Performance of two different digital evaluation systems used for assessing pre-clinical dental students' prosthodontic technical skills.

INTRODUCTION: Proper integration of newly emerging digital assessment tools is a central issue in dental education in an effort to provide more accurate and objective feedback to students. The study examined how the outcomes of students' tooth preparation were correlated when evaluated using traditional faculty assessment and two types of digital assessment approaches. Specifically, incorporation of the Romexis Compare 2.0 (Compare) and Sirona prepCheck 1.1 (prepCheck) systems was evaluated. Additionally, satisfaction of students based on the type of software was evaluated through a survey. MATERIAL AND METHODS: Students in a second-year pre-clinical prosthodontics course were allocated to either Compare (n = 42) or prepCheck (n = 37) systems. All students received conventional instruction and used their assigned digital system as an additional evaluation tool to aid in assessing their work. Examinations assessed crown preparations of the maxillary right central incisor (#8) and the mandibular left first molar (#19). All submissions were graded by faculty, Compare and prepCheck. RESULTS: Technical scores did not differ between student groups for any of the assessment approaches. Compare and prepCheck had modest, statistically significant correlations with faculty scores with a minimum correlation of 0.3944 (P = 0.0011) and strong, statistically significant correlations with each other with a minimum correlation of 0.8203 (P < 0.0001). A post-course student survey found that 55.26% of the students felt unfavourably about learning the digital evaluation protocols. A total of 62.31% felt favourably about the integration of these digital tools into the curriculum. CONCLUSIONS: Comparison of Compare and prepCheck showed no evidence of significant difference in students' prosthodontics technical performance and perception.

Effect of polyaramid fiber reinforcement on the strength of 3 denture base polymethyl methacrylate resins.

PURPOSE: The interactive effects of synthetic polyaramid reinforcement fibers on the transverse strength of intact and repaired heat-polymerized denture base acrylic resins were investigated. MATERIALS AND METHODS: Three polymethylmethacrylate (PMMA) polymers were tested: Acron MC (GC International Corp, Scottsdale, AZ), Lucitone 199 (Dentsply International Inc, York, PA), and Microlon (The Hygienic Corp, Akron, OH). With each polymer, there were 2 controls and 4 experimental groups (n = 9 per group). The treatment groups were intact heat-polymerized PMMA control, PMMA with unreinforced repair, PMMA with polyaramid reinforced repair, intact polyaramid reinforced heat-polymerized PMMA control, polyaramid reinforced PMMA with unreinforced repair, and polyaramid reinforced PMMA with polyaramid reinforced repair. The transverse fracture strengths of the samples were measured with a 3-point bending test on a Zwick Universal Testing Machine (Zwick of America, Inc, East Windsor, CT). RESULTS: The highest mean strength at fracture was recorded with intact polyaramid reinforced heat-polymerized PMMA controls for all resins. Analysis of variance showed significant differences in transverse strength (p < .05) by experimental group, by material, and by interaction of group and material. Tukey HSD (honestly significant difference) Multiple Comparisons Test (alpha = 0.05) showed that intact polyaramid reinforced heat-polymerized PMMA controls were significantly stronger than intact heat-polymerized PMMA controls and all the other treatment groups. Use of polyaramid reinforcement in repair of unreinforced PMMA or polyaramid reinforced PMMA did not result in significantly increased transverse strength. CONCLUSIONS: Polyaramid reinforcement significantly increased the transverse strength of intact heat-polymerized PMMA. Polyaramid fibers did not significantly increase strength to reinforce PMMA repairs.

Ten-year survival rates of teeth adjacent to treated and untreated posterior bounded edentulous spaces.

STATEMENT OF PROBLEM: Failure to replace a single missing posterior tooth may lead to a variety of dental problems, which may ultimately result in tooth loss. However, little is known about the fate of the adjacent teeth if a missing posterior tooth is not replaced. PURPOSE: This retrospective study evaluated the survival of teeth adjacent to treated and untreated posterior bounded edentulous spaces. MATERIAL AND METHODS: Data were obtained from electronic treatment records from the Kaiser Permanente Dental Care Program, Portland, Ore. A final sample of 317 patients who met the study inclusion criteria was identified. Each bounded edentulous space was placed in 1 of 3 treatment categories: untreated, restored with a fixed partial denture, or restored with a removable partial denture. Subsequent treatment and the status of the teeth adjacent to the bounded edentulous space were followed through December 1999. Ten-year Kaplan-Meier survival estimates were generated for each treatment group, and differences in survival were evaluated with the log-rank chi-square test (alpha=.05). RESULTS: There was a significant difference in survival among the 3 treatment categories (P=.005). Spaces restored with a fixed partial denture had longer 10-year survival estimates (92%) than those that remained untreated (81%). Spaces restored with a removable partial denture had the poorest 10-year survival rate (56%). CONCLUSION: Under the conditions and selection bias associated with this retrospective study, the survival of teeth adjacent to a single posterior edentulous space was negatively associated with removable partial denture placement compared with no treatment or the use of a fixed partial denture.

Tooth preparations for complete crowns: an art form based on scientific principles.

STATEMENT OF THE PROBLEM: No recent literature has reviewed the current scientific knowledge on complete coverage tooth preparations. PURPOSE: This article traces the historic evolution of complete coverage tooth preparations and identifies guidelines for scientific tooth preparations. MATERIAL AND METHODS: Literature covering 250 years of clinical practice was reviewed with emphasis on scientific data acquired during the last 50 years. Both a MEDLINE search and an extensive manual search were used to locate relevant articles written in English in the last 50 years. RESULTS: Teeth should be prepared so that they exhibit the following characteristics: 10 to 20 degrees of total occlusal convergence, a minimal occlusocervical dimension of 4 mm for molars and 3 mm for other teeth, and an occlusocervical-to-faciolingual dimension ratio of 0.4 or greater. Facioproximal and linguoproximal line angles should be preserved whenever possible. When the above features are missing, the teeth should be modified with auxiliary resistance features such as axial grooves or boxes, preferably on proximal surfaces. Finish line selection should be based on the type of crown/retainer, esthetic requirements, ease of formation, and personal experience. Expectations of enhanced marginal fit with certain finish lines could not be validated by recent research. Esthetic requirements and tooth conditions determine finish line locations relative to the gingiva, with a supragingival location being more acceptable. Line angles should be rounded, and a reasonable degree of surface smoothness is desired. CONCLUSIONS: Nine scientific principles have been developed that ensure mechanical, biologic, and esthetic success for tooth preparation of complete coverage restorations.

Micromotion and dynamic fatigue properties of the dental implant-abutment interface.

STATEMENT OF PROBLEM: Clinical loading may result in micromotion and metal fatigue in apparently stable implant screw joints. This micromotion may contribute to tissue inflammation and prosthesis failure. PURPOSE: This study investigated dental implant screw joint micromotion and dynamic fatigue as a function of varied preload torque applied to abutment screws when tested under simulated clinical loading. MATERIAL AND METHODS: Fifteen noble alloy single-tooth implant restorations, each containing a hexed UCLA-style gold cylinder, were randomly assigned to 3 preload groups (16, 32, and 48 N.cm). Each group consisted of 5 implants (each 3.75 x 15 mm) and 5 square gold alloy abutment screws. A mechanical testing machine applied a compressive cyclic sine wave load between 20 and 130 N at 6 Hz to a contact point on each implant crown. A liquid metal strain gauge recorded the micromotion of the screw joint interface after 100, 500, 1,000, 5,000, 10,000, 50,000, and 100,000 cycles. Baseline data at 0 N.cm were collected before the application of the specified preload torque. RESULTS: The 16 N.cm group exhibited greater micromotion (P<.001) than both the 32 and 48 N.cm groups at all cycle intervals (2-way ANOVA, Tukey HSD). Micromotion of the implant-abutment interface remained constant (P=.99) for each of the preload groups through 105 cycles. CONCLUSION: Under the loading parameters of this study, no measurable fatigue of the implant-abutment interface occurred. However, dental implant screw joints tightened to lower preload values exhibited significantly greater micromotion at the implant-abutment interface.

Strategies to achieve fit in implant prosthodontics: a review of the literature.

PURPOSE: This paper reviews the literature on advanced strategies that attempt to improve fit in implant prosthodontics with reference to the concept of the "distortion equation." MATERIALS AND METHODS: The majority of the articles reviewed were either clinical or technique articles that advocated strategies to improve fit in implant prosthodontics. A limited number of retrospective and prospective clinical trial studies were included as they related to the topic. Reviewed articles were limited to those that addressed advanced strategies to improve fit. All of the scientific studies included in this review used an in vitro experimental design. The advanced strategies were categorized into methods that address intraoral indexing and methods that use the implant master cast. RESULTS: Relatively few methods have been scientifically proven to improve fit in implant prosthodontics. Most of the tested strategies still resulted in a slight misfit of the frameworks to the implant abutments/analogues. CONCLUSION: Multiple factors preclude that the concept of "passive fit" can be achieved in implant prosthodontics, even with the use of advanced strategies. The use of meticulous, accurate implant prosthodontic procedures and the appropriate use of advanced strategies continue to be the recommended means of achieving precise fit of the implant prosthesis to the intraoral abutments.

Splinted all-ceramic laboratory technique for implant restorations.

The fabrication of esthetically pleasing implant restorations remains a common and frustrating challenge for dental technicians as well as clinicians. Poor implant placement and post surgical gingival response often creates a difficult foundation on which to build a successful restoration. Castable, cast-to and prepable abutments are employed to correct these problems as they allow some measure of control of angulation and depth of the clinical restoration. This article details the restoration of two implants with ceramic implant abutments and splinted crowns. The technique described achieves a high level of esthetics despite less than ideal conditions.

Use of low fusing alloy in dentistry.

STATEMENT OF PROBLEM: Low fusing alloy has been used in dentistry for remount procedures in both fixed and removable prosthodontics, in implant prosthodontics for the fabrication of solid implant casts, in maxillofacial prosthetics as oral radiation shields, and in dental research for its unique properties. Previously, the use of low fusing alloy was thought to offer a high degree of dimensional accuracy. However, multiple in vitro studies have shown that its presumed dimensional accuracy may be questionable. PURPOSE: This article reviews the physical properties, metallurgical considerations of low fusing alloy, its applications in dentistry, and a safe, simple method of using low fusing alloy.

Evaluation of the accuracy of solid implant casts.

PURPOSE: Materials used to fabricate the most dimensionally accurate implant casts have not been identified experimentally. The purpose of this investigation was to examine the dimensional accuracy of implant casts fabricated with different materials. Measurements of linear horizontal dimensional change and strain produced on a master framework were evaluated and correlated. MATERIALS AND METHODS: A master framework was fabricated to fit an aluminum five-implant model. Forty polyether implant impressions of the aluminium model were randomly grouped and poured in either Vel-mix, Die Keen, Resin Rock, or Low Fusing Alloy. A digital veneer caliper was used to measure linear distance between the most distal abutments on each of the experimental implant casts and the master model. In addition, strain values were recorded from strain gauges bonded in the mesiodistal axis of the framework, which was secured by prosthetic retaining screws torqued to 10 Ncm. RESULTS: A one-way ANOVA showed a significant difference among the four die materials in dimensional change of the experimental casts (p = .0001). A post-hoc Duncan's multiple-range test (p < .05) showed that casts fabricated with Low Fusing Alloy had the least linear dimensional change from the master cast, but the material exhibited the greatest dimensional variability. A MANOVA (Wilks' Lambda) showed significant differences in strain on the framework based upon die material (p = .015). A post-hoc Duncan's multiple-range test (p < .05) showed that Resin Rock casts induced significantly less strain on the framework than the other materials. Negligible correlation was found between the linear horizontal dimensional change and the total absolute strain on the framework. CONCLUSION: Experimental implant casts made of Resin Rock minimized strain on the master framework and decreased the amount of framework distortion on casts of this material. Low Fusing Alloy yielded accurate casts, but highly variable linear dimensional changes in the horizontal dimension may preclude its clinical benefit.

Evaluation of resilient abutment components on measured strain using dynamic loading conditions.

STATEMENT OF PROBLEM: Factors that affect transmission of strain from prostheses to bone may affect the long-term success of loaded implants. Current in vitro models are theoretically predictive (finite element modeling) or facsimile (photoelastic) in nature. A more clinically relevant in vitro model for strain evaluation should be investigated. PURPOSE: This study attempted: (1) to validate a human cadaver bone model for vitro measurement of cortical bone strain, and (2) to evaluate the effect on cortical strain measurements of a resilient plastic component incorporated within a titanium implant in response to variable dynamic loading. MATERIAL AND METHODS: Two IMZ (Interpore International) abutment alternatives were used: the titanium Abutment Complete and the polyoxymethylene Intra-mobile Element. The model system consisted of two implants placed in unfixed human cadaver ulna bone to simulate an implant bound edentulous region. Four biaxial rosette strain gauges simultaneously recorded cortical bone strain immediately mesial and distal to each implant. During experimentation a simulated prosthetic framework supported by either titanium or polyoxymethylene abutments was dynamically loaded 6 min from the terminal abutment along a cantilever extension. Cyclic nominal peak loads were applied with a materials testing machine at 20-N intervals from 20 to 200 N at a crosshead speed of 5 mm/minute. The protocol allowed frequency of load application to vary. A Newtonian linear correlation (r2 > or = 0.98) between load application and strain output was determined for each gauge position except for the terminal gauge located opposite the cantilever. RESULTS: Cortical strains recorded were within reported physiologic ranges involved in bone modeling and remodeling. Further, the polyoxymethylene abutment components did not result in reduction of peak microstrain at any gauge position. The Intra-mobile Element abutments, however, did increase the time required to complete 10 loading cycles when compared with the titanium Abutment Complete abutments for the crosshead speed and ultimate loads evaluated. CONCLUSIONS: Results indicate the cadaver bone behaved in an elastic manner within the load range evaluated, and as such represents a viable in vitro experimental model. Under these conditions, polyoxymethylene abutment components do not affect measurable bone strain in response to variable loading when compared with titanium.

Deflection and stress distribution in three different IMZ abutment designs.

PURPOSE: The purpose of this study was to compare the stress distribution in the resin element and the retaining screw for three different IMZ prosthetic systems: 1) original threaded Intra-Mobile Element (IME); 2) Abutment Complete (ABC); and 3) Intra-Mobile Connector (IMC). This stress distribution comparison was then related to variations in deflection of the prosthetic superstructure. MATERIALS AND METHODS: Employing the finite element method, a three-dimensional model simulating a cast gold restoration attached to an osseointegrated IMZ implant fixture was generated for each system. The representation of the implant fixture, the supporting structures, and the external contours of the crown were identical in the three models, while the configuration of the abutment varied to characterize the individual systems. Each model was discretized into axisymmetric finite elements representing the crown, the various implant system components, and supporting structures. A series of harmonic functions was written to define non-axisymmetric loads of 100 N and 500 N evenly distributed over the entire occlusal surface of the crown. Each load was applied individually to the models, first in a vertical direction, and then at a 45 degree angle to the median plane. Predicted deflection and stress distributions were computed and plotted for each loading condition of each model. RESULTS: Deflections measured at the buccal cusp tip ranged from 0.002 mm (100-N load applied vertically to the ABC model) to 0.802 mm (500-N load applied at 45 degrees to the IME model). Maximum effective stresses in the retaining screw ranged from 129 MPa (100-N load applied vertically to the ABC model) to 1,315 MPa (500-N load applied at 45 degrees C to the IMC model). A correlation was observed between the peak stresses in the screw and the deflection of the superstructure. CONCLUSIONS: Deflections and stress concentrations with the IMC were predicted to be in the same range as with the IME, but much greater than with the ABC.

Finite element stress analysis of IMZ abutment designs: development of a model.

PURPOSE: This article describes the development of a model system for use in finite element stress analysis of three different IMZ abutment designs: original threaded Intra-Mobile Element (IME), Abutment Complete (ABC), and Intra-Mobile Connector (IMC). MATERIALS AND METHODS: A three-dimensional model simulating a cast gold crown restoration attached to an osseointegrated IMZ implant fixture was generated for each abutment design. Each model was discretized into axisymmetric finite elements representing the crown, the various implant system components, and supporting structures. A convergence test was performed to optimize the mesh. Convergence test mesh refinement for the IME, the IMC, and the ABC abutment models resulted in 818 elements, 2,566 nodes; 738 elements, 2,362 nodes; and 663 elements, 2,051 nodes, respectively. Progressive tightening of the retaining screw (preload) was simulated; the degree of screw tightening necessary to prevent opening of the crown-abutment interface in extreme loading (500-N occlusal load at 45 degrees) was determined individually for each system. CONCLUSIONS: Models of three IMZ abutment designs have been refined and the appropriate relative screw preloads determined. This model system is to be used subsequently in stress analysis comparison for the three systems.

Effect of polyethylene fiber reinforcement on the strength of denture base resins polymerized by microwave energy.

This study investigated the effect of addition of polyethylene fibers on the transverse strength, deflection, and elastic modulus of two denture base acrylic resins polymerized by microwave irradiation. Specimens of each resin, Lucitone 199 and Acron MC, were fabricated with 0.0%, 0.5%, 1.5%, and 3.0% fiber loading. Specimens were tested with use of a three-point flexure apparatus with an Instron universal testing machine after storage in water for 10 days at 37 degrees C. The results showed that polyethylene fibers significantly decreased the transverse strength of Acron MC resin and slightly increased the transverse strength of Lucitone 199 resin at a loading of 0.5% by weight. Fiber loading did not significantly affect the deflection and the elastic modulus of either material.

Cantilever and implant biomechanics: a review of the literature, Part 2.

In Part 2 of this literature review, a summary of the literature regarding the determination of acceptable cantilever lengths for fixed implant prostheses is presented. Studies examining the possible effects of biomechanical stress on both the implant prosthesis and the supporting bone are also discussed.

Deflection of superstructure and stress concentrations in the IMZ implant system.

The finite element method was used to model a 4.0 x 13.0-mm IMZ implant restored with a cast gold crown to examine the relationship between deflection of the prosthetic superstructure and stress concentrations in the retaining screw using both the polyoxymethylene intramobile element and the analogous titanium element under various loading conditions. A strong correlation was observed between the peak stresses in the screw and the deflection of the superstructure. Deflections and stress concentrations generally increased with increases of either the load magnitude or the load angle. Greater deflection and stress concentrations within the coronal retaining screw were predicted with the use of the resin IME than with the titanium element.

Cantilever and implant biomechanics: a review of the literature. Part 1.

In Part 1 of this two-part literature review, the biomechanics of cantilever fixed partial dentures are reviewed. Theoretical constructs of implant biomechanics with special emphasis on implant-supported cantilevers are also discussed. Finally, an overview of the literature regarding occlusal forces generated by patients with implant-supported prostheses is presented.

Bond strengths of porcelain repair adhesive systems.

This study compared the shear bond strength of resin composite bonded to porcelain with six commercial porcelain repair adhesive systems. The porcelain repair systems tested were: All-Bond 2, Clearfil Porcelain Bond, Ceram-Etch, C&B Metabond with Etch-Free Primer, Scotchprime, and Ultradent Porcelain Etch. Resin composite cylinders were bonded to primed porcelain surfaces and the specimens were thermocycled and stored in distilled water at 37 degrees C for 36 days. After storage, a testing machine was used to deliver a shear force parallel to the porcelain/composite interface. Mean load to failure was calculated for each group and the fracture location was noted. Results demonstrated cohesive porcelain failures and high bond strengths for All-Bond 2, Clearfil, Metabond, Scotchprime, and Ultradent. Statistical analysis revealed significantly lower bond strengths for the group that failed at the porcelain/composite interface.