Abstract
Introduction
Fretting corrosion of the modular taper junction in total hip arthroplasty has been studied in several finite element (FE) studies. Manufacturing tolerances can result in a mismatch between the femoral head and stem, which can influence the taper mechanics leading to possibly more wear. Using FE models the effect of these manufacturing tolerances on the amount of volumetric wear can be studied. The removal of material in the FE model was validated against experiments simulating the clinical fretting wear process, subsequently the mismatch and assembly force were varied to study the effect on the volumetric wear.
Methods
An FE model was developed in which the geometry can be updated to account for material removal due to wear. In this model the geometry was updated based on Archard's Law, using contact pressures, micromotions and a wear factor, which was determined based on accelerated fretting experiments. The linear wear was calculated using H=k*p*S. Where H is the linear wear depth in mm, k is a wear factor (mm3/Nmm), p is the contact pressure (MPa) and S is the sliding distance (mm). 10 million cycles were simulated using 50 virtual steps. Using this scaling and the measured volumetric wear from the experiments a wear factor of 2.7*10−5 was applied.
Based on general manufacturing tolerances the resulting mismatch in taper angles were determined to be ± 1.26°. Using this mismatch a tip fit (figure 1a) and base fit (Figure 1b) model were created. In combination with a perfect fit, meaning no mismatch, and two different assembly forces of 4 kN and 15 kN, 6 different situations were studied.
Results
No mismatch proved to result in the least amount of wear after 10 million simulated cycles (Figure 2). Assembling with 15 kN instead of 4 kN reduced the total volumetric wear and the volumetric wear rate. A base fit mismatch resulted in less volumetric wear than a tip fit mismatch. The 15 kN assembled mismatch cases showed a large initial amount of material removal after which the wear rate was lower than the 4 kN assembled cases.
Discussion and conclusion
The results show that a perfect fit between the head and stem results in the least amount of wear. Furthermore a larger assembly force of 15 kN resulted in less wear than a 4 kN assembly force. The tip fit mismatch showed up to 144% more wear than the perfect fit where the base fit only had an increase in volumetric wear of 12%.
The relative large tolerances in this study may overestimate actual mismatch, but give good insight into the effect that manufacturing tolerances can have on the taper mechanics and volumetric wear.
Since manufacturing a perfect fit is impossible it is important to use a sufficiently high assembly force, when clinically possible, in order to reduce the amount of wear and wear rate significantly.
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