Stair stepping motion is important in daily living, similar to gait. In this study, we did a Kinematic Analysis of total knee arthroplasty during stair-stepping. A total of 20 patients implanted with Bi-Surface 5PS were assessed. The Bi-Surface knee is a posterior-cruciate substitute prosthesis with a unique ball-and-socket joint in the mid-posterior portion of the femoral and tibial components. This joint functions as a posterior stabilizing cam mechanism and as a load-bearing surface in flexion. Patients were examined during stair-stepping motion using a 2-dimensional to 3-dimensional registration technique. The kinematic pattern in step up was a medial pivot, in which the level of anteroposterior translation was very small. In step down, the kinematic pattern was neither pivot shift nor rollback. From minimum to maximum flexion, anterior femoral translation occurred slightly. However, it became clear in this study that the joint's stability during stair-stepping was affected by the design of the femorotibial joint rather than Post/Cam engagement.