Amorphous Carbon Coatings for Total Knee Replacements—Part II: Tribological Behavior

Abstract

Diamond-like carbon coatings may decrease implant wear, therefore, they are helping to reduce aseptic loosening and increase service life of total knee arthroplasties (TKAs). This two-part study addresses the development of such coatings for ultrahigh molecular weight polyethylene (UHMWPE) tibial inlays as well as cobalt-chromium-molybdenum (CoCr) and titanium (Ti64) alloy femoral components. While the deposition of a pure (a-C:H) and tungsten-doped hydrogen-containing amorphous carbon coating (a-C:H:W) as well as the detailed characterization of mechanical and adhesion properties were the subject of Part I, the tribological behavior is studied in Part II. Pin-on-disk tests are performed under artificial synovial fluid lubrication. Numerical elastohydrodynamic lubrication modeling is used to show the representability of contact conditions for TKAs and to assess the influence of coatings on lubrication conditions. The wear behavior is characterized by means of light and laser scanning microscopy, Raman spectroscopy, scanning electron microscopy and particle analyses. Although the coating leads to an increase in friction due to the considerably higher roughness, especially the UHMWPE wear is significantly reduced up to a factor of 49% (CoCr) and 77% (Ti64). Thereby, the coating shows continuous wear and no sudden failure or spallation of larger wear particles. This demonstrated the great potential of amorphous carbon coatings for knee replacements.

Amorphous Carbon Coatings for Total Knee Replacements—Part I: Deposition, Cytocompatibility, Chemical and Mechanical Properties

“…x /E IT y ratios to values published in literature [34,124,125], the coatings studied within this contribution were found to be in the upper range for Ti64:W and CoCr:W. For UHMWPE:H, the values even exceeded those from comparable studies in literature [47,111,126]. Therefore, it can be assumed that the developed a-C:H:W and a-C:H coatings will exhibit a highly advantageous wear behavior [42], which is also substantiated by the biotribological investigations in part II [50].…”

“…It can be assumed that the amorphous carbon coatings presented in this contribution are able to outperform uncoated metallic-polymeric reference pairings under biotribological stresses. This will be further elaborated in part II of this study [50].…”

“…The investigated substrate materials were UHMWPE [51] (Chirulen ® GUR 1020, Quadrant EPP, Vreden, Germany), Co28Cr6Mo [52] (CoCr, Peter Brehm, Weisendorf, Germany) and Ti6Al4V ELI [53] (Ti64, Jäckel + Co. Edelstahl Metalltechnik, Schöneck, Germany) with medical standards. The geometries of the samples to be coated encompassed both flat specimens for chemical, mechanical and cytological characterization, as well as disks and pins for tribometer tests (see [50]). The UHMWPE disks had a diameter of 45 mm and a height of 8 mm while the CoCr and Ti64 disks had a diameter of 25 mm and a height of 5 mm.…”

“…The average roughness values of the uncoated and coated surfaces of the metallic pins and the polymeric disks as used for the tribological experiments of part II [50] are summarized in Figure 3. Thereby, the values of uncoated reference specimens differed slightly by less than 0.04 µm.…”

“…Consequently, this two-part study focuses on the development of wear reducing DLC coating systems specifically for Ti64 and CoCr femoral as well as UHMWPE tibial components of TKAs. While the biotribological behavior is investigated in part II [50] using pin-on-disk model tests as well as a detailed wear analysis, this part I addresses the coating deposition and static characterization. The aim is to evaluate the in vitro behavior, mechanical and adhesive properties and to discuss the applicability to the articulating surfaces of knee joint replacements.…”