Broadly speaking, there are four basic categories of knee replacements depending on the degree of mechanical stability provided by the design of the artificial knee:
The highly successful non-constrained implant is the most common type of artificial knee. It is termed non-constrained because the artificial components inserted into the knee are not linked to each other and have no stability built into the system. It relies on the person's own ligaments and muscles for stability. This is the key feature of this group of artificial implants helping to maintain the stability of the knee.
The semi-constrained implant is a device that provides increasing stability for the knee. This type of artificial knee has some stability built into it. It is used if the surgeon needs to remove all of the inner knee ligaments(some surgeons prefer to do this), or if the surgeon feels the new knee will be more stable with this type of implant.
Constraint or hinged variety implants are rarely used as a first choice of surgical options. In this case, the two components of the knee joint are linked together with a hinged mechanism. This type of knee replacement is used when the knee is highly unstable and the person's ligaments will not be able to support the other type of knee replacements. It is useful in the treatment of severely damaged knees particularly in very elderly people undergoing a revision replacement procedure. The disadvantage of this type of knee joint is that it is not expected to last as long as the other types.
A unicondylar knee replacement replaces only half of the knee joint. It is performed if the damage is limited to one side of the joint only with the remaining part of the knee joint being relatively spared. It is now possible for the surgeon to replace only that area of the knee joint which is severely damaged. However, even with only half of the joint destroyed, many surgeons prefer doing a total knee replacement believing this is a better procedure than the half-knee (unicondylar) replacement. But equally, there are surgeons who believe it is more appropriate to perform a unicondylar knee in the right circumstance.