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Intraoperative assessment of the stability of the distal tibiofibular joint in supination-external rotation injuries of the ankle: sensitivity, specificity, and reliability of two clinical tests.

BACKGROUND
This study was designed to assess the sensitivity, specificity, and interobserver reliability of the hook test and the stress test for the intraoperative diagnosis of instability of the distal tibiofibular joint following fixation of ankle fractures resulting from supination-external rotation forces.


METHODS
We conducted a prospective study of 140 patients with an unstable unilateral ankle fracture resulting from a supination-external rotation mechanism (Lauge-Hansen SE). After internal fixation of the malleolar fracture, a hook test and an external rotation stress test under fluoroscopy were performed independently by the lead surgeon and assisting surgeon, followed by a standardized 7.5-Nm external rotation stress test of each ankle under fluoroscopy. A positive stress test result was defined as a side-to-side difference of >2 mm in the tibiotalar or the tibiofibular clear space on mortise radiographs. The sensitivity and specificity of each test were calculated with use of the standardized 7.5-Nm external rotation stress test as a reference.


RESULTS
Twenty-four (17%) of the 140 patients had a positive standardized 7.5-Nm external rotation stress test after internal fixation of the malleolar fracture. The hook test had a sensitivity of 0.25 (95% confidence interval, 0.12 to 0.45) and a specificity of 0.98 (95% confidence interval, 0.94 to 1.0) for the detection of the same instabilities. The external rotation stress test had a sensitivity of 0.58 (95% confidence interval, 0.39 to 0.76) and a specificity of 0.96 (95% confidence interval, 0.90 to 0.98). Both tests had excellent interobserver reliability, with 99% agreement for the hook test and 98% for the stress test.


CONCLUSIONS
Interobserver agreement for the hook test and the clinical stress test was excellent, but the sensitivity of these tests was insufficient to adequately detect instability of the syndesmosis intraoperatively.