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A prerobotic positioning system for retractors was developed to replace and improve upon the manual techniques and mechanical apparatus that are now commonly used during surgical procedures. The system is a lightweight, pneumatically powered, electronically controlled positioner having ball joints between the three members. The combination of ball joints and 90 degrees bent connecting shafts provides a versatile spherical geometry whereby redundant methods exist to achieve the same position of the end effector, thereby allowing the surgeon to avoid intermediate obstacles during setup and use. Besides being easy to control and position, the device's major advantage is a predictable and reliable locking strength of the ball joints (minimum 25 ft-lb at 100 psi).>
McEwen et al. (Mon,) studied this question.