The prevalence of metal allergy in the general population has been estimated to range between 10% and 15%.¹ About 14% of the population are actually sensitive to nickel if you use patch testing as the diagnostic tool. As it pertains to total knee arthroplasty (TKA) there is controversy over whether or not clinically significant metal allergy truly exists. The literature supports the presence of allergic reactions to other commonly used medical devices. These include those used in cardiovascular surgery,²,³,⁴ neurology,⁵ plastic surgery,⁶,⁷ and dentistry.⁸,⁹,¹⁰ Implantation of other orthopedic devices have similarly demonstrated immune reactions.¹¹,¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸ Case reports exist within the arthroplasty literature to support the presence of allergic reactions as well.¹⁹,²⁰,²¹,²²,²³,²⁴,²⁵,²⁶ With a growing body of literature around this topic, it is impossible to ignore. At a minimum, it is important to create an algorithm for addressing metal allergy in the clinical setting when the issue does arise.

Allergic responses to metal implants are generally thought to be type IV hypersensitivity reactions.¹³,¹⁷,²⁷,²⁸,²⁹,³⁰,³¹ These are cell-mediated, delayed-type hypersensitivity reactions that occur when sensitized T lymphocytes recognize an antigen and initiate a cascade that ultimately results in the release of cytokines that perpetuate an inflammatory response. Metal debris, both particulate and ionic, are generated from metal components, typically from a combination of wear and corrosion. It is known that all metals, when placed in contact with biologic systems, will experience some degree of corrosion.²⁷ Released metal ions can complex with local serum proteins to activate the immune response. In addition to these type IV hypersensitivity reactions, there is also a concomitant innate immune response to implant-derived wear and corrosion debris. This involves a nonspecific reaction, which is immediate and largely macrophage driven.³¹

When a patient presents as a candidate for arthroplasty, it is informative to determine if the patient has a previous history of a presumed metal allergy. Most surgeons routinely ask patients if they have allergies to medications or other environmental factors, but may not specifically inquire about a history of metal allergy. Nam et al reported on 1495 patients undergoing total hip and total knee arthroplasty (THA and TKA, respectively), of whom 1.7% self-reported a history of metal allergy. When specifically asked about a history of metal allergy, this number increased to 4%. Those with a reported metal allergy were associated with decreased functional outcomes after TKA and decreased mental health scores after THA when compared with patients not reporting a metal allergy.³² An additional, albeit potentially controversial, topic to consider is the psychological factors that may adversely impact clinical outcomes after TKA. Otero et al performed a prospective study on 446 patients undergoing THA and TKA and demonstrated that patients who report allergies have lower postoperative outcome scores.³³ Although this study did not specifically address the issue of metal allergies, this suggests that patients who report multiple allergies may be predisposed to have higher dissatisfaction after joint replacement. However, it was noted that there was a similar increase in Physical Component Summary (PCS) and Mental Component Summary (MCS), which also shows that even if they have a lower satisfaction rate, they still experience comparable improvement to patients who did not report any allergies.

Diagnosing a patient with metal allergy can be challenging and the symptoms may be vague. Typically, there will be a dermatitis (cutaneous reaction), urticaria, or vasculitis.²⁴,³⁴,³⁵,³⁶ In the immediate postoperative period it should be noted that patients may develop a reaction involving the skin adjacent to the surgical incision (Fig. 61-1). Often these reactions represent a superficial contact dermatitis in response to the dressing adhesive or the 2-octyl cyanoacrylate adhesive.³⁷,³⁸,³⁹ In these cases,
removal of the offending dressing or adhesive is required as well as routine surveillance. An oral antihistamine may also be helpful. In more severe cases, referral to a dermatologist is recommended; in general, however, these will resolve over time. In some instances the use of a topical or oral corticosteroid has been advised to facilitate resolution of the skin reaction.³⁹