Design and Bench Demonstration of a Robotic System for Cervical Spine Deformity Surgery

Precise manipulation using large forces is necessary to realign the cervical spinal joints during a cervical deformity surgery. However, this is difficult to achieve by hand in a limited surgical workspace. A weight-pulley system is currently used in clinical practices where discrete pulling forces via hanging weights are applied on the head through a rope and skull tongs. This method is also fundamentally flawed, as applying a static force from a single direction at one point on the head cannot fully constrain the head to achieve the optimal spinal alignment. The consequences of this can be severe; ligamentous injuries or long-term neurological complications may occur, and repeated surgeries due to under-correction may be necessary. We propose to challenge this conventional single-force, weight-pulley approach through a novel six degrees-of-freedom robotic system to manipulate the cervical spine through precise force and moment applied on the head. In this paper, we introduce the mechanical design of a bench version of the proposed robotic system. A cable-driven parallel mechanism was used to apply a desired wrench on the head by pulling seven cables attached to the head via servo motors mounted on a fixed frame. A simple open-loop control scheme was used to command target cable tensions in preliminary bench testing to demonstrate the force/moment application and position/orientation measurement accuracy. Results from these tests indicate a worst-case mean absolute force reproduction error of $< 23\mathrm{N}$ (5 lb) and moment reproduction error of $< 2$ Nm, and a forward kinematics measurement mean absolute error of $< 11$ mm (position) and $< 3.2^{\circ}$ (orientation). A user interface was built, and evaluated by a neurosurgeon, where comments of the surgeon will be used to inform future development.