Mechanisms of Failure

Alexander P. Sah

Implant Loosening and Wear

In order for a total knee replacement to function properly, an implant must remain firmly attached to the bone. During the initial surgery, it was cemented into position, the gold standard of care. Over time, however, an implant may loosen from the underlying bone, causing the knee to become painful.

In order for a total hip replacement to function properly, an implant must also remain firmly attached to the bone. During the initial surgery, the hip replacement components were either cemented into place or were "press fit" into the bone, to allow bone to grow onto them. Sometimes, however, bone may fail to grow onto press-fit components. In addition, cemented or press-fit components that were once firmly fixed to the bone can eventually loosen, resulting in a painful hip.

The cause of loosening is not always clear, but high-impact activities, excessive body weight, and wear of the plastic spacer between the two metal components of the implant are all factors that may contribute. Also, patients who are younger when they undergo the initial knee replacement may "outlive" the life expectancy of their artificial knee. For these patients, there is a higher long-term risk that revision surgery will be needed due to loosening or wear.

In some cases, tiny particles that wear off the plastic spacer accumulate around the joint and are attacked by the body's immune system. This immune response also attacks the healthy bone around the implant, leading to a condition called osteolysis. In osteolysis, the bone around the implant deteriorates, making the implant loose or unstable.

Infection

Infection is a potential complication in any surgical procedure, including total knee replacement. Infection may occur while you are in the hospital or after you go home. It may even occur years later.

If an artificial joint becomes infected, it may become stiff and painful. The implant may begin to lose its attachment to the bone. Even if the implant remains properly fixed to the bone, pain, swelling, and drainage from the infection may make revision surgery necessary.

An antibiotic spacer placed in the knee joint during the first stage of treatment for joint replacement infection.

Instability

If the ligaments around your knee become damaged or improperly balanced, your knee may become unstable. Because most implants are designed to work with the patient's existing ligaments, any changes in those ligaments may prevent an implant from working properly. You may experience recurrent swelling and the sense that your knee is "giving way." If knee instability cannot be treated through nonsurgical means such as bracing and physical therapy, revision surgery may be needed.

Injured ligaments can make the knee unstable.

Recurrent Dislocation

A hip replacement has a ball-and-socket structure like that of your natural hip. For a hip replacement to work well, the ball must remain inside the socket. Trauma or certain hip positions can sometimes cause the ball to become dislodged from the socket. This is called a "hip dislocation." If you experience recurrent hip dislocations, you may need revision surgery to better align your hip joint or to insert a special implant designed to prevent dislocations.

Stiffness

Sometimes a total knee replacement may not help you achieve the range of motion that is needed to perform everyday activities. This may happen if excessive scar tissue has built up around the knee joint. If this occurs, your doctor may attempt "manipulation under anesthesia."

To break down scar tissue, your doctor may perform manipulation under anesthesia.

Fractues

A periprosthetic fracture is a broken bone that occurs around the components of a total knee replacement. These fractures are most often the result of a fall, and usually require revision surgery.

The femur (thighbone) has broken in several places around the implant.

Reaction to Metal Ions and Allergy to Metal

Over time, the metals used in implants can break down or wear, causing tiny particles to fall off the device into the space around the implant. This is more common with "metal-on-metal" devices, in which both the ball and socket components are made of metal. In some patients, sensitivity to the metal ions in these particles can result in damage to the bone and soft tissues around the hip and lead to the need for revision surgery.

In very rare cases, a patient allergy to the metal used in implants may cause pain around the site of the implant. There is no definitive agreement among doctors regarding metal allergy in this setting, however, and more studies are needed.


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