In medical terms, metallosis is defined as aseptic fibrosis or local necrosis caused by corrosion of a metal device implanted in the body and the subsequent release of metal ion debris. Metallosis has also been described also as the macroscopic staining of the soft tissues…associated with abnormal wear. When metal surfaces come in contact with one another during periods of activity on the part of the patient—or when the polyethylene liner between the two metal components has degraded, allowing unintended contact between the metal parts—corrosion, fretting and excess wear can occur. In turn, microscopic metal ions shear away from the metal components and can either burrow into the surrounding hip tissues or may travel to the bloodstream. Metallosis can occur when this metallic debris begins to accumulate in the soft tissues of the body.
The human body perceives these metal ions as foreign bodies, marshalling its defenses and attempting to dispel the ions. The body sends macrophages—special white blood cells—to attack and obliterate the metal shards with a goal of trapping these ions within an inflamed area to prevent them from spreading. Unfortunately, this is simply not possible, and the longer the metals remain in the body, the more damage will accumulate. The bone surrounding the implant can deteriorate or suffer total death which can lead to implant loosening.
Physicians are not clear as to why some people will react much more strongly to heavy metals in the body while others can have a fairly high tolerance level. Metal sensitivity is essentially an allergic reaction to cobalt, chromium, or other metals, which causes the immune system to multiply its efforts to eliminate these tiny foreign bodies. Healthy tissue can get caught in this battle, becoming damaged in the process. When macrophages attack metal ions, they “remember” the invasion as they produce secretions meant to wipe out the invader. Because a metal hip implant is releasing a constant stream of metal ions into the hip tissues, the immune system becomes frazzled as its attempts to eliminate the cobalt and chromium are foiled time and time again.
A study in the Journal of Orthopedic Research followed 259 patients with metal-on-metal hip implants, then compared that group with a control group, making sure age and gender were closely matched. When compared to the control group, patients with an all-metal hip tested, on average, fifty times higher levels of cobalt and 100 times higher levels of chromium. Several studies have shown that among those with metal hip implants, women, those who are small in stature and patients who are obese tend to have a higher risk of metallosis. The primary symptoms of metallosis are pain—sometimes severe and chronic—inflammation, degradation and death of tissue and bone, gray discoloration of the tissues surrounding the hip and loosening of the hip implant. Should the implant loosen significantly, failure will occur and revision surgery will become necessary.
Synovitis - Another Type of Inflammation
Synovitis is similar in nature to metallosis, but is primarily inflammation of the joint lining among those with artificial hip implants, which can occur long before the patient notices any overt symptoms. In others, the joint may appear swollen and even feel “puffy” or warm to the touch (because of the increased blood flow).Because an MRI can identify this type of joint lining inflammation, it can be beneficial for those with a metal-on-metal hip implant to have regular MRI’s as a means of identifying an implant with a high probability of failure. In fact, a study in the Journal of Bone & Joint Surgery showed that synovitis existed frequently among those with no adverse health symptoms. Scans of 69 patients with metal hip implants were taken; patients were classified as those who were asymptomatic, those who had no symptoms other than pain, and patients who were symptomatic, with a mechanical cause, such as implant loosening or dislocation. Synovitis was identified through the MRI in 68% of the asymptomatic hips, in 75% of symptomatic hips with a mechanical cause and in 78% of asymptomatic hips with unexplained pain. Even the researchers were surprised by the number of asymptomatic patients who were found to have synovitis.