A semi-empirical model for prediction of how post-retained crowns will fail under compressive loading

The Use of the Finite Elements Method (FEM) to Determine the Optimal Angle of Force Application in Relation to Grooves Notched into a Zirconia Coping with the Aim of Reducing Load on a Connection with Veneering Ceramic

Objective of Study

To investigate, using the FEM, the influence of different notching angles on a zirconium dioxide coping with the aim of establishing the optimal connection conditions with veneering ceramic.

Materials and Methods

To calculate the stresses in the connection between zirconia coping and veneering ceramic, a model comprising grooves cut perpendicular was adopted. Such a notch profile was used to design the shape and spacing of the grooves on an FEM model simulating a zirconium dioxide coping. For discretization purposes we used twenty-node solid BRICK elements featuring intermediate nodes with three degrees of freedom in each node. The model was divided into 117 745 finished elements and 439 131 nodes. The problem was solved with a GLU type contact. The same load F = 1N divided by the number of nodes on the external surface was applied to each node of the outer surface of the base. In subsequent computing variants the F load changed the orientation by angle α from 0° to 45° every 15°.

Results

The highest level of material strain occurs at angle α = 0°  σ_red  max =309 MPa and the lowest at angle α = 45°  σ_red  max =220 MPa. The highest positive stress pressure occurs at angle α = 0°  p_max=251 MPa, p_min=-354 MPa and the lowest at angle α = 15°, p_max=171 MPa, p_min=-186 MPa. In the case of tangential stresses on the coping-veneering ceramic connection, the highest values were noted at angle α = 15°  τ_max=44,4 MPa and the lowest at angle α = 45°  τ_max=32,7 MPa.

Conclusions

To reduce the load on the zirconia-veneering ceramic connection, the notches should be made at an angle of α = 45°.

The Use of the FEM to Identify the Optimal Groove Dimensions Ensuring the Least Stressed Connection between a Zirconia Coping and Veneering Ceramic

Background: To examine the influence of coping notches with varying groove widths and depths on the quality of the connection with ceramic. Methods: Ten rectangular sintered zirconia (3Y-TZP) samples were etched with a neodymium-yag laser Nd:YAG. Then, a profilometer was used to test the depths and spacing of the grooves. A notch profile was used to design the shapes and spacing of the grooves based on a finite element method (FEM) simulating zirconia. The following situations were simulated: an increase in groove width from 100% to 180% and depth from 40% and 80%; and a 40% depth and width. Results: An increase of 10% in the baseline width caused an insignificant reduction of the strain in the connection. A further increase in this dimension led to a 50% increase in strain with a 40% increase in width. An increase in the groove depth by 40% reduced the strain level by 13%, while an increase in the groove depth by 80% reduced the strain level by 22%. Simultaneous deepening and widening of the groove by 40% had no significant impact on the strain level. Conclusion: Maintaining the width of the groove bottom while increasing the depth offers fewer advantages than deepening and narrowing the groove bottom.

Engineering Properties and Performance of Dental Crowns

Dental crowns are used to replace damaged natural crowns of teeth and are fixed to prepared teeth with luting cements, which should provide an adhesive bond to the tooth structure giving reliable retention and minimal microleakage. Mechanical testing of crowns in vitro gives failure load distributions that are well described by Weibull models, comparing probabilities of survival and reliability. Fatigue testing of crowns is time consuming, but regression analysis to interpolate functions through data points quoting probability limits or applying Weibull analysis is achievable. A complementary approach is to conduct materials tests with appropriate interfacial geometries. Luting cements are used in thin layers of 40–150 um. Contraction during polymerization is restrained by adhesion to substrates, allowing little relaxation of stresses. Conventional and resin-modified glass ionomer cements create thin zones of interaction with dentine and fail cohesively. The chevron notch short rod technique has been used to measure fracture toughness and rank cements. A development of this method, using chevron notch short bar specimens, permitted fracture toughness to be determined for luting cement-dentine substrate interfaces. Representative fracture experiments need to be developed to apply mixed mode conditions. The basic challenge to predict long-term performance from short-term laboratory tests remains.

An in vitro study into the effect of the ferrule preparation on the fracture resistance of crowned teeth incorporating prefabricated post and composite core restorations

AIM

This in vitro study investigated the effect of a ferrule preparation on the fracture resistance of crowned central incisors incorporating a prefabricated post (Parapost) cemented with Panavia-Ex and with a composite core.

METHODOLOGY

The test group consisted of 10 post crowned natural central incisor teeth with a 2-mm wide ferrule preparation, whilst the control group of 10 teeth had no ferrule. The specimens were mounted on a Lloyd universal testing machine and a compressive load was applied at an angle of 135 degrees to the palatal surface of the crown until failure occurred.

RESULTS

In both groups, failure occurred at higher loads compared with previous studies. The mean failure load was 1218 N for the unferruled group and 1407 N for the ferruled group. Statistical analysis showed no significant difference between the groups.

CONCLUSIONS

Under the conditions of this study, when composite cement and core materials are utilized with a Parapost prefabricated system in vitro the additional use of a ferrule preparation has no benefit in terms of resistance to fracture.

Esthetic restoration of endodontically treated teeth: factors that affect prognosis

Restoration of endodontically treated teeth involves a complex system of components and component interfaces designed to resist force. Dental materials, forces on teeth, clinical circumstances, and restorative design determine restoration success. A new classification evaluates number of canals, amount of remaining tooth surface, chamber space, canal quality, and canal orientation.

Capsulated versus hand-mixed glass-ionomer luting cements for post retention

OBJECTIVES

Glass-ionomer luting cements are supplied in two forms, as loose powder and liquid to be hand-mixed (HM) or pre-proportioned in a capsule to be mechanically mixed (MM). This study was to determine if post retention in pull-out tests was affected by the method of mixing the cement.

METHODS

Two hundred stainless steel posts of diameter 1.75 mm were cemented within post-channels prepared in stainless steel cylinders using two hand-mixed cements FJL and KCL (Fuji I Luting Cement and Ketac-Cem Luting Cement) and two capsulated cements FJC and KCM (Fuji Cap I and Ketac-Cem Maxicap). Three groups of test specimens were prepared. In Group I each cement was mixed as recommended by the manufacturer, Group II cements were placed within a capsule and mechanically mixed and in Group III cements were removed from the capsule and mixed by hand. Specimens were stored for 1 h at 37 degrees C and 100% humidity prior to post pull-out tests at a crosshead speed of 10 mm min-1. The maximum loads at failure were subjected to Weibull analysis and Mann-Whitney tests to determine probabilities of survival and significant differences between the groups.

RESULTS

Significant differences (P < 0.05) were found between all pairs of cements tested except KCM(MM) vs FJL(HM), FJL(MM) vs KCL(HM), KCL(MM), vs KCL(HM), FJC(HM) vs FJC(MM), KCL(MM) vs FJL(MM).

CONCLUSIONS

The capsulated cements as supplied by the manufacturers are preferable to the equivalent hand-mixed formulations, as they give higher probabilities of survival when subjected to a given load. Both capsulated and hand-mixed formulations of Fuji had higher probabilities of survival compared to the corresponding Ketac cements. The probability of post survival can be altered by the method of mixing the cement.

An in vitro investigation of cast post and partial core design

OBJECTIVES

Following endodontic therapy, the provision of a cast post and core restoration may be indicated. Many authors advocate retaining sound coronal dentine where possible, improving the resistance to fracture of the endodontically treated tooth. However, there is little conclusive experimental evidence to support this recommendation. This laboratory study was designed to investigate the effect of retained sound coronal dentine on the strength of a tooth restored with a cemented post and partial core restoration under direct loading.

METHODS

Specimens with retained coronal dentine buccally, lingually, or buccally and lingually were restored with cast post and partial cores and tested to failure using an Instron Universal Testing Machine, Model 1195. Control post and full core restorations were also tested. The mode of failure for all specimens was recorded photographically.

RESULTS

Teeth with retained buccal coronal dentine and post and partial cores were significantly less resistant to failure than controls with post and full cores (P < 0.01), failure occurring through crack initiation and propagation at the junction of the cast partial core and dentine core. There was no statistically significant difference in the strength of teeth with retained lingual coronal dentine or buccal and lingual dentine when compared to the control.

CONCLUSIONS

In conclusion, this in vitro model demonstrates that retained coronal dentine will not strengthen a tooth restored with a cast post and partial core. The clinical benefit of retaining buccal coronal dentine is brought into question when set against the increased technical effort and difficulty of producing the restoration in the dental technical laboratory and the possible overriding benefit of ferruling the crown margin onto a sound root structure.

The evaluation of post-retained crowns using a custom-designed fatigue machine

There are many published reports of static tests on dental materials that are required to perform clinically under a range of load amplitudes and cyclic frequencies. There are fewer reports in the dental literature of fatigue tests on dental structures and materials, and these are reviewed in this paper along with a description of the design, manufacture and commissioning of a machine which can apply chosen cyclic load/time characteristics up to a maximum amplitude of 700 N and frequency of 5 Hz. The machine design is not specific to particular test specimens but is widely applicable within dental materials testing. The machine is being used for a continuing study of post-retained crowns, cemented with glass-ionomer cement. The planning of fatigue tests and the interpretation of results is discussed with reference to the experience of previous authors, and it is concluded that a survival analysis approach offers a means of understanding materials' performances. The initial results from a random, blocked comparison of three glass-ionomer luting cements indicated that cyclic loads can be identified to give a 50% survival probability for specimens. It is expected that loads experienced in vivo have lower amplitude than those applied in this study, however, such loading cycles have been chosen to facilitate material comparisons. It is concluded that gaining significant results under laboratory fatigue conditions is difficult, but that they may usefully complement clinical trials.

Probability of failure of orthodontic brackets bonded with different cementing agents

OBJECTIVES

The aim of this study was to compare the maximum loads at failure and the probability of failure of three glass ionomer cements and a composite cement bonding orthodontic brackets to human premolar teeth.

METHODS

The cements studied included a conventional glass ionomer cement, two resin-modified glass ionomer cements and a composite cement. The roots of 200 human premolar teeth were embedded in acrylic resin and the buccal enamel surface of the crown prepared as required. Each cement used to bond the bracket to the enamel was weighted, and light-cured where required. The specimens were stored for 10 min or 24 h at 37 degrees C and 100% humidity. A tensile shear force was applied via a wire loop placed under the wings of the bracket. The maximum load at failure was noted and subjected to Weibull analysis to compare probabilities of survival for each cement. The data obtained was also analyzed using a Kruskal-Wallis test followed by comparison of the groups using Mann-Whitney tests.

RESULTS

Comparison of the loads at failure revealed that the composite cement was significantly stronger than the glass ionomer cements at 10 min and 24 h (p<0.05). Weibull analysis of the results gave values for the Weibull moduli and probabilities of survival for an orthodontic bracket under a given load for each cement at 10 min and 24 h.

SIGNIFICANCE

. Glass ionomer cements give a number of clinically significant advantages over composite cement in the retention of brackets. The resin-modified glass ionomer cements tested had a higher probability of survival than the conventional cement tested at 24 h. However, further improvements in their early bond strength would be clinically beneficial.

Factors influencing the failure of dental glass ionomer luting cement due to contraction

Glass ionomer cement used to fix a post within the root of a tooth undergoes cohesive failure on setting. This study aimed to determine the importance of humidity, substrate material and post design on cement failure. Nine groups of five specimens were prepared: groups 1-3 addressed the influence of post design and sectioning process on cement failure, while groups 4-9 examined the interaction between the cement, substrates and humidity. Replicas were prepared of all specimens and viewed using scanning electron microscopy. It was concluded that humidity and substrate influenced the failure of glass ionomer cement whereas post design did not.

Comparative study of four glass ionomer luting cements during post pull-out tests

OBJECTIVES

The aim of this study was to compare the maximum loads and modes of failure of four glass ionomer luting cements during post pull-out tests.

METHODS

The cements studied included two based on conventional glass ionomer cement chemistry, i.e., a hand-mixed cement and an encapsulated cement, and two based on the newer light-cured chemistry, i.e., a restorative cement used at a low powder:liquid ratio and an experimental luting cement. One hundred and forty bovine incisors were cut 13 mm from the apex and prepared with post channels, 1.75 mm in diameter and 9 mm long. Ni-Cr-Mb bonding alloy posts were cast and sandblasted prior to cementation into the roots with each of the cements. Twenty-four hours later, pull-out tests were carried out at a strain rate of 0.5 mm/min. The maximum loads and modes of failure were noted. The data obtained were analyzed using a Kruskal-Wallis test followed by comparison of groups using Mann-Whitney tests.

RESULTS

Comparison of the maximum loads revealed a significant difference between the conventional encapsulated cement and all other cements. Weibull analysis of the results gave values for the Weibull moduli and probabilities of survival for a post under a given load for each cement and indicated that the newer cements performed better than the conventional cements.

SIGNIFICANCE

Further research into the in vitro and in vivo behavior of the resin-modified glass ionomer cements in clinical situations where light-curing is not possible is justified.