Kaplan fibres of the knee revisited: Anatomical variants, MRI identification and surgical implications for ACL reconstruction and rotational instability

Kaplan fibres (KFs) are distinct capsulo-osseous connections between the iliotibial band and the distal femur that have re-emerged as critical contributors to anterolateral knee stability and the pivot shift phenomenon. This narrative review synthesises contemporary anatomical, radiological, biomechanical, and surgical data to clarify KF morphology, variants, and their implications in the setting of anterior cruciate ligament (ACL) injury. Anatomical studies consistently demonstrate a bipartite architecture composed of superficial and deep fibres, with variable femoral insertions and occasional accessory slips that help resist tibial internal rotation. These features may explain why some patients experience residual rotatory laxity even after anatomically positioned ACL reconstructions. On magnetic resonance imaging, KFs are best assessed with high-resolution 3 T protocols, fat-suppressed proton density or T2 sequences, and oblique planes aligned to the lateral femoral cortex. However, deep fibres are inconsistently visualised and interobserver agreement remains moderate. Clinically, KF disruption occurs in up to 60% of acute ACL tears and is strongly correlated with high-grade pivot shift. Biomechanical models confirm that KFs function as secondary stabilisers, particularly between 30° and 90° of flexion, complementing but not duplicating the roles of the ACL and anterolateral ligament. While no direct KF reconstruction technique exists, lateral extra-articular tenodesis (LET), notably the modified Lemaire procedure, replicates their tension vectors and reduces failure rates when combined with ACL reconstruction in young athletes, revision cases, or patients with generalised ligamentous laxity. Based on available evidence, we propose a preliminary Kaplan Fibre Injury Classification (KFIC) that integrates MRI and intraoperative features, and we outline a pragmatic decision-making algorithm linking KF injury patterns to augmentation strategies. Key gaps include a lack of standardised terminology, validated MRI grading, and prospective outcome studies stratified by KF status. By integrating anatomy, radiology, biomechanics, and operative perspectives, this review emphasises the critical role of KFs in rotational knee stability and provides a framework to improve preoperative planning, guide selective use of extra-articular augmentation, and optimise long-term outcomes after ACL reconstruction. LEVEL OF EVIDENCE: Basic.