The pathomechanism of isthmic lumbar spondylolisthesis. A biomechanical study in immature calf spines.

STUDY DESIGN: Anterior shearing force was applied to immature calf lumbar functional spinal units until failure. OBJECTIVES: To clarify the mechanism of slippage in immature calf lumbar spines with pars defects as a first step to understand the mechanism of spondylolisthesis in pediatric human lumbar spines. SUMMARY OF BACKGROUND DATA: Progression from lysis to olisthesis occurs during the adolescent growth spurt. However, the mechanism of slippage in the immature lumbar spine has not yet been understood clearly. METHODS: Bilateral pars defects were created at the rostral vertebra. The specimens then were assigned to one of the two groups: functional spinal units with intact disc (n = 5) and with disc dissected (n = 5). In the former group, the disc was left intact, whereas in the disc dissected group, the anterior longitudinal ligament and 75% of the anterior-to-posterior depth of the disc were incised along the mid-disc plane. Using a uniaxial MTS machine (MTS System, Minneapolis, MN), anteroposterior shearing force was applied to each specimen. Failure load and displacement at failure were calculated from the load--displacement curve. Failure sites also were assessed radiographically and histologically. RESULTS: The five functional spinal units in the intact disc group failed at 973.8 +/- 78.1 N, whereas specimens in the disc dissected group failed at 986.4 +/- 124.2 N. The data showed no significant differences between the two groups. All the specimens showed displacement through the growth plates on radiographs. Histologically, failure was observed to occur between the superior growth plate and osseous endplate of caudal vertebra, indicating that this site is the weakest link. CONCLUSIONS: The results suggest that in the pediatric immature lumbar spine with pars defects, slippage may occur between the growth plate and osseous endplate.