Broken Femur / Bone-Screw Prop Design

THINGIVERSE LINK: https://www.thingiverse.com/thing:4030004

(see bottom of post for Printer Settings)


INSPIRATION:

Hey, guys. Alex here.

So, naturally, I had to do a prop for my favorite professor, Dr. Donald Plante, and even though he tried to get me to pick someone else, I successfully pestered him enough about it that he eventually gave in and let me print something for one of his classes. It just goes to show, when in doubt: whine about it. Works every time. 😉

So Dr. Plante said that he needed a prop for his 3D bioprinting class next Spring that demonstrated how a bone-screw can be used to fix a break in a bone. Thus, I set out to design a bone-screw and a broken bone model to go with it.

DESIGN:

The design of the bone-screw was definitely the trickiest part. I was able to find an ISO standard that described the design parameters for a clinical-use bone-screw, linked here. The standard provides a schematic for the design of the threads and head of a standard shallow-thread bone-screw, which I have included below. These schematics were accompanied by tables that outlined the various numerical parameters corresponding to the different-size screws and threads.

Figure 1. Schematic for the thread of a standard shallow-thread bone-screw.
Table 1. Parameters for Figure 1.

For this project, I decided to design a code HA-4 shallow-thread bone-screw, with the corresponding parameters as outlined in the table above.

The schematic and value table for the head of a shallow-thread bone-screw are provided below. As with the threads of the screw, I used the parameters corresponding to a code HA-4 shallow-thread bone-screw.

Figure 2. Schematic for the head of a shallow-thread bone-screw.
Table 2. Parameters for Figure 2.

With the schematics and parameters thus acquired, I set about the task of creating the threads myself, first by sketching my own schematic of the thread with the correct parameters in the Fusion 360 sketch environment.

Now I can add schematic component design to my resume. 😀

With the basic thread fully dimensioned, I then set about actually coiling this 1D design around the body of a cylinder, to produce the completed screw. This required the invaluable assistance of Greg and Dr. Plante.

To begin, we created a basic coil of the correct diameter and with a wavelength corresponding to that of the threads themselves, using the Coil command. The purpose of this coil was to provide the basic path that the sketch would follow to create the threads of the bone-screw. Using the Sweep command, we then instructed the sketch to extrude along the path of this coil–specifically, we instructed it to follow along the outside edge of the coil, so that the sketch would follow the path in 3 dimensions and not just in 2. After a significant amount of troubleshooting, we (and by “we”, I mean “Greg”) were finally able to successfully extrude the schematic for the thread alongside the outside path of the coil, such that it generated a thread for the screw with the current shape and proportions. An image of this thread alone is included below.

Behold the product of much time and toil.

With the thread now complete, we merged it with a cylinder of the correct diameter and length, and ta-da! The body of the screw was complete.

The head of the bone-screw was designed in much the same way, although with significantly less difficulty owing to the lack of a need for the Coil and Sweep commands. Instead, we simply modeling the upper half of the head, and let the Revolve command take care of the rest.

Lines for days!

With the head of the screw thus complete, we added a nice 45-degree Fillet on the bottom of the screw to serve as the point, and voila! The completed bone-screw was assembled in all its glory!

Available for $4.99 for a 120-pc box, only at your local Home Depot. Wait, no, wrong screw.
The bit costs extra.

With the bone-screw itself complete, the next step was to design and construct a to-scale model of the human femur. Thankfully, the website Embodi3D has an online repository of hundreds upon hundreds of CT scans and other medical scans that can be used to generate accurate, to-scale .STL files using their proprietary, browser-based democratiz3D conversion program.

The CT scan that I used for the femur model was uploaded by the user vishakk and can be found here. An image from the scan is pictured below.

It kinda looks like two eyes staring into your soul.

With the help of the democratiz3D program, this scan was converted into a .STL model of a real femur!

If you’re reading this, thanks for the femur, whoever you are! 🙂

I then promptly broke this random person’s femur in half using the power of 3D modeling.

Break a leg!
Ouch. That doesn’t look too good.
That’s better. We might need a couple more screws, though.

After cutting out a space in the model for the screw to fit into, the model was deemed complete and ready for printing! Below are the design drawings for the bone-screw and the femur, respectively.

Screw this! (I’m sorry, I had to.)

And a download link to the .PDF drawing file:

You can tell by all the black lines that this model has a gazillion facets to it.

The corresponding .PDF drawing file:

Lastly, here are some screenshots of the two halves of the femur, and the screw itself in Cura. These models are scaled down to 80% of their actual size, in order to fit in the print-bed.

These models look really sharp, but thankfully they print very dull, so there’s no real danger of hurting yourself or anyone else with them. 🙂
~ Devil Horns ~

After printing, I realized that the bone-screw itself is pathetically small. The femur pieces printed well and assembled together nicely, but there was no way that 1 tiny little screw would have held the two halves together.

So, I went ahead and increased the scale of the screw by 200%. It is now 0.73 cm in diameter (roughly), which is about the diameter of a regular screw. Because the shaft of the screw was also increased in length by the scaling, I had to cut it in half to stop it from coming out the other side of the femur.

I also went ahead and added two additional holes, so that a total of 3 screws can be used to hold the femur fragments together. This should enable the two halves to be held firmly in place by the larger PBS bone-screws.

These new, finished versions of the femur halves and the bone-screws were printed out in orange PLA and assembled together. Since I forgot to make the holes for the screws slightly larger than the screws themselves, I had to file down the holes in the femurs to enable the screws to fit into the holes. The completed femur and screws were given to Dr. Plante for safekeeping.

FINISHED PRODUCT:

The final, printed versions of the femur and bone-screws are pictured below.

As you can see, the models all fit together nicely, and the three scaled-up bone-screws successfully held the two halves of the femur together. Huzzah! The prop is a success! 🙂

Below you can find the all-important printer settings.

CURA PRINTER SETTINGS:

Printer Model: Ultimaker 2+

Material: Generic PLA

Printing Time: 10 hrs 18 min (at 80% scale)

Amount of Material: 66 g / 8.33 m (at 80% scale)

Layer Height: 0.15 mm

Shell: Wall thickness = 1.05 mm; top/bottom thickness = 0.8 mm; top/bottom line directions = default; outer wall inset = 0.025 mm; fill gap between walls = everywhere; print thin walls = checked.

Infill: Infill density = 20%; infill pattern = zig zag; extra infill wall count = 0; infill overlap percentage = 10%.

Material: Enable retraction = checked.

Speed: Print speed = 50 mm/s.

Travel: Z-Hop when retracted = unchecked.

Cooling: Enable print cooling = checked; fan speed = 100.0%.

Support: Generate support = checked; support placement = everywhere; support overhang angle = 60%; support pattern = lines; support density = 15%.

Build Plate Adhesion: Build plate adhesion type = brim; brim width = 8.0 mm.

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