VERTICAL SHEAR FRACTURES OF THE MEDIAL MALLEOLUS - A BIOMECHANICAL STUDY OF 5 INTERNAL-FIXATION TECHNIQUES

Fifty embalmed human tibias were osteotomized to create a simulated ve rtical shear (supination-adduction) fracture of the medial malleolus a nd were stabilized using one of five internal fixation techniques. In offset axial testing, which simulated supination-adduction loading, th e fixation strength of tibias stabilized with either cortical or cance llous lag screws placed perpendicular to the osteotomy was over five t imes greater than the strength of those treated with an antiglide plat e and nearly two and a half times greater than those treated with canc ellous lag screws placed oblique to the osteotomy. The tibias stabiliz ed with cancellous lag screws placed perpendicular to the osteotomy ex hibited twice the fixation strength of the tibias stabilized with an a ntiglide plate and distal lag screw. The tibias stabilized with an ant iglide plate and distal lag screw and perpendicularly placed cortical or cancellous lag screws demonstrated three times greater resistance t o displacement to the applied supination-adduction load than those sta bilized with an antiglide plate alone. In offset transverse testing, t o simulate loading in external rotation, the mean failure load of the tibias stabilized with cancellous lag screws placed perpendicular to t he osteotomy was over two and a half times greater than those stabiliz ed with an antiglide plate and distal lag screw. No significant differ ences were observed in the resistance to displacement for these tests. These results support the use of lag screws placed perpendicular to t he fracture surface for stabilization of ertical shear fractures of th e medial malleolus and indicate that the use of an antiglide plate, wi th or without a distal lag screw, does not offer any advantage over la g screw fixation.