Abstract
Introduction: Low back pain (LBP) is an ailment affecting a large portion of the population and may result from degeneration of the intervertebral discs. Degeneration of the discs may be characterized by a loss of hydration, a more granular texture in the disc components and the presence of anular lesions which are tears in the anulus fibrosus. Research to date has been lacking in defining a relationship between LBP and anular lesions. In this study a materially and geometrically accurate finite element model (FEM) of an L4/5 intervertebral disc was developed in order to study the effects of anular lesions on the disc mechanics.
Methods: An anatomically accurate transverse profile for the disc FEM was derived from transversely sectioned human cadaveric discs. The anulus fibrosus ground substance was represented as an incompressible material using an Ogden hyperelastic strain energy equation. Material parameters were derived from experimentation on sheep discs. In order to separately assess the effects of degeneration of the nucleus and of the entire disc, four models were analysed. A healthy disc was modelled as reference and the three degenerate models comprised a degenerate nucleus (no hydrostatic nucleus pressure) with either a healthy anulus, or with a radial or rim anular lesion. Loading conditions to simulate the extreme range of physiological motions about the 6 axes of rotation were applied to the models and the peak rotation moments compared.
Results: The reduction in peak moment between the Healthy Disc FEM and the Healthy Anulus FEM ranged from 24% under flexion to 86% under right lateral bending. When the lesions were simulated, the rim and radial lesion resulted in variations in peak moment from the Healthy Anulus FEM of 1–10% and 0–4%, respectively.
Conclusions: The analysis suggested that loss of the nucleus pulposus pressure had a much greater effect on the disc mechanics than the presence of anular lesions. This indicated that the development of anular lesions prior to the degeneration of the nucleus would have minimal effect on the disc mechanics. But the response of an entirely degenerate disc would show significantly different mechanics compared to a healthy disc. With the degeneration of the nucleus, the disc stiffness will reduce and the outer innervated anulus may become overloaded and painful.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
