Christian W. A. Pfirrmann MD, MBA, Presenter: Advisory Board, Siemens AG Consultant, Medtronic, Inc

1) To learn about the epidemiology and risk factors for complications after metal on metal hip implants. 2) To know the role of different imaging modalities for the diagnostic assessment of sypmtomatic metal on metal hip replacements. 3) To recognize the typical imgaging findings symptomatic metal on metal total hip replacements.

In 2010 the Medicines and Healthcare products Regulatory Agency (UK) issued a Device Alert for certain metal on metal total (MoM) hip replacements: a small number of patients may develop progressive soft tissue reactions to metal wear. Data showed 5-year revision rate of approximately 12% on certain implants. Risk factors of MoM total hip arthroplasty (THA) are the use of large heads, hip resurfacings MoM THA with mal positioning (Cups with excessive anteversion/lateral inclination), high activity level of the patient, severe local / mechanical symptoms, change in gait (i.e. Limp), abductor weakness or swelling. Imaging workup of patients with MoM-THA should start with plain radiographs. Pseudotumors can be detected by US or MARS-MRI. A pseudotumor can be a solid or cystic mass, in continuity/communication with the hip joint. Pseudotumors are non-neoplastic and not infected. The majority of pseudotumors are associated with high wear. A minority of pseudotumors are associated with low wear and a prominent immune response. Two pathomechanism are discussed: Cytotoxic effect of phagocytosed metal particles on macrophages or a hypersensitivity response to metal wear particle. This leads to extensive necrosis and tissue destruction. Pseudotumors are not necessarily symptomatic. The presence of bone marrow edema and abductor tendon tears shows a higher correlation to symptoms than the presence or size of pseudotumors. Osteolysis is frequent, often undetected or underappreciated. Frequency of osteolysis in MoM hips similar to metal-on-polyethylene. However, early osteolysis is a concerning radiographic finding.  

Pfirrmann, C, MoM Hip Complications.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14000524.html