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Summary19710SummaryACL injury is a devastating injury for many young athletes, leading to prolonged absence or even cessation of sports activities. ACL reconstruction aims to restore knee kinematics and to return knee function to the preinjury level. An important outcome for successful ACL reconstruction is return to sports. Despite the increasing numbers of ACL reconstructions being performed around the globe, return to sports rates are poor. We hypothesised that persistent rotational laxity during sports activity could be a reason why athletes cannot return to their pre-injury level of sports.In Chapter 2 the results of a systematic review are described to assess the role of an ACL graft on range of tibial rotation. Most of the studies included used computer-assisted surgery (CAS) to assess range of tibial rotation before and after surgery. In an anaesthetised patient, a reduction of 17-32% of range of tibial rotation is achieved after ACL reconstruction. Included studies were mostly non-randomised and of low quality. Based on this review, we proposed a new measuring protocol that contains measurements at 0, 30 and 60 degrees flexion and a maximum of 5 Nm rotational force, in order to enhance comparability between studies.In Chapter 3 we report on a prospective cohort study to determine the range of tibial rotation within three months of ACL injury, and again one year after ACL reconstruction. It was hypothesised that, in line with the results from Chapter 2, after ACL injury the range of tibial rotation would increase compared to the contralateral intact knee. In search for a reason why athletes are unable to return to sports, we hypothesised that one year after ACL reconstruction increased range of tibial rotation would still be present during high-demand tasks, in comparison to the contralateral intact knee.Interestingly, this study showed that both within three months after ACL injury and one year after ACL reconstruction the range of tibial rotation was not increased during high-demand tasks. Both findings are indicative of a compensatory mechanism or protective strategy that is deployed by subjects. The underlying mechanism could not be determined based on the results of this study. Mark Zee.indd 197 03-01-2024 08:56