Knee implants are among the most common orthopaedic procedures. In Germany alone, more than 155,000 primary knee arthroplasties were performed in 2023. One of the most common causes of subsequent revision surgery is wear of the polyethylene inlay. As this material is radiolucent, the wear can hardly be recognised in the standard X-ray image. Current clinical practice therefore only uses indirect methods to assess wear.
Precise markings on knee implants should make diagnostics easier
The Institute of Production Engineering and Machine Tools (IFW) at Leibniz University Hannover is developing a technical solution to improve wear diagnostics together with the Laboratory for Biomechanics and Biomaterials (LBB) at Hannover Medical School as part of the Collaborative Research Centre SFB/TRR 298 “SIIRI – Safety-integrated and infection-reactive implants” in sub-project A01 “Implant Regeneration and Life Cycle Management”.
The aim is to integrate radiopaque markers directly into the surface of polyethylene inlays. These markers are intended to enable spatially resolved measurement of material loss on X-ray images and thus allow early detection of wear under everyday conditions.
To insert the markers into the inlay surface, the IFW uses microstructuring by five-axis micro-milling. This involves milling or drilling grooves and holes with widths of up to 0.8 and 1.6 mm respectively and a depth of 1 mm into the curved free-form surface of the inlay. These microstructures are then filled with a radiopaque polymer composite made of high-density polyethylene (HDPE) and barium sulphate. The composite was specially developed for this purpose and exhibits similar tribological properties to the surrounding ultra-high molecular weight polyethylene (UHMWPE).
The IFW has analysed more than 1,250 parameter combinations in test series in order to identify suitable process parameters for microstructuring. The formation of burrs at the edge of the microstructures is particularly critical, as these hinder the exact filling of the marker areas. The smallest burr was produced with a tool diameter of 0.6 mm, a cutting depth of 0.1 mm, a tooth feed of 0.06 mm and a cutting speed of 10 to 15 m/min.
IFW develops marker designs and systematic evaluation of visibility
The IFW analysed a total of eleven different marker designs. These differed in geometry, orientation, arrangement and positioning on the inlay surface. In addition to grooves parallel and perpendicular to the X-ray direction, structures with an angle to the implant surface and combinations of grooves and holes were also used. In addition, an anterior offset of individual patterns was integrated in order to test the mapping of localised wear in particularly stressed zones and depth differentiation in two-dimensional images.
The finished inlays were filled with the radiopaque composite material and x-rayed in a realistic phantom knee. Parallel grooves running in the direction of the X-ray beam achieved the best results in terms of visibility and measurability. Horizontally aligned grooves and holes with a diameter of 1.6 mm, on the other hand, showed significantly lower visibility.
To objectively assess the marker quality, the IFW developed an evaluation system with four main criteria: Visibility of the edges, measurability of the geometry, homogeneity of the filling and visibility when overlaid with metallic components. The best marker designs achieved more than 22 out of 27 possible evaluation points.
The highest-rated design consisted of grooves aligned longitudinally to the direction of the X-ray beam with a spacing of five millimetres. This arrangement enabled particularly clear delineation of the marker contours and precise geometric measurement of the cavities. The combination of good visibility and high dimensional accuracy makes this design particularly promising for further preclinical experiments.
Perspective: personalised partial revision instead of complete replacement of the implant
In the long term, the marker structure should enable a spatially resolved wear analysis on standard X-ray images. In the event of wear, a patient-specific replacement inlay could be produced that is specifically adapted to the wear geometry. This would not only promote the preservation of the surrounding tissue, but also reduce costs and stress for patients.
In further steps, the IFW wants to automate the filling of the markers, integrate simulation-based process adjustments in five-axis micro-machining and improve marker evaluation through AI-supported image analysis. The chosen path shows the potential of individualised implant care through precise manufacturing technologies in the context of personalised medicine.
