  For those experiencing severe-to-profound sensorineural hearing loss, the
cochlear implant (CI) is the preferred treatment. Augmented reality (AR) aided
surgery can potentially improve CI procedures and hearing outcomes. Typically,
AR solutions for image-guided surgery rely on optical tracking systems to
register pre-operative planning information to the display so that hidden
anatomy or other important information can be overlayed and co-registered with
the view of the surgical scene. In this paper, our goal is to develop a method
that permits direct 2D-to-3D registration of the microscope video to the
pre-operative Computed Tomography (CT) scan without the need for external
tracking equipment. Our proposed solution involves using surface mapping of a
portion of the incus in surgical recordings and determining the pose of this
structure relative to the surgical microscope by performing pose estimation via
the perspective-n-point (PnP) algorithm. This registration can then be applied
to pre-operative segmentations of other anatomy-of-interest, as well as the
planned electrode insertion trajectory to co-register this information for the
AR display. Our results demonstrate the accuracy with an average rotation error
of less than 25 degrees and a translation error of less than 2 mm, 3 mm, and
0.55% for the x, y, and z axes, respectively. Our proposed method has the
potential to be applicable and generalized to other surgical procedures while
only needing a monocular microscope during intra-operation.
