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

## TL;DR

L5 spondylolysis increases movement at the L4/L5 level but does not affect disc pressure, which could lead to faster disc degeneration.

## Contribution

This study is the first to show increased cranial segment mobility without disc pressure changes due to L5 spondylolysis.

## Key findings

- L5/S1 mobility increased in lateral bending and axial rotation with spondylolysis.
- L4/L5 mobility increased in all motion directions with spondylolysis.
- No significant changes in L4/L5 disc contact pressure or force were observed.

## Abstract

While lumbar spondylolysis has been biomechanically associated with subsequent spondylolisthesis and disc degeneration, its implications on cranial adjacent segments remain unclear. This in vitro experiment aims to quantify the segmental alterations in kinematics and contact mechanics at both L5/S1 and L4/L5 levels induced by L5 pars defects.

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.

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.

Isthmic defects induce segmental hypermobility at the cranial adjacent segment. This kinematic alteration may accelerate disc degeneration.