L5 spondylolysis increases segmental mobility at the cranial adjacent level without altering intervertebral disc contact pressure.

Objective: While lumbar spondylolysis has been biomechanically associated with subsequent spondylolisthesis and disc degeneration, its implications on cranial adjacent segments remain unclear. This Methods: Six fresh-frozen human lumbar cadaveric specimens (L1-S2) underwent pure moment loading (4 Nm) in flexion-extension, lateral bending, and axial rotation. Sequential testing compared intact specimens with simulated L5 bilateral spondylolysis models. Intervertebral kinematics were quantified using optical motion tracking, while L4/L5 disc contact parameters were measured using Tekscan pressure sensors. Results: L5/S1 segmental mobility increased in lateral bending (1.66°, p = 0.002) and axial rotation (1.45°, p = 0.007) in spondylolysis models. Motion increases were observed at the cranial adjacent L4/L5 segment: flexion-extension (1.89°, p < 0.001), lateral bending (2.15°, p = 0.002), and axial rotation (1.89°, p = 0.022). However, no significant differences were detected in the L4/L5 disc contact parameters for peak contact pressure, contact area, and contact force. Conclusion: Isthmic defects induce segmental hypermobility at the cranial adjacent segment. This kinematic alteration may accelerate disc degeneration.