The Small Molecule Components of Human Synovial Fluid and Bovine Calf Serum that Correlate with Cobalt Chrome (CoCrMo) Wear

Background:Implant wear in joint replacements is influenced by the chemical and physical properties of human synovial fluid (HSF). In vitro testing for implant wear uses 25% weight bovine calf serum (25BCS) to substitute for HSF, due to similar rheology and total protein content. However, previous studies have shown differences in the macromolecular composition. We aimed to evaluate the differences in small molecule composition between fluids and correlate these differences with their effects on implant material wear.Methods:HSF was harvested from osteoarthritis patients undergoing primary knee replacement (n=14). Nuclear magnetic resonance (NMR) spectroscopy with linear regression modelling analysed the metabolites present in HSF and commercially sourced 25BCS and investigated the differences. Wear properties of the fluids were evaluated using a validated quantitative laboratory bench-test utilising a cobalt/chromium/molybdenum (CoCrMo) ball oscillating against a CoCrMo disc and analysing the resulting wear scar. The variation in metabolite levels in both HSF and 25BCS was correlated to the wear properties of the fluids.Results:Differences in the levels of metabolites, lipids, and glycosaminoglycans (GAG) were observed between HSF and 25BCS: significance was confirmed by O-PLS-DA (p<0.05). The wear of CoCrMo was found to strongly correlate with the macromolecules GAG and proteins that potentially bind to glucose and citrate. Conclusions:The small molecule concentration differences between the fluids questions the validity of 25BCS as a model for wear analysis. The demonstration of variable metabolites present in HSF which correlate with material wear has implications for implant failure and targeted therapeutic manipulation of these metabolites. Trial Registration:Ethical approval was granted by the NRES Committee London, Chelsea, UK on the 12th May 2015. The study was performed in accordance with the ethical standards in the 1964 Declaration of Helsinki.

A simple method of suspending implant wear particles for more physiological modelling of cell-particle interactions in vitro

Arthroplasty implants e.g. hip, knee, spinal disc sustain relatively high compressive loading and friction wear, which lead to the formation of wear particles or debris between articulating surfaces. Despite advances in orthopaedic materials and surface treatments, the production of wear debris from any part of a joint arthroplasty implant is currently unavoidable. Implant wear debris induces host immune responses and inflammation, which causes patient pain and ultimately implant failure through progressive inflammation-mediated osteolysis and implant loosening, where the severity and rate of periprosthetic osteolysis depends on the material and physicochemical characteristics of the wear particles. Evaluating the cytotoxicity of implant wear particles is important for regulatory approved clinical application of arthroplasty implants, as is the study of cell-particle response pathways. However, the wear particles of polymeric materials commonly used for arthroplasty implants tend to float when placed in culture media, which limits their contact with cell cultures. This study reports a simple means of suspending wear particles in liquid medium using sodium carboxymethyl cellulose (NaCMC) to provide a more realistic proxy of the interaction between cells and tissues to wear particles in vivo, which are free-floating in synovial fluid within the joint cavity. Low concentrations of NaCMC dissolved in culture medium were found to be effective for suspending polymeric wear particles. Such suspensions may be used as more physiologically-relevant means for testing cellular responses to implant wear debris, as well as studying the combinative effects of shear and wear particle abrasion on cells in a dynamic culture environments such as perfused tissue-on-chip devices.