According to the American Academy of Orthopedic Surgeons (AAOS), approximately 500,000 knee replacement procedures are performed each year in the United States. This number is only expected to increase as the general population continues to age, and as more people are undergoing this procedure at earlier stages in life. Knee replacement surgery is the surgical removal and the resurfacing of arthritic joint surfaces that comprise the knee joint. To the person with chronic, end-stage arthritis, knee replacement is an opportunity for improved mobility.
Who Needs a Knee Replacement?
When the degenerative arthritic process becomes so severe that the ability to move about without pain, together with increased mechanical difficulties that could lead to falling (often called end-stage arthritis), a person becomes a prime candidate for joint replacement.
Arthritis is said to be end-stage when all other conservative measures have failed to give meaningful relief, and there is documented evidence, typically through x-ray, MRI and physical evaluation, that the joint surfaces have been irreparably damaged.
Partial Knee Replacement
Individuals who have localized, but severe degenerative arthritis, perhaps the result of a prior injury, may qualify for an abbreviated version of knee joint replacement called the partial, or uni-compartmental, knee replacement. This procedure is typically performed on somewhat younger patients, whose knee joint, in general, is healthy, with the exception of a focal area of severe degeneration that has become a hindrance to normal quality living and activity.
Partial knee replacements typically involve the replacement of only one portion of the knee joint, while leaving everything else alone. Since the knee is technically divided into three compartments; the patello-femoral (kneecap, thigh bone), lateral compartment (outside portion) and the medial compartment (inner, inseam side), the procedure is also called uni-compartmental, referring to the singular compartment that is replaced.
Total Knee Replacement
Total knee replacement refers to knee replacement surgery where all of the articulating surfaces (those surfaces that come in contact with opposing joint surfaces to create the surfaces necessary for motion) are replaced, or resurfaced, with artificial components. A total knee replacement procedure is performed when there is evidence of end-stage arthritis with almost complete loss of joint cartilage, and underlying bone surfaces rubbing together.
The femoral condyles, or knuckles, of the femur are shaved down to a particular configuration that will accept the implant. Then, the tibial (shin bone) surface is shaved to a smooth and flat nature, allowing it to accept its new implant. Following the tibial resurfacing , the under-surface of the patella (knee cap) is prepared in the same way.
Once all surfaces of the host knee are prepared, a dry fit is performed to verify size, tension, joint balance and flexibility. If all aspects of the fitting are acceptable, the new parts are permanently affixed to the host bone in one of two methods: cemented or press-fit.
Implant Materials
For the most part, joint replacement implants are manufactured using a variety of space-age metals, such as titanium or cobalt chrome. These metals are designed and used for longevity of the implant, durability and light weight.
Typically, the metallic surfaces are separated bay a spacer of sorts, commonly called the articulating surface. It is manufactured from ultra-high-molecular-weight polyethylene, or more simply put, space-age plastic. The plastic spacers come in a variety of thicknesses and configurations so that proper soft tissue tension and joint stability can be accurately restored, and it prevents the metallic surfaces from rubbing together. The implant used on the under-surface of the patella (knee cap) is the same plastic material and may also have a metallic base to allow for press-fit application.
Bone Cement
The most common method of permanently fixing the knee replacement implants onto the patient's host bone is with bone cement. The cement is placed on the backside of each implant, and some on the bone surface itself. The implant is then inserted onto the bone and pressed and held into place until the cement has hardened.
Because bone cement has no adhesive properties, it acts more like a grout that a glue, relying upon microscopic fixation to the small spaces in the bone marrow and less-than-smooth surfaces on the back side of the implants.
Press-Fit Fixation
Another method of permanently implanting knee replacement components is the press-fit method. Many cement-less implants are pressed into place on the cut host bone surfaces under significant pressure so the fit is extremely tight. Additionally, the back side of the implants are coated with a rough, gritty surface called a porous coating. This porous coating, when pressed into place under significant pressure, can stimulate the host bone to literally grow into the microscopic crevices on the implant, effectively healing itself to the implant.
Post-Surgery Care
The procedure typically lasts between 1 and 2 1/2 hours and the hospital stay is around two or three days. During the stay, and in addition to ice and medications, physical therapy is administered daily in an effort to reduce pain and swelling, as well as increase joint mobility, muscle strength and protected weight bearing.
Use of a walker or crutches can last several days to a couple of weeks, and there is a gradual progression to a cane and eventually no supportive devices. Out-patient therapy may last two to four weeks, depending upon the need and rate of progression. It make take six to 12 months before activities are felt to be normal.
