**4.2 Biotribology and biomechanics of OxZr and TiN/TiNbN monolayers**

One of the main goals of the oxidized zirconium is wear reduction, which has been tested in several wear simulation tests. The wear reduction compared to a standard CoCrMo femur against UHMWPE inserts ranges between 42% and 89%, depending on the wear simulation method, implant design and measurement protocol [59]. It also shows better scratch resistance than CoCrMo [60], as the nano-hardness of the zirconia layer ranges from 12 to 14 GPa, which is significantly harder compared to the 2–4 GPa from the CoCrMo standard material [56]. This has been demonstrated in three out of six OxZr retrieval studies from 1 to 5 years in-situ, which showed significantly less roughness of the femur component or less damage in the UHMWPE insert compared to CoCrMo implants. However, the other three retrieval studies showed no difference in the component analysis for femur and polyethylene insert damage [59].

A review of the literature on the effects of TiN coatings showed that TiN coatings have a positive effect on the biocompatibility and tribological properties of implant surfaces. TiN-coated implants showed also a high scratch resistance and less polyethylene wear. Nevertheless, several reports of third body wear due to delamination and cohesive failure also show negative effects of the TiN-coating [3]. The hardness of TiN and TiNbN coatings are with 24 GPa much higher than oxidized zirconium and supports the findings for lower polyethylene wear and high scratch resistance.

The metal ion release analysis of TiN and TiNbN coatings during an immersion study showed a reduction of cobalt ions of 80% and 76%, respectively, compared to an uncoated CoCrMo implant [61]. Similarly, another immersion study found a reduction of 80% for cobalt, 63% for chromium and 48% for molybdenum ion using a TiNbN coated implant in comparison to its uncoated CoCrMo version [62].

#### **4.3 Biotribology and biomechanics of the ZrN multilayer coating**

### *4.3.1 Wear articulating against UHMWPE*

Studies have shown a UHMWPE wear reduction of more than 50% (**Figure 1**) by using the ZrN multilayer coated femur and tibial components in comparison to their uncoated versions [17, 63]. A limitation of the standardized ISO 14243 test is that it only simulates the short-term performance of an implant, as it reproduces only about 3 years of in vivo service [64]. Based on in vivo measurements performed on eight patients with instrumented implants [65], a new highly demanding activities (HDA) knee wear simulation protocol was developed, which reproduces 15–30 years of in vivo service, depending on the activity level of the patient [66]. In the HDA wear protocol, the load and motion profiles of the high flexion activities of stairs ascending, stairs descending, chair raising, deep squatting and normal level walking are simulated. Moreover, the loading profiles were normalized to represent a patient weight of 100 kg. A wear test study performed with the HDA wear simulation protocol also demonstrated a 2.8-fold lower UHMWPE wear rate when articulated with ZrN multilayer coated implants compared to the uncoated version (**Figure 1**) [52].

Regarding the metal ion release, in vitro studies have shown that the ZrN multilayer coating (with a hardness of 25 GPa) is not impaired by failure modes such as delamination, surface disruption or flaking [17, 52, 53], even under third body particle contamination [17]. This has been confirmed by analysis of the metal ion concentration analysis performed on the test medium, where the metal ion release of the substrate material is several orders of magnitude lower than in the uncoated

**Figure 1.**

*UHMWPE wear rates comparison between uncoated CoCrMo and ZrN multilayer coated knee implants after ISO and HDA knee wear simulation protocols. Note: Reference [63] contains the results of the ZrN group only.*

components (**Figure 2**). Moreover, it can be seen that the use of an HDA knee wear simulation increased the metal ion release in the uncoated components (both studies were performed on Columbus CR, Aesculap AG, Tuttlingen), whereas the ion release of the ZrN multilayer components remained in substantially a lower level.
