Biomechanische Untersuchungen an der Ansatzsehne des M. Tibialis posterior

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In regions where a tendon curves around a bony pulley the structure of the tissue resembles to that of fibrocartilage. Most spontaneous ruptures of the posterior tibial tendon occur in the retromalleolar region where the tendon is subjected to com-pressive and shearing forces. The aim of the study was to determine the tensile and compressive properties of the retromalleolar zone of the human tibialis posterior tendon to gain further knowledge about spontaneous rupture of this tendon. We tested the tensile properties of segments from the retromalleolar region and segments from the proximal part of the tendon in a material testing machine. To determine compressive properties cylinders with a diameter of 4 mm and a height of 3 mm were taken from the center of both segments and tested in uniaxial compression. The ultimate load of specimen from the retromalleolar segment was found to be 45.4% lower than that of specimen tested from the proximal part (retromalleolar: 296±41 N, proximal: 543± 131 N). The load elongation curves show that the retromalleolar fibrocartilage had a lower linear stiffness than that of the typical tendon tissue from the proximal part (retromalleolar: 156±24N/mm, proximal: 213±46N/mm). The ultimate compressive properties of the retromalleolar fibrocartilage was found to be 69.3% higher than that of the dense connective tissue in the proximal part (retromalleolar: 1.6 ±0.5 N, proximal: 0.5 ±0.4 N). The stress-relaxation in the retromalleolar segment was significantly more effective. When the posterior tibial tendon was tested to failure by tensile stress, the tendon always ruptured in the retromalleolar region. The failure mode of both tendon regions was completely different. In the retromalleolar region most specimen failed by elongation without macroscopically visible damage. But microscopic examination and scanning electron microscopy revealed severe tissue damage such as partial ruptures at the anterior surface of the tendon which normally faces the medial malleolus. In the region where the tibialis posterior tendon curves around the medial malleolus, the tendon has biomechanical characteristics of fibrocartilage. The low ultimate tensile failure of the fibrocartilage might explain why degenerative changes and spontaneous ruptures occur in the region where the tibialis posterior tendon wraps around the medial malleolus.