Metal hip implants have received much attention in the past few years with several models being recalled. The Stryker ABG II and Rejuvenate were recalled July, 2012 after only a few short years on the market. Although the ABG II and Rejuvenate are not true metal-on-metal devices, they do have a metal neck juncture as well as metal trunnions which can lead to corrosion. In fact, when Stryker recalled the two hip implants they stated a higher-than normal risk of fretting, corrosion and implant failure. Although hip replacement surgery is considered one of the most significant medical device innovations of the past few decades, helping improve the quality of life for thousands of patients, there is significant potential for resulting medical issues.
Implant devices which implement a metal femoral component and plastic cup can create polyethylene particles which in turn lead to osteolysis or bone loss surrounding the hip implant. Osteolysis occurs when an implant device is placed in the body and the device releases particles. The body reacts by attempting to remove those particles, whether metal or polyethylene. Osteolysis is perhaps the most frequent, long-term difficulty from all-metal or metal-on-polyethylene hip implants and is the source for as many as three-quarters of all implant failures. Once the body detects a foreign object it releases enzymes and cytokines into the hip area with a goal of destroying the particles. In turn, this reaction leads to a persistent inflammatory disorder which can cause the surrounding bone to deteriorate to the point that total failure of the implant is inevitable.
In implants such as the Rejuvenate and ABG II, any activity on the part of the recipient can lead to particle debris shearing away from the implant and lodging in surrounding tissue or entering the bloodstream. It is believed that every step an implant recipient takes can produce between a hundred thousand and a million particulates of debris. Aside from osteolysis this debris can lead to other issues such as metal toxicity, pseudotumors, osteonecrosis, dislocation, heterotopic ossification, component loosening, and periprostetic fractures.
Metallosis occurs when the tiny metal ions build up in the soft tissue surrounding the hip causing a grey discoloration of the tissue indicating oxygen deprivation and death of the tissue. Patients who are particularly sensitive to metals are more likely to develop metallosis. Metallosis can cause severe pain, loosening and failure of the implant, tissue necrosis, formation of pseudotumors, osteolysis and bone deterioration. When the metal ions enter the bloodstream, neurological, cardiovascular, renal, thyroid and gastrointestinal disorders can occur as well as vision and hearing loss, headaches and even DNA disruption.
Dislocation of the implant can occur in one to five percent of hip implant procedures, however if revision surgery becomes necessary the dislocation rate rises to twenty percent or more. Heterotopic ossification occurs when bone forms outside the skeleton—the soft tissue calcifies. This calcification causes the joint to become stiff and painful. Bone death, known as osteonecrosis, can also occur following hip replacement surgery when the metal cap covering the femoral ball reduces the amount of blood reaching the ball. Once the bone is deprived of blood for an extended period of time it can collapse, causing the patient to lose the use of the bone. Component loosening is considered the most serious long-term complication as it is the strongest indicator of the necessity of a revision surgery. Periprosthetic fractures are bone breaks in the areas around the implant and generally occur when the implant is weakened because of osteoporosis, certain medications or implant pressure or stress. Recipients of a recalled ABG II or Rejuvenate should take care to have regular exams which include blood testing, x-rays and an MRI.