Abstract
Introduction: The accepted method of assessing wear following a hip simulator test has been to use a precision balance. As the MoM devices produce significantly less weight loss than hard-on-soft bearings, the measurements of MoM devices are now almost at the detection limit of many balances. There is a need for a method that can be used in conjunction with gravimetric analysis that will provide an accurate assessment of ion concentration levels that will support the gravimetric measurements.
Aim: To develop a method to assess wear using metal ion analysis in order to support gravimetric measurements of metal on metal devices.
Materials and methods: Hip simulator test: Three pairs of 50 mm diameter as cast CoCr MoM devices were tested in a ProSim hip wear simulator (SimSol Stockport/UK) under physiologically relevant conditions. The lubricant was new born calf serum with 0.2 % sodium azide concentration diluted with de-ionised water for protein concentration of 20 g/l. Stop-start motion was implemented every 100 cycles. Lubricant changed every 125 k cycles. The frequency was 0.5 Hz. Wear was assessed gravimetrically at every 0.5 million cycles (Mc) interval.
Ion analysis: Serum was collected from test station and allowed to settle for 12 hours. An aliquot of 20 ml from lubricant was collected. Each sample was centrifuged at 2500 g-force for 10 minutes. A 10 ml aliquot was collected from each sample and was further centrifuged at 2500 g-force for 10 minutes. 1.5 ml aliquot was collected and stored at −20 °C. A high resolution inductively-coupled plasma mass spectrometry instrument (ELEMENT, ThermoFinnigan MAT, Bremen/Germany) was then used for the analysis of metal ions.
Results and Discussion: The average cumulative metal ion levels at 0.5, 1 and 1.5 Mc showed similar trends in wear to that of the average cumulative weight loss assessed gravimetrically. There were similar biphasic wear trends in both metal ion levels and gravimetric weight losses. Other studies have also shown similar correlation between volume loss and ion concentration levels. The percentage distribution of Co, Cr and Mo in the metal ion samples are in close agreement with nominal chemical composition of the material tested.
Conclusion: This study showed that metal ion measurements can help to confirm gravimetrically measured material loss.
Correspondence should be addressed to: EFORT Central Office, Technoparkstrasse 1, CH – 8005 Zürich, Switzerland. Email: office@efort.org
