Weight-Bearing Computed Tomography Evaluation of Acute SER4a Ankle Fractures

Abstract

Category: Ankle; Trauma Introduction/Purpose: Recent studies have reported cadaveric and early clinical data regarding Weber B supination-external- rotation (SER)4a ankle fractures. SER4a ankle fractures have discordance of the medial clear space (MCS) between weight-bearing (WB) and gravity stress (GS) radiographs, showing instability on the GS view which normalizes with WB. Unfortunately, evaluation of the medial clear space only provides a two-dimensional evaluation of a three-dimensional structure. Prior cadaveric research has shown significant decreases in the tibiotalar joint contact area with only 1 mm of displacement of the talus. The purpose of this study is to utilize weight-bearing computed tomography (WBCT) to evaluate differences in the tibiotalar joint contact area and rotational alignment in patients with acute SER4a ankle fractures compared to their contralateral uninjured ankle. Methods: Adult patients presenting within ten days of injury with an isolated Weber B fibula fracture and a normal MCS ( < 5 mm) on injury radiographs were evaluated with weight-bearing and gravity stress radiographs. Patients determined to have isolated Weber B SER4a ankle fractures (GS positive but WB negative radiographs) and no history of injury to the contralateral ankle were included. The ability to place equal weight on both extremities was confirmed by standing on side-by-side scales. WBCT scans were obtained of both the healthy and injured ankles. The images were evaluated using Disior Bonelogic (Paragon28, Helsinki, Finland) software to calculate the tibiotalar joint contact area, axial tibiotalar rotation, and talocrural angle. An a priori power analysis was performed with six patients needed. Results: Five out of ten patients have been enrolled in the study to date (3 females, 2 males, average age 52.4). Preliminary data show an average 4.3% (standard deviation (SD) 4.9) increase in tibiotalar joint contact area between the injured and uninjured extremity. The average difference in axial tibiotalar angle is 3.6 (SD 6.9) degrees of internal rotation of the injured ankle. The talocrural in the injured and uninjured extremity have an average difference of 0.062 (SD 1.8) degrees. Conclusion: This study provides the first in vivo evaluation of the joint contact area changes seen in SER4 ankle fractures. Although limited in sample size, the results demonstrate that SER4a ankle fractures do not have decreased tibiotalar joint contact area relative to the healthy, uninjured side. The trend towards increased surface area in the injured ankles is hypothesized to be a result of the increased reliance on joint congruency with injury to the fibula and deltoid.