Introduction. With current in vivo imaging technologies used clinically, any high-resolution structural analysis of annular and endplate failure requires the intervertebral disc elements to be sectioned and assessed microscopically. Previous microscopic studies of sections from 10° flexed healthy ovine lumbar motion segments loaded to failure in vitro have shown that loading rates of 40 or 400 mm/min favour annular failure with nuclear herniation over endplate fracture1,2. The risk of herniation was shown to be highest for the 400 mm/min loading rate approximating ‘surprise’ trunk muscle activation2. Although traumatic impact loading of the lumbar spine is a common initiating factor in low back disorders, and simple low energy falls have been correlated with vertebral fractures3, it remains unclear whether impact loading increases further the risk of herniation or other damage. This study sought to determine whether flexed, healthy ovine lumbar motion segments subjected to controlled impact approximating a simple traumatic fall, failed preferentially by herniation or endplate fracture. Further, our structural analysis sought to establish whether a defined level of impact could induce subtle damage within the intervertebral disc elements, the answer to this question being largely inaccessible if reliant on data from in vivo clinical assessment tools.
Methods. 23 healthy ovine lumbar motion segments were fully hydrated and then creep loaded at ~300 N for 1 hour to reduce their hydration to a more physiologic level. Following creep loading, each segment was mounted in an impact loading rig, flexed 7° and impacted by dropping a mass of 4.3 kg from a height of 0.8 m. These parameters employed are sufficient to induce limited structural damage and resulted in an impact velocity of 3.9 m/s. Each tested segment was chemically fixed and decalcified, and then assessed for externally visible herniations before being sagittally bisected to assess for internal structural damage.
Results. 15 segments suffered endplate fracture, 7 samples herniated and 1 suffered both an endplate fracture and herniated. Of the 16 endplate fractured samples,13 had a major fracture of the superior endplate which was almost always accompanied by other subtle structural damage within the intervertebral disc and/or inferior vertebra. 8 of the superior endplate-fractured samples also contained subtle damage in the anterior annulus and/or inferior vertebra. Overall, both endplate fractured and herniated segments indicated an oblique failure trend from posterior-superior to anterior-inferior.
Discussion. While flexion with moderate compressive rates of 40 and 400 mm/min favoured herniation1,2, this study showed that impact loading of flexed, healthy ovine lumbar segments, approximating a simple traumatic fall, are more likely to fail via endplate fracture accompanied by more subtle structural damage in the disc and adjacent vertebra. Considering that in vivo endplate fractures lead to degeneration of the adjacent intervertebral disc4, the subtle, co-occurring, multi-element failure patterns observed in this impact study add to our understanding of the complexity of these injuries and the potential role they might play in the initiation and development of degeneration in the intervertebral disc.