The effect of implant design and bone density on maximum torque and holding power for femoral neck fracture devices.

Background and aims: Displacement of internally fixed femoral neck fractures due to implant migration is a common problem. This in vitro study was performed to compare maximum torque and holding power for five different implants.

Material and methods: Synthetic porous material with low, medium or high density was used to simulate cancellous bone. The tested implants included three conventional screws (AO, Olmed, Hansson), one screw with threads and a barb (Hybrid), and a pin with a hook (LIH).

Results: The Hansson screw provided higher maximum torque in low and medium density blocks when compared with the other implants (p < 0.0001) followed by LIH, Hybrid, Olmed, and AO. For high-density blocks there was no significant difference between Hansson and Hybrid screws, both with significantly higher torque than the other implants. The maximal pullout in low-density blocks differed significantly between all five implants with the Hansson screw providing the highest holding power. For medium and high-density blocks the conventional screws had significantly higher pullout resistance compared with the Hybrid and LIH.

Conclusion: The in vitro model used seemed to provide reproducible and clinically relevant results. There was a good correlation between material density and holding power for all implants. Screws inserted without predrilling provided higher maximal torque while maximal pull out load seemed less affected by predrilling being used or not.