The Normal Joint
From the point of view of arthritis, the most important thing about the normal joint is the articular cartilage that covers the surface of the bone within the joint. The two bones either side of the joint are joined together by a joint capsule. The surfaces of the bone that would be exposed within this joint capsule are covered in articular cartilage. This cartilage allows the two sides of the joint to articulate together with very low friction. It receives its nutrition from the fluid within the joint.
The Worn Joint
With age the quality of the cartilage will naturally deteriorate. Arthritis occurs when the joint becomes inflamed as the cartilage wears and eventually will allow the bone to be exposed. The arthritis will become increasingly symptomatic as more bone is exposed.
What is Joint Replacement?
A joint replacement is an operation which removes the failing articular cartilage from an arthritic joint and to replace it with an appropriate bearing surface. Joint replacement is perhaps most advanced in hips and knees although is now offered in several other joints around the body. In all joints the same four issues exist.
A joint replacement is an operation which removes the failing articular cartilage from an arthritic joint and to replace it with an appropriate bearing surface. Joint replacement is perhaps most advanced in hips and knees although is now offered in several other joints around the body. In all joints the same four issues exist.
Fixation
It is important that the replaced joint is adequately fixed. This normally means that the bearing surface has to be mounted on a prosthesis which goes into the bone and is either anchored there by bone cement or is of such a texture that bone can grow into the implant.
The bearing surface
The bearing surface has to be able to move through a satisfactory range of movement without the joint wearing. It is known that as the components wear they produce debris and this debris may cause a response from the body which would be detrimental to the success of the joint.
Stability
The two components of a joint replacement must articulate with each other. Often the movement in a joint is quite complex. The knee for example does not work as a simple hinge but also has to slide and rotate as well. To achieve this with a joint replacement it is not possible to constrain the components (joining together). This would produce forces that would tend to loosen the implants. The joint replacement therefore needs to be designed to allow one component to be mobile on the other but yet remain stable. The problems with stability are most commonly seen in the hip where the two components may come apart from each other or dislocate.
Durability
Success in joint replacements is now measured in tens of years. We now know for example that some hip replacement systems have a 97% success rate at 18 years. The National Institute of Health and Clinical Excellence have stated that a success rate below 90% survival at 10 years is unacceptable in a joint replacement system.
