1 Long-bone fractures such as those in the femur afflict more than 430,000 Americans per year and are 2 rising due to the aging population. Such fractures are serious injuries that require surgery. Aligning the long bone 3 fragments requires high precision in the presence of a huge traction force and is performed manually by 4 orthopedic surgeons before fixation. Surgeons have limited visual feedback even with repeated X-ray images, 5 and rotational malalignment of 10° or more after fracture fixation occurs in 28% of patients. Complications include 6 malalignment or nonunion of bone fragments, leg shortening, soft tissue damage, and high exposure to X-ray 7 radiation. 8 This work proposes a surgical robotic system that facilitates long-bone alignment currently performed 9 manually. The long-term objectives are to build and demonstrate a surgical robot that provides a large workspace 10 for the surgeon, can provide traction forces sufficiently large to align the bone fragments without damaging the 11 bones, and provides sub-millimeter precision for alignment. Patient outcomes will be improved by decreasing 12 procedure times and eliminating complications that require repeated operations, such as leg length 13 discrepancies and abnormal gait. The first Specific Aim focus is to develop an image-guided navigation system 14 to automatically align based on surgeon selected positioning on 3D model of the bone fragments. The second 15 Specific Aim focus is to demonstrate use of a force-feedback (haptic) controller for the surgeon to sense the 16 magnitude and direction of the muscle forces and manipulate the robot to align the femur segments. The third 17 Specific Aim is (i) to develop a clinical-grade robotic system that align bone fragments, eliminate the need for 18 manually applied force during alignment, and provide an open surgical field for the surgeon, (ii) to integrate robot 19 with the navigation system and haptic controller, and (iii) evaluate its performa