Surface Finish of Total Hip Arthroplasty Implants: Are We Evaluating and Manufacturing Them Appropriately?

Abstract

A recent change in the ISO 7206-2, Implants for Surgery – Partial and Total Hip Joint Prostheses – Part 2: Articulating Surfaces Made of Metallic, Ceramic and Plastics Materials, with regard to hip joint prostheses, altered the recommended cutoff for measuring the roughness of the articulating surfaces of the femoral head and acetabular cup but did not encompass two key sites: the bore head and the trunnion. The aim of this study was to evaluate whether the standardized parameters and sites established in ISO 7206-2 are appropriate for evaluating the quality of the surface finish of total hip arthroplasty implants. Three kits containing the polymeric acetabular components, metallic heads, and stems from 5 manufacturers were used for testing. Contact profilometry was performed on the internal acetabular cup, external head, head bore, and trunnion surfaces. Additionally, the external head surface was evaluated with an interferometer to increase the accuracy. The evaluation of the external surface of the femoral head with a profilometer seems to be inadequate, because of a limitation of the contact method associated with highly polished surfaces. The average roughness (Ra) values obtained with an interferometer were lower than those measured with the profilometer and revealed significant differences between the manufacturers. The use of an 0.80-mm cutoff is more appropriate for the Ra range normally found for acetabular cups, enhancing the power to detect acetabular cups with inadequate roughness. The absence of a standard for the surface finish of the stem-head interface generates heterogeneity between manufacturers and between samples produced by a single manufacturer, revealing a lack of quality control.

The role of roughness parameters in grading the machined surface quality in Ti-alloys

“…On the other hand, for articulating components, such as the femoral head, a R a below 0.05 mm is required (ISO 7206-2: 2011/AMD 1:2016) to avoid particle generation due to excessive wear. 19 This finish cannot be achieved solely through machining but rather requires grinding and polishing operations. Nevertheless, the better the finish provided by machining, the less time is required for these operations.…”

“…In our case, Ti-alloys turned under the same conditions. This statistical tool has been used by others to compare the material removal rate of 2 bars of steel machined by electric discharge 32 ; the roughness attained with two values of cut-off (0.08 and 0.25 mm) 19 ; the cutting efforts generated under two machining environments (dry and nanofluid), 33 and two tool coatings (AlTiN and TiAlCrN). 34 Besides, paired hypothesis testing was effective for inferring if a predictive model differs significantly (or not) from experimental measurements 35 or other models.…”

“…Indeed, contact profilometry has worldwide traceability, as many finish standards used by the industry were defined using it. 19 Other advantages of 2D profilometry include portability, flexibility in measuring small and large workpieces and the capacity to assess inside bores.…”