Background: Prosthetic alignment and size are important factors in achieving a long-term survival in TKA. Although two-dimensional and three-dimensional (3-D) planning for component sizing has been introduced, it sometimes is difficult to cut the bones accurately according to preoperative planning. It is unclear whether changing sagittal alignment of the distal femur affects the AP dimension and sizing of the prepared bone. Questions/Purposes: We therefore determined whether the AP dimension of the prepared distal femur increases if the distal femur is cut in extension and decreases if it is cut in flexion. Methods: One hundred knees were evaluated using 3-D imaging software. The AP dimension of the cutting surface was measured when the femoral component was aligned perpendicular to the anatomic axis. The measurement was repeated when the distal bone cut was planned in flexed positions of 3° and 5° and extended positions of 3° and 5°. Results: The AP dimension of the prepared femur was increased by 2 and 3 mm with 3° and 5° extension, respectively. The AP dimension of the prepared femur was decreased by 2 and 3 mm with 3° and 5° flexion, respectively. Conclusions: Our data suggest upsizing or downsizing of the femoral component can occur if the femoral osteotomy is performed in at least 3° extension or flexion. Level of Evidence: Level II, diagnostic study. See Guidelines for Authors for a complete description of levels of evidence.
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