The Kaplan fibers are the proximal (femoral) attachments of the iliotibial band to the lateral intermuscular septum and the lateral femoral cortex, approximately 5-6 cm proximal to the lateral femoral condyle. They hold the proximal IT band to the femur and create the characteristic IT band biomechanics: the band is slack in flexion and taught in extension, and the Kaplan fibers are the specific structure creating the IT band compression against the lateral femoral condyle at 30 degrees of knee flexion in IT band friction syndrome.
The Kaplan fibers are the biomechanical basis of IT band syndrome (runner's knee): the Kaplan fiber attachment prevents the IT band from sliding anteriorly beyond 30 degrees of flexion, creating a compression zone against the lateral femoral condyle as the IT band flicks across the condyle from anterior to posterior at this angle with each running stride. Surgical proximal IT band release (the Kaplan fiber release or Z-plasty) reduces the tension creating the compression zone. The Kaplan fibers are also a secondary stabiliser of the lateral knee against anterior tibial translation in combined ACL and lateral structure injury.
Repetitive knee flexion-extension in running causes the Kaplan fiber-anchored IT band to compress against the lateral femoral condyle at 30 degrees, producing lateral knee pain that is worst at this angle going downstairs or running downhill; conservative management includes hip abductor strengthening, foam rolling, and gait retraining with surgical IT band release reserved for truly refractory cases.
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