Metal on Metal and ASR Hips

In his clinical practice as an orthopaedic surgeon Dr Balakumar has not implanted any ASR (Articular Surface Replacement) Total hip replacements or resurfacings. He also does not implant Metal on Metal Bearing surfaces. He routinely performs ceramic on polyethelene or ceramic on ceramic. However the discussion and presentation on the recent ABC - Four Corners TV program - "Joint Reaction" (16/05/2011) may have misrepresented Total Hip Replacements.

So what does it all mean? There are many hip implant designs available to the surgeon. There is no universal agreement as to which design is best. Each surgeon selects what he believes is best for the patient, or what he was trained to use, or in some cases, what his hospital forces him to use. Find out what implant your surgeon plans to use. Older implants may have the proven track record. However newer implants have theoretical advantages that have only been proven in the laboratory.

The national joint registry in Australia provides data on survivorship and merely acts as an audit of prosthesis. You can ask your surgeon how your prosthesis compares in the joint registry data. Dr Balakumar uses an uncemented femoral component with 97% ten-year survivorship data.

The basic design of the implant is similar regardless of brand. It has a femoral side, an acetabular side and two sides to the bearing. The femoral side and acetabular side can be cemented or uncemented. IF it is cemented it is usually very stable but may have drawbacks with causing fat clots to the lung. The uncemented prosthesis usually is also stable but requires bone to grow into the prosthesis over 6 weeks.

The bearing surface is the weakest link. The options are metal on metal, metal on plastic (traditional), ceramic on ceramic, or ceramic on plastic. Dr Balakumar mainly uses ceramic on plastic.

The plastic socket (high-density polyethylene) is the hip implant’s weakest link. The plastic wears away at the rate of about one millimeter per year (about 1/40th of an inch), against a metal ball, giving the implant a life expectancy of 10 to 15 years.

Highly cross-linked polyethylene is one of several recently developed “cross-linked” polyethylenes. The wear rate is slower in the lab and in humans, perhaps extending the life expectancy of the socket to 15 to 20 years.

Microscopic plastic particles are produced by daily wear, even with the cross-linked poly. They migrate between the implant and the bone. The body reacts to these “foreign particles” by producing enzymes which slowly dissolve bone. This may eventually result in loosening of the implant. This is why surgeons follow up their total hip patients, even if they are doing well.

The metal parts of the implant are manufactured of Cobalt-chrome or Titanium. There is no agreement as to which is better. Dr Balakumar uses cobalt chrome. In some circumstances, each has advantages over the other. Cobalt-chrome has been used in the manufacture of orthopedic implants for 65 years, and is extremely well tolerated by the body.

In rare cases patients with metal allergies may have skin rashes, or chronic pain and swelling of the replaced joint which may be due to metal allergy. True rejection of the implant has never been reported. If you are allergic to any metals you need to let Dr. Balakumar know.

Metal-on-metal hips were introduced since the 1980s. It made a revival in the early 2002 era, where marketing forces made a resurgence of the metal on metal. This is similar to the anterior total hip replacement being advertised now. Both ball and socket are made of Cobalt-chrome. It is believed that these will never wear out. Patients are allowed greater freedom of activity than with plastic sockets. There is some concern that the long-term frictional release of cobalt or chrome ions from the joint may be harmful to the human body. So far, after over 25 years of use in Europe, there is no evidence that these metal ions might cause cancer. A very small number of patients with metal-on-metal hips develop what seems to be an allergic reaction to the metal, called ALVAL. If that happens, the metal socket my have to be replaced with a plastic socket, or the hip converted to a ceramic-on-ceramic hip. The main symptom of ALVAL is persistent pain after hip replacement. It can result in what they call a “BURSA” or pseudo-tumor as per the ABC Four Corners program.

Ceramic-on ceramic-hips the ball and socket are made of ceramic, which is not pottery, but the oxide of any metal, in this case, aluminum oxide. Wear is even less than with metal-on-metal surfaces, and there are no metallic ions to worry about. However, there is a 1 in 25,000 risk of the ceramic components fracturing. Dr Balakumar will use this in very young patients as there is theoretical chance of lifelong retention of the prosthesis. It certainly has the best wear rates, however it can leave patients with the annoying problem of “squeaky” hips.

Re-operation may be required if either the ceramic ball or ceramic socket fractures. Ceramic fracture leaves behind shards of ceramic, which may damage the joint installed to replace the fractured component.

No one knows the effect of circulating metallic ions on a fetus in the uterus. For this reason a ceramic-on-ceramic or ceramic-on-plastic joint is advisable in women of childbearing age.

Large-Head-Technology.

The ball (“head”) of hip replacements has customarily been much smaller than the normal head of the femur because a small ball has less friction against plastic (which therefore wears out slower). A possible complication of surface replacement is fracture of the neck of the femur, which is managed by installing a standard hip replacement stem. Since the socket does not need to be removed, a large head is needed to match the large socket. A few years ago it was realized that these large heads are much less prone to dislocation, a problem that has been most troublesome for the hip surgeon.

Surgeons en masse switched to the larger heads and sockets for metal-on-metal replacements. Unfortunately, for reasons not yet clear, these larger sockets have had a higher incidence of loosening and ALVAL and have now fallen out of favor. This is what the ASR and other resurfacing hip replacements offer.

Overall however the hip replacement is still one of the best operations available today. Orthopaedic surgeons in Australia maintain some of the highest ethical and clinical care standards anywhere in the world. There was an insinuation that sometimes prosthesis can be implanted in patients, where they become the “guinea pigs”.

This is NOT TRUE. Surgeons would have to inform the hospital ethics committee and the patient if this was the case. All products in Australia need to be approved by the TGA prior to their use in patients. The TGA in Australia certainly enforces stringent practices, however they also cannot predict how some prosthesis may fair in clinical practice. To keep up with the demand of the most exciting and newest technology the TGA uses in vivo data to approve implants.

Surgeons then make decisions based on evidence presented to them and should declare any financial interests in any implant company. Dr Balakumar or any member of his family does not receive financial incentives or royalties from any implant company.

May 17, 2011