OTA 1997 Posters - Foot & Ankle Fractures

Minimal Fixation of Lateral Malleolar Fractures: A Biomechanical Study

Jeff Traub, MD, Paul P. Hospodar, MD, Richard L. Uhl, MD, Edward Ashman, Eric Ledet, MS, Chris Manseau, MD

Albany Medical College, Albany, New York, USA

Purpose: To compare fixation of lateral malleolar fractures with two isolated interfragmentary lag screws with a single interfragmentary lag screw used in combination with a one-third tubular neutralization plate.

Materials and Methods: Fifteen matched sets of cadaveric fibulae were harvested from formalin preserved specimens. The cortical surface of each fibula was scored with a saw blade and a two to one length to width fracture pattern was created so as to duplicate the minimum ratio recommended by Muller et al. for isolated interfragmentary lag screw fixation. One of each pair of fibulas was randomly assigned fixation with two lag screws, and its partner was fixed with a lag screw and one third tubular plate using standard AO/ASIF technique. Each bone was then cut to a uniform length, and then potted at each end in epoxy resin. The distal end of each fibula was secured to a torsional testing machine (Instron model TTC-M) and the proximal end was attached to a torsional load cell. Each fibula was tested to torsional failure at a loading rate of 90 degrees per second, and the maximal torque to failure and the mode of failure under external rotation were determined.

Results: The mean ultimate torsional strength for the specimens fixed with two interfragmentary lag screws was 1.53 N-m (13.6 inch-lbs). The mean ultimate torsional strength for the specimens fixed with a single interfragmentary lag screw and one third tubular plate was 2.19 N-m (19.0 inch-lbs). The single interfragmentary lag screw and one third tubular plate was 1.4 times as strong as two interfragmentary lag screws used alone. A paired t-test showed a statistically significant difference (p=0.05) in the ultimate torsional strength between the two methods of fixation.

Discussion: Ankle fractures treated with surgical fixation require anatomical reduction with compression of the fracture surfaces. Compression is achieved with an interfragmentary lag screw. This diminishes the load at the fracture site, and increases the strength of fixation. The neutralization plate is added to protect the lag screw from torsional, bending and shearing forces. The cost of this additional security is increased exposure for placement, and the common postoperative complaint of long term tenderness from hardware. Muller et al. state that two conditions must be met to achieve stability in diaphyseal bone with lag screws alone. The length of the fracture must be at least twice the diameter of the bone and at least two lag screws must be used. This study shows that two interfragmentary screws have less torsional strength than a single interfragmentary screw used with a neutralization plate.

Conclusion: A neutralization plate increases the torsional strength of fracture fixation. This added strength may allow the surgeon to feel more comfortable with early motion or weight bearing.