Kaplan fiber injuries in the anterior cruciate ligament-injured knee: current concepts

Kaplan fibers (KFs) are the deep femoral attachments of the iliotibial band and are increasingly recognized as important secondary restraints to anterolateral rotatory instability in the anterior cruciate ligament (ACL)-injured knee. However, their clinical significance, magnetic resonance imaging (MRI) diagnosis, natural history after ACL reconstruction, and indications for surgical treatment remain incompletely defined. This current concepts review summarizes contemporary evidence on KF anatomy, biomechanics, imaging, healing potential, clinical outcomes, and surgical management. Current evidence suggests that KFs contribute substantially to restraint of internal tibial rotation, particularly at higher flexion angles, but available biomechanical data are derived largely from cadaveric studies and show considerable inter-specimen variability. MRI studies report KF injury rates ranging from 18.6% to 43% in acute ACL tears, although comparisons across studies are limited by heterogeneity in imaging protocols, diagnostic criteria, and poor inter-rater reliability in some grading systems. Recent clinical data suggest that many KF abnormalities resolve after ACL reconstruction, and short-term comparative studies have not shown consistent differences in laxity, functional outcomes, or return to sport between knees with and without preoperative KF injury. At present, the literature does not support routine KF reconstruction on the basis of MRI findings alone. Lateral augmentation procedures that incorporate KF anatomy may be considered in selected high-risk situations, including persistent high-grade pivot shift, revision ACL reconstruction, and generalized ligamentous laxity, but direct evidence supporting isolated KF reconstruction remains limited. Current best practice is therefore a selective, individualized approach that integrates imaging findings, intraoperative assessment, and patient-specific risk factors. Future work should prioritize standardized MRI criteria, prospective natural history studies, and comparative clinical trials evaluating KF-based augmentation against established lateral extra-articular procedures. LEVEL OF EVIDENCE: V, Expert Opinion-Narrative Review).