Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Supervisor

Jenkyn, Thomas R.

Abstract

Resonant frequencies have been suggested as a mechanism of brain injury since these vibrations can transfer energy into the brain. Study of the vibrational response of the craniofacial skeleton to impact is limited in literature. In this research, four cadaver specimens were impacted at five locations on the craniofacial skeleton. The mechanical response to each impact was compared in the time and frequency domains. Impacts to the maxilla and its associated soft tissues tended to be attenuated, while impacts to the cranial vault, specifically to the occipital, produced the most severe response. Results suggest that the facial skeleton and its soft tissues act as an energy absorbing zone. Overall skull resonant frequencies were dominated by peaks between 113 and 521Hz. Minor peaks were also excited at frequencies above 1000Hz. Results demonstrated that the overall resonant frequency response was not significantly influenced by impact height or location.

Summary for Lay Audience

Brain injury places a substantial economic and social burden on society. Injury to the brain can occur from indirect and direct blows to the head. This research examines the response of the craniofacial skeleton (CFS) and its transient vibration during impact. Firstly, the role of the CFS in mitigating injury was investigated. Secondly, the time and frequency response throughout the CFS was compared between specimens.

The CFS, also known as the skull, is comprised of the cranial and facial bones. The geometry, material properties, and structures of the facial skeleton are proposed to mitigate brain injury by acting as a zone that absorbs the energy of impact. Fresh-frozen cadaver heads were impacted at five different bone locations to investigate the behaviour of the CFS due to impact. Our findings indicate that impacts to the maxilla – upper jaw – attenuates impact force due to thick soft tissue and fat present on the face. Impacts to the cranium of the skull were more severe, with the occiput – the back of the head – producing the most severe impacts. Comparing impact sites, results indicate that the facial skeleton is capable of reducing impacts for forces below fracture level, but it is suggested that there are limitations to the amount of force the facial bones and soft tissues are capable of absorbing before failure.

For very short impacts, it is hypothesized that the force propagates through the skull causing vibration at resonant frequencies specific to each skull. Measuring the impact responses at nine locations throughout the skull, an overall frequency profile was developed for each specimen. Comparing the rate at which the force travels throughout the skull, there was weak correlation between distance and response time. This suggests that there are force propagation pathways and vibration modes that are specific to each skull and not equally excited by impacts on all bone locations. Our results indicate that the dominant frequencies of the skull are not significantly influenced by impact height and location, but varied between specimens. This suggests that each individual has a specific natural frequency response.

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