An experimental evaluation of loads occurring during guided drilling for cochlear implantation

During guided drilling for minimally invasive cochlear implantation and related applications, typically forces and torques act on the employed tool guides, which result from both the surgeon's interaction and the bone drilling process. Such loads propagate through the rigid mechanisms and result in deformations of compliant parts, which in turn affect the achievable accuracy. In this paper, the order of magnitude as well as the factors influencing such loads are studied experimentally to facilitate design and optimization of future drill guide prototypes. The experimental setup to evaluate the occurring loads comprises two six degree of freedom force/torque sensors: one mounted between a manually operated, linearly guided drill handpiece and one below the specimens into which the drilling is carried out. This setup is used to analyze the influences of drilling tool geometry, spindle speed as well as experience of the operator on the resulting loads. The results reveal that using a spiral drill results in lower process loads compared with a surgical Lindemann mill. Moreover, in this study, an experienced surgeon applied lower interaction forces compared with untrained volunteers. The measured values further indicate that both the intraoperative handling of the bone-attached drill guide as well as the tool removal after completing the hole can be expected to cause temporary load peaks which exceed the values acquired during the drilling procedure itself. The results obtained using the proposed experimental setup serve as realistic design criteria with respect to the development of future drill guide prototypes. Furthermore, the given values can be used to parameterize simulations for profound stiffness analyses of existing mechanisms.