In a normal knee, four ligaments help hold the bones in place so that the joint works properly. When a knee becomes arthritic, these ligaments can become scarred or damaged. During knee replacement surgery, some of these ligaments, as well as the joint surfaces, are substituted or replaced by the new artificial prostheses. Two types of fixation are used to hold the prostheses in place. Cemented fixation uses a fast-curing bone cement (polymethylmethacrylate) to hold the prostheses in place. Cementless fixation relies on bone growing into the surface of the implant for fixation.
Most knee replacements done today are cemented into place. Cemented fixation has a generally excellent track record and may last more than 20 years. The longevity and performance of a knee replacement depends on several factors, including activity level, weight, and general health.
Cemented fixation relies on a stable interface between the prosthesis and the cement as well as a solid mechanical bond between the cement and the bone. Metal alloy components rarely break, but they can occasionally come loose from the bone. Two processes, one mechanical and one biological, can contribute to loosening.
- During natural movement, the knee is subject to considerable loads and stresses, which the prostheses must transfer to the underlying bone. Because the hard subchondral bone of the shinbone (tibia) is removed during a knee replacement, loads are absorbed by the softer cancellous bone and the peripheral cortical bone that remains. If loads are heavier than the underlying bone can bear over a long period, the prosthesis will begin to sink into or loosen from its attachment to the bone. Additionally, if the load applied to the knee during walking is uneven, one side of the implant may lift off the bone as the other side is pressed into it, resulting in uneven wear of the polyethylene liner between the metal components. This wear creates debris particles of polyethylene that can trigger a biologic response and further contribute to loosening of the implant and sometimes to bone loss around the implant.
- The microscopic debris particles are absorbed by cells around the joint and initiate an inflammatory response from the body, which tries to remove them. This inflammatory response can also cause cells to remove bits of bone around the implant, a condition called osteolysis. As wear continues, so does the bone loss. The bone weakens, and the loosening of the implant from bone increases. Despite these recognized failure mechanisms, the bond between cement and bone is generally very durable and reliable. Cemented fixation has been used successfully in all patient groups for whom total knee replacement is appropriate, including young and active patients with advanced degenerative joint disease.
In the 1980s, implants were introduced that were intended to attach directly to bone without the use of cement. These implants have a surface topography that is conducive to attracting new bone growth. Most are textured or coated so that the new bone actually grows into the surface of the implant. They may also use screws or pegs to stabilize the implant until bone ingrowth occurs. Because they depend on new bone growth for stability, cementless implants require a longer healing time than cemented replacements. Some cementless total knee replacement implants have been as successful as cemented implants in relieving pain and restoring function.
Cementless implants, however, have not solved the problems of wear and bone loss. In all knee replacement implants, metal (usually a titanium- or cobalt/chromium-based alloy) rubs against ultra-high-density polyethylene. Even though the metal is polished smooth and the polyethylene is treated to resist wear, the loads and stresses of daily movements will generate microscopic particulate debris. This debris, in turn, can trigger the inflammatory response that results in osteolysis.
Because cementless implants have not been used as long as cemented implants, comparisons of long-term use is not possible. However, short-term outcome studies have shown that cementless fixation has success rates comparable to those of cemented fixation.
In a hybrid fixation for total knee replacement, the femoral component is inserted without cement, and the tibial and patellar components are inserted with cement. This technique was introduced in the early 1980s; long-term results are just now being measured and are generally positive.
Knee replacement operations, whether they use cemented or cementless fixation, are highly successful in relieving pain and restoring movement. However, the ongoing problems with wear and particulate debris may eventually necessitate further surgery, including replacing one or more parts of the knee replacement (revision surgery).
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