Abstract
Purpose
Current methods for inserting a press fit hemispherical metal-backed acetabular component within the acetabula are uncontrolled, relying on the surgeon to generate the necessary forces required for sufficient introduction. While previous studies have recorded impact forces of 2–3 kN necessary to seat an acetabular cup using visual observation[1], some researchers have observed users imparting as high as 8.9 kN of force[2]. The aim of this study is to quantify the forces required to generate optimal implant primary stability, as well as compare force delivery methods.
Method
The experiments were carried out using prepared bone substitute. A high frequency force sensor was rigidly mounted under the substitute to measure impact force and duration. An acetabular cup was inserted using successive reproducible impacts of varying magnitude (2.5 kg falling 17, 34, 43, 51, 68, or 85 mm). Impacts were repeated until the cup was no longer advancing. Each test recorded the number of impacts, maximum impact force, impact duration, and extraction force of the cup after insertion. The results were then compared against manual insertion (tapping) and high frequency vibratory insertion (50–500 Hz).
Results
As shown in figure 1, an exponential relationship was found between the maximum impact force and cup extraction force (R2 = 0.97), with a mean impact force of 4200 N at full insertion. By contrast, manual insertion resulted in maximum impacts 30% greater on average, with no discernible increase in extraction force. High frequency vibratory insertion resulted in a linear relationship (R2 = 0.86) with a maximum extraction force of 335 N.
Conclusion
Manual insertion has been shown to result in excessive force being used. This may result in additional stress to the acetabula, although additional study is needed to determine the clinical relevance. High frequency vibratory insertion has shown promise of reducing the impact forces required, with ongoing study of the effect at higher impact forces.
