**2.2 Hypersensitivity and immunological reactions**

There is a debate if patients with skin allergies to implant materials may react to metal implants, resulting in incompatibility reactions including persistent swelling, pain and early loosening. Moreover, there is still a controversy if hypoallergenic implants may be advantageous for these patients [18–22]. Despite these controversies, the prevalence of contact allergies against implant materials is high (especially nickel), and affected patients usually ask for hypoallergenic implants [23].

It has been demonstrated that patients with metal implants have more often skin allergies against implant materials than patients without such implants [24]. It has been discussed that the long contact time to the metal implant may result in a hypersensitization. However, only very few patients present with symptoms. There are reports about the improvement of symptoms after the revision of standard TKA to coated TKA implants [25]. On the contrary, it has also been reported that there is no increased risk of failure when implanting a standard TKA in skin-sensitive patients [19, 26]. However, functional outcome after TKA is worse in patients with reported metal allergies [27], which might be explained by psychological aspects [28]. Even if there is to date no clear evidence that patients with reported metal allergies need hypoallergenic implants, this psychological advantage might be helpful for patient satisfaction.

Several proinflammatory cytokines have been reported to play a role in healing and pain after TKA [29, 30]. It has been demonstrated that an increased inflammatory reaction after TKA results in less favorable results [31] and that cytokines were lower after being coated compared to standard TKA [32]. Surface modification of TKA may therefore result in less inflammation and better results.
