We used buckle transducers in conjunction with a mathematical model to investigate the mechanics of the cruciate ligaments during flexion of knees containing prostheses with differing component geometries. With normal component placement and both cruciate ligaments intact, the full range of flexion is restricted in a knee with the conforming, single radius-of-curvature geometric prosthesis because of large constraint forces in the posterior cruciate ligament. Ligament incompatibility is avoided with multicurvature designs, as represented by the kinematic prosthesis. Low conformity of components allows anterior-posterior displacements to accompany flexion, which provides for a full range of flexion and allows cruciate ligament forces to approach those of a normal knee. This analysis points out that special precautions must be taken to avoid ligament constraint forces (and potential loosening) if an implant with highly conforming components is used when both cruciate ligaments are intact. The information presented here, relating implant geometry to cruciate ligament mechanics, must be considered when choosing an appropriate prosthetic design to ensure ligament compatibility in the presence of one, both, or no cruciate ligaments.