ROEN0210

10 I I special _ endo-implant algorithm Dentine is the most abundant mineralised tissue in thehumantooth.Inspiteofthisimportance,overhalf a century of research has failed to provide consistent values of dentine’s mechanical properties. In clinical dentistry, knowledge of these properties is pivotal to any number of variables, ranging from innovations in preparation design to the choice of bonding materials and methods. The Young’s modulus (the measure of the stiffness of an isotropic elastic material) and the shear modulus (modulus of rigidity) are diminished by viscoelastic behaviour (time-dependent stress relax- ation)atstrainratesofphysiological(functional)rele- vance.Thereportedtensilestrengthdatasuggeststhat failure initiates at flaws. These flaws may be intrinsic, perhaps regions of altered mineralisation, or extrinsic, causedbycavityorpost-channelpreparation,wear,or damage. There have been few studies of fracture toughnessorfatigue.18 Finally,littleisknownaboutthe biomechanical properties of altered forms of dentine subsequent to decay, the influence of irrigants and chemicals,andthechoiceofcuringtechniquesusedfor bondedrestorations.19 Studies suggest that there are at least two forms of transparent or sclerotic dentine: a form associated with caries and a form associated with age-related changesintheroot.Theimpactupontoothstrengthas afunctionofthesealteredformsofdentineisnotwell understood. The long-term predictability of residual coronal tooth structure to function in a manner com- mensurate with the demands of the orofacial ecosys- temmayneedtobereassessedinlightofobservations that sclerotic dentine, unlike normal dentine, does not exhibityieldingbeforefailureandthatthefatiguelife- time is deleteriously affected at high stress levels.20 Mechanisms for energy dissipation and crack growth resistance present in young dentine are not present in old dentine. Restorative methods and techniques, particularly regarding ferrule creation for endodonti- callytreatedteeth,mayneedtobeamplifiedtoaddress thefactthatfatiguecrackgrowthresistanceofdentine decreaseswithage(Fig.3).21 Understanding the mechanical properties of teeth is essential in order to address the most common clinical problem affecting all endodontically treated teeth,fracturing,whichinspiteofevenminimallossof tooth structure may be severe enough to necessitate removal.22–24 The hypothesis that dentine brittleness increases with diminished moisture content has been debunked;conservingbulkdentineisthesinequanon of fracture prevention. Kuttler et al. reported that dentine thickness correlates inversely to post-space diameter in the distal roots of mandibular molars.25 Asize#4Gates-Gliddendrillcausedstripperforations in7.3%ofcanalsstudied.Theauthorsrecommendthat Gates-Glidden drills no larger than a size #3 be used. After endodontic treatment, dentine thickness on the furcation side was less than 1mm in 82% of the distal rootsstudied(Fig.4). There are primary causes that predispose teeth to fracturingandsecondarycausesthatpredisposeteeth tofracturingafteraperiodoftime(Fig.5).Endodontics is a component of an interdisciplinary process and a chain is only as strong as its weakest link. Subse- quenttoanyendodonticprocedure,intensityofstress concentrationandtensilestresseswithinanendodon- ticallytreatedtoothwilldependupon: 1)the material properties of the crown, post, and core materialchosen; 2)theshapeofthepost; 3)the adhesive strength at the crown–tooth, core– tooth,core–post,andpost–toothinterfaces; Fig. 6_Flat field peri-apical radiograph (left); small focal field CBCT (right; Kodak 90003D, Kodak Dental Systems). The differential in visualisation of peri-apical pathology from a 3-D to a 2-D image is as much as 2:1.62 Fig. 7_The initial endodontic treat- ment procedure was inadequate and failing. Re-engineering (inclusive of interim calcium hydroxide therapy) ensured optimal eradication of microflora from the root-canal space, and the obturation produced definitive closure of the apical termini. Surgery was performed to redress persistent symptoms. Fig. 8_The strength of the egg-like coronal structure of a tooth can support substantial occlusal stress and force; however, disrupting the integrity of the ‘dome’ or roof of the pulp chamber with an access prepa- ration will invariably lead to a statisti- cally significant degree of fracturing after endodontic therapy.16 roots2_2010 Fig. 7 Fig. 8 Fig. 6