Cranial Suture Three-Dimensional Analysis: An Image Processing, Morphometry, and Finite Element Study

Abstract

Cranial sutures are a network of vasculature, extracellular matrix, and fibers that join the bones in the skull of most vertebrates. Not only do sutures play an important mechanical role in the skull, but they are also vital for growth and development. Sutures respond to mechanical stimuli, reorganizing and transforming on a cellular level, which in turn shapes the bulk morphometric form of the suture. Sutures exhibit non-linear viscoelastic material properties that cause them to behave differently in tension and compression. Morphometry of sutures ranges from a straight butt-ended form to a highly interdigitated tortuous form; both of which may even be found within a single suture structure in different regions. Due to the enclosed location in the skull, suture structures are typically viewed in a cross-sectional planar form. This cross-sectional view of the suture is often taken to represent the complexity of the entire structure, despite the fact that sutures can be highly spatially variable in three dimensions (3D). Previous works have typically noted morphometric and cellular variability within sutures qualitatively or analyzed the planar morphometry quantitatively. However, little attention has been given to quantifying 3D suture morphometry. The extent that a sutures’ form changes through the skull thickness, or how through-thickness variability could affect mechanical behaviour represents a gap in literature. The first objective of this work was to develop methods that quantify bulk suture complexity through the skull thickness by processing and segmenting X-ray computed tomography images to a binary form where morphometric properties could be quantified by an automated method. Second, the potential implications of treating sutures as 2D structures in mechanical simulations as opposed to using the true 3D geometry was studied using finite element analysis. Third, an age-based 3D morphometric analysis that focused on through thickness interdigitation and width variability, using the methods developed within this thesis was considered. The results of this thesis showed morphometric variability within a given suture and between sutures in a rat model. The geometric models generated using swine X-ray computed tomography datasets assumed cross-sectional as well as 3D suture representations. The geometric models were exposed to tensile loading by finite element methods, where it was found that 2D models can approximately represent the bulk average mechanics of the 3D model in certain instances. However, the 2D representations were typically found to have more uniformly distributed mechanical parameters and are unable to provide comparable resolution along the suture-bone interface to the 3D model. The age-based 3D morphometry analysis showed interdigitation and width variability of more than two times within a single suture, which highlights the necessity to treat highly interdigitated sutures as 3D structures. With one exception, there was no statistical significance found in mean suture interdigitation between age groups, and none found in mean suture width between age groups. The methods and general ideas presented in this thesis focus on treating suture complexity as a 3D problem, this has potential applications in studies that are interested in suture morphometry, development, mechanics, or simulations. The advancement of these fields could improve and optimize treatments and instruments related to orthodontics and pathological conditions involving sutures.