Reaction to metal
Reaction to metal, or adverse reaction to metal debris, is a general term describing pathologic response of tissue to metal-containing implants, most commonly described complicating hip arthroplasty. Reaction to metal can have a systemic or a local form, the latter of which is termed adverse local tissue reaction and is the focus of this article .
Terminology
The nomenclature of this entity is variable. Adverse reaction to metal debris is, as an umbrella term, sometimes interchanged with metallosis . More commonly, metallosis refers either to deposition of metallic wear debris (without specifying the nature of the tissue reaction) , or to the less aggressive end of the range of possible biologic reactions . See the separate article: metallosis.
Pathology
Reaction to metal is a spectrum of periprosthetic aseptic inflammation triggered by products of corrosion and metallic debris from bearing wear. The reaction occurs around metal-on-metal implants or femoral components with a modular stem (trunnionosis), not from cement or polyethylene wear.
Macroscopic appearance
The soft tissue surrounding the implant appears thickened, fibrotic, and partly necrotic (often "cheesy" in texture) and may be discolored due to metallic deposits . There is variable fluid collection. Extensive reaction may form a solid and cystic mass, clinically termed a pseudotumor.
Microscopic appearance
Histopathology demonstrates surface necrosis and a spectrum of inflammatory responses including macrophages containing fine metal debris without chronic lymphocytic infiltration ("pure metallosis"), aseptic lymphocyte-dominant vasculitis-associated lesions (ALVAL) with or without germinal centers, and sarcoid-like granulomatous inflammation .
Radiographic features
Both ultrasound, CT and MRI can depict a pseudotumor as a mixed fluid/solid soft-tissue mass centered around the prosthesis and distending the trochanteric or iliopsoas bursa.
MRI
Metal artifact reduction techniques can help better depict additional features, such as periprosthetic osteolysis, synovitis, debris, pseudocapsule distention or dehiscence. A contrast-enhanced sequence can show non-enhancing periprosthetic tissue, consistent with necrosis.
Treatment and prognosis
Surgery is the only effective method of treating changes associated with reaction to metal. If the condition is diagnosed and treated early, the surgery is often successful.
The presence of a pseudotumor of tissue necrosis is a poor prognostic factor, which has a negative impact on planned revision surgery.
See also
