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ENDODONTICS: Comparison of the Photoelastic Stress Properties for Different Post-Core Combinations

October 1, 2001
by Brett I. Cohen, Ph.D, Yekaterina Volovich, BS, Barry Lee Musikan


The main purpose of a post and core restoration is to provide a substructure to which the final restoration can be attached and to anchor to the root. If the post and / or core material fails, the artifical crown will also fail. Therefore, retention of the post and the stability of the core are important factors in preventing restorative failures. There are many factors that influence retentive properties of a post/core constructions; type of a post used,1-15 accuracity of post-hole preparation,1-15 cementation,16-25 type of core material used.26-30

The use of photoelastic stress analysis has been shown to illustrate the stress distribution for various prefabricated and cast post systems.13,31-39 Numerous investigators have shown a relative qualitative ranking between stress distribution of both threaded and passive prefabricated post systems and their relationship with retention.22,31-33,37,39-40 This study compared the stress distribution under function using photoelastic blocks for oblique loading for the two different core materials (Ti-Core [Essential Dental Systems, S. Hackensack, NJ] and Miracle Mix [GC Corporation, Tokyo, Japan]) supported by two different endodontic post systems (Flexi-Post a split-shank prefabricated post design [Essential Dental Systems] and Para-Post a passive prefabricated post [Coltene/Whaledent, Mahwah, NJ]).

The Flexi-Post is a split-shank threaded, multiple-tiered, post that actively engages the dentin and has been shown to have excellent retention with minimal stress and resistance to in vitro fatigue and torsional forces.19-20,37,41-45 The Para-Post is a single-tiered stainless steel prefabricated parallel passive post that relies on cement to retain the post in the root canal. It is important to note, that retention of the Flexi-Post is far greater (4 to 5 times greater) than other post systems, which are passive (average retention of 60 pounds) which rely only on cement for retention.13-15,18-19,22,43-44,46 Photoelastic studies have shown the stress distribution for Para-Post was different for vertical and oblique loadings compared to Flexi-Post.36,39 The stress distribution for Para-Post displayed asymmetric stress patterns with greater stress fringes apically compared to coronally,36,39 that will result in uneven stress distribution and can cause premature failure of the post. For the Flexi-Post, even symmetrical stress distribution is obtained when loaded in vertical or an oblique angle due to the multi-tiered post design.36,39-40

Ti-Core is a hybrid auto polymerizing composite resin (Bis-GMA based) reinforced with titanium. The addition of titanium was to improve the compressive and diametral tensile strength of the composite resin to make it comparable to dentin.12,26-27 This reinforced composite system also includes a peroxide and an amine chemical initiation system. Miracle Mix is classified as glass ionomer cement restorative with amalgam alloy (silver) particles added which do not chemical interact with the ionomer matrix (polymaleic acid based).12,30,47 The system is a powder/liquid where an acid-base curing reaction occurs between the ionomer liquid(polymaleic acid) and the powder (salts of calcium and aluminum).

Numerous studies are found in the literature where loads of 20 to 30 pounds in a variety of angles and loading conditions were used during photoelastic testing in order to simulate the oral environment invitro.36-40 A survey of the literature has shown that oblique loading at an angle of 26 can be used to approximate the oral condition.36-40

The purpose of this study was to compare two different core materials (Ti-Core and Miracle Mix) supported by two different endodontic post systems (Flexi-Post and Para-Post) for photoelastic stress distribution in an oblique loading condition with a 26 angle at 20 pounds.

MATERIALS AND METHODS

Photoelastic stress characteristics were evaluated under function for oblique loading at a 26 degree angle to the long axis for four endodontic post-core systems. This in vitro experiment was divided into four groups: group 1, Flexi-Post No. 1 (Essential Dental Systems, S. Hackensack, NJ)/ Ti-Core (Essential Dental Systems, Lot No. 120699), group 2, Flexi-Post No. 1 / GC Miracle Mix (GC Corporation, Tokyo, Japan, Lot No.Liquid 011081, Lot No. Powder 220481), group 3, Para-Post No. 1 equivalent (Coltene / Whaledent, Mahwah, NJ) / Ti-Core and group 4, Para-Post No. 1 equivalent/GC Miracle Mix. The following stainless steel (Flexi-Post and Para-Post) posts were used: Flexi-Post No.1 (post diameter of 1.40 mm with post length in the root canal of 10 mm) a split-shank prefabricated post design and Para-Post (post diameter of 1.27 mm with Para-Post cut apically 5 mm to give a post length in the root canal of 10 mm) a passive prefabricated post.

Photoelastic stress analysis is concerned with qualitative visual observations based on the ability of transparent plastic materials to exhibit interference fringes when stressed in polarized light.36-40 The fringes observed in the transparent plastics represent zones of stress intensity or concentration and can be identified by a sequence of color bands. The colors of the bands can range from black (no stress) to yellow, red, blue, etc. Stress fringes with the colors yellow, red, and blue represent maximum stress patterns.36-40 These color fringes allow the observer to qualitatively evaluate a post-core system by interpreting the color stress patterns obtained.

The photoelastic blocks (Block No. PSM-5, Measurements Group, Raleigh, NC) used had a modulus of elasticity approaching that of human dentin.36-40 A typical block sample measured 38.0mm x 38.0mm x 6.5mm. These blocks were not standard size and were sized to fit the appropriate test jig apparatus. Square polarizer sheets (Sheet No. 90770, Measurements Group) were then placed on each block. A simulated endodontic stress-free channel were prepared with endodontic instrument according to manufacturer’s instructions for Flexi-Post No. 1 and Para-Post. A corresponding reamer was used for each post system to create a post-hole space. The preparation for Flexi-Post post-holes also involved a secondary drill to create a spacing for the second tier for the post.

Posts were trial seated into their post-hole preparations and luted in their corresponding post-hole using zinc phosphate cement (Fleck’s cement Mizzy, Cherry Hill, NJ, Lot. No. Liquid JI5 103091, Lot. No. Powder B93 050288). Zinc phosphate cement was used (as the post cement for all groups) in order to eliminate the cementation variable. After the cement was allowed to set for at least 1 hour, core preparations were performed with Ti-Core and Miracle Mix.

The fabrication of the core was accomplished with the use of a copper band matrix (No. 1 hard). Core materials were mixed and placed into the supporting matrices according to manufacturer’s instructions. The core specimens were trimmed and cut to a standard height of 6 mm with the use of a dental high speed instrument (FG-557 Bur, SS White Burs, Inc., Lakewood, NJ). After core preparation the specimens were allowed to set for 24 hours before being photographed in the unloaded and loaded state.

Specimens (photoelastic block/ cemented post/core combinations) were then positioned in a special jig on a universal testing machine (Model CETM922MTT20/1, Comten Industries, St Petersburg, FL) with oblique loading at an angle of 26 relative to the long axis of the post. In Figure 1 a schematic drawing of the special jig used for this experiment with a test sample (Flexi-Post/Ti-Core) in the unloaded oblique state is shown. Test loads of 20 pounds were applied to each test sample and recorded directly from a digital display on the universal testing machine used. A photograph of each specimen was recorded in the unloaded state and then under a 20 pound load at a 26 angle relative to the long axis of the post (Figs. 2 & 3). It should be noted that a 20 pound load was chosen for both testing conditions due to the inherent weakness of the GC Miracle Mix glass ionomer core material. Earlier studies involved post (cast and prefabricated) photoelastic stress analysis utilizing a 30 pound load38-40 but in this in vitro study a 30 pound load applied to the glass ionomer (GC Miracle Mix) core material resulted in core failure and the photoelastic results were not consistent. As a result, a 20 pound load was used for all groups in this study.

RESULTS

A 20 pound load was applied to each testing condition (Figs. 2 & 3). It should be noted, that a 30 pound load applied to the GC Miracle Mixglass ionomer cores resulted in core failure with inconsistent photoelastic results. In a previous study by Cohen et al.,12 a load of 37.7 (18.2) pounds to a Para-Post/silver reinforced glass ionomer core also resulted in failure. Numerous studies are found in the literature where loads of 20 to 30 pounds in a variety of angles and loading conditions were used during photoelastic testing in order to simulate the oral environment invitro.36-40 As a result, a 20 pound load was chosen as the test loading condition. Despite this loading condition, after loading of 20 pounds for the GC Miracle Mix core groups failure of the core was observed for groups 2 and 4. Photoelastic data was possible to obtain at this loading condition (20 pounds), however the GC Miracle Mix cores for groups 2 and 4 were damaged in the coronal portion. No core damage was observed for the Ti-Core groups (groups 1 and 3) when loaded at 20 or 30 pounds.

Figures 2 and 3 illustrate the stresses that are present after cementation (unloaded) and loading obliquely for Flexi-Post and Para-Postcore systems. Clearly minimal to no stresses were observed for each system in the unloaded cemented state. Additional stresses occurred during loading obliquely at 20 pounds.

The stress distribution for Flexi-Post/Ti-Core and Flexi-Post/ GC Miracle Mix systems were evenly distributed coronally for the length of post (symmetrically distributed) with limited apical stress patterns. The distribution for the Flexi-Post systems were consistent throughout the coronal length of the shaft. The stress concentrations are observed to be radiating from the coronal part of the shaft of the Flexi-Post (Fig. 2).

However, the stress distribution for Para-Post/Miracle Mix and Para-Post/Ti-Core groups both displayed asymmetric stress patterns with greater stress fringes apically compared to Flexi-Post (Fig. 3).

Asymmetric stress fringes from the Para-Post single tiered system were observed apically compared to the symmetrically distributed stresses observed for the Flexi-Post/Ti-Core and Flexi-Post/ GC Miracle Mix groups.

Visual observations for figures 2 and 3 reveals that loading at a 26 angle with GC Miracle Mix as the core material for both Flexi-Post and Para-Post resulted in more photoelastic stresses than for the core material Ti-Core. In Figure 2 clearly more stress coronally is generated for the Flexi-Post when loading with GC Miracle Mix as compared to Ti-Core. In Figure 3, again more stress is generated when the Para-Post was loaded with GC Miracle Mix as compared to Ti-Core.

DISCUSSION

Minimal stresses were observed for both the Flexi-Post/core and Para-Post/core systems in the cemented unloaded state (see Figs. 2 and 3). Minimal stresses were observed in the loaded state coronally for the Flexi-Post shaft. The stress distribution for the Para-Post concentrated stresses apically as compared to Flexi-Post. These stresses observed for Para-Post seems to be more asymmetric than for the Flexi-Post/ core groups. A possible explanation for this is the effect of the single tiered Para-Post system. Similar stress concentration patterns were observed for the single tiered (Para-Post) prefabricated cast post systems as well as the Para-Post itself.38 Greater ‘bottoming out’ stresses are possible for the Para-Post because of a lack of the second tier. Burns et al. reported the stress distribution for the Para-Post under a load of 30 pounds, and stated that ‘the Para-Post has a tendency towards a concentration of stress at the apex.’36 This apical concentration was confirmed in a study by Cohen, et al. with the Para-Post burnout post system.38 In a similar study, Cohen, et al. reported stress distribution patterns for the prefabricated endodontic Para-Post system, where extreme apical asymmetric stresses were also observed (loaded vertically and obliquely).39

Stress distribution for the Flexi-Post system concentrated stresses more evenly and coronally as compared to apically in the loaded state. These stresses observed seem to be symmetric and were different from the stress patterns observed for the Para-Post/core systems. Apical stress patterns were infrequent under loading for the Flexi-Post/core systems, and this is attributed to the split-shank and the coronal secondary tier design (The Flexi-Post is a multi-tier design with a primary shaft and second tier). Similar stress concentration patterns were observed for the Flexi-Post where only the post was loaded in a photoelastic block.39-40 Cohen et al., observed similar stress concentration patterns for the tripled tier prefabricated endodontic post system (Flexi-Flange).39 They have reported that the stress distribution patterns for the Flexi-Flange prefabricated endodontic post system were coronal symmetric and evenly distributed.39

It should be noted, that Ti-Core (a titanium reinforced composite) and GC Miracle Mix are very popular core restorative materials and numerous studies are found in the literature concerning their physical properties. Cohen et al. measured the compressive and diametral tensile strength for both Ti-Core and GC Miracle Mix (compressive, diametral tensile strength)Ti-Core 41,131 3662 psi, 5,219 529 psi; Miracle Mix 14,197 1150 psi,1,408 450 psi.26 Ti-Core had statistically greater compressive and diametral tensile strength then the glass ionomer GC Miracle Mix.26 Ti-Core’s strength found in the study by Cohen et al. was approaching the compressive and diametral tensile strength of dentin.26

The Young’s modulus (measures the rigidity of a material) was also determined for Ti-Core and GC Miracle Mix. Values range was from 9.40 Gpa for GC Miracle Mix to 18.53 GPa for Ti-Core.30 Cohen et al. examined the cyclic fatigue characteristics of Ti-Core and GC Miracle Mix supported by five endodontic posts (Flexi-Post and Para-Post were included in that in vitro study).48 All posts/ core samples with Ti-Core completed the 4,000,000 test cycle configuration without core failures, whereas all posts/core samples with GC Miracle Mix failed before the 4,000,000 test cycle configuration (average Flexi-Post 321,000 cycles, average Para-Post 113,000 all with core failure).48 All posts/core sample failures with GC Miracle Mix resulted in core failure. Therefore, since the glass ionomer GC Miracle Mix lacks the strength of Ti-Core and is inherent weak, failures of the GC Miracle Mix cores in this in vitro photoelastic study is consistent with the literature.

This study enabled a relative qualitative ranking between loads and stresses that are observed for different post/core systems. The most stable post/core system would be the system that distributes stresses symmetrically and evenly. This is because a specific amount of stress distributed over a greater surface area results in less stress at one particular site. A post/core system that distributes stress symmetrically can apply stress more evenly at a specific site on the post shaft. Asymmetric stress patterns will result in uneven stress distribution and can cause premature failure of the post/core. This is because one portion of the post/ core may have an uneven stress concentration. Therefore, the Flexi-Post a multi-tiered split shank post offers a more stable system than the single-tiered Para-Post system. Posts loaded with Ti-Core a titanium reinforced composite were more stable than posts loaded with GC Miracle Mix a silver reinforced glass ionomer.

CONCLUSION

Two different core materials (Ti-Core and GC Miracle Mix) supported by two different endodontic post systems (Flexi-Post and Para-Post) were tested for photoelastic stress distribution under function and the following conclusions were drawn:

1) Minimal stresses were observed for the Flexi-Post/core systems in the unloaded state, while symmetric, even patterns of stresses concentrated coronally were present in the loaded oblique state.

2) Minimal stresses were observed for the Para-Post/core systems in the unloaded state, while asymmetric, uneven patterns of stresses concentrated conorally and apically were present in the loaded oblique state.

3) The presence of the multiple tiered design for the Flexi-Post clearly directs stresses in a symmetric pattern, while the single tiered Para-Post system directs stresses in an asymmetrical pattern.

4) Posts loaded with Ti-Core a titanium-reinforced composite were more stable than posts loaded with GC Miracle Mix a silver reinforced glass ionomer.

5) The symmetric, even stresses for the Flexi-Post/core systems are more favorable than the asymmetric, uneven stresses for the Para-Post/core systems.OH

Brett I. Cohen, Ph. D, is Vice-President of Dental Research; Yekaterina Volovich, BS, is a Research Chemist; and Barry Lee Musikant, DMD and Allan S. Deutsch, DMD are Co-Directors of Dental Research for Essential Dental Laboratories.

Oral Health welcomes this original article.

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