Assessing Fibular Length Using Bimalleolar Angular Measurements

Abstract

Shortening of the fibula after fracture is common and often difficult to appreciate. Loss of lateral malleolar anatomy causes significant biomechanical changes in the ankle and correlates with poor clinical results. We studied angular measurements of distal fibular length to serve as a guide for assessing fibular reduction after ankle fracture. Mortise view X-rays of 50 normal ankles from 25 healthy volunteers were obtained. The average talocrural angle measured 78.5°. However, individual variation was high with values ranging from 75 to 86°. Comparing contralateral ankles demonstrated an average difference of 1.3° (range 0 to 4°). A new, simpler bimalleolar angle was devised which compares the long axis of the fibula with a line drawn between the tips of the malleoli. The average bimalleolar angle measured 77.8° (range 72 to 86°). The contralateral difference averaged 1.2° (range 0 to 3°). This angle was simpler to use and more reproducible. Angular measurements were tolerant of usual radiographic techniques. Internal or external rotation of the ankle up to 5° caused an insignificant change in the angular measurements. One degree change in the talocrural or bimalleolar angle was found to correspond with a 1 mm change in fibular length for the average ankle, calculated radiographically and confirmed in a cadaver study. Abnormal fibular shortening is detected with an angular difference between injured and contralateral sides of 3.0° using the talocrural angle or 2.5° using the bimalleolar angle (95% confidence limits). Thus, a 2.5 to 3.0° contralateral difference should serve as a minimum value required to direct a change in therapy. We conclude that comparing angular measurements of the injured with the contralateral ankle provides the most accurate guide for assessing fibular length.