Spondylolisthesis

Spondylolisthesis is the forward slip of a vertebra on the one below. Of the Wiltse types, isthmic and degenerative are the two common ones, and the examination turns on telling them apart: the isthmic slip is a pars defect in a young patient, most commonly at L5-S1, and the degenerative slip is facet arthropathy with an intact pars in an older patient, most commonly at L4-L5.

Risk factors for an isthmic slip. Extension sports such as gymnastics and cricket fast bowling, repetitive hyperextension, the adolescent growth spurt and a genetic predisposition. Males are affected more than females.

Risk factors for a degenerative slip. Age over 50 and female sex, with a female-to-male ratio of 3:1. Sagittally oriented facets, disc degeneration and postmenopausal hormonal factors add to the risk.

The pars defect. The pars interarticularis, or isthmus, is the bridge of bone between the superior and inferior articular processes. Spondylolysis is a fatigue fracture of the pars from repetitive hyperextension and rotation. A unilateral defect is spondylolysis without listhesis; bilateral defects allow the vertebra to slip forward.

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How the vertebra slips. In an isthmic slip the pars breaks, the posterior elements separate from the vertebral body, and the body slides forward on the intact disc. In a degenerative slip the pars is intact but the facets degenerate, become incompetent and allow forward translation.

Which root is compressed. In the isthmic slip it is the L5 root in the foramen, the exiting root. In the degenerative slip it is the L5 root in the lateral recess, the traversing root, together with foraminal stenosis. Understanding the compression pattern guides the decompression.

Wiltse-Newman. The classification sorts slips by mechanism. Type II, isthmic, is the most common and has three subtypes: IIa lytic (fatigue fracture), IIb elongated intact pars and IIc acute pars fracture.

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Meyerding. The grade is the forward slip of L5 as a percentage of the S1 endplate, read in quarters; how to measure it is set out below. Grades I and II are low-grade. Grade III and above, a slip of more than 50%, is high-grade and requires special consideration for reduction.

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Marchetti-Bartolozzi. This classification helps predict behaviour and guide treatment in high-grade slips. The developmental slip is dysplastic, with a dome-shaped sacrum and a high slip angle, and carries a higher risk of progression. The acquired slip is isthmic with a more horizontal sacrum, and carries a lower risk.

Why the pelvis matters. Pelvic incidence is a fixed parameter, and a high pelvic incidence allows more slip before imbalance. In sagittal imbalance the patient leans forward and flexes the hips and knees to compensate, and high-grade spondylolisthesis often causes significant imbalance requiring correction. Sacral inclination affects neural tension and posture.

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The SDSG classification sorts L5-S1 slips by grade and by spinopelvic alignment.

Balanced or unbalanced. In a high-grade slip the key distinction is the pelvis. A balanced pelvis has a high sacral slope and low pelvic tilt; an unbalanced pelvis retroverts to compensate for the slip, with a low sacral slope, high pelvic tilt and a "vertical sacrum".

What it decides. An unbalanced or retroverted pelvis (types 5 to 6) is the setting in which reduction to restore spinopelvic balance is most strongly considered. A balanced high-grade slip may be managed with in-situ fusion.

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History. Back pain is worse with extension, and leg pain follows a radicular pattern. Claudication points to associated stenosis. Take a sports history for the isthmic slip, and note the age of onset: young suggests isthmic, old suggests degenerative.

Examination. Look at posture, which may show hyperlordosis or a flexed stance, and test the L5 and S1 roots. Tight hamstrings are found especially in high-grade slips.

Signs of a high-grade slip. Besides tight hamstrings, look for:

• A palpable step-off
• Flattened buttocks (vertical sacrum)
• A waddling (pelvic waddle) gait, if severe
• A shortened trunk
• Severe sagittal imbalance, which may be present

Differential diagnosis. Mechanical and radicular back pain has a broad differential. The key discriminators are the presence of a slip on the standing lateral radiograph, dynamic motion on flexion-extension and the pattern of neural compression.

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Plain radiographs. Take AP and lateral views, standing preferred, because standing laterals better demonstrate the true slip. The lateral gives the Meyerding grade and slip angle, the oblique view shows the pars defect, and flexion-extension views show dynamic motion and instability.

The Scotty dog. The oblique view is the one for the pars, whose posterior elements outline the dog below.

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CT shows pars defects better than MRI. It gives detail of the defect, assesses healing potential (acute versus chronic), defines the bony anatomy and serves preoperative planning.

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MRI assesses neural compression, nerve root impingement, stenosis and disc degeneration, and rules out other pathology. It is essential for surgical planning when neurological symptoms are present.

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Two measurements from the standing lateral radiograph drive grading and prognosis: the amount of forward slip and the kyphotic angulation across the slip.

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Algorithm

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The acute pars stress reaction. In a young athlete the defect may heal if caught early (bone scan or MRI oedema positive). Treat with a Boston overlap brace for 6-12 weeks, activity modification and physiotherapy (flexion-based, core).

The established lesion. Chronic or established lesions are managed with physiotherapy and core strengthening, activity modification and weight management, with monitoring for progression. Many patients with low-grade slips remain asymptomatic.

When to operate. Surgery is for refractory symptoms or objective instability. The indications:

Choosing the operation. Match the technique to the pathology; the choice depends on grade, symptoms and surgeon experience.

• Low-grade isthmic (I-II). Posterolateral fusion, TLIF or PLIF, or ALIF. Isthmic slips often go to ALIF or circumferential fusion; direct pars repair is described below.
• Degenerative. TLIF with decompression, or posterolateral fusion; a posterior approach is often sufficient.
• High-grade. Whether to fuse in situ or reduce is controversial (see Controversies); consider partial reduction.

Direct pars repair. In young patients with single-level isthmic spondylolysis, no significant disc degeneration and no or minimal slip, direct repair (Buck repair, Scott wiring or a pedicle screw-hook construct) is an option, and it preserves motion.

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Instrumented posterolateral fusion. Indicated for most low-grade listhesis. The key point is adequate decompression of the affected roots while achieving solid fusion.

1. Prone positioning
2. Midline approach, exposing the transverse processes
3. Pedicle screw placement
4. Decompression if needed (laminectomy, foraminotomy)
5. Decortication and bone grafting to the transverse processes

Interbody fusion. Three routes to the disc space:

• TLIF (transforaminal). Unilateral facetectomy and an interbody cage through the foramen; good for degenerative listhesis.
• PLIF (posterior). Bilateral cages, with more retraction of the neural elements.
• ALIF (anterior). The best disc height restoration, preserving the posterior muscles, and often combined with posterior instrumentation. It is excellent for isthmic L5-S1, where it provides excellent access.

High-grade slips. Circumferential fusion, anterior and posterior, is often needed. The decision is how far to reduce:

• In-situ fusion fuses the slip in its current position, with lower neurological risk, and does not correct the deformity.
• Partial reduction gives some correction of slip and angle, balancing correction against risk, and is the most commonly used.
• Full reduction gives the best sagittal correction, with the highest L5 nerve root risk, and may require sacrifice of the L5 root.

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What affects fusion. Fusion rates are affected by the number of levels, smoking, biology, technique and bone graft quality; the interbody question is taken up under Controversies.

Prognostic factors. Outcomes are better with a low-grade slip, predominant leg pain, good bone quality, a non-smoker and single-level disease. They are worse with a high-grade slip, predominant back pain, osteoporosis, smoking and multi-level disease.

Global Epidemiology

• Isthmic spondylolysis/spondylolisthesis: affects roughly 4-6% of the general adult population; the classic Fredrickson cohort found pars defects in about 4.4% of children by age 6 and 6% by adulthood, most at L5.
• Marked ethnic variation: prevalence is far higher in some Inuit/Alaska Native populations (reported up to 30-50%) and lower in some Black African populations - evidence for a strong genetic and biomechanical contribution.
• Sport exposure: rates rise sharply with repetitive lumbar hyperextension - cricket fast bowlers, gymnasts, divers, weightlifters and rowers.
• Degenerative spondylolisthesis: predominantly over age 50, female-to-male around 3:1, classically L4-L5, associated with sagittally-oriented facets and disc degeneration.

Side-by-Side Guideline Positions

Registry & Practice Variation

• Registry signal: large spine registries (e.g. Swespine, the UK British Spine Registry) consistently show good patient-reported gains after decompression for degenerative listhesis, with wide centre-to-centre variation in fusion rates that is not fully explained by case mix - underscoring practice-pattern (rather than evidence) driven fusion.
• High-resource settings: ready access to MRI, intraoperative neuromonitoring, navigation/robotics and interbody implants; greater use of circumferential reconstruction for high-grade slips.
• Limited-resource settings: reliance on plain radiographs and clinical assessment; in-situ posterolateral fusion (without expensive interbody cages or neuromonitoring) remains a safe, durable default, particularly for high-grade slips where reduction risk is highest.

Decompression alone or with fusion. The defining modern debate, and it is narrower than it sounds. SLIP (Ghogawala, 2016) favoured adding fusion in grade I degenerative listhesis on its primary endpoint, the SF-36 physical component, and on reoperation. The Swedish Spinal Stenosis Study (Forsth, 2016) found no benefit, including in its listhesis stratum. On the Oswestry Disability Index both trials were null, so the two agree wherever they can be compared directly.

Instability does not explain the trials. The common reconciliation, fuse the unstable segment, decompress the stable one, is clinically sensible, but it is not what separates these trials: SLIP explicitly enrolled patients with stable degenerative spondylolisthesis and still favoured fusion. Treat instability as a reasonable decision rule in its own right, not as the explanation for the trial results.

In-situ fusion or reduction in high-grade slips. Reduction improves sagittal and spinopelvic alignment but carries a real risk of L5 nerve root injury, highest during the final stage of reduction. No RCT settles this; many surgeons favour partial reduction with neuromonitoring over both full reduction and pure in-situ fusion.

Interbody or posterolateral fusion. Interbody fusion (TLIF or ALIF) raises radiographic fusion rates, but the clinical advantage over posterolateral fusion is small and of debated significance, while adding cost and potential morbidity.

Healing and return to sport in spondylolysis. The optimal brace type, duration and timing of return to sport in adolescent athletes remain poorly standardised, and the value of bracing over activity restriction alone is contested.

The three NEJM randomised trials below are the most-quoted evidence in any spine viva, and the way they are usually quoted is wrong twice over. SPORT did not show that surgery beats non-operative care in its randomised comparison - the intention-to-treat analysis of the randomised cohort showed no statistically significant effect on any primary outcome, and the large figures everyone quotes come from an as-treated analysis after roughly 40% crossover in each direction. And SLIP and the Swedish Spinal Stenosis Study did not reach opposite conclusions on the same question. Read the endpoints in the cards below: on the Oswestry Disability Index, which both trials measured, both were null - P=0.06 in SLIP and P=0.24 in the Swedish study. SLIP favoured fusion on its own primary endpoint, the SF-36 physical-component summary, and on reoperation. The famous disagreement is largely a disagreement about which endpoint was made primary.