• Kelly R Wade, Peter A Robertson, Ashvin Thambyah, and Neil D Broom.
• *Experimental Tissue Mechanics Laboratory, Department of Chemical and Materials Engineering, University of Auckland, Auckland, New Zealand; and †Department of Orthopaedic Surgery, Auckland City Hospital, Auckland, New Zealand.
• Spine. 2015 Jun 15;40(12):891-901.

Study DesignMicrostructural investigation of compression-induced herniation of the flexed lumbar disc.ObjectiveTo provide a microstructural analysis of the mechanisms of annular wall failure in healthy discs subjected to flexion and a rate of compression comparable with the maximum rate at which the muscles of the spinal column can generate a force.Summary Of Background DataClinical evidence indicates the involvement of the endplate in herniation. It is known that both an elevated rate of compression and a flexed posture are necessary to cause disc failure either within the midspan of the annulus or at the annular-endplate interface. However, the question of what effect a sudden or "surprise" loading might have on the mode of failure is, as yet, unanswered.MethodsTwenty-four healthy mature ovine lumbar motion segments were compressed to failure in high physiological flexion (10º). This occurred over approximately 5 mm of crosshead displacement in 0.75 seconds that resulted in a displacement rate of 400 mm/min (defined as a "surprise" rate) and was intended to simulate the maximum rate at which the muscles of the spinal column can generate a force. The damaged discs were then analyzed microstructurally.ResultsFifty-eight percent of discs suffered annular-endplate junction rupture, 25% suffered midspan annular rupture, and the balance of 17% endplate fracture. Microstructural analysis indicated that annular rupture initiated at the endplate apical ridge in the mid-to-outer region of the annulus in both annular-endplate and midspan annulus rupture.ConclusionMotion segments subjected to a "surprise" loading rate are likely to fail via some form of annular rupture. Failure under such sudden loading occurs mostly via rupture of the annular-endplate junction and is thought to arise from a rate-induced mechanostructural imbalance between the annulus and the endplate.Level Of EvidenceN/A.

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