CT-scans, fracture displacement/angulation cplane/world

Hi everyone,

For a research project I am analyzing displaced fracture fragments of bones.
I create .stl files of fractured bones to analyze in Rhino.

I have measured volume of fragments and articular area of fragments

Now I also want to know the displacement of the fragments. I’ve been doing it by using ‘volume centroid’ and then calculating the distance between these coordinates of the fragment in its location on CT-scan and it’s location when I put the fragment back in place.

This does give a general idea of displacement but it doesn’t give me information about the direction the bone fragment has travelled. For this I tried lining up the bones with the Cplane in a standardized way so that the axes correspond with normal anatomical position. For example, the z axis is completely vertical and a difference in z coordinates gives me the vertical displacement of a bone. However, when I calculate ‘volume centroid’ this gives me the world coordinates in the left down corner of the screen, not the cplane coordinates I am looking for. I’ve spent hours to find a solution but it doesn’t seem possible.

Does anyone know a way to bypass the c-plane? (aligning the world axes with my model for example in a simple way)

Another question, this one might be more difficult. I’m also looking to find a way to analyze the way a fragment angulates in comparison with its normal anatomical position. This should describe all 3 dimensions. I’ve been thinking about drawing a triangle that I put back into positiont together with the fragments. With some tangens, sinus etc. it should be able to use coordinates of the triangle in both locations and find degrees of rotation for each dimension. Does anyone know if there is an easier way?

Kind regards, Dirk

Dirk -

It would be best to see and example model of this. So, I cannot answer many of the general questions without looking at what you have specifically.

As for how to line up the world coordinate system with the bones, I would actually move the bones to the world construction plane. We do that with the Orient 3 point command. I would pick three points on the bones and then three points on the world cplane.

Can you send us an example? tech@mcneel.com

Hi Scott,

I’ve sent you an email with images and a description, thanks!

Hi Dirk- AnalyzePt, snappng to the point object from VolumeCentroid should give info in CPlane and World coordinates.


Ha great, thanks!
I’ve been orienting with 3points orient as Scott advised, but this also works really well.