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Treatment of Degenerative Hip Disease in Younger Patients

Posted on: 04/23/2009
Most people would be surprised to know that total hip replacements (THRs) have been around since the 1930s. Today's successful use of THRs reflects the many implant changes in design and materials that have taken place since those early attempts.

In this article, orthopedic surgeons from the New York University Hospital for Joint Diseases review the history of THRs up to and including today's surface replacement arthroplasty (SRA). SRA is a type of hip replacement that replaces the arthritic surface of the joint. But it removes far less bone than the traditional total hip replacement.

Because the hip resurfacing removes less bone, it may be preferable for younger patients who are expected to need a revision (second) hip replacement surgery as they get older and wear out the original implant.

The surface replacement arthroplasty (SRA) is done by dislocating the femoral head out of the socket. Special powered instruments are used to shape the bone of the femoral head so that the new metal surface will fit snugly on top of the bone. The cap is placed over the smoothed head like a tooth capped by the dentist. The cap is held in place with a small peg that fits down into the bone. The patient must have enough healthy bone to support the cap.

The hip socket may remain unchanged but more often it is replaced with a thin metal cup. A special tool called a reamer is used to remove the cartilage from the acetabulum and shape the socket to fit the acetabular component. Once the shape is correct, the acetabular component is pressed into place in the socket. Friction holds the metal liner in place until bone grows into the holes in the surface and attaches the metal to the bone.

A major advantage of a surface replacement arthroplasty (SRA) is its success in patients under age 65. The rate of implant wear with total hip replacements (THRs) make the use of THRs very limited in younger, more active adults. SRAs provide a solution to the problem of disabling arthritic hip pain in younger adults.

SRAs are made from high-carbide cobalt chrome. This is a metal-on-metal (MOM) design. It gives the hip smooth action. But with repeated motions, flecks of metal ions are released into the joint and into the blood stream. Metal ion release may be a factor in implant loosening. Some patients are hypersensitive to these particles and develop hip pain as a result.

And there's been some question about the possibility of an immune system response to the foreign debris being linked with cancer.Particles of both cobalt and chromium have been found in organs of the immune system (e.g., spleen, lymph nodes) and in red blood cells and the liver. This will bear warching in future studies.

The SRA surgery is done from a posterior (behind the hip) approach. This allows the surgeon to see the blood supply to the hip and avoid cutting any blood vessels by mistake. It also gives the surgeon better access to the hip casule and soft tissues, which must be released to remove the femoral head. And the posterior approach allows the surgeon to position the femoral cap with just the right amount of rotation.

Long-term outcomes of SRA are not available yet. Since these implants have only been in use a limited amount of time, studies have only reported early-to-midterm results. Survival rates are high (98 per cent) for the first five years. When failure occurs, it's because of femoral neck fractures, infections, and implant loosening.

Sometimes there's a loss of blood supply to the bone called osteonecrosis. Osteonecrosis can also lead to implant failure. When osteonecrosis occurs, the patient may need a second surgery to convert the SRA to a total hip replacement (THR).

How does the SRA compare to a THR? Studies show that patients with a SRA have more motion and engage in higher levels of activity. Patients with a SRA can walk faster than those with a THR. Gait (walking) pattern is closer to normal after SRA compared with THR. Resuming normal movement patterns called kinematics with SRA may be due to the larger-sized femoral head with the cap in place.

Problems after surgery and the need for a revision (second) surgery are about the same for both procedures (SRA and THR). Typical complications after either surgery include blood clots, dislocation, and damage to blood vessels or nerves in the hip area.

The authors conclude by pointing out that modern orthopedics have made it possible to treat painful, limiting hip arthritis in young patients. Improvements in metal-on-metal SRAs have reduced failures and extended the life of the implant. There are still some potential problems (e.g., effects of metal ions) that require further study.

Aaron K. Schachter, MD, and Justin G. Lamont, MD. Surface Replacement Arthroplasty of the Hip. In Bulletin of the NYU Hospital for Joint Diseases. March 2009. Vol. 67. No. 1. Pp. 75-82.

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