CAD Modeling Instructions for Pelvic Hip Fracture

Note: Ensure that you follow the steps faithfully to avoid issues and achieve desired results

Step 1: Opening and Preprocessing in Autodesk Meshmixer

  1. Open M11_cg_bone_lpelvis.obj in Autodesk Meshmixer.
  2. Select the entire geometry.
  3. Navigate to Edit > Remesh to reduce the chance of overlapping during offsetting.
  4. Accept the Remesh operation.
  5. Continue with Edit > Extrude > Offset +1.77 mm (direction: normal).
  6. Select the geometry again and perform Edit > Remesh (optional but beneficial).

Step 2: Exporting and Transferring to Siemens NX

  1. Export the model as an .obj file.
  2. Transfer the file to Siemens NX on SCOREC computers using WinSCP or hard drive.
  3. Save and close the file to individually save convergent bodies as separate .prt files.

Step 3: Performing Subtractions in Siemens NX

  1. Open one file and import the second (outer and inner).
  2. Navigate to File > Start > Modeling.
  3. Select the outer part and use the Subtract operation with settings:
    • Subtract Tool from Target. Keep Tool operation.*
  4. Repeat the subtraction with the sacrum until a full shell model is created.

Step 4: Processing the Sacrum Shell

  1. To remove the right half of the sacrum: Place a Datum CSYS in the middle of the sacrum shell using Home > Datum CSYS and position the datum such that there is left-right symmetry.
  2. Trim the body using Trim Body command after selecting the appropriate plane. You may have to select the Reverse Direction option to leave the desired left half remaining.
    • It is advised to trim the cancellous and cortical parts separately.
  3. Save as a .prt.

Step 5: Facet Cleanup and Hole Insertion

  1. In NX, Open the cartilage (m11_cg_jnt_lsi.obj) and save it as a part file.
  2. Create a new model in Siemens NX and ensure that units are in mm.
  3. Import the Sacrum, Cartilage, and Pelvis parts.
  4. Use Facet > Cleanup Facet Body to clean mesh.
  5. Delete all minor extra parts that may appear.
  6. Add a fracture to the sacrum (Note: trim body command would be more efficient, but has proven to be problematic with regard to the meshing)
    • First, duplicate the entire geometry as it currently exists and make sure its copy is superimposed upon the original
    • Create another Datum CSYS, place it where shear would occur
    • Create a sketch of a large square of arbitrary size, as long as it encompasses the geometry
    • Extrude it in one direction (right if observing from the front) such that it covers half of the sacrum and all of the pelvis
    • Use the subtract command to subtract the box from the part of the bone (choose one of the superimposed copies) which it overlaps
    • Repeat the process for the other half, this time deleting from the other superimposed half
    • This will leave you with one right side and one left side, leaving you with the geometry you had after Step 4, but with a “fracture”
  7. Insert a hole:
    • Place a Datum CSYS and then Create Sketch on Plane.
    • Draw a rectangle going into/through sacrum (~100mm length, 3.66 – 3.67mm radius).
    • Revolve the rectangle
    • Select the cylinder and use Facet Body from Body to make the new cylinder convergent.
    • Use Subtract Tool to remove the cylinder from the bone model (one part at a time).
    • Repeat for the second hole, which should not be parallel to the first.

Step 6: Inserting Screw and Final Steps

  1. Open the screw (screw-7-74.prt) and remove the Split Bodies and Datum Planes.
  2. Import the updated screw part using Assemblies > Add Component.
  3. Align the screw to the hole using Assembly constraints (Touch Align the centerlines first and then use Distance Align).
  4. Save the entire model as a .prt file.
  5. Export the final model as a Parasolid binary file .x_b