OBJECTIVE: To compare cast immobilisation with surgery using open reduction and internal plate fixation for unimalleolar Weber B ankle fractures with a congruent mortise on initial radiography but deemed unstable by external rotation stress testing.

DESIGN: Randomised, pragmatic, non-inferiority, clinical trial.

SETTING: One specialist university hospital trauma centre in Finland, 16 January 2013 to 7 July 2021.

PARTICIPANTS: 840 skeletally mature patients (age =16 years) with an isolated Weber B fibula fracture on static radiographs. Fracture instability was assessed by standard external rotation stress test under fluoroscopy. 714 participants were excluded (569 with stable fracture, mortise incongruency, or fracture dislocation) and the remaining 126 patients with a congruent but unstable ankle mortise were randomised.

INTERVENTIONS: Participants were randomly allocated to receive either conventional cast immobilisation for six weeks (n=62) or surgical treatment with open reduction and internal plate fixation followed by cast immobilisation for six weeks (n=64).

MAIN OUTCOME MEASURES: The primary, non-inferiority outcome was the Olerud-Molander Ankle Score (OMAS; range 0-100 points; higher scores indicating better outcomes and fewer symptoms) at two years. The predefined non-inferiority margin for the primary outcome was set at -8 points. Secondary outcomes were ankle function, pain, health related quality of life, ankle range of motion, and radiographic outcome. Treatment related adverse events were also recorded.

RESULTS: 121 out of 126 randomised participants (96%) completed the study. In the primary intention-to-treat analysis of 121 participants completing two year follow-up, the mean OMAS was 89 in the cast immobilisation group and 87 in the surgery group (between group mean difference 1.3 points, 95% confidence interval -4.8 to 7.3). No statistically significant between group differences were observed in any secondary outcomes. One participant in each group had radiographic evidence of non-union. In the surgery group, one participant had a superficial wound infection, one had delayed wound healing, and nine underwent procedures to remove hardware, two of whom developed postoperative infections (one deep and one superficial).

CONCLUSIONS: Cast immobilisation proved non-inferior to surgery for the treatment of unimalleolar Weber B ankle fractures with a congruent mortise on initial radiography but deemed unstable by external rotation stress testing. Overall, fewer treatment related harms occurred with cast immobilisation compared with surgery.

TRIAL REGISTRATION: ClinicalTrials.gov NCT01758796.

Emergency Medicine rater

As an emergency physician, Weber type 2 fractures are a encountered often. With unstable lateral malleolus fracture and disrupted deltoid ligament, surgery is the choice. However, when the mortise is maintained, this study suggests conservative management to be less inferior to surgery with fewer treatment related complications. The implications would be to consider cast immobilization in a case-by-case basis with frequent follow-ups to look for complications - both short term and long term, including post-traumatic osteoarthritis. More evidence would be welcome.

Pediatric Emergency Medicine rater

Most patients seen in the pediatric ED are skeletally immature, but this study is applicable to older adolescents. It is new information that may educate providers on alternate treatment options.

Surgery - Orthopaedics rater

This randomised, pragmatic, non-inferiority clinical trial compared cast immobilisation with surgery using open reduction and internal plate fixation for unimalleolar Weber B ankle fractures with a congruent mortise on initial radiography but deemed unstable by external rotation stress testing. 126 patients were randomly allocated to receive either conventional cast immobilisation for six weeks (n=62) or surgical treatment with open reduction and internal plate fixation followed by cast immobilisation for six weeks (n=64). The primary, non-inferiority outcome was the Olerud-Molander Ankle Score (OMAS) at two years. The mean OMAS was 89 in the cast immobilisation group and 87 in the surgery group. Cast immobilisation proved non-inferior to surgery for treating unimalleolar Weber B ankle fractures but deemed unstable by external rotation stress testing. Overall, fewer treatment-related harms occurred with cast immobilisation compared with surgery.