Delayed presentation of cobalt allergy to a shoulder prosthesis presenting 4 years post‐implantation

A 77-year-old male experienced a work-related injury which resulted in a right shoulder joint replacement (Comprehensive® Total Shoulder System – Zimmer Biomet) in 2019. The prosthetic shoulder joint materials included cobalt (58%–63%), chromium (27%–30%), molybdenum (5%–7%), nickel (1%), manganese (1%), and other metals (<1% each). Following the surgery, the joint was always painful. Four years later, he developed blotchy areas of erythema over the joint which became confluent and more marked over the following months (Figure 1). He also had worsening pain in the joint and was unable to abduct his shoulder greater than 45 degrees post-surgery. Over the course of 2023, he underwent two shoulder washouts under general anaesthetic because of concerns of an intra-articular infection. There were no signs of deep infection on either occasion, but heavy granulation tissue and black discoloration were noted at operation which was reported to be consistent with metallosis. The patient has no previous history of metal allergy. The patient was reviewed by the infectious diseases team and treated with multiple courses of antibiotics for presumed intra-articular infection, but remained systemically well throughout. The patient's lab results also displayed no signs of systemic infection or inflammation. Despite all these therapeutic attempts, there was still no improvement in the patient's erythema, pain or mobility. It was decided to remove the prosthesis in December 2023 and a spacer composed of antibiotic-loaded polymethyl methacrylate was put in place, with a plan for staged revision. The patient underwent patch testing to the Skin Health Institute's prosthetic joint series. Days 2 and 4 readings displayed a positive (+) reaction as per the International Contact Dermatitis Research Group criteria to titanium oxalate hydrate with a doubtful/ irritant reaction on Day 7. A second titanium oxalate hydrate test displayed irritant reactions (IR) on Days 2, 4, and 7. Cobalt chloride hexahydrate displayed a positive (+) reaction on Days 4 and 7. The patient also had a raised serum cobalt chloride level of 33 nmol/L (normal range <20 nmol/L) 3 months post-cobalt implant removal which reduced to 22 nmol/L 6 months post-cobalt implant removal. However, he displayed no systemic symptoms from cobalt toxicity. The patient was diagnosed with allergic contact dermatitis to cobalt chloride, which was present in his joint prosthesis. In April 2024, the spacer was removed, and a titanium shoulder implant was inserted. On review at 3 months post-surgery, the patient's erythema and pain had greatly improved, and his range of movement had increased (Figure 1). Metal implant hypersensitivity may present with localised dermatitis over the implant or more widespread dermatitis from systemic allergic contact dermatitis and urticaria.1 It may also present with non-cutaneous manifestations, including pain, joint effusion, and a reduced range of movement.1 This case was unusual for the relatively delayed presentation of a rash over the joint, although delays in the diagnosis of metal implant hypersensitivity are not uncommon as symptoms often mimic more common complications.2 In fact there are very few documented cases of metal implant hypersensitivity to shoulder prostheses and cutaneous changes have not been described previously.3 This patient showed a positive patch test reaction to titanium oxalate at Days 2 and 4, which reduced to a doubtful or irritant reaction on Day 7. This demonstrated the decrescendo effect often seen in testing with titanium oxalate.4 It is important to be aware of this decrescendo effect when testing titanium oxalate. To the inexperienced patch tester, this could have been misinterpreted as a false positive result, which would have prevented the successful therapeutic result achieved with the replacement titanium shoulder implant. The second titanium oxalate reaction provided reassurance in the decision-making by remaining doubtful throughout the 7 days. It is common practice in our institute to perform duplicate patch tests of metal allergens, given the difficulty often associated with interpretating results. The patient displayed no systemic symptoms from cobalt toxicity despite having an elevated serum cobalt. It is important to be aware that signs and symptoms of cobalt toxicity can appear between 3 and 72 months after arthroplasty (median 19 months).5 However, the association between the level of serum cobalt and pathological effects is complex and not well understood.6 It is difficult to confirm a direct correlation between serum levels and symptoms that are non-specific.6 Mildly elevated serum cobalt levels have been reported in patients with total shoulder replacements compared with healthy controls, as a result of corrosion at the surface of the joint replacement, which may have occurred here.7 This may have led to cobalt sensitization in this patient given no prior history of metal allergy. James Fuller: Writing—original draft; writing—review and editing; conceptualization. Rosemary Nixon: Conceptualization; writing—review and editing. The authors declare no conflicts of interest.