Daniel,
My understanding is that you have two CT sets (preop for planning and postop), and you could segment sacrum in this dog. I assume difficulty you have is to overlap these two to compare planned drill trajectory/starting point with actual surgery. I previously did the same work as yours.
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Since positioning for CT is different between the two sets, you need registration. You can use one of many registration modules or extensions to do this. One easy way is to place a few fiducials on anatomical landmarks on the sacrum and use those for registration and add further shape matching.
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Then you can compare the planned and actual trajectory.
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You may use a line mark-up and measure an angle. Or you can place fiducials at the origin and end of each planned and actual drill hole. Then, you will have coordinates of fiducials and make 2 vectors. Then, you can calculate angle difference and displacement of the origin of the drilling.
You may also want to measure angles from each plane or axis. Then, you can reformat (reslice) based on axes you assign on the sacrum.
Sun.
Sun Young Kim, DVM, MS, DACVS-Small Animal
Associate Professor of Small Animal Surgery
Veterinary Clinical Sciences
College of Veterinary Medicine
625 Harrison Street
West Lafayette, IN 47907
U.S.A.