There are many factors involved in choosing the right implant for each patient. The goal is to use the implant that can give the most motion and function without losing stability. Some implants have more constraint or limits than others.
The condition of the soft tissues in and around the joint is a big factor in implant selection. The surgeon must try and balance these structures to allow as much flexion and extension as possible.
Sometimes the ligaments are torn or the cartilage is damaged. The surgeon may try to save these structures. Or it may be necessary to remove them. Certain knee implants are designed based on whether the ligaments are present or absent.
The patient's activity level can also make a difference. Implants with greater constraint puts more stress and load on the implant-bone interface. A very active person may experience implant loosening and failure with this type of joint replacement.
There are other things the surgeon must think about. It makes a difference if the patient is bow-legged or knock-kneed. If the kneecap must be removed (or has already been taken out), the load on the ligaments increases. Choosing the right implant is a challenging and important task for any surgeon.