Abstract
Introduction
Total hip arthroplasty (THA) is a commonly performed procedure to relieve arthritis or traumatic injury. However, implant failure can occur from implant loosening or crevice corrosion as a result of inadequate seating of the femoral head onto the stem during implantation. There is no consensus—either by manufacturers or by the surgical community—on what head/stem assembly procedure should be used to maximize modular junction stability. Furthermore, the role of “off-axis” loads—loads not aligned with the stem taper axis—during assembly may significantly affect modular junction stability, but has not been sufficiently evaluated.
Objective
The objective of this study was to measure the three-dimensional (3D) head/stem assembly loads considering material choice—metal or ceramic—and the surgeon experience level.
Methods
A total of 29 surgeons of varying levels (Attending, Fellow, Resident) were recruited and asked to perform a benchtop, head/stem assembly using an instrumented apparatus simulating a procedure in the operating room (Figure 1). The apparatus comprised of a 12/14 stem taper attached to a 3D load sensor (9347C, Kistler® USA, Amherst, NY). Surgeons were randomly assigned a metal or ceramic femoral head and instructed to assemble the taper using their preferred surgical technique. This procedure was repeated five times. Surgeons were brought back to test the opposite material after four weeks. Output 3D load data was analyzed for differences in peak vertical load applied, angle of deviation from the stem axis—termed off-axis angle, variability between trials, and impaction location.
Results
Preliminary results suggest no significant differences between the loads applied to the metal heads and the ceramic heads. Across the two materials tested, both attendings and residents applied greater loads than fellows (p=0.33; Residents=9.0 kN vs Fellow=7.2 kN: p=0.27; Attendings=8.9 kN vs 7.2 kN) with significantly less variability (Attendings: σ= 1.58; Fellows: σ= 3.26; Residents: σ= 2.86). Attending surgeons also exhibited applied loads at significantly lower off-axis angles compared to fellows (p=0.01; 4.6° vs Fellow=7.2°) (Figure 2). However, all of our clinicians assembled ceramic head tapers with a greater off-axis angle as compared to assembling metal heads. In addition, metal heads were impacted more on-axis for all surgeon experience levels (Figure 3). While the impaction load plots suggest that the first impact strike is the most crucial for head stability, it was determined that the number of strikes is not as important as the maximum impaction load applied.
Conclusion
Differences in impaction load when assembling metal and ceramic femoral heads were not apparent; however, variability of technique and load was observed across the different surgical experience levels as well as within surgeons of the same level. Understanding assembly mechanics and surgical habits for THA will provide insight to the best assembly procedures for these implants.
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