Abstract
Introduction: A computer model of the L4/5 human intervertebral disc is currently under development. An integral aspect of this model is the material properties assigned to its components. Detailed data on the material properties of the anulus fibrosus ground matrix are not available in the existing literature. To determine these properties, mechanical tests were carried out on specimens of anulus fibrosus harvested from sheep spines. The tests included unconfined uniaxial compression, simple shear and biaxial compression. Data on the strain required to cause permanent damage in the anulus ground matrix and data on the mechanical response of the anulus to repeated loading were obtained.
Methods: Intervertebral discs were isolated from the lumbar spines of recently sacrificed sheep. These discs were sectioned into test specimens ensuring there were no continuous collagen fibres bearing load. The edge dimensions of the cubic specimens were 3 ± 0.2mm. To ascertain the strain to initiate tissue damage, the specimens underwent successive loadings, which were carried out 1 hour apart to allow recovery. The maximum strain in each test was increased incrementally by 5% until a reduction in stiffness was observed in the following test. Separate tests were carried out to quantify and characterise the response of the anulus ground matrix in the three modes of loading and to strains greater than that which initiates damage.
Results: The strains at which permanent tissue damage occurred were between 20 and 27% in uniaxial compression and between 25 and 35% in simple shear. Testing the specimen beyond these strains showed an obvious reduction in stiffness. The biaxial compression tests showed similar changes but did not result in such pronounced losses in stiffness. The material characteristics were reproducible up to 20% strain. Following deformation to higher strains the altered mechanics were also shown to be reproducible indicating that the matrix had been deranged but not failed.
Discussion: Average physiological strains in the L4/5 intervertebral disc are in the order of 10–50% based on maximum deformations observed in vivo. The current results demonstrate that this strain will cause some permanent damage to the anulus ground matrix, however, the matrix will still be capable of carrying stress upon repeated loading. Thompson et. al.1 found that people over the age of 35 all exhibited signs of disc degeneration. We hypothesise that the regenerative ability of the anulus ceases to function effectively as we age and the continual damage caused to the anulus tissue by daily activities may lead to the degenerative changes seen in the anulus.
Knowledge of the material characteristics up to 20% strain and following exposure to higher strains will enable a more realistic model of the intervertebral disc and the effects of degeneration to be studied.
The abstracts were prepared by Dr Robert Moore. Correspondence should be addressed to him at Spine Society of Australia, c/o the Adelaide Centre for Spinal Research, Institute of Medical and Veterinary Science, PO Box 14, Rundle Mall, Adelaide SA 500, Australia.
1 Thompson, R,Pearcy, M,Downing, K.,Manthey, B.,Parkinson, I., Disc Lesions and the Mechanics of the Intervertebral Joint Complex, Spine, 25:23, 3026–35, 2000 Google Scholar
