Fixed-bearing and mobile-bearing implants are the two major types of knee prosthetics that facilitate total replacement, but each FDA-approved manufacturer offers specific levels of flexion, according to Healthline. Two important subcategories are cruciate-retaining and posterior-stabilized implants, and choosing between them depends on whether the patient’s cruciate posterior ligament is preserved or removed. For partial knee implants, some prosthetics are unicompartmental devices designed to fit one knee section, while modular designs allow surgeons to choose which components to implant.
In a healthy knee, the bottom of the femur rests against the top of the tibia, and the joint is held in place by supportive ligaments and cartilage that enable flexible gliding motion, as the American Academy of Orthopaedic Surgeons explains. Full-knee prosthetics typically provide protective components for the femur, tibia and patella, or kneecap. In a fixed-bearing implant, a polyethylene insert attaches to the tibial component, providing a fixed and cushioned platform for the femoral component to glide upon. In a mobile-bearing implant, the polyethylene insert has a small degree of independent rotation to increase the joint’s flexion. The latter system may be more expensive, and it is only suitable for patients with enough undamaged soft tissue to support the prosthetic.
As of 2015, doctors use two major methods of fixation, cemented and cementless, as Healthline describes. A cemented prosthetic relies on a bonding agent to stay attached to the bone, while a cementless device stimulates bone growth that helps hold the prosthetic in place. Gender-specific prosthetics are also available to accommodate the different size proportions of men and women, but doctors debate whether there is any difference in effectiveness.