Design and analysis of a head-mounted parallel kinematic device for skull surgery.

PURPOSE: Precision skull surgery requires specialized instrumentation to satisfy demanding requirements in cochlear array implantation, deep brain stimulation electrode placement, and related applications. A miniaturized reconfigurable parallel kinematic mechanism which can be directly mounted on a patient's skull was designed, built, and tested for precision skull surgery.
METHODS: A Stewart-Gough platform is attached to a patient's skull so no optical tracking affecting the overall accuracy in keyhole surgery is required. Six bone anchors comprising the mechanism base joints are implanted at positions with sufficient skull thickness. Since no fixation frame is required, intervention planning flexibility is increased. The centers of the spherical shaped bone anchors can be localized accurately in the image space. An implicit registration to the physical space is performed by assembling the kinematics. Registration error is minimized compared to common optical tracker-based approaches. Due to the reconfigurable mechanism, an optimization of the hexapod's configuration is needed to maximize accuracy and mechanical stability during the incision. Mathematical simulation was conducted to estimate system performance.
RESULTS: Simulation results revealed significant improvement in accuracy and stability when exploiting the redundant degrees of freedom and the implemented reconfigurability. Inaccurate localization of base points in the image data set was identified as the main source of error. A first prototype with passive prismatic actuators, i.e. micrometer calipers, was successfully built.
CONCLUSIONS: A head-mounted parallel kinematic device for high precision skull surgery was developed that provides submillimetric accuracy in straight-line incisions. The system offers enhanced flexibility due to the absence of a rigid fixation frame.