Stress analysis of a tooth restored with a post and core

Survival of fixed prosthetic restorations on vital and nonvital teeth: A systematic review

PURPOSE

To evaluate the survival rate of full-coverage tooth-supported fixed prosthetic restorations, single crowns (SCs), and fixed dental prostheses (FDPs), taking into consideration the potential influence of tooth-vitality, presence and type of post, and type of prosthetic restoration material.

MATERIALS AND METHODS

In October 2022, two authors independently conducted a search in PubMed, Web of Science, and Scopus electronic databases as well as a hand search to identify clinical human studies on full-coverage SCs and FDPs supported by vital and/or non-vital abutments and/or a combination of both, with a minimum observation period of 24 months.

RESULTS

Out of 4198 studies identified through the database search and 22 through hand searching, 26 studies fulfilled the inclusion criteria and were included in the analysis of the present systematic review. Included studies scored six points and more according to Newcastle-Ottawa Scale (NOS). The highest estimated 5-year survival rate was observed for (metal-ceramic and all-ceramic) SCs on vital teeth (98.3%; 95% CI [98.1, 98.6%]) and all ceramic SCs on non-vital teeth with fiber post (95.0%; 95% CI [94.5-95.4%]). Metal-ceramic SCs on vital teeth (97.1%; 95% CI [95.6-98.7%]) showed a statistically significant higher estimated 5-year survival rate compared to metal-ceramic SCs with cast metal post (90.7%; 95% CI [87.4-94.0%], P < 0.001), fiber post (91.3%; 95% CI [90.9-91.6%], P < 0.001) and without post (85.7%; 95% CI [80.7, 90.6%], P < 0.032). All-ceramic SCs with fiber post had a statistically significant higher estimated 5-year survival rate (95.0%; 95% CI [94.5-95.4%]) compared to metal-ceramic SCs on non-vital teeth with fiber post (91.3%; 95% CI [90.9-91.6%], P < 0.001). SCs (all-ceramic and metal-ceramic) with fiber post had a statistically significantly higher estimated 5-year survival rate of (92.7%; 95% CI [92.4-92.9%]) than SCs made of metal-ceramic and retained by cast metal post (90.7%; 95% CI [87.4-94.0%], P < 0.001). For FDPs, the 5-year survival rate was significantly higher for FDPs on vital abutments (84.9%; 95% CI [75.9, 93.9%]) compared to FDPs retained by non-vital abutment/s (81.3%; 95% CI [80.3, 82.2%], P = 0.049) irrespective to presence, type of post, and FDPs material. The results are limited by the limited number of studies and the presence of uncontrolled confounding clinical variables.

CONCLUSIONS

Within the limitations of the study, tooth vitality is suggested to contribute positively to the survival of SCs and FDPs.

Retrospective clinical study of tooth-supported single crowns: A multifactor analysis

The purpose of the study was to investigate the influence of multiple factors on the survival of tooth‐supported single crowns and assess the biological and technical complications. This retrospective study included patients rehabilitated with single crowns with a minimum follow‐up time of 6 months after delivery. The cumulative survival rate was calculated over the maximum period of follow‐up time and reported in a life‐table survival analysis. Univariate and multivariate Cox regression was used to evaluate the associations between clinical covariates and crown failure. The included cohort group consisted of 1037 single crowns delivered in 401 patients and followed for a mean of 134.8 ± 80.2 months. Cumulative survival rate was 89.9% and 80.9% after 5 and 10 years and 70.5% and 61.8% after 15 and 20 years, respectively. The main reasons for single crown failure were loss of retention, tooth loss, and fracture. Anterior placement, non‐vital abutments, and bruxism significantly influenced the survival of single crowns. The survival of single crowns was not influenced by patient's age and sex, location of the crowns in relation to the jaws, type of tooth, presence of post and core, and type of crown material, treatment providers, or smoking. Anterior placement, non‐vital abutments, and bruxism are factors suggested to increase the risk of single crown failure and the prevalence of technical and biological complications.

Comparative Evaluation of the Effect of Core Type and Antirotational Post on Stress Distribution in an Endodontically Treated Maxillary First Molar: FEA

Background

This study aims to analyze and compare the stress distribution in an endodontically treated maxillary first molar restored with various post and core systems and assess the benefit of the presence of an antirotational post and the effect of its length using finite element analysis.

Materials and Methods

Five 3D models of maxillary first molar restored with variable designs of post and core were constructed using CT scanning and information obtained from textbook and FEM software. Variables were types of core, presence or absence of an antirotational post, and length of the antirotational post. A load of 480 (N) vertically and a load of 240 (N) with 45° to the occlusal plane were applied. Results were analyzed using 3D von Mises criteria.

Results

The results showed that the most homogeneous stress distribution pattern along with dentin and the post-core system was observed in the model with one main post in the palatal canal and in the model with a two-piece core and one main post in the palatal canal and shorter antirotational post in the distal-buccal canal. However, models with one-piece core and antirotational post (either the same length or shorter than the main post) in the distal-buccal canal had the most stress concentration sites.

Conclusion

In this study, it was found that the presence of an antirotational post, its length, and type of core affected the distribution of stress along the dentin and post-core system. This study indicated that if the antirotational post is considered in the post-core design, it is better to use a two-piece core type due to more homogeneous stress distribution along the dentin and post-core system.

Influence of elastic modulus mismatch between dentin and post-and-core on sequential bonding failure

PURPOSE

Clinical failures of teeth restored with post-and-core are critical issues for the survival of teeth and maintenance of oral functions. A tooth with post-and-core restoration is a complex structure. Cement adhesion is believed to be the weakest component, and breakage in this component leads to changes in stress distribution in the complex structure. The tested hypothesis was that cement breaking processes of prosthetic treated teeth were affected by elastic properties of post-and-cores.

METHODS

Finite element analysis focused on sequential adhesion failure between the dentin and cement; the penalty function method was used to analyze stress during each stage of bonding conditions. Failure patterns of adhesion and stress distribution within dentin under load of different materials of post-and-core was observed.

RESULTS

Although, an initial failure of cement was observed at the palatal crown margin regardless of the material. Different patterns of adhesion failure between dentin and post-and-cores were observed by different elastic properties of post-and-cores. Stress concentration was observed at the corresponding areas of interface between adhesion failure and continued elements using both post-and-cores.

CONCLUSIONS

Using failure criteria for cement adhesion, sequential changes of adhesion failure between dentin and post-and-cores were observed. Local stress concentrations leading to severe destruction of dentin were caused by not only materials of post-and-cores but their adhesive conditions to dentin. Nonlinear finite element analysis (FEA) using complex structure model which deals with alterations of interfacial condition between components could provide the simulation for the clinical failure of teeth restored with post-and-cores.

Effect of three metal priming agents on the bond strength of adhesive resin cement to Ag-Zn-Sn-In alloy and component metals

The aim of this study was to investigate the effect of three metal priming agents on the bond strength of adhesive resin cement to Silver-Zinc-Tin-Indium (Ag-Zn-Sn-In) alloy and pure Ag, Zn, Sn, and In. The specimens were air-abraded with alumina and then primed with one of three metal priming agents: V-Primer, Estenia Opaque Primer, or Alloy Primer. The metal disks were bonded with adhesive resin cement (Super-Bond Bulk-mix technique). Shear bond strengths (n=10/group) were determined before and after 50,000 thermocycles for Ag-Zn-Sn-In alloy as well as after 5,000 thermocycles for pure Ag, Zn, Sn and In. For Ag-Zn-Sn-In alloy, the post-thermocycling bond strength of the Alloy Primer group was significantly higher than that of the other primers. It can be concluded that Alloy Primer containing both the vinyl-thione monomer (VBATDT) and hydrophobic phosphate monomer (MDP) is effective for bonding Ag-Zn-Sn-In alloy and pure Ag, Zn and Sn.

A finite element study of teeth restored with post and core: Effect of design, material, and ferrule

Background:

Different postdesigns and materials are available; however, no consensus exists regarding superiority for stress distribution. The aim of this study was to evaluate the effect of design and material of post with or without ferrule on stress distribution using finite element analysis.

Materials and Methods:

A total of 12 three-dimensional (3D) axisymmetric models of postretained central incisors were made: Six with ferrule design and six without it. Three of these six models had tapered posts, and three had parallel posts. The materials tested were titanium post with a composite resin core, nickel chromium cast post and core, and fiber reinforced composite (FRC) post with a composite resin core. The stress analysis was done using ANSYS software. The load of 100 N at an angle of 45΀ was applied 2 mm cervical to incisal edge on the palatal surface and results were analyzed using 3D von Mises criteria.

Results:

The highest amount of stress was in the cervical region. Overall, the stress in the tapered postsystem was more than the parallel one. FRC post and composite resin core recorded minimal stresses within the post but the stresses transmitted to cervical dentin were more as compared to other systems. Minimal stresses in cervical dentine were observed where the remaining coronal dentin was strengthen by ferrule.

Conclusion:

A rigid material with high modulus of elasticity for post and core system creates most uniform stress distribution pattern. Ferrule provides uniform distribution of stresses and decreases the cervical stresses.

Retention of Root Canal Posts: Effect of Cement Film Thickness, Luting Cement, and Post Pretreatment

The aim of this study was to investigate the effect of the cement film thickness of a zinc phosphate or a resin cement on retention of untreated and pretreated root canal posts. Prefabricated zirconia posts (CosmoPost: 1.4 mm) and two types of luting cements (a zinc phosphate cement [DeTrey Zinc] and a self-etch adhesive resin cement [Panavia F2.0]) were used. After removal of the crowns of 360 extracted premolars, canines, or incisors, the root canals were prepared with a parallel-sided drill system to three different final diameters. Half the posts did not receive any pretreatment. The other half received tribochemical silicate coating according to the manufacturer's instructions. Posts were then luted in the prepared root canals (n=30 per group). Following water storage at 37°C for seven days, retention of the posts was determined by the pull-out method. Irrespective of the luting cement, pretreatment with tribochemical silicate coating significantly increased retention of the posts. Increased cement film thickness resulted in decreased retention of untreated posts and of pretreated posts luted with zinc phosphate cement. Increased cement film thickness had no influence on retention of pretreated posts luted with resin cement. Thus, retention of the posts was influenced by the type of luting cement, by the cement film thickness, and by the post pretreatment.

Three-dimensional finite element analysis of stress distribution in a tooth restored with metal and fiber posts of varying diameters: An in-vitro study

OBJECTIVE

To compare stress distribution in a tooth restored with metal and fiber posts of varying diameters (1.2 and 1.4 mm) by means of three-dimensional finite element analysis (3D-FEA).

MATERIALS AND METHODS

Four 3D-FEA models were constructed: (1) fiber post (1.2 and 1.4 mm) and (2) metal post (1.2 and 1.4 mm). The material properties were assigned and a force of 100 N was applied at 45° angle to the longitudinal axis of the tooth onto the palatal surface incisal to the cingulum. Analysis was run and stress distribution pattern was studied.

RESULTS

Maximum stresses in the radicular tooth structure for fiber post were higher than that for metal post. In the former models, stresses in the tooth structure were slightly reduced with increase in fiber post diameter.

CONCLUSIONS

To reduce stress in the remaining radicular tooth structure, it is better to use a fiber post of a large diameter.

A Comparative Study of the Stress Distribution in Different EndodonticPost-RetainedTeeth with and without Ferrule Design-A Finite Element Analysis

Purpose. To analyze the stress distribution in an endodontically treated maxillary central incisor restored with various post-core systems and assess the benefit of ferrule using finite element analysis. Material and Methods. Twelve models with metal ceramic crown were created based on the combination of three types of post-core systems (titanium post-composite resin core, nickel-chromium post-core, and fiber reinforced composite resin post-composite resin core), two varieties of posts (tapered, parallel), and with or without ferrule. 100 N load was applied in three directions and the von Mises stress was compared. Results. Ferrule made no difference in stress distribution for the titanium and nickel-chromium posts, though it showed some stress reduction in fiber-reinforced composite resin posts. Nickel-chromium cast post-core transmitted the least amount of stresses to the dentin despite producing the maximum stress. Conclusion. Incorporation of ferrule offered some degree of stress reduction in nonmetal post, and it increased the stresses within cervical dentin.

Stress analysis of endodontically treated teeth restored with post-retained crowns: A finite element analysis study

BACKGROUND

The authors conducted a study to analyze the stress concentration areas in a tooth restored with a post-retained crown under various conditions.

MATERIALS AND METHODS

The authors constructed a three-dimensional finite element model describing a maxillary second premolar restored with an all-ceramic crown supported by a titanium post and a resin-based composite core. They applied static vertical and horizontal loads of 100 newton to the cusp tip of the crown and recorded Von Mises and tensile stress values. The variables investigated were the presence of the post, coronal and apical post extensions, post diameter, post shape, and post and core material.

RESULTS

The study results showed that horizontal loading generated higher levels of stress than did vertical loading. The greatest stress levels were concentrated at the cervical region and at the post-dentin interface in all models. Under both loads, a higher modulus of elasticity of the post material and a wider post diameter were associated with increased stress values at the post-dentin interface. Reduction of the post extension above the level of bone was associated with increased dentinal stresses near the apex of the post.

CONCLUSIONS

Although endodontic posts provide retention for coronal restorations, they result in dentinal stress values higher than those of crowns without posts. Posts that had a similar modulus of elasticity to dentin and smaller diameters were associated with better stress distribution. Resting coronal restorations on sound dental tissues affected stress distribution more than did the core material or the length of the coronal post extension.

CLINICAL IMPLICATIONS

Many factors influence the distribution of stress within dentin and, consequently, the fracture resistance of teeth restored with post-retained crowns. Clinicians need to keep these factors in mind when performing endodontic procedures that involve placement of post-retained crowns to ensure optimal success.

Three-dimensional finite element analysis of strain and stress distributions in endodontically treated maxillary central incisors restored with diferent post, core and crown materials

OBJECTIVES

The present comparative analysis aimed at evaluating which combination of restorative materials resulted in the most homogeneous stress and strain distributions.

METHODS

A three-dimensional finite element analysis was performed. All the nodes on the external surface of the root were constrained in all directions. Eighteen experimental models with different material properties and configurations were simulated. An arbitrary load of 10N was applied at 60 degrees angle with tooth longitudinal axis on the palatal surface of the crown. Von Mises (equivalent stresses) energetic criterion was chosen.

RESULTS

In all the models the values of both strain and stress recorded at the middle third of the buccal aspect of the root surface were at their maxima. On the contrary, the minimum values were noticed at level of both the apical portion of the post and the root apex. The maximum stresses were evidenced at level of the cemento-enamel junction (CEJ) on both the buccal and palatal aspects of root cement and dentin. Stress progressively decreased from the outer to the inner part of the root and from the CEJ towards the incisal margin of the crown as well.

SIGNIFICANCE

The results of the present study would allow clinicians to make an informed choice from among available materials to restore endodontically treated teeth.

A survey of failed post-retained restorations

Survival of endodontically treated, post-restored teeth depends on a multitude of factors, all of which are practically impossible to include in a randomized, controlled clinical study. The purpose of this survey was to characterize and analyze reported failures of post-retained restorations to identify factors critical to failure and to type of failure. A questionnaire was mailed to private practitioners in Denmark with a request to complete the questionnaire whenever a patient presented with a failed post-retained restoration. Information was gathered on factors related to the patient, the tooth, the restorative materials, and the techniques. Two-hundred and sixty questionnaires were collected from 171 practitioners over a 3-year period. Functioning time until failure varied between 3 months and 38 years. Mean survival time until failure was 11 years. Of the failed restorations, 61% had functioned for 10 years or less. Fracture of the tooth was the most common type of failure reported, followed by loosening of the post and fracture of the post. Tapered posts implied an increased risk of tooth fracture compared to loosening or fracture of the post, and the relative risk of tooth fracture increased with the functioning time until failure. Fracture of the post was more common among male than female patients. On the basis of this survey of failed post-retained restorations, it was concluded that tapered posts were associated with a higher risk of tooth fracture than were parallel-sided posts.

Fracture resistance of crowned incisors with different post systems and luting agents

This study evaluated the fracture resistance of crown-restored incisors with different post-and-core systems and luting cements. Fifty intact maxillary central incisors were randomly assigned to five groups of 10 teeth each. Group 1 was restored with fibre posts (Snowpost) luted with an adhesive composite resin cement (Panavia F). Group 2 was restored with titanium alloy posts (Parapost) luted with the resin cement, and Group 3 with titanium alloy posts and a glass-ionomer cement (Fuji I). Composite cores (Clearfil Photocore) were built up in groups 1, 2 and 3. Group 4 was restored with cast post-and-cores luted with the resin cement, and group 5 with the cast post-and-cores and the glass-ionomer-cement served as a control group. All teeth were restored with metal-ceramic crowns. After thermal stressing, the specimen was then secured in a universal testing machine. Fracture loads and modes were recorded. One-way ANOVA and a Tukey test were used to determine significant differences between the failure loads of groups. Chi-square test was conducted for evaluation of the fracture modes. The fracture loads of groups 1 and 2 were significantly higher than that of the control group (P<0.05). Group 1 had a significantly higher number of repairable fractures than the other four groups (P<0.001). Within the limitations of this study, the results suggest that fibre posts can be recommended as an alternative to cast and prefabricated metallic posts. Composite resin cement cannot significantly improve fracture resistance of metallic post and crown-restored incisors.

Evaluation of the biomechanical behavior of maxillary central incisors restored by means of endocrowns compared to a natural tooth: A 3D static linear finite elements analysis

OBJECTIVE

The present study aimed at evaluating different restoring configurations of a crownless maxillary central incisor, in order to compare the biomechanical behavior of the restored tooth with that of a sound tooth.

MATERIALS AND METHODS

A 3D FE model of a maxillary central incisor is presented. An arbitrary static force of 10 N was applied with an angulation of 125 degrees to the tooth longitudinal axis at level of the palatal surface of the crown. Different material configurations were tested: composite, syntered alumina, feldspathic ceramic endocrowns and glass post resorations with syntered alumina and feldspathic ceramic crown.

RESULTS

High modulus materials used for the restoration strongly alter the natural biomechanical behavior of the tooth. Critical areas of high stress concentration are the restoration-cement-dentin interface both in the root canal and on the buccal and lingual aspects of the tooth-restoration interface. Materials with mechanical properties underposable to that of dentin or enamel improve the biomechanical behavior of the restored tooth reducing the areas of high stress concentration.

SIGNIFICANCE

The use of endocrown restorations present the advantage of reducing the interfaces of the restorative system. The choice of the restorative materials should be carefully evaluated. Materials with mechanical properties similar to those of sound teeth improve the reliability of the restoartive system.

Stress distribution in a post-restored tooth using the three-dimensional finite element method

Clinicians are opting ever more frequently for restorative materials which have an elastic modulus similar to that of dentin when reconstructing endodontically treated teeth. Metallic posts, which are capable of causing dangerous and non-homogenous stresses in root dentin, are slowly being abandoned. Ideal posts may be those made of various types of fibre (carbon, mineral and glass) and which are adhesively luted into the canal. Among the different methods for evaluating the mechanical behaviour of posts in root canals (progressive loads and photo-elastic technique) the finite element method (FEM) presents many advantages. The aim of this paper is to evaluate, utilizing three-dimensional analysis of the finite elements, what the effect of material rigidity, depth of insertion and post diameter could be on the stress distribution in the different components of the single tooth-post-core reconstruction unit. The results of the FEM analyses, expressed as the distribution of Von Mises stress values, has allowed us to conclude that (i) fibreglass-reinforced composite distributes stress better than titanium alloy or stainless steel; (ii) fibreglass-reinforced composite posts should be inserted as deeply as possible (but maintaining 5-6 mm of gutta-percha apical seal); (iii) fibreglass-reinforced composite post diameter does not affect stress distribution, therefore, as much radicular dentin as possible should be preserved.

3D FEA of cemented steel, glass and carbon posts in a maxillary incisor

OBJECTIVES

A comparative study on the stress distribution in the dentine and cement layer of an endodontically treated maxillary incisor has been carried out by using Finite Element Analysis (FEA). The role of post and cement rigidity on reliability of endodontic restorations is discussed.

METHODS

A 3D FEM model (13,272 elements and 15,152 nodes) of a central maxillary incisor is presented. A chewing static force of 10 N was applied at 125 degree angle with the tooth longitudinal axis at the palatal surface of the crown. Steel, carbon and glass fiber posts have been considered. The differences in occlusal load transfer ability when steel, carbon and glass posts, fixed to root canal using luting cements of different elastic moduli (7.0 and 18.7 GPa) are discussed.

RESULTS AND SIGNIFICANCE

The more stiff systems (steel and carbon posts) have been evaluated to work against the natural function of the tooth. Maximum Von Mises equivalent stress values ranging from 7.5 (steel) to 5.4 and 3.6 MPa (respectively, for carbon posts fixed with high and low cement moduli) and to 2.2 MPa (either for glass posts fixed with high and low cement moduli) have been observed under a static masticatory load of 10 N. A very stiff post works against the natural function of the tooth creating zones of tension and shear both in the dentine and at the interfaces of the luting cement and the post. Stresses in static loading do not reach material (dentine and cement) failure limits, however, they significantly differ leading to different abilities of the restored systems to sustain fatigue loading. The influence of the cement layer elasticity in redistributing the stresses has been observed to be less relevant as the post flexibility is increased.

Resistance to cyclic loading of teeth restored with posts

This in vitro study evaluated the effect of presence of post, presence of core, and of shape, type, and surface treatment of posts on resistance to cyclic loading of crowned human teeth. For all teeth, crowns designed without ferrule were cast in sterling silver and luted with resin cement (Panavia F). Each tooth underwent cyclic loading of 600 N at two loads per second until failure. Teeth that had only been crowned showed significantly higher resistance to cyclic loading than teeth with cores or with post and cores. No significant differences were found between teeth restored with cores only or with post and cores, irrespective of surface-treatment of the posts. Teeth restored with parallel-sided cast post (ParaPost XP) and cores showed significantly higher resistance to cyclic loading than teeth with either tapered cast posts or untreated prefabricated posts of titanium alloy (ParaPost XH) or glass fiber composite (ParaPost Fiber White). No significant difference was found between teeth restored with parallel-sided cast post and cores and teeth restored with untreated prefabricated posts of zirconia (Cerapost). Surface treatment of posts significantly increased the resistance to cyclic loading compared with untreated posts. When posts are used, surface treatment is recommended.

Factors determining post selection: a literature review

Most endodontically treated teeth require a post-and-core build-up for restoring the teeth to optimum health and function. Selection of an appropriate post-and-core system from the wide variety of those available may be a clinical dilemma. Therefore, the purpose of this article is to review the literature to identify the various factors that influence the selection of the post-and-core assembly. English-language peer-reviewed articles between 1961 and 2002 were identified using MEDLINE, as well as a hand search. The following key words were used: post, design, retention, fracture resistance, survivability, and esthetics. Selection of a post and core system should satisfy many interrelated biologic, mechanical, and esthetic factors to optimally restore the endodontically treated tooth to adequate form and function. This review may serve as a guide to aid the clinician in the selection of a post-and-core system.

The restoration of endodontically treated, single-rooted teeth with cast or direct posts and cores: a systematic review

Direct post-and-core restorations with prefabricated posts are becoming increasingly popular. A literature review was conducted to compare the clinical and in vitro performance of cast posts and cores to that of direct cores with prefabricated posts in single-rooted teeth. Research on the restoration of endodontically treated teeth was identified through a search of electronic databases. The search yielded a total of 1773 references. After these references were subjected to strict inclusion criteria, 10 in vitro and 6 in vivo studies remained and critically reviewed. A comparison of fracture loads in the in vitro studies revealed no significant difference between cast and direct posts and cores. Meta-analysis of the data suggested that there is no difference in fracture behavior associated with the 2 treatment modalities. An overall survival analysis was not possible for the in vivo studies. The survival for cast posts and cores in 2 studies ranged from 87.2% to 88.1% and in a third study reached 86.4% for direct cores after 72 months. Randomized clinical trials on this topic were not available but should be conducted to verify published findings.

Qualitative assessment of stress distribution during insertion of endodontic posts in photoelastic material

OBJECTIVES

The purpose of this study was to investigate the stress patterns associated with prefabricated endodontic posts during the various stages of insertion according to a number of design characteristics.

METHODS

In a photoelastic material with elastic properties comparable to dentin, analyses were performed of the overall stress patterns with polarized light revealing substantial differences in stresses generated by the various posts. The effects of variations in design for certain configurations of the posts were also assessed.

RESULTS

One geometric feature was the retentive thread of the post. The stress patterns within the photoelastic material revealed a homogeneous distribution of stress along the entire length of the thread, and more threads induced additional stress. The stress recorded with a vent when the pitch of the thread was 0.8-1.0 mm was classified as minimal-to-mild stress. Another geometric feature considered was the head (coronal extension) of the post. Minimal stress was recorded in the material in contact with the head and the apical end of the post when the contact surface of head was more than 3 mm2.

CONCLUSIONS

This study suggests that during insertion of threaded posts the least stress occurs when the head contact surface is sufficient (> or = 3 mm2). A thread with a pitch of 0.8-1.0 mm is most desirable in stress reduction. The number of windings should also be limited (less than six windings) as samples with a substantial number of windings (N = 13 or 30) produce severe stress.

Analysis of stress distribution in a maxillary central incisor subjected to various post and core applications

In this study, stress distribution in a maxillary central incisor that was subjected to endodontic treatment and various post and core applications was determined by using a three-dimensional finite-element method. Cast gold post and gold core and different combinations of prefabricated stainless steel post, or a prefabricated titanium post with amalgam or composite cores were considered. In all cases, the tooth was assumed to have a porcelain crown. The primary purpose of the study was to evaluate the stress distribution due to simulated biting forces and thus investigate the reliability of different post and core applications. It was found that, within the limitations of the study, the cast gold post and gold core application yielded the best result. The second purpose of the study was to highlight the importance of using a unique stress value, which contains the effects of all the possible stress components existing upon loading in the determination of a possible failure. A resultant stress value and all the constituent components were shown in detail, and it was proved that an estimation independently based on an individual component might lead to erroneous results.

Modeling the mechanical behavior of the jaws and their related structures by finite element (FE) analysis

In this paper, we provide a review of mechanical finite element analyses applied to the maxillary and/or mandibular bone with their associated natural and restored structures. It includes a description of the principles and the relevant variables involved, and their critical application to published finite element models ranging from three-dimensional reconstructions of the jaws to detailed investigations on the behavior of natural and restored teeth, as well as basic materials science. The survey revealed that many outstanding FE approaches related to natural and restored dental structures had already been done 10-20 years ago. Several three-dimensional mandibular models are currently available, but a more realistic correlation with physiological chewing and biting tasks is needed. Many FE models lack experimentally derived material properties, sensitivity analyses, or validation attempts, and yield too much significance to their predictive, quantitative outcome. A combination of direct validation and, most importantly, the complete assessment of methodical changes in all relevant variables involved in the modeled system probably indicates a good FE modeling approach. A numerical method for addressing mechanical problems is a powerful contemporary research tool. FE analyses can provide precise insight into the complex mechanical behavior of natural and restored craniofacial structures affected by three-dimensional stress fields which are still very difficult to assess otherwise.

Apical root strain as a function of post extension into a composite resin core

This study tested the hypothesis that there is no difference in the amount of strain measured in the root of an extracted tooth at the apical end of a prefabricated post, whether a composite resin core completely covers the head of the post or the post is exposed at the occlusal surface of the core. Twenty extracted teeth were measured (1) after endodontic treatment (baseline), (2) after a composite resin core was fabricated over a prefabricated post, and (3) after a cast crown was cemented. The teeth were randomly divided into two groups after baseline testing. In group 1, the post was buried within the core, and in group 2, the head of the post was exposed on the occlusal surface of the core. Analysis of results indicated a statistically significant decrease in strain from the baseline to the crown when 1 mm of composite resin covered the head of the post. However, based on the small strain values measured, the difference may not be clinically significant.

The prosthodontic management of endodontically treated teeth: a literature review. Part III. Tooth preparation considerations

The final part of this literature review series presents guidelines for the optimal preparation of teeth to receive posts and cores.

The prosthodontic management of endodontically treated teeth: a literature review. Part I. Success and failure data, treatment concepts

Part I of this three-part literature review discusses the incidence of endodontic treatment required after prosthodontic procedures, whether crowns are needed on endodontically treated teeth, the primary purpose of posts, the causes of post and core failure, root fracture incidence data, and post design considerations. Pertinent questions are addressed based on the past 25 years of dental literature.

Finite element analysis of quasistatic and fatigue failure of post and cores

Finite element (FE) analysis of the mechanical behaviour of materials and structures facilitates the investigation of their internal stress distributions. However, the validity of the model is not always ascertained. In this study a three-dimensional (3D) FE model was developed, representing a laboratory set-up of direct post and core restored upper premolars. These restorations, using either composite or amalgam for core material, have been the subject of study in previous quasistatic and fatigue strength tests. The aim of this study was to validate the FE model for prefailure and failure modelling, by comparing the computational results with the laboratory observations and failure results. Two failure criteria were selected for investigation: Modified Von Mises and Drücker-Prager equivalent stress. Four model variations were carried out, representing different conditions at the core-tooth interface. Prefailure modelling was found to be adequate. The calculated failure results could only partly be fitted to the quasistatic tests. The best fit was effected with a model using partial bonding of the core, for the composite core. Fatigue failure was reproduced somewhat better by a model using no bonding at all, again to a higher degree for the composite core. Calculations of post stress using a model simulating increased core mobility supported an observation made previously (M. C. D. N. J. M. Huysmans et al., in press; Int. Endodont. J. XX, XXX-XXX), implying that a composite core raises the demands made on the post. The conclusion is made that validation of FE calculations is essential. A 3D model as presented here shows a satisfactory fit to fatigue data but not to quasistatic results.(ABSTRACT TRUNCATED AT 250 WORDS)

Stresses induced by different loadings around weak abutments

Stresses and deflections of abutments induced by various loadings were analyzed with a two-dimensional finite element model. The biomechanic system consisted of the mandibular posterior three-unit fixed partial denture (FPD). Four different loading types were analyzed: (1) a distributed force of 600 N; (2) concentrated nonaxial and (3) axial 300 N forces at the marginal ridge of the molar; and (4) a concentrated vertical 300 N force at the center of the pontic. All computations were conducted for three different alveolar bone levels. The premolar exerted a greater pressure during occlusal loadings (except axially) on the alveolar bone than the molar. According to the stresses induced in the alveolar bone, the most critical loading was the distributed force. With diminishing periodontal support, stresses elevated in the biomechanic system and critical increases were noted for the concentrated nonaxial load on the molar.

Root fracture in endodontically treated teeth related to post selection and crown design

Standardized plastic analogues simulating an endodontically treated maxillary central incisor root were used to investigate the resistance to root fracture in endodontically treated teeth. Three different post and core systems were used: (1) cast post and core, (2) Para-Post Plus post, and (3) Flexi-Post post. The core build-up material selected in this study was Ketac Silver material, after which a crown preparation was made on each analogue. Two types of preparations were used: a wide chamfer margin (butt-joint) and a wide chamfer margin with a 1.0 mm circumferential bevel. An overcasting was fabricated and was permanently cemented on each preparation. Assemblies were subjected to an increasing lingual oblique force until fracture occurred. Results were tabulated using the analysis of variance (F test) and Student's t test. Beveled preparations with a concomitant final restoration provided a significant increased resistance to root fracture. Furthermore, vertical fracture occurred twice as often with nonbeveled preparations.

Failure characteristics of endodontically treated premolars restored with a post and direct restorative material

Ninety-one extracted maxillary premolar teeth were restored with a prefabricated post and amalgam, composite resin or glass-cermet core. Each group was again divided into three groups of 9-13 teeth to be subjected to an increasing load in one of three standardized directions (10, 45 and 90 degrees to the long axis of the tooth). Failure load and characteristics of failure were recorded. The glass-cermet-restored teeth had a lower strength than the other groups for every load direction (Student's t-test: P less than 0.01). Amalgam and composite resin groups showed a significant difference only for the 10 degrees loading condition (Student's t-test: P less than 0.02). Teeth restored with amalgam cores displayed a higher mean failure load, in combination with a 46% occurrence of root fracture.

The dentin-root complex: anatomic and biologic considerations in restoring endodontically treated teeth

The restoration of endodontically treated teeth has been the focus of considerable controversy and empiricism. Time-tested methods have been highly successful in some respects, but failures are still apparent. The inherent causes of failure are rarely evaluated and the limitations of specific restorative systems are seldom identified. Regardless of the system, there should be a thorough understanding of the anatomy and biology of the dentin and root supporting the restoration on the part of the practitioner, because both endodontic and restorative procedures alter the hard tissues.

Clinical implications of the response of enamel and dentin to masticatory loads

The success of restorative procedures is dependent on comprehension of the responses of enamel and dentin, including responses to masticatory forces. The regional variation resulting from masticatory forces is critical because clinically it relates to the thickness of enamel and dentin occlusogingivally. Three-dimensional finite element models of an intact mandibular molar were developed to analyze stresses in enamel and dentin occlusogingivally, buccolingually, and mesiodistally. There were dramatic regional variations in the magnitude and character of different stresses caused by masticatory forces, and despite being organically "bonded," enamel and dentin responded independently. This unique behavior with regional variations of these tissues could have serious clinical implications during restorative procedures.

Three-dimensional photoelastic stress analysis of the ferrule effect in cast post and cores

The effect of a metal collar on stress distribution with cast post and cores was studied by using three-dimensional photoelastic models of maxillary canine teeth of average dimensions. Standardized parallel post and cores were cemented into the models, with half of the samples incorporating a 1.5 mm metal collar. A 400 gm load was applied to the cingulum of the cores and stresses were frozen in the models. The posts were removed, the teeth were sectioned, and stresses were measured with a circular polariscope. In both groups the greatest stress concentration was found at the lingual apex of the post. On a point by point basis, stresses were higher in the collared specimens. Variation in stress magnitude among five preselected points was greater within the noncollared group.

Load transfer of posts and cores to roots through cements

This study evaluated differences in load transfer when cast posts are fixed to roots with different cements. Cast posts and cores were cemented with 40 endodontically prepared teeth by using four different cement mediums. The load exerted on the root surface through the post was evaluated using a strain gauge. The increased flexure after cementation was statistically analyzed. In conclusion, there was load transfer from post to root structure when posts were cemented, but no difference was found between cementation mediums.

The restoration of endodontically treated teeth. Part 2. Posts

Posts are normally used to aid core retention and to provide tooth support, although a significant minority question the use of posts in the latter role. A direct relationship is thought to exist between the quantity of sound tooth structure and tooth strength and care is required when preparing teeth to accept posts. Post configuration is of over-riding importance in determining retention and support and the effect on retention of small increases to post length should not be over emphasized. Because accurate post fit is important to maximize both retention and support, direct fabrication techniques using matched post/reamer systems are recommended.

A comparison of two post systems under applied compressive-shear loads

This study compared a new parallel-tapering, threaded, split-shank post with a well-accepted parallel serrated post under applied compressive-shear loads. The posts were placed in paired, contralateral human teeth to attempt to minimize variation in the tooth model. Both initial and ultimate failure modes were observed, and the clinical significance reported. The Flexi-post system compared favorably with the Para-Post system under the conditions of this research.

Hemisection and root amputation

The disciplines of endodontics and periodontics fuse when molars that have bifurcation or trifurcation involvements are treated with hemisection and root amputation. Attempts to save parts of teeth go back 100 years or more, but it is the increased predictability of success of endodontic therapy and the increased sophistication of periodontal treatment that have given us the means to save molars with furcation problems that, otherwise, would be lost. Even when less invasive modes of therapy have failed (scaling, root planing, occlusal adjustment, and flap surgery perhaps with osseous recontouring and synthetic or natural bone grafting material where indicated), it is no longer necessary to lose a molar with complete furcation problems. When restorative dentistry has already been finished, and the retention of part of the tooth will extend the life of a crown or fixed partial denture, the patient certainly deserves the option of hemisection or root amputation rather than extraction.