In this paper, we present a new approach to construct a 3D human skeleton model, which is then used to quantify gait pathologies, using a depth camera. First, thanks to the depth map, we obtain a human depth silhouette in 3D, from which our method is based to estimate each body part position. Second, the angle between the upper and lower legs of the 3D skeleton model is calculated. Finally, we show that using only this angle information is enough to quantify motion asymmetry. This result has been verified through an experimental study with 3 different subjects. Due to its advantages (simple, markerless and low-cost), this method is a promising solution for gait clinics in the future.