NZ National Joint Reigstry

Total hip replacement Total hip replacement is an operation designed to replace a hip joint that has been damaged by some form of arthritis. The hip joint is a ball and socket joint. The ball or head is at the top end of the thigh bone (femur) and fits the socket (acetabulum). This is shown in the diagram entitled "Normal Hip". The surfaces of these bones are coated by a smooth substance known as articular cartilage. This acts as a shock absorber for the forces passing through the joint.
	Arthritis occurs when the cartilage wears out and the underlying bone is unable to adapt to the forces it is directly exposed to. This is shown in the diagram entitled "Arthritic Hip". Arthritis of the hip causes pain, stiffness and deformity. When arthritis has caused severe damage to the joint, a total hip replacement may allow the patient to return to everyday activities.
In a total hip replacement operation, the surgeon replaces the worn head of the thigh bone with a metal or ceramic ball mounted on a stem. This is inserted into the top part of the shaft of the thigh bone. The socket is resurfaced with a cup that can be plastic (polyethylene) or metal, with a plastic liner, or metal alone. The components of the artificial joint may be cemented in place with a filler or grout similar to dental cement, or securely pressed into the bone without using cement. In this situation, bone can gradually grow in, to anchor the components in place. Studies have shown that hip replacements with cemented stems and cementless sockets ( "hybrid hips") are also very satisfactory.
Total knee replacement Total knee replacement is an operation designed to replace a knee joint that has been damaged by some form of arthritis. The knee joint is basically a hinge joint, but some rotary and sliding motion does occur. It is formed by the two bony convex projections (condyles) from the bottom end of the thigh bone (femur), that articulates with the biconcave top end of the shin bone( tibia). The knee cap (patella) lies in the front part of the joint and is held within the large muscle of the front of the thigh (quadriceps). The patella moves up and down a groove in the femur as the knee straightens and bends. The surfaces of these bones are coated by a smooth substance known as articular cartilage. This acts as a shock absorber for the forces passing through the joint. Arthritis occurs when this cartilage wears out, and the underlying bone is unable to adapt to the forces it is directly exposed to. Arthritis causes pain, swelling, stiffness, deformity and sometimes instability.
	In a total knee replacement, the surgeon resurfaces the worn out condyles at the bottom end of the femur with a matching metal part and resurfaces the condyles of the upper end of the tibia by a plastic (polyethylene) lined biconcave metal plate. The back of the patella may or may not be resurfaced by a plastic component, depending on how badly it is affected by the arthritis. The various components can be cemented in place with a filler or grout similar to dental cement, or securely pressed into place without cement, so the bone grows in to anchor the components.