Changes in the morphology of microgrooved stem tapers with differing assembly conditions

Abstract

Background Fretting and corrosion at the head-stem taper junction has been cited as a potential clinical concern. Material loss measurement is a vital tool for quantifying changes due to in vitro, in vivo, and/or ex-vivo implant experience. Material loss measurement requires reconstruction of pre-implantation geometry to delineate damage. This is straightforward in principle for plain machined tapers, integrating between a fitted interpolated cone and the measured data. Mathematical filtration methods have been developed to remove this texture and facilitate measurement. The extent to which the assembly process could influence filtration accuracy is not currently known. Methods An engagement/ disengagement study was performed on 27 head/ stem pairs using 2, 4, and 8kN impact loads. Impact was delivered in three locations; axial, 10 o anterior, and 10 o anteroproximal. Pull-off force was measured, and stems were measured pre and post assembly using a Talyrond 365 roundness measurement machine. Results An increase in the plastic deformation of microgrooves and pull-off force was noted with increasing assembly loads. Off-axis impaction resulted in reduced pull-off strength and reduced uniform microgroove distortion. Volumetric change between pre and post assembly was below the noise threshold on stem and head surfaces. Discussion The measurement method was shown to be capable of capturing linear microgroove deformation of 1 µm peak-peak and volume loss above 0.1mm 3. Lower peak load was noted when impacting a seated head when compared with engagement of discrete heads and stems, under the same assembly conditions.

Interaction of surface topography and taper mismatch on head‐stem modular junction contact mechanics during assembly in modern total hip replacement

“…The rationale for implementing microgrooves in the Morse Taper design was to allow for more effective collapse of the microgrooves under loading from impaction. As shown in our simulations and another benchtop study 34 , the peaks of the microgrooves undergo plastic strain under impact load and varies depending on the surface profile. For these reasons we are confident that we can conclude that the presence of topography on the head taper significantly affects plastic strain, and that the observed quadratic behavior of the effect of taper mismatch angle is valid.…”

An Overview of the Stability and Fretting Corrosion of Microgrooved Necks in the Taper Junction of Hip Implants

“…The two main components are usually manufactured from ceramic (alumina/zirconia), and Co28Cr6Mo, 316L stainless steel, and Ti6Al4V alloys [ 4 , 5 ]. There are various types of heads/trunnions on the market with different design characteristics (geometry, material, and surface finish) [ 25 ]. In terms of surface finish (which is the focus of this review), the engaged surfaces of the junction components can be machined smoothly or with microgrooves.…”

“…In some of these studies, this influence was combined with the influence of other variables such as the assembly force, trunnion length, and taper angle mismatch. In a recent study by Dransfield et al [ 25 ], the contact situation was monitored by measuring the deformations of the microgrooves using a method called “roundness measurement”. Impaction forces of 2, 4, and 8 kN were selected to assemble 27 mm CoCr heads onto Ti trunnions.…”

“…Surface finish/roughness is one of the key parameters affecting the interface engagement which is of interest because manufacturers traditionally believe that it can improve the junction integrity and durability [ 4 ]. There are two main classes of surface finish: (1) trunnions of the same roughness as their head counterparts, and (2) trunnions produced with purposely designed microgrooves (also called ridges/undulations) [ 25 ]. The periodic pattern of such microgrooves is sometimes classified with an amplitude and wavelength of more than 4 µm and 100 µm, respectively [ 25 ].…”

“…There are two main classes of surface finish: (1) trunnions of the same roughness as their head counterparts, and (2) trunnions produced with purposely designed microgrooves (also called ridges/undulations) [ 25 ]. The periodic pattern of such microgrooves is sometimes classified with an amplitude and wavelength of more than 4 µm and 100 µm, respectively [ 25 ]. These microgrooves were originally created on the trunnion’s surface to minimize the risk of brittle fracture which may naturally occur in ceramic heads.…”