Session VII - Foot and Ankle

*Syndesmotic Instability in Weber B Ankle Fractures: A Clinical Evaluation

Paul Tornetta, III, MD; Erik Stark, MD; William R. Creevy, MD; Boston University Medical Center, Boston, Massachusetts, USA (a-Stryker Howmedica Osteonics)

Purpose: Syndesmotic instability may coexist with Weber B (occurring at the level of the joint) lateral malleolar fractures. Cadaveric and clinical studies have documented that restoration of medial stability in bimalleolar injuries will prevent lateral displacement of the talus. Likewise, criteria for syndesmotic fixation in the face of lateral malleolar fractures with deltoid rupture have been developed based on a cadaveric model (Boden SD, Pano AL, McCowin P, et al: Mechanical considerations for the syndesmosis screw. JBJS 71: 1548 - 1555, 1989). However, these models make assumptions that have not been clinically proven. On the basis of current recommendations, no Weber B injury should have associated syndesmotic instability after anatomic open reduction internal fixation of the bony injuries. The purpose of this study was to evaluate syndesmotic stability with respect to the current recommendations for syndesmotic fixation in Weber B lateral malleolar and bimalleolar fractures in a large clinical series.

Methods: Over a 7-year period, 291 skeletally mature patients with unstable SE pattern Weber B ankle fractures were evaluated. There were 115 bimalleolar injuries and 176 lateral malleolar injuries associated with deltoid ligament incompetence. Each patient was treated by standard open reduction internal fixation of all fractures. Lateral fixation included antiglide plate (47%), lateral plate (35%), and lag screws only (18%). Medial malleolar fractures were treated with one or two lag screws. After all bony fixation was complete, syndesmotic stability was evaluated with use of a standard stress radiograph or direct observation with the foot forced into external rotation + abduction or direct observation with use of a clamp to pull on the distal fibula. All fibular fractures were anatomically reduced and stably fixed. Two medial malleolar fractures were not anatomic because of comminution. Any subluxation of the talus or more than 2 mm of syndesmotic widening or posterior translation were used as criteria for the diagnosis of syndesmotic instability, and trans-syndesmotic fixation was added to the construct.

Results: Syndesmotic instability was found in 104 of the 292 (36%) fractures after bony stability was restored. Lateral malleolar fractures associated with deltoid injury (ligamentous SE4 type) had a 40% rate of syndesmotic instability as compared with 30% for the bimalleolar SE4 fractures (P = 0.07). The relative risk of syndesmotic instability with a ligamentous SE4 compared with a bimalleolar SE4 was 1.4 (95% CI .91 to 1.92). The vast majority of the bimalleolar fractures that had associated syndesmotic instability were anterior collicular fractures (94%) rather than supracollicular fractures (6%). A diagnosis of syndesmotic instability was made in the operating room for 103 of the 104 ankles; a diagnosis for the one ankle missed during surgery was made radiographically because of demonstrated widening 2 weeks after surgery. This patient was returned to the operating room for reduction and trans-syndesmotic fixation. All patients were followed to union without displacement.

Discussion: Recommendations for syndesmotic fixation have been in large part based on cadaveric studies. Criteria published by Boden, et al. have been widely quoted and generally accepted. These authors suggested that, in the face of anatomic and stable bony fixation, syndesmotic mobility would not exceed 2 mm unless the fracture was a minimum of 3 to 4.5 cm above the joint if the lateral malleolar fracture was associated with a deltoid rupture and more than 15 cm above the joint if the injury was bimalleolar. That study and others made assumptions that the interosseous membrane, interosseous ligament, deltoid, and capsular tissues do not stretch prior to fracture. Additionally, embalmed cadaveric specimens were used. Our findings in the current clinical series clearly dispute the cadaveric findings. We found syndesmotic instability to be common after anatomic and stable bony fixation in Weber B, SE type unstable ankle injuries. On the basis of this study we recommend that these criteria be abandoned in favor of individual examination of the syndesmosis after open reduction internal fixation. This can be performed by use of a stress radiograph or by direct examination. This observation is strengthened by our experiences treating patients referred for late subluxation after anatomic open reduction internal fixation of Weber C injuries.

Conclusions: Previously published criteria for syndesmotic instability based on cadaveric studies are not representative of the clinical situation. Syndesmotic instability is common after bony fixation in unstable Weber B, SE type ankle fractures and must be sought out and treated.

* If noted, the author indicates something of value received. The codes are identified as a-research or institutional support; b-miscellaneous funding; c-royalties; d-stock options and e-consultant or employee.