  Accurate and consistent mental interpretation of fluoroscopy to determine the
position and orientation of acetabular bone fragments in 3D space is difficult.
We propose a computer assisted approach that uses a single fluoroscopic view
and quickly reports the pose of an acetabular fragment without any user input
or initialization. Intraoperatively, but prior to any osteotomies, two
constellations of metallic ball-bearings (BBs) are injected into the wing of a
patient's ilium and lateral superior pubic ramus. One constellation is located
on the expected acetabular fragment, and the other is located on the remaining,
larger, pelvis fragment. The 3D locations of each BB are reconstructed using
three fluoroscopic views and 2D/3D registrations to a preoperative CT scan of
the pelvis. The relative pose of the fragment is established by estimating the
movement of the two BB constellations using a single fluoroscopic view taken
after osteotomy and fragment relocation. BB detection and inter-view
correspondences are automatically computed throughout the processing pipeline.
The proposed method was evaluated on a multitude of fluoroscopic images
collected from six cadaveric surgeries performed bilaterally on three
specimens. Mean fragment rotation error was 2.4 +/- 1.0 degrees, mean
translation error was 2.1 +/- 0.6 mm, and mean 3D lateral center edge angle
error was 1.0 +/- 0.5 degrees. The average runtime of the single-view pose
estimation was 0.7 +/- 0.2 seconds. The proposed method demonstrates accuracy
similar to other state of the art systems which require optical tracking
systems or multiple-view 2D/3D registrations with manual input. The errors
reported on fragment poses and lateral center edge angles are within the
margins required for accurate intraoperative evaluation of femoral head
coverage.
