Modelling and surgical analysis of fracture repair of the facial skeleton using generic and patient matched finite element models

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This thesis is aimed at finite element modelling of the fractured and repaired facial skeleton, specifically the mandible and surgical aspects that influence a rapid and complete recovery process without complications that otherwise lead to significant patient morbidity and sometimes mortality. The mandible, as a site of specific investigation, was chosen due to the following factors. Firstly, as a prominent part of the facial skeleton, the mandible is a highly complex structure, being composed in its hard tissue elements by cancellous and cortical bone and the dental complex consisting of enamel, dentine and cementum. These hard tissue components house the associated soft tissues consisting of major nerves and blood vessels, fat and other connective tissues. Already one can see the complexity of the mandible but its unique anatomical character does not stop there as the entire structure articulates with the remainder of the facial skeleton via the temporomandibular joints. The mandible is also unique because it is being commonly affected by all of the major disease classifications, being congenital, traumatic, neoplastic, infective, inflammatory and iatrogenic disease. As a consequence, clinicians ranging from dentists through to maxillofacial surgeons are often confronted with complex disease patterns that require complex reconstructive methods to restore form and function. At the terminus of reconstruction, these elements of form (cosmesis) and function (speech, facial expression, mastication, deglutition, taste and airway maintenance) are what our patients demand and should be entitled to expect from modern day surgical techniques. The aim of this work is to look at the fractured mandible using finite element modelling and ascertain what provides the surgeon with the best results when performing osteosynthesis of the fracture and hence lead to rapid, complete and complication free healing. The models used in this thesis are based on a well established and validated model, initially taking its heritage from the works of Ben-Nissan in 1987, followed by Choi in 2005. In this thesis, it is termed the ‘generic (GEN) model’. A second model is also used throughout this thesis, which is based on computed tomography scans of a human mandible, termed as the ‘patient specific (PS) model’. With the GEN and PS models, various fracture fixation systems were designed and applied to both FE models and analysed for their efficacy. This brings to the study a unique attribute of comparing a GEN model with a PS model, thus allowing multiple fracture fixation systems to be analysed. In doing so, a better correlation is provided with the pathophysiology and mechanics of trauma as it occurs in vivo. The results of this study show that most importantly we have been able to produce a valid models that closely mirrors trauma and its repair as occurring in real life, not just as it appears in laboratory analysis. Not only are the models valid, but equally important, they are predictably reproducible. Specifically, the PS model could be applied in PS scenarios to allow a customised design of surgery that best suits the individual in question. In the analysis, we studied various plate shapes ranging from traditional linear plates to mesh pattern fixation plates. We also looked at various plate material ranging from titanium alloy (Ti-6Al-4V) to zirconia (ZrO₂) and polylactic acid (PLA) and also in various dimensions. With the analysis of these variants we found that whilst there were variations in performance between the various designs, they all performed equal to the physical parameters required for fracture fixation and subsequent repair. Across all variants of fixation methods, one significant factor is that the fractured ends of the bone are applied as closely as possible but not compressed. The final product of this work is a reproducible valid model where production, performance and results represent a dialectic improvement over past mandibular models and which reproduces the most realistic scenario of constraints and forces during functional movements. With further improvements, such as patient specific bone mineral density and the use of dedicated software for the manipulation of the computed tomography data, this model is destined to become a functional tool for clinical diagnosis and treatment in maxillofacial surgery and dentistry.
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