Almost everyone knows or has heard of someone who has had a hip, knee or other joint replaced with a metal or plastic replacement.
Hip replacement has been considered one of the most successful procedures in modern spare parts surgery. Prior to this, people with joint damage had to manage to cope with painful medication and heat therapy, requiring the use of crutches or walkers. But since the late 1960s, surgeons have performed more than a million hip replacements, demonstrating the powerful ability of the implant to reduce pain and increase mobility. According to the American Academy of Orthopedic Surgeons, in 2007 (the most recent statistics available), surgeons in the United States replaced 500,000 knees and 550,000 hips.
A variety of causes can contribute to joint deterioration, such as fractures, repeated dislocations, bone tumors, and other rare degenerative joint diseases. In contemporary times, however, there are other risks associated with artificial joints besides the loosening of cemented grafts, including infection and possible breakage of the artificial joint.
Any implanted joint must be securely attached to the existing bone in order to function effectively. The connection can be accomplished with or without bone cement. Most orthopedic surgeons agree that both methods can serve their purpose equally well, but no method is perfect.
In an implant with bone cement, the surgeon anchors the joint with a special bone cement (which functions like the cement that holds the bricks together). Cemented joints became common in the late 1960s and early 1970s, and are still common today for knee and hip replacements because the cement is very strong, fits well, and is quick to use. As a result, cemented joints are still the preferred method of keeping knee implants in place and are common in hip replacements. Over 90&;#xFF05; of knee implants are secured with bone cement, which works well for a period of time, but the cement and metal used to attach the bone inevitably loosens and cracks in the bone cement can develop. Cracked bone cement can cause serious problems for patients with artificial joints, such as loosening of the implant. These patients may opt for a new implant through a procedure called a “revision”. In this procedure, the surgeon first needs to remove all remaining bone cement from the inside of the bone, a process that requires care. In hip revision surgery, there are many small holes inside the thigh bone that make the bone suitable for absorbing and attaching the bone cement. If the residual cement inside the bone is removed, it leaves a large hole and exposes the bone to a smooth, porous permeable layer that is not conducive to cementing. As a result, a larger implant is needed for the next implant; and in many cases, the re-implant is a bone graft. The bone for grafting usually comes from a donor bone bank.
Non-cemented joints.
In response to the limitations of using bone cement, surgeons have turned to other methods of connecting the implant to the bone. In the late 1970s, surgeons and biomedical engineers at Dartmouth Medical School in Hanover, New Hampshire, introduced a cement-free surface coating for implants. This overlay, usually composed of cobalt or titanium beads for grafting, creates a bump on the implant’s surface. Once the implant is in place, bone or soft tissue grows over or around the raised surface and the metal connection.
Bone cement is recommended to hold the implant in place for patients over the age of 75. This is because older bones are not as dense and are weaker relative to younger bones and are not suitable for uncemented fixation. Non-cemented implants require bones that are strong enough for the implant to work.
For people aged 65 to 75 years, a hybrid artificial hip implant is the best choice. In the hip bone, bone cement holds the implant in the thigh bone, and a plastic cup attached to the top of the thigh bone is held securely in place using a non-cemented method. Experts say that most people in the 65- to 75-year-old age group have healthy enough bone in their pelvis to withstand a non-cemented joint, but that the thigh bone may not have enough bone growth to facilitate its connection to the non-cemented joint.
Both with bone cement and without bone cement have their own advocates. The uncemented implant is slightly more expensive because it requires an additional device. On the other hand, uncemented implants are easier to disassemble or reset if they have to be removed or reset.
Those who want joint surgery should ask for details about the procedure, as well as its risks and benefits. To avoid post-operative disappointment, patients should have a clear understanding of their expectations for this new joint. Other questions may include preoperative preparation and postoperative recovery. In most cases, for example, the patient will be asked not to take aspirin or other anti-inflammatory medications for several days prior to surgery, as such medications may increase bleeding during surgery.
Patients should also be aware of the details of the recovery process, including the length of the recovery period, and the amount of physical therapy needed. After hip or knee replacement, the standard for physical therapy is at least three months. However, recovery varies on an individual basis, depending on preoperative health status and postoperative recovery. There may also be pain during the recovery period, especially for patients with knee replacements.
Experts believe that the rate of infection in the tissue around the implant is only 0.5&;#xFF05; to 1&;#xFF05; in patients who have undergone joint replacement surgery. But when complications strike, the situation can be serious. Surgical devices or implants that are not properly sterilized can cause infections while the patient is still in the hospital. Most patients who get infected this way have weakened immune systems or are taking drugs that suppress the immune system, such as corticosteroids.
Infections can also occur years after the surgery. Researchers do not yet fully understand why the immune system is less effective in fighting infection near plastic or metal implants. Antibiotics can be mixed with bone cement to prevent infection, but this manipulation makes the bone cement less effective.
If bacteria reside in the tissue near a joint implant, the implant must be removed. Because germs can enter the implant through the bloodstream, people with artificial joints should take antibiotics as a precaution when faced with the possibility of bacterial contamination of the bloodstream. An example of a situation that may arise is a visit to the dentist, a dental cleaning or surgery that may allow bacteria from the mouth to enter the bloodstream. Patients with joint implants should also seek prompt treatment if bacterial infections are present in the skin, urinary tract or elsewhere.
Bone stress, blood clots (thrombosis) and other risks.
Another risk comes from the gravitational removal of the thigh bone. The tissue in the bone is constantly dissolving and reorganizing. The process of bone formation is stimulated by the weight-bearing pressure on the bone, and too little pressure can make the bone weak. Once the artificial hip bone is cemented in place, the implant takes on most of the body weight, usually putting pressure on the thigh bone. As a result, the bone tissue in the thigh bone becomes thinner. This removal of weight-bearing pressure from the bone is called “stress masking.”
Severe stress shading can cause the thigh bone to become weak and, in more severe cases, to break. If the thigh bone breaks, the extra weight is transferred to the metal implant inside the bone, which is also at potential risk of damage. Doctors use X-rays to see the extent of bone loss in the thigh bone. If the x-ray shows significant loss, the artificial hip bone may need to be replaced. Stress obscuration occurs mostly in non-cemented joints.
Blood clots are another potential complication of joint surgery, especially in knee resurfacing. In the middle of surgery, blood flow to the leg stops. Some blood flows down the leg and may clot and form a blood clot. Although blood clots can cause chronic swelling and leg pain, most people do not show symptoms. In some rare cases, the clots come loose and move with the blood flow. They become dangerous when they take up residence in the lungs or block blood vessels. As many as 50&;#xFF05; of patients with knee implants form clots during the procedure. However, most clots do not produce symptoms.
Doctors put patients on postoperative blood-thinning medications and use compression stockings and boots during and after surgery to combat clots. These devices squeeze the legs and feet tightly as a way to speed blood flow and reduce swelling.
A major problem for many implant patients is a condition called “particle disease”. Microparticle disease occurs when the plastic or metal of the implant breaks down and small fragments irritate the surrounding tissue, causing inflammation of the tissue. As part of the inflammation process, white blood cells called macrophages move in and release anti-inflammatory chemicals that can damage the bone and cause the implant to loosen.
Although microparticle disease occurs together with knee resets, it usually appears less problematic because the pieces that peel off from an artificial knee are somewhat larger than those that peel off from a hip bone. The researchers believe that macrophages, the purifying cells of the immune system, may not be able to absorb these large microparticles so that they don’t start releasing chemicals to attack and cause consequences such as bone loss in the hip bone.
Although the joint may be more durable than the recipient, all joint implants eventually fail, say life cycle orthopedic surgeons. The two most common causes of failure are stress masking in uncemented joints and bone cement breakdown in cemented joints. Most studies have shown that artificial knees last somewhat better than artificial hips.
Researchers at the Mayo Clinic in Rochester, Minnesota, USA, studied 9,000 knee replacements performed between 1971 and 1987. They found that 91&;#xFF05; of the new knee joints were intact after five years, the rate was 80&;#xFF05; after 10 years and 69&;#xFF05; after 15 years. The study found that patients with bone cement implants fared best, with about 98&;#xFF05; of the new knee joints still intact after 5 years and 91&;#xFF05; after 10 years. patients over 60 years of age and those with rheumatoid arthritis fared somewhat better, presumably because they exercised less and therefore put less stress on the knee. Plastic surgeons agree that technological improvements in artificial joint design, materials and connections have made implants more durable for knees and hips in the 1990s than they were in the 1970s and 1980s.
When do implants fail?
Implant failure leads to chronic pain and sclerosis, and may require reoperation. However, the surgeon does not need to reset every failed implant. The age of the patient is a critical consideration. If the patient is over 75 or 80 years old and the pain is not very intense, then the surgeon may not perform a resurfacing. A small percentage of people request another surgery because the average age for hip and knee implants is about 70 years old. But younger, more active patients are more likely to undergo reoperations. As the age of patients undergoing implant surgery continues to decrease, reoperations may become common. One lesser-known example is the case of baseball player Bo Jackson, who had his hip and knee implanted at the time of his original surgery. Bo Jackson requested reoperation three years after his original hip implant because of the increased stress on the artificial joint from playing professional baseball.