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Child Orthopedics
Spine - Cervical
Spine - Lumbar
Spine - Thoracic

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I'm going to have to change out my half-knee replacement for a full replacement. What kinds of problems can develop during this change over?

We are assuming by half-knee replacement, you are referring to a unicompartmental implant. Unicompartmental knee replacement (also known as a unicondylar knee replacement) is less invasive than a full knee replacement. The operation is designed to replace only the portions of the joint that are most damaged by arthritis. This can have significant advantages, especially in younger patients who may need to have a second artificial knee replacement as the first one begins to wear out. Removing less bone during the initial operation makes it much easier to perform a revision artificial knee replacement later in life. But problems can develop. Conversion from a unicompartmental implant to a total replacement can be a challenging procedure because the joint has been changed as a result of the first surgery. Anatomic landmarks the surgeon normally uses to line up the implant correctly aren't always there. Balancing the pull of the muscles and tendons around the joint can also pose some problems. There are ways for the surgeon to avoid these problems during conversion. Some surgeons leave the unicompartmental implant in place at first. Cuts necessary on the other side of the femur (medial in this case) are made first. The amount of bone removed to put the unicompartmental implant in is checked by the surgeon to see if it was over- or under-cut. Then the bone is removed on the opposite side of the unicompartmental implant. This step is called bone resection. It is designed to make room for the total knee replacement implant. The surgeon tries to avoid making the two ends of the femur (thigh bone) even with each other. Instead, bone graft can be used to lengthen the unicompartmental side if it is too short. A special tool called a distal femoral cutting jig can be used to remove just the right amount of bone from the end of the femur. Bone on the tibial side (upper end of the lower leg bone) is measured and removed next. Again, the surgeon tries to cut as little bone away as possible. A metal wedge can easily accomplish the same thing without compromising the bone and potentially weakening the tibia. The third step involves using a special sizing guide to judge the amount of external (outward) rotation of the implant as it is placed in the bone. Keeping the unicompartmental implant in place for this step makes it possible to get a better measurement of the rotation and any gap in the bone that will have to be filled in. When everything lines up properly and all the guidelines match, then the femoral side of the unicompartmental implant can be removed and the total knee replacement put in place. Fewer errors are made when this approach is used to create the right amount of external rotation of the implant and proper alignment of the joint. The surgeon will still have to check the muscular/tendon tension on the joint and make sure there is an even pull that mimics normal motion. Even balancing of the soft tissues is just as important as getting the right amount of rotation. Any surgery can result in infection, blood clots, or other complications. These are rare but they do happen. Ask your surgeon to go over the procedure with you and outline any problems that could arise. That will give you a chance to ask any other questions you may have.


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