Response of the end-plates to compression of the spine.

The instantaneous displacement and creep of the caudal end-plate were measured during compression of six segments of cadaveric lumbar spine. Each segment consisted of a disc attached to the two adjacent vertebrae. It was subjected to an applied load of 1.6 kN which was supposed to be within the normal physiological range. End-plate displacement was measured using a displacement transducer. When the load was applied there was an almost instantaneous displacement of the end-plate of 0.14 +/- 0.04 mm (mean +/- standard deviation). The creep displacement in the specimens which remained undamaged was very small (0.023 +/- 0.006 mm) and associated with a retardation time of 1.9 +/- 1.1 s, i.e. the endplate displacement in these specimens reached an equilibrium value within a few minutes. Appreciable endplate displacement (exceeding 4 mm) was associated with damage which included fracture of the trabecular bone. In the damaged specimen, the end-plate continued to displace under a constant load. This result suggests that sustained compression could aggravate mechanical damage to the vertebra and end-plate in vivo.