BIOMECHANICAL COMPARISON OF THE SLIDING HIP SCREW AND THE DOME PLUNGER

We studied the biomechanical behaviour of three sliding fixation devic es for trochanteric femoral fractures, These mere a titanium alloy sid eplate and lag screw, a titanium alloy sideplate and dome plunger with cement augmentation, and a stainless-steel sideplate and lag screw. W e used 18 mildly osteoporotic cadaver femora, randomly assigned to one of the three fixation groups, Four displacement and two strain gauges were fixed to each specimen, and each femur was first tested intact ( control), then as a two-part fracture and then as a four-part intertro chanteric fracture. A range of physiological loads was applied to dete rmine load-bearing, load-sharing and head displacement. The four-part- fracture specimens were subsequently tested to failure to determine ma ximum fixation strengths and modes of failure. The dome-plunger group failed at a load 50% higher than that of the stainless-steel lag-screw group (p < 0.05) and at a load 20% higher than that of the titanium-a lloy lag-screw group (NS), All 12 lag-screw specimens failed by cut-ou t through the femoral head or neck, but none of the dome-plunger group showed movement within the femoral head when tested to failure, Strai n-gauge analysis showed that the dome plunger produced considerably le ss strain in the inferior neck and calcar region than either of the la g screws, Inferior displacement of the femoral head was greatest for t he dome-plunger group, and was due to sliding of the plunger. The dome plunger with cement augmentation was able to support higher loads and did not fail by cut-out through the femoral head, Its sliding potenti al was maintained, retaining the biomechanical advantages of the slidi ng screw.