Absorbable Springs for Craniosynostosis
Ella Arendes Sara Jodayri Elizabeth Quinlan Vera Wong
Department of Mechanical and Aerospace Engineering
University of California, San Diego
Sponsored by Dr. Benjamin B. Massenburg
[1] Kajdic, Nina et al. “Craniosynostosis - Recognition, clinical characteristics, and treatment.” Bosnian journal of basic medical sciences vol. 18,2 110-116. 20 May. 2018, doi:10.17305/bjbms.2017.2083. https://pmc.ncbi.nlm.nih.gov/articles/PMC5988529/
[2] Spazzapan, Peter, and Tomaz Velnar. “Isolated Sagittal Craniosynostosis: A Comprehensive Review.” Diagnostics (Basel, Switzerland) vol. 14,4 435. 16 Feb. 2024, doi:10.3390/diagnostics14040435. https://pmc.ncbi.nlm.nih.gov/articles/PMC10887665/
[3] Prusa3D. “Infill Patterns.” Prusa Knowledge Base, https://help.prusa3d.com/article/infill-patterns_177130. Accessed 4 June 2025.
[4] da Silva, Dana et al. “Biocompatibility, biodegradation and excretion of polylactic acid (PLA) in medical implants and theranostic systems.” Chemical engineering journal (Lausanne, Switzerland : 1996) vol. 340 (2018): 9-14. doi:10.1016/j.cej.2018.01.010
We would like to thank Professor Maziar Ghazinejad, Professor Jennifer Mullin, and Jackie Amorin for all their mentorship throughout the project, Can Uysalel and Edward Pogue for their assistance with prototype testing, and Dr. Benjamin Massenburg for his sponsorship and guidance.
Future Recommendations
Spring-assisted sagittal suturectomy is a surgical technique used to treat craniosynostosis, a condition where premature fusion of skull sutures impairs development in infants. Currently, stainless steel springs are implemented which require a second surgery for removal. The objective of our project is to develop a bioabsorbable spring to eliminate the additional removal procedure.
Each iterative design was analyzed through ANSYS to determine its estimated force exertion and high stress concentration locations when displaced to 1 cm compression.
Secondary Design Requirements
The final design reached 16.5 Newtons of force at 1 cm of compression, exceeding the original objective of 10 Newtons of force at 1 cm of compression. The maximum force recorded was 16.7 Newtons.
Recommendation | Justification |
Grid over rectangular infill | The grid infill has higher strength (and has better layer adhesion) than the rectilinear infill. |
Printer experimentation | Each printer produces various results so experiment with infill, layer height, and temperature. |
Conduct in vitro studies | Will determine degradation rates. |
Figure 2. End of spring test on Instron machine.
Figure 3. Custom force gauge testbed.
Printer Cost
Material Cost
The bulk of the cost to replicate our design comes from the 3D printer which will range from $700-$1100 depending on the model, while a stainless steel spring can cost upwards of $2000.
Material Selection
Figure 15. ANSYS Simulation of reaction force given displacement. 18 N at 1 cm compression.
Figure 14. Stress concentrations at 1cm compression.
Overview
Figure 19. Currently used Osteomed smartflex pediatric cranial expander from Acumed.
Design Requirements
The springs were tested on an Intron 34TM-30 and placed into a custom testbed to hold them between the compression plates. Each spring was compressed until the hooks were 1 cm apart. The optimal design options were then duration tested using a custom force gauge testbed.
Test Procedure
Cost Analysis
Design Evolution
Acknowledgements
References
Impact on Society
Final Design
Results
Finite Element Analysis
Figure 5. Unsuccessful PCL prototype.
Figure 4. Failed magnesium prototype.
0.375 cm
0.175 cm
Table 1. Material selection criteria and overall ranking
The top three materials after criteria evaluation were taken to testing. PCL was unsuitable due to its flexibility and lack of mechanical strength, and magnesium lacked mechanical strength and was too brittle. PLA produced the best results and has the best manufacturability.
Radius of Curvature
Leg Length
Thickness
Figure 12. Annotated final design prototype CAD.
Figure 6. Final PLA design selection post 10 hour duration testing.
Figure 1. Infant skull depicting craniosynostosis.
Table 2. Material property degradation after 3D printing standard PLA on the Prusa MK4
Radius of Curvature (cm) | 3.5 |
Leg Length (cm) | 3.1 |
Thickness (cm) | 0.275 |
Hook Sizing
Hooks will be customized for each patient based on their skull thickness, measured from a 3D printed model provided for each surgery. This will be incorporated into the CAD design to ensure a precise, patient specific fit.
PLA properties change after being processed through a 3D printer due to melting and extrusion. A dogbone test following ASTM standards was conducted to understand its qualities for accurate simulations.
Figure 8. Polylactic acid (PLA) degradation rate and degradation process in various parts of the human body.
Figure 9. Dogbone test on PLA sample.
Figure 10. Grid infill pattern.
Figure 7. The chemical synthesis and natural biodegradation pathway of polylactic acid (PLA) in vivo.
Figure 11. Final design prototype CAD.
In the 10 hour duration testing window the final design prototype lost 8.84 Newtons of force.
Polylactic acid (PLA) served as the most suitable and accessible material option due to its high mechanical strength, affordability, manufacturability, and biocompatibility.
The infill pattern that provided the best results was grid. The type of selection made for the infill does have an effect on force testing and getting a good evaluation of material properties.
Figure 17. Beginning of duration test.
Figure 18. End of duration test.
Figure 13. CAD of adjustable hook sizes.
Figure 16. Force versus displacement graph of final spring design taken from Instron testing.
Figure 20. Evolution of spring design detailing various iterations tested.
Table 4. Force exerted and maximum stress of the spring when hooks displaced to 1 cm
Table 3. Final Design Measurements
Because the spring design was below 6 Newtons of force after duration testing, and earlier design with higher mechanical strength could be tested and implemented using a satinsky clamp with a ratcheting mechanism to slowly compress the spring to 1 cm. These higher mechanical strength spring designs snap when compressed by hand, but do not snap during slow compression Instron testing. This could provide better initial strength (previous design had approximately 28 Newtons) and remain above or within 6-10 Newtons for a longer period of time.