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Metallosis denotes the tissue deposition of metallic debris as a result of wear. It is a complication that most commonly occurs following a total joint replacement.
Metallosis is sometimes used as a synonym for the umbrella term adverse reaction to metal debris 4. However, the nature of the biologic response that accompanies metal deposition is variable. When used to describe the biologic reaction (as opposed to the inciting metal deposition), "pure metallosis" has been defined histologically as the predominance of macrophages containing metallic particles (i.e., histocytic foreign body reaction), without the presence of a chronic lymphocytic infiltrate to suggest an aseptic lymphocyte-dominant vasculitis-associated lesion (ALVAL) and type IV delayed type hypersensitivity reaction 5,6.
Metallosis can cause pain.
Metallosis is the deposition of metallic debris into periprosthetic soft tissues. Metallic microparticles are generated from repetitive abrasion of metallic hardware, such as from metal-on-metal arthroplasty, modular reconstruction components (such as at the prosthetic femoral head–neck interface, called trunnionosis), or fractured fixation plates 1. The debris can involve the joint capsule, cavity, and/or extra-articular tissues.
Metallosis should not be confused with particle disease, which is caused by the release of nonmetallic polyethylene microparticles from the prosthesis liner 3.
Metallosis tends to involve large, weight-bearing joints, such as knees and hips.
As metallosis is a result of hardware failure, signs of it are most commonly present.
Several plain film signs have been described:
- bubble sign: curvilinear radiodensity surrounding the replacement
- cloud sign: presence of an amorphous cloudy density within the soft tissues adjacent to the prosthesis
- metal line sign: thin rim of increased linear density outlining a portion of the joint capsule.
CT is better at demonstrating the dense collections of microdebris 3.
Magnetic susceptibility (blooming) artifacts might be present. Microdebris collections are typically low on T2W sequences unlike, for example, seromas. Resultant tissue necrosis and mass-like pseudotumors are indicative of adverse local tissue reaction to the metallic microparticles 3.