The AdditiveLab team brings you an update on the next stage of the spinal fusion case design. If you have read our first article, you can skip to the latest advancement in this case.
We at AdditiveLab develop software for Additive Manufacturing (AM) process simulation and get to talk to users of our software on a regular basis. We often get to learn about their applications and challenges they face on a daily basis. Over the past few years, we got the opportunity to learn how our users manufacture their parts with high success rates. However, our users are innovators who push the limits of AM beyond imagination knowing that not all design concepts may manufacture as intended. Particularly the creation and occurrence of complex geometrical distortions post manufacturing, can sometimes be hard to explain by intuition and can leave one quite baffled.
Below we would like to share a case from Tangible Solutions of a novel spinal fusion cage concept which revealed residual deformations that were hard to make sense of. Such cases are commonly great candidates for simulation with AdditiveLab. The simulation with AdditiveLab allows users detailed insight into process characteristics at any point in time and location and helps to explain effects such as unexpected excessive deformation. With this insight users are able to further optimize their designs to get more control over the manufacturing process and its outcomes.
Matt the Director of Engineering at Tangible Solutions and his crew recently allowed us a glimpse into one of their key applications, the manufacturing of spinal fusion cages. Such cages are used to fuse vertebrae in the lower back to treat degenerative disc disease. New studies presented in literature demonstrate that cage designs with patient-specific, bone-like stiffness promote bone in-growth and segmental stability.
Example of a spinal fusion cage design additively manufactured. (Courtesy of Tangible Solutions Inc.)
What holds great potential for the adaption of the spinal cage stiffness is the utilization of sub-structures, such as lattices or helices, in order to tailor the compressional stiffness to the patient’s needs. The research that Matt’s team is doing around finding suitable sub-structure candidates is extensive and definitely requires out of the box thinking. While using non-traditional design concepts can lead to breakthrough designs that help patients, it can also mean such designs may not be manufactured as intended. One case that did not go as planned that Matt allowed us to share is illustrated below.
Example of an innovative spinal fusion cage design. (Courtesy of Tangible Solutions Inc.)
What’s so interesting about this design is how it deformed and why it deformed the way it did. And if you have not glimpsed at the pictures below, please take a few moments and make your best guess about how the above design may deform and why?
When presented with the actual manufacturing results, we at AdditiveLab were most baffled by the distortion in the central region of the printed cage since no geometrical irregularities pointed towards such a contraction (see figure below). We had several theories, including thermal issues, particularly due to the limited heat flow capabilities and down-facing surfaces of the individual helices, however, even looking at the manufactured distorted design the deformation made little intuitive sense prior to running the simulation.
Spinal fusion cage design additively manufactured showing considerable distortion in the central region of the design, left: lateral view, right: frontal view. (Courtesy of Tangible Solutions Inc.)
To get more insight into what the issue may be, we ran a simulation in AdditiveLab and this is what we saw after running a thermo-mechanical analysis:
Spinal fusion cage design simulated with AdditiveLab showing the same distortion in the central region of the design, left: lateral view, right: frontal view.
As mentioned above, our AdditiveLab team was set towards temperature accumulation in the central region which may have led to unforeseen thermal expansions and contractions that caused the distortions. However, looking at the accumulated average temperature we saw the following result:
Spinal fusion cage design simulated with AdditiveLab showing the calculated average temperatures with higher temperatures in the central region of the design, left: lateral view, right: frontal view.
In general, our expectations were met with the design having limited heat flow capabilities in the central region, however, the highest average temperatures were calculated locally at the corners of the individual helices. (We saw a difference of factor 2 between the lowest average temperature at the top and the highest one in the central region). Even though it was a good indicator that temperatures may contribute to the considerable displacement, the thermal pattern of the average temperatures did not perfectly follow the deformation pattern. I.e. the highest values of the average temperatures start to occur slightly below rather than exclusively in the maximum deformation.
From there we started looking at the displacements during the manufacturing and noticed something interesting; we saw that the deformation was fairly low during a predominant period of the building process as illustrated below:
Spinal fusion cage design simulated with AdditiveLab showing the calculated displacements at different stages of the manufacturing process (frontal view).
However, as we continued the buildup animation, we saw that as soon as the building process reaches the upper socket (where the helices connect with the top cone), the upper socket introduces a considerable bending which is subsequently transferred to the individual helices introducing the pictured global deformations.
Spinal fusion cage design simulated with AdditiveLab showing the calculated displacements at different, final stages of the manufacturing process (frontal view).
And there you have it. Based on the simulation we could clearly see that not the design of the individual helix elements caused the issue but rather the interface to the solid, upper cone. To us, it was quite a surprise because we were not expecting that the upper cone element would have such a considerable influence and caused the presented deformation.
Our next step was to determine if the Counter-Deformation approach can compensate for the occurring displacements. The Counter-Deformation approach in AdditiveLab utilizes the simulated displacements and subtracts them from the original CAD design., i.e., creating a negatively-pre-deformed structure. With this approach, we created counter deformed designs in AdditiveLab.
Simulation-driven counter deformed design of spinal fusion cage by AdditiveLab, showing mid-sections pre-deformed in the outward direction, left: lateral view, right: frontal view.
The team from Tangible Solutions produced the counter deformed spinal fusion cage designs using their standard production parameters, and sent us the production results.
Counter-deformed spinal fusion cage design additively manufactured, showing the distortions have been compensated and a successful production result is achieved, left: lateral view, right: frontal view. (Courtesy of Tangible Solutions Inc.)
This case is a clear example that simulation can not only aid in the determination of distortions and issues in the parts but can also help design parts that meet their production tolerances better.
If you have any questions feel free to get in touch with us and check out Tangible’s website. Special thanks to Matt and Tangible Solution for providing this case and pictures of the manufactured design.
Do you need more info or a demo? Write us an email via info@additive-lab.com