Tooth Preparations For Crowns Research Articles

Effect of the Crown Preparation Margin and Die Type on the Marginal Accuracy of Fiber-reinforced Composite Crowns

The objective of this laboratory investigation was to determine the effect of different preparation designs (light chamfer, deep chamfer, and shoulder) and die-making materials (stone and epoxy) on the resulting margin misfit for fiber-reinforced composite crowns using a measuring microscope. Sixty standardized FibreKor crowns were made on stone and epoxy resin dies (n=30 each) duplicated from three metal master dies representing complete crown tooth preparation with a total convergence of 5 degrees . For each die group, three of the tooth preparation designs were established in relation to the type of finish line (n=0 each) as follows: Group A (0.5-mm light chamfer finish line); Group B (1.0-mm deep chamfer finish line); and Group C (1.0-mm shoulder finish line). Marginal accuracy was evaluated by measuring the distances between each of four pairs of indentations on the crowns and on the dies with a Nikon measuring microscope. Analysis of seating measurements with parametric analysis of variance and Tukey's Studentized Range (HSD) disclosed a statistically significant difference for both tooth preparation design and die material (p< 0.001). However, the interaction effect was not significant (p=0.9073). The least marginal opening value was for FibreKor crowns made on epoxy resin dies with a light chamfer finish line (57 microm), but the difference was not statistically significantly different from crowns made on epoxy resin dies with a deep chamfer light chamfer finish line (61 microm). However, crowns made on epoxy resin dies with a shoulder finish line (81 microm) had significantly higher values (p< 0.05). FibreKor crowns made on stone dies with the shoulder finish line (95 microm) had statistically higher marginal opening values (p< 0.05). The least marginal opening value was for crowns made on stone dies with a light chamfer finish line (66 microm), but the difference was not statistically significantly different from crowns made on stone dies with a deep chamfer light chamfer finish line (70 microm). Significant differences were found among the die material used for the shoulder margin design. However, there was no significant difference between light chamfer and deep chamfer margin designs for both die materials.

Frequently encountered errors in tooth preparations for crowns

Frequently encountered errors in tooth preparations for crowns

Axial-wall inclination angle and vertical height interactions in molar full crown preparations

This study has been designed to evaluate the interaction of axial wall heights with inclination angles in full crown tooth preparations. The interaction of these parameters was related to the resulting preparation surface area. A right regular pyramid was used to simulate a single mandibular molar preparation with known convergence angles and vertical heights. Various combinations of these two variables allowed the calculation of surface areas with a formula for the area of a pyramid and right triangles through trigonometric manipulations. The pyramidal model system had a 9-mm square base with vertical heights from 3- to 5-mm and single-side inclination angles from 2 to 25 degrees. The occlusal surface was a flat, square or rectangular surface and was included in the total area. A percentage of surface area lost or gained served as the dependent variables. The significance levels were set at 10.0% or greater magnitude of loss/gain in a surface area compared to the ideal 2 degree-level. Significant area loss was demonstrated in all alpha-level comparisons. The largest change was found in the 5-mm height grouping compared to the 3-mm height grouping, -36.6% difference between groups at the 2 degree-level. Axial single-side inclination angles greater than 10 degrees in 3- and 4-mm height-molars are detrimental to maximum surface area in full crown restorations. The 5-mm axial wall height with < or = 10 degree single wall has been shown to maximize the luting agent surface area between restoration and tooth structure.

Effect of adhesive resin cement on design of partial veneer crowns

Statement of problem. Procedures involved in the tooth preparation, impression, and casting of partial veneer three-quarter crowns are considered to be complex. Purpose. This study assessed whether a simplified partial veneer crown design, when cemented with a 4-META adhesive resin, had a comparable resistance to displacement forces compared with a conventionally prepared partial veneer crown cemented with a traditional luting agent. Material and methods. Conventional partial veneer crowns were prepared on 20 natural premolars, and 20 other premolars received modified partial veneer crown tooth preparations without proximal grooves. Artificial crowns were made for these teeth with Students alloy. Half the crowns in each of the 2 groups were luted with 4-META resin; the other half were luted with a zinc phosphate cement. Loads required to displace the partial veneer crowns were recorded after 24 hours and thermocycling, then the mode of separation was recorded for each crown. Average force required to displace partial veneer crowns for the 4 groups were subjected to 2-way analysis of variance and Scheffé test. Results. The 2 groups luted with 4-META resin and the group with proximal grooves luted with zinc phosphate were not significantly different. However, they were all greater than the group without proximal grooves luted with zinc phosphate cement. Conclusion. Forces required to displace partial veneer crowns prepared in a traditional manner and those teeth prepared without proximal grooves were similar if the artificial crowns were luted with adhesive resin cement. (J Prosthet Dent 1999;82:157-60.)

Evaluation of the clinical performance and effectiveness of adhesively-bonded metal crowns on damaged canine teeth of working dogs over a two- to 52-month period.

In this clinical study, 41 metal full crown restorations of canine teeth were placed in 18 working dogs. Twenty-six canine teeth had severe attrition with no involvement of the pulp cavities; 15 fractured canine teeth were endodontically treated. With the exception of one tooth, at least one-third of the coronal part of each canine tooth was available for a supragingivally performed, minimal tooth crown preparation. A dental resin luting cement technique was used to bond the electrolytically etched crown (made from an alloy of cobalt-chrome-molybdenum) to the tooth. The metal crowns were slightly shorter and with a more rounded tip than the original tooth. Posts or post-and-core techniques were not used. Median follow-up period was 30 months (range 2 to 61 months), at which time 36 crowns were found to be intact and functional. Five crowns were lost; three as a result of subsequent injury and fracture of the tooth below the crown; one as a result of use of less than one-third of the coronal portion of the tooth for retention of the crown; and one as a result of an oblique fracture of the root.

TOOTH PREPARATION AND PULP DEGENERATION

Conscious attention must be directed toward providing atraumatic tooth preparations for crowns, and to constructing adequate provisional restorations. If meticulous care is not taken, pulpal damage or death can occur. Patients should be warned that pulpal death and endodontic therapy can result from crown placement.

Long‐term success rate of resin‐bonded metal crowns on the canine teeth of working dogs

Summary In this clinical study, 19 full metal crown restorations of canine teeth were placed in seven working dogs. Thirteen canine teeth were severely abraded with no involvement of the pulp cavities; six fractured canine teeth were endodontically treated. At least 1/3 of the coronal part of the canine tooth was available for a supragingivally performed, minimal tooth crown preparation. An adhesive technique to bond the electrolytically etched crown (an alloy of cobalt‐chrome‐molybdenum) to the tooth was used. The metal crowns, slightly shorter and with a rounder tip than the original tooth, were bonded to the enamel and dentine by using a resin luting cement. Posts or postand‐core techniques were not used. Mean follow‐up period was 32 months (range 24 ‐ 52 months), at which stage 17 crowns were found to be intact and functional. Two crowns were lost as a result of trauma resulting in a fracture of the tooth below the crown.

Influence of tooth surface roughness and type of cement on retention of complete cast crowns

Statement of Problem. Bond strength of luting cements to dentin is a critical consideration for success of cast restorations. Purpose of Study. This study determined the relationship between surface characteristics of teeth prepared for complete cast crowns and retention of respective cemented restorations. Material and Methods. Ninety artificial crowns were cast for standardized complete crown tooth preparations accomplished with the use of a milling machine on extracted human teeth. Diamond, tungsten carbide finishing, and cross-cut carbide burs of similar shape were used. The crowns in each group were randomly subdivided into three subgroups of 10 for the three luting cements selected for this study: zinc phosphate cement (Fleck's), glass ionomer cement (Ketac-Cem), and adhesive resin cement (Panavia-EX). Retention was evaluated by measuring the tensile load required to dislodge the artificial crowns from tooth preparations with an Instron testing machine. Results. Analysis of forces with parametric analysis of variance and Tukey's Studentized Range (HSD) disclosed a statistically significant difference for both luting cement and finishing burs ( p < 0.001). A statistically significant interaction effect ( p < 0.001) was also found. The greatest retention value (372.9 N) was for tooth preparations refined with carbide burs and cemented with Panavia-EX cement. However, the least retention value (201.6 N) was for tooth preparations completed with finishing burs and luted with zinc phosphate cement. Conclusions. Significant differences were found among all three cements for finishing burs. However, there was a difference only between Panavia-EX cement and the other two cements for tungsten carbide burs. For diamond rotary instruments, zinc phosphate cement was significantly different from glass ionomer and Panavia-EX cements.(J Prosthet Dent 1997;77:116-21.)

WHEN TO USE FILLERS, BUILD-UPS OR POSTS AND CORES

After preliminary tooth preparation for crowns, all previously placed tooth restorative material should be removed from teeth before making final tooth preparations. Small areas of missing teeth should be replaced with fillers (compomer); larger pieces of missing teeth should be replaced with build-ups (bonded composite resin or bonded amalgam); and endodontically treated teeth with half or more of the coronal tooth structure missing should receive a titanium or titanium alloy post and core and a bonded composite resin or bonded amalgam build-up.

Improved resistance and retention of a short coronal tooth preparation for a complete crown

It is a challenge to construct a complete veneer crown for a tooth with a short coronal tooth preparation such as a posterior tooth with only 3 mm occluso-gingival (vertical) height. This clinical situation can be improved sur&ally by crown lengthening1 to increase the vertical height of the tooth preparation. Patients occasionally reject the surgical treatment because of discomfort, additional treatment time, and cost, Conversely, some features such as axial wall parallelism, groove, and box2 can be added in the tooth preparation for a crown to increase resistance and retention. Unfortunately, these measures are commonly ineffective for a tooth with a short coronal preparation. Occlusal wells are an internal feature designed to solve these clinical situations when they occur in 0) endodontitally treated teeth with castings or amalgam cores, and (2) vital teeth with recessed pulps. Occlusal wells are contraindicated for vital teeth with large pulp because of the risk of pulpal damage.

The relationship between preparation convergence and retention of extracoronal retainers.

Although traditional ideal convergence (the sum of taper of the opposite sides) for crown preparation has been arbitrarily set at 4 degrees to 10 degrees, some believe absolute parallelism yields the highest retention. This study examined the relationship between the degree of convergence of a machined metal die and the retention of its casting. The method used was that of cementing cast metal crowns onto full crown preparations on brass dies with varying convergence angles, and then recording the force required to remove the crowns from the dies in a vertical direction using a Tate-Emery Testing Machine and Load Indicator. It was found that retention (i.e., the force needed to remove the cemented castings from the die in their common long axis) increases from 0 degree convergence to peak between 6 degrees to 12 degrees convergence. It also seems that a critical film thickness does exist for optimum retention, and that film thicknesses smaller than the critical thickness may be responsible for the phenomenon that we have observed and directly related to the convergence angle itself. There seems to be experimental data supporting the use of traditionally taught convergence. Our study found that convergence angles between 6 degrees and 12 degrees seem to be optimum for tooth crown preparation when one plans to use zinc phosphate cement. Convergence angles of less than 6 degrees may not be desirable even if they can be clinically achieved. The results of our study indicate that a relationship exists between the convergence angle and the critical cement thickness that is necessary to realize the maximum strength properties of zinc phosphate cement.

Voids in a mixed elastomeric impression material

V oids or bubbles are always aggravating to the clinician who is attempting to obtain a perfect impression with an elastic impression material. Critical areas such as margins and internal grooves of tooth preparations for crowns, onlays, or inlays and regions for rests and clasp arms for removable partial dentures require exact duplication of detail. An air blast reputedly reduces or eliminates the number of bubbles subgingivally. ‘,2 Others feel that “operator skill is a more important factor in bubble entrapment than is the syringe or material used.“3 Although other suggestions appear from time to time in the literature, few scientific studies have been done on the cause and elimination of bubbles in elastomeric impressions. The objective of this in vitro study was to determine the influence of mixing techniques and operator experience in controlling the qumber and volume of voids that occur during the mixing and manipulation of a representative rubber impression elastomer.

Periodontal Considerations in Tooth Preparation for Crowns and Bridges

Correct tooth preparation and adequate reconstruction of the anatomy of the crown are essential to the maintenance and preservation of a healthy periodontium. Adequate tooth preparation requires careful attention to many details and clear knowledge of the features of the finished product. The anatomic reconstruction of the crown with a perfect marginal adaptation will provide an adequate environment for maintaining the health of surrounding periodontal tissues.