IN-VITRO FEMORAL STIFFNESS AFTER FEMORAL-NECK OSTEOTOMY AND OSTEOSYNTHESIS WITH DEFINED SURGICAL ERRORS

In our search for an osteosynthesis device that would tolerate the sur gical errors of the inexperienced surgeon, we tested in vitro femoral stiffness in 75 human osteoporotic femora after internal fixation of a cervical neck osteotomy using three commonly used devices: two von Ba hr screws (A. Ericsson AB, Sweden), two cannulated screws (Uppsala typ e, Olmed AB, Sweden), and two hookpins (LiH, PSAB, Sweden). The first device has its main grip in the cancellous bone by threads; the second has grip in cancellous and subchondral bone by threads; and the third , which has no threads, has its grip in cancellous bone by a hook pin. The intact specimen was in all instances stiffer (22-63%) than the os teosynthesized specimen (p < 0.001). An osteosynthesized femur with pe rfectly reduced bone ends was 14-23% stiffer than when reduction of th e bone ends was insufficient, irrespective of device malposition (p ( 0.001). Insufficient reduction of the osteotomy leaving a 20 degrees d orsal angulation of the femoral head combined with too far ventrally p laced screws resulted in the lowest femoral stiffness. If reduction of osteotomy was sufficient, screws placed too far ventrally or convergi ng screws did not result in decreased stiffness compared with optimal screw placement. Irrespective of the quality of reduction, osteosynthe sis with the Uppsala screw resulted in all instances in a higher stiff ness than using the other devices (p < 0.01). With the Uppsala screw d esign, femoral stiffness after optimal osteosynthesis was reduced by 2 2% compared with the intact femur, and in the most unfavorable positio n with combined malreduction and malpositioning it was reduced by 42%. Corresponding values for the von Bahr screws were 29% and 46%, respec tively, and for the LiH screws 47% and 63%, respectively. Use of a dev ice with threads and grip in the subchondral bone is recommended for f ixation of femoral neck fractures in osteoporotic bone. Furthermore, t he importance of anatomical reduction for fracture fixation is emphasi zed.