  Objective: State of the art navigation systems for pelvic osteotomies use
optical systems with external fiducials. We propose the use of X-Ray navigation
for pose estimation of periacetabular fragments without fiducials. Methods: A
2D/3D registration pipeline was developed to recover fragment pose. This
pipeline was tested through an extensive simulation study and 6 cadaveric
surgeries. Using osteotomy boundaries in the fluoroscopic images, the
preoperative plan is refined to more accurately match the intraoperative shape.
Results: In simulation, average fragment pose errors were 1.3{\deg}/1.7 mm when
the planned fragment matched the intraoperative fragment, 2.2{\deg}/2.1 mm when
the plan was not updated to match the true shape, and 1.9{\deg}/2.0 mm when the
fragment shape was intraoperatively estimated. In cadaver experiments, the
average pose errors were 2.2{\deg}/2.2 mm, 3.8{\deg}/2.5 mm, and 3.5{\deg}/2.2
mm when registering with the actual fragment shape, a preoperative plan, and an
intraoperatively refined plan, respectively. Average errors of the lateral
center edge angle were less than 2{\deg} for all fragment shapes in simulation
and cadaver experiments. Conclusion: The proposed pipeline is capable of
accurately reporting femoral head coverage within a range clinically identified
for long-term joint survivability. Significance: Human interpretation of
fragment pose is challenging and usually restricted to rotation about a single
anatomical axis. The proposed pipeline provides an intraoperative estimate of
rigid pose with respect to all anatomical axes, is compatible with minimally
invasive incisions, and has no dependence on external fiducials.
