Implants for Knee Replacement

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For simplicity, the knee is considered a hinge joint because of its ability to bend and straighten like a hinged door. In reality, the knee is much more complex because the surfaces actually roll and glide as the knee bends.. Newer implant designs, recognizing the complexity of the joint, attempt to replicate the more complicated motions and to take advantage of the posterior cruciate ligament (PCL) and collateral ligaments for support.

The decision to use an implant type is based on the need of the patient and the habituation of the surgeon. I prefer sticking to one or two implant instrumentations only.

Implant Components

Total knee replacement is essentially a surface replacement. three bone surfaces are replaced during the total replacement of the knee: the lower ends (condyles) of the thighbone, the top surface of the shinbone, and the back surface of the kneecap. Components are designed so that metal always articulates against plastic, which provides smooth( low friction) movement and results in minimal wear.

Femoral Component

The metal femoral component curves around the end of the thighbone and has an interior groove for the patella (knee cap) can move up and down smoothly against the bone as the knee bends and straightens. Usually, one large piece is used to resurface the end of the bone. If only one side of the thighbone is damaged, a smaller piece may be used (unicompartmental knee replacement) to resurface just that part of the bone. Some designs, called posterior stabilized, have an internal post with a center cam. This works with a corresponding tibial component to prevent the thighbone from sliding forward too far on the shinbone when you bend the knee. Designs which do not have the center cam are called cruciate-retaining.

Tibial Component

The tibial component is a flat metal platform with a polyethylene cushion. The cushion may have either a flat surface (cruciate-retaining) or a raised surface with a center cam (posterior-stabilized).

Patellar Component

The patellar component is a dome-shaped piece of polyethylene that duplicates the shape of the patella (kneecap).

Fixed- vs. Mobile-Bearing Prosthesis
Fixed-Bearing Prosthesis

Most people get a fixed-bearing prosthesis.

• Femoral: a polished, strong metal shell on the lower end of the thighbone.
• Tibial: a high-density polyethylene piece on top of a metal tray.
• Patellar: a high-density polyethylene piece replacing the underside of the kneecap in the center of the knee.

In certain cases, excessive activity and extra weight can accelerate the process of wear to parts of a fixed-bearing prosthesis, causing it to loosen from the bone and become painful. Loosening is a major reason some artificial joints fail. If you are younger, more active and/or overweight, sometimes a doctor may recommend a rotating platform/mobile-bearing knee replacement designed for potentially longer performance with less wear.

Mobile-Bearing Prosthesis

Like fixed-bearing replacements, mobile-bearing knees use three components to provide a relatively natural and even interface. The difference between them is in the bearing surface. In a mobile-bearing knee, the femoral component and tibial tray move across a polyethylene insert to create a dual-surface articulation. This helps reduce the amount of wear to the bearing and helps prevent loosening in places where the prosthesis attaches to bone. Mobile-bearing knees are also designed to allow greater rotation of the knee.

Advantages.

Mobile-bearing knee replacements can reduce early wear failure caused by high contact stress and early loosening failure caused by over-constraint. The insert's mobility ensures congruentcontact between the femoral and tibial components and conformity of the surfaces that move together when you bend and rotate your knee during activity. The mobile-bearing insert lets you move the knee from both the thighbone and shinbone. You can also rotate the shinbone slightly.

Disadvantages.

Compared with fixed-bearing designs, mobile-bearing knee implants are less forgiving of imbalance in soft tissues. They may increase the chance of dislocation and may cost more than fixed-bearing implants.

Implant Construction

The metal parts of the implant are made of titanium- or cobalt/chromium-based alloys. The plastic parts are made of ultrahigh-density polyethylene. All together, the components weigh between 15 and 20 ounces, depending on the size selected. The construction materials used must meet several criteria:

• They must be biocompatible; that is, they can function in the body without creating either a local or a systemic rejection response.
• Their mechanical properties must be able to duplicate the structures they are intended to replace; for example, they are strong enough to take weightbearing loads, flexible enough to bear stress without breaking, and able to move smoothly against each other as required.
• They must be able to retain their strength and shape for a long time. The chance of a knee replacement lasting 15 to 20 years is about 95 percent.
  Implant Wear

To date, manmade joints have not solved the problem of wear. Every time bone rubs against bone, or metal rubs against plastic, the friction creates microscopic particulate debris. Just as wear in the natural joint contributed to the need for a replacement joint, wear in the prostheses may eventually require a second (revision) surgery.

Implant Insertion

During a TKA, the knee is in a bent position so that all the surfaces to be replaced can be exposed. The usual approach is lengthwise over the front of the knee, or just to the inside of the kneecap. After the incision is made, the large quadriceps muscle and the kneecap are moved to the side to reveal the bone surfaces.
After taking several measurements to ensure that the new implant will fit properly, the surgeon begins to smooth the rough edges of the bones. Depending on the type of implant used, the surgeon may begin with either the thighbone or the shinbone.

Special cutting guides are used to accurately trim the damaged surfaces at the end of the thighbone. The devices shape the end of the thighbone so it configures to the inside of the prosthesis. The shinbone is cut flat across the bone and a portion of the bone's center is drilled out. The surgeon removes just enough of the bone so that when the prosthesis is inserted, it recreates the joint line at the same level as prior to surgery. If any ligaments around the knee have contracted due to pain and deformity before the surgery, the surgeon carefully releases them so that they function as close to the normal state as possible.

The prostheses are inserted, tested and balanced. The surgeon wants to be sure that the joint line is in the right place and the kneecap is accurately aligned for proper joint movement. If it is necessary to resurface the kneecap, the surgeon will apply a shaped piece of polyethylene that maintains the original width of the kneecap.
The knee replacement may be "cemented," "cementless," or "hybrid," depending on the type of fixation used to hold the implant in place. Although there are certain general guidelines, each case is individual and your surgeon will evaluate your situation carefully before making any decisions. Do not hesitate to ask what type of fixation will be used in your situation and why that choice is appropriate for you.