Weight Loss Technique for Measurement of Wear of Polymeric Orthopedic Implants

Low wear properties of ultra-high molecular-weight polyethylene (UHMWPE) and carbon reinforced polyethylene prosthesis require 0.1-mg accuracy for a valid quantitative wear test. Buoyancy of air displaced by the volume of the wear specimen will have an influence on the specimen's weight. Density variation of air due to barometric pressure, temperature, and relative humidity during a wear test will alter the succeeding weight measurements. The average amount of wear debris is very small compared to the total mass of the wear specimen, approximately 0.005 to 0.05% or 1 to 2 mg per million cycles for a 4000 to 22 000-mg component, respectively. Displaced air mass variation for a volume equivalent to a wear component over a test period can be as high as 1 mg. Buoyancy compensation should be considered for addition to ASTM F 732 (Practice for Reciprocating Pin on Flat Evaluation of Friction and Wear Properties of Polymeric Materials for Use in Total Joint Prostheses) and any test procedure utilizing weight loss measurement for wear.