Abstract
INTRODUCTION
Wettability of bearing couples has always been related to the tribological performance of implants, and it is understood to affect lubrication of surfaces. So far researchers fail to understand the real mechanisms governing the lubrication process of prostheses. Different models attempt to explain the phenomena, but more research is needed. To add more difficulties, some classical measuring techniques have provided inaccurate values of surface properties. For instance, wettability may seem a priori a simple technique capable of providing easy-to-read cost-effective information. However, ignoring surface preparation may lead to wrong values of wettability and mis-understanding of the results.
OBJECTIVES
The dependence of wettability of commercial bearing materials used in arthroplasty has been studied as a function of the cleaning procedure, showing the variability of the results, and providing a series of guidelines to understand and perform wettability measurements.
EXPERIMENTAL METHODS
Three different materials were tested: BIOLOX®forte and BIOLOX®delta (CeramTec GmbH, Plochingen), and Cobalt Chrome Molybdenum alloy (CoCrMo; ISO 5832-12). All samples were polished to comparable mirror finish. For wettability assessment, water sessile drop contact angle tests both in static and dynamic mode in a goniometer (KRÜSS model DSA25) with automatic drop dozer were performed, and the images analysed with a proprietary computer software (KRÜSS DSA4 version 1.0). The samples were cleaned using different methods found in the literature (organic solvents, sonication, wipers) and compared with the industrial procedure CeramTec uses before packaging and sterilizing (“Production Cleaning”).
RESULTS AND DISCUSSION
Wettability assessment is very sensible to contamination. High variability of the contact angles was found among the three materials investigated, with values ranging from 25 to 90°.
By studying the difference between advancing and receding angles (hysteresis) when performing dynamic contact angle tests, the best cleaning method was achieved by the one yielding the lowest hysteresis. Production Cleaning was the best cleaning method, meaning it is capable of removing most sources of heterogeneities (i.e. surface contamination), yielding values of static contact angle of 28.7 ± 3.9° for BIOLOX®forte, 32.0 ± 4.0° for BIOLOX®delta, and 49.8 ± 7.4° for CoCrMo alloy. Cleaning with ethanol in sonication was not sufficient to remove adsorbed contaminants in the surface and left hydrophobic traces. Also usage of wipers in combination with organic solvents is discouraged as a single cleaning method because they are incapable of removing well-adsorbed contaminants.
CONCLUSION
Wettability measurements are easy to perform but difficult to interpret, and the effect of contamination must be respected. The results of this work demonstrate a major influence of the cleaning process on the results. When cleaned properly, BIOLOX® ceramics exhibit a more hydrophilic response than CoCrMo.
ACKNOWLEDGMENTS
The authors would like to acknowledge the European Commission funding under the 7th Framework Program (Marie Curie Initial Training Networks; grant number: 289958, Bioceramics for bone repair).
