Stephen C. Bayne

Effect of in vivo crown margin discrepancies on periodontal health

Numerous reports have suggested a relationship between marginal adaptation of dental castings and periodontal tissue health, and this study examined this relationship quantitatively. Forty-two crown restorations in 29 randomly selected patients were selected for this study using three criteria. (1) The crowns were placed at the University of North Carolina School of Dentistry; (2) the crowns were in service for a minimum of 4 years; and (3) the crown margins were within the intracrevicular crevice (subgingival). Replica impressions of the facial margins of specific crowns were made with a vinyl polysiloxane impression material, and poured casts were prepared for scanning electron micrograph evaluation. Marginal discrepancy measurements were identified on each micrograph at 10 equally spaced locations along the margin and averaged for each specimen. Periodontal indices of pocket depths, crevicular fluid volume, and gingival index were accumulated for clinical measurements. Pearson correlation and Bonferroni adjusted probability tests were performed, but no significant correlation was found between marginal discrepancy (0.16 +/- 0.13 mm) and pocket depth (2.4 +/- 0.9 mm). However, a strong correlation (p less than 0.001) existed between marginal discrepancy and gingival index (2 +/- 0.8) and between marginal discrepancies and crevicular fluid volume (49.9 +/- 31.1). These results established that a significant quantitative relationship existed between the marginal discrepancy and periodontal tissue inflammation for subgingivally located crown margins.

Examining Tooth Flexure Effects on Cervical Restorations: A Two-Year Clinical Study

This study evaluated the clinical performance of dentinal adhesives in seven various material/technique combinations. No statistically significant differences were observed among the restorative combinations, or among technique variables after two years in terms of retention, sensitivity or USPHS categories. However, other factors related to tooth flexure--such as occlusal stress, patient age, restorative material and restoration location--showed statistically significant associations with retention failures. These results support a tooth flexural theory of restoration retention.

Three-dimensional finite element analysis of stress-distribution around single tooth implants as a function of bony support, prosthesis type, and loading during function☆

The elastic limit of bone surrounding implants may be surpassed and thus produce microfractures in bone. The purpose of this study was to use computer simulations to examine clinical situations with IMZ implants in edentulous mandibles and to identify loading conditions that could lead to bone microfractures. Three-dimensional finite element analysis models were used to examine effects of: (1) types of edentulous mandibles, (2) veneering materials, (3) the absence of cortical bone, (4) different intramobile elements, (5) loading directions, and (6) loading levels. Stress distribution patterns were compared and interfacial stresses were monitored specifically at four heights along the bone-implant interface. Stresses were concentrated toward cortical bone (0.8 to 15.0 MPa). There were no differences between types of veneering materials and the absence of cortical bone increased interfacial stresses. The use of a titanium intramobile element decreased stresses. Minor stress increases were associated with smaller mandibles. Oblique loads increased stresses 15 times, and 200 N loads increased stresses 10 times. Conditions for bone microfracturing were associated with oblique loads, high occlusal stress magnitudes, and the absence of cortical bone.

A review of adhesion science.

OBJECTIVE Adhesion or cohesion includes an adherend, adhesive, and intervening interface. Adhesive joints may include one or more interfaces. Adhesion science focuses on understanding the materials properties associated with formation of the interfaces, changes in the interfaces with time, and events associated with failure of the interfaces. METHODS The key principles for good interface formation are creation of a clean surface, generation of a rough surface for interfacial interlocking, good wetting of the substratum by the adhesive/cohesive materials, adequate flow and adaptation for intimate interaction, and acceptable curing when phase changes are required for final joint formation. RESULTS Much more effort is needed in the future to carefully assess each of these using available testing methods that attempt to characterize the energetics of the interfaces. Bonding involves potential contributions from physical, chemical, and mechanical sources but primarily relies on micro-mechanical interaction for success. Characterization of the interface before adhesion, during service, and after failure would be much more useful for future investigations and remains as a great challenge. SIGNIFICANCE Scientists should more rigorously apply techniques such as comprehensive contact angle analysis (rather than simple water wettability) for surface energy determination, and AFM in addition to SEM for surface texture analysis.

Annual review of selected dental literature: Report of the Committee on Scientific Investigation of the American Academy of Restorative Dentistry

This review was conducted to assist the busy dentist in keeping abreast of the latest scientific information regarding the clinical practice of dentistry. Each of the authors, who are considered experts in their disciplines, was asked to peruse the scientific literature in their discipline published in 2016 and review the articles for important information that may affect treatment decisions. Comments on experimental methodology, statistical evaluation, and the overall validity of conclusions are included with many of the reviews. The reviews are not meant to stand alone but are intended to inform the interested reader about what has been discovered in the past year. The readers are then invited to go to the source, if they want more detail.

FDI World Dental Federation: clinical criteria for the evaluation of direct and indirect restorations—update and clinical examples

In 2007, new clinical criteria were approved by the FDI World Dental Federation and simultaneously published in three dental journals. The criteria were categorized into three groups: esthetic parameters (four criteria), functional parameters (six criteria) and biological parameters (six criteria). Each criterion can be expressed with five scores, three for acceptable and two for non-acceptable (one for reparable and one for replacement). The criteria have been used in several clinical studies since 2007, and the resulting experience in their application has led to a requirement to modify some of the criteria and scores. The two major alterations involve staining and approximal contacts. As staining of the margins and the surface has different causes, both phenomena do not appear simultaneously. Thus, staining has been differentiated into marginal staining and surface staining. The approximal contact now appears under the name “approximal anatomic form” as the approximal contour is a specific, often non-esthetic issue that cannot be integrated into the criterion “esthetic anatomical form”. In 2008, a web-based training and calibration tool called e-calib (www.e-calib.info) was made available. Clinical investigators and other research workers can train and calibrate themselves interactively by assessing clinical cases of posterior restorations which are presented as high-quality pictures. Currently, about 300 clinical cases are included in the database which is regularly updated. Training for eight of the 16 clinical criteria is available in the program: “Surface lustre”; “Staining (surface, margins)”; “Color match and translucency”; Esthetic anatomical form”; “Fracture of material and retention”; “Marginal adaptation”; “Recurrence of caries, erosion, abfraction”; and “Tooth integrity (enamel cracks, tooth fractures)”. Typical clinical cases are presented for each of these eight criteria and their corresponding five scores.

Marginal fit of castable ceramic crowns

The objective of this study was to measure marginal fit of castable ceramic versus gold crowns. Full veneer gold and ceramic crowns were made on Ivorine dies. Crowns were cemented, embedded, sectioned faciolingually and mesiodistally, and photographed for measurement of absolute marginal discrepancies (cavosurface angle to casting margin) to evaluate fit. Results revealed no statistically significant differences in fit among four locations around the margins of either ceramic or gold crowns. There was no statistically significant difference in the combined absolute marginal discrepancy (fit) between ceramic and gold crowns. The variance of the combined absolute marginal discrepancy (fit) of the ceramic crowns was significantly different than that of the gold crowns (p = 0.01 level). The standard deviation (variability) of the gold crowns was more than twice that of the ceramic crowns. Randomized block ANOVA demonstrated statistically significant differences among individual gold crown specimens, but none among individual ceramic crown specimens.

Finite element analysis estimates of cement microfracture under complete veneer crowns

Long-term clinical failures of complete veneer crowns are commonly attributed to microleakage of the cement. Excessive stress or fatigue cycling may create cement microfractures and promote microleakage. Two-dimensional (2D) finite element analysis (FEA) was selected to determine stress levels and distributions on dental cements resulting from 10 MPa occlusal loads on single-unit complete artificial veneer crowns during various clinical conditions. Sixteen 2D-FEA computer models were generated for a mandibular first premolar to study the effects of (1) marginal configuration (shoulder for all-ceramic crown versus chamfer for type III gold alloy crown), (2) four types of cement (zinc phosphate, polycarboxylate, glass ionomer and composite resin), and (3) two thicknesses of cement (25 and 100 microns) for single-cycle loads and fatigue loading. There was almost no difference between a chamfer and shoulder marginal configuration except at the edge of the margin where the chamfer finish lines reached 2 to 8 times greater stresses. There were minimal effects for thickness of cement and marginal configurations. Stresses were slightly less for thicker cement. Fatigue analysis was based on estimated stress versus number of cycle curves for cements and resulted in stresses below the estimated endurance limit. If the average occlusal loading levels were 10 MPa, there did not appear to be a risk of microfracture in dental cement because of mechanical loading.

Bonding to resurfaced posterior composites

Despite significant improvements in the wear resistance of posterior composite restorations, they still undergo occlusal wear, color change, and surface staining with time. One method of repairing these restorations is re-surfacing the old composite. This study investigated the bonding of new composite to the corresponding old composite material by several different mechanical conditioning steps, chemical conditioning steps, primer conditioning steps, and four posterior composites. Aged composite surfaces were conditioned, re-bonded to new composite, stored in artificial saliva for seven days at 37 degrees C, and tested in shear. The mean shear strength for unrepaired composites was 27 MPa. Optimal re-bond strengths were 88% (Estilux), 77% (Ful-Fil), 92% (Occlusin), and 102% (P-10) of original bulk shear strengths. General linear modeling revealed that the best combination tested was roughening with a D830 diamond bur, conditioning the surface with water, and priming with Scotchbond-1 dentin bonding agent.

Fracture resistance of yttria-stabilized zirconia dental implant abutments

PURPOSE An in vitro study was performed to assess the effect of different degrees of clinical reduction of zirconia abutments on the failure load of clinical assemblies. MATERIALS AND METHODS Zirconia abutments (Y-TZP Ceramic Abutment, Astra Tech) were prepared with 0, 0.5, or 1 mm of external axial reduction starting 1 mm above the height-of-contour. Abutments (n = 10) were attached to implant analogs (25 Ncm torque) embedded in a stainless steel cylinder using Fields metal. Fracture loads (N) were determined when assemblies were loaded at 60 degrees off-axis until failure (Instron, CHS = 0.1 mm/min). Groups were statistically compared using ANOVA (p < 0.05). RESULTS Fracture loads for all assemblies displayed large variations within groups. There were no statistically significant differences (p > 0.05) among different abutment groups with a mean fracture load of 429 N (+/-140) for the control group, 576 N (+/-120) for 0.5-mm margins, and 547 (+/-139) for 1.0-mm margins. All fractures occurred at the interface where the abutment was connected to the analog. CONCLUSION In this in vitro study of simulated ultimate assembly strength, the preparation of zirconia abutments did not significantly impair the fracture resistance of simulated implant assemblies. All implant abutments fractured at rates higher than the maximum incisal forces (90-370 N) estimated to occur in the anterior region of the mouth.