A more realistic disc herniation model incorporating compression, flexion and facet-constrained shear: a mechanical and microstructural analysis. Part II: high rate or ‘surprise’ loading
Part I of this study explored mechanisms of disc failure in a complex posture incorporating physiological amounts of flexion and shear at a loading rate considerably lower than likely to occur in a typical in vivo manual handling situation. Given the strain-rate-dependent mechanical properties of the heavily hydrated disc, loading rate will likely influence the mechanisms of disc failure. Part II investigates the mechanisms of failure in healthy discs subjected to surprise-rate compression while held in the same complex posture.
37 motion segments from 13 healthy mature ovine lumbar spines were compressed in a complex posture intended to simulate the situation arising when bending and twisting while lifting a heavy object at a displacement rate of 400 mm/min. Seven of the 37 samples reached the predetermined displacement prior to a reduction in load and were classified as early stage failures, providing insight to initial areas of disc disruption. Both groups of damaged discs were then analysed microstructurally using light microscopy.
The average failure load under high rate complex loading was 6.96 kN (STD 1.48 kN), significantly lower statistically than for low rate complex loading [8.42 kN (STD 1.22 kN)]. Also, unlike simple flexion or low rate complex loading, direct radial ruptures and non-continuous mid-wall tearing in the posterior and posterolateral regions were commonly accompanied by disruption extending to the lateral and anterior disc.
This study has again shown that multiple modes of damage are common when compressing a segment in a complex posture, and the load bearing ability, already less than in a neutral or flexed posture, is further compromised with high rate complex loading.
KeywordsOvine lumbar motion segments Surprise-rate compression Complex posture Microstructural analysis Annular disruption Mechanism of herniation Intervertebral disc Herniation Radial fissure Mechanics
- 1.Marras WS, Lavender SA, Leurgans SE, Rajulu SL, Gary Allread W, Fathallah FA, Ferguson SA (1993) The role of dynamic three-dimensional trunk motion in occupationally-related low back disorders: the effects of workplace factors, trunk position, and trunk motion characteristics on risk of injury. Spine 18(5):617–628CrossRefPubMedGoogle Scholar
- 18.Osti OL, Vernon-Roberts B, Moore R, Fraser RD (1992) Annular tears and disc degeneration in the lumbar spine: a post-mortem study of 135 discs. J Bone Joint Surg Ser B 74(5):678–682Google Scholar