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Biomedical subjects

E D Rekow

Publications and source records attributed to E D Rekow.

15 recordsLinked to original sources
PubMed2002

Materials design of ceramic-based layer structures for crowns.

Radial cracking has been identified as the primary mode of failure in all-ceramic crowns. This study investigates the hypothesis that critical loads for radial cracking in crown-like layers vary explicitly as the square of ceramic layer thickness. Experimental data from tests with spherical indenters on model flat laminates of selected dental ceramics bonded to clear polycarbonate bases (simulating crown/dentin structures) are presented. Damage initiation events are video-recorded in situ during applied loading, and critical loads are measured. The results demonstrate an increase in the resistance to radial cracking for zirconia relative to alumina and for alumina relative to porcelain. The study provides simple a priori predictions of failure in prospective ceramic/substrate bilayers and ranks ceramic materials for best clinical performance.

Aluminum Oxide↗
PubMed1998

Indentation damage and mechanical properties of human enamel and dentin.

Understanding the mechanical properties of human teeth is important to clinical tooth preparation and to the development of "tooth-like" restorative materials. Previous studies have focused on the macroscopic fracture behavior of enamel and dentin. In the present study, we performed indentation studies to understand the microfracture and deformation and the microcrack-microstructure interactions of teeth. It was hypothesized that crack propagation would be influenced by enamel rods and the dentino-enamel junction (DEJ), and the mechanical properties would be influenced by enamel rod orientation and tooth-to-tooth variation. Twenty-eight human third molars were used for the measurement of hardness, fracture toughness, elastic modulus, and energy absorbed during indentation. We examined the effect of enamel rod orientation by propagating cracks in the occlusal surface, and in the axial section in directions parallel and perpendicular to the occlusal surface. The results showed that the cracks in the enamel axial section were significantly longer in the direction perpendicular to the occlusal surface than parallel. The cracks propagating toward the DEJ were always arrested and unable to penetrate dentin. The fracture toughness of enamel was not single-valued but varied by a factor of three as a function of enamel rod orientation. The elastic modulus of enamel showed a significant difference between the occlusal surface and the axial section. It is concluded that the cracks strongly interact with the DEJ and the enamel rods, and that the mechanical properties of teeth are functions of microstructural orientations; hence, single values of properties (e.g., a single toughness value or a single modulus value) should not be used without information on microstructural orientation.

Analysis of Variance↗
PubMed1998

Mechanical characterization of dental ceramics by hertzian contacts.

Hertzian indentation testing is proposed as a protocol for evaluating the role of microstructure in the mechanical response of dental ceramics. A major advantage of Hertzian indentation over more traditional fracture-testing methodologies is that it emulates the loading conditions experienced by dental restorations: Clinical variables (masticatory force and cuspal curvature) identify closely with Hertzian variables (contact load and sphere radius). In this paper, Hertzian responses on four generic dental ceramics systems-micaceous glass-ceramics, glass-infiltrated alumina, feldspathic porcelain, and transformable zirconiaare presented as case studies. Ceramographic sectioning by means of a "bonded-interface" technique provides new information on the contact damage modes. Two distinct modes are observed: "brittle" mode, classic macroscopic fracture outside the contact (ring, or cone cracks), driven by tensile stresses; and "quasi-plastic" mode, a relatively new kind of deformation below the contact (diffuse microdamage), driven by shear stresses. A progressive transition from the first to the second mode with increasing microstructural heterogeneity is observed. The degree of quasi-plasticity is readily apparent as deviations from ideal linear elastic responses on indentation stress-strain curves. Plots of threshold loads for the initiation of both fracture and deformation modes as a function of indenter radius constitute "damage maps" for the evaluation of prospective restoration damage under typical masticatory conditions. The degree of damage in both modes evolves progressively with load above the thresholds. Strength tests on indented specimens quantify sustainable stress levels on restoration materials after damage. The most brittle responses are observed in the fine glass-ceramics and porcelain; conversely, the most quasi-plastic responses are observed in the coarse glass-ceramics and zirconia; the medium glass-ceramics and alumina exhibit intermediate responses. Implications of the results in relation to future materials characterization, selection, and design are considered in the clinical context.

Aluminum Oxide↗
PubMed1997

Force degradation in elastomeric chains.

Elastomeric chains are a frequent choice for delivering forces required to close spaces orthodontically. Unfortunately, these forces degrade over time. Open and closed chains from six orthodontic suppliers were evaluated over time. For both types and for all suppliers, the greatest loss of force occurred within the first hour. During the next 2 to 4 days, forces delivered continued to fall but at a slower rate. Beyond that time, in general, forces delivered remain nearly constant but at a level lower than originally available. The amount of the force delivered at 28 days ranged from 85% to 30% of that available at the time of placement. At 28 days, gray chains from all suppliers delivered forces greater than 100 g.

Analysis of Variance↗
PubMed1997

Enamel subsurface damage due to tooth preparation with diamonds.

In clinical tooth preparation with diamond burs, sharp diamond particles indent and scratch the enamel, causing material removal. Such operations may produce subsurface damage in enamel. However, little information is available on the mechanisms and the extent of subsurface damage in enamel produced during clinical tooth preparation. The aim of this study, therefore, was to investigate the mechanisms of subsurface damage produced in enamel during tooth preparation by means of diamond burs, and to examine the dependence of such damage on enamel rod orientation, diamond particle size, and removal rate. Subsurface damage was evaluated by a bonded-interface technique. Tooth preparation was carried out on two enamel rod orientations, with four clinical diamond burs (coarse, medium, fine, and superfine) used in a dental handpiece. The results of this study showed that subsurface damage in enamel took the form of median-type cracks and distributed microcracks, extending preferentially along the boundaries between the enamel rods. Microcracks within individual enamel rods were also observed. The median-type cracks were significantly longer in the direction parallel to the enamel rods than perpendicular to the rods. Preparation with the coarse diamond bur produced cracks as deep as 84 +/- 30 microns in enamel. Finishing with fine diamond burs was effective in crack removal. The crack lengths in enamel were not significantly different when the removal rate was varied. Based on these results, it is concluded that subsurface damage in enamel induced by tooth preparation takes the form of median-type cracks as well as inter- and intra-rod microcracks, and that the lengths of these cracks are sensitive to diamond particle size and enamel rod orientation, but insensitive to removal rate.

Dental Enamel↗
PubMed1994

Confirmation of Leinfelder clinical wear standards.

OBJECTIVES: Accuracy of composite wear studies based on Leinfelder standards has been disputed. There are differences with other well-calibrated systems such as the M-L and Vivadent wear standards. The objective of this study was to reevaluate the margin height at key regions along the restoration margins for each of the 6 Leinfelder standards using laser profiling techniques. METHODS: The Leinfelder standards were profiled in parallel paths 100 microns apart and measured in x-y-z position every 20 microns along those paths using a laser profilometer. RESULTS: Rounding of cavosurface enamel margins from intraoral wear greatly increased the uncertainty of the true enamel margin location and step height measurements, precluding unequivocal measurements for standards #2 and #3. Values for other standards for the original report, newly measured means and standard errors, and measured ranges were: #4 (322 microns, 333 +/- 34 microns, 171-507 microns), #5 (382 microns, 459 +/- 44 microns, 202-649 microns), and #6 (493 microns, 584 +/- 91 microns, 315-1022 microns). There were no statistically significant differences (p < or = 0.10) between these and original values. Large standard errors may have obscured small differences that may exist. The Leinfelder cast conversion scale seems to be the correct relative magnitude. SIGNIFICANCE: Differences between Leinfelder casts and other standards may be due to differences in shadow production. Clinical wear may be systematically underestimated by other cast evaluation methods that have well-defined margins. This emphasizes the need for standard casts with margin morphology similar to the clinical casts being evaluated for wear.

Bicuspid↗
PubMed1993

Location of the mandibular center of autorotation in maxillary impaction surgery.

Controversy exists about the location of the center of autorotation of the mandible after maxillary impaction surgery. This investigation focuses on the problems associated with locating that center of autorotation and identifies factors that can increase the probability of accurately identifying its location for predicting surgical outcomes. The reliability of the Rouleaux technique for calculating the centers of rotation is established and is shown to be acceptable, as long as the landmarks used for determining the center are properly selected, and the magnitude of the rotation required is sufficient. The location of the centers of autorotation of the mandibles after maxillary impaction surgery for 46 patients was used to investigate the errors associated with landmark selection and amounts of rotation. Although there is much variation in its location, the center does not lie within the body of the condyle but instead lies away from the condyle. Guidelines for maximizing the reliability of predicting surgical outcomes on the basis of autorotation of the mandible after maxillary impaction surgery are given.

Adult↗
PubMed1993

High-technology innovations--and limitations--for restorative dentistry.

Technology has brought some exciting advances to dentistry--restorations that can be produced in one appointment and fit as well as cast restorations; laser procedures that arrest early caries, etch enamel, and that the patient finds more comfortable; and improved diagnostic capabilities through computer-enhanced images. Dental applications of these technologies are in their infancy and most of the limitations discussed in this article will be eliminated in the near future. More and more applications and possibilities are continually evolving. Technology promises to enhance dentistry for the clinician and patient alike.

Ceramics↗
PubMed1992

A review of the developments in dental CAD/CAM systems.

At least 1000 CAD/CAM (computer-aided design and computer-aided manufacturing) systems for producing restorations are in dental offices. At least nine different systems have been described; the Cerec system (Siemens, Bensheim, FRG) is the best known and most widely used. Clinical results to date suggest that the automated CAD/CAM systems can provide restorations which perform at least as well as cast restorations. There are some technique sensitivities that must be managed with the new technology. The importance of fit at the margins is a controversial issue that remains to be settled. Bond strength of composite-luted restorations is dependent on margin location and luting material properties, and on the combination of silanating, etching, and curing mechanism of the luting composite. Surface finishes equivalent to cast gold and glazed porcelain can be achieved for machinable CAD/CAM materials.

Ceramics↗
PubMed1992

CAD/CAM in dentistry: a historical perspective and view of the future.

What can we look forward too? Lots of fun with new CAD/CAM systems that will enhance dentistry, providing quality restorations quickly. The evolution of an array of new versions of already available systems as well as altogether new systems will provide improved quality, expanded capabilities, and increasing user friendliness. And new materials will be more esthetic, wear more nearly like enamel, and strong enough for full crowns and bridges. We can also look forward to lots of change. Because of the cost of CAD/CAM systems, many clinicians are likely to collaborate by sharing a single system. Laboratories and clinicians may collaborate as well, with data being gathered in the operatory and sent to a laboratory via modem. The fabrication would then be done by the laboratory. Other changes that we cannot even predict are likely to occur in dentistry. Exciting times are here. Automation through dental CAD/CAM systems will, most certainly, change the profession. The impact of that change will only be known in the future. But as the future approaches, the systems and materials available to us will continue to evolve, improve, and enhance dentistry.

Artificial Intelligence↗
PubMed1991

CAD/CAM for dental restorations--some of the curious challenges.

Computer-aided design and manufacturing for dental restorations has opened a new world of possibilities--some that appeal to engineers and clinicians and some that have created some interesting challenges. The objective of this overview is to briefly describe a system being developed by the Universities of Maryland and Minnesota which is capable of producing dental crowns. Some of the challenges and difficulties that have arisen during the development activities will be addressed. The final focus will be on some of the questions that, because of the new technology, can now be addressed and are presenting new challenges.

Computer Graphics↗
PubMed1991

CAD/CAM in dentistry.

The advent of computers and interactive computer graphics laid the groundwork for a revolution in dentistry. Dental CAD/CAM systems can at least match the quality of cast restorations. With the automation, consistency in quality is also provided. The dental CAD/CAM system can simplify the effort required to produce a restoration. With new technologies for fabrication (like milling and edm), a whole array of new materials, which were historically difficult or impossible to manipulate, suddenly become viable possibilities. When used in the dental office, many of the systems eliminate the need for impressions and can produce a restoration while the patient waits. A dental revolution is underway, providing exciting possibilities for restorative dentistry.

Crowns↗
PubMed1985

Treatment-induced errors in occlusion following orthognathic surgery.

Posttreatment occlusion following orthognathic surgery is often different from that predicted in the treatment plan. Differences between intended and actual occlusion may be treatment-induced occlusal errors caused by mismatches between the centers of rotation of the mandible and of the articulated models. Discrepancies in the position of the articulator center of rotation (relative to the position of the center of rotation of the patient's mandible) influence the magnitude of occlusal errors. A computer model was developed to quantify these errors. As the center of rotation of the articulated models becomes more divergent from the patient's center of rotation, the magnitude of the occlusal errors increases. This magnitude increases most rapidly along the line that is perpendicular to the line joining the patient's center of rotation and a preselected mandibular landmark (incisor tip or molar cusp, for instance). For small changes in vertical dimension, clinically insignificant errors result, independent of the degree of mismatch between the centers of rotation. Clinical implications of these findings are discussed.

Computers↗
PubMed1980

Growth contributions to class II corrections based on models of mandibular morphology.

Various morphologies of human models are modeled with various growth patterns to demonstrate the role of mandibular morphology on growth contributions to Class II corrections. Growth patterns are described by centers of mandibular rotation relative to the cranial base. Centers of rotation are used to determine several parameters of growth generated by a computer programmed to show growth effects. The direction and amount of condylar growth are held constant. With condylar growth constant, various centers of rotation of the mandible reveal that maximum Class II molar correction is present when the condyle is vertically located farthest from the molar. Of lesser importance, Class II corrections are greater when the condyle is anteroposteriorly closest to the molar.

History, 18th Century↗