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Bone Bending: Developing An Animal Model For Mechanical Properties Of Human Infant Calvarium
Devansh Saini, PhDc1, Lee Alkureishi, MD1, Russell Reid, MD PhD2, Pravin Patel, MD1, Linping Zhao, PhD3, Roberto Leonardo Diaz, PhD1, Farid Amirouche, PhD1.
1University of Illinois at Chicago, Chicago, IL, USA, 2The University of Chicago, Chicago, IL, USA, 3Shriners hospital for children, Chicago, IL, USA.

PURPOSE: Manipulation of the shape of infant calvarium is an integral part of surgery to correct craniosynostosis, and may involve cutting, drilling, weakening, or bending the bone selectively in order to achieve the desired outcome. Little is known of the mechanical properties of infant calvarium and its response to mechanical manipulation, beyond the surgeonís "feel" in the operating room. Knowledge of these mechanical properties can provide insight into its response to physical loads, limits for plastic/elastic deformation, and may inform future development of novel surgical techniques. As such, we sought to develop an animal model which most closely resembles the biomechanical characteristics of human infant calvarium, as the foundation for future research into cranial bone shape manipulation. In this study, we analyzed the boneís response to bending stimulus.
METHODS: Candidate species models included macaque monkey, neonatal swine, beagle, and human adult calvarium. 8 specimens were tested for each of the planned tests. A universal test machine (UTM) with custom fixtures was used to perform a 3-point bend test on each specimen at a preload of 10N and a rate of 10mm/min until specimen fracture. Load-displacement curves and break forces such as peak load, stress, and strain were recorded during the test. The bending modulus was computed as the linear slope of the stress-strain plot, while the maximum point in this curve is defined as the bending strength. Our results were then compared with historical data for the human infant calvarium.

RESULTS: An unpaired t-test was used to compare bending modulus and strength from different species. The mean bending modulus was found out to be 3.03 GPa (SD 1.4) for adult human calvarium, 1.73 GPa (SD 0.85, p=0.06) for canine, 1.71 GPa (SD 0.73, p=0.05) for macaque, and 0.82 GPa (SD 0.14, p=0.001) for pig. The mean bending strength for the adult human calvarium was 61 MPa (SD 12.8), 74.12 MPa (SD 37.85, p=0.4) for canine, 74.43 MPa (SD 27.6, p=0.26) for macaque, and 29.05 MPa (SD 7.49, p=0.001) for pig. The elastic modulus of composite human cranial bone in bending is 1.37 GPa at around the time of birth.

CONCLUSION: Our results in this study show that the neonatal piglet model shows bending properties closest to that of a 1-week-old human infant (E=820.9 MPa at a loading rate of 2540mm/min). While canine and macaque models show similar properties, the adult human has a bending modulus that is 3.7 times that of the pig. This study is the first in a series examining the mechanical properties of various animal skull bone models. These results will be collated with finite element analysis and subjective testing to inform the optimal choice of model for future development of surgical techniques for cranial bone shape manipulation.

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