Lumbar Spinal Endplate Damage And Its Related Factors

Study design:Cadaveric motion segment experiment. Objective:Show how disc degeneration might cause spinal instability. Summary of Background Data:Aging and disc degeneration have a profound effect on the mechanism of load transfer through the disc. The morphology of fatigue fractures of lumbar motion segments has been investigated in laboratory experiments, the weakest part of the vertebral body was shown to be the endplate. If the endplate was damaged it would lose the ability of intervertebral disc to distribute load evenly ,then under load the nucleus pulposus bulges into the vertebral body .reducing overall disc height and causing radial bulging of annulus fibrous so that the spine can become more flexible to the extent that spine become instable. However, there is no study to evaluate the contribution of disc dehydration and damage of vertebral endplate in the lumbar spinal segmental instability in aging spine Methods.-Twenty-one cadaveric thoracolumbar motion segments aged 48-90 yrs were loaded to simulate full flexion, extension and lateral bending. Vertebral movements, recorded using a "MacReflex" motion analysis system, were analysed to calculate neutral zone (NZ), range of motion (ROM), Ratio of NZ/ROM(NZR) bending stiffness (BS), horizontal translational movement, and the location of the centre of rotation (COR). Testing was repeated after two treatments which simulated physical aspects of disc degeneration: creep loading to dehydrate the disc, and compressive overload to disrupt the endplate. BMD and BMC were also tested respectively. Results were analysed using repeated measures of ANOVA and linear regression and paired-t test.Results.-The height of spinal segment was reduced by 1.0mm (SD 0.3) following creep and by further 1.6mm(SD 0.5mm) following endplate fracture(p<0.01).Mean bending stiffness in flexion, for before creep(BC), after creep(AC) and after failure(AF) were 19.9,17.0 and 10.3Nm/degree(p<0.05 or more); in lateral bending were 8.3, 7.2 and 4.9 Nm/degree respectively, compared with AC and BC, AF is reduced significantly(p<0.01); in extension were 27.6, 41.2 and 28.5 Nm/degree respectively, AC is increased significantly compared with BC. Mean NZ in flexion for BC, AC and AF were 2.8°,4.0°and 6.8° (p<0.01); in extension were 1.3°,1.5°and 1.6°respectively(NS); in lateral bending were 0.65°,1.2°and 2.6°respectively, compared with BC.AC and AF is significantly increased. Mean ROM in flexion for BC, AC and AF were 6.1°,8.rand 11.6°respectively(p<0.001); in extension were 3.2°,3.4°and 3.4°respectively (NS); in lateral bending were 5.3°,7.0°and 9.8°respectively(p<0.001). Mean ratio of NZ/ROM (NZR) in flexion for BC, AC and AF were 0.39, 0.44 and 0.56 respectively, compared with BC and AC, NZR of AF is increased significantly; and in lateral bending were 0.15, 0.17 and 0.23 respectively, compared with BC, NZR of AF is significantly increased. Correspondingly, the BS of BC, AC and AF passively correlated with NZ and ROM respectively, and the stress and failure load of lumbar motion segment correlated with BMD and BMC positively. No correlation existed between height loss and BS and BMD.In flexion and lateral bending, the combined treatments increased NZ and RoM by 89-298%, and increased the "instability index" (NZ/ROM) by 43-61%. Translational movements increased by 58-86%, whereas BS decreased by 42-48%. In extension, ROM and NZ were little affected, although the COR moved closer to the apophyseal joints. Measures of instability increased most in lateral bending, and following endplate disruption. Stress concentrations in the posterior annulus fibrosus increased markedly after endplate disruption.Conclusions:Two physical aspects of disc degeneration - dehydration and endplate disruption - lead directly to marked segmental instability. Back pain associated with instability may be attributable to disc degeneration rather than abnormal segmental movements...