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Image guided navigation systems (IGNS) have been implemented successfully in orthopaedic trauma surgery procedures because of their ability to help surgeons position and orient hand-held drills at optimal entry points. However, current IGNS cannot prevent drilling tools or instruments from slipping or deviating from the planned trajectory during the drilling process. A method is therefore needed to overcome such problems.A novel passive/active hybrid robot (the HybriDot) for positioning and supporting surgical tools and instruments while drilling and/or cutting in orthopaedic trauma surgery is presented in this paper. This new robot, consisting of a circular prismatic joint and five passive/active back-drivable joints, is designed to fulfill clinical needs. In this paper, a system configuration and three operational modes are introduced and analyzed. Workspace and layout in the operating theatre (OT) are also analyzed in order to validate the structure design. Finally, experiments to evaluate the feasibility of the robot system are described.Analysis, simulation, and experimental results show that the novel structure of the robot can provide an appropriate workspace without risk of collision within OT environments during operation. The back-drivable joint mechanism can provide surgeons with more safety and flexibility in operational modes. The mean square value of the positional accuracy of this robot is 0.811 mm, with a standard deviation (SD) of 0.361 mm; the orientation is accurate to within 2.186°, with a SD of 0.932°. Trials on actual patients undergoing surgery for distal locking of intramedullary nails were successfully conducted in one pass using the robot.This robot has the advantages of having an appropriate workspace, being well designed for human–robot cooperation, and having high accuracy, sufficient rigidity, and easy deployability within the OT for use in common orthopaedic trauma surgery tasks such as screw fixation and drilling assistance. Copyright © 2012 John Wiley & Sons, Ltd.