# Model registration as measurement of displacement

**URL:** https://discourse.slicer.org/t/model-registration-as-measurement-of-displacement/26104
**Category:** Support
**Created:** [November 7, 2022, 9:28am UTC](https://discourse.slicer.org/t/model-registration-as-measurement-of-displacement/26104 "2022-11-07T09:28:32Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![jegberink](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/jegberink/32/16031_2.png) [@jegberink](https://discourse.slicer.org/u/jegberink)
#### Post date: [November 7, 2022, 9:28am UTC](https://discourse.slicer.org/t/model-registration-as-measurement-of-displacement/26104/1 "2022-11-07T09:28:32Z")

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Hey Everyone,

I’m using ALPACA from Slicermorph to measure the rotation and translation from a preoperative, postoperative and planning of the femoral head after an osteotomy.

Explanation of my methods:

I have a preoperative CT and a postoperative CT of a patient. I also have a 3d model of a planning.  
I segment the femur for both preoperative and postoperative.

I alligned all of the femoral shafts (with ALPACA) and created a coordinate system so i can measure all patients from a similar point. as shown here:

 ![front view femur alligned](https://us1.discourse-cdn.com/flex002/uploads/slicer/original/3X/7/c/7cf05665f9423c8c8ed4f11d3712e69938666545.png)

Now i overlap the proximal femur above the saw cut with ALPACA, to see how it has changed in terms of rotations and translations like this (yellow= PreOp, Blue= PostOp, Red=planning):

 ![front view femur alligned overlap proximal](https://us1.discourse-cdn.com/flex002/uploads/slicer/original/3X/0/2/02863b1fdc00816e3abcf7497e2eecf914e8707d.png)

I thought it would be possible to subtract the difference in each plane to get the difference between postoperative and planning. However, this does not seem to coincide.  
when i use ALPACA to overlap postoperative to the planning like this:

 ![post to planning](https://us1.discourse-cdn.com/flex002/uploads/slicer/original/3X/1/6/1609d4d1d3a600d428c9e6cfc1f5b021bf1714bf.png)

I get the following rotations and translations:

 ![output slicer](https://us1.discourse-cdn.com/flex002/uploads/slicer/original/3X/1/f/1f17e1b7974b464f4ade45e2435fb9983208042c.png)

My understanding of this is clearly flawed, if there is anyone who could explain to me where i’m going wrong in my thinking i would greatly appreciate it.

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### Author: ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)
#### Post date: [December 1, 2022, 7:54am UTC](https://discourse.slicer.org/t/model-registration-as-measurement-of-displacement/26104/2 "2022-12-01T07:54:03Z")

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Comparing pose differences is a very complex topic and there are many potential approaches. Based on the clinical requirements, you need to specify exactly what differences you want to quantify, such as rotation around a specific axis, surface-to-surface distance in a specific region, absolute orientation difference, orientation difference of vectors projected on a specific plane,…