Dual-taper modular hip implant: Investigation of 3-dimensional surface scans for component contact, shape, and fit.

Background: The etiology of wear particle generation and subsequent corrosion in modular total hip arthroplasty implants likely begins with mechanical fretting. The purpose of this study was to determine geometric features of the male and female taper surfaces that drive stability within the neck-stem junction.

Methods: Eighteen modular hip components received 3-dimensional surface scans to examine the neck-stem taper junction using an optical scanner. The normal distance between the surfaces of the neck taper as seated in the stem slot was measured and produced a color map of the contact proximity. Contour plots identified surface shape variation and contact. Angle measurements and neck seated depth were analyzed by regression.

Results: The typical features observed were (1) a vertical line of contact at one end of the transition from the flat surface to the radius surface; (2) a vertical line of contact in the radius surface just past the centerline; (3) a concavity along the flat surface between the neck and stem components; and (4) one of the neck flat surfaces was closer to its mating surface on the stem. The seated depth of the neck was dependent on the taper angles in the flat section of the neck (R2  = 0.5000, P  = .0332).

Conclusions: The shape of the neck and stem tapers deviate from ideal design dimensions, contributing to relative motions between the neck and stem. While these processes are not proven to directly cause implant failure, they may place the implants at higher risk for failure.