The case: your diagnosis? Calcific tendonitis of the fibular collateral ligament.

This article describes calcific tendonitis within the fibular collateral ligament, presumably from hydroxyapatite deposition, a rare cause of acute, severe lateral knee pain. Imaging findings confirmed calcifications in an intact but thickened fibular collateral ligament with adjacent soft-tissue reaction, consistent with calcific tendonitis. Magnetic resonance imaging can appear aggressive, and therefore the findings often can be mistaken for other knee abnormalities; specifically, the presentation following a twisting injury that requires avulsion fracture or ligamentous injury be excluded. Confirmation of crystal deposition with thin-section CT is helpful when suspected on MRI because it is unequivocal in depicting calcifications and bony detail.