Background
We noticed that subcircumferential periosteal oedema above the ankle joint was frequently present on MRI with syndesmosis injuries but was not previously reported. Fluid height within the interosseous membrane has also not been shown to be associated with syndesmosis injury severity.

Purpose
Investigate whether a new sign on MRI and measurement of the length of fluid within the interosseous membrane above the ankle may be used to enable early identification of a syndesmosis injury and allow differentiation from lateral ligament injury.

Methods
Three groups of patients were identified from a database and the MRI scans retrieved – those with an isolated syndesmosis injury (SI group), isolated lateral ligament injury (LLI group) and or no injury (NI group) who had an ankle MRI for another reason. The scans were anonymized and independently assessed by eight clinicians (surgeons and radiologists) who were blinded to the diagnosis. The maximum length of fluid above the ankle within the intraosseous membrane was measured for each patient. The presence or absence of distal anterior, lateral and posterior tibial periosteal oedema was recorded (‘Ring of Fire’).

Results
Measurement of the length of fluid above the ankle had excellent intra-observer reliability (ICC=0.97 [0.93-0.99]) but poor interobserver reliability. Fluid extended higher in both the LLI group (p=0.0043) and SI group (p=0.0058) than the NI group but there was no significant difference between the LLI and SI groups (p=0.3735) indicating that this measurement cannot differentiate between the injuries. The presence of the ‘Ring of Fire’ around the distal tibia was significantly more frequent in the SI group when compared to both LLI and NI groups (p<0.00001). The sensitivity of this sign is 49% but when present this sign has a 98% specificity for syndesmosis injury.