Control of Prosthetic Arm Rotation by Sensing Rotation of Residual Arm Bone

Arm rotation is very useful for unilateral amputees and essential for bilateral amputees to perform tasks of daily living. We propose a new approach for improving the control of prosthetic arm rotation in amputees. This new approach involves inserting a small permanent magnet into the distal end of the residual bone of subjects with upper limb amputations. When a subject rotates the residual arm, the magnet will rotate with the residual bone, causing a change in magnetic field distribution. This field change can be detected by magnetic sensors in the prosthetic socket, from which information on the residual bone rotation is derived and used as an input signal to control a powered prosthetic rotator. Proprioception remains intact for residual limb skeletal structures, thus this control approach should be natural and easy to use. Studies have been conducted in both simulation and physical experimental models to assess the feasibility and performance of sensing the voluntary rotation of the residual bone with an implanted magnet. The results from the studies are encouraging, suggesting potential clinical applications to improve the control of powered prostheses with preservation of physiological proprioceptive feedback. INTRODUCTION Internal and external rotations of the arm are very useful for upper limb amputees [1, 2]. Most commercially available upper limb prostheses only provide passive arm rotators using a friction turntable incorporated into the prosthesis. Thus, control of current rotators is cumbersome, slow, unintuitive, and lacks proprioceptive feedback. One approach to improve voluntary control of artificial arm rotation is to physically couple it to the rotation of the bones remaining in the residual arm. Two interfacing mechanisms, osseointegration [3] and artificial epicondyles [4], have been developed to create physical connections between the residual humerus and the prosthesis for control of prosthesis rotation. These approaches are promising, but have drawbacks: Direct skeletal attachment is challenged by infections at the skin interface, and loading of the skin over artificial epicondyles can cause skin breakdown, in addition, there is potential for loosening of the implants. In this study, we have proposed an alternative approach for improving the rotational control of artificial limbs. This new approach involves inserting a small permanent magnet into the distal end of the residual bone of subjects with upper limb amputations (Figure 1). The permanent magnet generates a magnetostatic field, and when the amputee rotates the residual bone, the magnet rotates relative to the surface of the arm. This rotation causes a change in the magnetic field distribution around the surface of the arm that can be detected by magnetic sensors fixed within the prosthetic From “MEC '08 Measuring Success in Upper Limb Prosthetics,” Proceedings of the 2008 MyoElectric Controls/Powered Prosthetics Symposium, held in Fredericton, New Brunswick, Canada, August 13–15, 2008. Distributed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License by UNB and the Institute of Biomedical Engineering, through a partnership with Duke University and the Open Prosthetics Project.