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CRP of 6 mg/L
1%
6/875
Presence of ossified material in the abductor tendons on plain radiographs
9/875
Hip aspiration with 900 white blood cells
2%
15/875
Serum cobalt levels 5 ppb
13%
118/875
MARS MRI with periarticular fluid collection and abductor destruction
83%
723/875
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Metal artifact reduction sequence (MARS) MRI with periarticular fluid collections and abductor destruction suggests an adverse local tissue reaction due to metallosis. In this setting, the implant should be revised to a ceramic-on-polyethylene prosthesis with the removal of recalled components. Metal-on-metal total hip arthroplasty design was predicated on increasing the femoral head size and thus improving hip stability by maximizing the head-neck ratio and jump distance. However, early results demonstrated metal corrosion and adverse local tissue reactions with lymphocytic necrosis of the surrounding tissue. This adverse local tissue reaction often degraded surrounding soft tissue with resultant abductor musculature insufficiency. Similarly, modular dual-tapered stems provided additional customization for offset and soft tissue tensioning in adult hip reconstruction. But these implants were associated with high rates of trunnionosis that manifested similarly to metal-on-metal implants. When treating these patients, revision operations can be fraught with complex reconstruction options. Patients that present with hip pain and swelling in the presence of a metal-on-metal implant should be assessed for local soft tissue reactions. A battery of tests including cross-sectional imaging and serum metal ion levels is better than single testing alone. MARS MRI of the affected joint is highly sensitive to detecting adverse local tissue reactions. Kwon et al. described a risk-stratification algorithm for managing patients with dual modular taper total hip arthroplasty implants. They stated that the detection of adverse local tissue reactions based on single tests is unreliable. They recommended having a low threshold for performing systemic tests in patients with dual taper stems. Kwon reviewed cross-sectional imaging for the evaluation of soft tissue reactions due to metal debris in metal-on-metal implants. The author described the usefulness of ultrasound imaging in detecting adverse local tissue reactions. The author concluded relying on single investigative tools with concerns for metallosis is not recommended and a comprehensive workup is needed. Jacobs et al. reviewed serum metal ions for monitoring the status of metal-on-metal total hip arthroplasties. The authors described the potential of metal ion testing for metal-on-metal hip arthroplasty prognosis. However, they stated the current work included in the article was premature for making concentration recommendations for clinical decision-making. Incorrect answers Answer 1: A CRP >10 mg/L is concerning for a chronic prosthetic joint infection. Answer 2: Heterotopic ossification (HO) is common after total hip arthroplasty and may impair functional outcomes depending on the extent of involvement. The presence of HO is not an absolute indication of revision surgery. Answer 3: The presence of >3,000 cells per μL is concerning for a prosthetic hip infection. In metal-on-metal hips, the threshold is >4,350 cells per μL. Answer 4: Patients with well-functioning metal-on-metal hips may have increased serum metal ions. A cobalt ion level >7 ppb is concerning for metallosis, but an elevated ion level is not an absolute indication for a revision operation.
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