slip forward on the one below, which is spondylolisthesis.

Why L5. L5 is the usual level. Its pars is susceptible because shear forces are highest at the lumbosacral junction, and because it sits at the transition from the mobile spine to the fixed pelvis.

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Spinopelvic Parameters

The geometry of the pelvis plays a crucial role in the aetiology and progression of lysis and listhesis.

Pelvic incidence (PI) is an anatomical constant, fixed by morphology. It is the angle between the line perpendicular to the sacral plate and the line joining the sacral midpoint to the femoral head axis. A PI over 60° predisposes to spondylolisthesis, because the sacrum is more vertical and shear at L5-S1 is higher. A low PI protects against slip but may cause impingement.

Sacral slope (SS) is the angle of the sacral plate to the horizontal. A high PI usually brings a high SS, over 50°, and a steep sacral slope increases the anterior shear component of gravity. This is the "ski-slope" effect that turns a pars crack into a slip.

Pelvic tilt (PT) is compensatory. As the body tries to keep its balance over a high slip, the pelvis retroverts and PT rises, and this produces the crouched gait of a high-grade listhesis. The three are linked: PI = SS + PT.

Understanding PI is essential for surgical planning, especially if fusion is considered.

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Wiltse is the standard classification; Meyerding grades the degree of slip.

Pain. Low back pain that is worse in extension, the position that closes the facet joints and loads the pars, which is why a young gymnast, fast bowler or lineman is the classic patient. It is usually localised to the belt line, occasionally radiating into the buttocks or posterior thighs.

Radicular pain. True radiculopathy is rare in an isolated lysis. It should make you look for a higher-grade slip, since the L5 root is stretched over the back of the sacrum as L5 translates forward.

Examination. Look for hyperlordosis and tenderness at L5/S1. Hamstring tightness is close to universal and is the most reliable physical sign in this condition. It is a protective response rather than a true contracture, which is why it recovers: measure the popliteal angle and follow it, because it often improves as the pain settles.

The stork test. Pain on standing on one leg and extending the spine implicates the pars on the standing side. In the prospective series that formally assessed it against bone scintigraphy with SPECT, the test was neither sensitive nor specific, and the authors concluded it should not be relied on to exclude the diagnosis.

A positive test may direct imaging, but a negative test does not rule out an active pars lesion in an athlete whose history fits. Image anyway.

An isolated lysis often looks normal on inspection. A high-grade isthmic or dysplastic slip produces a characteristic phenotype that can be examined, and it marks spondylolisthesis rather than isolated spondylolysis. Finding it should prompt assessment of slip grade and spinopelvic balance.

The Phalen-Dickson sign is a crouched gait with the hips and knees flexed, the knees held flexed through stance and gait. It results from hamstring tightness or spasm and the lumbosacral kyphosis of a high-grade slip. The hamstring spasm limits the popliteal angle and is protective splinting rather than true neural tension. In severe slips the gait may be frankly waddling.

The back and trunk. As L5 translates forward, the posterior sacral prominence becomes visible, giving heart-shaped ("flat") buttocks, and the L5 spinous process is felt as a palpable step-off at the lumbosacral junction. The trunk shortens, with a low rib-to-pelvis distance and a transverse abdominal crease. A compensatory hyperlordosis above the slip and pelvic retroversion (a high pelvic tilt) keep the trunk balanced.

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MRI first. STIR MRI comes first in children because it spares the gonads radiation and shows marrow oedema before there is a cortical break. That makes it the best first test. SPECT is still the more sensitive for the earliest stress reaction, which is why a normal MRI does not end the work-up when suspicion is high.

A normal radiograph does not exclude an early stress lesion.

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The Scottie dog. The Scottie dog is visible on an oblique film, and a collar across its neck is the lysis. Oblique views are high-dose and are often skipped in children in favour of MRI, but the map still wins vivas.

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CT for healing. CT defines the chronicity of a lysis and whether it can still unite: rounded sclerotic margins mean non-union. Do not CT first in a painful child. MRI comes first; CT is for the question of healing.

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Adolescent low back pain has a different differential from the adult's. Mechanical and discogenic pain is far less common and an organic cause is found more often, so red flags must be actively excluded.

For high-grade slips the central, examinable debate is whether to reduce the slip or fuse it in situ, and the dominant hazard is to the L5 nerve root.

In-situ fusion. Posterolateral fusion in situ, often L4-S1, is the traditional option with the lower neurological risk. It accepts the deformity but reliably arrests progression and relieves pain in most patients, and it remains the safe default.

Reduction. Reduction with instrumented fusion improves the sagittal profile by correcting the lumbosacral kyphosis and restoring balance, but carries a substantially higher risk of L5 root injury. That injury occurs predominantly during the final part of the reduction, when correcting the slip and kyphosis stretches the root over the sacral dome.

Many surgeons therefore perform a deliberate partial reduction, prioritise correcting the slip angle (the lumbosacral kyphosis) over the translation, and use intra-operative neuromonitoring. Reduction remains controversial.

Spondyloptosis and the Gaines procedure. When L5 has translated completely in front of the sacrum (Meyerding grade V), one described salvage is the Gaines procedure: a staged L5 vertebrectomy, excising the L5 body anteriorly and then the L5 posterior elements from behind, with L4 reduced directly onto S1. It is technically demanding, has a high neurological complication rate and is reserved for severe, fixed deformity.

Prognosis is excellent for most adolescents. Giving up the sport is the hardest part.

Which defects heal. Unilateral defects heal far more often than bilateral, 71% against 18% in the pooled data of Klein's meta-analysis, but even unilateral healing is not universal. Bilateral defects have low healing rates but high asymptomatic rates, and terminal (sclerotic) defects essentially never unite, which is acceptable.

Global epidemiology

• Overall prevalence of the pars defect is approximately 6% of the adult population (Fredrickson), with a strong hereditary component and a marked association with spina bifida occulta.
• Highest prevalence is reported in some Inuit/circumpolar populations (historically up to ~40-50%), suggesting a genetic predisposition layered on activity.
• It is the single most common identifiable cause of low back pain in the young athlete worldwide; hyperextension/rotation sports (gymnastics, diving, cricket fast bowling, pole vault, dance, throwing, American football line play) carry the greatest risk.

Side-by-side guidance

Registry and surveillance notes

• There is no dedicated spondylolysis implant registry; outcomes derive from sport-injury surveillance cohorts (e.g. junior cricket fast-bowler programmes) and observational meta-analyses rather than arthroplasty-style registries.
• Sport-governing-body workload surveillance (limits on bowling/throwing frequency, mandatory rest, screening of skeletally immature athletes) is the main population-level prevention lever and is broadly convergent across cricketing nations.

High- vs limited-resource practice variation

• Well-resourced settings: MRI STIR as first-line, low-dose/limited CT for healing assessment, and access to motion-preserving repair where indicated.
• Limited-resource settings: diagnosis often rests on plain radiographs and clinical findings (extension pain, tight hamstrings, stork test); management is overwhelmingly activity modification and physiotherapy, which is appropriate given the benign natural history and the high rate of good clinical outcomes without union.
• Persistent athletic back pain beyond 2-3 weeks warrants imaging and specialist input in any setting; early recognition reduces progression to a terminal non-union.