People operated for a rupture of the Achilles tendon receive a concentrate of their own platelets injected into the operated area. The study is randomized and single-blinded, with 20 treated patients and 20 controls, who get no injection. The primary endpoint is a mechanical property of the tendon after 12 weeks (modulus of elasticity), as measured by a special type of radiography. This radiography uses 0.8 mm metal markers, which are injected into the tendon. The tendon is mechanically loaded during radiography and its strain can be measured as increased distance between the markers. Other mechanical variables at 6, 12 and 52 weeks are secondary variables, as well as a validated patient-administrated questionaire at 1 year.