|Year : 2011 | Volume
| Issue : 6 | Page : 879
|Rigidity, retention, and resistance of titanium versus double taper light root canal posts: An in vitro evaluation
Ramneek Khatter1, Ripu Daman Singh1, Renu Sroa2
1 Department of Conservative Dentistry and Endodontics, Sri Guru Ram Das Institute of Dental Sciences and Research, Amritsar, Punjab, India
2 Department of Conservative Dentistry and Endodontics, Punjab Government Dental College and Hospital, Amritsar, Punjab, India
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|Date of Submission||10-Oct-2010|
|Date of Decision||28-Apr-2011|
|Date of Acceptance||08-Aug-2011|
|Date of Web Publication||5-Apr-2012|
| Abstract|| |
Aim: to evaluate and compare the physical properties of titanium posts and double taper (DT) light posts.
Materials and Methods: Sixty posts (30 titanium post and 30 DT light post) were selected and divided into three groups. In Group I: Ten posts of each type were subjected to a three-point bending test. In Group II: 20 posts extracted maxillary central incisors were restored with ten posts of each type and subjected to tensile loading. In Group III: 20 posts extracted maxillary central incisors were restored with ten posts of each type and subjected to compressive loading at an angle of 135°.
Results: The results showed that DT light posts were significantly less rigid (P <0.001), more retentive and significantly less resistant to fracture (P <0.001) as compared to titanium posts.
Conclusion: It is suggested that although the DT light post system does not completely fulfill the requirements claimed by manufacturer but it would meet the requirements to combat the physiological forces operating in the oral cavity.
Keywords: DT light post, resistance, retention, rigidity, titanium post
|How to cite this article:|
Khatter R, Singh RD, Sroa R. Rigidity, retention, and resistance of titanium versus double taper light root canal posts: An in vitro evaluation. Indian J Dent Res 2011;22:879
The crux of endodontically treated teeth revolves around its restoration and reinforcement, since failure to do this may invite "embarrassing mishaps." Endodontically treated teeth have a higher risk of biomechanical failure than teeth with vital pulps. , If endodontic treatment is inevitable and there is insufficient dentin to support a crown restoration, a post is essential to retain the core. However, the placement of a post and core do not increase the fracture resistance of endodontically treated teeth.  Several investigators reported that posts do not strengthen teeth but actually weaken them. , Ideally, a post should minimize the stress on a tooth by distributing occlusal loads evenly.  In addition, the elastic modulus of post should be similar to dentin to avoid root fracture. 
|How to cite this URL:|
Khatter R, Singh RD, Sroa R. Rigidity, retention, and resistance of titanium versus double taper light root canal posts: An in vitro evaluation. Indian J Dent Res [serial online] 2011 [cited 2019 Jul 20];22:879. Available from: http://www.ijdr.in/text.asp?2011/22/6/879/94694
Common post designs include the tapered, parallel, and threaded designs. Tapered posts follow the prepared canal shape and conserves the tooth structure; however, they are less retentive and have shown to cause non-restorable fractures.  Parallel-sided posts provide superior retention; however, more dentin from apex of root canal must be removed. This may result in higher incidence of perforation failure and generates greatest stress at apex of canal preparation.  Threaded posts are most retentive but the incidence of vertical fracture is increased with the use of these posts. In addition to post design, dowel retention is also influenced by length and diameter of the post.  However, all these prefabricated metal posts result in final heterogeneous combination of dentin, metallic post, cement, and core material. The major disadvantage of these post systems is the stress concentration in uncontrolled areas that are sometimes vital to root, thereby leading to fracture. , The major objective in dentistry is restoration of endodontically treated teeth with metal free physiochemically homogeneous material that has physical properties similar to those of dentin.
In 1990, a non-metallic material, based on carbon fiber reinforcement principle was introduced for the fabrication of post.  The addition of fibers to the polymer for the prefabricated posts offer number of advantages: The biocompatibility, resistance to corrosion, mechanical properties that closely match those of tooth, and easy removal of post from the root canal.  Moreover, the core resin and luting cement resin are BIS-GMA-based materials and, hence, form two homogeneous combinations.  Thus, this post system has been advocated to reduce the risk of root fractures from wedging effect of metallic posts. , Although carbon fiber posts offer significant advantages as compared to metal posts, the carbon fiber post system shows limitations of material's radiolucency and masks difficulties because of its grey color.
The double taper quartz fiber post (Double taper (DT) light post) was introduced with better properties such as radiopacity, translucency, better light reflection, and light transmission. This post system has double taper, so that, they confirm closely to shape of endodontically treated canals, hence, renders the restorative work more resistant to fracture; decreases the thickness of cement at coronal level; reduces polymerization shrinkage; and reduces the microleakage.  These advantages combined with other positive findings for the nonmetal post system have lead to increased patient acceptance of this type of restoration. 
Numerous benefits were claimed by this post system. Therefore, the aim of this study was to compare rigidity, retention, and resistance of titanium posts (metallic) with DT light posts (quartz fiber).
| Materials and Methods|| |
Two post systems were selected, namely DT light post (Recherches Techniques Dentaires, RTD, France) and titanium post (Unimetric, Dentsply, Maillefer, Ballaigurs, Switzerland). Post systems with 12 mm length and tip diameter of 1 mm were chosen. Sixty posts (30 Titanium and 30 DT Light Post) were selected and three groups, that is, Group I, Group II, and Group III, consisting of ten posts of each type, were formed.
Group I: Test for rigidity
Ten titanium posts and ten DT light posts were tested for rigidity in a three-point bending test (3 PBT) [Figure 1]. The posts were subjected to three PBT in the Instron Universal testing machine (Lloyds, England) at a crosshead speed of 5 mm/min and load applied was continued until elastic limit of posts was reached.
Stress versus deflection was plotted for each post and elastic modulus (E) was derived with the following formula.
E = PL 3 /48Id
Where P = load applied in Newton
L = length of span in mm.
d = deflection in mm.
I = moment of inertia.
The moment of inertia was calculated with following formula.
I = d 4 /64, d = diameter of post in mm.
For Group II and Group III, 40 freshly extracted maxillary central incisors were collected and restored with 20 titanium posts and 20 DT light posts. Specimens were free from cracks, caries, fractures, and required extraction, due to periodontal involvement from various age groups of both the sexes. The crowns of selected teeth were severed away 2 mm coronal to the most apical extension of cervical line on labial aspect of tooth and straight access to pulp was made. All canals were sequentially prepared within 1 mm of the end of the canal enlarging it to size 50 using Hedstrom files (Dentsply, Maillefer, USA). The prepared canals were obturated using AH 26 sealer (Detrey, Konstanz, Germany Dentsply) and gutta-percha points. Teeth were stored at room temperature for 48 h for the complete setting of sealer.
Post holes were prepared to a depth of 8 mm with appropriate post hole drills (supplied by the manufacturer) to create the form that matched the structure of the post. Canals were etched with 35% phosphoric acid (Scotchbond Etching Gel, 3M Dental Products Division, St. Paul, MN, USA) for 15 s, rinsed with distilled water for 10 s, were blot dried without rubbing to keep moist. The prime and bond dual cure adhesive system (Dentsply International, USA) was applied to the post space and the post using microbrushes and was cured according to manufacturer's instructions. A mix of Calibra (Dentsply International, USA) dual cure resin cement was made on mixing pad according to manufacturer's instructions. Mixed cement was spread on post space with lentulospiral (Mani, Japan) and the post with brush. The post was immediately seated with hand wrench and pressure was applied until the post was stable. Cement was light-cured for 30 s (Max; Dentsply-Caulk, Milford, DE, USA). Radiographs were made to ensure conformity of the dowel to dentinal walls. Roots of all the teeth were coated with an air thinned layer of polyvinylsiloxane (President light body, Coltene-Whaledent AG, Switzerland) to simulate periodontal ligament and embedded in methyl methacrylate acrylic resin supported by metal cylinders to ensure pulling along the long axis of the tooth. After resin polymerizes, specimens were divided into two groups, consisting of 20 specimens each and restored with ten titanium posts and ten DT Light Posts.
Group II: Test for retention (tensile strength)
Specimens were placed in a fixture that grasped the base of these metal cylinders. The 4 mm portion of post extending from the post preparations was grasped by Jacob's chuck attached to an Instron Universal testing machine. The retention of each post was determined by tensile loading of post at cross head speed of 5 mm/min. Tensile force required to initially break the cement bond and partially or totally remove the post was recorded [Figure 2]. Determination of the displacement was recorded at certain release of load.
Group III: Test for resistance (compressive strength)
Specimens in this group were restored with composite resin (Z-350, 3M Dental, USA) cores, using the individual custom moulds from matrix band (Hahnenkratt, Germany). The core of about 4 mm in height was made with the post in the center of core. All specimens underwent thermocycling at 10 000 thermal cycles between 5°C and 55°C, with 30-s dwell time and a 5-s transfer between temperature baths. Using the flame-shaped diamond point in the hand piece, core build up was removed at 45° to the lingual aspect, so that breaking probe could be placed directly on the edge of the post. The specimens were mounted in test jig aligned at an angle of 45° so that the ball probe attached to Instron Universal testing machine could apply pressure at an angle of 135° with the long axis of teeth [Figure 3]. This angle was selected on the basis to simulate the average of contact between maxillary and mandibular incisors. The pressure was applied as close to the end of post as possible, at across head speed of 5 mm/min, until the sample reached ultimate failure.
|Figure 3: Device to secure specimen and metal plunger of the Instron tester supplying compressive load|
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The readings obtained from all the three tests were tabulated and put to statistical analysis using Student's t test.
| Results|| |
The modulus of elasticity, retention, and fracture resistance of titanium versus DT light posts were compared. The statistical comparison between titanium posts and DT light posts in all the groups were calculated [Table 1], [Table 2] and [Table 3].
|Table 1: Statistical evaluation of the comparison of the mean modulus of elasticity (GPa) for titanium posts and DT light posts|
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|Table 2: Statistical evaluation of the comparison of the mean retention forces (Newton) for titanium posts and DT light posts|
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|Table 3: Statistical evaluation of the comparison of the mean fracture forces (Newton) for titanium posts and DT light posts|
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On statistical analysis using Student's t -test, the results of Group I showed that titanium posts were significantly more rigid as compared to DT light posts at 1% level of significance [Table 1]. Whereas, in Group II DT light posts were significantly more retentive than titanium posts at 1% level of significance [Table 2]. In Group III, the titanium posts were significantly more resistant to fracture than DT light posts at 1% level of significance [Table 3].
| Discussion|| |
Endodontically treated teeth with an inadequate remaining coronal tooth structure require foundation restorations to increase the retention and resistance form for definitive restoration. Posts are essential to allow the clinician to rebuild an enough tooth structure to retain restoration.  Despite the steady evolution in the range of post and core materials and techniques, failure of postretained crowns is relatively common, as highlighted in a number of clinical studies. Hence, the purpose of this study was to compare rigidity, retention, and resistance of titanium posts with double taper and light transmitting quartz fiber posts.
In this study, the titanium (metallic) post system was selected because of its good biomechanical properties, easy usage, biocompatibility, and radiopacity.  The DT light post system was chosen, with the manufacturers claiming several interesting advantages over existing products like the double taper design, Young's modulus and mechanical properties approximating that of natural dentin. Besides these posts have excellent esthetics, homogenous mechanical and chemical bonding of all components serve to reinforce the tooth. All the tests were performed in the Instron Universal testing machine. This machine is simple to use and is most commonly used for measuring mechanical properties. In Group I, results of present study revealed that the mean modulus of elasticity of titanium posts (30.97 Gpa) was significantly higher than DT light posts (6.11 Gpa). These results were in concurrence with study in which metallic posts proved to be significantly more rigid than carbon fiber posts under 3PBT.  Contrary to the claims of manufacturers, the modulus of elasticity of DT light posts was very low (6.11Gpa) as compared to titanium posts (30.97 Gpa). It was even lesser than dentin (18 Gpa). The difference in the rigidity of two posts is most likely because it is the inherent property of material. However, less stiffness has been a recently accepted measure to preserve endodontically treated tooth. It is because the more rigid component (dentin) is able to resist forces without distortion, as stress is expected to get transferred to a less rigid substrate; hence, reducing the stress in the root. 
The results of Group II demonstrated that the mean tensile strength of titanium posts (257.54 N) was significantly less as compared to DT light posts (316.63 N). This finding agrees with the study,  which revealed that mean tensile bond strength of DT light posts luted with Calibra dual cure resin was 338.19 N, which is comparable to our results. It was not in agreement with the study that demonstrated higher mean retentive values for titanium posts as compared to fiber posts. 
In our study, the higher retention of DT light posts over the titanium posts could most likely be due to the fiber post containing epoxy resin matrix that integrates with adhesives, resin cements (with similar properties) to create a singular "monoblock" assembly  and these complementary materials create an exceedingly strong bond with high tensile strength.
Moreover, in comparison to other fiber posts, the DT light posts used in our study are double taper in design, which confirms closely to the shape of the prepared canal. Therefore, this setup prevents the loss of dentin during post space preparation and is expected to increase the retention and resistance. Lang et al.  concluded that if the excessive amount of tooth structure were removed and the natural geometry of the root canals were altered, this would have a destabilizing effect on root-filled teeth.
In Group III, the results showed that the mean compressive strength of titanium posts (489.02 N) was significantly higher than that of DT light posts (251.76 N). This study was not in concurrence with other laboratory studies, , in which significantly higher failure loads were evaluated in the root-canal-treated teeth restored with quartz fiber post and core followed by the crown. The higher fracture load in these studies could be because these teeth were finally restored with custom-made laboratory crown with a circular ferrule. The ferrule design of the crowns is generally considered to be one of the most important factors to improve the load resistance of root-filled teeth. ,, da Silva NR et al.  evaluated the effect of the different post, core type and ferrule presence on the deformation and fracture resistance of endodontically treated teeth. The authors found lower strains and higher fracture resistance of the teeth restored with crowns.
This study shows that DT light post system does not completely satisfy the properties claimed by the manufacturer as it has modulus of elasticity less than dentin, good retention properties but less fracture resistance. However, in clinical situation the effect of full coverage crown as a part of definitive restoration to restore the badly mutilated teeth should be considered. Nauman et al.  found that the incorporation of the ferrule had increased the fracture resistance of titanium post and fiber-reinforced composite post to statistically similar values. Uy et al.  suggested that placement of foundation restoration on teeth with a short ferrule height improves the fatigue performance of definitive restoration.
We agree that the results in vitro, as in this study may not accurately reflect the situation in vivo. However, considering the above values and giving leeway for the difference in vivo and in vitro forces, we assume that the DT light posts evaluated in this study would meet the requirements of compressive and tensile strength required to combat physiologic forces operating in the oral cavity. These factors, combined with the less destructive nature of tooth damage, would allow the possibility of salvage and repreparation with the minimum of complex treatment.
| Conclusion|| |
Within the limitations of this study, the following conclusions were drawn.
- The rigidity of DT light posts was significantly less than titanium posts, as more rigid component (dentin) is able to resist the forces without distortion, strain is transferred to the less rigid substrate and causes its failure; hence, tooth structure is saved at the expense of the post.
- DT light posts had significantly higher retention as compared to titanium posts due to increase reliance on bonding mechanism, thus permitting more conservative tapered preparation.
Teeth restored with the DT light post system exhibited inferior strength properties in comparison to the titanium post system.
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Department of Conservative Dentistry and Endodontics, Sri Guru Ram Das Institute of Dental Sciences and Research, Amritsar, Punjab
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3]
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