Rejuvenate – What Are The Medical Problems?

A total hip replacement (THR) is performed about 400,000 times a year in the U.S. An aging population will turn to a THR to correct inflammatory degenerative joint disease such as rheumatoid arthritis, to correct a deformity that affections function, to address osteoarthritis, or to fix painful hip dysplasia.

Increasingly the THR is being performed to revise a procedure that has failed.

Also known as a total hip arthroplasty, a THR involves removing the damaged bone and cartilage and replacing a diseased femoral head with a femoral (hip) component.

Modular hip

The latest and greatest of hip prosthesis are called ‘modular’ meaning they contain multiple femoral and acetabular components. The femur or femoral components contain a metal stem adhered to the middle of the femur with either cement or fitted into the bone when is attached to a metal or ceramic ball in the upper part of the stem. This replaces the damaged femoral head that was removed in surgery.

The acetabular socket or receiving end is replaced with a metal cup in the socket which may contain a liner of plastic, ceramic or metal attached with screws or cement holding the socket in place. These different ‘modular’ components allow for a variety of combinations of articulating surfaces.

The alternative to the THR is hip resurfacing where the upper surface of the femur head is resurfaced leaving more bone behind. It is also less invasive into the femur and is usually marketed to the younger adult.

What are the medical conditions that are emerging from this newer, improved and now recalled Stryker modular hip prosthesis? Unfortunately, they are very similar to the outcomes being felt by patients who were implanted with metal-on-metal (MoM) hip implants.

In the July 6, 2012 recall notice from Stryker, the company said “given the potential risks associated with fretting and corrosion at the modular neck junction, Stryker Orthopadics decided to take this voluntary action.”

Fretting is wear that comes from surfaces that come in contact, generally where there is a load involved and repetitive movement. The abrasive contact can cause mechanical wear and material transfer. This is a term seen in the steel industry for example, where fine metal dust can result from fretting.

As more and more implants are made for an aging population, the implications of corrosion are a problem. Bodily fluids may hasten corrosion and metals are inherently susceptible to corrosion, especially cobalt and chromium alloys.

A question that must be asked with all implants is – are they biocompatible? While carbon, copper, aluminum, zinc and carbon steels, nickel and magnesium were considered too reactive for a long-term implantation, cobalt-chromium molybdenum alloys were first used in dental and orthopedic applications with the most corrosion-resistant using titanium and its alloys in the 1960s.

And cobalt-chromium alloys are used in orthopedic implants due to their hardness and strength and resistance to corrosion and wear. However, we are seeing that in the presence of water, organic compounds, dissolved oxygen, sodium chloride, bicarbonate potassium, calcium, magnesium, proteins, plasma, and saliva in the human body, corrosion or breakdown will result at the cellular or direct contact level.

Corrosion can include pitting, galvanic, stress-corrosion cracking, corrosion fatigue, and fretting corrosion. Wear at the bearing surfaces seems to be responsible for generating metal debris of cobalt into the surrounding tissue among hip implant patients.

Corrosion is one of the major issues resulting from the breakdown of a biomedical implant.

Stryker says the incidents are “extremely low,” despite the fact that Searcy Denney is talking to patients who have presented with pain, swelling at the local joint site, and a loosening of the hip implant.

The Vice President and General Manager Stuart Simpson said the recall occurred and, “Following this action, we will work with the medical community to better understand this matter as we continue to evaluate the data.”

It’s too bad the company didn’t evaluate the data before the hips were marketed and implanted, not after.