Protocol amendments 6/9-2017: At the third outcome assessment (at hospital discharge, 3-8 days after surgery) only the outcomes isometric knee-extensor strength, 6MWT, SCT and current knee pain are assessed. The KOOS and OKS questionnaires as well as knee pain during the last week are omitted at this end-point as they are not validated to assess acute post-operative conditions, and use a too long recall period.