Abstract
Introduction
This retrieval analysis study consisted of two goals. The first goal was to determine if there was a difference in the corrosion and fretting damage along the taper interface between large femoral heads in comparison to monopolar hemiarthroplasty heads. The second goal was to examine if the diameter of monopolar hemiarthroplasty heads can influence corrosion and fretting damage along the taper interface.
Patients and Methods
This retrieval analysis compared the corrosion and fretting behaviour of 40 mm femoral heads (n = 13) to monopolar hemiarthroplasty heads (n = 17 for a diameter < 50 mm; n = 6 for a diameter ≥ 50 mm) such that all implants had a minimum implantation period of three months, a 12/14 mm taper, and the heads and stems consisted of CoCr alloy. The 40 mm heads articulated with a polyethylene cup whereas the monopolar hemiarthroplasty heads articulated with cartilage. The 40 mm heads were manufactured from one company whereas the monopolar hemiarthroplasty heads were manufactured from four different companies. Corrosion and fretting damage were assessed using a previous technique [1]. Table 1 lists the patient information and reasons for revision whereas Table 2 provides the implant information.
The Mann Whitney U test and the Kruskal-Wallis test were performed for identifying significant differences for corrosion and fretting scores that were not normally distributed (α = 0.05). An unpaired student's t-test was conducted for comparing the head corrosion scores for the two head size groups of monopolar hemiarthroplasty implants since these scores were normally distributed.
Results
As shown in Table 3, the 40 mm heads had an average head corrosion score of 6.4 ± 9.9 whereas the monopolar hemiarthroplasty heads had an average score of 13.8 ± 13.4 resulting in a significant statistical difference (p = 0.049). In regards to head fretting, the scores were 1.3 ± 1.9 for the 40 mm heads and 10.5 ± 18.2 for the monopolar hemiarthroplasty implants, but the difference was not statistically significant (p = 0.058). There were no significant statistical differences in the corrosion and fretting scores between the four manufacturers. A significant statistical difference was found for the head corrosion scores of the two monopolar hemiarthroplasty head size groups. Monopolar hemiarthroplasty heads with a diameter < 50 mm had an average score of 11.0 ± 10.2 whereas the heads with a diameter ≥ 50 mm yielded scores of 21.8 ± 18.6 (p = 0.043).
Discussion/Conclusion
Monopolar hemiarthroplasty heads exhibit more corrosion damage along the taper interface than 40 mm heads. This study also revealed that monopolar hemiarthroplasty implants exhibit more corrosion damage with a larger diameter. This supports the argument that a greater head diameter increases the torque acting along the head-stem taper interface. This would increase the micromotion, which can deteriorate the protective oxide layer, and make the taper interface more vulnerable to crevice corrosion.
