The entrapment of corrosion products from CoCr implant alloys in the deposits of calcium phosphate: A comparison of serum, synovial fluid, albumin, EDTA, and water

Abstract

Physical wear of orthopedic implants is inevitable. CoCr alloy samples, typically used in joint reconstruction, corrode rapidly after removal of the protective oxide layer. The behavior of CoCr pellets immersed in human serum, foetal bovine serum (FBS), synovial fluid, albumin in phosphate-buffered saline (PBS), EDTA in PBS, and water were studied using X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS). The difference in the corrosive nature of human serum, water, albumin in PBS and synovial fluid after 5 days of immersion was highlighted by the oxide layer, which was respectively 15, 3.5, 1.5, and 1.5 nm thick. The thickness of an additional calcium phosphate deposit from human serum and synovial fluid was 40 and 2 nm, respectively. Co and Cr ions migrated from the bulk metal surface and were trapped in this deposit by the phosphate anion. This may account for the composition of wear debris from CoCr orthopedic implants, which is known to consist predominantly of hydroxy-phosphate compounds. Known components of synovial fluid including proteoglycans, pyrophosphates, phospholipids, lubricin, and superficial zone protein (SZP), have been identified as possible causes for the lack of significant calcium phosphate deposition in this environment. Circulation of these compounds around the whole implant may inhibit calcium phosphate deposition. ß

Effects of soluble cobalt and cobalt incorporated into calcium phosphate layers on osteoclast differentiation and activation

“…Ions released from the implant surface are coprecipitated and entrapped in the newly deposited layers of bone mineral [20]. In this study, coprecipitation of Co 2þ ions was mimicked by incorporation of Co 2þ into synthetic CaP layers.…”

“…CaP is found to develop naturally on Co-based alloy surfaces [19]. Furthermore, Co and Cr ions are observed to migrate from the bulk CoCr metal surface and are incorporated in the layer of calcium phosphate deposit, and phosphate anions may be responsible for the integration of released metal ions [20]. Upon implantation in vivo, reaction of ions at the surface of metallic prostheses with body fluids and surrounding tissues occurs, ostensibly by binding of Co, Cr, or Fe ions with the adsorbed protein layer that lubricates the contact zone.…”

Orthopaedic metals and their potential toxicity in the arthroplasty patient

“…18 The deposition of calcium phosphate and the subsequent formation of metal phosphates (CrPO 4 , Co 3 (PO 4 ) 2 , etc) occur in nonsynovial environments. 19 This may significantly alter the biological and chemical properties of free particulate metals outside the effective joint space. THE JOURNAL OF BONE AND JOINT SURGERY Prosthesis-derived metal wear products are found extensively within the synovial fluid and peri-prosthetic tissues of arthroplasty patients.…”

Simple isolation method for the bulk isolation of wear particles from metal on metal bearing surfaces generated in a hip simulator test

“…The differences in these ratios may be explained by the different chemical properties of Co and Cr. Cr metal readily forms a thin passive oxide surface layer when exposed to oxygen, stabilising the metal [25]. Conversely Co is highly soluble and will therefore rapidly move into an ionic form.…”