Metal-on-metal hip resurfacings—a radiological perspective
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It is important to be aware of the various complications related to resurfacing arthroplasty of the hip (RSA) and the spectrum of findings that may be encountered on imaging. The bone conserving metal-on-metal (MOM) hip resurfacing has become increasingly popular over the last ten years, especially in young and active patients. Initial reports have been encouraging, but long-term outcome is still unknown. Early post operative complications are rare and have been well documented in the literature. Medium and long term complications are less well understood. A rare but important problem seen at this stage is the appearance of a cystic or solid periarticular reactive mass, which occurs predominately in women and usually affects both hips when seen in patients with bilateral RSAs. The following imaging findings are illustrated and their significance discussed; Uncomplicated hip resurfacing arthroplasty, radiolucency around the femoral peg, femoral neck fracture, loosening and infection, suboptimal component position, femoral notching, dislocation, heterotopic ossification, femoral neck thinning and reactive masses. The radiologist should be aware of the normal radiographic appearances and the variety of complications that may occur following RSA and should recommend ultrasound or MRI in patients with an unexplained symptomatic hip and normal radiographs.
KeywordsHip Resurfacing Arthroplasty Complications MRI
Current designs of hip resurfacing arthroplasty consist of cobalt chrome acetabular and femoral components. The acetabular component is a thin shell and the femoral component is a shell with a narrow stem which is placed in the femoral neck. The prosthesis is mainly used in men under the age of 65 and women under 60 with good bone stock. Resurfacing arthroplasty has the advantage of improved stability when compared to conventional total hip replacement and, because the native femoral neck is preserved, revision with a conventional primary hip prosthesis is possible in the event of failure. The design of resurfacing prostheses has evolved over the years, from Charnley’s Teflon-on-Teflon bearing in the 1950s to the variety of currently available metal on metal devices, which include the Cormet 2000 (Corin Medical), the Birmingham Hip (Midland Medical Technologies), the Conserve Plus (Wright Cremascoli) and Recap (Biomet). In order to prevent excessive wear, the prosthesis needs to be made from a hard material and with precision engineering to ensure a perfect fit between the components. Approximately 45% of patients under 55 years of age undergoing primary hip replacement received resurfacing in the UK in 2004 .
The current literature indicates that resurfacing arthroplasty is an effective intervention in the short to medium term. There are a number of recognised complications with which surgeons and radiologists should be familiar. Plain radiography is the primary method of monitoring progress following surgery, but further imaging techniques including magnetic resonance imaging (MRI) and ultrasound may be required to assess purely soft tissue abnormalities.
In this article, the normal post-operative radiographic appearances and the imaging of complications of resurfacing arthroplasty are illustrated.
Uncomplicated resurfacing arthroplasty
Femoral neck fracture
Loosening and infection
Suboptimal component position
Femoral stem fracture
Femoral neck thinning
Complications following resurfacing arthroplasty are rare and are mainly related to suboptimal surgical technique resulting in impingement or femoral neck fracture. Infection and aseptic loosening are unusual. Reactive masses, occurring predominately in women, are increasingly being recognised as a cause of symptoms.
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