New Technique for Converting From Unicompartmental to Total Knee Replacement

This is the case of one 83-year-old woman who had a unicompartmental knee replacement that failed and had to be removed and replaced with a total knee replacement. The surgeons who did the revision surgery describe their technique. It is designed to help place the new implant with the right amount of rotation to prevent future uneven wear and tear.

The authors provide a bit of history about this patient, include X-rays of the unicompartmental implant, and describe their surgical technique for the conversion. Color photos taken during the procedure offer a step-by-step visual aid in understanding what they did.

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.

Since the time that unicompartmental replacements were first used, the design and surgical technique have improved quite a bit. The result has been an implant that lasts almost as long as the complete knee replacement. That's good news for most patients.

But problems can develop as this case shows. The patient had significant osteoarthritic changes along the lateral side of the knee joint. That's the side away from the other knee. With only one side affected by the degenerative changes, a unicompartmental knee replacement was a good idea. But three years later, she had such pain and disability that the decision was made to convert her to a total knee replacement.

Conversion 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.

The authors offer one way to avoid these problems during conversion. They suggest leaving 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 authors encourage other surgeons to avoid trying to even the two ends of the femur (thigh bone). Instead, bone graft can be used to lengthen the unicompartmental side if it is too short. As in this case, 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.

In the procedure described, the bone on the tibial side (upper end of the lower leg bone) was measured and removed next. Again, the surgeon urges other surgeons to cut as little bone away as possible. They say don't try to create an even or level surface from one side of the joint to the other. 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.

The authors also give alternate ways to measure femoral rotation during this part of the procedure. 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.

Although there aren't studies to compare this surgical technique with others, this case study showcases the technique described. Other surgeons may find this method helpful in judging rotation of the femoral part of the implant when converting from a unicompartmental to total knee replacement.

Reference: 

Thomas F. Moyad, MD, and Jeffery Marxen. Failed Unicompartmental Knee Replacement: A Useful Technique for Assessing Proper Rotation During Subsequent Conversion to Total Knee Arthroplasty. In Current Orthopaedic Practice. July/August 2011. Vol. 22. No. 4. Pp. 364-369.

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