Biceps femoris avulsion fracture is an avulsion fracture of the knee involving the insertion of the biceps femoris tendon onto the lateral aspect of the fibular head (or, less commonly, its proximal origin at the ischial tuberosity) 1-3. At the knee, it forms part of the spectrum of posterolateral corner (PLC) injuries and is frequently associated with anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) disruption 1-3.
Clinical presentation
Patients present with acute posterolateral knee pain, localised tenderness and swelling over the fibular head, and posterolateral rotatory or varus instability 1,3. Because the common peroneal nerve courses immediately posterior and inferior to the biceps femoris tendon insertion at the fibular neck, patients should be carefully assessed for foot drop or sensory deficit over the dorsum of the foot 1,3.
Pathology
Distal biceps femoris avulsion fractures at the fibular head result from a forceful varus stress applied to a flexed, externally rotated knee, or sudden eccentric contraction of the biceps femoris muscle against forced knee extension 1,3. High-energy varus force avulses a larger osseous fragment from the lateral margin of the fibular head (conjoined insertion of the biceps femoris tendon and fibular collateral ligament), differing from the smaller horizontal fibular styloid tip avulsion of the arcuate complex (arcuate sign) 1-3.
Radiographic features
Plain radiograph & CT: displaced osseous fragment arising from the lateral aspect of the fibular head, larger and more vertically oriented than an isolated arcuate complex avulsion 1-3
MRI: demonstrates marrow oedema in the lateral fibular head, continuity of the biceps femoris tendon and fibular collateral ligament with the avulsed fragment, and associated ACL/PCL or common peroneal nerve injury 1-3