Retention of metal ceramic crowns cemented with resin cements: effects of preparation taper and height

Feasibility of software-based assessment for automated evaluation of tooth preparation for dental crown by using a computational geometric algorithm

The purpose of this study was to propose the concept of software-based automated evaluation (SAE) of tooth preparation quality using computational geometric algorithms, and evaluate the feasibility of SAE in the assessment of abutment tooth preparation for single-unit anatomic contour crowns by comparing it with a human-based digitally assisted evaluation (DAE) by trained human evaluators. Thirty-five mandibular first molars were prepared for anatomical contour crown restoration by graduate students. Each prepared tooth was digitized and evaluated in terms of occlusal reduction and total occlusal convergence using SAE and DAE. Intra-rater agreement for the scores graded by the SAE and DAE and inter-rater agreement between the SAE and DAE were analyzed with the significance level (α) of 0.05. The evaluation using the SAE protocol demonstrated perfect intra-rater agreement, whereas the evaluation using the DAE protocol showed moderate-to-good intra-rater agreement. The evaluation values of the SAE and DAE protocols showed almost perfect inter-rater agreement. The SAE developed for tooth preparation evaluation can be used for dental education and clinical skill feedback. SAE may minimize possible errors in the conventional rating and provide more reliable and precise assessments than the human-based DAE.

Bonding Strength of Lithium Disilicate Adhesive Crowns with Different Occluso-Cervical Preparation Heights and Cement Types

The aim of this study was to evaluate if adhesion technology with CAD/CAM can compensate for the reduction of occluso cervical preparation heights using different types of dental cement. The de-bonding failure types were then assessed. Here, 72 caries-free extracted human premolar teeth were prepared to have a remaining occlusal height of two, three, and four mm. IPS e.max lithium disilicate CAD/CAM crowns were cemented with adhesive resin cement Panavia SA, self-adhesive resin cement, RelyX Unicem Aplicap, and zinc phosphate cement. The cementation techniques were based on the manufacturer’s instructions. After thermocycling, all samples were tested for tensile bond strength via an Instron machine. One-way analysis of variance (ANOVA) with post hoc testing (P < 0.05) was performed. The means TBS for the two, three, and four-mm OCHP groups were 2.72±0.69, 3.06±0.82, and 3.25±0.79.0 MPa; ARC, SARC, and ZPC were 3.41±0.51, 3.45±0.41, 2.08±0.35 MPa, respectively with significant differences in both. The mixed cement had failures in the resin cement groups. Failure was predominantly cohesive in the zinc phosphate group. Resin cement had the highest SBS values versus ZPC values when both bonded to lithium disilicate crowns with different occlusal heights. The failure of the adhesive to the crown and/or to the tooth were the highest for the four types of resin cement. Around 25% were cohesive failures with resin cement, but this was predominately adhesive in crowns in zinc phosphate regardless of the preparation heights.

Long-term retention of lithium disilicate crowns with a current bioactive cement

OBJECTIVE

To determine if a recent bioactive cement provides acceptable lithium disilicate crown retention after long-term aging with monthly thermocycling.

MATERIALS AND METHODS

Extracted molars prepared with flat occlusal, 20° taper, ~4 mm axial. Prepared teeth assigned to two groups for equal mean surface areas per group. Lithium disilicate crowns fabricated with occlusal bar to facilitate removal. Crowns etched with 9.5%HF and cleaned. Cements were Ceramir Crown & Bridge QuikCap (CM) and Ketac Cem Maxicap (KC). Before cementation, specimens stored in 37°C water. Crowns cemented with 196 N force, placed in 37°C, 100% humidity oven for setting. Specimens thermocycled (5-55°C) 5000 cycles monthly for 6 months; otherwise stored in phosphate buffered saline solution. Crowns removed axially at 0.5 mm/min. Removal forces recorded and stresses calculated using areas. Independent t-test (α = 0.05).

RESULTS

Levene test not significant (P = 0.649). CM removal stresses and forces (P < 0.001) were higher (1.93 MPa, 261.4 N) compared to KC (1.06 MPa, 139.4 N). CM cement found principally on crown intaglio, KC found with most cement on prepared tooth. Chi-square significant (P < 0.001).

CONCLUSIONS

Following long-term aging with monthly thermocycling, lithium disilicate crowns were best retained by CM cement, however both cements are capable of retaining lithium disilicate crowns with preparations of ideal taper and length.

CLINICAL SIGNIFICANCE

Results serve as a basis for bioactive cement selection for retaining lithium disilicate crowns. Without optimal axial length, taper of preparation or retentive features, Ceramir Crown and Bridge QuikCap offers a bioactive cement with improved long-term retention when compared to Ketac Cem Maxicap for lithium disilicate crowns.

In Vitro Comparison of Porcelain Fused to Metal Crown Retention after Endodontic Access and Subsequent Restoration: Composite, Amalgam, Amalgam with Composite Veneer, and Fiber Post with Composite

INTRODUCTION

The aim of this study was to evaluate the effect of different access opening restorative materials on crown retention.

METHODS

Thirty-eight extracted molars were mounted in resin and prepared for porcelain fused to metal (PFM) crowns. The crowns were fabricated and cemented with zinc phosphate, and the force to displace it was measured with a tensile-testing machine before and after endodontic access preparations. The endodontic access area, crown preparation axial wall, and preparation surface area were measured for comparison. The crowns were then recemented, and access openings were restored with either amalgam or composite before displacement force was remeasured. The restorative material was removed from each access opening; access area was measured and restored again (amalgam with composite or fiber post with composite) for displacement force to be remeasured. Paired t test was used to compare the means of displacement between groups. One-way analysis of variance was used to compare the mean outcome measure within the groups.

RESULTS

Statistical analyses showed retention after unfilled access was significantly lower than intact crowns. Amalgam, composite, amalgam + composite, and fiber post + composite increased retention beyond the original value. There was no statistical difference among the different restorative protocols. Qualitative results indicate that the restorative material remains in the crown after displacement regardless of the material used to restore the access.

CONCLUSIONS

The results from this study suggest that an endodontic access cavity decreases retention of a PFM crown. However, subsequent restoration with amalgam, composite, amalgam + composite, or post + composite may increase the original retention of the crown.

Long-term retention of zirconia crowns cemented with current automixed cements

STATEMENT OF PROBLEM

Automixing and dispensing cements is a straightforward approach with consistent dosing. Previous studies have demonstrated clinically significant differences in crown retention between power-liquid and paste-paste forms of the same cement, as the composition between the 2 differs. A self-adhesive modified-resin (SAMR) and a resin-modified glass ionomer (RMGI) cement, originally offered as a powder-liquid, are now in common use as paste-paste automixed cements. With the increased use of zirconia restorations, the long-term retention of zirconia crowns for these 2 automixed cements should be evaluated.

PURPOSE

The purpose of this in vitro study was to determine whether zirconia crowns cemented with 2 automixed cements provided clinically acceptable retention after 6 months of aging with monthly thermocycling.

MATERIAL AND METHODS

Extracted molars were mounted in resin and prepared with a flat occlusal surface, 20-degree taper, approximately 4-mm axial length, and with the axio-occlusal line angle slightly rounded. Prepared teeth were equally distributed into 3 cementation groups (n=12) to achieve nearly equal mean preparation surface areas for each group. Zirconia crowns (IPS ZirCAD LT) were fabricated with an added occlusal bar to facilitate removal of the cemented crowns. Cement space was set at 45 μm axially and 55 μm occlusally. After sintering and before delivery, the intaglio surfaces were airborne-particle abraded with 50-μm alumina at 275-kPa pressure for 3 seconds and then steam cleaned. Cements were the original powder-liquid RelyX Luting (RMGI; RXL) as the control, paste-paste, automixed systems RelyX Luting Plus Automix (RMGI; RXLA), and RelyX Unicem 2 Automix (SAMR; RXUA). Crowns were cemented under 196 N force, placed in an oven at 37 °C and 100% humidity during setting and then thermocycled (5 °C-55 °C) for 5000 cycles monthly for 6 months. The crowns were removed axially with a universal testing machine at 0.5 mm/min. Removal forces were recorded and dislodgement stress calculated by using the surface area of each preparation. One-way ANOVA was used for dislodgement stress and force. Chi-square test was used for cement location after testing (α=.05).

RESULTS

RXLA demonstrated considerably lower crown retention (1.3 MPa) and differed significantly (P<.001) from RXUA (3.1 MPa) and RXL (3.1 MPa). Modes of failure showed most of the cement remaining only in the crown intaglio for RXLA for all specimens, whereas half of the crowns for RXL and RXUA demonstrated cement adhesion to both dentin and the intaglio surface, indicating cohesive failure of the cement at separation. As the Levene test was significant, the Games-Howell test was used for mean differences. The χ² analysis was significant.

CONCLUSIONS

After long-term aging with monthly thermocycling, high-strength zirconia crowns were strongly retained by 2 (RXL, RXUA) of the 3 cements. Crown retention for RelyX Luting Plus Automix was less than half in comparison and with cement found only on the intaglio surface after separation.

Convergence Angle of Preparations for Lithium Disilicate Glass-Ceramic Crowns by Dental Students and Its Effect on Crown Retention

The aims of this study were to determine the convergence angles of posterior teeth prepared by dental students at the University of Toronto for lithium disilicate glass-ceramic (LDGC) CAD/CAM crowns and to investigate their effect on loss of retention rate. A total of 280 preparations for posterior monolithic LDGC CAD/CAM crowns were performed on 270 patients (169 women and 101 men). Crowns were cemented with RelyX Unicem and Calibra Universal resin cements. Mesial, distal, and angle of convergence were measured on the bitewing radiographs. Cemented crowns were followed for up to six years. Data were analyzed for tooth type and location and for operator experience. The results showed the majority of convergence angles were greater than the recommended guidelines but fell within a clinically acceptable range (20 to 24 degrees). However, angles of convergence for mandibular molar preparations were highest (28.06±5.50 degrees), while maxillary premolars exhibited the lowest values (24.72±6.59 degrees). No significant difference was found between the results of dental students and foreign-trained dentists. Over a six-year observation period, only two crowns lost retention. The findings of this study indicated that ideal taper angles were impractical and difficult to achieve in clinical education settings.

Crown Taper Angles Achieved by Dental Students: A Systematic Review

The aim of this systematic review was to examine the literature on clinical taper angles achieved by dental students during crown preparation to determine the theoretical and clinically acceptable values identified in research studies. Medline, Embase, Web of Knowledge, the Cochrane Library, the British Dental Journal, and the Journal of the American Dental Association were searched to identify relevant studies. Studies were included if they were in vivo research on full crown preparations by dental students and published in English. Data extracted were country, year of publication, model selection and measurement methods, tests for reproducibility, tooth type, number of teeth assessed, and tapers achieved. The search resulted in 12 included articles from 11 countries published between 1978 and 2014 featuring a total of 2,306 preparations. In those studies, students failed to achieve ideal convergence angles (between 4° and 14°) but produced clinically acceptable results (between 10° and 20°). These findings should be taken into account when assessing dental students during their training programs.

Failure load effect of molar axial wall height with CAD/CAM ceramic crowns with moderate occlusal convergence

OBJECTIVES

To evaluate the significance of axial wall height (AWH) in molar fracture resistance involving CAD/CAM adhesively bonded, all-ceramic full coverage restorations on preparations with moderate total occlusal convergence (TOC) (16°).

MATERIALS AND METHODS

60 newly extracted maxillary third molars were divided into 5 groups (n = 12). Specimens were prepared for full-coverage, all ceramic restorations with occlusal cervical AWHs of 4, 3, 2, 1 as well as a flat preparation (0 mm AWH) with all preparations with AWH containing a moderate 16° TOC. Scanned preparations were fitted with a lithium disilicate restoration with a self-adhesive resin luting agent after intaglio surface preparation with hydrofluoric acid and silanation. Specimens were stored at 37°C/98% humidity for 24 hours and tested to failure at a 45° angle applied to the palatal cusp on a universal testing machine. Mean results were analyzed using ANOVA/Tukey's (P = .05).

RESULTS

Preparations containing 2, 3, and 4 millimeters of AWH demonstrated similar and higher resistance to fracture than the 1 and zero millimeter AWH groups.

CONCLUSIONS

Under the conditions of this study, results suggest that adhesive CAD/CAM technology may compensate for reduced axial wall height. However, more definitive results depend on fatigue testing.

CLINICAL SIGNIFICANCE

These in vitro results suggest that adhesive CAD/CAM technology may compensate for less than optimal AWH.

Quantitative Comparison of Mathematical Models to Measure Surface Area of Canine Teeth Prepared to Receive Full Veneer Crowns in Dogs

OBJECTIVE

This study was performed in order to determine if mathematical modeling of the canine teeth in dogs could be utilized to provide an accurate and reliable estimation of crown surface area that could be used in both a research and a clinical setting.

MATERIALS AND METHODS

Actual surface area (aSA) calculations for 32 stone dies of clinical crown preparations were acquired utilizing a tridimensional (3D) laser scanner and 3D computer-aided design and manufacturing (CAD/CAM) software applications. These calculations were used as a control. Seventeen unique mathematical models from eight geometric shapes were used to calculate estimated surface area (eSA) of each stone die. Linear association and agreement between eSA and aSA calculations were assessed with multiple statistical methods.

RESULTS

All methods of eSA showed a significant linear association with aSA. Five of the mathematical models [right elliptical frustum (H3), right elliptical cone (G3), right pyramidal cone (A3), right circular frustum (F2), and right circular cone (E1)] were superior to the other 12 models.

CONCLUSION

The H3 mathematical model based on the right elliptical frustum provided the most accurate estimate of crown surface area of dog teeth. However, H3 requires the use of laser scans and a 3D CAD software program. As a result, this model would be recommended for research applications. The E1 mathematical model was similar in accuracy to H3 and, given it requires only two measurements and a comparatively simple equation for calculation, this method would be recommended for clinical chair-side use.

Effect of preparation convergence on retention of multiple unit restorations - An in vitro study

BACKGROUND AND AIM

Convergence angle (CA) is one of the major determinant factors in the retention of single as well as multiple units. Hence, the aim of the present study was to determine the effect of preparation convergence on the retention of multiple unit restorations.

MATERIALS AND METHODS

Nickel-chromium alloy single crowns as well as three, six and nine multiple unit fixed partial dentures (FPDs) were casted for standardized milled dies that simulate canine, premolar and molar teeth machined with two different degrees of convergence, 12 (n = 55) and 20 (n = 55). The dies were threaded on rectangular metal platforms with the help of retainer shaft in the proper position to model either single or multiple preparations. The casted crowns and FPDs were cemented and retention tested by securing the units in an Instron universal testing machine. The tabulated values were analyzed statistically using Mann-Whitney test.

RESULTS

Single and multiple units with 12° CA needed greater force to dislodge compared to 20°, but this difference was not statistically significant for single units of premolar (P > 0.05). Irrespective of the angle, there was a simultaneous statistically significant increase in retention as the number of units increased in FPDs, except for the difference between 3 and 6 units in 20° group (P > 0.05).

CONCLUSION

This study highlights the importance of emphasizing on CA, during canine and molar preparation for single units as well as preparation of canines for 6 units FPDs.

Evaluation of the Natural Crown Convergence Angle of Dog Carnassial Teeth

A limiting factor in achieving an ideal crown preparation design is the natural morphology, principally the convergence angle, of the tooth. The natural convergence angle of the carnassial teeth in the dog has not been reported. The aim of the study reported here was to establish the natural crown convergence angle of the maxillary fourth premolar and mandibular first molar teeth in the dog. The natural convergence angle of the carnassial teeth in dogs was large and was not significantly associated with patient weight, age, or skull type.

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.

Crown preparation design: an evidence-based review

Evidence-based clinical guidelines for full metal crown preparation design are seldom encountered in the veterinary literature. The veterinary literature regarding prosthodontic treatment in companion animals is sparse and consists primarily of case reports and expert opinion based primarily on clinical experience. The goal of this manuscript is to present concepts of evidence-based practice, review common recommendations on crown preparation design from the veterinary literature, and to provide an assessment of the evidence for or against those recommendations.

Retentiveness of metal coping luted to teeth of uremic patients undergoing hemodialysis using five different luting cements

OBJECTIVES

This study aimed to assess the retention of metal copings luted to uremic teeth with five different luting agents.

METHODS

A total of 35 sound natural molars was collected from uremic patients and randomly assigned into five groups (n=7). The teeth were prepared for metal copings using diamond tips and water coolant. Metal copings with a loop on the occlusal surface were fabricated using base metal alloy (Rexillium III). The copings were luted using Fuji I, glass ionomer (GI); Fuji Plus, resin-modified glass ionomer (RMGI); Panavia F 2.0, resin cement; Rely X Unicem, self-adhesive cement (SA); and Adhesor, zinc phosphate cement (ZPh). All specimens were incubated at 37°C for 24 hours, conditioned in artificial saliva for 7 days, and then thermocycled for 5000 cycles (5°C-55°C). The dislodging force was measured using a universal testing machine at a crosshead speed of 2 mm/min. The mode of failure of the loaded adhesive copings was evaluated. Statistical analyses were performed using one-way analysis of variance and Tukey post hoc test.

RESULTS

GI and SA cements had the highest and the lowest mean retentive strength, respectively (580.90±17.3, 406.6±12.7). There was no significant difference between ZPh, SA, and resin cements. These cements were inferior to GI and RMGI cements (p<0.05), which showed statistically similar retentive strengths.

CONCLUSIONS

The results of this study support the use of glass ionomer and resin-modified glass ionomer cements for luting of metal copings to uremic teeth with retentive preparations.

Computer-aided evaluation of preparations for CAD/CAM-fabricated all-ceramic crowns

OBJECTIVE

The aim of this study was the evaluation of preparations from general dental practitioners for zirconia crowns and their correlation with clinical recommendations using a digital approach.

MATERIAL AND METHOD

Seventy-five datasets of left first upper molars (FDI 16) prepared for single zirconia crowns by general dental practitioners were analyzed using a computer-aided design software (LAVA(TM) Design; 3M ESPE, Seefeld, Germany) and a 3D-inspection software (COMETinspect®plus version 4.5; Steinbichler Optotechnik, Neubeuern, Germany). Evaluated parameters were convergence angle, undercuts, interocclusal reduction, abutment height, and design of preparation margin.

RESULTS

The mean convergence angle was determined to be 26.7°. The convergence angle in the mesiobuccal to distopalatal dimension was significantly the highest (31.7°), and the abutment height showed a mean value of 4.1 mm. Convergence angle and abutment height showed a negative correlation. Seventy-three percent of the evaluated locations revealed a margin design conforming to ceramic restorations. In over 30 % of the cases, the interocclusal reduction was insufficient. Generally, no preparation fulfilled all recommendations. Five (6.66 %) of the preparations fulfilled four criteria, 16 (21.33 %) preparations fulfilled three criteria, 31 (41.33 %) fulfilled two criteria, 17 (22.66 %) preparations fulfilled one criterion, and 6 (8 %) fulfilled no criterion.

CONCLUSIONS

Within the limitations of this study, most general dental practitioners seem to have difficulties fulfilling all clinical recommendations given for the preparation of zirconia crowns. The presented digital approach seems to be a useful method to evaluate the preparation geometry.

CLINICAL RELEVANCE

The correct preparation geometry represents an important prerequisite for the success of all-ceramic full crowns. As preparations clearly need to be improved, the approach presented could be the basis of a future tool to increase preparation quality in practice and education by direct objective feedback.

Achievable convergence angle and the effect of preparation design on the clinical outcome of full veneer crowns in dogs

It is widely accepted that the convergence angle of a full veneer crown preparation should be as close to parallel as possible to attain adequate retention/resistance. The shape of the dog's canine tooth limits the veterinary dentists' ability to achieve the recommended convergence angle. However, the clinically achievable convergence angle of the canine tooth in dogs has not been evaluated. In addition, the convergence angle and other physical properties of a preparation, such as height and base diameter, have been shown to affect the retention/resistance of full veneer crowns, in vitro. This effect has not been evaluated clinically in the dog. Physical properties of 32 stone dies from full veneer crowns of canine teeth were studied to evaluate the clinically achievable convergence angle and the potential effect physical properties of the preparation had on the clinical outcome of the restoration. The clinically achievable convergence angle was much higher than the current recommendation. There was an association, albeit not statistically significant, between physical properties of a preparation (convergence angle, height, base diameter) and the clinical outcome of the restoration.

Effect of temporary cements on the shear bond strength of luting cements

Objective

The purpose of this study was to evaluate, by shear bond strength (SBS) testing, the influence of different types of temporary cements on the final cementation using conventional and self-etching resin-based luting cements.

Material and Methods

Forty human teeth divided in two halves were assigned to 8 groups (n=10): I and V (no temporary cementation); II and VI: Ca(OH)₂-based cement; III and VII: zinc oxide (ZO)based cement; IV and VIII: ZO-eugenol (ZOE)-based cement. Final cementation was done with RelyX ARC cement (groups I to IV) and RelyX Unicem cement (groups V to VIII). Data were analyzed statistically by ANOVA and Tukey's test at 5% significance level.

Results

Means were (MPa): I - 3.80 (±1.481); II - 5.24 (±2.297); III - 6.98 (±1.885); IV - 6.54 (±1.459); V - 5.22 (±2.465); VI - 4.48 (±1.705); VII - 6.29 (±2.280); VIII - 2.47 (±2.076). Comparison of the groups that had the same temporary cementation (Groups II and VI; III and VII; IV and VIII) showed statistically significant difference (p<0.001) only between Groups IV and VIII, in which ZOE-based cements were used. The use of either Ca(OH)₂ based (Groups II and VI) or ZO-based (Groups III and VII) cements showed no statistically significant difference (p>0.05) for the different luting cements (RelyX^TM ARC and RelyX^TM Unicem). The groups that had no temporary cementation (Groups I and V) did not differ significantly from each other either (p>0.05).

Conclusion

When temporary cementation was done with ZO- or ZOE-based cements and final cementation was done with RelyX ARC, there was an increase in the SBS compared to the control. In the groups cemented with RelyX Unicem, however, the use of a ZOE-based temporary cement affected negatively the SBS of the luting agent used for final cementation.

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

STATEMENT OF PROBLEM

The minimum amount of resistance form required for the success of a clinical crown is unknown.There is little information on the fatigue performance of complete coverage restorations on natural tooth preparations cemented with different luting cements.

PURPOSE

The purpose of this study was to evaluate the effect of tooth preparation height and luting agent on resistance form using unidirectional load fatigue testing. For a given tooth preparation with a clinically relevant total occlusal convergence (TOC), the adequacy of resistance form was investigated.

MATERIAL AND METHODS

Sixty-four human maxillary premolars were prepared with occlusal-cervical dimensions of 2, 3,4, or 5 mm and a TOC of 20 degrees. Complete metal crowns were cemented using either zinc phosphate cement (HY Bond;ZP groups) or resin cement (Panavia F; PF groups). Cyclic load fatigue testing was done with an applied load of 6.0 kg at 2.6 Hz. Load cycles to preliminary failure were detected with a strain gauge at the palatal crown-tooth interface. Results were subjected to the Kruskal-Wallis test and the Wilcoxon post-hoc rank sum test (alpha=.05).

RESULTS

Groups ZP4, ZP5, PF2, PF3, PF4, and PF5 had the highest mean number of cycles to preliminary failure,while group ZP2 had the lowest mean number of cycles to failure. Group ZP2 was significantly different (P<.001) from all other test groups for the number of cycles to failure.

CONCLUSIONS

For the 2- and 3-mm preparation height groups, zinc phosphate cement exhibited a poorer fatigue performance compared to Panavia F. There was no significant difference in the number of cycles to failure for groups ZP4,ZP5, PF2, PF3, PF4, and PF5. For both cements, the number of cycles to failure increased with increasing resistance length. (J Prosthet Dent 2009;102:155-164)

Effect of metal type and surface treatment on in vitro tensile strength of copings cemented to minimally retentive preparations

STATEMENT OF PROBLEM

Due to the potential lack of ideal preparation form, the type of alloy and its surface pretreatment may have clinically relevant correlations with the retentive strength of castings to minimally retentive preparations.

PURPOSE

The purpose of this study was to evaluate the effect of alloy type and surface pretreatments of base and noble metal copings on their tensile strength to minimally retentive preparations.

MATERIAL AND METHODS

Minimally retentive, standardized crown preparations were made on recently extracted human third molars (n=68). Noble (IPS d.SIGN 53) and base metal (Rexillium NBF) copings were fabricated. All copings received heat treatment for oxide formation. Three experimental groups were then developed for each metal type (groups ranging from 10 to 12 specimens each): oxide only, airborne-particle abraded, or metal-primed. Copings were cemented using a self-adhesive universal resin cement (RelyX Unicem) and were thermal cycled (500 cycles between 5 and 55 degrees C) and stored (24 hours, 37 degrees C) before debonding using a universal testing machine. Frequency of debond location was compared among specimen groups. A 2-way ANOVA was used to test for interaction between the metal type and surface treatment, and, if no significant interaction was found, to test the main effects for metal type and surface treatment (alpha=.05). A multinomial logit model using the likelihood ratio test was used to describe the effect of metal type and surface treatment on failure site location (alpha=.05).

RESULTS

The 2-way ANOVA indicated no significant influence of any factor on debond load: metal type (P=.885), surface treatment (P=.555), or their interaction (P=.644). The multinomial logit statistical model showed that noble metals and metal primers significantly (P<.05) shifted debond failures to occur more frequently at the resin/tooth interface or within the tooth itself.

CONCLUSIONS

Neither metal type nor surface pretreatment affected bond strength. However, alloy type and surface treatment affected site of debond location. (J Prosthet Dent 2007;98:199-207).

The Effect of Die Spacer on Retention and Fitting of Complete Cast Crowns

PURPOSE

This study evaluated the effect of die spacer on the fit and retention of complete cast crowns by using three different cements.

MATERIALS AND METHODS

Standardized full crown restoration preparations were completed on 99 extracted molar teeth, impressions were made with poly(vinyl siloxane), and stone dies were made. Dies were covered with four layers of die spacer using three techniques: (1) covering the occlusal and 1/3 of the axial surfaces, (2) covering the occlusal and 2/3 of the axial surfaces, and (3) covering the entire preparation except the apical 0.5 mm of the preparation. Complete metal crowns were cast using Pors-on 4 alloy. Crowns were then assigned to one of three luting agent groups: resin modified glass ionomer cement, resin cement, or zinc phosphate cement. The castings were placed on their respective teeth and the marginal opening was recorded by two methods: 72 specimens were examined before and after cementation using optical microscopy with 0.001 mm resolution, and 27 specimens were examined after cementation with scanning electron microscopy. After cementation, the teeth were thermocycled for 700 cycles between 5 degrees C and 55 degrees C. The tensile retentive strength was measured on a universal testing machine with a crosshead speed of 0.5 mm/min. The data obtained for the fitting were recorded in millimeters and the data for the tensile retentive strength were recorded in KgF. The statistical analysis was performed by analysis of variance and post hoc Tukey's test (p< 0.05).

RESULTS

Before cementation, better marginal fit was obtained when the die spacer covered all but the area 0.5 mm short of the margin of the preparation; however, after cementation, the resin modified glass ionomer cement group had the best fit with the same application of die spacer. Castings luted with resin cement required the greatest tensile force to produce cement failure.

CONCLUSIONS

Increasing the area of the die surface covered with spacer improved the fit of the cast restoration. After cementation, the resin modified glass ionomer showed better adaptation; however, the optical microscopy and scanning microscopy correlate well. Resin cement had the highest resistance to tensile forces.

[Impact of various luting cements on the fixed dentures bonding strength]

BACKGROUND/AIM

Various luting cements are used to fix dental crowns to prepared teeth, and should provide an adhesive bond to the tooth structure giving reliable retention. The aim of this study was to establish in vitro which type of the tested luting cement provided the strongest adhesive bond of the prepared teeth to the fixed denture.

METHODS

Testing was carried out on the sample of 100 intact human premolars extracted for orthodontic reasons. The preparation of the teeth was performed by a heavy-duty machine. The surfaces of the prepared teeth were mathematically calculated. Dental crowns from the Nickel-Chromium-Molybenum (Ni-Cr-Mo) alloy were made in a standard fashion, and fixed to prepared teeth (two samples of each group) with 5 different types of luting cements. The strength of force applyed to separate the cast crowns from the prepared teeth was measured by an electronic dynamometer, after 7 days.

RESULTS

The obtained results revealed the connection between the type of luting cement and the values of retention power. The best adhesive bond under the constant convergence angle of the prepared teeth was provided by the resin cement.

CONCLUSION

When choosing a luting cement for fixing dental crowns to prepared teeth, the advantage should be given to the resin cement in case the glassionomers are not available.

Load-fatigue performance of gold crowns luted with resin cements

STATEMENT OF PROBLEM

Resin cements have gained popularity over the past few years because of their improved physical properties. There is a need to test these cements under fatigue loading, as there is growing recognition that fatigue testing is more clinically relevant than traditional monotonic static tests.

PURPOSE

This study investigated the load-fatigue performance of complete gold crowns cemented with 4 types of resin cement.

MATERIAL AND METHODS

Four resin cements (C & B Opaque [CBO], Calibra Esthetic [CE], RelyX Unicem [RU], and Panavia F [PF]) and a control, zinc phosphate cement (HY-Bond [HBZPC]) were tested. Fifty human maxillary premolars were prepared to an occluso-cervical dimension of 4 mm and a convergence angle of 20 degrees. Complete gold crowns were cast (Strator 3) and cemented with 1 of the 5 cements (n=10). A fatigue load of 73.5 N was applied at an angle of 135 degrees to the long axis of each tooth-crown specimen. Preliminary failure was defined as the propagation of a crack in or around the crown luting cement layer. The number of cycles to preliminary failure and the cement failure location were determined. Significant differences in cycles to preliminary failure were assessed by the nonparametric Kruskal-Wallis test, with follow-up Mann-Whitney tests (alpha=.05).

RESULTS

Group CE had the highest rank of cycles to preliminary failure, while HBZPC had the lowest cycles to preliminary failure. Group CE had a significantly higher failure cycle count compared to PF (P=.016), RU (P=.001), and HBZPC (P<.001), but was not significantly different from CBO (P=.112). There was no significant difference in the failure cycle count between RU and HBZPC (P=.070).

CONCLUSION

Not all tested resin cements had a superior fatigue life when compared with zinc phosphate cement. Of the 4 resin cement groups, Groups CE, CBO, and PF were significantly superior to HBZPC.

Assessment of convergence angles of tooth preparations for complete crowns among dental students

OBJECTIVES

Convergence angles for complete crown preparations have been recommended at 4-12 degrees . However, practitioners have difficulty meeting these recommendations. This study measured and compared the convergence angles of tooth preparations for complete crowns prepared at three Colleges of Dentistry: the University of Tanta, Egypt, King Abdulaziz University, Saudi Arabia, and The Ohio State University, Columbus, Ohio.

METHODS

The convergence angles of 499 tooth preparations for complete crowns were evaluated. These comprised of a random sample of 262 teeth prepared by third-year dental students at the University of Tanta on extracted molars under normal preclinical conditions, 37 preparations on molar teeth on typodonts done by first-year dental students. The Ohio State University and 200 molar typodont complete crown preparations done by fourth-year dental students at King Abdulaziz University, Saudi Arabia. The bucco-lingual and mesio-distal convergence angles of each preparation were measured with a goniometer microscope. Mean convergence angles were calculated, and differences between groups were tested for statistical significance with analysis of variance (ANOVA) at 5% level of confidence.

RESULTS

Convergence angle measurements were significantly different between the groups and the dimensions (P<0.001). The greatest convergence value (19.8+/-10.0) was for bucco-lingual measurements prepared by Egyptian dental students. The smallest convergence value (14.1+/-3.8) was for mesio-distal measurements prepared by Saudi dental students.

CONCLUSIONS

This study emphasized the difference between what is taught at dental schools (what is theoretically possible) and the school results of actual practice. There was a considerable disparity between the convergent angles recorded in this study and the ideal configurations recommended in fixed prosthodontic textbooks and the dental literature.

Diametral tensile strength of a resin composite core with nonmetallic prefabricated posts: an in vitro study

STATEMENT OF PROBLEM

A number of prefabricated nonmetallic posts are currently available for use in conjunction with resin composite cores before fabrication of crowns for endodontically treated teeth. Information is needed regarding the strength of the composite and the nature of attachment between its components.

PURPOSE

The aim of this study was to determine the influence of different types of posts on the fracture resistance of a resin composite core material using the diametral tensile strength (DTS) test.

MATERIAL AND METHODS

Cylindrical specimens, 6 mm in diameter and 3 mm high, were prepared from resin composite (Tetric Ceram) and a group of prefabricated posts (n=10) as follows: resin composite only (control); Vectrispost (VTS); FiberKor (FKR); AEstheti-Plus post (ATP); Light-Post (LTP); Dentorama post (DRM), and Para-Post (PRP) as a second control. Specimens were stored for 7 days in water at 37 degrees C and then subjected to DTS test in a universal testing machine until failure occurred and load was recorded (N). Mean values and SD for DTS values (MPA) were calculated, and data were analyzed statistically with 1-way analysis of variance, followed by the Tukey test (alpha=.05). Representative specimens from each group were examined with SEM to determine nature of failure.

RESULTS

Mean values (SD) in MPa for DTS were as follow:

CONTROL GROUP

49.64 (3.36); VTS: 29.77 (3.36); FKR: 31.9 (2.39); ATP: 28.92 (2.2); LTP: 34.26 (3.37); DRM: 33.45 (2.46), and PRP: 27.90 (2.40). Analysis of variance indicated significant differences among the groups (P<.05). SEM examination indicated that for PRP failure was adhesive in nature, whereas with all nonmetallic posts, cohesive failure was more predominant.

CONCLUSION

The use of posts did not result in reinforcement of resin composite core when diametral tensile force was applied. When used with the core material, LTP, DRM, and FKR resulted in the highest DTS values, whereas PRP resulted in the lowest values.

Mechanical and physical properties of contemporary dental luting agents

STATEMENT OF PROBLEM

New luting agents, particularly with adhesive capability, are being introduced in an attempt to improve clinical success. Independent studies of basic comparative data are necessary to characterize these materials in relation to mechanical and physical properties.

PURPOSE

The purpose of this study was to compare the flexural strength, modulus of elasticity, and radiopacity and pH of representatives of 5 types (categories) of luting agents.

MATERIAL AND METHODS

The luting agents included a zinc phosphate, a conventional and a resin-modified glass ionomer, 2 dual-polymerizing resins ("photopolymerized" after mixing and "unphotopolymerized" conditions), and an auto-polymerizing resin. The specimens were prepared and the testing was conducted by 1 person to maximize standardization. Flexural strength (MPa) and modulus of elasticity (GPa) were determined on bar-shaped specimens (2 x 2 x 20 mm) at 24 hours and 3 months (n = 8). Radiopacity (mm Al) was measured by exposing 1 mm thick specimens along with an aluminum step wedge (n = 4). pH was measured using a pH electrode immediately after mixing; at 1, 5, 15, 30 minutes; and at 1, 2, 4, 6, and 24 hours (n = 4). The data were subjected to statistical analyses with analysis of variance and Duncan's multiple range test (P<.05).

RESULTS

The resin luting agents (64 to 97 MPa) showed higher flexural strength than all other materials tested (7 to 27 MPa), with the "photopolymerized" (83 to 97 MPa) conditions higher than "unphotopolymerized" (64 to 81 MPa) (P<.0005). Zinc phosphate was the most radiopaque (6.4 mm Al) (P<.0001) and provided the highest rigidity (9.2 GPa) (P<.05). The autopolymerization resin cement was the most radiolucent (1.1 mm Al) (P<.0001). Zinc phosphate and conventional glass ionomer cements were the most acidic immediately after mixing (pH 1.5 to 2.2) but were the least acidic after 24 hours (pH 6.4 to 6.8) (P<.0001).

CONCLUSIONS

Within the limitations of this study the data showed a wide variation of material properties. The dual-polymerization resin luting agents tested showed the best combination of mechanical and physical properties combined with the highest setting pH. Photopolymerization of these resin-based materials was necessary to maximize strength and rigidity.

Effect of cement types on the tensile strength of metallic crowns submitted to thermocycling

The relationship between metallic cast crowns and tensile strength according to cement types submitted to thermocycling was studied. Seventy-two metallic crowns were cast with Verabond II Ni-Cr alloy and cemented in standardized preparations with 10º tapering. Three types of finishing line (45-degree chamfered, 20-degree bevel shoulder and right shoulder) were made with diamond burs on bovine teeth. Twenty-four metallic crowns in each group were randomly subdivided into three subgroups of 8 samples each according to the cement used: SS White zinc phosphate cement, Vitremer resin-modified glass ionomer cement, and Rely X resin cement and were submitted to thermocycling. Retention was evaluated according to tensile load required to displace the metallic cast crowns from tooth preparations with an Instron testing machine. ANOVA and Tukey's test showed a statistically significant difference among luting materials, with greater results for Rely X resin cement (24.9 kgf) followed by SS White zinc phosphate cement (13.3 kgf) and Vitremer resin-modified glass ionomer cement (10.1 kgf). The finishing line types did not influence the tensile resistance of the crowns fixed with the three cements. Increased tensile resistance of metallic crowns fixed on bovine teeth was obtained with resin cement, independent of the finishing line types.

Retentive properties of five different luting cements on base and noble metal copings

STATEMENT OF PROBLEM

The retention of indirectly fabricated restorations can be compromised by short or over-tapered tooth preparations.

PURPOSE

The aim of this study was to evaluate the retentive properties of 5 different luting cements on base and noble metal copings to short and over-tapered preparations.

MATERIAL AND METHODS

Eighty extracted mandibular premolars were prepared to receive full cast copings with a flat occlusal surface, 33 degrees taper, and 3-mm axial length. Half of the standardized metal copings were cast in an AuAgPd alloy, whereas the other half were cast in an NiCr alloy. Cementation was performed with 5 different luting cements through use of 5 kg of pressure in 90% relative humidity. Specimens were stored in distilled water at 37 degrees C for 24 hours and thermocycled between 5 degrees C and 55 degrees C for 5000 cycles, with a dwell time of 30 seconds. After thermocycling, vertical tensile force was applied in a Zwick universal testing machine with a constant speed of 1 mm/min until separation was noted. A 2-factor analysis of variance was used to analyze the data, with a significance level of alpha =.05.

RESULTS

Mean dislodgement forces for AuAgPd crowns and NiCr crowns were 120.88 N and 143.09 N, respectively, for zinc phosphate cement; 135.45 N and 150.38 N for Principle; 145.88 N and 220.71 N for Meron; 276.85 N and 225.61 N for Avanto; and 300.92 N and 381.02 N for Fuji Plus.

CONCLUSION

Within the limitations of this study, Fuji Plus and Avanto showed significantly higher retentive strengths for AuAgPd copings in comparison to the other cements tested (P <.05). The retentive strength of Fuji Plus was significantly higher than those of the other cements tested with NiCr copings (P <.05).

The use of reinforced composite resin cement as compensation for reduced post length

STATEMENT OF PROBLEM

Cements that yield high retentive values are believed to allow use of shorter posts.

PURPOSE

This study investigated the use of reinforced composite resin cement as compensation for reduced dowel length.

MATERIAL AND METHODS

The retention values of stainless steel posts (parallel-sided ParaPost and tapered Dentatus in 5-, 8-, and 10-mm lengths) luted with Flexi-Flow titanium-reinforced composite resin and zinc phosphate cements were evaluated. Single-rooted extracted human teeth with crowns (n = 120), removed at the cementoenamel junction, were randomly divided into 4 groups of 30 samples each. Different post lengths were luted with either Flexi-Flow or zinc phosphate. Each sample was placed into a specialized jig and on a tensile testing machine with a crosshead speed of 2 mm/min, applied until failure. The effect of different posts and cements on the force required to dislodge the dowels was evaluated with multiple analyses of variance (ANOVA). One-way ANOVA with Scheffé contrast was applied to determine the effect of different post lengths on the retentive failure of posts luted with the 2 agents.

RESULTS

Flexi-Flow reinforced composite resin cement significantly increased retention of ParaPost and Dentatus dowels (P<.001) compared with zinc phosphate. One-way ANOVA revealed no statistically significant difference (P>.05) between mean retention of both dowels luted with Flexi-Flow for all posts length used (5 mm = 8 mm = 10 mm). Mean retention values of the groups luted with zinc phosphate showed a statistically significant difference (P<.001) for the different post lengths (10 > 8 > 5 mm). Parallel-sided ParaPost dowels demonstrated a higher mean retention than tapered Dentatus dowels (P<.001).

CONCLUSION

In this study, Flexi-Flow reinforced composite resin cement compensated for the reduced length of shorter parallel-sided ParaPost and tapered Dentatus dowels.

Current status of luting agents for fixed prosthodontics

STATEMENT OF THE PROBLEM

The long-term clinical outcome of fixed prosthodontic treatment depends, in part, on the use of adhesives that can provide an impervious seal between the restoration and the tooth. There are several types of available luting agents, each possessing unique properties and handling characteristics. No one product is ideal for every type of restoration.

PURPOSE

The purpose of this article is to review available dental luting agents, discuss improvements and drawbacks in newly formulated adhesives, and present the indications for their use.

RESULTS

Each cement type is physically and chemically unique. A single adhesive will not suffice in modern day clinical practice.