Novel lap test determines the mechanics of delamination between annular lamellae of the intervertebral disc

Delamination between lamellae of the annulus fibrosus is a crucial stage of intervertebral disc herniation, and to better understand the mechanics of the delamination process, a novel lap test was devised. Specimens consisting of two adjacent, naturally bonded lamellae were obtained from the cervical region of frozen porcine spines. They were cut into specimens nominally 3.5 mm wide by 7 mm long and tabs of the deep and superficial layers were removed from opposite ends of the specimens so that a 4.5–5.0 mm long intact interface remained between the lamellae. Specimens were mounted in a BioTester tensile instrument using BioRake attachments having 5 sharpened points side-by-side, and they were strained at 2%/s. Force–time curves were obtained and, using tracking software, a detailed map was made of the time course of the displacements within the specimens. Extensibility of the lamellae themselves was found to substantially complicate interpretation of the data. The experiments, together with mathematical analyses and finite element models, show that much of the shear load is transferred between lamellae at the ends of the bonded region, a finding of clinical importance. The inter-lamellae bond was found to have a peak strength of 0.30±0.05 N/mm of specimen width (not to be confused with lap length), and the remarkable ability to carry substantial load even when lamellae had displaced up to 10 mm relative to each other.