Ultrastructural response of tendon to excessive level or duration of tensile load supports that collagen fibrils are mechanically continuous

The basic collagen fibril structure of tendons continues to be debated in the literature. Some studies have proposed that collagen fibrils are longitudinally discontinuous, with the load-bearing ability of tendon dependent on interfibrillar shear strength. Other evidence indicates that collagen fibrils are probably structurally continuous, running uninterrupted from osteotendinous to myotendinous junction. In this study we explored the question of collagen fibril continuity in tendon by examining fibril response to tendon loading. Tendons were subjected to high stress and/or long duration tensile loading routines, after which we examined the ultrastructure of the tendons using differential scanning calorimetry and scanning electron microscopy, comparing the results from the loaded tendons to control samples taken from the same tendons prior to loading. Our results show that under ramp loading, collagen fibril damage begins near the end of the linear region in the stress-strain response (i.e., near the yield point). When tendons are allowed to gradually elongate under static load, tendon rupture is caused by failure of collagen fibrils, not uncontrolled slippage between fibrils. Our findings indicate that the collagen fibrils of tendon are at least sufficiently long to be mechanically continuous, meaning that tensile failure of tendon does not occur as the result of uncontrolled slippage between fibrils, but by failure of the fibrils themselves.