In this article, Dr. James W. Pritchett, orthopedic surgeon from Orthopaedics International in Seattle, Washington provides us with an in-depth review of a condition known as metallosis. Metallosis following metal-on-metal hip resurfacing is the focus of this report.
Metallosis is defined as the body’s reaction to the presence of wear debris in the joint from metallic corrosion. This metallic corrosion occurs as a result of two metal surfaces rubbing against each other. In the case of hip resurfacing, there is a metal implant covering the head of the round femur (thigh bone) that sits inside the hip socket. The material is usually cobalt chromium as titanium and stainless steel are no longer used.
Symptoms of metallosis include hip pain, noises coming from the hip, joint swelling, and elevated metal levels in blood. Noise by itself is not very diagnostic. All joint implants make some noise — most of the noise is vibrational and at a frequency the human ear cannot detect. But with enough friction, the noise may become louder. Squeaking sounds are not as diagnostic of metallosis as clunking sounds that can also be felt as a clunking sensation.
Implant loosening is often a natural consequence of metallosis. When examined, the joint tissues are thickened (fibrotic) and have a grey discoloration. Soft tissue masses called pseudotumors (meaning they are like tumors but aren’t malignant or infected) often develop.
Anyone with metal-on-metal (cobalt) implants will have some increase in cobalt in their hair, blood, urine, and organs. It has even been detected in the placenta of pregnant women with this type of hip implant. The placenta is the organ that connects the developing fetus to the uterine wall. It allows nutrients in and other substances (such as metal debris) in and waste out.
To monitor for metallosis, anyone with metal-on-metal hip resurfacing implants is tested periodically to look for rising levels of cobalt in the blood. The diagnosis of metallosis is made based on patient symptoms, blood testing, and fluid taken from the joint. Early diagnosis is important in effective treatment.
Why does metallosis develop in some patients but not others? It remains a mystery why metallosis can develop in someone with a perfect surgical result but not in others who have less than satisfactory implant placement. And regardless of the implant placement, not all patients develop metallosis. So what is the key factor or factors involved?
Experts suggest there may be many different contributing factors making this a multifactorial problem. There could be specific patient characteristics, features of the implant itself, and possibly surgical technicalities at fault.
Some patients may develop hypersensitivity to the cobalt leading to the death and breakdown of bone (a process referred to as osteonecrosis). Since there is no way to test for cobalt hypersensitivity before putting the implant in place, surgeons have no way to predict or avoid the problem.
Others may have a reduced ability to absorb and excrete the cobalt through lymphatic flow, blood circulation, and kidney function. Anyone with impaired circulatory systems involving any of these systems may not be a good candidate for a metal-on-metal hip resurfacing implant.
Treatment is by surgical means. Sometimes it’s just a matter or repositioning the implant to reduce an uneven wear pattern. In other cases, it becomes necessary to replace the metal-on-metal implant to one that is metal-on-polyethylene (plastic). Advanced or progressive metallosis may only respond by replacing the entire hip joint with an entirely new joint.
For surgeons who may be interested in hip resurfacing procedures, Dr. Pritchett offers the benefit of his extensive experience with this technique. Photos, case reports, patient X-rays, and details of surgical technique are offered and discussed. Specific tips for close follow-up and assessment for complications and instability are also provided.