COMSOL 6.3 - STL Import 2 — Combining Geometry with an Imported Mesh

STL Import 2 — Combining Geometry with an Imported Mesh¹

When working with imported STL meshes, you may run into applications where you want to combine your imported mesh with an imported CAD part, CAD assembly, or with geometry created in COMSOL Multiphysics.

This tutorial series consists of two parts, where the first part, STL Import 1 — Repairing and Combining STL Files, focuses on using available tools to edit imported STL meshes: the different operations for repairing the meshes, combining imported meshes with each other, and generating a volume mesh from the imported surface mesh. This second part of the series describes a workflow for combining the mesh obtained in the first part with a parameterized geometry.

Figure 1: Left: Tetrahedral mesh after repairing, combining, and partitioning imported surface meshes in the first part of the tutorial series. Right: Tetrahedral mesh after combining the mesh from the first part with parameterized geometry created in the second part of the tutorial series.

The STL file format is one of the standard file formats for 3D printing, and it is also often used as a format for exchanging 3D scan data. STL files contain only the triangulated surface, which can also be called a surface mesh, of a 3D object. The triangles in the file are identified by their normals and vertex coordinates and are stitched together using a tolerance to form a faceted representation of the object.

COMSOL Multiphysics supports a variety of operations, for example, moving, scaling and rotating an imported mesh, combining the imported mesh with parameterized geometry to run parametric sweeps, and generating unstructured, structured, and boundary layer mesh in the domains. When creating geometry or importing CAD files for combining with an imported mesh, you can view the imported mesh with the geometry to help with positioning the design. This procedure is shown in the instructions of this tutorial.

Lastly, it is important to mention that the workflows presented in this tutorial series apply to any type of imported surface meshes, such as the formats PLY and 3MF. They also apply when creating a mesh from a Filter or Partition dataset, which you would do when using the results of a simulation as the mesh for a new simulation, for example during a topology optimization study.

Model Definition

The goal of this tutorial is to combine the mesh of the vertebrae and discs, imported in the first part of this tutorial series, with the geometry of the three pedicle screws and a rod. The screws are created as parameterized geometry parts with the possibility of varying the screw angles and entry points.

Figure 2: The transverse angle (left) and sagittal angle (right) of the polyaxial screws.

The workflow for the model includes the following steps:

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Setting up the parameters for the geometry

•

Importing the mesh of the vertebrae and discs into the geometry sequence as a construction object to help with the positioning of the screws

•

Inserting the geometry parts for the screws into the geometry sequence and positioning them in relation to the imported mesh of the vertebrae and discs

•

Creating the geometry for the rod that connects the screws by leveraging measurement features for the correct positioning

•

Importing the geometry of the screws and rod into the meshing sequence and combining it with the mesh of the vertebrae and discs

•

Building a physics-controlled mesh

COMSOL_Multiphysics/Meshing_Tutorials/stl_2_combine_geom_mesh

Modeling Instructions

COMSOL Desktop

In the window, click the root node.

From the menu, choose .

Browse to the model’s Application Libraries folder and double-click the file stl_1_repair_imported_meshes.mph.

This file contains the prepared mesh of the vertebrae and discs. For the step-by-step instructions to import, repair and combine the STL meshes, see the tutorial STL Import 1 — Repairing and Combining STL Files.

Global Definitions

Under Global Definitions, import the parameters and load the geometry part of a polyaxial screw.

Parameters 1

In the window, under click .

In the window for , locate the section.

Click

Browse to the model’s Application Libraries folder and double-click the file stl_2_combine_geom_mesh_parameters.txt.

Polyaxial Pedicle Screw

In the toolbar, click and choose .

Browse to the model’s Application Libraries folder and double-click the file polyaxial_screw_geom_sequence.mph.

Selection List

In the toolbar, click

to open the window.

Keep the open to check the created and imported domains and selections throughout the tutorial.

Geometry 1

Import 1 (imp1)

You will now continue with importing the mesh of the vertebrae and discs as a construction geometry that you can use as an aid when positioning the screws.

In the toolbar, click

Notice that according to the default option in the Combine Geometry with Mesh dialog, an Import node will also be added to the Mesh 1 sequence.

In the dialog, click .

In the window for , locate the section.

From the list, choose .

Locate the section. In the text field, type 0.001.

Locate the section. Select the checkbox.

Click

Component 1 (comp1)

Inspect the Import node in Mesh 1 now before you continue with the geometry. This Import node was added to prepare for combining the geometry and mesh once you are done creating the geometry. Physics interfaces that you add to this model will use the domains and boundaries defined by , not the geometry under . In the , this is indicated by the green rectangle around the icon for

In the window, expand the > node.

Mesh 1

Import - Geometry

In the window, expand the > > node, then click .

In the window for , type Import - Geometry in the text field.

In the window, click .

In the window for , type Combined Vertebrae and Screws in the text field.

Geometry 1

Before adding an instance of the screw, create a work plane along which to align the part. The work plane also sets the entry point of the screw.

The angles and entry point coordinates are parameterized in this tutorial so that it is easy to set up a parametric study to compare different screw fixation techniques. The screw dimensions can of course also be parameterized, but in this tutorial, you will only vary the length of the screw thread for the different vertebrae and keep the other parameters fixed.

C3 Pedicle Screw

Add node groups for the two screws and the rod to organize the feature nodes.

In the window, right-click and choose .

In the window for , type C3 Pedicle Screw in the text field.

C3 screw position

In the toolbar, click

In the window for , type C3 screw position in the text field.

Locate the section. From the list, choose .

In the text field, type c3_zw.

Click to expand the section. In the text field, type -c3_xw.

In the text field, type -c3_yw.

Polyaxial Pedicle Screw 1 (pi1)

In the toolbar, click

and choose to add an instance of the screw that you will align with the newly created work plane.

In the window for , locate the section.

In the table, enter the following settings:


Name	Expression	Value	Description
thread_l	c3_L	11 mm	Screw thread length
sagittal_angle	s_angle	35 °	Sagittal angle
transverse_angle	t_angle	10 °	Transverse angle

Locate the section. Find the subsection. From the list, choose .

Find the subsection. From the list, choose .

Click to expand the section. In the table, select the checkbox for .

Click to create a cumulative selection for the help points that you want to delete once the geometry is done.

In the dialog, type Help Points in the text field.

Click .

Make sure was added under the column for the selection. Otherwise, select it manually.

In the window for , click to expand the section.

In the table, select the checkbox for .

Click .

In the dialog, type Screw Domains in the text field.

Click .

In the window for , click

Make sure was added under the column for the selection. Otherwise, select it manually.

Click to expand the section. In the table, enter the following settings:


Name	Keep	Physics	Contribute to
Thread boundaries (Thread domain)	√		None

Click .

In the dialog, type Simplify threads in the text field.

Click .

In the window, click .

In the window for , click

and rotate the geometry to verify that it looks like the image below.

C4 Pedicle Screw

Repeat the procedure for the C4 vertebra screw.

In the window, right-click and choose .

In the window for , type C4 Pedicle Screw in the text field.

C4 screw position

In the toolbar, click

In the window for , type C4 screw position in the text field.

Locate the section. From the list, choose .

In the text field, type c4_zw.

Click to expand the section. In the text field, type -c4_xw.

In the text field, type -c4_yw.

Polyaxial Pedicle Screw 2 (pi2)

In the toolbar, click

and choose .

In the window for , locate the section.

In the table, enter the following settings:


Name	Expression	Value	Description
thread_l	c4_L	12 mm	Screw thread length
sagittal_angle	s_angle	35 °	Sagittal angle
transverse_angle	t_angle	10 °	Transverse angle

Locate the section. Find the subsection. From the list, choose .

Find the subsection. From the list, choose .

Locate the section. In the table, enter the following settings:


Name	Keep	Contribute to
Rod help points	√	Help Points

Locate the section. In the table, enter the following settings:


Name	Keep	Physics	Contribute to
Screw domain	√	√	Screw Domains

Locate the section. In the table, enter the following settings:


Name	Keep	Physics	Contribute to
Thread boundaries (Thread domain)	√		Simplify threads

Click

C5 Pedicle Screw

Repeat the procedure for the C5 vertebra screw.

Right-click and choose .

In the window for , type C5 Pedicle Screw in the text field.

C5 screw position

In the toolbar, click

In the window for , type C5 screw position in the text field.

Locate the section. From the list, choose .

In the text field, type c5_zw.

Click to expand the section. In the text field, type -c5_xw.

In the text field, type -c5_yw.

Polyaxial Pedicle Screw 3 (pi3)

In the toolbar, click

and choose .

In the window for , locate the section.

In the table, enter the following settings:


Name	Expression	Value	Description
thread_l	c5_L	12 mm	Screw thread length
sagittal_angle	s_angle	35 °	Sagittal angle
transverse_angle	t_angle	10 °	Transverse angle

Locate the section. Find the subsection. From the list, choose .

Find the subsection. From the list, choose .

Locate the section. In the table, enter the following settings:


Name	Keep	Contribute to
Rod help points	√	Help Points

Locate the section. In the table, enter the following settings:


Name	Keep	Physics	Contribute to
Screw domain	√	√	Screw Domains

Locate the section. In the table, enter the following settings:


Name	Keep	Physics	Contribute to
Thread boundaries (Thread domain)	√		Simplify threads

Click

Check the bottom section of the to see that there are now selections created that you can use in geometry operations and some of them also when setting up the physics.

Rod

To create the geometry for the connecting rod, sweep a circular face along a polygon that goes through the four help points of the screw heads.

Right-click and choose .

In the window for , type Rod in the text field.

Rod cross section

In the toolbar, click

In the window for , type Rod cross section in the text field.

Locate the section. From the list, choose .

On the object , select Point 1 only by clicking on the topmost vertex of the screw head help points or by selecting and adding it from the .

Locate the section. From the list, choose .

On the object , select Point 1 only.

Locate the section. Select the checkbox.

Locate the section. Select the checkbox to make the rod cross section a construction object that is automatically deleted when finalizing the geometry.

Click

In the toolbar, click .

In the window, place the center in the origin of the work plane, then move the mouse cursor, and click once more in the to draw the circle.

Rod cross section (wp4) > Circle 1 (c1)

In the window for , locate the section.

In the text field, type d_rod/2.

Click

Rod

In the window, under > click .

In the window for , click

Centroid Measurement

In the window, click to select the topmost of the six help points for the screw heads.

Right-click in the window and select from the Graphics context menu.

In the window, you can change the name of the generated coordinate parameters to something more descriptive, if you like, but for this tutorial, keep the default names.

Repeat steps and for the other five help points for the screw heads.

Polygon 1 (pol1)

Next, draw a polygon through the help points of the screw heads.

In the toolbar, click

and choose .

In the window for , locate the section.

Click the button. From the menu, choose .

Browse to the model’s Application Libraries folder and double-click the file stl_2_combine_geom_mesh_rod_coord.txt.

The naming and numbering assumes that you added the Centroid Measurement features for the points top-down and kept their original names. If you changed the names, you need to insert the parameter names manually. To do so, in the table for the polygon, paste in, for example, geom1.cm and then use Ctrl+space to easily select and fill in the table.

Locate the section. Select the checkbox.

Click

Rod domain

In the toolbar, click

In the window for , type Rod domain in the text field.

Locate the section. From the list, choose .

Clear the checkbox.

Locate the section. From the list, choose .

Locate the section. Select the checkbox.

Locate the section. From the list, choose .

Click

Delete Entities 1 (del1)

The geometry is almost finished, just some cleanup left to do. First, delete the help points as they are no longer needed.

In the window, right-click and choose .

In the window for , locate the section.

From the list, choose .

Click

Form Union (fin)

In the window, click .

In the window for , click

This removes all construction geometry and you are left with the screws and rod. Check the that there are now four domains, one for each screw and one for the rod.

Simplify the boundaries of the screw threads to one single boundary using virtual operations.

Form Composite Faces 1 (cmf1)

In the toolbar, click

and choose .

In the window for , locate the section.

From the list, choose .

Click

Combined Vertebrae and Screws

The geometry is now completed. Next, import it into the meshing sequence and combine it with the mesh of the vertebrae and disc.

Import - Geometry

In the window, under > > click .

In the window for , locate the section.

From the list, choose .

Select the checkbox to use a finer element size to represent the screws and rod.

This step is done to ensure that the intersecting mesh surfaces of the pedicle screws and the vertebrae have a similar element size. This will help with forming the union of the meshes further ahead.

Click .

This imports the geometry of the screws and rod. Make sure that the screws are indeed fully inside the vertebrae by examining the mesh in the window. If not, go back and check that the geometry is set up correctly.

Information

A subnode to the node informs that the meshes are intersecting.

In the window, click .

In the window for , locate the section.

From the list, choose .

Click to zoom in on a reported problem coordinate to inspect the intersection more closely. If the mesh is more complex, it can help to also clip around the coordinates to remove everything except the mesh closest to the coordinates from the view and you can also rotate the mesh such that you see at least some of the reported points more clearly. Note that the list of reported points is filtered so that only a smaller number of intersecting elements are listed.