Weightbearing Radiographs Reliably Predict Normal Ankle Congruence in Weber B/SER2 and 4a Fractures: A Prospective Case-Control Study

Background: In Weber B/SER2-4 ankle fractures, assessment with weightbearing radiographs to ascertain stability of the ankle mortise has been advised. However, no previous studies report whether this method leads to preservation of normal ankle congruence. The purpose is to evaluate equivalence of ankle congruence of injured ankles after fracture union, vs the uninjured side, for stable SER2 and partially unstable SER4a fracture types. Methods: We conducted a prospective case-control study designed as an equivalence trial to evaluate if weightbearing radiographs predict whether stable/SER2 and partially unstable/SER4a Weber B ankle fractures reach union with preserved normal tibiotalar congruence and without a concomitant increase of treatment failure. A total of 149 patients with a Weber B fracture stable on weightbearing radiographs were recruited into the trial. All participants were treated with a functional orthosis and weightbearing allowed. Results from gravity stress radiographs classified ankles as SER2 or SER4a fracture types. We defined an equivalence margin in medial clear space difference of 1.0 mm. We also evaluated the reliability of obtaining measurements from weightbearing radiographs. Results: No differences in medial clear space between the injured and uninjured ankles were observed after fracture union for the SER2 group (mean difference 0.1 mm, 95% confidence interval [CI] −0.3, 0.0; P = .056), or the SER4a group (mean difference 0.0 mm (95% CI −0.1, 0.1; P = .797). No between-group differences were observed (mean difference 0.0 mm, 95% CI −0.2, 0.2; P = .842). These findings were consistent with equivalence. CIs for the intraclass correlation coefficients indicated excellent reliability. Conclusion: Assessment of stability of Weber B SER2/4a ankle fractures, with weightbearing radiographs, also predicts preservation of normal ankle congruence in those deemed stable, with no difference between SER2 and SER4a fracture types. Further, excellent reproducibility of the method of obtaining medial clear space measurements was demonstrated. Level of Evidence: Level III, case-control.