Endplate fractures with subtle structural damage in impact loaded flexed ovine lumbar segments: a potential initiator of disc degeneration

Abstract Purpose To (a) investigate patterns of structural damage in flexed, healthy ovine lumbar motion segments subjected to controlled impact loading approximating a low energy traumatic fall; and (b) determine the relative risk of herniation, endplate failure and other subtle modes of structural disruption as potential mechanical initiators of disc degeneration. Methods Twenty-three healthy ovine lumbar motion segments were flexed 7° and then impact loaded by dropping a weight of 4.3 kg from a height of 0.8 m. Each sample was macroscopically assessed externally for herniations and then sagittally bisected to assess internal structural damage. Results Fifteen segments suffered endplate failure, 7 herniated, and 1 suffered both an endplate fracture and herniation. Thirteen segments had a major fracture of the superior endplate and 8 of these also contained subtle damage in the anterior annulus and/or inferior vertebra. Overall, both herniated and endplate fractured samples indicated an oblique mode of failure tending from posterior-superior to anterior-inferior. Other forms of subtle tissue damage were observed across all visible regions of the disc, these coexisting with the major modes of failure. Conclusion Flexed ovine lumbar motion segments subjected to impact loading approximating a low energy traumatic fall, are more likely to fail via endplate fracture accompanied by more subtle structural damage in the disc and/or adjacent vertebra. Failure by disc herniation was less common. The subtle, co-occurring, multi-component failure modes add to our recognition of the complexity of these injuries, and the potential role they might play in the initiation and development of the degenerative cascade in the intervertebral disc.