Motion of a mobile bearing knee allowing translation and rotation

The kinematics of a mobile bearing knee, which allowed +/-20 degrees of rotation and 4.5 mm of anteroposterior translation, was measured for ascending and descending a step, deep-knee bend, normal walking, and twisting. A fluoroscopic technique was used, analyzed by 2 different methods. The rotations and displacements during the activities were similar to those of moderate-to-high constrained fixed bearing knees. The motion patterns were variable among test subjects and in general did not reproduce normal knee motion. Because of the freedom of anteroposterior translation and rotation in the design, however, each knee could determine its own neutral position and its own axis of internal-external rotation, depending on the activity.