Total knee replacement implants are not one-size-fits-all or even one-style-fits-all. Prosthetic implants vary greatly by design, fixation and materials.
Your implant may be a fixed- or mobile-bearing implant; a PCL-retaining design or a PCL-substituting style. It may be fixed with bone cement or could be a cementless fixation design. The choice of implant will ultimately be made by your surgeon, based upon your physical situation, your age and lifestyle, the surgeon’s experience and the level of familiarity and trust he or she has with a specific piece of equipment and/or manufacturer.
Total Knee Implant Components: Although the overall designs of total knee implants vary, as discussed below, typical total knee replacement implants have three basic components: femoral, tibial and patellar.
The most common knee replacement implant is referred to as a fixed-bearing implant. It is referred to as “fixed” because the polyethylene cushion of the tibial component is fixed firmly to the metal platform base. The femoral component then rolls over this cushion.
The fixed-bearing prostheses provide a good range of motion and just as long lasting as other implants for most patients. In some cases, excessive activity and/or extra weight can cause a fixed-bearing prosthesis to wear down more quickly. This wear can cause loosening of the implant, causing pain and joint failure – though this is not common.
If you are younger, more active, and/or overweight, your doctor may recommend a rotating platform/mobile-bearing knee replacement. These implants are designed for potentially longer performance with less wear.
The difference between a fixed-bearing implant and a medial-pivot implant is in the bearing surface. Fixed-bearing implants and mobile-bearing implants use the same three components. In a mobile-bearing knee, however, the polyethylene insert in the tibial component can rotate short distances inside the metal tibial tray.
This rotation allows patients a few degrees of greater rotation to the medial and lateral sides of their knee.
Because of this mobility, mobile-bearing knee implants do require more support from the ligaments surrounding the knee. If the soft tissues are not strong enough, though, the knee is more likely to dislocate. Mobile-bearing implants may also cost a bit more than fixed-bearing implants.
There are no studies and no consensus that indicate that mobile-bearing implants have better durability, improvement in pain, or improvement of function than a fixed-bearing design.
The Medial-Pivot knee replicates the rotating, twisting, bending, flexion, and stability of your natural knee, so it feels more like your natural knee. A normal knee actually pivots on its medial (inner side) condyle. When the knee flexes, the lateral (outer side) side rolls back while the medial side rotates in one place. This design “stays put” or is more stable during normal knee motion as opposed to sliding forward slightly.
However, compared with Fixed-Bearing designs, Mobile-Bearing knee implants are less forgiving of imbalance in soft tissues. They may cost more than Fixed-Bearing implants.
The Posterior Cruciate Ligament is one of the major ligaments in the knee. It provides support and stable movement of the knee, preventing the femur from rolling back on top of the tibia when flexed. Depending upon its condition, the type of knee implant and the surgical approach of your surgeon, this ligament can be kept or removed during total knee replacement surgery.
Posterior Cruciate Retaining: In PCL Retaining designs, rearward movement of the tibia continues to be resisted by an intact PCL, which creates stability. The femoral and tibial prostheses have notches to accommodate the ligament and the plastic insert also has a flat central surface.
Posterior Cruciate Substituting: PCL Substituting knees (also called posterior stabilized knees) have raised surface on the tibial component cushion with a raised sloping cam or post which compensates for the missing PCL to give your knee stability in the absence of this ligament. This “cam and post” interaction mimics the normal function of the PCL by limiting the forward roll of the femur over the tibia and supporting backward movement of the femur as you bend or flex your knee.
Again, the choice of implant is generally made by your surgeon, based upon several factors – some of which pertain to you and your circumstances, others of which pertain to the surgeon himself (or herself) and the level of familiarity and trust with a specific piece of equipment. The patient though is well-advised to have a working understanding though of the various options in order to understand the recommendations made and the reasons behind recovery and discharge instructions.
The other big difference between types of knee replacements is fixation. There are cemented, cementless, and hybrid (combination of cemented and cementless) designs.
Cemented prostheses utilize a special kind of bone cement that helps hold the components of the artificial joint in place. The majority of knee replacements are generally cemented. Cemented knee replacements have been used successfully in all types of knee replacement patients. Historically, some younger, more active patients had problems with loosening cement but the material is now much improved.
Cementless implant designs become fixed by bone growth into the surface of the implant. Most implant surfaces are textured or coated with a porous material so that the new bone actually grows into the surface of the implant. Screws or pegs may also be used to stabilize the implant until bone ingrowth occurs. Recovery does generally take a bit longer as the bone grows into the prosthesis. Cementless implants are not usually a viable for patients with osteoporosis, as the existing bone must be in good shape in order to grow into the implant.
Some surgeons also use a hybrid of the two methods of fixation, though this is more common in hip replacement surgery.
The good news is that your surgeon will make the decision regarding which implant to use, but talk with your him or her about which implant will work best for your body and lifestyle. Although the decision is ultimately your surgeon’s, you are well-advised to have a working understanding though of the various options in order to understand the recommendations made as well as the reasons for your recovery and discharge instructions.
Partial knee replacement (PKR) or resurfacing is necessitated when the arthritic damage to the joint is confined to one, two, or even three compartments of your knee: (1) Medial: the inner side of the joint next to the other leg; (2) Lateral: the outer side of the joint and (3) Patellofemoral: the area between the knee cap and the upper front surface of the femur (thigh bone). There are prosthetic implants designed to replace each arthritic compartment.
A unicompartmental knee system retains the healthy compartments of your knee joint and only replaces the damaged surfaces of one compartment. The implants can either be stock items in a range of sizes or custom made from MRI and computer generated scans.
There are three areas within the knee served by partial replacements
Generally, partial knee implants are embedded in shallow, prepared areas in the bone and secured with cement. Custom made implants may also be secured with cement but bone preparation is minimal and not to the extent that standard implants require.
Both medial and lateral types of partial knee replacement have plastic devices inserted onto the tibial component which serve as a spacer and provide smooth articulation or bearing between the two metal devices. The plastic is designed to replace the semi-lunar cartilages in the knee that act as shock absorbers and aid the activity of the knee joint which is why they have a tendency to get damaged during intense sports activities such as football, rugby and ice hockey.
Most partial knee implants are metal-on-plastic. This means that a metal implant caps the femur and the tibia. Between those metal implants, your surgeon places a plastic insert or spacer which may be fixed or mobile. The most common method of fixation of both implants to the bone is with bone cement. Fixed spacers are press-fitted into the tibial tray and the femoral component glides across the surface.
In joints with mobile bearings, the plastic spacer isn’t fixed but allowed to move back and forth with the action of the joint. Many believe that this allows greater freedom of movement in the joint and also reduces the amount of wear that could take place. However, traditional PKRs retain good movement even in joints with fixed bearings and demonstrate a limited history of wear.
Where there is a small area of damage in the knee, it may be possible to resurface just that point with plastic implants but the data on outcomes is limited.
Patients with arthritic changes in more than one compartment are not generally considered candidates for partial replacements. In such cases, a total replacement is recommended as there will be considerable erosion and deformity on both sides of the knee joint and behind the patella.
Long term results for partial replacements are still the subject of some debate. Some surgeons are very committed to the procedure and even recommending that if either or both the other compartments develop problems, they too can be resurfaced so the patient ends up with a tri-compartmental resurfacing instead of a total joint replacement. Other surgeons decline to perform partial replacements or have given up doing them because of their experience of a high rate of conversion to total replacement. There is, though, sufficient evidence of long-lasting partial replacements to make PKR a viable option for many patients.
As in every orthopedic situation, it is best for a patient to consult with his or her knee surgeon to determine whether or not a specific situation necessitates partial knee replacement, total knee replacement or a more conservative treatment.
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