Abstract
Introduction
The use modular total hip arthroplasty is associated with potentially serious local and systemic complications. Each modular interface introduces a source for wear particle generation. Research suggests the etiology of wear particle generation and subsequent corrosion begins with mechanical fretting and disruption of the protective oxide layer leading to release of metal ions at the taper interface. The purpose of this study was to conduct three dimensional (3D) surface scans of the mating surfaces of the neck-stem taper to identify features that may contribute to the fretting and surface corrosion.
Methods
Eighteen modular hip implant components (9 stems and 9 necks) received 3D surface scans to examine the neck-stem taper junction. The study analyzed the neck-stem taper in an as assembled condition so relative surface positions and surface features could be studied. The 9 stems and 9 necks were scanned using an optical scanner. The implant image volume was resolved to a point spacing of 0.5 mm. Measurements were made to determine the normal distance between the surfaces of the neck taper as seated in the stem slot. These measurements were used to produce a color map of the contact proximity between the neck and stem surfaces (Figure 1). Circumferential surface points from the neck and stem at corresponding taper axis heights were used to create surface contour plots to identify surface shape variation and contact. The angle measurements and neck seated depth were analyzed by regression.
Results
The typical features observed in these contact maps were: 1) a distinct vertically running line of contact at one end of the transition from the flat surface section to the radius surface section, and present on opposite surfaces in the same location; 2) distinct vertically running line of contact in the radius surface section just past the centerline on the side further away from the transition contact, and also present on the opposite radius section in the same location; 3) a concavity or area of no contact along the flat surface exists between the neck and stem components; and 4) one of the neck flat surfaces was closer to its mating surface on the stem. The plot colors show contact proximity ranging from 0 – 0.025 mm, 0.025 – 0.050 mm, and 0.050 – 0.075 mm. The average neck seated depth in the stem was 14.181 mm, ranging from 13.796 mm to 14.422 mm. Regression analysis showed that the seated depth of the neck was dependent on the taper angles in the flat section of the neck (R2 = 0.5000, p = 0.0332).
Conclusions
Three dimensional scans and analysis suggest that the shape of the neck and stem tapers deviate from ideal design dimensions, which results in a contact pattern and component fit with gaps between the mating surfaces and rotated alignment. The probable cause of the dimensional deviation is due to machine tool deflection during manufacture. The combination of the contact and fit is expected to contribute to relative motions between the neck and stem, exacerbating the MACC.
