Currently, most patients with an anterior cruciate ligament (ACL) injury undergo surgery. There is a general belief that surgical reconstruction is necessary to return to sport safely and to limit premature knee osteoarthrosis or additional meniscal damage. However, there is unsufficient scientific evidence for this belief. Moreover, several studies show that a reconstruction does not guarantee successful return to sports or the prevention of osteoarthritis or meniscal injuries at all. Therefore, an immediate surgery after an ACL injury is more and more questioned. The only qualitative RCT that exists (KANON trial) could not demonstrate that an immediate reconstruction is an added value (in terms of symptoms, knee function, activity level, osteoarthritis or incidental meniscal damage) compared to a conservative approach consisting of progressive rehabilitation and delayed surgery if there was persistent knee instability.

In a future multicenter RCT the investigators want to 1) verify these results and 2) search for predictors that predict which patients from the conservative group do well without delayed surgery. This information is invaluable to physicians as it allows them to decide which treatment is best for the patient.

Before performing a large, adequately-powered RCT that compares both treatment options, the investigators will run a pilot study that assesses the feasibility to recruit ACL patients for such RCT. This seems necessary, as many patients still believe that timely surgery is a prerequisite for restoring knee function, for returning to sports and for preventing cartilage degeneration. These preferences for surgery might affect recruitment and adherence to the protocol. Therefore, a pilot study will performed that demonstrates whether a large RCT is feasible with regard to 1) participant recruitment, 2) adherence to the treatment arm they were allocated to and 3) protocol feasibility. The findings of this pilot study will help deciding about progressing to a future definitive RCT.

Inclusion Criteria:

• Rotational trauma to a previously uninjured knee within the preceding 4 weeks
• Medical diagnosis of ACL insufficiency including MRI (both partial and complete ruptures)
• Minimum of 18 years

Exclusion Criteria:

• Patient has history of a previous ACL injury or knee surgery to the index knee.
• Indication for acute surgery because of related injuries to the knee
• Any disorder, which in the investigator's opinion might jeopardise participant's safety or compliance with the protocol.
• Female who is pregnant. Since MRI assessment cannot be performed.