Surface roughness quantification of CoCrMo implant alloys

Abstract

The articulating surfaces of CoCrMo alloy wear specimens and retrieved femoral components of artificial total knee joints are subject to uneven wear. A repeatable and reliable measurement method is necessary to evaluate the surface damage. In this study, the surface roughness of CoCrMo alloy specimens subjected to in vitro third-body wear, and retrieved femoral components of knee joints were analyzed using a white light interference surface profilometer. Each third-body wear specimen was divided into a 19x19 grid of 1-mm(2) squares (361 squares) and each femoral condyle of retrieved specimens was divided into two 10x10 grids of 1-mm(2) squares (100 squares). The surface roughness average (Ra) and root mean square roughness (RMS) were measured for each of the squares. The average of all points measured was defined as the true surface roughness mean (TSRM). Measurements were then performed on 40-60 (in vitro specimens) or 30 (retrieved specimens) randomly selected points on each surface and a cumulative average was calculated. The cumulative average surface roughness value from only a few (5-15) measurement points generated large deviations (>40%) from the TSRM, but converged to the TSRM as the number of measurements increased. The number of randomly selected points necessary for the cumulative average roughness to be within 10% of the TSRM was defined as the representative measurement number (RMN). The RMN for the third-body wear specimens (surface area of 573 mm(2)) was 40 points, and the RMN for the retrieved femoral components (surface area of 100 mm(2)) was 20 points. To obtain the cumulative surface roughness average within a desired percentage of the TSRM, it is important to define or experimentally determine the critical minimum number of measurements, RMN. Several types of measurements may be necessary to understand wear and damage on metal components of artificial knee joints. The TSRM represents a consistent and reproducible measure of surface damage, and a starting point to develop appropriate measurement protocols to quantify damage on a specific surface.

Oxidized Zirconium versus Cobalt-Chromium in TKA: Profilometry of Retrieved Femoral Components

“…Sixty measurements (four rows of 15 points, each point covering 600 9 800 lm of the component's bearing surface) were taken per condyle, approximately at the location of contact during 308 of flexion (Fig. 2) as suggested by Que et al [24,25]. One pristine, unimplanted OxZr and one pristine CoCr component were measured to provide an estimate of the initial roughness before implantation.…”

“…In the current study, we limited our selection of retrieved femoral implants to a single design and manufacturer to focus the comparison between implants based primarily on the bearing material: OxZr versus CoCr. We used three-dimensional surface profilometry, a highly accurate technique used in previous studies of CoCr components [19,[23][24][25], to provide objective measurements of the surface roughness of the components.…”

On the Roughness Measurement on Knee Prostheses

“…Among these, surface roughness and friction between metal and polyethylene play a very important role. The finishing surface of the metal components is a vital factor for minimizing the polyethylene wear rate and the gradual production of metal and plastic debris (6, 13, 14). Wear-produced debris and third-body wear may promote further abrasive wear of the polyethylene (9-13).…”

“…The finishing surface of the metal components is a vital factor for minimizing the polyethylene wear rate and the gradual production of metal and plastic debris (6, 13, 14). Wear-produced debris and third-body wear may promote further abrasive wear of the polyethylene (9-13). For this reason, the minimization of the surface roughness, surface profile, and finishing of the metal components are believed to be of great importance to minimize wear of the insert (12-15).…”

“…Several studies have performed surface roughness analysis on femoral TKR components using contact profilometry (17, 18) and non-contact surface roughness measurement methods (13, 16). However, these authors report the roughness measurements only in terms of average surface roughness, Ra.…”