Quantification of the Contact Area at the Head-Stem Taper Interface of Modular Hip Prostheses

Abstract

Corrosion of modular taper junctions of hip implants may be associated with clinical failure. Taper design parameters, as well as the intraoperatively applied assembly forces, have been proposed to affect corrosion. Fretting corrosion is related to relative interface shear motion and fluid ingress, which may vary with contact force and area. It was hypothesised in this study that assembly forces modify the extent and distribution of the surface contact area at the taper interface between a cobalt chrome head and titanium stem taper with a standard threaded surface profile. Local abrasion of a thin gold coating applied to the stem taper prior to assembly was used to determine the contact area after disassembly. Profilometry was then used to assess permanent deformation of the stem taper surface profile. With increasing assembly force (500 N, 2000 N, 4000 N and 8000 N) the number of stem taper surface profile ridges in contact with the head taper was found to increase (9.2±9.3%, 65.4±10.8%, 92.8±6.0% and 100%) and the overall taper area in contact was also found to increase (0.6±0.7%, 5.5±1.0%, 9.9±1.1% and 16.1±0.9%). Contact was inconsistently distributed over the length of the taper. An increase in plastic radial deformation of the surface ridges (-0.05±0.14 μm, 0.1±0.14 μm, 0.21±0.22 μm and 0.96±0.25 μm) was also observed with increasing assembly force. The limited contact of the taper surface ridges at lower assembly forces may influence corrosion rates, suggesting that the magnitude of the assembly force may affect clinical outcome. The method presented provides a simple and practical assessment of the contact area at the taper interface.

Does Surface Topography Play a Role in Taper Damage in Head-neck Modular Junctions?

“…The machining mark height and spacing are almost identical, indicating that similar machining parameters have been applied. Other studies have reported values for head and stem taper topographies that fit well within our results [18,23,30,34,39,40].…”

Surface and subsurface changes as a result of tribocorrosion at the stem-neck interface of bi-modular prosthesis

“…The findings complemented those of Lanting et al [8]. Over the past 20 years extensive research has aimed to elucidate biomechanical, electrochemical, patient and system factors in the ultimate performance of conical modular taper systems [3,17,4,18]. Whilst a lot of work has been conducted in this area a full understanding of the degradation processes occurring within the local regions of the interfaces still does not exist around.…”

Prediction of taper performance using quasi static FE models: The influence of loading, taper clearance and trunnion length

“…A 1995 FDA recommendation for ceramic head tapers suggested that a stem taper must be in direct contact with at least than 50% of the vertical length of the taper . Yet, in a study assessing the contact area at the taper interface, the proportion of area in contact for a 11 mm long trunnion with 12/14 taper was reported to be <20% even at impact loads of 8 kN . These values are analogous to the contact area generated following mating reported in the current study.…”

“…These values are analogous to the contact area generated following mating reported in the current study. Accordingly, it may be deduced that the FDA recommendation as well as the contact area reported by Witt and colleagues would result in a contact area of 130 and 106 mm 2 , respectively, at the point of maximum taper gap under stair climb loading. These values would be predicted to generate taper gaps of over 60 μm.…”