OPTIMUM CONFIGURATION OF CANNULATED HIP SCREWS FOR THE FIXATION OF INTRACAPSULAR HIP FRACTURES: A BIOMECHANICAL STUDY

Abstract

In our region, we found six different radiological configurations of cannulated hip screws fixation in patients with intracapsular fracture of the femoral neck (AO type 3,1,B). These configurations, produced at the time of the screws insertion were I: Triangular, consisted of two parallel screws with a third screw placed either superiorly, inferiorly, anteriorly or posteriorly. II: Two or three screws in a vertical line. Current literature suggests that parallel lag screws and subchondral fixation are important for stable fixation, but there are no current guidelines about the optimum configuration to achieve the best fixation.

In a laboratory setting, using standard synthetic bones and ordinary AO cannulated hip screws, six different configurations were fashioned similar to clinical practice. Each specimen was subjected to a single progressive vertical load until failure. Displacement curves, in relation to the load (peak and ultimate), were recorded on the computer based data acquisition system.

The most significant result of the study was that there is a significant difference between the superior ‘single screw triangle’ [mean difference 627 (Newton), 95% CI (66.72, 1187.28)] and ‘two screws vertical’ configurations [mean difference 744 (Newton), 95%CI (183.72, 1304.28)]. No other significant differences were detected.

If cannulated hip screws are chosen for internal fixation of femoral neck fractures we would, based on our study, recommend the use of triangular configurations with two parallel screws and a third screw placed either anteriorly, posteriorly, superiorly or inferiorly as they afford better strength and stability of fixation. The configuration of two or three vertical screws should be avoided as they provide lower grade of stability and a high incidence of failure. This suggests surgical technique can influence mechanical stability and thus outcome. This needs to be emphasized, particularly during training, in the hope of improving overall results in the future.