Abstract
Introduction
Recent literature demonstrates that the assembly load to connect ball head and femoral stem affects the taper junction fretting wear evolution in THR [1]. During assembly the surface profile peaks of the mostly threaded tapers are deformed. This contributes to the taper locking effect. Very little is known about this deformation process and its role in the evolution of fretting and wear. Therefore, this study aimed to experimentally determine the deformation of the profile peaks after the initial assembly process.
Materials and Methods
36 tapers of three different stem materials acc. to ISO5832-3 (titanium), ISO5832-9 (steel), ISO5832-12 (cobalt chromium) and 36 ceramic ball heads were tested under quasi-static (4kN) and dynamic (impaction) (3.7±0.3kN) axial assembly. Before and after loading 4 surface profiles in 90° offset were measured on each taper. Height differences of profile peaks and areas under profile curves were calculated and compared. Both parameters provide insights into the deformation behavior of the surface structure. Additionally, subsidence of tapers into ball heads was measured and subsidence rates were calculated with regard to varying impaction forces. Due to different thermal expansion coefficients tapers could be disconnected from ball heads by utilizing liquid nitrogen. Thus, further surface damage due to disassembly was avoided. Statistical analysis was performed using a Wilcoxon test (p<0.05).
Results
Almost no differences of the subsidence rate were found among the taper materials in both assembly groups while it was higher for dynamic assembly (Figure 1). Peak height difference decreased with increasing number of profile peak (Figure 2) and increased in the dynamic assembly group. Largest peak height differences were found for titanium tapers, while steel and cobalt chromium tapers showed almost identical values, especially in the dynamic assembly group. Differences in area under profile showed varying results for the three taper materials (Figure 3). Almost no changes were found for steel tapers, while titanium and cobalt chromium tapers showed distinct differences.
Discussion
This study describes the deformation behavior of the taper surface structure of three commonly used metallic materials coupled with ceramic ball heads. Since titanium has the lowest Young's modulus it seems reasonable that highest subsidence rates and peak height differences were found. Nevertheless, this material also showed the largest differences in area under profile which could be interpreted as a parameter of material removal. In contrast, steel tapers showed lowest material removal, but also lowest peak height differences and subsidence rates corresponding to the finding of almost none metal transfer at the ceramic counterface. These low subsidence rates could be influenced by frictional forces since this material combination has the highest friction coefficient [2,3]. The results provide insights into the mechanical behavior of stem tapers from commonly used metallic materials in THR and will be used for calibration of finite element models examining interface contact mechanics and wear.
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