Comparative analysis of the microstructure of the hamstring tendons: an electron microscopic, histologic, and morphologic study

Semitendinosus and gracilis tendons taken from 25 cadaveric knees were investigated using light and electron microscopy, immunohistochemistry, and morphometry. Thickness of the collagen fibrils, fibril/interstitium ratio, density of blood vessels, density of fibroblasts, and distribution of the collagen fibrils (types I, III, and V collagen and elastic fibers) were analyzed. It was hypothesized that the difference in biomechanical stability between the gracilis and semitendinosus tendons could be reflected by different morphologic features. The results of this study showed that the gracilis tendon, in comparison with the semitendinosus tendon, provides a significantly higher fibril/interstitium ratio and a higher density of collagen III fibers. Conversely, the semitendinosus tendon provides a higher density of blood vessels and collagen I fibers. No differences regarding the density of fibroblasts, thickness of collagen fibrils, and elastic and type V collagen fibers were found. In conclusion, the gracilis tendon graft can provide approximately 15% more collagen than the semitendinosus tendon graft with the same thickness. This fact can play an important role for better biomechanical stability of the gracilis tendon.