Computational simulation of cranial soft tissue expansion on the cranium during early postnatal growth in humans

The importance of interactions between neighbouring rapidly growing tissues of the head during development is recognised, yet this competition for space remains incompletely understood. The developing structures likely interact through a variety of mechanisms, including directly genetically programmed growth, and are mediated via physiological signalling that can be triggered by structural interactions. In this study, we aimed to investigate a different but related potential mechanism, that of simple mechanical plastic deformation of neighbouring structures of the head in response to soft tissue expansion during human postnatal ontogeny. We use computational modelling and normative real-world data to evaluate the potential for mechanical deformation to predict early postnatal cranial shape changes in humans. We test some aspects of the spatial packing hypothesis applied to the growing brain and masticatory muscles, and their effects on the cranium, with a particular focus on the basicranium and face. A simple finite element model of an early postnatal human cranium, brain and masticatory muscles was created from CT and MRI. Growth of the brain and muscles was simulated using a tissue expansion material. The effect of the expanding soft tissues on the cranium was assessed using geometric morphometrics, comparing the baseline model to simulation results, and also to normative cranial shape data collected from neonatal MRI (0-4 months of age). Findings revealed that cranial shape changes present in the normative sample were consistent with cranial base flexion and were largely allometric (size-linked). Simulation of brain expansion produced broadly similar shape changes of the basicranium with most growth occurring in the cranial vault, while masticatory muscle expansion produced smaller and more widespread changes throughout the cranium. When simulated together, expansion of the masticatory muscles exerted a constraining effect on the results of brain expansion. Our findings that the simple growth simulations were able to mimic biological growth suggest that the relationship between regions of the developing head may be partly structural within the first few months of postnatal ontogeny in humans.