Proper positioning of the joint line is essential for normal kinematics. The distal femoral alignment guide is applied and further stabilized by dialing the medial or lateral adjustable screw down to the defective distal femoral condyle. If both condyles are defective (i.e., with rheumatoid arthritis), both adjustable screws are dialed down slightly to compensate for the lost cartilage (2 to 3 mm). This maneuver avoids elevation of the joint line.
If the patient has normal proximal tibial varus, which ranges from 0 to 6 degrees,32 it is preferable to make a 2-degree varus cut to allow resection of a more symmetrical wedge of proximal tibia. This will significantly improve soft tissue balancing and allow for proper orientation of the joint line. A caliper is used to measure the resected tibia in areas of relatively normal cartilage. Adding 1 mm to this measurement for bone loss from the saw blade will predict the thickness of the tibial replacement.
Before making any bone cuts, the maximum thickness of the patella is determined using a caliper. The total patellar resection should equal the thickness of the patellar insert, except in cases of severe patellar wear. Increasing the overal thickness of the patella-prosthesis construct will increase the patellofemoral joint forces and cause tracking problems and excessive wear, necessitating a lateral release. For improved fixation of the patella, countersinking the 10 mm components 2 to 3 mm is a routine procedure in our clinical practice.
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