DTUK2010

D entin is the most abun- dant mineralised tissue in the human tooth. In spite of this importance, over half a century of research has failed to provide consistent values of dentin’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 visco- elastic behaviour (time-depend- ent stress relaxation) at strain rates of physiologic (functional) relevance. The reported tensile strength data suggests that fail- ure initiates at flaws. These flaws may be intrinsic, perhaps re- gions of altered mineralisation, or extrinsic, caused by cavity or post channel preparation, wear, or damage. There have been few studies of fracture toughness or fatigue (18) . Finally, little is known about the biomechanical proper- ties of altered forms of dentin subsequent to decay, the influ- ence of irrigants, chemicals and the choice of curing techniques used for bonded restorations (19) . Studies suggest that there are at least two forms of transpar- ent or sclerotic dentin; a form associated with caries and a form associated with age-related changes in the root. The impact upon tooth strength as a function of these altered forms of dentin is not well understood. The long-term predictability of residual coronal tooth struc- ture to function in a manner commensurate with the de- mands of the orofacial ecosys- tem, may need to be reassessed in light of observations that sclerotic dentin, unlike normal dentin, exhibits no yielding be- fore failure and that the fatigue lifetime is deleteriously affected at high stress levels (20) . Mecha- nisms for energy dissipation and crack growth resistance present in young dentin are not present in old dentin. Restorative meth- ods and techniques, particularly as it relates to ferrule creation for endodontically treated teeth, may need to be amplified to ad- dress the fact that fatigue crack growth resistance of dentin de- creases with age (21) (Fig 3). Addressing clinical problems Understanding the mechanical properties of teeth is essential in order to address the most common clinical problem af- fecting all endodontically treated teeth, fracturing, which in spite of even minimal loss of tooth structure may be severe enough to necessitate removal (22-24) . The hypothesis that dentin brit- tleness increases with dimin- ished moisture content has been debunked; conserving bulk dentin is the sine qua non of fracture prevetion. Kuttler et al reported that dentin thickness correlates in- versely to post space diameter in the distal roots of mandibular molars (25) . A #4 Gates-Glidden drill caused strip perforations in 7.3 per cent of canals studied. The authors recommend that Gates-Glidden drills no larger than a size #3 be used. After en- dodontic treatment, the furcation side dentin thickness was less than 1mm in 82 per cent of the distal roots studied (Fig 4). There are primary causes that predispose teeth to fracture and secondary causes that pre- dispose fracture after a period of time (Fig 5). Endodontics is a component of an interdiscipli- nary process and a chain is only as strong as its weakest link. Subsequent to any endodon- tic procedure, intensity of stress concentration and tensile stress- es within an endodontically treated tooth will depend upon (1) the material properties of the crown, post, and core material chosen, (2) the shape of the post, (3) the adhesive strength at the crown–tooth, core–tooth, and core– post, post–tooth interfaces, (4) the magnitude and direction of occlusal loads, (5) the amount of available tooth structure and (6) the anatomy of the tooth. Any combination of vectored stress concentration and high tensile stresses will predispose these teeth to fracture without an adequately engineered restora- tive design. Reengineering Reengineering negative treat- ment outcomes is a significant part of the contemporary en- dodontic oeuvre. The presence of apical periodontitis may or may not affect the outcome of initial endodontic treatment (26) ; however, there is a general con- sensus that apical periodontitis Back to the Egg; Part II Kenneth Serota continues his look at the Endo- dontic Implant Algorithm Fig 3. Two different retreated teeth; two different potential treatment outcomes. The root canal system of both teeth has been reengineered in its anatomic entirety; however, the treatment outcome after restoration for both is unlikely to be the same. Regenerative technologies incorporating mesenchymal stem cells derived from dental tissues may one day obviate the concern. ‘There are primary causes that predispose teeth to fracture and secondary causes that predispose fracture after a period of time ’ August 2-8, 201024 United Kingdom EditionClinical The UK’s leading supplier of dental anaesthetic, Septodont, already bring you the high quality Septoject and Septoject XL needles and would now like to introduce a new development to Ultra Safety Plus. Ultra Safety Plus syringe is a sterile, disposable and self aspirating syringe system with a pre-mounted needle. Its use means needle stick injuries can be virtually eliminated. With the option of a NEW single use handle (non sterile), Ultra Safety Plus is now 100% disposable. For more information please contact your dental retailer www.septodont.co.uk Anaesthetics • Endodontics Restorative Dentistry • Dental Surgery Prosthetic Dentistry • Disinfection & Hygiene