Knee Arthroplasty Implants
Knee replacement surgery is one of the most commonly performed surgery by orthopedic surgeons. The surgery offers relief from knee pain and increases mobility. Common causes for knee pain include arthritis caused by primary osteoarthritis, rheumatoid arthritis, psoriatic arthritis, and secondary arthritis. The surgery is performed when all conservative forms of management are exhausted.
A total knee replacement involves the replacement of the ends of the bones forming the knee joint with artificial parts. The prosthetic parts are made of metal and plastic components. The type of implants used in the surgery depends upon the anatomy of the patient, patient’s medical conditions, age, physical activity, and surgeon’s expertise.
During the surgery, the joint is opened from the front and the damaged bone along with cartilage is removed. Bone cuts are made with specially designed jigs to prepare the joint surface. Both the anterior and posterior cruciate ligaments are usually sacrificed in posterior stabilized implants. The medial and lateral collateral ligaments are retained and surgically released to balance the alignment.
The metal parts are then introduced and are glued with acrylic bone cement. A special high-grade plastic (Polyethylene) is inserted between the metal parts acting as a stabilizer and cushion. The metal parts are made of a cobalt-chromium alloy and are designed to allow smooth gliding. The undersurface of the patella is resurfaced and a special plastic is cemented to the undersurface.
The joint is thoroughly irrigated, cleaned, and closed. Intraoperative medications may be injected to reduce post-op pain. The patient is shifted to the recovery room where their vitals are closely monitored.
The biomechanics of the knee joint are complicated rather than simply bending and straightening. Movements such as rollback and gliding occur in a natural knee to provide stability.
Ideally, any prosthetic implant replacing the knee joint should have an anatomic femoral and tibial surface. The implant should be able to reduplicate the function of menisci and cruciate ligaments. The implant should be able to maintain contact throughout the normal range of motion. The implant should be stable through 110 degrees of flexion and full extension.
There are 3 compartments in the knee joint, the inner and the outer compartment and the patellar compartment. All three are usually replaced in a total knee replacement. However, some patients with limited arthritis may benefit from a unicompartmental or bicompartmental arthroplasty. The implant designs are made for the compartment there are to replace.
Constrained and Non-constrained Implants
The constraint is the ability of the implant to provide stability in the front-back and sideways. The ligaments, bone anatomy, and various soft tissues provide stability to the knee during motion. In advanced arthritis, there may be loss of the stabilizing structures causing instability. The constraints implants are used in severely unstable knees.
The most commonly used implants are non-constrained, posterior-stabilized (PS), and cruciate-retaining (CR).
Posterior cruciate ligament retaining (CR) implants utilize the native PCL to provide stability in flexion and extension. The native PCL also allows for some femoral rollback, but because the ACL (anterior cruciate ligament) is sacrificed, the rollback is not anatomical.
The CR implant is used mainly in knees with minimal arthritis and soft tissue laxity. The implants require a functioning posterior cruciate ligament.
The posterior cruciate substituting (PS) also known as posterior stabilized implants are more commonly used than CR. The implant design includes a special post on the polyethylene insert and a cam mechanism on the femoral component. The cam-post mechanism prevents the thigh bone from dislocating forward when straightening the knee.
The above images show different types of polyethylene inserts. The cruciate retaining and medial congruent polyethylene inserts lack a tibial post as the native posterior cruciate ligament is intact. The posterior stabilized and constrained polyethylene inserts have a tibial post for providing femoral rollback. The constrained insert has a tall and wide tibial post which provides small amount of side to side and rotational stability.
The cam-post mechanism also allows femoral rollback that helps in bending the knee without impingement. The cam-post mechanism, however, does not provide any sideways stability and requires the integrity of the stabilizing soft tissues.
In patients with a sideways instability, implant designs are modified to include a larger central post. The wider post substitutes for the medial collateral ligament (MCL) and the lateral collateral ligament (LCL). These are known as constrained non-hinged designs. The implant design however requires more resection of the femoral bone.
Constrained hinged are implant designs that involve the femoral component connected with a bar to a rotating platform of the tibia. These are used in cases of severe knee instability on all the sides of the knee.
A mobile-bearing polyethylene design includes a mobile bearing that is not attached to the tibial component. The mobile polyethylene can rotate freely on the tibial component. It may reduce the wear and tear of the polyethylene by increasing the surface area of contact. However, there may be a greater risk of dislocation of the mobile bearing implant designs.
The implants used in the knee replacement may be fixed to the bone with bone cement or maybe press-fit to the bone. The implants utilizing uncemented fixation have a special porous surface to allow bone ingrowth on the implant. Hybrid fixation involves an uncemented femoral component with a cemented tibial component. The patellar component is usually cemented due to high cases of failure with uncemented patellar fixation.
Successful knee replacement surgeries last on an average of 15 – 20 years or more. But like all mechanical machines, knee replacement implants may fail after many years. They may also fail due to other causes such as infections, misplacement, or trauma.
Modular implant designs are used in revision surgeries. The revision surgeries require the extraction of the old implants and implantation of the newer implants. The revision surgeries require implants with a longer stem to fit in the femur and tibia. The longer stem allows for greater stability and fixation of the implant.
The modular implants may also allow attachments for metal parts known as augments to substitute for bone loss during extraction. The revision designs also typically have a wider cam post mechanism to allow for greater stability.
Read more about Total Knee Replacement here.