Abstract
Introduction:
Large diameter metal on metal total hip arthroplasty (MOM THA) have shorter lengths of implantation due to increased failure caused by wear either at the articulating surface as well as the taper-trunnion interface. Taper-trunnion wear may be worse in large diameter MOM THA due the increased torque at the taper-trunnion interface. However little has been done to understand how differences in taper-trunnion geometry and trunnion engagement effects wear. The purpose of this study was to (1) measure the differences in taper geometry and trunnion engagement on the head-taper of 11/13, 12/14, and Type 1 taper designs and (2) to determine if taper geometry affects fretting, corrosion, and wear at the taper interface.
Methods:
We identified 54 MOM THA primary revision implants with head diameters greater than 36 mm from our retrieval archive. Patients' charts were queried for demographic information and pre-revision radiographs were measured for cup inclination and cup anteversion. To measure taper geometry and wear the head tapers were imaged using Redlux©. The point clouds obtained from this were analyzed in Geomagic©. Taper angles and contact length where the trunnion engaged with the female taper of the head-tapers were measured. The diameter of the taper at the most distal visual area of trunnion engagement was also measured. Best fit cones were fit to the unworn regions to approximate the pristine surface. Differences between the raw data and the unworn surface were measured and volumetric wear rates were calculated. Fretting and corrosion of the head-taper was graded using the Goldberg Scoring.
Results:
Geometric differences were found between the three designs with the Type 1 being the narrowest with an average taper angle of 3.97 ± 0.09° and an average distal diameter of 12.42 ± 0.35 mm; 11/13 was the second narrowest with a taper angle of 5.97 ± 0.03° and a distal diameter of 13.13 ± 0.27 mm. The widest taper was 12/14 with a taper angle of 5.58 ± 0.21° and a distal diameter of 13.91 ± 0.35 mm. Contact lengths were greatest for 11/13 tapers, 18.96 ± 1.51 mm, then 12/14, 13.31 ± 3.46 mm and least for Type 1, 11.98 ± 4.44 mm (Table 1).
Differences in geometry did not significantly affect volumetric wear rate or corrosion but did affect fretting. Type 1 tapers had significantly lower fretting scores (2.9 ± 1.5, p < 0.05) than 12/14 tapers (5.0 ± 1.6) and 11/13 tapers (6.4 ± 2.1).
Discussion:
We were able to measure determine geometric differences between three common taper designs which may affect taper damage. Tapers which are narrower and have less contact length (i.e. Type 1) had less head-taper fretting than those which are wider and have longer contact lengths. This may be a function of less surface of the trunnion contacting the head taper interface. While we could not demonstrate any differneces in trunnion wear rates among taper types, volumetric wear and corrosion may be independent of taper geometries.
