The challenging thing for us as clinicians seems to be the many definitions of what a “partial ACL tear” is. Noyes et al (1989) defined it as tear to 50-75% of the fibres. Hong et al (2003) defined a partial tear as less than 50% of ligament fibres torn. (DeFranco & Bach, 2009) have described it as: an asymmetric Lachman test, a negative Pivot-Shift test under anaesthesia,
The problem with using a 3mm difference between legs as a cut-off for either a full tear or a partial tear is that, more than 3mm difference between limbs in anterior translation suggests ACL deficiency; but less than 3mm does not definitely correlate with a partial tear, and it does not give any meaningful information regarding the quality or functional capacity of the intact tissue.
The other clinical challenge for us to appreciate is, if the ACL is partially torn, is it partially torn right down the middle and are some fibres holding on? Or is it that only 1 of its 2 bundles are torn? If you don’t know, the ACL has 2 functional bundles that add 2 different types of stability to the knee; the anteromedial (AM) bundle and the posterolateral (PL) bundle.
The AM bundle tightens in knee flexion (whilst the PL bundle relaxes) and restrains tibial translation >45deg knee flexion, whereas the PL bundle tightens in knee extension and tibial external rotation and as a result restrains tibial translation near full knee extension.
As you can then probably guess, depending on the knee position at the time of injury, it would reflect the ACL bundle that is partially torn; an AM bundle is more likely to be torn whilst the knee is flexed and pivoting, and the PL bundle is more likely to be injured in a hyperextended knee or a pivoting injury when the knee is in near full knee extension.
So with so much inconsistency in its definition, and the unknown about whether a partial tear progresses over time, or if it can heal, one thing that is consistent in the literature is that nonoperative rehabilitation should be the first line of treatment for partial tears to the ACL (DeFranco & Bach, 2009).
Like all injuries, especially ACL injuries, treatment needs to be individualised to the patient. Any associated injuries that may have been sustained at the time need to be considered and respected, as well as the demands on the knee and the goals of the patient. Patients who have a partial tear, and are involved in low risk activities, are most likely to have a good outcome without any further reinjury to the ACL or require operative treatment. DeFranco et al (2009) suggest that higher demand patients should pursue a nonoperative pathway initially, but have it explained that instability and symptoms may develop overtime and operative treatment may need to be performed in the future.
Bracing of the partial tear is up to the discretion of the treating orthopaedic surgeon and physiotherapist, but active rehab should be initiated as soon as possible after a partial tear following the same principles of a complete tear with the decision to return to sport based on strength test, function tests and psychological readiness as per ACLR. At this stage there is no evidence to suggest that bracing prevents progression of a partial to a full tear during rehab or upon return to sport (DeFranco et al 2009).
From an exercise perspective, it also makes biological sense that full AROM knee extension exercises should be avoided for the first 3 months to give the partial ACL the best chance to potentially heal. We know the greatest amount of strain the ACL encounters is during 0-30eg knee flexion, so I would recommend the same restrictions on ROM as per ACLR patients in that knee extensions be limited to 45-90deg knee extensions during the first 3 months, the progressed to full AROM exercises from 3 months onwards. Squats, lunges, leg press etc (closed chain exercises) have no restrictions on load and ROM.