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The three-dimensional ultrastructure of human tendons has been studied. Epitenon and peritenon consis of a dense network of longitudinal, oblique and transversal collagen fibrils crossing the tendon fibres. The interna structure of tendon fibres is also complex. The collagen fibrils are oriented not only longitudinally but also transversel· and horizontally. The longitudinal fibrils do not run only parallel but also cross each other forming spirals (plaits) These fibril bundles are bound together by a three-dimensional collagen fibril network of endotenon. In the myotendi nous junction the surface of the muscle cells form processes. A network of tendineal collagen fibrils fills the recesses between the muscle cell processes penetrating the basement membrane of these processes. This complex ultrastructur < of human tendons most likely offers a good buffer system against longitudinal, transversal, horizontal as well as rotational forces during movement and activity.

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During the last few decades, the incidence of tendon ruptures has increased in civilized countries. Our material comprises 749 patients who had 832 tendon ruptures treated surgically between 1972 and 1985. There were no competitive athletes among the patients studied. There were 292 single ruptures of the Achilles tendon, 274 of the proximal biceps brachii, 113 of the extensor pollicis longus, and 70 of other tendons. Forty-eight patients had multiple ruptures and 35 patients had reruptures. Achilles tendon ruptures often occurred in recreational sports activities (59%), in contrast to other tendon ruptures (2%; P less than 0.001). The mean age for patients who had Achilles tendon rupture was 35.2 years and for patients with other ruptures, 50.7 years (P less than 0.001). There was a connection between the high incidence of blood group O and tendon ruptures (P less than 0.001). In cases of multiple ruptures and reruptures, the frequency of blood group O was 71%. Sixty-two point three percent of the patients with Achilles tendon rupture were professionals or white collar workers, which is markedly more than in the Hungarian population (12.7%; P less than 0.001). Two hundred and six Achilles tendon ruptures were studied histologically, and all cases displayed pathological alterations. The results indicate that complete rupture of the Achilles tendon is usually a sequel to a sedentary life-style and participation in sports activities.

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