Introduction. The influence of two different compressive rates (40 and 400 mm/min) on load induced failure modes in flexed healthy ovine lumbar motion segments has been studied and both rates favour herniation1,2. However, these investigations were confined to segments containing one intervertebral disc (IVD) even though the lumbar spine is a multi articulating structure. The aim of this new study was to investigate the combined effect of flexion and compressive overloading at 40 mm/min on the structural failure of segmental samples containing two IVDs, these better approximating the behaviour of the multi-segment spine.
Methods. Five L3-L4-L5 ovine multi-segments containing two IVDs and intact facet joints were fully hydrated, mounted in cups and then creep loaded at 300 N for 15 minutes. A 7° wedge was positioned at each end of the sample and reflective position markers were attached enabling kinematic measurement of the three vertebrae during subsequent mechanical testing in an Instron loading machine. Each sample was then flexed, secured and compressed to failure at 40 mm/min. Following external inspection for herniations, each sample was chemically fixed and decalcified, sagittally bisected and assessed microscopically for evidence of internal structural failure.
Results. Although none of the 5 samples contained externally visible herniations, bisectioning indicated that all had suffered endplate failure. Four of the 5 multi-segments revealed fracture of an endplate in one IVD, the 5th suffered an endplate fracture in both IVDs. The multi-segments failed at an average load of 8.75 kN (7.30-11.07). With reference to the sagittal plane, the kinematic measurements revealed that during compression to failure one of the two flexion angles between the vertebra increased while concurrently the other decreased at similar magnitude. In the 4 samples demonstrating an endplate fracture in one IVD, the failed IVD correlated with the vertebrae that experienced an increase in flexion angle.
Discussion. While flexed healthy ovine lumbar segments containing one IVD compressed at 40 mm/min preferentially fail by herniation1, this study shows that flexed multi-segments containing two IVDs compressed at the same rate suffered only endplate fractures. The loads at failure of the multi-segments in this study compare favourably with those for the single IVD segments suffering endplate failure1. Compressing flexed multi-segmental samples that contain a vertebra with relative freedom to move seem to indicate that an increase in flexion angle makes the correlating IVD more prone to failure.