The effect of preparation height and luting agent on the resistance form of cemented cast crowns under load fatigue

Posterior restoration rotational torque associated with cuspal incline angulation: A proof of concept

STATEMENT OF PROBLEM

Off-axis, external forces with a moment arm on posterior restorations have not been investigated in experimental studies.

PURPOSE

The purpose of this proof-of-concept study was to determine the interaction between occlusal force directed toward cuspal incline angulations with variations in base width and preparation vertical heights. Torque forces on a single crown restoration in simulated premolar and molar tooth forms were calculated for 3 different rotational axes.

MATERIAL AND METHODS

Trigonometric calculations were made to determine the amount of torque generated in a simulated-crown restoration in premolar and molar tooth forms. Restorations with different cuspal incline angulations were loaded with an off-axis force of 200 N. This force was applied to 5 different cuspal incline angulations in both tooth forms at varying preparation heights. Right triangles were used to enable trigonometric computations of the resulting moment arms that accompanied the 3 rotational axes.

RESULTS

The total torque values were calculated with a range from 7.5 to 372.8 Ncm. The highest levels of torque were generated in the 5-mm-high molar tooth form with a rotational axis located within the root form, perpendicular to the 45-degree cuspal incline. In general, large moment arms were generated with steep cuspal incline angulations and mid-root axis locations; the lowest torque values in all cuspal incline angulations were found in the rotational axis locations at the crown finish line. The torque values at the rotational axis finish line location were found to be greatest in the largest vertical tooth form height category (5 mm) in both tooth model sizes.

CONCLUSIONS

The crown restoration cuspal incline angulations, vertical preparation heights, base widths, and rotational axis locations all played a role in the torque force levels generated, probably influencing restoration stability.

Management of short clinical crowns by utilizing horizontal groove retentive technique in crown/tooth or both with different luting cements - An analysis on extracted teeth

Purpose:

The aim of the present study was to compare and evaluate the retention of dislodged crowns by addition of one horizontal circumferential groove (HCG) to preparation and/or casting and with two luting cements.

Materials and Methods:

A total of 80 recently extracted human maxillary first molar teeth of appropriate sizes were collected and mounted in the resin blocks using surveyor. Standardized full coverage tooth preparations were prepared and impressions were obtained. Dies were prepared for casting and were then subjected to tensile loading using UTM. The data obtained was statistically analyzed.

Results:

Obtained values were statistically analyzed using ANOVA, STUDENT “t” TEST and BONFERRONI TEST. The results showed that the mean tensile force (TF) was in the range from 49.05 to 264.87 for group A and 255.06 to 588.60 for group B. The highest TF was recorded for subgroup 3B, whereas the lowest for subgroup 1A.

Conclusion:

Within the limitations of the study, it was concluded that by addition of one HCG to tooth preparation or casting and to both showed significant increase in retention, when compared to control groups. The highest retention value was obtained for sample with groove on the internal surface of crown luted with SARC.

An auxiliary factor for increasing the retention of short abutments

BACKGROUND

Retention is an essential factor in the sustainability of the prosthesis, especially for short abutment. Despite, the availability of several auxiliary applications, achieving a clinically good retention for cast crowns in prepared short teeth remains a major challenge for the practicing dentist.

OBJECTIVES

This study tests a new method for applying frustum-shaped grooves on short prepared abutments and compares them with traditional setting grooves.

METHODS

Forty-eight copper machine-milled short dies with 3.5 mm length and 12o conversion with a chamfer finish line of 0.7 mm were prepared to mimic short prepared molars and distributed into 4 groups. A control group with two opposing setting grooves and two groups with dual opposing frustums, one of which was prepared to receive two integrated metal protrusions inside the casting that fit the frustums. Frustums were prepared with Komet bur No. 807 with dimensions 0.9 mm at the base and 0.7 mm at the occlusal surface. A direct wax-up was carried out on all dies and all castings were cemented with zinc phosphate cement. A pull-off test was applied until separation. Data were analysed to compare the correlation of the four groups using one-way ANOVA and Bonferroni test (p ≤ 0.05).

RESULTS

The mean retentive values in Newton were as follows: the control group, 457.80; the setting grooves group, 461.07; the frustum group, 597.59; and the frustum group with the two metal protrusions, 919.80. The retention was increased by 65% in frustum group and doubled in frustum group with the metal protrusions.

CONCLUSIONS

The results showed a statistical significance in retention in the frustum groups with and without metal protrusion over the retention of the control and the setting grooves groups.

Choice of cement for single-unit crowns: Findings from The National Dental Practice-Based Research Network

BACKGROUND

In this article, the authors present clinical factors associated with the type of cement practitioners use for restoration of single-unit crowns.

METHODS

A total of 202 dentists in The National Dental Practice-Based Research Network recorded clinical details (including cement type) used for 3,468 single-unit crowns. The authors classified crowns as bonded if the dentist used a resin cement. The authors used mixed-model logistic regression to assess the associations between various clinical factors and the dentist's decision to bond.

RESULTS

A total of 38.1% of crowns were bonded, and 61.9% were nonbonded; 39.1% (79 of 202) of dentists never bonded a crown, and 20.3% (41 of 202) of dentists bonded every crown in the study. Crowns with excessive occlusal reduction (as judged by laboratory technicians) were more likely to be bonded (P = .02); however, there was no association with bonding and excessive taper (P = .15) or axial reduction (P = .08). Crowns were more likely to be bonded if they were fabricated from leucite-reinforced glass ceramic (76.5%) or lithium disilicate (70.8%) than if they were fabricated from layered zirconia (38.8%), full-contour zirconia (30.1%), full metal (14.7%), or porcelain-fused-to-metal (13.8%) (P < .01) restorative material. There was no significant association between choice to bond and crown margin location (P = .35). Crowns in the anterior maxilla were more likely to be bonded (P < .01).

CONCLUSIONS

Excessive occlusal tooth preparation, anterior location of a crown, and the use of glass ceramic crowns were associated significantly with the decision to bond.

PRACTICAL IMPLICATIONS

In this study, the authors identified factors significantly associated with the clinical decision made by practicing dentists when selecting a cement for restoration of single-unit crowns.

Premolar Axial Wall Height Effect on CAD/CAM Crown Retention

OBJECTIVE

To evaluate the significance of reduced axial wall height on retention of adhesively luted, all-ceramic, lithium disilicate premolar computer-aided design/computer-aided manufacturing (CAD/CAM) crowns based on preparations with a near ideal total occlusal convergence of 10°.

METHODS

Forty-eight recently extracted premolars were randomly divided into four groups (n=12). Each group received all-ceramic CAD/CAM crown preparations featuring axial wall heights of 0, 1, 2, and 3 mm, respectively, all with a 10° total occlusal convergence. Scanned preparations were fitted with lithium disilicate all-ceramic crowns that were luted with a self-etching resin cement. Specimens were tested to failure at a 45° angle to the tooth long axis with failure load converted to megapascals (MPa) based on the measured bonding surface area. Mean data were analyzed using analysis of variance/Tukey's post hoc test (α=0.05).

RESULTS

Lithium disilicate crowns adhesively luted on preparations with 0 axial wall height demonstrated significantly less failure resistance compared with the crowns luted on preparations with axial wall heights of 1 to 3 mm. There was no failure stress difference between preparations with 1 to 3 mm axial wall height.

CONCLUSIONS

Under conditions of this study, adhesively luted lithium disilicate bicuspid crowns with a total occlusal convergence of 10° demonstrated similar failure resistance independent of axial wall height of 1 to 3 mm. This study provides some evidence that adhesion combined with an ideal total occlusal convergence may compensate for reduced axial wall height.

A Comparative Study of the Retentive Strengths of Commercial and Indigenously Developed Luting Cements using Both Lathe-cut and Clinically Simulated Specimens

INTRODUCTION

Superior adhesive strength in luting agents is of paramount significance in fixed partial denture success. In this in vitro study five cements were tested for retentive qualities, using both lathe-cut and hand-prepared specimens.

MATERIALS AND METHODS

A total of 104 freshly extracted tooth specimens were prepared. Seventy of them were lathe-cut and 30 specimens were hand-prepared to simulate clinical conditions. Five different cements were tested, which included a compomer, a composite, a zinc phosphate, and 2 glass-ionomer luting cements. Of the 5, 2 trial cements were indigenously developed by Sree Chitra Tirunal Institute for Medical Sciences and Technology (SCTIMST), Trivandrum, India - a glass-ionomer cement (Chitra GIC) and a chemical-cure composite (Chitra CCC). All cements were compared within each group and between groups (lathe-prepared and hand-prepared).

RESULTS

GC Fuji 1 (GC America) exhibited superior retentive strengths in both lathe-cut and hand-prepared specimens, whereas the compomer cement displayed the lowest values when tested. In lathe-cut specimens, statistical analysis showed no significant difference between GC Fuji 1 and indigenously developed Chitra CCC.

CONCLUSION

Both Chitra CCC and GC Fuji 1 have comparable strengths in lathe-cut samples, making Chitra CCC a potential luting agent. Statistical analysis reveals that all cements, except GC Fuji 1, exhibited a significant decrease in strength due to the change in design uniformity. The chemical bonding of GC Fuji 1 proves to be quite strong irrespective of shape and precision of the tooth crown.

CLINICAL SIGNIFICANCE

The indigenously developed Chitra GIC and Chitra CCC showed promising results to be used as a potential luting agent.

Reporting numeric values of complete crowns. Part 2: Retention and resistance theories

STATEMENT OF PROBLEM

Determining the retention and resistance of a tooth preparation for a complete crown has only existed in theory, and these theories have never been measured on tooth preparations performed in vivo.

PURPOSE

The purpose of this study was to measure the theoretical retention and resistance of clinically produced complete crown preparations by using an objective measuring method.

MATERIAL AND METHODS

Stone dies from 236 complete crown preparations were collected from dental laboratories. The dies were scanned and analyzed with the coordinate geometry method. Cross-sectional images were captured, and the surface area was measured with a cone frustum and right truncated pyramid formula. Two different theories of resistance form, the "on" or "off" theory (limiting taper) and the linear model (resistance length), were calculated for premolar and molar preparations.

RESULTS

The mean surface areas ranged from 33.97 mm(2) to 105.44 mm(2) for the cone frustum formula and 41.75 mm(2) to 117.50 mm(2) for the right truncated pyramid formula. The facial side of maxillary premolars exhibited the highest percentage of resistance form with the limiting taper, at 58%, and the mesial side of the mandibular molars exhibited the lowest percentage of resistance form, at 6%.

CONCLUSIONS

The objective method used in this study provides a way for retention and resistance theories to be tested and for further clinical implications to be investigated.

Effect of Preparation Taper, Height and Marginal Design Under Varying Occlusal Loading Conditions on Cement Lute Stress: A Three Dimensional Finite Element Analysis

To assess the effect of preparation taper, height and margin design under different loading conditions on cement lute stress. A 3-D FE model of an upper second premolar and molar was developed from CT scan of human skull using software programmes (MIMICS, Hypermesh and ANSYS). 10° and 30° taper, 3 and 5 mm preparation height and shoulder and chamfer finish lines were used. Type 1 Glass ionomer cement with 24 μm lute width was taken and the model was loaded under 100 N horizontal point load, vertical point load distributed axial load. The maximum shear stress and Von Mises stress within the cement lute were recorded. The maximum shear stresses ranged from 1.70 to 3.93 MPa (horizontal point loading), 0.66 to 3.04 MPa (vertical point loading), 0.38 to 0.87 MPa (distributed loading). The maximum Von Mises stresses ranged from 3.39 to 10.62 MPa (horizontal point loading), 1.93 to 8.58 MPa (vertical point loading) and 1.49 to 3.57 MPa (distributed loading). The combination of 10° taper and 5 mm height had the lowest stress field while the combination of 30° taper and 5 mm height had the highest stress field. Distributed axial loading shows least stress, better stress homogenization and gives a favorable prognosis for the fixed prostheses. Smaller preparation taper of 10° is biomechanically more acceptable than a 30° taper. It is desirable to decrease taper as height increases. The chamfer margin design is associated with greater local cement stresses toward the margins that could place the cement at greater risk for microfracture and failure.

Coordinate geometry method for capturing and evaluating crown preparation geometry

STATEMENT OF PROBLEM

A validated universal method requiring no human input is needed to capture and evaluate preparation geometries in a manner that can be used to see the correlation of different parameters.

PURPOSE

The purpose of this study was to present a method of capturing and evaluating crown preparation geometry.

MATERIAL AND METHODS

One manually machined acrylic resin block and 9 randomly selected preparations for ceramic complete crowns prepared by general dentists were selected and prepared. The specimens were scanned (3D scanner; Nobel Biocare), and buccolingual and mesiodistal cross section images were collected. The images were imported into digitizing software (Engauge Digitizer 4.1) to convert the outlines into x and y coordinates. Six points were chosen by using a set of algorithms, and the resulting parameters were calculated.

RESULTS

The acrylic resin block was milled with a 12 degree total occlusal convergence (TOC) instrument producing a 12.83 degree TOC. For the other specimens, average TOC values ranged from 18 degrees to 52 degrees. The mean average margin width was 0.70 mm, and the mean average base dimension was 6.23 mm. The surface area/volume ratio, resistance length, and limiting taper were also calculated.

CONCLUSIONS

The method described provides a basis for accurately evaluating preparation geometry without human input.

Influence of masticatory fatigue on the fracture resistance of the pulpless teeth restored with quartz-fiber post-core and crown

To investigate whether masticatory fatigue affects the fracture resistance and pattern of lower premolars restored with quartz-fiber post–core and full crown, 44 single rooted lower premolars recently extracted from orthodontic patients were divided into two groups of 22 each. The crowns of all teeth were removed and endodontically treated and then restored with quartz-fiber post–core and full crown. Twenty-two teeth in one group were selected randomly and circularly loaded at 45° to the long axis of the teeth of 127.4 N at a 6 Hz frequency, and the other group was not delivered to cyclic loading and considered as control. Subsequently, all teeth in two groups were continually loaded to fail at 45° to the long axis of the teeth at a crosshead speed of 1 mm⋅min⁻¹. The mean destructive force values were (733.88±254.99) and (869.14±280.26) N for the experimental and the control group, respectively, and no statistically significant differences were found between two groups (P>0.05). Bevel fracture and horizontal fracture in the neck of root were the major fracture mode of the specimens. Under the circumstances of this study, it seems that cyclic loading does not affect the fracture strength and pattern of the quartz-fiber post–core–crown complex.

Strategic considerations in treatment planning: deciding when to treat, extract, or replace a questionable tooth

Prosthodontists face the difficult task of judging the influence and significance of multiple risk factors of periodontal, endodontic, or prosthetic origin that can affect the prognosis of an abutment tooth. The purpose of this review is to summarize the critical factors involved in deciding whether a questionable tooth should be treated and maintained, or extracted and possibly replaced by dental implants. A MEDLINE (PubMed) search of the English, peer-reviewed literature published from 1966 to August 2009 was conducted using different keyword combinations including treatment planning, in addition to decision making, periodontics, endodontics, dental implants, or prosthodontics. Further, bibliographies of all relevant papers and previous review articles were hand searched. Tooth maintenance and the acceptance of risks are suitable when: the tooth is not extensively diseased; the tooth has a high strategic value, particularly in patients with implant contraindications; the tooth is located in an intact arch; and the preservation of gingival structures is paramount. When complete-mouth restorations are planned, the strategic use of dental implants and smaller units (short-span fixed dental prostheses), either tooth- or implant-supported, as well as natural tooth abutments with good prognoses for long-span FDPs, is recommended to minimize the risk of failure of the entire restoration.