Abstract
Introduction: The basic architecture of the annulus fibrosus has long been established; successive lamellae containing parallel collagen fibers cross obliquely as you move through the annular wall, with the lamellae anchored in the endplates to form a multi-ply structure. Less is known of the interactions between fiber populations in the multi-laminate annulus fibrosus. Their significant contribution to the material behaviour was highlighted in Elliot and Setton’s 2001 attempt to build a material model based on experimental measurements of properties of the annulus. Recent research has confirmed a localized rather than a homogeneous or dispersed mode of interconnectivity between lamellae. Whilst clearly indicating localized bridging structures these studies have allowed only a glimpse of how these bridging elements fit within the overall lamellar architecture. The aim of this investigation was to analyse the interlamellar interconnectivity in its full 3-dimensional form and in complete segments of the annular wall.
Methods: Anterior segments of ovine lumbar discs in two age groups were sectioned along the oblique fiber angle. A 3-dimensional picture of the translamellar bridging network (TLBN) is developed using structural information obtained from fully hydrated unstained serial sections imaged by differential interference contrast optics.
Results: A high level of connectivity between apparently disparate bridging elements was revealed. The extended form of the bridging network is that of occasional substantial radial connections spanning many lamellae with a subsidiary fine branching network. The fibrous bridging network is highly integrated with the lamellar architecture via a collagen-based system of interconnectivity.
Discussion: This study demonstrates a far greater complexity to the interlamellar architecture of the disc annulus than has previously been recognised. Our findings are clearly relevant to disc biomechanics. Significant degrading of the TLBN may result in annular weakening leading potentially to disc failure. Most importantly this work opens the way to a much clearer understanding of the micro-anatomy of the disc wall.
Correspondence should be addressed to Dr Owen Williamson, Editorial Secretary, Spine Society of Australia, 25 Erin Street, Richmond, Victoria 3121, Australia.
