location is anterior to the center of the tibial plateau resembling the screw home mechanism of the femur on the tibia as the femur comes into terminal extension the lower surface of the composite envelope of the femoral surfaces will be conformal with the tibial surface and is consistent with the required neutral path of motion however it will be understood that for purposes of tolerances and to allow some laxity to occur the tibial surface will be relieved slightly to avoid a tight femoral tibial fit in any case laxity is inherent in this tibial surface except at the extremes of the flexion range to produce this behavior extra femoral surfaces can be added to provide the required laxity to the composite at the extremes after generating the composite femoral positions a modification in the sagittal plane is carried out whereby the profiles are placed on arcs on the medial side the arc is of small radius for example 40 50 mm while on the lateral side the arc is of large radius for example 70 100 mm the anterior parts of the arcs will preferably be of smaller radius than the posterior to allow for a high flexion range and posterior displacement of the lateral femoral condyle in flexion the final step is to mathematically smooth the composite of the corrected femoral surfaces using a drape function this resulting smoothed surface defines that portion of the tibia which is contacted by the condyles the tibial surface also includes the central ramp or post surface which is similarly generated by the envelope of successive positions of the.