**4.5.1 Epidemiology**

Metal allergy, as defined by a positive patch reaction to a given metal, is frequent in the general population since up to 17% of adult women and 3% of adult men are nickel sensitized (Thyssen et al. 2007). Chromium and cobalt allergy are less frequent and occur in about 1-2%. Other prevalent causes of metal allergy include gold and palladium resulting in positive patch test reactions in about 10% of dermatitis patients albeit these are rarely clinically relevant (Faurschou et al. 2011). The prevalence of other metals, e.g. titanium, platinum, molybdenum, manganese and other has not yet been evaluated in general populations and are general infrequent in dermatitis patients.

It has often been debated whether metal allergy increases the risk of developing delayed type hypersensitivity reactions, e.g. loosening of THA. The literature was recently reviewed and it was concluded that metal allergy might in a minority increase the risk of complications caused by a delayed type hypersensitivity reaction (Thyssen et al. 2011a). Also, we do not know how to identify the subgroups of metal contact allergic patients with a potentially increased risk of complications following insertion of a metal implant. However, it seems to be certain that insertion of metal bearings increase the prevalence of metal allergy as a review showed it was 25% among patients with well-functioning hip arthroplasties and 60% among patients with a failed or a poorly functioning implants (Hallab et al. 2001); considerably higher frequencies than general population estimates (Thyssen et al. 2007). Despite the higher prevalence in the latter patient group, it remains unknown whether the higher prevalence is caused by implant loosening or whether metal hypersensitivity results in loosening.
