Abstract
Introduction
Modularity in total hip arthroplasty offers many potential benefits, however the consequences of mechanically associated corrosion continue to be concerning. Micromotion and settling of the modular components at the taper interface are thought to contribute to the etiology of this problem. The purpose of this study was to investigate the effect of hammer blows delivered in different directions on the force transmitted to the head-neck and neck-stem interface in modular hip implants.
Methods
One-hundred and forty-four impact tests were performed in six different directions: one on axis and five 10° off axis. Four different simulations were performed measuring the head-neck only and three different necks: 0°, 8°, and 15°. A constant height delivered on-axis hammer blows at a constant 4,500 Newton (N). Load cells positioned in the hammer and at the neck-stem junction transmitted voltage to an oscilloscope which measured forces.
Results
Impact force of the hammer on the head ranged from 3800N – 4500N. Blows delivered in line with the axis had the highest force. There was a trend toward decreased force as direction moved in the proximal, anterior and posterior direction. Impact forces measured at the head-neck junction were roughly proportional to those measured at the neck-stem junction in all 3 different necks, albeit slightly attenuated. The 15° neck resulted in variable force transmission at all angles off-axis.
Discussion/Conclusion
The location of the impact significantly influences the force transmitted to the head-neck and neck-stem junction in dual modular hip implants. Input loads appear to be proportional, although slightly attenuated at the taper joints. Increasing the version of the neck results in increased variability of forces at the taper junctions.
