Table 1.

Differential diagnosis table for metallosis following total knee arthroplasty

Differential diagnosis	X-ray	CT	MRI T1	MRI T2	Pattern of contrast enhancement	Scintigraphy
Metallosis	First line of diagnosis; metal-line sign; bubble sign; cloud sign (see discussion for details of signs); joint effusion often present; periprosthetic osteolysis, may see malposition of polyethylene liner	Radiodense rim outlining joint cavity, joint effusion, sometime with increased density within the joint fluid, periprosthetic osteolysis, soft tissue masses and complex fluid collections can be seen [5]	Not routinely used but may see low signal rim outlining joint capsule or in areas of metal deposition, low signal regions of lysis in bone	Not routinely used but may show signal rim outlining joint capsule or in areas of metal deposition *note: susceptibility artifact is seen on spin echo or gradient echo images [5]	Associated synovitis may enhance	Not routinely used
Small particle disease	First line of diagnosis; Periprosthetic osteolysis that is not loosening; may see evidence of component wear or malposition; no secondary bone response changes; progressive with eventual destruction and fracturing	Same as X-ray but with more accurate quantification of bone loss; look carefully for source of debris, particularly polyethylene wear; used for preoperative planning for extent and precise location of bone destruction	Low signal regions of synovitis and lysis of bone	May see high signal in osseous cysts	Associated synovitis may enhance	Not routinely used
Infected prosthesis	May be negative or indistinguishable from prosthetic loosening; lytic bone destruction; ill- defined bone resorption; periosteal reaction; occasional periprosthetic fluffy dystrophic bone formation	Same as X-rays	Not routinely used			Bone scan: cold defect associated with prosthesis and generalized increased activity around prosthesis, especially the stem; Indium 111 white blood cell scan shows activity around prosthesis, without corresponding marrow activity on sulfur colloid marrow scan.
Prosthesis loosening	First line of diagnosis; lucency at bone-prosthesis or bone-cement interface	Same as x-rays, with more accurate depiction	Not routinely used			Bone scan: cold defect associated with prosthesis, increased activity at site of loosening; Sulfur colloid scan shows uptake accumulation around stem of prosthesis
Metastatic periprosthetic lytic lesion	Lytic lesions and/or associated soft tissue masses may be seen	Same as X-rays with more accurate depiction	Lytic lesions often T1 dark	Lytic lesions may have variable T2 appearance	Usually associated with enhancement	May show increased uptake or cold defects on bone scan