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PRODUCT PROFILE: Retentive Properties of a Split-Shank Threaded Post with a Core Using Three Different Multi-Step Bonding Agents

October 1, 2000
by Brett I. Cohen, Ph.D, Yekaterina Volovich, BS, Barry Lee Musikan


ABSTRACT: The use of dentinal bonding agents has achieved wide acceptance in dentistry. However, there are limited studies on their retention of a post with a core bonded to tooth structure using various dentinal bonding agents. This study determined the retentive values of a Flexi-Flange post with a Ti-Core Natural core using various dental multi-step bonding agents (Tenure, All-Bond 2 and 3m Scotchbond) compared to a control where no bonding agent was used. The experiment was divided into 3 groups and a control with 10 specimens per group. The retentive strengths were evaluated after 24 hours on a 810 MTS universal testing machine with a crosshead speed of 0.638 cm/min. A one-way ANOVA illustrated that none of the groups differed statistically from each other and from the control (P< 0.2448). The values for the retention of the Flexi-Flange/ Flexi-Flow/Ti-Core using various dentinal bonding agents ranged from a low of 264.5 pounds (Tenure) to a high of 297.9 pounds (3M Scotchbond) with a control (without bonding agent) of 270.4 pounds. All of the retention for the Flexi-Flange post resulted from the retention of the split-shank threaded portion (mechanical retention) of the post which mechanically engages the root canal. The phenomenon where mechanical retention is greater and more significant than chemical retention (from dentinal bonding agents) has been described previously in the literature.

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. Many articles in the literature have described techniques and methods for the construction of post and cores.1-15

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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 and also the use of bonding agents, which increase survivability of restoration.31-34 In this study we analyzed the retentive properties of a split-shank threaded post with a core using 3 different multi-step bonding agents.

The Flexi-Flange post (Essential Dental Systems, South Hackensack, NJ) is a threaded, multiple-tiered, split-shank post that mechanically engages the dentin and has been shown to have great retention with minimal insertional and functional stresses and maximun resistance to in vitro fatigue and torsional forces.14,35-38 The Flexi-Flange design has been reported to produce minimal functional stresses when a 30-pound load was applied.35 The core material used in this study was Ti-Core Natural. Ti-Core Natural (Essential Dental Systems, Inc.) is a hybrid autopolymerizing composite resin (Bis-GMA based) reinforced with lanthanide. The addition of lanthanide was to improve the compressive and diametrial tensile strength of the composite resin to make it comparable to dentin.26,27 Flexi-Flow Natural was also used as the post cement and it too is a hybrid autopolymerizing composite resin (Bis-GMA based) reinforced with lanthanide. The addition of Lanthanide improved the compressive and diametrial tensile strength for the post cement Flexi-Flow Natural making it comparable to dentin.26,27 Both Ti-Core Natural and Flexi-Flow Natural are shaded A3 for natural color.

In this study “multi-step” bonding agents were used. Scotchbond 2 dentin bonding agent (3M Dental Products Division, St. Paul, MN.) consists of an aqueous solution of maleic acid and HydroxyEthyl Methacrylate (HEMA). This solution is applied to alter the smear layer and encourages chemical adhesion to the subsequent composite resin adhesive. After treatment of the smear layer, Scotchbond 2 light-cured dental adhesive (containing HEMA and Bis-GMA monomers) is then applied to the dentinal surface.39 Next, Tenure multi-step (Den-Mat Corp., Santa Maria, CA.) contains a dentinal conditioner aluminum oxalate, an “A” acetone solution of the adduct of N (p-tolyl-glycine) and glycidyl methacrylate (NTG-GMA) and a “B” acetone solution of the adduct of pyromellitic acid dianhydride and 2-hydroxyethyl methacrylate (PMDM). The last application for the dental adhesive consists of Visar Seal (Den-Mat Corp.) light cured unfilled composite resin.40 All-Bond 2 (Bisco, Inc. Itasca, IL) bonding agent consists of a phosphoric acid (10%) conditioner with benzalkonium chloride with an “A” solution of the adduct of N (p-tolyl-glycine) and glycidyl methacrylate (NTG-GMA) (similar to Tenure bonding agent) and “B” consisting of a proprietry monomer (BPDM) also with a light cured Bis-GMA and HEMA (unfilled) adhesive.41-42

The purpose of this study was to determine the retentive value of a Flexi-Flange post with a Ti-Core Natural core using various dental multi-step bonding agents compared to a control where no bonding agent was used.

MATERIALS AND METHODS

This study consisted of the preparation of 30 tooth/bonding agent/ post/core specimens. The specimens were divided into 3 groups (according to bonding agent used) with 10 specimens per group. In all specimens a core material was used Ti-Core Natural Lot No. 022100 (Essential Dental Systems, Inc., So. Hackensack, N.J.) and a stainless steel post , Flexi-Flange No. 2 (Essential Dental Systems). The outer diameter of a No. 2 Flexi-Flange with threads is 1.65 mm and the full length of the post in the root canal is 10.5 mm. Table 1 lists the tooth/bonding agent/post/ core combinations for each group with bonding agent Lot Nos. and manufacturer.

A total of 30 recently extracted single rooted human teeth (premolars and incisors) were used for this study. All teeth had their crowns removed at the cementoenamel junction (CEJ). The preparation for Flexi-Flange post holes were performed according to manufacturer’s instructions with the recommended No. 2 reamer, used to create the post space (and a No. 2 countersink/rootfacer drill was used to provide spacing for a second tier and flange).

Before post cementation, preliminary tooth preparation was accomplished using a bonding agent; Group 1, Tenure Lot No. 03114600071 (multi-step) (Den Mat Corp. Santa Maria , Calif.); Group 2, All-Bond 2 Lot No. 069187 (Bisco, Inc. Itasca, IL) and Group 3, 3m Scotchbond Lot No. 19970207 ( 3M Dental Products Division, St.Paul, Minn). All bonding agents were used according to manufacturer’s instructions which included the use of a dentin conditioner, primer and a dental adhesive. The dental adhesive for each bonding agent was light cured for 60 seconds with an Optilux 400 curing light (Demetron Research Corporation, Danbury, Ct).

All posts were cemented with lanthanide reinforced Bis-GMA composite cement Flexi-Flow Natural Lot No. 090999 (Essential Dental Systems) which was mixed in a 1:1 ratio (catalyst/base) according to manufacturer’s recomendations. After the cement was allowed to set for 30 minutes, core preparations was performed with Ti-Core Natural. Ti-Core Natural is also a lanthanide reinforced Bis-GMA composite, but it is thicker than Flexi-Flow Natural because it is a core build-up material. The fabrication of a core was accomplished with the use of a copper band as a matrix. The core specimens were trimmed and cut to a standard height of 3mm with the use of a diamond instrument (Two Striper 703-8C, Premier Dental Company, Norristown, PA) (Fig. 1). The core samples were allowed to set for 1 hour and then they were immersed in 100% humidity for 24 hours. The final tooth/bonding agent/post/core combinations were mounted in acrylic resin blocks (Formatray, Kerr Manufacturing Co., Romulus, MI) with 6 mm of the tooth and core material remaining above the acrylic blocks (Fig.1).

The test specimens were placed in a specialized jig and tensile strength tests were performed using a 810 MTS testing machine (MTS Systems Corp., Eden Prarie, MN) with a crosshead speed of 0.638 cm/min until failure (Fig. 2). Failure occurred when the post was removed from the to
oth or with post head breakage.

STATISTICAL METHODS

One way analysis of variance (ANOVA) was used to compare the retention strengths of the three bonding agents and control. Results were considered statistically significant if P < 0.05. Summary statistics are given in Table 2.

RESULTS

Results for this study are summarized in Table 1. The values for the retention of the Flexi-Flange/ Flexi-Flow/Ti-Core using various dentinal bonding agents ranged from 264.5 pounds (Tenure (multi-step)) to 297.9 pounds (3M Scotchbond). One-way ANOVA analysis indicated that none of the groups differed statistically from each other (P < 0.2448). The mean bonding strengths in decreasing order corresponded to: 3M Scotchbond, All Bond 2, Control (without bonding agent), and Tenure (Multi-Step) (Table1). In group 1, Tenure (multi-step) 7 posts were removed from their root canals with 3 Flexi-Flange post head failures. For group 2, All-Bond 2 there were 8 posts removed from the root canals with 2 Flexi-Flange post head failures and in group 3, 3m Scotchbond, 2 posts were removed from the root canals with 8 Flexi-Flange post head failures.

DISCUSSION

The retention of the Flexi-Flange post with Flexi-Flow Natural cement with Ti-Core Natural using three different bonding agents resulted in no statistical differences compared to the control without a bonding agent (Table 1). This is probably due to the weak retentive bond for these dentinal bonding agents. For example, Cohen, et al.34 and other researchers39-42 have measured bond strengths of various dentinal bonding agents and the results vary widely with bond strengths of approximately 1000 psi (7 mpa) to 2000 psi (14 mpa). All of the retention for the Flexi-Flange post (control group with retention of 270.4 34.3 pounds) in this study resulted from the retention of the split-shank threaded portion of the post which mechanically engages the root canal. This results in a mechanical engagement of the dentin and is much stronger and more significant than the chemical bond found between dentin tooth structure and the composite, Ti-Core. This phenomenon where mechanical retention (resulting from dental post use) is greater and more significant than chemical retention (resulting from dentinal bonding agents) has been described previously in the literature.23,25,31-32,34,40-41,43

It is important to note that the retention of the Flexi-Flange split-shank threaded post is far greater (4 to 5 times greater) than other post systems which are passive (average retention of 60 pounds) and rely only on cement for retention.13-15,18-19,22,36-38,43 In this study a lanthanide composite resin Flexi-Flow Natural was used to cement each Flexi-Flange post. The compressive and tensile strength of Flexi-Flow Natural have been measured and shown to be similar to dentin.26,30 The type of cement used will effect the retention for the post and the stronger the cement cohesively the more retentive the post/cement interface.16-25 For example, Cohen et al.19 illustrated that Flexi-Flow cement was statistically stronger in retention using the Flexi-Post (Essential Dental Systems, Inc.) than using zinc phosphate cement.

It must be noted that the retention for the 3M Scotchbond group resulted in more Flexi-Flange post head failures (8 out of the sample size of 10) than for Tenure (3 out of the sample size of 10) or All-Bond 2 (2 out of the sample size of 10). Once the Flexi-Flange heads were deformed by the tensile forces they could not be tested again. It could be concluded that the tensile failure results for 3M Scotchbond were probably lower than they would have been if the Flexi-Flange post were removed from their root canals as was done in most of the specimens for the Tenure and All-Bond groups.

CONCLUSION

Within the limits of this in vitro study for the retentive values of a Flexi-Flange post with a Ti-Core Natural core using various dental multi-step bonding agents (Tenure, All-Bond 2 and 3m Scotchbond) compared to a control where no bonding agent was used, the following conclusions were made.

1. The values for the retention of the Flexi-Flange post with Ti-Core using various dentinal bonding agents ranged from a low of 264.5 pounds (Tenure) to a high of 297.9 pounds (3M Scotchbond) with a control (without bonding agent) of 270.4 pounds.

2. A one-way ANOVA illustrated that none of the groups differed statistically from each other and from the control.

3. All of the retention for the Flexi-Flange post resulted from the retention of the split-shank threaded portion (mechanical retention) of the post which mechanically engages the root canal. The phenomenon where mechanical retention is greater and more significant than chemical retention from dentinal bonding agents has been described previously in the literature.

Brett I. Cohen, PhD, is vice-president of Dental Research, Essential Dental Laboratories.

Yekaterina Volovich, BS, is Research Chemist, Essential Dental Laboratories.

Barry Lee Musikant, DMD, and Allan S. Deutsch, DMD are co-directors of Dental Research, Essential Dental Laboratories.

Oral Health welcomes this original article.

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Table 1: Materials and Results

Group Bonding agent/ Retention No. Manufacturer Lot No. (pounds) SD
1 Tenure(multi-step)Den Mat Corp. 03114600071 264.5 52.7
2 All-Bond 2/Bisco, Inc. 069187 291.4 42.1
3 3m Scotchbond/3M Dental Division 19970207 297.9 37.9
Control No Bonding Agent 270.4 34.343
Ti-Core Natural/Essential Dental Systems 022100
Flexi-Flow Natural/Essential Dental Systems 090999

Table 2: Summary of Statistics

Dependent Variable: RETENTION Sum of Mean Source DF Squares Square F Value Pr>F
Model 3 7779.5548 2593.1849 1.45 0.2248
Error 36 64439.1190 1789.9755
Corrected Total 39 72218.6738
R-Square C.V. Root MSE Retention Mean
0.107722 15.05426 42.308 281.04
Source DF Type I SS Mean Square F Value Pr>F
3 7779.5548 2593.1849 1.45 0.2448
Source DF Type III SS Mean Square F Value Pr>F
3 7779.5548 2593.1849 1.45 0.2448