Damage assessment
The results of the visual semi-quantitative assessment method have shown that the surface damage of the TiN coating did not correlate with the implantation period. Four different types of damage were detected on the coated articulation surfaces, whereby narrow parallel scratches, similar to former retrieval studies, were primarily observed [
13,
14,
17]. Scratches occur when scattered hard particles interact between the counterfaces within the sliding gap and abrade the coating [
10]. The scratches that were evaluated were formed in the main direction of sliding (anterior–posterior) and differed in their orientation from the untypical cross-passing ones resulting from implantation or revision procedure. A coating breakthrough was detected at the bottom of separated narrow scratches in three retrievals. Severe third-body wear was responsible for the occurence of this coating failure. Due to its small dimension, the influence on the ion release behaviour may be extremely low. However, the recorded damage feature score for scratches was ten times lower than the one reported in a previous study [
3] with femoral knee components made from CoCr and nearly three times lower than the score for retrieved, oxidized zirconium (OxZr) TKAs, despite them being implanted for a longer period (TiN: 30.7 months, CoCr: 23.1 months, OxZr: 21.9 months). TiN coatings are therefore assumed to be more resistant against third-body particles compared to CoCr or OxZr. In view of the clinical relevance of third-body particles [
6], this could be an advantage for TiN-coated TKAs.
Besides scratches, indentations were the type of damage detected most frequently. In comparison with uncoated TKAs made of CoCr or OxZr, the indentation score was nearly 50% lower with the TiN coating [
3]. Indentations were primarily located in the range of low flexion angles. The left pits may indicate a plastic deformation of the coated substrate, caused by third bodies during the stance phase. Notches were hardly observed and may be attributed to a heavy impact between the femoral component and the tibial tray, during movement at high flexion angles, as already mentioned by [
4]. Generally, the TiN-coated retrievals revealed a notch score two times lower than uncoated CoCr and OxZr [
3].
In two retrievals however, the TiN coating was partly delaminated on the bottom of an indentation and also at the bottom of a notch in one retrieval. These signs of delamination developed as a result of a massive plastic substrate deformation which was not fully covered by the elasticity of the TiN coating. However, this kind of delamination differed significantly from former findings, where wide regions of flaked coating fragments were detected as a result of insufficient adhesion at the substrate/coating interface [
22]. Further, the sparse delaminations evaluated in this study were not located directly at the articulating surface but rather inside a pit and, thus, had no direct contact to the counterface. Therefore, surface asperities caused by delamination may not be associated with adhesive wear on the articular surface as mentioned in a former TiN retrieval study [
12].
A coating breakthrough was observed in the patellofemoral joint region of three retrievals. In two cases, the adverse direct contact between the patellar metallic base plate and the metallic femoral component led to abrasive wear, resulting from a patellar maltracking and a patellar dislocation. The restoration of the patellofemoral joint has often been reported as a clinical problem after TKA with instability rates in the range of 1–12% [
9,
18]. However, besides pain, dislocation and revision surgery, incorrect loading of the TKA is known to cause patella maltracking.
In the third case of a coating breakthrough, the surgery report showed that revision was necessitated by ligament instability. Moreover, no patella resurfacing was used in combination with the examined TiN-coated mobile-bearing TKA. Therefore, it can be assumed that the native patella eventually led to the coating breakthrough. Two factors might have played an important role within this failure mode: first, the given geometrical nonconformity between the native patella and the femoral component right after primary surgery. This nonconformity might have prevented the retropatellar bone surface from achieving even contact with the coated femoral component. Moreover, the articulation in the beginning of the running-in phase took place only at separated points or along a line, respectively. As a result, the contact surface of the retropatellar joint was reduced. Second, the retropatellar pressure increased significantly after TKA compared to a natural knee [
25]. This might have led to high-level contact stresses in combination with the reduced contact surface. Both factors affected wear of the retropatellar bone until the geometrical conformity to the femoral component was achieved. Up to this point, the coating was heavily stressed, considering the applied local stress and the generated third bodies, resulting in a coating abrasion process.
It should be mentioned that the analysis of the clinical data and the evaluation of the corresponding retrievals have demonstrated that the reasons for revision cannot be attributed to the coating breakthroughs. Previous TiN studies with implants made of titanium alloy have suggested that a coating breakthrough was associated with the uncovered substrate material being prone to third-body wear [
12,
22]. However, the vulnerability of the CoCrMo TKAs observed within our study was reduced by the increased strength of the substrate material compared to titanium alloys commonly used in total hip arthroplasty. Nonetheless, the barrier for the metal ion release is damaged by a coating breakthrough. Objectively, comparing the total area of the intact TiN surface to the area of coating breakthrough, only a slightly increased ion release could be expected.
Roughness
Despite a longer implantation period [
3], the arithmetical mean surface roughness of the retrievals coated with TiN was comparable with retrieval surfaces made of OxZr or CoCr. Based on the initial mean surface roughness, the absolute roughness levels during in vivo service increased to the range of 0.01 µm for TiN (30.7 months) and doubled to 0.02 µm for uncoated CoCrMo (23.1 months). Thus, the surface roughness remained smoother with the TiN coating, which could be attributed to a reduced polyethylene wear rate, as demonstrated in different knee simulator tests [
20,
21].
The absolute values for the maximum profile peak were higher with TiN in contrast to CoCr or OxZr due to the coating technology (e.g. deposition of titanium droplets on the surface). Nevertheless, the decrease in the peak height for TiN and OxZr as a result of in vivo loading could be attributed to a smoothening effect which occurred during the running-in phase. An increase in the maximum peak height, as seen with CoCr retrievals [
3], might indicate the poor scratch resistance of CoCr in contrast to TiN and OxZr.
Finally, the condyles of the retrievals were found to be slightly rougher on the medial side than lateral. This was consistent with former retrieval studies of Heyse et al. [
13] and Brandt et al. [
3] and could be attributed to the primarily medial applied contact forces, as mentioned by Halder et al. [
11]. However, this behaviour was already observed in our control group with new, unused surfaces and indicated a difference as a result of the polishing process.
Coating thickness
According to Lützner et al. [
16], TKAs are subjected to a number of motion cycles that amount to approximately 6.500 steps taken daily. During a combination of the sliding and rollback knee movement, wear occurs at the counterfaces of the bearing. Our study demonstrated low abrasive wear of the TiN coating even after several years of in vivo loading. All measured thickness values were found to be within the initial coating thickness range. Unfortunately, this study was not able to make a general statement about the coating thickness decrease rate per year because on the one hand, the initial coating thickness of new femoral components that had never been implanted was not measured due to the destructive measurement method. On the other hand, wear to the counterface is always dependent on specific patient-related factors such as implant position or level of activity [
30]. Further, it should be noted that the initial coating thickness is not a general defined value. Rather, the initial coating thickness is batch related (e.g. depends on the number of implants in the coating chamber) and is therefore defined as a range. However, the coating thickness decrease rate per year was minimal, which was confirmed by the fact that the measured thickness values did not deteriorate in line with the implantation period.
A conclusion was not reached about the most stressed side due to the minimal deviations in coating thickness between the medial and lateral condyles. The smallest coating thickness was measured on the femoral component that was revised due to a patella dislocation after it was used for more than 5 years. The corresponding retrieved patella base plate showed patterns of a metal-on-metal contact with the femoral component. Thus, a severe third-body wear scenario on the counterfaces could be expected, representing a worst-case scenario in TKA.
The present study dealt with some limitations that should be explained as follows: all evaluated parameters were based on prematurely revised implants. Thus, the results of this study did not exactly represent the in vivo performance of TiN-coated TKAs without any complications. Another limitation was the limited number of retrievals, which was attributed to the limited number for this type of TKA in the retrieval archive of the hospital.
In addition, uncoated, retrieved femoral components of identical design would have presented the ideal group when comparing the surface characteristics and abrasive wear of CoCrMo substrates with those of coated, retrieved components. However, this reference group was not available. Further, the level of patient activity as well as the reasons for revision surgery was not considered within the analysis. Nonetheless, the 25 analysed retrieved femoral components gave an exclusive insight into the midterm wear resistance of TiN coatings in TKA. Finally, as radiological images were not available, malalignments, particularly in the patella, could not be proven in all cases.