Follow these steps:

1. Import the Geometry: Load the pelvis and sacrum bone geometry file into a 3D modeling software or a CAD program capable of handling complex geometries. Ensure that the file format is compatible with the software you are using.

2. Duplicate the Bone Geometry: Create a duplicate copy of the original bone geometry to work on. This will allow you to preserve the original bone geometry while creating the cortical bone shell.

3. Scale the Duplicate Geometry: Scale up the duplicate bone geometry uniformly by 4mm in all directions. This will create a larger version of the bone geometry, which will serve as the outer boundary for the cortical bone shell.

4. Offset the Duplicate Geometry: In the CAD software, use the "offset" or "shell" feature to create a new surface that is 2mm away from the outer surface of the scaled duplicate bone geometry. This will generate the cortical bone shell with the desired thickness.

5. Boolean Operation: Perform a Boolean subtraction operation between the original bone geometry and the cortical bone shell geometry. This will remove the original bone geometry from the cortical bone shell, leaving behind the shell itself.

6. Clean and Refine the Geometry: After the Boolean operation, you may need to clean and refine the resulting geometry. Check for any overlapping or intersecting surfaces and make necessary adjustments to ensure a watertight and smooth cortical bone shell.

7. Create Holes for Screw Insertion: Identify the locations where you want to insert screws and create holes in the cortical bone shell geometry accordingly. The size and shape of the holes will depend on the specifications of the screws you intend to use.

8. Export the Final Geometry: Once you have completed the cortical bone shell and added the necessary holes, export the final geometry in a suitable file format (such as STL) that can be imported into a finite element analysis (FEA) software.

I am NOT a doctor, it should be used only for models.

Hope it helps: partial credit AI