Abstract
Introduction: We hypothesise that disc degeneration is a major cause of segmental instability in elderly spines. Accordingly, we simulated two mechanical features of disc degeneration on cadaveric spines, and measured their effects on spinal movements.
Methods: Twenty-one motion segments (T8–9 to L4–5) were obtained from spines aged 48–90yrs. Specimens were loaded rapidly to simulate full spinal bending movements in vivo, while vertebral movements were tracked using an optical MacReflex system. Intradiscal stresses were investigated using “stress profilometry”. Experiments were repeated following compressive creep loading (which reduced disc water content by an amount similar to the aging process) and again following a compressive overload cycle which fractured a vertebral endplate and decompressed the nucleus. MacReflex data were used to quantify the neutral-zone (NZ), the range of motion (ROM), and the range of translational (gliding) movements.
Results Creep and endplate fracture both reduced disc height, and generated stress concentrations within the posterior annulus. Both treatments increased NZ, ROM and translational movements in flexion and lateral bending, but not in extension. Endplate fracture markedly increased the “instability index” (NZ/ROM) in flexion.
Discussion Disc “degeneration” increased all measures of spinal instability during flexion and lateral bending. Disc decompression in particular created a large NZ in which the spine had negligible resistance to bending. In life, muscle action would prevent the spine “wobbling” within this range of movement. Results in extension suggest impaction between the neural arches. Back pain associated with spinal instability could arise from stress concentrations in the annulus and neural arches, or from abnormal muscle activity.
Correspondence should be addressed to SBPR c/o Royal college of Surgeons, 35 - 43 Lincoln’s Inn Fields, London WC2A 3PN
