Kinds of Knee Replacements
In a normal knee, four ligaments help hold the bones in place. Arthritis can cause these ligaments to become scarred or damaged. During knee replacement surgery, the ligaments and joint surfaces are replaced by a knee implant.
There are two common methods of holding a replacement knee in place.
Cemented Implants
Most knee replacements are cemented into place. A fast-curing bone cement attaches the metal to the bone. By filling all the spaces, the cement allows the implant to fit perfectly to the irregularities of the bone. This method has an excellent track record and may last more than twenty years. |
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Cementless Implants
Cementless implants were introduced to attach implants directly to bone without cement. The Zimmer NexGen CR-Flex Porous Femoral knee replacement system uses this method.
The NexGen CR-Flex relies on the bone naturally fusing with an implant mesh surface engineered to promote new bone growth. Bone is expected to actually grow into this surface. The surface coating layer is only a few millimeters thick, fast sintered to the surface of the
Bone tissue cannot grow into mesh openings that are very small, and will grow very slowly into openings that are too wide. The precise dimension of openings in the mesh coating varies with the manufacturer. |
| A surgeon "press fits" the implant against the bony surfaces to which it must attach. Faultless positioning of the implant is essential for attachment. Metal pegs and screws temporarily hold the implant in place. If there is insufficient "good bone" for attachment, new bone growth will be hampered. This can cause knee failure.
With time, bone tissue grows into the sponge-like surface of the knee implant. If everything goes well, it becomes an integrated part of the skeleton. This is called biological fixation. For some unknown reason, the Zimmer NexGen CR-Flex knee replacement may not allow adequate bone growth into its porous surface. The result can be a loosening of the knee. |
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