Ideal length of thread forms for screws used in screw fixation of nondisplaced femoral neck fractures

Background

It is common practice when placing cannulated screws within the femoral head when treating femoral neck fractures to avoid the thread-forms from crossing the fracture line. Despite the widespread use of cannulated screws in internal fixation of femoral neck fractures, there is no study to our knowledge that describes the ideal length of thread-forms.

Purpose

The purpose of this study is to determine the thread length that will maximize purchase within the femoral head while minimizing risk of crossing the fracture line. Additional analysis was conducted to identify factors associated with the maximal possible length of treads in minimally and non-displaced femoral neck fractures.

Methods

We performed a retrospective study of all patients treated for a minimally or non-displaced femoral neck fracture from April 1, 2004 through December 31, 2017. Only patients who had received a pre-operative CT or MRI scan were included. Fixation was then templated using radiographs and the distance from the subchondral bone to the fracture line was then measured.

Results

The study included 127 patients. The average estimated length of lag screw threads was 33.2?±?6.67?mm, with lower quartile of 29.1?mm and higher quartile of 37.2?mm. The median was 32.0?mm and most frequently encountered estimate was 29?mm. Estimated lag screw size did not differ significantly based on age or BMI, but both height (p?<?0.001) and race (0.04) were positively correlated with estimated lag screw size and males had longer measurements compared to females, 37.2?±?7.0?mm vs 31.4?±?5.7?mm (p?<?0.001), respectively.

Conclusion

In conclusion, we propose an additional lag screw thread form with length 26.0?mm to capture 90% of femoral neck fractures.