**5. Clinical results and register data**

The theoretical advantages of these improved implant surfaces need to be confirmed in patients. There are several clinical studies and several implants are monitored in arthroplasty registries. Because coated and standard implants are often used in different patient populations, a direct comparison of the implant performance is difficult. The more expensive implants with a surface modification or coating are more often used in patients with metal allergies. These patients have a high level of psychological distress when undergoing a TKA surgery [77], and it has been

suggested that anxiety is the main reason for less favorable results in these patients [28]. It is therefore likely that patients with allergies to implant materials have a higher overall risk for revision because of an unsatisfactory result after TKA. This needs to be considered when looking at arthroplasty registry data.

### **5.1 Clinical studies**

There are only a few studies comparing coated and standard implants. **Table 1** summarizes studies with a minimum 5-year follow-up. If the performance of the implant is the outcome of interest, coated and standard implants should be tested in similar patient populations. As it is difficult to randomize patients with allergies against implant materials, in most studies only patients without known allergies were included. This is not the target population for hypoallergenic implants, but investigation of the mechanical properties of implants in these patients is reasonable.

Studies are not available for all implants on the market. Published studies demonstrated no relevant differences between coated and standard TKA and overall good survival rates. Based on that data, modern coatings which have been tested in clinical studies seem to work well and there is no reason for concern. Because technologies are different, the published results cannot be transformed into different implants or coating technologies without additional clinical studies.

#### **5.2 Arthroplasty registry results**

The results of arthroplasty registries provide "real-world" information and are therefore an important source of data. However, it needs to be considered that, in most countries, patients who receive a coated TKA are somehow different from patients who receive a standard TKA, which results in patient groups with a different probability of revision. Therefore, as always with registry data, revision rates of an implant or implant group are not entirely be caused by the implant, but might also be influenced by the patient population.

There is a significant difference in the hazard ratio (HR) of revision between all the alternative surfaces (coatings and surface modifications) and standard CoCrMo femoral components in the Australian Arthroplasty Registry (AOANJRR). After 15 years, the CoCrMo implants have a revision rate of 6.3%, whereas the alternative surfaces have a revision rate of 9.4%. However, there are differences between the alternative surfaces. ZrN has lower revision rates than OxZr and TiN implants. After 5 years, the revision rate for the ZrN is 2.1% and for the uncoated components is 3.1%.

A review of more than 17,000 cases out of the AOANJRR evaluated outcomes of up to 12 years for the OxZr [88]. They found no significant difference for the 12-year cumulative percent revision (CPR) due to all causes (4.8% for CoCr and 7.7% for OxZr); non-septic causes, or osteolysis or loosening (0.6% for CoCr and 1.1% for OxZr). The only age-related difference was found with patients who were > 75 years old, for whom OxZr TKA had an increased CPR due to osteolysis or loosening.

In the National Joint Registry (NJR) of the United Kingdom, OxZr and ZrN have similar results in both versions [39]. AS Columbus (ZrN) had a cumulative revision rate of 2.42 compared to 2.05 of its uncoated version after 5 years. Genesis II Oxinium (OxZr) had a cumulative revision rate of 3.57 at 5 years and 7.67 at 15 years compared to the standard CoCr implants (2.05 at 5 years, 3.49 at 15 years). Both designs have a remarkably lower patient median age at primary TKA (AS Columbus 5 years lower, Genesis II

*Modern Coatings in Knee Arthroplasty DOI: http://dx.doi.org/10.5772/intechopen.105744*


*RCT = randomized controlled trial; TAS = Tegner activity scale; KSS = knee society score; and KSF = knee society function score.*

#### **Table 1.**

*Clinical studies involving monolayer and multilayer coatings as well as the OxZr surface modification.*

Oxinium 12 years lower). The younger age in TKA is a higher risk for earlier revision, which could explain the slightly higher revision rates of both designs in the register [38].

Hypoallergenic implants demonstrated higher overall revision rates in the German Arthroplasty Registry after 3 years [89]. In this report, the main differences between coated and standard implant groups were the higher rate of metal allergies in female

patients in the coated TKA group. Looking at the data in detail, it was recognized that for the most often used coated implants (Columbus, Vanguard, e.motion), there were no differences between coated and standard implants and that for these implants, revision rates were favorably lower than all implants. It seems that the higher revision rates are caused by less frequently used implants and that a general revision rate for "coated implants" is not sufficient. Each implant needs to be looked at separately.

An analysis of 62,177 primary TKAs from the Total Joint Replacement Registry in the USA compared OxZr to traditional CoCr TKA implants and showed no statistically significant higher risk for revision after a mean follow-up time of 2.8 years [90].
