Internal fixation of femoral neck fractures. A comparative biomechanical study of Knowles pins and 6.5-mm cancellous screws.

In a comparative biomechanical study of the stability of experimentally produced femoral neck fractures fixed with Knowles pins or with 6.5-mm cancellous screws, 12 matched pairs of anatomic specimen femurs were divided into four groups of standardized osteotomies. One member of each matched pair was fixed with three or four 6.5-mm cancellous screws, whereas the other member was fixed with the same number of Knowles pins. Mechanical testing was performed on an Instron testing machine. There was no statistically significant difference in maximum load to failure or in fixed stiffness between the two methods of fixation. When examining maximum load to failure, three of either device provided fixation equivalent to four. Although there was a difference in absolute fixed stiffness between specimens fixed with three Knowles pins compared to four, when stiffness was expressed as a percentage of intact stiffness, there was no difference between any fixation group. From an analysis of the observed modes of failure and from calculations of the flexural rigidity provided by the devices, it is proposed that the quality of the trabecular bone of the femoral neck is a major factor in the experimental fixation of femoral neck fractures.