An In vitro Animal Study of the Biomechanical Responses of Anulus Fibrosus With Aging

Abstract

In vitro biomechanical tests were performed on aged (group A) and young (group B) porcine intervertebral discs. The in vitro biomechanical responses of aged and young porcine intervertebral discs were measured under designated axial compressive loads and analyzed. From the biomechanical point of view, the major biomechanical functions of intervertebral discs are to absorb and distribute external loadings. Although the histological observation of intervertebral discs on the effect of aging and related degeneration has been extensively studied and described, the changes in those biomechanical functions attributable to aging are still left to be studied. Two groups were set for mechanical tests. Group A consisted of 24 motion segments obtained from female porcine lumbar (44.0 +/- 2.8-months old). The group B consisted of 30 motion segments from female porcine lumbar (6.2 +/- 1.3 months old). The specimens were chosen randomly from all levels. For the measurements of biomechanical responses, a pressure transducer was placed on anterior and posterolateral locations of anulus fibrosus. Morphological and histological observations were carried out to confirm any age-related changes in both groups. Intradiscal pressures and relaxation times were measured and calculated at points in the anulus fibrosus under designated axial compressive loads. Morphological and histological difference between group A and group B were confirmed with H&E staining and other measurements. Group A showed a lower ratio of nucleus pulposus area to total disc area than did group B. There was no significant difference in the intradiscal pressure between groups as measured in the anterior zone, except at an axial load of 740 N. However, a significant pressure difference was found in the posterolateral zone when the axial load was 542 N or greater (P < 0.05). At 740 N, the average relaxation time for group A was significantly longer than that for group B (P < 0.05). Differences in biomechanical responses between groups were confirmed. Group A was less flexible and slower at energy relaxation under axial loading. A larger proportion of the external load was taken by the posterolateral part of the degenerative discs. These results were consistent with clinical experiences: 1) most hernias are observed more often at the posterolateral side than other sides, and 2) the degeneration attributable to age reduces the function of absorbing and distribution of external loadings.