(B-57) Static Simulation within Three-Dimensional Finite Element Model of the Neonatal Brachial Plexus

Introduction:: Limited research has been conducted on the neonatal brachial plexus (BP) due to the vulnerability of the subjects. Finite element modeling (FEM) is an appropriate way to analyze injury mechanisms of the brachial plexus, but no brachial plexus properties or mechanical response data for human infants have been published. Thus, the development of the first known 3D, anatomically-accurate, neonatal brachial plexus FEM was validated against data from an in vitro neonatal piglet study. This model was created to provide insight into the progression of injury to the plexus during the birthing process – as 1-2/1000 infants are susceptible to an injury known as Neonatal Brachial Plexus Palsy (NBPP). The objective of this study is to (1) identify when stresses reach a set injury threshold (0.22 MPa) at specific levels of the plexus, (2) compare the FEM results to clinical patterns of injury, and (3) demonstrate where high stress levels occurs when the entire BP is loaded.

Materials and Methods:: The BP model was analyzed in Solidworks. As full neonatal BP dimensions cannot be determined from standard medical images, the model dimensions were assigned based on measured dimensions of neonatal BP nerve roots along with complete dimensions from a cadaveric adult BP. The developed model was reviewed by a pediatric neurosurgeon to confirm that it represented reality. An encastré B.C was used within the spinal cord of the model. A fixture constraint was used along the plexus to prescribe zero displacements in the ± y and z directions to mimic the effect that surrounding soft tissue has on limiting out of plane displacements. A static analysis was conducted to analyze the amount of stress needed to cause a BP injury as determined by a nerve root reaching an experimentally-established injury threshold. Once the necessary load was calculated, the model analyzed how stresses at various levels align with the pattern of clinically predicted strains.

Results, Conclusions, and Discussions:: A common NBPP injury includes an injury at the C5 nerve root. The load needed to simulate an injury-causing stress of 0.22 MPa was determined to be 1.55 N, distributed equally between the distal nerves of the BP. Cadaveric research has demonstrated that the cephalad nerve roots (C5/C6) experience higher stress values than the lower roots when the brachial plexus is intact, and C5 is clinically shown to be the site of the initial injury. Our model confirms these findings. Furthermore, a non-injurious stress value (0.36 MPa) was calculated within the cords of the plexus during loading that would cause root C5 injury. By simulating the progression of BP injury (sequential damage of nerve roots), the changes in stress along the plexus can be analyzed. NBPP can involve rupture or avulsion of one or more nerve roots. An anatomically accurate model of the plexus allows a more accurate assessment of how specific injuries (Erb’s Palsy, etc.) may affect the stress within the plexus as a whole.

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