Vertical root fracture and fracture-related properties of dentine

© 1999 Dr. Veera LertchirakarnThe prognosis of vertical root fracture (VRF) is unfavourable and endodontic procedures, especially lateral condensation, have been suggested as a cause of VRF. In addition, the mechanislTI of VRF resulting in a typical buccolinguaI fracture has not been investigated. The possibility of lateral condensation as a cause of VRF was investigated by comparing loads and strains during obturation with those at fracture. The mechanism of VRF was also investigated, in association with stress and strain distribution. Finally, selected tensile properties of dentine were examined in relation to patterns of fracture. Lateral condensation alone should not be a cause of VRF because loads and strains on the outer root surface generated during lateral condensation were much lower than at fracture in each tooth type. However, to avoid VRF, finger spreaders should be used to obturate root canals, because the results showed that loads and strains generated during lateral condensation using finger spreaders were significantly lower than when a hand spreader (D 11 T) was used. The mechanism of VRF was investigated using finite element analysis (FEA), and the results were validated by experimentally measuring outer root surface strains using strain gauges. The strain measurement results correlated well with predicted circumferential strain distribution from FEA models. FEA results showed that stress distribution was not uniform, and the highest tensile stresses were found on root canal surfaces in a buccolingual direction, concentrated in areas of greatest curvature of the root canal. The results suggested that the dentine thickness, root morphology and root canal shape, including root canal irregularities, all contributed to this pattern of non-uniform stress distribution. In addition, a root canal shape predisposing to stress concentration areas is a more influential factor than root morphology, and a severe oval root shape is more susceptible to VRF than less oval or circular root shape. The root with a combination of these factors is the most susceptible to VRF. The SEM photographs of fracture surfaces suggested that the microstructure of dentine may affect the tensile strength of dentine. Dentine was shown to behave as an anisotropic material with regard to ultimate tensile strength (UTS), with the highest UTS when the tensiIe force was perpendicular to tubule orientation. Thus, buccolingual fractures, which are the predominant pattern of VRF in all teeth, occur in a direction requiring the greatest energy to fracture. This suggested that the mechanisms which create local high tensile stress concentration or non-uniform stress distribution play a stronger role in VRF rather than dentine structure. UTS of dentine was shown to depend on the tubule orientation and location (distance from pulp cavity) of dentine. These suggested that both orientation and microstructure (hydroxyapatite and collagen fibrils) of intertubular and peri tubular dentine contributed to the anisotropy of dentine. Therefore, UTS of dentine should be described in relation to location (distance from pulp cavity) as well as tubule orientation. However, because of the complexity of dentine components and the small size of specimens, developing an improved testing system is necessary to achieve more information to understand fracture behaviour and other mechanical properties of dentine