Abstract
Performance and durability of total knee arthroplasty is optimised when bone surfaces are prepared with the knee in neutral varus-valgus alignment in the anteroposterior (AP) plane. For the femur, this means resecting the surface perpendicular to the mechanical axis of the femur, which passes through the center of the femoral head and center of the knee. Because the center of the femoral head is not a reliable landmark during the operation, the distal femoral surface can be resected at 5 degrees valgus to the long axis of the femur using an intramedullary (IM) alignment rod to establish the position of the femur's long axis. The IM rod also provides the landmark for alignment of the femoral component in the flexion-extension position. Tibial alignment is established by cutting the upper surface of the tibia perpendicular to the long axis. An IM rod is not necessary for alignment since the ankle is accessible for reference. An extramedullary (EM) rod easily can span the distance between the centers of the tibial surface at the knee and ankle to establish a reference for upper tibial surface resection via the long axis of the tibia. In cases with femoral deformity or bone disease that prevents use of an IM rod as a landmark for the long axis of the femur, computer-assisted alignment can be helpful to establish the mechanical axis of the femur and to determine the level of resection of the femoral surface to create a plane that is perpendicular to the mechanical axis of the femur and positioned to place the joint surface at the correct level. Whereas this can be done with CT scan or MRI imaging and robotic instrumentation, the cost in time and money is substantial. Rather, plane film radiographs can be used along with intra-operative measurements and hand-held tools that are readily available in the standard total knee instrument set.
Using an AP radiograph taken to include the femoral head and knee: Mark the centers of the femoral head and knee. Draw a line to connect the centerpoints. Mark the high points of the medial and lateral femoral condylar joint surfaces. Draw a line perpendicular to the mechanical axis that crosses the mark on the high point of the most prominent femoral condyle. This line marks the position and alignment of the femoral implant surface. Next, measure the distal thickness of the femoral component and add 10% to account for magnification of the radiograph. Draw a parallel line this distance proximal to the femoral surface line. This is the femoral resection line. Less than the thickness of the implant will be resected from the least prominent condyle. On the low side, measure the thickness of bone to be resected and the distance between the bone surface and distal surface line. Insert a threaded pin into the bone surface with the measured distance protruding from the surface to set this position. Seat the distal femoral cutting guide against the protruding pin and against the surface of the femur on the high side. Resect with the cutting guide fixed perpendicular to the long axis of the femur. This resects the thickness of the implant from the prominent side and resects the prescribed amount from the low side to set the distal cut perpendicular to the mechanical axis of the femur. Draw the AP axis from the center of the intercondylar notch posteriorly to the deepest point of the patellar groove, and use the combined cutting guide to finish the femur. Make the anterior, posterior, and bevel cuts perpendicular to the AP axis. Finally, align the tibial surface, with an IM or EM rod, to resect perpendicular to the long axis of the tibia in the AP plane and sloped 4 degrees posteriorly in the lateral plane. Once the bone surfaces are resected at the proper angle, insert the trials or spacer blocks and finish the arthroplasty with release of tight ligaments.
