David A. Felton

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.

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.

Edentulism and Comorbid Factors

INTRODUCTION Complete edentulism is the terminal outcome of a multifactorial process involving biological factors and patient-related factors. It continues to represent a tremendous global health care burden, and will for the foreseeable future. The purpose of this review is to determine what comorbid factors exist for the completely edentulous patient. METHODS This literature review evaluated articles obtained via the National Library of Medicines PubMed Website, using keywords of edentulism with various combinations of the terms comorbidity, incidence, health, nutrition, cancer, cardiovascular health, diabetes, osteoporosis, smoking, asthma, dementia, and rheumatoid arthritis. Abstracts were selected and screened, and selected full-text articles were reviewed. Articles were limited to those with adequate patient cohorts and a minimum of 2-year follow-up data. RESULTS Edentulism was found to be a global issue, with estimates for an increasing demand for complete denture prostheses in the future. Completely edentulous patients were found to be at higher risk for poor nutrition, coronary artery plaque formation (odds ratio 2.32), to be smokers (odds ratio 2.42), to be asthmatic and edentulous in the maxillary arch (odds ratio 10.52), to being diabetic (odds ratio 1.82), to having rheumatoid arthritis (odds ratio 2.27), and to having certain cancers (odds ratios varying from 1.54 to 2.85, depending on the type of cancer). Chronic residual ridge resorption continues to be the primary intraoral complication of edentulation, and there appear to be few opportunities to reduce bone loss in the edentulous patient. CONCLUSIONS While the completely edentulous patient seems to be at risk for multiple systemic disorders, whether development of these disorders is causal or casual has not been determined. To minimize the loss of residual alveolar ridges, exemplary complete denture therapy, along with the establishment of routine recall systems, should be the ultimate goal of treatment of this patient cohort.

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.

3D-FEA of osseointegration percentages and patterns on implant-bone interfacial stresses.

OBJECTIVES The degree of osseointegration and its patterns are important for the success of implants. 3D-FEA was used to determine interfacial stresses on a single tooth implant (IMZ) for four degrees of osseointegration (100, 75, 50 and 25%), and five patterns at 50% osseointegration (locally alternating, coronal only, apical only, facial only and lingual only). METHODS The implant was restored with a metal-ceramic crown and subjected to 10 MPa axial or oblique applied stress. Resolved stresses were examined at four heights along the implant-bone interface. RESULTS The degree of osseointegration did not affect resolved stress levels or distributions. Oblique loads elevated interfacial stresses 5 to 20 times. Stresses were always higher at the bone crest. CONCLUSIONS Osseointegration patterns with crestal bone reduced both crestal and apical stresses. Apical only osseointegration produced much higher apical stresses. Crestal osseointegration and axial loads minimized overall stress.

Threaded endodontic dowels: Effect of post design on incidence of root fracture

The use of threaded endodontic dowels is a controversial issue. The purpose of this study was to compare the potential for root fracture resulting from the cementation of nine threaded and three nonthreaded endodontic dowel systems. The clinical crowns of 140 extracted premolars were removed at the cementoenamel junction. The teeth were randomly assigned to the following treatment groups of 10 teeth each: Group 1, endodontically instrumented but not obturated; group 2, instrumented and obturated; group 3, instrumented, obturated, and restored with custom-cast gold dowel and cores; groups 4 and 5, instrumented, obturated, and restored with prefabricated, nonthreaded dowels; and groups 6 through 14, instrumented, obturated, and restored with one of nine prefabricated, threaded dowels. All dowels were inserted according to manufacturers directions, removed, and cemented with vinyl polysiloxane impression material. Each specimen was demineralized and cleared. Photographs at 1:1 magnification were taken to assess dowel fractures. Fishers test and chi square analysis were performed to evaluate the differences between post types, and between posted and nonposted controls (p less than 0.05). The results indicate no statistically significant differences between dowel types when compared with each other, regardless of dowel shape, taper, or presence or absence of threads, or when compared to instrumented, nonobturated controls. The amount of remaining dentin and existing root morphology may be a determining factor for endodontically treated teeth to resist fracture during dowel placement.

Finite element stress analysis on the effect of splinting in fixed partial dentures

STATEMENT OF PROBLEM Long-span fixed partial dentures usually require splinting of multiple abutments to overcome mechanical problems associated with the long edentulous span. Most information and indications for the use of multiple splinted abutments have been empirically derived. PURPOSE This study analyzed the stress levels in the teeth and supporting structures of a fixed prosthesis and ascertained how the addition of multiple abutments in a fixed prosthesis modifies the stresses and their deflection. MATERIAL AND METHODS The finite element method was used to analyze mechanical behaviors of a prosthesis and its supporting structures when a fixed prosthesis with several designs replaced a mandibular second premolar and a first molar. Variations of the standard finite element model were made by changing the number of splinted teeth and the level of bone support. RESULTS A reduction of stress and deflection was observed in the supporting structures when a fixed partial denture was fabricated and teeth were splinted together. Increasing the number of splinted abutments did not reveal a proportional reduction of stress in the periodontium. Stress concentrations were seen in the connectors of prosthesis and in the cervical dentin area near the edentulous ridge. CONCLUSION Increasing the number of the splinted abutment did not compensate for the mechanical problems of a long-span fixed partial denture sufficiently.

Pulpal response to chemically cured and experimental light-cured glass ionomer cavity liners

This investigation evaluated the effects of an experimental light-cured glass ionomer (LCGI) cavity liner and chemically cured Ketac-Bond glass ionomer restorative material on the pulpal tissues of monkeys. Class V cavities were prepared in 71 teeth of three adult Macaca mulatta monkeys at 7- and 35-day intervals. Ketac-Bond or LCGI cavity liners were placed in cavities with the smear layer intact (groups I and II) or with the smear layer removed (groups III and IV) and were restored with composite resin. Tissues were acquired, sectioned at 7 microns, stained for microscopic evaluation, or prepared for SEM evaluation. No statistically significant differences between materials existed at either time interval, regardless of smear layer presence. Histopathologic results demonstrated minimal pulpal reactions for groups I through IV. SEM analysis showed tenacious bonding of the LCGI material to prepared cavity walls, with absence of contraction gap formation. The results indicated excellent pulpal responses to both materials.

Pulpal response to threaded pin and retentive slot techniques: A pilot investigation

This investigation compared pulpal response to threaded pin techniques with response to retentive slot techniques. The teeth were restored with composite resin. Twenty-four teeth were assigned to three treatment groups in one Macaca mulatta monkey. Ten teeth (group 1) received 32 TMS 0.021-inch self-threading pins. Ten teeth (group II) received circumferential slot retention 1 mm deep, 0.5 mm inside the dentinoenamel junction. Four teeth (group III) served as controls. Groups I and II were restored with composite resin. Fourteen days later, the teeth were removed, demineralized, serially sectioned, and stained with hematoxylin and eosin. Chi-square analysis indicated more pulp inflammation when self-threading pins were used (p less than 0.5). Pins placed within 0.5 mm of the pulp elicited severe inflammatory responses, and those placed further than 1 mm had minimal effect. Little correlation existed between remaining dentin thickness and adverse pulp response when slot retention was used.

Effect of cavity varnish on retention of cemented cast crowns

C avity varnish is applied to prepared teeth before cementation of a cast restoration with zinc phosphate cement for pulpal protection and attenuation of postoperative sensitivity. Dentinal permeability to constituents in acid-containing cements is significantly reduced by the application of a cavity varnishle3 and marginal microleakage is decreased.4 It is unknown, however, whether these advantages are outweighed by the effects of varnish on the retention of the restoration or prosthesis. Cavity varnish has been shown to reduce the retention of cemented pins’ and decrease the tensile bond between two opposed dentinal surfaces when zinc phosphate is used as the luting agent.’ Others have reported no significant difference in the retentive values for inlays cemented into varnished and unvarnished preparations.’ The purpose of this investigation was to compare the retention of full veneer crowns cemented on prepared teeth to which a cavity varnish had been applied with that of crowns cemented on prepared teeth that were unvarnished.