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.

The component partial: a new RPD construction system.

Most removable partial denture frameworks constructed today are one-piece castings. This article presents the Component Partial, a removable partial denture in which the framework is constructed in pieces. Such construction ensures a more accurate fit of the RPD.

The swing-lock removable partial denture.

The swing-lock removable partial denture is a good treatment alternative for maxillofacial prosthodontics, periodontally compromised dentitions and where some conventional designs for removable partial dentures cannot be applied. This paper presents clinical applications of the SLRPD. Indications, contraindications, advantages and disadvantages are discussed.

A comparison of an accelerated technique for casting post-and-core restorations with conventional techniques.

PURPOSE: This project compares an accelerated technique for the casting of post-and-core restorations with four traditional techniques. The accelerated technique uses two phosphate-bonded investments and the traditional techniques use a gypsum- and a phosphate-bonded investment. The study measures and compares the differences between the seating of the casting and the seating of the acrylic resin pattern. The effects of the techniques on the fit of castings with and without a ferrule are also compared. MATERIALS AND METHODS: Six groups of 10 castings were made from plastic patterns formed on a stainless steel test die. A different investment and/or burnout method was used for each group. Each group had two subgroups: ferruled and nonferruled. The fit of the plastic patterns was measured at two time intervals after forming, 2 weeks and 3 months. The patterns were invested immediately after the 3-month measurement, and the difference in fit of the castings was calculated. An ANOVA and Tuckey-Kramer test were done to determine the statistical validity. RESULTS: The seating of the patterns after 3 months of storage was consistently worse than the 2-week measurements of fit. The ferrule and nonferrule patterns were not statistically different in seating. Measurement of the castings showed that the ferruled castings seated significantly worse than the nonferrule castings. The difference in the seating of the castings as compared with the patterns was considered clinically unacceptable, showing a range of 0.301 mm to 0.528 mm. The nonferrule castings showed a significant difference in seating among groups. The difference ranged from -0.099 mm to 0.322 mm. CONCLUSIONS: The castings of the ferrule subgroups were considered clinically unacceptable and were not analyzed for significance. Among the nonferrule castings, the group using a gypsum investment and conventional technique for investing and burnout but no ring liner showed the best seating. The accelerated technique was intermediate in seating with a difference of 0.148 mm from the seating of the patterns. This group was significantly different from the two best groups but not from the remaining three groups.

An accelerated technique for casting post and core restorations.

An accelerated technique for casting a direct-pattern post and core restoration is presented. The entire clinical and laboratory procedure may be completed within 1 hour. The necessity for two appointments for completion of the restoration as well as the necessity of making two provisional restorations may be eliminated.

A comparison of the stress transfer characteristics of a dental implant with a rigid or a resilient internal element.

It has been suggested that there is a unique set of problems associated with joining an implant and a natural tooth with a fixed partial denture. The manufacturer of the IMZ implant system claims that this procedure can be accomplished successfully because of the planned stress-distributing characteristics of their resin internal (intramobile) element. This study compared the difference in the stress patterns generated in photoelastic plastic by an IMZ implant with a resilient or a rigid internal element. Under a standardized cantilever load, the stress patterns were photographed in the field of a circular polariscope. The total stress areas were calculated and a statistical comparison performed. The static load conditions of the model demonstrated no statistical difference between the area of stress pattern generated by an IMZ implant with or without a resilient internal element. Moreover, a single load produced the same deflection of the cantilever beam regardless of which element was interposed.

The effects of a liquid dispersing agent and a microcrystalline additive on the physical properties of type IV gypsum.

This study evaluated the effects of a liquid dispersing agent (LDA) and a microcrystalline additive (MCA) on selected physical properties of type IV gypsum. Working consistency, setting time, setting expansion, and compressive strength (1 hour and 7 days) were determined, following ADA Specification No. 25, on a standard, LDA (0.5, 1, 1.5, and 2 mL), MCA (21.1, 24.1, and 27.1 gm), and combination (LDA 0.75 mL + MCA 12.05 gm) mixes per 300 gm of gypsum. Results indicate that the additives affect the consistency of the mix, but consistency can be kept close to that of the standard by lowering the water/powder ratio. Statistical analysis of the data indicated that the additives significantly affected the setting time, setting expansion, and both the 1-hour and the 7-day compressive strengths. SEM examination of fractured surfaces of test mixes indicated improved crystal packing. The properties of type IV gypsum can be improved by optimizing the amount of LDA and MCA additives.

Effect of die spacer on the seating of complete cast gold crowns with grooves.

Fifty crowns were constructed with an indirect technique for a standardized Ivorine tooth preparation to determine whether casting relief would improve the seating of complete cast crowns with zinc phosphate cement. The variables were two grooves in the preparations and die spacer casting relief of approximately 20 to 40 micron. The tooth preparations of two groups had no grooves, but castings were made with internal relief for only one group. Three groups had buccal and lingual grooves in the preparation. One group had no relief, a second group had complete relief, and a third group had relief but not in the grooves. The discrepancies noted before and after cementation of the castings were measured. The average discrepancy for each group and the significance between groups was determined. Results of this study demonstrated the following. Die relief significantly improved the seating of complete cast-gold crowns with or without grooves. Omitting die spacer from grooves did not reduce the benefits of relieving the remainder of the die. Partial internal relief was better than no relief. The grooves did not disrupt seating when die spacer application was complete or specifically omitted from the grooves alone.

Simplified techniques for the preparation and use of reversible hydrocolloid impression material.

The use of a simple technique with an inexpensive syringe loaded with heavy-body tray material resulted in a savings of preparation time and storage space in the conditioning unit. Only one type of material needs to be liquefied. The use of the heavy-body material in syringe and tray in conjunction with a hydrophilic technique have practically eliminated the annoying problem of tearing of the material in the gingival crevice on removal of the impression. Moisture control is no longer an aggravating problem.

Technique for cementation of provisional restorations.

A technique for cementation of a provisional resin splint or crown has been presented. The splint is lubricated with silicone grease and cemented with a noneugenol cement mixed with antibiotic ointment. The grease aids in the complete removal of the cement. The use of a sterile composite resin syringe and preloaded disposable tip allows delivery of the grease to the splint with minimal chance of bacterial cross contamination to the patient.

Use of paint-on die spacers in preparations with grooves.

Film thickness of paint-on die spacer materials was measured on axial walls and in grooves. Two different materials and two different modes of application were tested. From the data presented, the following conclusions can be drawn: There are potentially significant differences in film thickness produced with different die spacing materials. The technique used to apply die spacer can affect the film thickness produced. Careful application of Pactra airplane dope will not result in significant accumulation of die spacer in the groove. Tru-fit die spacer produced generally erratic film thickness and demonstrated a potentially significant amount of pooling in the groove base. Further study is warranted to determine if use of die spacer in grooves will improve the seating of castings.

Measurement of paint-on die spacers used for casting relief.

Three paints used for die spacing were measured for film thickness. Two of the materials are marketed as die spacers, and the third is sold as a model airplane paint. The materials were applied to stone dies in multiple coats. They were embedded in resin, sectioned, and measured photographically through a microscope. The three die spacing materials were capable of achieving clinically acceptable relief in the range of 20 to 40 mu by using a variable number of coats. Different paints produced different thicknesses. The two paints sold specifically for die spacing gave 11 to 13 mu per coat. The hobby shop paint had the smallest film thickness per coat, yielding approximately 5 mu per layer. It also had the smallest standard deviation. This control plus the relative low cost and availabilty make it attractive for this purpose.