An Investigation Of The Physical Formation Of Post Bonding Interface

IntroductionPosts and cores are frequently used in endodontically treated teeth that suffered excessive loss of coronal tooth structure. The post is necessary for the retention of the crown. The common reason lead to the failure of the post restoration is displacement of post. This study was to investigate the physical structure of the dentine - cement - post bonding interface by Laser scanning confocal microscope to reveal the relation between the micro - conformation and the macro-scopical retention force.Material and Methods1. Eighteen extracted intact human maxillary anterior teeth were sectioned at 2mm coronal to the cementoenamel junction that were then for regular endodontic treatment. After one week, the roots were prepared for post insertion. The canal space of each root was enlarged with ParaPost twist drills to a final diameter of 1.5 mm and a depth of 9 mm from the cervical surface.2. All roots were embedded with transparent epoxy resin in 18 mm * 15 mm * 9mm rectangle block, the prepared post space located in center and vertical tothe 18 mm * 9 mm surface. Two different prefabricated posts were cemented respectively with the three different luting cements.3. All specimens were attached to the grips of a low speed saw and sectioned parallel to long axis of the posts at a speed of 5 w/min. Three locating notches for standardized examination of the interface were made by a scalpel, 2, 5, 8 mm apical to the dentin - core junction.4. Observations were performed by confocal microscope of the dentin - cement - post interface. Six areas were observed in each root half. Random software was used to measure the cement thickness.5. A Latin Square Design analysis of variance ( ANOVA) was performed to compare the results.ResultsDifferent kind of cement did not affect the cement thickness; but there was obvious difference between the different part of the root, the part of apical < the part of middle < the part of neck. And different posts affected the cement thickness ; the cement layer of ParaPost Fiber White was thicker than the that of Para-Post Ti. The cement layer of ParaPost Cement was uniformity, no microgap, and there were much hybrid layer; the cement layer of Glass Ionomer Cement was less uniformity, and there were some microgap; the cement layer of Zinc Phosphate Cement was very roughness, there were many microgaps and some big bubbles. There was hybrid layer neither in the GIC group nor in the ZPC group. There were fluorescence bundle in the structure of ParaPost Fiber White, but there was nothing in the structure of ParaPost Ti.DiscussionThe most important factor to the retention of the post is the friction between post and the dentin wall, and the second is the bond force of the cement. Therefore , in order to gain more retention, we have to increase the closeness of fit and the contact area. This study indicate that the cement thickness from the apical to the coronal is apical < middle