Buck's direct repair of lumbar spondylolysis restores disc stresses at the involved and adjacent levels.

Background: Lumbar spondylolysis was reported to cause disc degeneration at both caudal and cranial adjacent level. However, basic disc biomechanics in the spondylolytic spine is not fully understood. Purpose of this study was to analyze the disc stresses at cranial and caudal adjacent level of lumbar spondylolysis. Also, the biomechanical effects of Buck's technique on disc stresses at these two segments were evaluated.

Methods: An experimentally validated three-dimensional non-linear finite element model of the intact ligamentous L3-S1 segment was used. Bilateral lumbar spondylolysis was simulated by creating bilateral pars defects with 1.0 mm gap at L5. Buck's direct repair model was simulated with 4.0 mm cannulated Titanium screws, placed bilaterally across the defect. Von Mises stresses in the annulus fibrosus and nucleus pulposus at L4/5 (cranial adjacent) and L5/S (caudal adjacent) disc levels were analyzed in flexion, extension, lateral bending and axial rotation in response to 400 N of axial compression and 10.6 Nm moment. The highest values were compared among the three models, intact, spondylolysis and Buck's technique.

Findings: After spondylolysis occurred at L5, annulus fibrosus and nucleus pulposus stresses at L4/5 increased to 111% and 120%, respectively. After the Buck's technique it recovered to 102% and 105%, correspondingly. On the other hand, at L5/S, annulus fibrosus stress increased to 168%, and nucleus pulposus, 155%, which was much higher when compared to the stresses at L4/5. After the Buck's technique the stresses were decreased to 125% and 120%, correspondingly. During rotation motion, especially, the operation normalized the disc stress completely.

Interpretation: Spondylolysis increases disc stresses at the affected as well as cranial adjacent level, and it may lead to disc degeneration at both levels. However, the increase in stresses is higher at the affected caudal level, when compared to the cranial level. Buck's technique may restore the disc stresses back to normal at both disc levels. Thus, this technique may be beneficial from a biomechanical perspective as well.