Position of the distal fibular fragment in pronation and supination ankle fractures: a CT evaluation.

BACKGROUND: Although classically the fibula has been reported to be in external rotation after supination-external rotation (SER) or pronation-external rotation (PER) ankle fractures, a previous CT study demonstrated that what had traditionally been interpreted as external rotation of the distal fibular fracture fragment is actually internal rotation of the proximal fibular fragment. The purpose of this study was to evaluate a series of CT scans in patients who have suffered type IV SER or PER ankle fractures to assess the rotational deformity of the fibular fragment.

MATERIALS AND METHODS: CT scans of the injured and uninjured extremities were performed on 30 extremities which had sustained either SER (21) or PER (9) injuries. The rotational relationship between the tibia and fibula was determined by a measured rotational ratio. A qualitative assessment of the rotational relationship between the tibia and fibula above, at, and below the fracture site at the level of the mortise was also performed. The difference in the ratio (calculated by subtracting the rotation ratio of the normal side from the fracture side) demonstrated whether the fractured fibula is externally or internally rotated compared to the uninjured side.

RESULTS: The average rotational ratio difference above the fracture compared to below the fracture for the SER group demonstrated significant external rotation (p < .001). The PER fracture also demonstrated external rotation of the distal fragment compared to the proximal fragment (p = .002). Additionally, qualitative assessment of the relationship demonstrated no obvious change in the rotational relationship in any patient above the fracture site except one where mild internal rotation of the proximal fragment was noted. However, at the level of the mortise, all had a normal talofibular rotational relationship while 24 of 30 had widening of the medial clear space with external rotation clearly evident on 15 of these 24 scans.

CONCLUSION: Our study demonstrated that the distal fibular fragment in both SER and PER fractures is externally rotated relative to both the contralateral normal side and compared to the proximal fibular fragment.