Judging Stability in the Isolated Weber B Fracture
An isolated Weber B fracture with a hidden deep deltoid tear can let the talus drift out of the mortise and wear out the ankle, and a stable one heals in a cast without the wound and infection risks of surgery.
Key terminology
- Mortise
- This socket holds the talus between the tibial plafond and the two malleoli. A congruent mortise shows even joint space all the way around the talus.
- Lateral malleolus
- This bony knob is the distal end of the fibula on the outside of the ankle. It is the bone that breaks in a Weber B fracture.
- Syndesmosis
- These ligaments bind the distal tibia and fibula together just above the ankle joint. The Weber level of a fibula fracture is judged against it.
- Weber B
- The fibula breaks at the level of the syndesmosis. Stability hinges on whether the deep deltoid ligament is intact.
- Deltoid ligament
- This fan of ligaments runs from the medial malleolus to the talus and calcaneus. Its deep layer is the main restraint against lateral talar shift.
- Supination-external rotation (SER)
- This Lauge-Hansen pattern names the foot position and the force at injury. Stage 2 is a fibula fracture alone, and stage 4 adds a medial injury.
- Medial clear space
- This gap lies between the medial malleolus and the medial talus on a mortise view. Widening means the talus has shifted laterally.
- Noninferior
- This trial asks whether a new treatment is no worse than the standard by more than a preset amount. That amount is the noninferiority margin.
Takeaways
- The ankle is stable when the talus stays centered in the mortise. In 23 cadaver ankles, the first 1 mm of lateral talar shift cut tibiotalar contact area by an average of 42%. Less contact area means higher joint pressure and later arthritis. A later cadaver model found the highest talar dome pressures with fibular shortening (Ramsey and Hamilton, J Bone Joint Surg Am 1976;58:356-357, PMID 1262367; Thordarson et al, J Bone Joint Surg Am 1997;79:1809-1815, PMID 9409794).
- Weber B is the most common ankle fracture, and its stability depends on the medial side. About 2/3 of ankle fractures are unimalleolar, and a lateral malleolus break at the syndesmosis (Weber B) is the most common type. An intact deep deltoid holds the talus (SER stage 2), and a torn one lets it drift laterally (SER stage 4 equivalent) (Kortekangas et al, BMJ Surg Interv Health Technol 2021;3:e000098, PMID 35047809; Gougoulias and Sakellariou, Bone Joint J 2017;99-B:851-855, PMID 28663388).
- Medial tenderness does not predict a deep deltoid tear. In 55 Weber B fractures with a normal medial clear space, medial tenderness had a sensitivity of 57% and a specificity of 59% for a positive stress radiograph. In 97 isolated fibula fractures, tenderness, ecchymosis and swelling were not predictive of instability. Stability is judged on the radiograph (DeAngelis et al, J Orthop Trauma 2007;21:244-247, PMID 17414551; McConnell et al, J Bone Joint Surg Am 2004;86:2171-2178, PMID 15466725).
- A medial clear space of 5 mm or more on external rotation stress points to a deep deltoid tear. In a cadaver model, ≥5 mm on a dorsiflexion external rotation view was the most reliable cutoff. A review of 10 studies (698 patients) supported 4 to 5 mm. Gravity and manual stress views gave the same readings in 25 patients (Park et al, J Orthop Trauma 2006;20:11-18, PMID 16424804; Yousaf et al, Foot Ankle Orthop 2019;4:2473011419890861, PMID 35097353; Gill et al, J Bone Joint Surg Am 2007;89:994-999, PMID 17473136).
- Stress views may overestimate instability that a weight-bearing film does not show. Of 104 SER fractures, 44 were borderline on gravity stress (4 to 7 mm) and congruent standing, and all healed congruent in a cast. In 38 stress-positive fractures, only 3 (8%) widened on weight-bearing films and went to surgery. Gravity stress views appear to overrate the need for surgery (Seidel et al, Foot Ankle Int 2017;38:736-744, PMID 28511569; Hoshino et al, Foot Ankle Int 2012;33:92-98, PMID 22381339).
- An undisplaced fibula does not prove a stable ankle. In 241 SER fractures that were stress tested, 55% of the unstable ankles had ≤2 mm of fibular displacement and 15% had none. A fibula displaced ≤2 mm had a positive predictive value of only 0.37 for a stable mortise. The medial clear space answers the stability question and the fibular gap does not (Shahien and Tornetta, J Orthop Trauma 2022;36:e1-e5, PMID 33878070).
- A cast was noninferior to ORIF for stress-unstable Weber B fractures with a congruent mortise. SUPER-FIN randomized 126 Weber B fractures that were congruent on plain films and unstable on external rotation stress. At 2 years the Olerud-Molander Ankle Score (OMAS) was 89 vs 87 (cast vs ORIF), inside the 8 point margin, with 1 nonunion per arm. It was a single Finnish center, and guidelines still call these fractures operative (Kortekangas et al, BMJ 2026;392:e085295, PMID 41534905; Kortekangas et al, BMJ Surg Interv Health Technol 2021;3:e000098, PMID 35047809).
- A stable Weber B fracture may need only 3 weeks of immobilization. In 247 patients with an isolated Weber B and a congruent mortise on plain films, 3 weeks in a cast or a simple orthosis was noninferior to 6 weeks in a cast at 1 year. OMAS was 91.7 for the 3 week cast and 89.8 for the 3 week orthosis vs 87.6 for the 6 week cast (Kortekangas et al, BMJ 2019;364:k5432, PMID 30674451).