The Effect of Impact Location on Force Transmission to the Modular Junctions of Dual-Taper Modular Hip Implants.

Background: The purpose of this study was to investigate the effect that off-axis impaction has on stability of dual-taper modular implants as measured by forces delivered to and transmitted through the head-neck and neck-stem tapers, respectively.

Methods: One hundred forty-four impact tests were performed using 6 different directions: one on-axis and five 10° off-axes. Four different simulations were performed measuring the head-neck only and 3 different neck angulations: 0°, 8°, and 15°. A drop tower impactor delivered both on- and off-axis impaction from a constant height. Load cells positioned in the drop mass and at the head-neck (HN) or neck-stem (NS) junction measured the impact and joint forces, respectively.

Results: Impact force of the hammer on the head ranged from 3800-4500 N. Greatest impact force delivered to the head was typically with axial impact. However, greatest force transmission to the neck-stem junction was not necessarily with axial impacts. There was limited variability in the force measured at the NS junction for all impaction directions seen in the 8° neck, whereas the 15° neck had greater forces transmitted to the NS junction with off-axes impactions directed in the proximal and posterior-proximal directions.

Conclusion: The location of the impact significantly influences the force transmitted to the head-neck and neck-stem junctions in dual-taper modular hip implants. Although axial impacts proved superior to off-axis impacts for the straight 0° neck, greater force transmission with off-axis impacts for the angled necks suggests that off-axis impacts may potentially compromise the stability of dual-taper components.