Metal-on-Metal Hip Replacements: A Short Review for Chiropractors
Hip implants used to be limited to patients who had hip fractures when surgical pinning of the fracture was considered unlikely to result in the healing of the fracture; and for patients who were older and suffered from severe degenerative arthritic disease. This population was generally less active and older, less likely to outlive their hip replacement. (Hip replacements are expected to last an average of 15 to 20 years.)
Advances in hip replacement design, materials used and surgical procedures have made hip arthroplasty one the most common orthopedic procedures performed today. With the advent of metal-on-metal implants, which experimentally appeared to indicate they would last longer, the theory was that these new implants could be used for younger, more active patients. In fact, surgeons are now offering hip implants to patients younger than age 55 to correct or relieve a number of conditions which in the past would not have been considered severe enough to warrant surgery.
These new implants are being made from stronger materials and have been developed to more closely mimic the natural motion of the hip joint. This has made hip implants available to a larger population of patients; however, complications are more common and in some cases, more severe. It appears that hip implants are being pushed beyond their design limitations.
Even though chiropractors are not in the business of hip implants, we need to be aware of the recent complications that have resulted from the use of metal-on-metal hip implants. A high percentage of our patients are older and many have hip implants or will require one in the coming years. Some may have issues related to their implants. We need to be able to differentiate symptoms that are indicative of hip implant complications and those that are a result of other musculoskeletal disorders.
Hip Replacement Procedures
There are three types of hip replacement surgery. In a total hip replacement, the entire hip joint is replaced; there is a femoral component and acetabular component. A partial hip replacement is when only the femoral head is replaced. In a hip resurfacing procedure, the acetabulum is replaced, but the femoral head is not; instead, it is reshaped and covered with a metal cap.
All resurfaced hip replacements use metal-on-metal components. Each procedure is performed to accomplish slightly different goals, but they all have some of the same complications.
Complications to Watch For
One of the most important complications is caused by the materials the implant components are made of – debris from the stresses placed on the implant. All hip replacement components create debris due to wear-and-tear, but the large-diameter, metal-on-metal components used in hip replacement and hip resurfacing have been found to have the most problems in terms of creating metal debris. This is despite the fact that earlier evidence suggested metal-on-metal implants were more resistant to wear-and-tear and dislocation.
The type of debris produced depends on the material the components are made of. Different materials can cause different long-term complications. For example, the metal-on-metal hip replacement creates debris primarily made of cobalt and titanium ions. This metallic debris can cause metallosis, which is a buildup of metallic debris in the soft tissue adjacent to the implant; and in some cases even a cyst or pseudotumor. Hip implants known as metal-on-plastic, made of a metal femoral component and a plastic acetabular component, create polyethylene particles that can lead to osteolysis of the adjacent bone.
Metallosis is relatively rare and is more often seen in hip resurfacing patients than in total hip replacement patients. Primarily this is due to two issues. First, most hip resurfacing implants are used in younger, more active recipients, who tend to place a higher level of wear-and-tear on the implant than older recipients. Second, hip resurfacing implants have a larger femoral ball than total hip replacement implants. Because the femoral ball is larger, there tends to be more abrasive wear within the joint, which produces more metallic debris. However, metallosis can occur in metal-on-metal total hip replacements; usually due to the failure of the articular interface, which is a plastic insert that separates the metal femoral component from the metal acetabular cup.
The effects of metallosis can be particularly acute in implant recipients who are sensitive to metal. In patients with metal sensitivity, metallosis can lead to metal poisoning. An inflammatory reaction to metal wear debris has been called an inflammatory pseudotumor. This adverse reaction to metal debris is associated with aseptic lymphocytic vasculitis, which creates a mass that may be cystic or solid; with histopathological findings consistent with inflammation.
Another problem created by the metallic debris is that metal ions can spread from the surrounding tissues into the blood, which cause a host of other conditions such as mental cognitive problems, emotional imbalance, severe headaches and problems with the nervous system.
Subtle differences between different prosthetic designs are believed to play a role in increased wear and subsequent metal debris. One particular hip resurfacing device, the articular surface replacement (ASR) hip resurfacing device, has been associated with a higher incidence of metallic debris. In fact, this device was recalled in 2010 since some studies demonstrated adverse reactions to metal debris – as high as 50 percent of recipients at six years.
The true incidence of adverse reactions to metal wear debris is not known, nor is the full impact of metallosis really known. Adverse reactions to metal debris are difficult to diagnose and therefore probably underreported. Four percent of masses (pseudotumors) are thought to be asymptomatic initially and magnetic resonance imaging studies have shown that "silent" pathology exists. Adverse reactions to metal debris have be demonstrated with MRI in 25 percent of patients with a best possible Oxford Hip Score.
To reiterate, the worst results have been with the resurfacing procedures and large-head, metal-on-metal total hip replacements. The performance of these devices is even worse for women than for men. The incidence of adverse reactions to metal debris may be even greater than that predicted by examining the revision rate for the replacement (ASR) hip resurfacing device. The cumulative revision rate for adverse reactions may increase progressively with time. Remember, hip implants are expected to last 15 to 20 years; some metal-on-metal hips are failing in as little as five years, with serious complications.1-5
Osteolysis: The other major complication associated with hip implants is osteolysis, which is the cause of nearly 75 percent of all hip implant failures. Osteolysis, or periprosthetic osteolysis, is the loss of bone around the hip replacement. It is the body's attempt to clean up particle debris, plastic or metal, produced by the implant. Osteolysis is the most common long-term hip replacement complication.
Osteolysis occurs when the body recognizes the implant debris as foreign and reacts to remove the particles. The body releases enzymes, cytokines and other cellular reactions into the area, all in an attempt to either expel or dissolve the particulates, which causes a chronic inflammatory condition. This autoimmune reaction causes bone reabsorption, which eventually leads to loosening of the prosthesis and failure of the implant. This problem in inherent with any implant, be it shoulder, knee or hip.5-8
Although metallosis and osteolysis are two of the more serious complications associated with hip implants, they are not the only complications associated with hip replacement surgery. More than 450,000 hip arthroplasties are performed annually in the U.S. It is considered by many to be routine surgery, but the associated complications can be serious, even deadly.
Mortality: A 2003 study published in the Iowa Orthopedic Journal reviewed the results of nearly 5,000 hip arthroplasty surgeries.9 It found that the mortality rate for patients having the surgery for the first time was nearly 1 percent. For patients undergoing a revision surgery, the mortality rate more than doubled.
Researchers also found that the most important determining factor of mortality was age. Patients older than age 70 were three times more likely to die from the surgery than younger patients.
Dislocation: Although dislocation is not common, it occurs following 1 percent to 5 percent of initial surgeries. Following revision surgery, the risk for dislocation rises to as high as 20 percent. Dislocation leading to implant failure is most likely to occur within the first few months after surgery.
Heterotopic ossification around the joint replacement is considered one of the most common hip replacement complications, occurring in nearly 50 percent of patients. However, only about 10 percent of those suffer any side effects from the condition, including tenderness, swelling and a decreased range of motion. The condition can be treated with low-dose radiation and anti-inflammatory drugs. In severe cases, surgery is required to remove the calcified tissue.
Infection: As with any surgery, infection is another problem that can occur. With hip replacements the rate of infection is about 1 percent depending on the study. If the infection is caught within four weeks after surgery, the wound has to be reopened, cleaned and injected with antibiotics. If that does not work or if the infection is not discovered until later, the implant may need to be removed and later reimplanted after the infection has been eradicated. To avoid scar tissue and bone degeneration, reimplantation needs to occur within three months.
Avascular necrosis is another complications that can occur. It is more often associated with hip resurfacing than total hip replacement. With hip resurfacing, the metal cap that covers the femoral ball can reduce the amount of blood reaching the ball. If the bone is deprived of blood for an extended period of time, it will collapse, which will destroy the bone. Use of the joint will be lost. In extreme cases, the implant will have to be replaced. With the femoral ball no longer usable, a total hip replacement will be required.
Periprosthetic fractures can also occur around the implant and could cause the implant to fail. These fractures typically occur because the bone around the implant has been weakened by osteoporosis, medications, pressure from the implant or stress placed on the implant. Periprosthetic fractures occur around hip implants following a little more than 1 percent of first surgeries and 4 percent of revision surgeries.
In summary, the problems associated with hip implants can be serious and even life threatening. In some circles it may be considered routine surgery. It is not to say that most recipients don't do well and regain function they had lost due to painful symptoms; but it is not a surgery without potential problems. It appears that the new metal-on-metal devices need to be reassessed and recipients of these devices need to be followed more carefully.