A potential mechanism for age-related declines in patella tendon biomechanics

M R Dresssler, D L Butler, R Wenstrup, H A Awad, F Smith, F Smith, G P Boivin

Journal of Orthopaedic Research

Volume 20, 2002, Page 1315 - 1322

Dressler et al stated there has been an increased incidence of sports-related soft tissue injuries in older individuals. They believe that this is mainly due to increased sports participation in the over 50 age group, encouraged by health promotion initiatives. Previous research has suggested that age-related changes within soft tissues may predispose older individuals to conditions such as tendonopathies. This lab-based biomechanical study investigated the age-related changes in the patella tendon of rabbits.

Collagen fibril structure, collagen type and tensile strength of one year-old rabbits and four year-old rabbits were tested, with the researchers unaware of the age of each sample. Dressler et al found that the tensile strength of the older rabbit tendons was 27% less than in the younger specimens. The ultrastructure of the tendons was examined using an electron microscope to determine the diameter of the individual collagen fibrils. This showed that the younger rabbits had more 'larger' (>150nm) diameter collagen fibrils compared to the older rabbits, with the authors using distribution analysis to show that this difference was statistically significant. Analysis of the collagen biochemistry showed that the older rabbits demonstrated Type V collagen, which was not present at all in the younger specimens. Overall, there are significant changes between older and younger specimens, but how do these findings extrapolate to the clinical situation?

In previous studies, Type V collagen has been shown to regulate the diameter of Type I collagen. Dressler et al propose that increased Type V collagen may therefore have an adverse affect on patella tendon strength. The finding that as we age, so our tendon structure changes and loses elasticity, has good face validity. However, there are methodological issues which mean this conclusion is not so straightforward. In common with most biomechanical studies, these findings are based on an animal model and it may not be reliable to assume the same findings in human tissue. This is because tissue loading profiles and tendon function is different between species. In this study the authors acknowledge that the older rabbits were 'retired breeders' and the effect of multiple pregnancies may have led to hormonal changes in soft tissue, rather than the changes being age-related.

Another common problem with lab-based biomechanical studies is the method of tissue loading. In this study the tendons were mechanically gripped and pulled to failure through elongation. This does not fully replicate the in vivo stresses which, during sport, tend to be repetitive rather than maximal load to failure. The findings of this study are therefore more relevant to traumatic tendon rupture than to the development of tendonopathy due to cyclic loading.

The authors indicate that future genetic research may be able to determine the direct effect that Type V collagen has on fibril diameter and the material properties of tendon tissue.

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