Optimisation of radiographic visibility and wear detection of total knee arthroplasty inlays using radiopaque markers

Wear of the inlay in total knee arthroplasty (TKA) contributes to implant failure and the need for revision surgery. In vivo wear assessment is challenging owing to the radiolucency of the inlay in standard radiographs. This study aimed to investigate the basic feasibility of integrating radiopaque X-ray markers on standard inlays to enhance their radiographic visibility and enable wear evaluation. Preliminary experiments identified optimal process parameters for micro-milling cavities on ultra-high molecular weight polyethylene (UHMWPE). A total of 450 parameter combinations were evaluated, with burr formation serving as the quality criterion. A process chain, comprising pre-contouring, micro-milling, filling cavities with radiopaque composite, and final contouring, was developed for inlay production. Eleven inlays with varying marker alignments, orientations, and geometries were manufactured, featuring grooves ([≤]0.8 mm wide) and holes (diameter = 1.6 mm), both 1 mm deep. Three HDPE + BaSO composites (10, 20, and 30 wt.% BaSO) were formulated and assessed for radiopacity per ASTM F640-20. Final marker cavities were filled with HDPE + 20 wt.% BaSO via pellet extrusion. The inlays were positioned in a phantom knee setup and radiographed in the anteroposterior view. Projected markers were evaluated based on edge visibility, measurability, homogeneity, and obscuration by the implant. None of the parameter combinations resulted in burr-free cavities, indicating an unstable five-axis process. X-ray images revealed that grooves aligned in the X-ray direction and drilled holes exhibited the best visibility for wear markers. Pin-on-plate tribological experiments revealed that BaSO addition to pure HDPE reduced its CoF from 0.25 to 0.1, reaching a value comparable to UHMWPE (0.15), while also enhancing wear resistance. This study demonstrated the feasibility of integrating wear markers on standard TKA inlays by micro-milling cavities at different positions and orientations on the inlay surface and filling them with a radiopaque composite. Further research is required to optimise process parameters and investigate marker wear.