Review Question - QID 215615

This patient has a symptomatic metal-on-metal (MoM) total hip arthroplasty (THA), MRI evidence of a pseudotumor, and elevated metal ions. Treatment for a bimodular (a taper for the stem and a taper for the femoral head), also known as modular neck, should include revision arthroplasty of both the acetabular and femoral components to include a ceramic on polyethylene bearing.

A metal-on-metal THA pseudotumor, also known as aseptic lymphocyte-dominant vasculitis-associated lesion (ALVAL), is a mass-forming tissue reaction caused by metal-on-metal wear. The diagnosis involves obtaining metal-subtraction MRI studies in the setting of elevated serum metal ion levels (ie. cobalt, chromium). Following a bimodular THA, elevation of the metal ion levels possibly may arise from the head-liner articulation, the trunnion, or the modular neck and stem junction. Therefore, in this symptomatic pseudotumor with elevated metal ions, the treatment should include revision of both acetabular and femoral components to include a ceramic on polyethylene bearing. Revision of the femoral stem and conversion to dual mobility maybe an option depending on the type of monoblock acetabular component in place and the positioning of implants.

Molloy et al. retrospectively reviewed a consecutive cohort of patients who had undergone THA with a modular-neck hip system with ceramic-on-ceramic bearings. They reported cobalt-ion levels were elevated in all patients, while chromium levels were within the normal range. They also noted medial femoral calcar erosion in 47% of cases. They concluded that surgeons using modular hip systems with a titanium stem and cobalt-chromium neck should be vigilant about annual follow-up with radiographs, and use of MRIs as indicated.

Bolognesi et al. reviewed MoM hip articulations. They noted that high implant failure rates attributed to the release of metal particles into the periprosthetic space, creating macroscopic necrosis, corrosive osteolysis, large sterile hip effusions, and periprosthetic solid and cystic masses (ie. pseudotumors). They concluded that a thorough clinical history and physical examination, along with laboratory data and imaging surveillance of these patients, is critical for appropriate diagnosis and treatment, while the decision to perform revision hip arthroplasty of a MoM implant is multifactorial and should be based on documented, objective clinical indications.

Pour et al. reviewed the results of the bimodular femoral neck implants used in THAs. They report that the most common modes of failure were loosening, neck fracture, metallosis, and periprosthetic fracture. They concluded that long neck lengths, greater offset, and larger head diameters contribute to bimodular neck failure by creating a larger moment about the neck's insertion in the stem. The authors did not recommend the use of bimodular femoral neck implants.

Figure A is an MRI of the right hip demonstrating a marked, lobular mass-like thickening of the synovium of the right hip with complete fatty atrophy of the adductor muscles. Illustration A is a clinical photograph of a pseudotumor.

Incorrect Answers:
Answer 1: Treatment of a symptomatic pseudotumor with elevated metal ions is revision arthroplasty. Observation may be appropriate in the asymptomatic patient, in which serial hip radiographs should be obtained to assess for wear, or component loosening
Answer 2: No way exists to perform a head and liner exchange, because this cup is monoblock, with a highly polished metal bearing surface
Answer 4: Revision THA to ceramic-on-polyethelyne components is indicated.
Answer 5: I&D alone would not adequately treat this problem.