Surface roughness quantification of CoCrMo implant alloys

The articulating surfaces of CoCrMo alloy wear specimens and retrieved femo ral components of artificial total knee joints are subject to uneven wear. A repeatable and reliable measurement method is necessary to evaluate the s urface damage. In this study, the surface roughness of CoCrMo alloy specime ns subjected to in vitro third-body wear, and retrieved femoral components of knee joints were analyzed using a white light interference surface profi lometer. Each third-body wear specimen was divided into a 19 x 19 grid of 1 -mm(2) squares (361 squares) and each femoral condyle of retrieved specimen s was divided into two 10 x 10 grids of 1-mm(2) squares (100 squares). The surface roughness average (Ra) and root mean square roughness (RMS) were me asured for each of the squares. The average of all points measured was defi ned as the true surface roughness mean (TSRM). Measurements were then perfo rmed on 40-60 (in vitro specimens) or 30 (retrieved specimens) randomly sel ected points on each surface and a cumulative average was calculated, The c umulative average surface roughness value from only a few (5-15) measuremen t points generated large deviations (> 40%) from the TSRM, but converged to the TSRM as the number of measurements increased. The number of randomly s elected 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)) mas 40 points, and the RMN for the retrieved femoral components (surface area o f 100 mm(2)) was 20 points. To obtain the cumulative surface roughness aver age within a desired percentage of the TSRM, it is important to define or e xperimentally determine the critical minimum number of measurements, RMN. S everal types of measurements may be necessary to understand wear and damage on metal components of artificial knee joints, The TSRM represents a consi stent and reproducible measure of surface damage, and a starting point to d evelop appropriate measurement protocols to quantify damage on a specific s urface. (C) 1999 John Wiley & Sons, Inc.