The patients are randomized in two groups, one with the use of PRP and the other group without PRP. This information for the patients will be granted just at the end of the research. After the surgery the rehabilitation protocol is the same for both groups. An isokinetic testing is done before the surgery and after six months. Questionnaires about the knee function are asked before the surgery and after six months. A magnetic resonance imaging is performed after six months to evaluate the tendon regeneration.