Isthmic Spondylolisthesis

Educational information only. This page describes the condition and the published evidence and does not give medical advice.

Isthmic spondylolisthesis is a forward slip of one vertebra on the one below that happens because of a small defect (a stress fracture) in a bony bridge called the pars interarticularis. It is the type of spondylolisthesis most often linked to a defect acquired in youth, and it most commonly affects the lowest level of the spine, where L5 sits on the sacrum.

Spondylolisthesis is the general term for one vertebra sitting slightly forward of the one beneath it. There are several distinct types, and isthmic spondylolisthesis has one defining feature that sets it apart: a defect in the pars interarticularis, the narrow bridge of bone that connects the upper and lower joints of a vertebra. This defect is called spondylolysis, and it is usually a fatigue (stress) fracture that develops during childhood or adolescence.

This is the key distinction from the other forms. In degenerative spondylolisthesis the pars is intact and the slip comes from wear of the discs and facet joints in later life. In congenital (dysplastic) spondylolisthesis the slip arises from a bony abnormality present from birth. Because in isthmic disease the back part of the vertebra is separated from the body by the pars defect, the vertebral body can drift forward while the bony arch behind it stays put. This tends to open out the central spinal canal rather than narrow it, which is why nerve irritation in isthmic slips is usually felt at the exit tunnel (the foramen) for the L5 nerve, rather than as the central narrowing seen in degenerative slips.

In the established classification of Wiltse, Newman and Macnab, isthmic disease is Type II, with three sub-patterns: a fatigue fracture of the pars (the commonest), an elongated but intact pars, and a rare acute fracture.

The pars interarticularis is the most heavily loaded part of the bony arch when the lower back is repeatedly arched backwards and twisted. Repetitive loading of this kind is thought to produce a stress fracture in a susceptible person. There is a recognised hereditary tendency, and an association with spina bifida occulta (a minor, usually harmless gap in the bony arch).

Activities that involve repeated hyperextension and rotation of the lower back are associated with a higher rate of pars defects. Reported examples in the literature include gymnastics, cricket fast bowling, diving, weightlifting and throwing sports. A defect on one side alone does not allow the vertebra to slip; a slip generally requires the bridge to be affected on both sides.

In the general population, a pars defect is present in roughly six in every hundred people. The long-running study by Fredrickson and colleagues, which followed 500 children into adulthood, found a defect in about 4.4 per cent at age six, rising to around six per cent by adulthood. Many of these defects never cause symptoms and are discovered by chance. The level involved is most often L5, then L4.

The same long-term studies provide reassuring information about how isthmic disease tends to behave over a lifetime. Defects on one side alone essentially did not slip. Where a slip did develop, it usually occurred early, and progression slowed with each passing decade of adult life. The degree of slip seen on an x-ray correlated poorly with how much back pain a person had, and adults in the cohort had quality-of-life scores similar to the general population.

It is important to read these findings in context: they describe unselected community populations, in whom the defect is often an incidental finding. They do not describe the outlook for someone who has come to a clinic specifically because a symptomatic slip is causing trouble.

The pattern most often reported is mechanical lower back pain, frequently brought on by activity and by arching backwards. Where the slip narrows the exit tunnel for the L5 nerve, leg symptoms in an L5 distribution can occur, such as pain spreading into the buttock and leg. Tightness of the hamstrings and a change in posture or gait are classically described, particularly with larger slips. In young athletes, an aching extension-related back pain is a frequently identified pattern.

Assessment is descriptive here, not a set of instructions. A standing side-on x-ray shows the degree of any slip, which is commonly described using the Meyerding grade (grade I is up to a quarter, grade II up to a half, and so on). Oblique x-rays can show the pars defect as a break in the outline often likened to a collar on a "Scottie dog". CT shows the bony detail of the defect particularly well, including whether it is on one or both sides and whether it appears fresh or long-standing. MRI shows the discs, the nerve tunnels and any active bone reaction, and helps assess the L5 nerve. Surgeons also assess the shape of the pelvis (the pelvic incidence), because a higher value is associated with this condition.

The information below describes the evidence base and is not advice for any individual.

For incidental or mild cases, the natural-history data above underpin a watchful, non-surgical approach, and many lesions never require treatment. For symptomatic adolescents with an active pars stress lesion, the literature describes activity modification, physiotherapy and sometimes bracing, with bony healing more likely when the lesion is caught early.

Where a young person has a symptomatic defect that does not settle, a direct repair of the pars (preserving movement) has been described, with reported healing rates broadly in the range of two-thirds to over nine in ten in selected cases.

For a symptomatic low-grade adult slip that has not responded to non-surgical care, the strongest comparative evidence comes from Scandinavian randomised trials, in which fusion surgery gave better pain and function than a structured exercise programme, with the early advantage narrowing over the longer term. Adding an interbody support (a cage placed in the disc space) tends to raise the proportion of solid fusions but provides only a small and uncertain additional clinical benefit over a simpler fusion. In higher-grade slips, whether and how far to reduce (realign) the slip is genuinely debated, because realignment carries a higher risk of stretching the L5 nerve.

For incidental and community-detected isthmic lesions, the outlook over a lifetime is generally favourable, and the slip rarely progresses after adolescence. For symptomatic adolescents, most do well, with bony healing more likely in early lesions. For a symptomatic low-grade adult slip that has not settled with non-surgical care, fusion offers durable benefit in the trials, with the choice of technique making a smaller difference than the decision to operate at all.

• —Wiltse LL, Newman PH, Macnab I. Classification of spondylolysis and spondylolisthesis. CORR. 1976. PMID 1277669.
• —Fredrickson BE et al. The natural history of spondylolysis and spondylolisthesis. JBJS Am. 1984. PMID 6373773.
• —Beutler WJ et al. The natural history of spondylolysis and spondylolisthesis: 45-year follow-up. Spine. 2003. PMID 12768144.
• —Andrade NS et al. Systematic review: no association between low back pain and lumbar spondylolysis with or without isthmic spondylolisthesis. Eur Spine J. 2015. PMID 25833204.
• —Labelle H et al. Spondylolisthesis, pelvic incidence, and spinopelvic balance: a correlation study. Spine. 2004. PMID 15371707.
• —Ekman P, Moller H, Hedlund R. The long-term effect of posterolateral fusion in adult isthmic spondylolisthesis: a RCT. Spine J. 2005. PMID 15653083.
• —Choi JH et al. Management of lumbar spondylolysis in the adolescent athlete: review of over 200 cases. Spine J. 2022. PMID 35504566.
• —Soler T, Calderon C. The prevalence of spondylolysis in the Spanish elite athlete. Am J Sports Med. 2000. PMID 10653544.
• —Floman Y et al. Instrumented slip reduction and fusion for painful unstable isthmic spondylolisthesis in adults. J Spinal Disord Tech. 2008. PMID 18836358.
• —Moller H, Hedlund R. Surgery vs conservative management in adult isthmic spondylolisthesis: RCT (Part 1). Spine. 2000.
• —Lee GW et al. Spondylolysis outcomes in adolescents after direct screw repair of the pars. J Neurosurg Spine. 2014. PMID 24949906.

This information is for general patient education and is not a substitute for an individual consultation. Always seek personalised advice from a qualified spinal specialist.