To Give a Dog a Bone
Can you imagine holding a 3D-printed model of your dog’s bone in your hand? Not just their favorite toy, but their actual anatomy! I’ve been learning of the process that makes this incredible thought a reality. Janet Sinn-Hanlon is a medical illustrator media specialist at the University of Illinois College of Veterinary Medicine, and she took the time to speak with me and share her kindness and expertise on creating 3D models of patient specific anatomy.
Janet walked me through the process, and the example she showed me was of a puppy who had one radial bone that stopped growing due to a defect in the growth plate, which resulted in a bowed leg for the puppy, and ultimately prevented it from walking properly. To create a model of the deformed bone, Janet will receive the puppy’s computed tomography (CT) scans, which are just two-dimensional cross sections of the patient’s bones, blood vessels, and soft tissues. The vet requiring the print will tell Janet which specific bone she needs to produce, and she can load the CT scans into Amira, which is an incredible software that creates 3D renderings from the slices that the CT scan produces.
The system cannot always distinguish what is and what is not bone from the CT scan alone. Janet explained that she uses a threshold of between 150 to 250 Hounsfield units to distinguish which sections of CT scan are actually bone. Hounsfield units are the grayscale used in medical imaging, a reading of -1024 is black, and that represents air, and 3071 is the densest tissue, which for humans happens to be tooth enamel. Once the bone matter is isolated, a polygonal mesh can be applied, ideally aiming for a resolution just higher than what the printer is capable of producing. From there, the file is uploaded into 3D modeling software that can convert the isolated bone shape into a .stl file so it is ready for the 3D printers.
Janet has worked on many projects involving 3D modeling of the anatomy of animals. She showed me the model of an eagle’s humerus that was shattered after the eagle was shot and she was found unable to fly. The other humerus was printed as well, and the surgeon, Dr. R. Avery Bennett, was able to use these as reference before he operated on the eagle. You can read more about the eagle’s story here.
Janet also showed me a series of 3D-printed turtle skeletons, which includes the shell of the turtle, that are used in a surgical simulation workshop for wildlife veterinarians run by Dr. Matt Allender, of the College of Vet Med. The participants will come in, smash the turtle skeleton model, and have the opportunity to practice patching them together as they would in a surgery. Janet strategically places holes inside the walls of the shell of the skeleton models to ensure that the models will fail in the same places actual turtle shells are expected to break upon impact.
The question of which material most accurately models bone is another huge subject. The PLA filament in 3D models proved to be a bit too strong to accurately reflect the way a turtle shell will break. Janet found that ABS failed most similarly to the turtle shell, however it took too much time to print and process. Ultimately, the ideal turtle shell simulator was deemed to be a PLA powder, which provided the proper combination of strength and brittleness. In fact, ABS has proven to be the material that surgeons like the most in other projects as well, as they find it to feel the most like actual bone with roughly the same hardness and the best resolution of the materials and printers available in the Rapid Prototyping Lab in MEL.
This idea of 3D printing patient specific anatomy is not limited to the animal world. Janet worked on a collaborative research project with the Beckman Imaging Technology Group, Illinois researchers, and a Carle surgeon to create a prototype procedure that will eventually pave the way for producing implants for bone defects in human patients. This particular project involved reconstruction of the mandible, and you can learn more about it here.
MechSE Professor Mariana Kersh is utilizing synthetic bone in her orthopedic biomechanics course to give students a better understanding of the mechanics of bone as they design their own implants. With the idea of utilizing rapid prototypes to become more physically acquainted with a biological system before operating on it or designing for it, there is hope for human application in the future.