Bill Kahler

Fracture-toughening mechanisms responsible for differences in work to fracture of hydrated and dehydrated dentine

This study investigates the nature of deformation and differences in the mechanisms of fracture and properties of dentine where there has been a loss of moisture, as may occur with removal of the pulp in the endodontic treatment of teeth. Controlled fracture toughness testing was conducted on bovine teeth to determine the influence of hydration on the work of fracture of dentine. Significant differences (p<0.01) were observed between the fracture toughness of hydrated (554+/-27.7J/m2) and dehydrated (113+/-17.8J/m2) dentine. Observations of the crack tip region during crack extension revealed extensive ligament formation occurred behind the crack tip. These ligaments provide considerable stability to the crack by significantly increasing the work of fracture, thereby acting as a fracture-toughening mechanism. Micro-cracking, reported as a fracture-toughening mechanism in bone, is also clearly seen. A zone of in-elastic deformation may occur as hydrated specimens revealed upon crack extension, a region about the tip that appeared to suck water into the structure and to exude water behind the crack tip. In dehydrated dentine, no in-elastic zone was observed. Micro-cracking is present though the cracks are smaller, straighter and with less opening than hydrated dentine. Only limited ligament formation just behind the crack tip was observed. These differences resulted in a significantly lower work of fracture with unstable brittle fracture characteristics. Based on these results, several fracture-toughening mechanisms were identified in dentine, with micro-cracking not considered the most important. These findings may be relevant for bone, a similar mineralised hydrated tissue.

Revascularization outcomes: a prospective analysis of 16 consecutive cases.

INTRODUCTION Recent reviews lament the lack of evidence on the efficacy of regenerative procedures to induce further root maturation despite claims of a paradigm shift in the way infected, necrotic immature teeth with arrested root development can be endodontically treated. The majority of reports are either case series or successful case reports where nonstandardized images may make interpretation uncertain. METHODS This prospective clinical study reports on preliminary outcomes of regenerative endodontic procedures carried out on 16 teeth, 3 mandibular premolars and 13 traumatized central incisors, after 18-month reviews. Qualitative analysis of resolution of periapical radiolucencies and apical closure was undertaken. Quantitative analysis compared preoperative and recall radiographs by using a geometrical imaging program that calculated percentage changes in root length and dentin wall thickness. RESULTS Qualitative assessment showed 90.3% resolution of the periapical radiolucency. Apical closure was assessed as incomplete in 47.2% and complete apical closure in 19.4% of cases. Quantitative assessment showed change in root length varying from -2.7% to 25.3% and change for root dentin thickness of -1.9% to 72.6%. CONCLUSIONS Patterns of continued root maturogenesis were variable at 18-month review. Reviews at 36 months showed continued root maturogenesis for 2 cases. Quantitative analysis can control for changes in angulation but may introduce other measurement errors. However, not all anterior teeth were suitable for TurboReg assessment because overlapping of the cementoenamel junctions and/or further eruption of teeth often precluded stable landmark positioning. Discoloration of the crown was a common consequence, with unaesthetic results in 10 of the 16 cases.

Pulpal Response after Acute Dental Injury in the Permanent Dentition: Clinical Implications-A Review

INTRODUCTION Pulpal reactions after acute dental injury have been puzzling for many clinicians. The management of dental trauma and an understanding of clinical and treatment factors in outcomes arose from multivariate statistical analyses of archive material from Copenhagen. METHODS The aim of this article was to review the works of this period with respect to pulpal reaction after acute mechanical trauma. These traumas include luxation, avulsion, root fracture, and crown fracture. A PubMed search identified other literature where multivariate analysis was used, and these results were compared with earlier pioneering studies. RESULTS AND CONCLUSIONS This article will describe pulpal responses after the said acute injuries and outline the competition that takes place between ingrowth of a new neurovascular system into the traumatized tissue versus bacterial invasion. If there is an intact neurovascular supply to the pulp, then the same immunologic defenses that are found in the rest of the body can function and defend against infection. If this is disturbed in any way, alterations in the pulp (eg, pulp canal obliteration, resorption processes) or pulp death (pulp necrosis) will occur. Intermediary stations in pulpal response (ie, transient apical breakdown) mimicked the cardinal signs of pulp necrosis, which could be reversible and lead to pulpal healing. These processes will also be addressed with respect to a more conservative treatment approach. In young patients, it is of the utmost importance that pulp vitality be maintained to ensure continued root growth and development and an intact dentition.

A Review of Tooth Discoloration after Regenerative Endodontic Therapy.

INTRODUCTION The American Association of Endodontists clinical considerations for regenerative therapy (April 2015) advise of the risk of the possible adverse effect of staining of teeth. It is widely acknowledged that before these guidelines were developed there was no standardized protocol for regenerative therapy, and different approaches using different materials have been reported. The purpose of this review was to undertake a systematic review of published cases to determine the incidence of reported discoloration and examine whether there was any particular association with any material. METHODS A PubMed search was performed using key words for regenerative endodontic therapy consistent with prior published systematic reviews. RESULTS There were 80 studies identified with 379 teeth treated. Many of the studies did not report on the presence or absence of discoloration. It was noted that there was a strong association of discoloration with the use of triple antibiotic paste containing minocycline; however, discoloration was also noted when other materials were used. Unfortunately, bleaching of stained teeth was not predictably achieved in some studies. CONCLUSIONS The use of triple antibiotic paste with minocycline as the intracanal medicament should be reconsidered. Either calcium hydroxide or the double antibiotic paste of metronidazole and ciprofloxacin may limit discoloration. Mineral trioxide aggregate was also associated with discoloration. However, there is little evidence of an alternative superior material as a coronal barrier. Because discoloration is a patient-orientated outcome, further research should document and identify the incidence of discoloration in regenerative endodontic treatment.

Depth and Distance Perception of Dentists and Dental Students

The quality of work carried out by dentists is dependent, among other things, on experience, training, and manual dexterity. Historical focus on the latter as a predictor of dental performance has failed to recognize that dental competence also requires good perceptual and visual skills, not only for gathering information but also for judging positions, distances, and the size of objects and shapes. Most predictive tests ignore visual and interpretative deficiencies that could make individual acquisition of skills and interpretation of instructions difficult. Ability to estimate depth and distance, the manner in which students learn this ability, whether and how it can be taught, or whether there is an association among ability, stereopsis, and dental performance has not been thoroughly examined; nor has the perception that dental students fully understand verbal and written instruction relating to depth and distance. This study investigated the ability of dentists and dental students to estimate and reproduce small depths and distances and the relationship of this ability to stereopsis, dental experience, and student performance. A total of 163 undergraduate dental students from three year groups and 20 experienced dentists and specialists performed three tasks. A depth-perception task involved estimation of the depth of two sets (2-mm or 4-mm wide) of nine computer milled slots ranging in depth from 0.5 to 4.0 mm. A distance task involved estimation of the width of specially prepared printed square blocks. In a writing task, participants recorded distances across a printed line on separate sheets of paper. All tasks were conducted at set positions in custom-made transportable light boxes. Stereopsis and visual acuity were also measured. Ability to perform perceptual tasks varied enormously, with the level of accuracy dependent on the type of task and dental experience. Many students had considerable difficulty in estimating depth. Inexperienced students performed poorly. Most participants overestimated depth and distance estimation tasks, but underestimated when required to draw distances. Smaller depths and distances were easier to estimate than larger ones. All groups overestimated depth more in 4-mm-wide blocks than in 2-mm-wide blocks. There was no correlation found between depth and distance estimation and stereopsis scores or with the overall grades tested. This study highlights that some dentists and many dental students, particularly early in their course, have great difficulty in accurately gauging depths and distances. It is proposed that that this could impact significantly on a students ability to interpret verbal and written preclinical instruction and could make the acquisition of manual skills and interpretation of clinical instruction difficult. Routine testing of all undergraduate dental students for perceptual and visual difficulties is recommended, so that those with difficulties can be identified and problems remedied, if possible, early in their course.

Tooth discolouration: staining effects of various sealers and medicaments.

The aim of this investigation was to quantify the discolouration of tooth roots caused by various medicaments and sealers. The roots of extracted teeth were chemo-mechanically prepared and the smear layer removed. The roots were filled with either Multi-Cal (Pulpdent Corporation, Watertown, MA, USA), Ledermix (Lederle Laboratories, Wolfratshausen, Germany), 2% chlorhexadine acetate in polyethylene glycol (PEG), 5% clindamycin in PEG, PEG alone (as a vehicle control), sealers AH26 (De Trey Dentsply, Zurich, Switzerland), AH Plus (De Trey Dentsply), an experimental epoxy resin with zirconium dioxide (Millenium Chemicals, Rockingham, Western Australia) or Araldite (Selleys Pty Ltd, Sydney, Australia). The roots were maintained at 37°C in dark humid conditions and standardised images of the coronal surfaces recorded over 12 months. Ledermix caused the most darkening of the introduced medicaments, while calcium hydroxide and the materials in PEG did not cause darkening. AH26 caused more darkening than did the other sealers. It was concluded that the use of alternatives to Ledermix and AH26 should be considered when aesthetics are important.

Analysis of interfacial fracture in dental restorations

OBJECTIVES To provide a brief summary of the background theory of interfacial fracture mechanics and develop an analytical framework that identifies the critical factors for the analysis of the initiation and propagation of adhesion failure in composite restorations. METHODS A conceptual framework utilizing interfacial fracture mechanics and Toyas solution for a partially delaminated circular inclusion in an elastic matrix, which can be applied (with caution) to approximate polymer curing induced cracking about composite resins for class 1 cavity restorations. RESULTS The findings indicate that: (1) most traditional shear tests are not appropriate for the analysis of the interfacial failure initiation; (2) material properties of the restorative and tooth material have a strong influence on the energy realize rate; (3) there is a strong size effect; and (4) interfacial failure once initiated is characterized by unstable propagation along the interface almost completely encircling the composite. SIGNIFICANCE The work is important for the analysis of the reliability of composite class I restorations and provides an adequate interpretation of recent adhesion debonding experimental results utilizing tubular geometry of specimens. The approach clearly identifies the critical parameters including; curing strain, material modulii, size and interfacial strain energy release rate for reliable development of advanced restorative materials.

The cervical wedge-shaped lesion in teeth: a light and electron microscopic study

BACKGROUND The cervical non-carious wedged-shaped lesion is controversial in that its aetiology may involve attrition, erosion, abrasion and stress-corrosion (abfraction). This study examined the histopathology of anterior teeth with cervical wedge-shaped lesions by light and electron microscopy to elucidate their pathogenesis. METHODS Ten undecalcified human teeth with cervical lesions were available for investigation. Patency of the dentine tubules was tested using red dye penetration from the pulp chamber. The morphology of normal and sclerotic dentine adjacent to the cervical wedge-shaped lesions was investigated by scanning electron microscopy. The numbers and diameters of dentinal tubules were measured at different levels beneath the surfaces of the lesions. RESULTS The gross and microscopic features of the worn teeth were described. Red dye penetration tests showed white tracts of sclerotic tubules contrasted with red tracts of patent tubules. Numbers of tubules per square area and diameters of patent and sclerotic tubules varied at different levels within the dentine due to deposits of intratubular dentine. CONCLUSIONS The cervical wedge is shaped by interactions between acid wear, abrasion and dentinal sclerosis. No histopathological evidence of abfraction was found. Clinical diagnosis, conservation and restoration of non-carious cervical lesions need to take into account the extent of sclerotic dentine beneath wedge-shaped lesions.

Particle Size Changes in Unsealed Mineral Trioxide Aggregate Powder

INTRODUCTION Mineral trioxide aggregate (MTA) is commonly supplied in 1-g packages of powder that are used by some clinicians across several treatments against the manufacturers instructions. ProRoot MTA cannot be resealed after opening, whereas MTA Angelus has a resealable lid. This study assessed changes in particle size distribution once the packaging had been opened. METHODS Fresh ProRoot MTA and MTA Angelus powder were analyzed by using laser diffraction and scanning electron microscopy and compared with powder from packages that had been opened once and kept in storage for 2 years. The ProRoot packet was folded over, whereas the MTA Angelus jar had the lid twisted back to its original position. RESULTS After 2 years, ProRoot MTA powder showed a 6-fold increase in particle size (lower 10% from 1.13 to 4.37 μm, median particle size from 1.99 to 12.87 μm, and upper 10% from 4.30 to 34.67 μm), with an accompanying 50-fold change in particle surface area. MTA Angelus showed only a 2-fold increase in particle size (4.15 to 8.32 μm, 12.72 to 23.79 μm, and 42.66 to 47.91 μm, respectively) and a 2-fold change in particle size surface area. CONCLUSIONS MTA reacts with atmospheric moisture, causing an increase in particle size that may adversely affect the properties and shelf life of the material. Smaller particles have a greater predisposition to absorb moisture. Single-use systems are advised.

D90: The Strongest Contributor to Setting Time in Mineral Trioxide Aggregate and Portland Cement

INTRODUCTION The setting times of commercial mineral trioxide aggregate (MTA) and Portland cements vary. It was hypothesized that much of this variation was caused by differences in particle size distribution. METHODS Two gram samples from 11 MTA-type cements were analyzed by laser diffraction to determine their particle size distributions characterized by their percentile equivalent diameters (the 10th percentile, the median, and the 90th percentile [d90], respectively). Setting time data were received from manufacturers who performed indentation setting time tests as specified by the standards relevant to dentistry, ISO 6786 (9 respondents) or ISO 9917.1 (1 respondent), or not divulged to the authors (1 respondent). In a parallel experiment, 6 samples of different size graded Portland cements were produced using the same cement clinker. The measurement of setting time for Portland cement pastes was performed using American Society for Testing and Materials C 191. Cumulative heat release was measured using isothermal calorimetry to assess the reactions occurring during the setting of these pastes. In all experiments, linear correlations were assessed between setting times, heat release, and the 3 particle size parameters. RESULTS Particle size varied considerably among MTA cements. For MTA cements, d90 was the particle size characteristic showing the highest positive linear correlation with setting time (r = 0.538). For Portland cement, d90 gave an even higher linear correlation for the initial setting time (r = 0.804) and the final setting time (r = 0.873) and exhibited a strong negative linear correlation for cumulative heat release (r = 0.901). CONCLUSIONS Smaller particle sizes result in faster setting times, with d90 (the largest particles) being most closely correlated with the setting times of the samples.