Factors Affecting Fracture Susceptibility of Tooth Root: A Laboratory and Finite Element Analysis (FEA) Study Chankhrit Sathorn Doctor of Clinical Dentistry November 2004 Endodontic Unit School of Dental Science, Faculty of Medicine, Dentistry and Health Sciences Submitted in partial fulfillment of the requirements of the degree of Doctor of Clinical Dentistry Abstract From a fracture mechanics viewpoint, structural defects, cracks or canal irregularities are likely to play a major role in fracture susceptibility of the roots, because stresses can be exponentially amplified at these sites. By incorporating defects into a smooth round canal using rotary NiTi, theoretically the roots could be strengthened. The aims of the study were to determine whether rotary NiTi canal preparation strengthens roots, and whether the fracture pattern can be predicted by finite element analysis (FEA) models. 25 teeth were prepared using hand file and another 25 using rotary NiTi. After obturation, all teeth were subject to loading until fracture; load and patterns were recorded. Four FEA models were created from fractured roots. No significant difference of fracture load between the two techniques was found. Mesio-distal fracture occurred more often in the rotary NiTi group. Stress patterns in three of the four FEA models correlated well with the observed fracture patterns. The aim of the subsequent study was to determine the extent to which canal size, radius of curvature and proximal root concavity influence fracture susceptibility and pattern. A standardized cross-section of the mid-root region of a mandibular incisor was created by averaging the dimensions of ten extracted teeth, and the basic FEA model was created. By varying canal diameter, shape and proximal concavity, these factors could be examined for roles in fracture susceptibility and pattern. The factors all interact in influencing fracture susceptibility and pattern, with dentine thickness not the ii only determining factor. The removal of dentine does not always result in increased fracture susceptibility. iii Declaration This is to certify that the thesis comprises only my original work except where indicated in the preface; due acknowledgement has been made in the text to all other material used; the thesis is 12,858 words in length, inclusive of footnotes, but exclusive of tables, maps, appendices and bibliography. Dr Chankhrit Sathorn iv Acknowledgements Mr Dariusz Walter, a PhD candidate in engineering, for his expertise in LUSAS software. Dr Michael Swain, Professor of Aerospace Mechanical, and Mechatronic Engineering at University of Sydney, for his invaluable suggestions and insights in biomechanical properties of dentin. Dr Sabu John, Associate Professor of Engineering at RMIT, for his in depth understanding of engineering aspects of this research project. Dedication To my family, I couldn’t have done all this, without you. To my high school teacher, Ms Poonsup Mitsumpun, who has always inspired me with her endless enthusiasm and innate curiosity. A great example of how one fine teacher could shape many more student lives. To my mentor, Prof Harold H Messer, a wonderful scientist and a great human being. v Preface This thesis is a part of ongoing research at the University of Melbourne into different aspects of vertical root fractures and endodontics. It consists of four chapters; the first two chapters are introduction and literature review, the latter two are manuscripts that have already been accepted for publication in the Journal of Endodontics. Spelling and referencing formats differ in Chapters 3-4 according to the requirements of the Journal of Endodontics. References are included at the end of Chapters 2-4 because of the structure of the thesis. vi Table of Contents Chapter 1: Introduction .............................................................................................1 Chapter 2: Literature review .....................................................................................2 Vertical Root Fracture (VRF)................................................................................. 2 Clinical problems................................................................................................2 Prevalence...........................................................................................................2 Prognosis.............................................................................................................3 Etiology ...............................................................................................................4 Endodontic procedures.................................................................................. 4 CO 2 test........................................................................................................ 4 Access opening ........................................................................................... 5 Calcium hydroxide medication.................................................................5 Effects of NaOCl ......................................................................................... 6 Canal preparation....................................................................................... 6 Obturation................................................................................................... 8 Retropreparation......................................................................................... 9 Bleaching ..................................................................................................... 9 Restorative procedures ................................................................................ 10 Management ..................................................................................................... 13 Fracture related biomechanical properties of dentine ...................................... 14 Moisture content............................................................................................... 15 Hardness ........................................................................................................... 15 Young’s modulus ............................................................................................. 16 Ultimate strength.............................................................................................. 17 Fracture mechanics .............................................................................................. 18 Stress.................................................................................................................. 18 Stress vs. Strength......................................................................................... 19 Strain.................................................................................................................. 20 Young’s modulus ............................................................................................. 20 Fracture toughness........................................................................................... 21 Mode of Fracture .............................................................................................. 21 Brittle fracture............................................................................................... 21 Ductile fracture............................................................................................. 22 Why do fractures occur below a material’s ultimate tensile strength?........ 22 Fatigue failure............................................................................................... 23 Crack, flaw size concept............................................................................... 23 Crack arrest ................................................................................................... 27 Fracture prediction........................................................................................... 28 The study of stress and fracture...................................................................... 28 Experimental method................................................................................... 28 Theoretical method....................................................................................... 28 Numerical method ....................................................................................... 29 Aims ...................................................................................................................... 30 References ............................................................................................................. 31 Chapter 3: Article I Laboratory study and FEA verification................................ 38 vii Abstract................................................................................................................. 39 Introduction.......................................................................................................... 40 Materials and Methods........................................................................................ 41 Results................................................................................................................... 46 Discussion............................................................................................................. 48 References ............................................................................................................. 52 Figure legends ...................................................................................................... 54 Figure 1 ................................................................................................................