Lateral ankle injuries are extremely common, most commonly injury to anterior talofibular (ATFL) and calcaneofibular ligaments (CFL). High grade "sprain" injuries consist of either intrasubstance ligament rupture, or small avulsion fractures at the ligamentous insertions. It is hypothesized that the latter are more common at slower injury velocity and of a bipolar age distribution (more common in young and elderly patients) ¹.

Typically, avulsion injuries involving the ATFL and CFL result in fracture of the fibular tip. This case is unusual because it is the calcaneal attachment of the CFL which is avulsed.

A few specific points:

• small avulsion fractures may be quite subtle and easily missed on x-ray. Always focus on site of common avulsion, with special attention to regions of soft tissue swelling
• small avulsion fracture fragments are difficult to appreciate by MRI, and so avulsion injuries may be mislabeled as (intrasubstance) ligament rupture. Always re-review available x-rays when reading an MRI to detect subtle osseous defects and improve your awareness