Biomechanics of the Achilles tendon.

PURPOSE: Tendons behave viscoelastically and exhibit adaptive responses to conditions of increased loading and disuse. Strain patterns in tendons may not be uniform, as tendons show stress-shielded areas and areas subjected to compressive loading at the enthesis. These areas correspond to the sites where tendinopathic characteristics are typically seen. METHOD: We review the biomechanic literature on Achilles tendon, trying to interpret it on the basis that classic inflammatory changes are not frequently seen in chronic athletic tendon conditions. RESULTS: Biomechanical studies show that the strains within the tendons near their insertion site are not uniform. If the material properties are similar throughout the tendon, forces transferred through the insertion site preferentially load the side of the tendon that is usually not affected initially in tendinopathy. In that case, the side affected by tendinopathy is generally 'stress shielded'. CONCLUSION: The presence of differential strains opens the possibility of alternative biomechanical explanations for the pathology found in these regions of the tendon. The traditional concept of tensile failure may not be the essential feature of the pathomechanics of insertional tendinopathy. Additional work is needed ascertain whether such principles should be incorporated in current rehabilitation.