Constraint implants are linked together with a hinged mechanism. This type of knee replacement is used when the knee is very unstable. The patient's ligaments won't support the other type of knee replacements.
Surgeons choose the constrained implant in cases of severely damaged knees. Sometimes it's used when an elderly adult is having a second joint replacement on the same knee. The disadvantage of this type of knee joint is that it doesn't last as long as the other types.
A non-constrained implant is the most common type of knee joint replacement. Non-constrained means the artificial parts are inserted into the knee but aren't linked to each other. There's no stability built into the system. The patient uses his or her own ligaments and muscles for stability.
Two other types are the semi-constrained and unicondylar knee replacements. The semi-constrained implant has some stability built into it. It's used when the surgeon has to take out all of the inner knee ligaments. Some surgeons use it when they feel the new knee will be more stable with this type of implant.
The unicondylar implant just replaces half of the joint. This type is used when only one side of the joint is damaged or worn down.