NATURAL HISTORY AND CLASSIFICATION

Lytic spondylolisthesis initially must be defined and classified. The focus of this review is on lytic spondylolisthesis in the adult patient. It should be established, however, that the lytic lesion (defect in the pars interarticularis) develops in childhood. The lesion is not present at birth but has been noted in children as young as 4 months. The pathologic lesion occurs from 5.5 to 7 years of age and during increased activity from ages 11 to 16.¹ The prevalence of a lysis is estimated to be 4.4% at age 6 and increases to 6% in adulthood.² In skeletally immature individuals, the tendency of lumbosacral slip progression is most likely to occur in adolescents younger than 15 years. The majority of skeletally mature individuals with a mild lumbosacral slip are asymptomatic, and slippage after adulthood is uncommon. In a long-term follow-up study, Osterman and colleagues³ note that 90% of the slip had occurred by the time the patient was first seen, and when evaluating long-term outcomes, it was difficult to prove the connection between the radiographic findings and pain.

Most adolescents and young adults with spondylolytic spondylolisthesis have no radicular symptoms. When symptoms do occur, it is due to irritation of the exiting nerve root (L5 in a L5-S1 spondylolisthesis). This develops generally after two to three decades and is secondary to disc degeneration with facet arthropathy leading to lateral recess and foraminal stenosis. This compounds the compression of the L5 nerve root caused by the fibrocartilaginous material formed at the edges of the pars defect^4–6 (Fig. 4-1).

FIGURE 4-1 Axial view of vertebrae with bilateral pars defect with fibrocartilage creating lateral zone stenosis. B, L5-S1 lateral view demonstrating secondary degenerative changes causing compression of the L5 nerve root at the subarticular and extraforaminal zones.

When comparing treatment options, it is critical that similar pathologic lesions are being compared. Spinal level involved and degree of slip are clinical features that are important in categorization. L5-S1 accounts for 82% of the occurrences of lytic spondylolisthesis; L4-L5 level is involved in 11% of cases. In contrast with the L5-S1 isthmic lesion, the L4-L5 level is more prone to be unstable and subject to further slip progression in adulthood. Sagittal rotation, shear translation, and axial rotation are all greater at the L4-L5 level with a pars defect.⁷ This can accelerate disc degeneration, further compromise mechanical stability, and lead to greater and earlier onset symptoms compared with the L5-S1 level.³ The L5-S1 level has greater inherent stability, and hence a lower rate of slip progression and symptoms.

Degree of slip (anterior displacement of L5 on S1) is categorized on a scale I to V. Sagittal rotation or slip angle describes the rotational relation between L5 and the sacrum. Higher slip angles commonly create increased lumbosacral kyphosis and are generally associated with higher degree slips. Generally, the spectrum of developmental lytic spondylolisthesis is divided into low and high grades. Low grade encompasses no slip (spondylolysis alone) to less than 50% (grades I and II). High grade is a slip greater than 50% (grades III, IV, V). High- and low-grade slips, although manifestations of the same pathology, require different treatment strategies. Evaluation of these needs to be independent.

HIGH-GRADE SPONDYLOLISTHESIS

High-grade spondylolisthesis is more commonly treated in the adolescent population when the symptoms develop. Few adults are seen with symptomatic severe slips, which were untreated at a younger age. Most studies in adults that include both high- and low-grade spondylolisthesis report no difference in the outcomes; however, the numbers of high-grade slips included are small.^8,9

Most authors suggest posterior fusion to include L4 to S1. Numerous approaches to fusion are reported; however, low cohort numbers and no comparative study groups are available for critical evaluation of these various techniques. In summary of the described techniques, these include in situ posterior fusion with instrumentation, transvertebral screws (S1 pedicle screw transgressing the S1 superior end plate to the L5 body), fibular dowels for L5/S1 interbody fusion with L4-S1 instrumentation, titanium cages from either an anterior or posterior approach for interbody L5/S1 fusion, iliac screw supplementation, and L5 vertebrectomy. Good clinical outcomes and fusion rates are described by the advocates of each; however, all studies are class Level IV and V evidence. The role of reduction has inconsistent data to support or refute this, although the risk for neurologic injury is greater with reductions compared with fusions in situ.¹⁰

LOW-GRADE SPONDYLOLISTHESIS

Greater number and more comprehensive studies are available for low-grade slips in the adult population.