Improved Compensation Of Gravitational Torque At The Shoulder

Abstract

A simple analysis of prosthetic shoulders reveals that they are mainly used to position the orientation of the elbow in relatively few fixed positions. For some tasks the elbow needs to be positioned forward of the shoulder (Forward flexion). For desk and table tasks the preferred positions are both forward and out (flexion and abduction). For a few tasks straight out angles are appropriate (abduction). What all of these positions have in common is the need to compensate for the gravitational load of the prosthesis as the angle with respect to the vertical is increased. The paper on the Z-axis shoulder joint shows that by first moving away from the vertical in the abduction direction one need only provide one strong locking joint to take care of the gravitational load. However, the amputee still requires considerable effort to move the arm out, and the lock mechanism must be made stronger to accommodate the load. These two problems can be addressed by compensating for all or part of the gravitational load.