How does a joint replacement work? By replacing parts of the joint, or by replacing the whole joint.
Joint replacement surgery is called arthroplasty. It is made from the Greek word “arthro”, which means joint and “plasty”, which means to mold or form. It is carried out when a joint is no longer usable or causes too much pain. We have 360 joints in our bodies, but most of them don’t need to be replaced. The main joints that are replaced are shoulders, elbows, hips, and knees. The shoulder and the hip joint are ball and socket joints. The knee and the elbow are hinge joints. It is possible to replace ankle and wrist joints as well, but that is more a case of removing and replacing damaged bones, rather than a complete joint replacement.
Joints can become unusable because of injury or disease, but the most common reason is age. Several factors can cause problems with the joints. The first is that connective tissue and tendons in the joints get stiffer as people get older. This coupled with a loss of muscle strength can put more pressure on the joints, increasing the pain. Another reason is that people start to lose bone density as they age, making the joints weaker. And the most likely reason is that the cartilage in the joint wears down. Cartilage is a strong, flexible tissue that sits on the ends of the bones in a joint. It acts as a shock absorber and it reduces friction because the bones don’t rub on each other. It holds the joints together as well and maintains the shape of the joint. As people age, the cartilage can wear down. It has had a lifetime of rubbing against another piece of cartilage with all of your body weight on it and this will slowly wear it down. When the cartilage is gone, bone will rub on bone. This is painful, breaks the bones, and also means that the joints don’t move smoothly because there is too much friction. This is called osteoarthritis and it is not only painful, but it can also make joints unusable.
It is possible to replace cartilage, and this is an operation that is carried out if there is not much cartilage damage. Cartilage doesn’t have its own blood supply, so it cannot heal itself when it is damaged. The broken bits of cartilage are replaced with bits of cartilage taken from non-weight bearing parts of the patient’s knee. This is why it is not possible to replace all of the cartilage.
A complete joint replacement is carried out if the joint is too far gone to be fixed, or if the patient is in too much pain or can’t use the joint. The shoulder and hip joints are both replaced in a similar way. They are ball and socket joints. A ball at the top of the humerus in the arm and at the top of the femur in the legs rests in a socket in the shoulder and in the pelvis. This allows the joint to rotate. When a ball and socket joint is replaced, the ball and some of the bone at the top of the humerus or femur are cut off and a metal ball is added. The ball is pinned into the soft center of the humerus or femur. Then the socket part of the pelvis or shoulder is removed, and a metal socket with a lubricating layer is added. After a few months of rehabilitation, most hip and shoulder replacement patients have fairly good use of the joint. It will never be as good as a real and perfect joint, but it is usually better than the joint that they had.
A knee and an elbow joint are replaced in a similar way. The knee joint is where the femur and the tibia come together. They are connected by strong muscles and tendons that cross behind the kneecap. When the cartilage goes, the bones rub on each other, and walking can be all but impossible. With a knee replacement, the damaged part of the femur is trimmed away, and a metal cup is fixed over it. The damaged part of the tibia is trimmed away, and a plate is fixed onto it. Then, if the kneecap is still ok, everything is put back together. If the kneecap is not in working order, that will be replaced with a plastic one.
As more people are living for longer, the number of joint replacement surgeries is increasing. A replacement joint will be good for about twenty years before it needs to be replaced. However, technology and materials are improving so joints will last longer and longer. The holy grail of joint surgery, though, will be to stop the joints getting damaged in the first place. Research is ongoing into helping cartilage fix itself. And this is what I learned today.
Photo by Towfiqu barbhuiya: https://www.pexels.com/photo/person-holding-their-knee-11969599/
Sources
https://my.clevelandclinic.org/health/body/23173-cartilage
https://www.webmd.com/healthy-aging/features/joints-change-age
https://orthoinfo.aaos.org/en/treatment/total-knee-replacement-animation/
https://orthoinfo.aaos.org/en/treatment/total-joint-replacement/
https://www.webmd.com/osteoarthritis/features/joint-replacement-risks-benefits
https://www.mayoclinic.org/tests-procedures/knee-replacement/about/pac-20385276
https://www.uofmhealth.org/conditions-treatments/cmc/total-joint-replacement