Lumbar intervertebral disc mobility: effect of disc degradation and of geometry

Abstract

Effects of disc degeneration (DD) on mobility under loads have been demonstrated, but they are not taken into account when providing experimental corridors for ranges of motion (ROM); this could explain that these corridors are usually very large. Other influencing factors such as lumbar level or geometry have seldom been studied, and never conjointly. The objectives of the study were to understand relationships between intervertebral discs (IVDs) mobility disparity and disc characteristics using medical imaging. For that purpose, an in vitro biomechanical and imaging protocol was conducted on human lumbar IVDs: 22 specimens (14 L1-L2 and eight L4-L5) were considered. Radiography, MRI, discography and macroscopic slices were obtained. Flexion/extension, lateral bending and torsion 10 Nm moments were applied on each specimen, with and without posterior elements. 3D motion of the upper vertebra was recorded. After testing, morphometric data was measured, DD was assessed and pathological singularities were identified. Mobility corridors for each spinal level in all loading modes were very large. For example, lateral bending ROMs within L1-L2 group ranged from 0.5° to 10.5°. Abnormal behaviours such as excessive ROMs or extension greater than flexion were listed. In most cases, imaging could identify the reasons of these erratic behaviours (scoliosis, calcification, retrolisthesis, etc.). After discarding six discs carrying major anomalies identified by imagery, mobility corridors were drastically reduced (min 3°, max 6.5° for the same L1-L2 lateral bending example). In specimens without abnormalities, relationship between geometry and ROMs could be revealed. As a conclusion, this study provided more relevant data for ROM of lumbar spine functional units. For the first time relationships could be considered between image attributes and mechanical properties.