title: Synovial fluid replication in knee wear testing: an investigation of the fluid volume.
abstract: Wear testing cannot replicate the variations in wear rates and wear mechanisms seen in vivo, which may be related to differences between in vivo and in vitro conditions. A considerable difference exists between the in vivo synovial fluid volume (few milliliter) and the in vitro substituted bovine serum volume (several hundred milliliter). The aim of this study was to analyze the effects of a reduced fluid volume on the wear behavior in a knee wear simulator study. Four wear tests with decreasing fluid volumes (250, 150, 75, and 45 ml) were carried out. Using a large fluid volume of 250 ml for wear testing resulted in a wear rate of 9.7±1.2 mm3/10(6)  cycles. Decreasing the fluid volume consecutively reduced the wear rate to down to 8.8±1.4 mm3/10(6) for 150 ml (p=1.00), 5.6±1.2 mm3/10(6) for 75 ml (p=0.01), and 1.0±0.2 mm3/10(6) cycles for 45 ml fluid volume (p≤0.01). Additionally, higher serum degradation and larger wear particles were observed with smaller fluid volumes used for testing. This study demonstrates the high relevance of the protein-based lubricant on the wear behavior and the technical limitation to replicate the synovial fluid in simulator tests. Wear testing should be carried out using larger fluid volumes (e.g., 250 ml) to generate physiological relevant wear masses.
authors.affiliation: Laboratory of Biomechanics and Implant Research, Clinic for Orthopedics and Trauma Surgery, Center for Orthopedics, Trauma Surgery and Spinal Cord Injury, Heidelberg University Hospital, Schlierbacher Landstrasse 200a, 69118, Heidelberg, Germany.
authors.forename: Jörn
authors.initials: J
authors.lastname: Reinders
authors.forename: Robert
authors.initials: R
authors.lastname: Sonntag
authors.forename: Jan Philippe
authors.initials: JP
authors.lastname: Kretzer
mesh_terms: D000818:Animals; D019645:Arthroplasty, Replacement, Knee / Q000295:instrumentation*; D002417:Cattle; D007719:Knee Joint / Q000601:surgery; D007720:Knee Prosthesis / Q000592:standards*; D054327:Lubricants; D008422:Materials Testing / Q000379:methods*; D023421:Models, Animal; D011475:Prosthesis Failure; D013582:Synovial Fluid
chemical_list: D054327:Lubricants
keywords: knee replacement; particle analysis; wear; wear testing