Design and Performance Evaluation of a Novel Compliant Wrist for Minimally Invasive Middle Ear Surgery

Objective: To present the design choices, modelling of bending stiffness and experimental validation of a novel notched Nitinol compliant wrist that enables tip articulation and suction during endoscopic and robotic middle ear surgery. Methods: The wrist consists of an inner wrist tube (IWT) assembled within an outer wrist tube (OWT). The OWT was selected from five concept designs that were evaluated based on stiffness and compact bending. An analytical model was developed based on Castigliano's 2^{nd} Theorem and a finite element model (FEM) was developed to predict tip stiffness under soft-tissue surgical interactions and compared to experimental results. Results: The FEM and analytical models predicted tip displacement due to an applied force within 25% and 12 % of the experimental results, respectively. The complete wrist prototype withstood 700-1000 cycles before fracture when it was articulated up to 145° - 178°, corresponding to 9.6 N - 12.4 N of cable tension. Conclusion: A wristed tool is presented to facilitate endoscopic ear surgery and its mechanical behaviour in surgically relevant loading conditions. Significance: This instrument could be adapted to provide suction and soft tissue dissection capabilities for future minimally invasive endoscopic or robotic surgery, through the ear canal or other natural orifices.