Mechanically- and Electrochemically- Induced Damage at Metal Interfaces of Joint Replacements and the Biological Consequences

Retrieval analysis has emerged as one of the most direct and informative ways to analyze mechanisms of orthopaedic device failure. With a rising projected demand for revision joint arthroplasty, 268,200 revision total knee replacements (TKR) and 96,700 revision total hip replacements (THR) in 2030, metal interfaces in joint replacements are currently under intense scientific focus as the interactions among mechanical and electrochemical processes continue to challenge the orthopaedic device industry. The objective of these studies was to use implant retrieval and in in vitro analyses to determine the effects of material and mechanical properties on wear and corrosion, and to quantify the biological consequences of the degradation products. A combination of non-destructive and destructive characterization techniques were developed to characterize the wear and corrosion on retrieved joint replacements. Surface degradation modes including abrasive two-body wear, adhesive two-body wear, third body abrasion, local surface fracture, fretting corrosion, and crevice corrosion were identified on articulating and non-articulating metal components. The key predictors of the type and extent of surface degradation were material hardness, fracture toughness, and implant design. The clinical outcomes and reasons for revision varied for the implant retrieval sets studied, though a positive correlation was found between the extent of surface corrosion and incidence of metallosis, and a negative correlation was found between extent of surface corrosion and inter-component stability. In vitro testing was undertaken to identify additional electrochemical degradation mechanisms that were mechanically induced, as well as the possible effects on bone remodeling. The ability of titanium orthopaedic alloys to resist corrosion is derived from the passivation behavior, in which an oxide film serves as a kinetic barrier to the thermodynamically favorable electrochemical process. The introduction of stress at physiologically-relevant levels was found to elevate the active and passive corrosion rates and inhibit the passivation process to a degree. Osteoblasts exposed to titanium ions, released during corrosion, had reduced viability, proliferation, and alkaline phosphatase production at 10 ppm concentrations. At lower concentrations of 0.1 ppm, osteoblasts had reduced osteocalcin production, suggesting mineralization and bone remodeling may be affected even at the low end of the clinically relevant range. Through these studies, several advances were made in the characterization of wear and corrosion, and how these processes affect clinical outcomes of failed joint replacements. The results from these post-market surveillance and retrieval analysis findings suggest that mechanical and electrochemical interactions do have a significant effect on otherwise wear-resistant and corrosion-resistant orthopaedic metals. The in vitro investigations find that physiologically-relevant stresses affect corrosion behavior and that physiologically-relevant metal ion concentrations may affect bone mineralization on a cellular level