Retrieval study of commercially available knee implant coatings TiN, TiNbN and ZrN on TiAl6V4 and CoCr28Mo6

BACKGROUND

Coated implant components for total knee arthroplasties are primarily used for metal-sensitive patients and are offered by different manufacturers. However, there is only little knowledge with respect to their coating design and supposed superior tribological performance. Our aim was to compare retrieved coated implants by identifying present damages, critical factors influencing the coating durability and their correlation to the clinical performance.

MATERIALS AND METHODS

28 retrieved knee endoprostheses from nine different manufacturers were analyzed for potential surface defects as well as the coating strategy for each manufacturer. The coating designs were investigated on preserved regions with regard to substrate and coating material, layer thickness and roughness using scanning electron microscopy and confocal microscopy. Furthermore, the mechanical properties and adhesive strength of the layer were evaluated by nanoindentation and scratch testing. The friction performance of the coatings against ultra-high molecular weight polyethylene (UHMWPE) was investigated in a tribological test. In addition, clinical data were collected and evaluated for all patients.

RESULTS

Our cohort of 28 retrieved knee endoprostheses exhibited different damage patterns in the articulating area with an incidence of 79% for discoloration and 21% for coating delamination. All coatings presented droplets, macropores and pinholes in preserved areas, which can be attributed to the coating and post-polishing processes. Interestingly, the adhesive strength was significantly increased by 60.4% for titanium nitride coatings on TiAl6V4 alloy in comparison to CoCr28Mo6 substrates. The friction behavior of titanium nitride coatings against UHMWPE is similar to uncoated CoCr28Mo6 alloy and lowest for the ZrN multi-layer coating with a reduction of 14%.

DISCUSSION

This study shows that manufacturing related coating deposition defects can cause wear due to adhesive failure and corrosion underneath the coating layers. Adhesive strength was identified as a critical factor for coating durability. Minor adhesive strength was present on CoCr28Mo6 cast alloy in comparison to good adhesion of Ti-based coatings on TiAl6V4 wrought alloy. Based on our findings, this is consistent to higher prevalence rates of CoCr28Mo6/TiNbN coatings for gross delamination and pitting damage with increasing implantation time.

Diffusional motion as a gauge of fluidity and interfacial adhesion. Supported alkylphosphonate monolayers

We report on the use of diffusion measurements to gauge the fluidity and surface binding properties of a molecular monolayer. The monolayer film consists of octadecyl-1-phosphonic acid (ODPA) and controlled amounts of a lyso-phosphatidic acid tagged with the fluorescent probe BODIPY (BLPA). The monolayer films were formed using a Langmuir-Blodgett (LB) trough and deposited onto a glass slide. Monolayer morphology was characterized during film formation using Brewster angle microscopy (BAM). Fluorescence Recovery After Photobleaching (FRAP) microscopy was used to measure translational diffusion of BLPA and Fluorescence Anisotropy Decay Imaging (FADI) was used to measure rotational diffusion of the BLPA chromophore. These results provide information on the motional freedom of the probe and, importantly, on the strength of interaction between the probe and the support. Compositional variations in the monolayer give rise to changes in constituent dynamics that reflect intermolecular interactions.