Abstract
Introduction The annulus fibrosus of the intervertebral disc is composed of a series of concentric collagenous lamellae that constrain the highly pressurised fluid of the nucleus pulposus. With advancing age and even after physical injury in youth the disc almost invariably becomes progressively degenerate due to the combined effects of dehydration of the nucleus and disruption of the annulus. There is conjecture however, about which of the two compartments shows degenerate changes first.
Methods A histological and biochemical review is presented, based on a review of the literature and work carried out in our laboratories.
Results Three distinct types of annulus tears are seen histologically. Rim tears are formed by detachment of the peripheral annular fibres from the vertebral rim. Autopsy studies show that these lesions are rare in subjects younger than 30 years but the incidence increases significantly with advancing age. Although granulation tissue grows into the outer layers of the annulus in a normal healing response, these lesions frequently extend deeper into the disc. Radiating tears course radially across several lamellae, most often extending from the vertebral rim across the nucleus to involve the posterior side of the disc. These lesions are seen mostly between the ages of 30 and 50 years. Concentric tears are characterised by separation of adjacent lamellae and may appear as early as the second decade of life. In advanced degeneration nuclear changes may be seen with any combination of annular lesions. Biochemical changes, including dehydration and reduced proteoglycan content of the nucleus, parallel the morphologic changes to the disc. Altered biochemistry is further reflected by MRI studies in which some scanning sequences can detect even modest loss of fluid from the nucleus as early as three months after experimental annular incision. Annular lesions also compromise the biomechanical properties of the disc. While internal fixation that aims to immobilise the injured disc may promote some recovery of the mechanical integrity, degeneration nevertheless advances in the long term. Attempts to seal peripheral annular defects in experimental studies using biocompatible glue have also failed to promote healing and to prevent progression of even minor structural defects.
Conclusions Technological developments such as gene transfer into disc cells and direct implantation of either stem cells or more mature cells are emerging as potential candidates for the treatment of disc degeneration.
The abstracts were prepared by Mr Jerzy Sikorski. Correspondence should be addressed to him at the Australian Orthopaedic Association, Ground Floor, William Bland Centre, 229 Macquarie Street, Sydney NSW 2000, Australia.
None of the authors have received any payment or consideration from any source for the conduct of this study.
