Influence of the shape of a forearm support on the range of pronation and supination in the context of a surgical arm assistance system
Authors: Langer Ferdinand , Maier Thomas 
Published: June 2023

Surgery often results in static, uncomfortable arm and upper body postures, which lead to high stress on the surgeons' upper extremities [Szeto 2012]. To counteract this, an interaction-based arm assistance system has been developed to physically unload the surgeon's upper extremities during laparoscopic procedures. This is achieved by actively supporting the forearms. Thereby, a supporting force acts on the forearms, which follows the natural forearm movements without restrictions [Karlovic 2015]. The assistance system is controlled exclusively by a form fit and frictional connection of the forearms with the arm support. Investigations show an influence of the anthropomorphic shape of the form on objective and subjective operability and an advantage of the anthropomorphic shape over no support and a flat form [Langer 2022]. In this study (funded by the German Research Foundation), the range of pronation und supination within anthropomorphic forms is investigated to design the human-machine interface of the arm assistance system for precise surgical tasks with forearm rotation. Within the scope of this research project, the interface parameter shape of the forearm support is therefore investigated on the basis of five shape variants (gender-specific negative shapes of 05-, 25-, 50-, 75- and 95-percentile adults). To investigate the range of motion of pronation and supination, the 32 subjects grasp a round handle, which is oriented orthogonally to the ground (start position). The forearm is bent 90° to the upper arm (flexion). On instruction, the subjects rotate their forearm maximally in the pronation direction and then in the supination direction. The maximum angles achieved in each case are measured starting from the start position. The subjects then rate the comfort and further properties of the shape on a 7-point Likert scale. This is repeated with all shape percentiles. The study shows that the percentile of the anthropomorphic form has an influence on the range of motion in pronation and supination of the forearm. Compared to no support of the forearm (Ø-pronation = 76.4°, Ø-supination = 114.3°), the average range of motion for support with the 05-percentile form (smallest shape) decreases by 8.4% (70.6°) for pronation and by 22.5% (93.3°) for supination. In contrast, the average range of motion for support with the 95-percentile form decreases by 5.3% for pronation and 18.1% for supination. For pronation and supination, the trend shows that the smaller the shape of the forearm support, the smaller the average range of motion. The average range of rotation that can be achieved without support and without form is not achieved with any form. The subjective survey shows that comfort is lowest for the smallest form. Furthermore, the correlation of the range of motion of the different forms is evaluated depending on the forearm percentile of the subjects. Design recommendations for the shape of the forearm support are derived from the results. It is deduced that a forearm support with rotational degrees of freedom for pronation and supination could offer an advantage for the preservation of the range of motion and consequently for the performance of tasks.