Circumferential Strains in Roots of Premolars/Incisors under Internal Hydrostatic Pressure

Objective: Maxillary second premolars have a higher incidence of vertical root fracture (VRF) post-endodontic treatment compared to maxillary incisors. The aim of this study was to determine the circumferential strains of entire roots of maxillary second premolars (MSP) compared to maxillary central incisors (MCI) by a pressurization technique. Methods: The crown-down technique with final apical filing to No. 50 was used to endodontically treat 7 MSP and 9 MCI. Circular stainless steel plates with two small tubes extending from both sides connected the tooth to a pressure system. One tube was bonded to the access cavity preparation to the level of the CEJ; the other was connected to a piston. The apical (flowable composite resin) and coronal (Penloc acrylic resin) ends of the root canal were sealed. Four miniature strain gauges (031-EC or 031-DE) were bonded on the buccal (B), Lingual (L), mesial (M) and distal (D) sides of the center of the roots. A pressure transducer directly measured the pressure that developed in the system. The piston was pressed in a constant speed that elevated the internal hydrostatic pressure until water was observed on the root surface (=VRF). Results: Pressure at failure for MCI and MSP was 246±73 bar and 167±22 bar respectively (p=0.04). In all teeth the circumferential strains highly correlate the internal hydrostatic pressure (r>0.9, p<0.01). The mean of circumferential maximal tensile strains at failure on the proximal sides (M=1208±491µs; D=1031±676µs) were significantly higher than those on the bucco-lingual sides (B=441±360µs; L=369±370µs) (p=0.001). Conclusions: Premolars are more prone to VRF compared to incisors. The internal hydrostatic pressure at failure does not predict the magnitude of the circumferential maximal strains