Objective

The anterolateral soft tissue envelope of the knee is frequently injured at the time of anterior cruciate ligament (ACL) rupture. The significant structures here are referred to as the ‘anterolateral complex’ (ALC) and these have been extensively studied again over the last decade. There is disagreement in regard to the importance of components: deep capsulo-osseous layer of the ITB (connecting Gerdy’s tubercle on the tibia to the lateral distal femur via the Kaplan Fibers (KF)), and anterolateral ligament (ALL)) with various authors championing the importance of one versus the other structure in resisting anterolateral rotatory instability (ALRI). The purpose of this study was to investigate the MRI injury patterns to the ALC and their associations in patients with acute ‘isolated ligament’ ACL ruptures.

Material And Methods

Patients who underwent isolated ACL reconstruction for complete ACL rupture between September 2015 and April 2019 were included in this study. Patients’ characteristics and intraoperative findings were retrieved from clinical and surgical documentation. Preoperative MRIs were evaluated and the injuries to respective structures of the ALC and their associations were recorded. All patients were clinically assessed under anaesthesia with standard ligament laxity tests and isolated ACL laxity confrmed. Binomial logistic regression was used to analyse risk factors for exhibiting injuries to ALL and KFs.

Results

ALC injuries were noted in 63% of cases. The majority were to KF (39% isolated injury and 19% combined with ALL injury). There was a very low incidence of isolated ALL injuries (2%). KF injuries are associated with the presence of LFC bone edema, and injuries to the superficial MCL, deep MCL, and ramp lesions. High grade pivot shift test was not associated with the presence of KF or ALL injuries.
Patients with an intact ALC sustained injuries to other knee structures (13% to medial ligament complex, 14% to medial meniscus, and 16% to lateral meniscus).

Conclusion

There is a high incidence of concomitant ALC injuries in combination with ACL ruptures, with KF (and therefore the deep casulo-osseous layer of the ITB) being the most commonly injured structure. ALL injuries occur much less frequently. High grade pivot shift was neither associated with KF or ALL injuries and is hence the magnitude of a pivot shift must depend on a number of factors – not just anterolateral laxity. Given the high injury incidence to KFs / ITB, and much lower ALL injury the former would seem the more important restraint to ALRI. This study also highlights the fact that a truly isolated ACL rupture is a truly rare occurrence and may never exist.