Metal On Metal (MoM) Hip Replacement

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Total hip replacement is the most successful treatment available for symptomatic osteoarthritis of the hip2. The operation involves removing the painful arthritic surfaces of the hip joint and replacing it with artificial components (usually metal + plastic). See Figure 1 below:

Figure 1

mom-x-ray-web

Osteoarthritis Right Hip Total Hip Replacement Left Hip.

The Components Of Total Hip Replacement

Hip replacements have a metal cup seated into the hip socket in the pelvis, and a metal stem which is seated in the femur (thigh bone). See figure 2 below:

Figure 2

mom-web

Bearing Surfaces

The cup has a liner (usually made of polyethylene or ceramic) which locks tight into the cup. Securely attached to the femoral stem is a ball (usually metal or ceramic) which fits snugly in the liner. This ball is designed to rotate within the liner during walking and hip movement.

The term “bearing surface” describes the type of materials used in the ball and liner of the hip replacement. These are the surfaces that rub on one another during walking and activities. In figure 2 the bearing surface is a ceramic ball (pink colour) on a polyethylene liner (white colour).

The most common bearing surfaces used are a metal or ceramic ball, which sits in a polyethylene liner. Another common bearing surface is a ceramic ball on ceramic liner. These bearing surfaces are all proven and successful designs with longterm follow up and excellent results1.

Metal on Metal (MoM) Hip Replacements-Design and Development

Hip replacement has been in use since the 1960s. The techniques and implants (prostheses) used have evolved since, allowing hip replacements to last longer and perform better.

In the 1990s an attempt was made to improve the results of total hip replacement further by using “metal on metal” hips. These hip replacements have a bearing surface of a metal ball in a metal socket.

The initial results of MoM hip replacements were encouraging (especially the “Birmingham” hip prosthesis). Other manufacturers therefore developed their own versions. Unfortunately, some of these other metal on metal hip designs have been unsuccessful, most notably the “ASR” hip. These problems are thought to be due to the metal ball wearing on the metal cup and producing metal ions which are released into the tissues adjacent to the hip replacement. These ions (chromium and cobalt) can cause a local toxicity to the bones and soft tissues of the hip joint.

Problems with Metal on Metal Hips

Most patients with metal on metal hips are quite satisfied with their function but some have described problems, including:

  • Hip pain and discomfort
  • Hip stiffness
  • Swelling around the hip replacement
  • Squeaking
  • Difficulty with ambulation and function

It is widely thought that these problems are associated with metal ion toxicity affecting the hip tissues.

Some patients also describe systemic symptoms (including malaise, lethargy, nausea, a feeling of being generally unwell). It is sometimes less clear if these symptoms are directly associated with metal ion levels within the body and this is a subject of some debate currently.

Assessment Of Patients After MoM Total Hip Replacement

It is recommended these patients should be assessed annually by an orthopaedic surgeon3; this will involve clinical review and x-ray. Other testing (blood tests for metal ion levels, ultrasound, MRI) may be indicated after this review.

Management of Patients after MoM Total Hip Replacement

Management depends upon patients’ symptoms and review findings:

Asymptomatic patient

Many patients are very satisfied with the results of their metal on metal hip replacements and require observation only.

Symptomatic patients

Treatment will depend upon the severity of symptoms and the wishes of the patient. Revision hip surgery (removal of total hip replacement and insertion of a new hip replacement of a different type) may be indicated if symptoms dictate, after appropriate discussion and informed consent.

I am happy to see and advise your patients regarding their concerns after metal on metal total hip replacement. Please contact my rooms to organise this on [site_phone].

References:

  1.  Learmonth, I, The Lancet 2007:370;1508-19.
  2. Australian Orthopaedic Association National Joint Replacement registry 2013, www.aoa.org.au.
  3. Australian Orthopaedic Association Hip Arthroplasty Society – position statement on follow up of routine total hip and knee replacement.

Richard Harbury

July 2014