In this paper, a computed-torque control and disturbance-observer-based control were evaluated for an orthopedic surgery robotic system called OrthoRoby, to track bone-cutting trajectory. Robust motion control is required to control a medical robotic system in an environment where model uncertainty and disturbances exist. Experiments were performed to demonstrate the performance of the tracking of the desired cutting trajectory when a computed-torque controller and disturbance-observer-based controller were used for OrthoRoby. It was observed that a computed-torque controller can become unstable with inexact cancellation due to parameter uncertainties and unmodeled dynamics. A disturbance-observer-based control was shown to ensure stability with inexact cancellation due to parameter uncertainties and unmodeled dynamics.