# How to determine the maximum posible size of a inside shell before extruding the external surface?

**URL:** <https://discourse.slicer.org/t/how-to-determine-the-maximum-posible-size-of-a-inside-shell-before-extruding-the-external-surface/23175>\
**Category:** Support\
**Created:** [April 28, 2022, 4:46pm UTC](https://discourse.slicer.org/t/how-to-determine-the-maximum-posible-size-of-a-inside-shell-before-extruding-the-external-surface/23175 "2022-04-28T16:46:43Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Ricardo\_Vega](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/ricardo_vega/32/15124_2.png) [@Ricardo\_Vega](https://discourse.slicer.org/u/Ricardo_Vega)\
**Post date:** [April 28, 2022, 4:46pm UTC](https://discourse.slicer.org/t/how-to-determine-the-maximum-posible-size-of-a-inside-shell-before-extruding-the-external-surface/23175/1 "2022-04-28T16:46:43Z")

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Hello,  
I need to make some measurements for bone tunnels.  
Right now we are exporting a bone model to meshmixer, then we create a cilindre aligned on a defined axis inside the bone. We then manually increase the diameter until it extrudes the surface of the bone model.

I wanted to ask for some tips to make this process a little more automated. We need to measure 60 models and each with the cilindres at different angles.

Thank you.  
Ricardo Vega

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**Author:** ![mau\_igna\_06](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/mau_igna_06/32/9056_2.png) [@mau\_igna\_06](https://discourse.slicer.org/u/mau_igna_06)\
**Post date:** [April 29, 2022, 5:48pm UTC](https://discourse.slicer.org/t/how-to-determine-the-maximum-posible-size-of-a-inside-shell-before-extruding-the-external-surface/23175/2 "2022-04-29T17:48:17Z")

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Im my opinion this can be automated.

First you need to get the anatomical axis of the long bone. Look at this code to get an idea of something you could improve:

> <https://github.com/SlicerIGT/SlicerBoneReconstructionPlanner/blob/699c9970f163a14c7289dd8c8a7aa8b7346969d3/BoneReconstructionPlanner/BoneReconstructionPlanner.py#L3042>

You can see my post about finding the anatomical axis also:  
[https://discourse.slicer.org/t/approximate-anatomical-axis-curve-of-long-bone/](https://discourse.slicer.org/t/approximate-anatomical-axis-curve-of-long-bone/)

Please let me know if you achieve it

Hope it helps

Mauro