A common finding in patients with and without symptoms is hamstring spasm. This is a manifestation of a postural reflex that stabilizes the painful segment. With further slippage, the tight hamstrings readjust the anteriorly displaced center of gravity by extending the pelvis.30 In the rare case that progresses to a high grade olisthy, the spine will hyperextend above the slip, even on occasion effecting a thoracic lordosis. With more than 25% slippage, the cauda equina may be compressed over the dome of the sacrum.
RADIOGRAPHIC ASSESSMENT
When spondylolysis or spondylolisthesis is suspected, routine radiographic examination of the lumbar spine should include antero-posterior, lateral, and oblique views. The lateral view should be taken with the patient standing, which will increase the percentage slip and the slip angle4 (Fig. 3). Slippage will be seen on the routine lateral view; a 25 to 45 degree oblique view is usually needed to demonstrate spondylolysis. This may be unilateral in 20% of cases, mandating good imaging technique.5 Reactive sclerosis of the opposite pars, lamina, or pedicle may be found, along with asymmetry of the neural arch and vertebral body .42 Instability at the defect may be demonstrated on fiexion-extension views (Fig. 4).
Figure 3.