Joints in the human body are subjected to stress every time we move or apply force. Heavy loads, excessive use, and/or disease can lead to deterioration of joints. In joint replacement surgery, a damaged joint is replaced with an artificial joint. These artificial joints are engineered to perform the function of the original joint. The most common joint replacements are for the hip and knee. The shoulder, finger, ankle, and elbow joint can also be replaced. When the entire joint is replaced, it is called a total joint replacement.
In hip joint replacement surgery, a hollow cavity is cut into the center of the femur (thigh bone) and the metal shaft of the implant cemented into this hole. This forms a strong connection between the femur and the implant. The implant contains the artificial femoral neck and head (ball). Similarly, a cavity is cut into the hip bone and a metal acetabular cup is cemented into the cavity (socket). There is a plastic liner, usually consisting of ultra-high molecular weight polyethylene (UHMWPE), on the interior of the metal cup. After both sides are prepared the femoral head of the implant is inserted into the acetabular cup.
Implants are available in a variety of designs, sizes, and biomaterials. The material options are a metal ball fitted into a plastic socket; a ceramic ball and a plastic socket; a ceramic ball and socket; a ceramic ball and a metal socket; or a metal ball and socket. Surgeons recommend designs and materials based on factors such as the patient’s age, weight, activity level, and cause of replacement.
Joint replacements are common and the success rate if good, with about 95% of people experiencing a reduction in pain and satisfaction with the results. However, over time, daily abrasion causes wear on plastic and metal, creating debris known as wear particles that can initiate inflammation, an immunological response. Although ceramic is a more biocompatible option, ceramic-on-ceramic implants can produce noise when walking. Implant loosening is another possible complication. The rigid shaft that is inserted into the bone can cause stress shielding. This phenomenon reduces stress levels around the implant, leading to bone absorption and implant loosening. In addition, the higher rate of implant failure with metal-on-metal devices highlights the ongoing need to research and develop improved biomaterials.
