Fundamental differences in implant design and the relative size of the flexion and extension spaces suggest that subtle, different surgical techniques should be used for cruciate-retaining and posterior-substituting total knee arthroplasties. Sacrificing the posterior cruciate ligament (PCL) selectively opens the flexion space approximately 2 mm more than the extension space, which accounts for some of the technical differences. When performing a cruciate-retaining knee, the key points are to avoid stiffness due to an extra tight flexion space caused by a tight PCL and to avoid flexion instability from overrelease of the PCL. When performing a posterior-substituting knee, the key points are to avoid flexion instability due to an extra large flexion space caused by PCL sacrifice and to avoid postoperative knee flexion contracture by underresection of the distal femur. For posterior-sacrificing knees, cam-post impingement is a potential problem and can be avoided by avoiding excessive femoral component flexion and tibial component slope.