| Abstract|| |
Aim: The aim of this study was to evaluate the fracture resistance of endodontically treated maxillary premolars with wide mesio-occluso-distal (MOD) cavities restored with either composite resin, or composite resin reinforced with different types of fibres.
Materials and Methods: Fifty human maxillary premolars were selected. Five intact teeth served as positive controls. Endodontic therapy was carried out in the remaining forty-five teeth. Standardized MOD cavities were prepared in all the teeth. The teeth were restored with a nanocomposite using an incremental technique. These forty five teeth were randomly divided into three experimental groups (Group A, B and C) (n = 15). The teeth in Group A did not undergo any further procedures. The teeth in Group B and C were reinforced with composite impregnated glass fibre and polyethylene fibre, respectively. Fracture resistance was measured in Newtons (N).
Results: The positive controls showed the highest mean fracture resistance (811.90 N), followed by Group B (600.49N), Group A (516.96N) and Group C (514.64N), respectively. One Way analysis of variance (ANOVA) test revealed a statistically significant difference between all the groups (P = 0.001). Post-hoc Tukey test revealed a moderately significant difference (P = 0.034) between Control and Group B, and a strongly significant difference between Control and Group A (P = 0.002), and Control and Group C (P = 0.001).
Conclusions: Endodontic therapy and MOD cavity preparation significantly reduced the fracture resistance of endodontically treated maxillary premolars (P = 0.001). No statistically significant difference was found between the experimental groups (Group A, B and C) (P > 0.1). However, the fracture resistance of the composite impregnated glass fibre reinforced group was much higher than the others.
Keywords: Composite impregnated glass fibre; fracture resistance; mesio-occluso-distal cavity; polyethylene fibre
|How to cite this article:|
Luthria A, Srirekha A, Hegde J, Karale R, Tyagi S, Bhaskaran S. The reinforcement effect of polyethylene fibre and composite impregnated glass fibre on fracture resistance of endodontically treated teeth: An in vitro study. J Conserv Dent 2012;15:372-6
|How to cite this URL:|
Luthria A, Srirekha A, Hegde J, Karale R, Tyagi S, Bhaskaran S. The reinforcement effect of polyethylene fibre and composite impregnated glass fibre on fracture resistance of endodontically treated teeth: An in vitro study. J Conserv Dent [serial online] 2012 [cited 2019 Jun 16];15:372-6. Available from: http://www.jcd.org.in/text.asp?2012/15/4/372/101914
| Introduction|| |
Following endodontic treatment, there is a decrease in the fracture resistance of the teeth. This is also seen after the preparation of wide mesio-occluso-distal (MOD) cavities. An ideal restoration for a tooth is able to maintain the aesthetics and function, preserve the remaining tooth structure and prevent the microleakage.  Indirect restorative procedures such as metallic post and core fabrication followed by full crown are customary.  However, there may be cases where the tooth is still erupting, or where the tooth or root canal therapy has a questionable prognosis, or where the clinician wants to wait and evaluate the healing of a periapical lesion before proceeding with full crown restorations. To prevent the failure of root canal treatment, a simple, quick, high strength, direct and cost effective restorative procedure may be desirable. Adhesive technology is advancing by leaps and bounds every day, making it possible to create conservative and highly aesthetic restorations with direct bonding to the teeth. A significant increase in the fracture resistance of root filled teeth was observed when they were intracoronally restored with a resin composite material. , Reinforcing composites with polyethylene fibres and glass fibres has successfully provided superior results. ,, However, there are no studies comparing the two in direct composite restorations.
The aim of this study was to evaluate the fracture resistance of endodontically treated maxillary premolars with wide mesio-occluso-distal (MOD) cavities restored with either composite resin, composite resin reinforced with a polyethylene fibre, or composite resin reinforced with a composite impregnated glass fibre.
| Materials and Methods|| |
50 intact human maxillary premolars, extracted for orthodontic reasons were collected. All the specimens were scaled to remove the adhering soft tissue and calculus, and were stored in physiologic saline. Teeth with caries, morphological defects or fracture lines were discarded. Five intact teeth were used as positive controls and endodontic access cavities were prepared in forty five teeth using a water cooled diamond bur (Endo Access Bur; Dentsply) with an airotor hand piece, and the pulp tissue was removed with barbed broaches. A size 15 K-file (Mani Prime Dental Pvt. Ltd.) was introduced into each canal until it could be seen at the apical foramen. The working length was then determined by subtracting 1 mm from this length. The canals were prepared with ProTaper (Dentsply Maillefer) nickel titanium (Ni-Ti) instruments using a 1:64 reduction handpiece (Anthogyr NiTi Control; Dentsply) at a speed of 300 revolutions per minute. Coronal flaring was done with the Sx file. Canals were shaped till the working length using the S1 and S2 files, and the apical third of the canal was finished using finishing file F1 and later F2. Copious irrigation using 5 ml of 5.25% sodium hypochlorite was carried out throughout the procedure. One set of ProTaper files™ was used for the five teeth.
The tooth root canals were flushed with saline to remove traces of sodium hypochlorite. The canals were then dried with absorbent paper points (Dentsply Maillefer). Obturation was donewith ProTaper gutta percha pointsÔ and AH Plus root canal sealer (Dentsply De Trey, Konstanz, Germany ) using a cold lateral condensation technique. MOD cavities were prepared in the teeth using an airotor hand piece with a long straight fissure diamond point (SF-12C; Mani Dia Burs). The diamond abrasive was changed after every 10 preparations. The dimensions of the cavity were decided with the help of a periodontal probe to standardize the remaining tooth structure. The remaining thickness of the buccal and lingual wall of the teeth was 2.5 mm from the height of contour of the buccal and lingual surface till the buccal and lingual cavity walls, and the gingival cavosurface margin was 1.5 mm above the cement-enamel junction (CEJ) [Figure 1]a. 
|Figure 1: (a) Standardized dimensions of mesio-occluso-distal cavity (b) Preparation of bucco-occluso-lingual groove|
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Subsequently, the teeth were randomly divided into three groups (n = 15):
Preparation of specimens
- Group A - Preparations restored with composite, without fibre (n = 15)
- Group B - Preparations restored with composite impregnated with glass fibre (n = 15)
- Group C - Preparations restored with composite impregnated with polyethylene fibre (n = 15)
- Positive Control - Intact teeth (n = 5)
Group A (No fibre group)
Cavities were cleaned, dried, and etched with 35% phosphoric acid (Scotch Bond Multi Purpose Etchant; 3M ESPE, St. Paul, MN, USA) for 15 seconds. The tooth surfaces were then rinsed for 10 seconds and gently dried for 1-2 seconds. Subsequently, a single bottle total etch adhesive (Adper Single Bond 2; 3M ESPE, St. Paul, MN, USA) was applied and photocured using light-emitting diode (LED) (LEDition; Ivoclar Vivadent) at 600 mW/cm 2 for 10 seconds. Light intensity output was verified after every 10 samples using a digital read out light meter. Tofflemire retainer and matrix band were applied and the preparations were restored with a nanocomposite (Filtek Z350 XT; 3M ESPE, St. Paul, MN, USA) by using an incremental technique. The layers were placed at a 1.5 mm thickness and cured for 40 seconds according to the manufacturer's instruction. 
Group B (Composite impregnated glass fibre)
After restoring the preparation as described for group A, a groove 2 mm wide and 1 mm deep was prepared buccolingually on the occlusal surface of the restoration and cusp tips. The ends of the groove were on the occlusal third of the buccal and lingual surfaces of the tooth [Figure 1]b. After etching the grooves, single bottle total etch adhesive was applied and cured for 10 seconds. Flowable composite with 10 mm of composite impregnated glass fibre (Interlig; Angelus, Londrina PR, Brazil) was placed on the floor of the groove and light cured for 20 seconds. A layer of the nanocomposite was added to cover the exposed fibre surface and cured from the occlusal direction for 40 seconds according to manufacturer's instructions. 
Group C (Polyethylene fibre)
Buccolingual grooves were prepared, etched and bonded in a manner similar to Group B. Flowable composite resin with 10 mm of polyethylene fibre (Ribbond; Ribbond Inc., Seattle, WA, USA) which had been saturated with the unfilled resin (Scotch Bond Multi Purpose Plus; 3M ESPE, St. Paul, MN, USA) was added to the groove and cured from occlusal direction for 20 seconds. The exposed surface of the fibre was covered with a layer of the nanocomposite and cured for 40 seconds from the occlusal direction according to the manufacturer's instructions.  All restorations were finished and polished (Shofu Dental). The teeth were then mounted in a block of cold cure acrylic measuring 2 cm in diameter, at CEJ. A layer of polyvinyl siloxane was adapted around the root surfaces to simulate the periodontal ligament. Thermocycling was carried out for 500 cycles at 5 to 55°C, 30 second dwell time and 5 second transfer time. All the specimens were stored in an incubator (iBod) at 37°C and 100% humidity for 24 hours. , The fracture resistance of the teeth was measured using a universal testing machine (LR 50K; LLOYD Instruments), under a compressive force at a strain rate of 0.5 mm/min. A 5 mm diameter round stainless steel ball was positioned parallel to the long axis of the teeth and centered over the teeth until the bar just contacted the occlusal surface of the restoration and the buccal and lingual cusps. The force needed to fracture each tooth was recorded in Newtons. ,, The study design was a three arm comparative study where the outcome variable was fracture resistance. Statistical analysis was performed using descriptive statistical methods, One Way ANOVA test and the Post-hoc Tukey test. The Statistical software namely SAS 9.2, SPSS 15.0, Stata 10.1, MedCalc 9.0.1, Systat 12.0 and R environment ver.2.11.1 were used for the analysis of the data and Microsoft word and Excel were used to generate graphs, tables etc.
| Results|| |
Highest mean fracture resistance was observed with positive control followed by Group B and group A respectively. Group C recorded the lowest mean fracture resistance. One Way ANOVA test revealed a significant difference (P = 0.001) between the groups [Table 1] [Figure 2]. On intergroup comparison, Post-hoc Tukey test revealed a moderately significant difference between Control and Group B (P = 0.034) whereas a strongly significant difference was recorded between Control and Group A (P = 0.002) and between control and group C (P = 0.001). No significant difference was recorded between any of the experimental groups (Group A, B and C) (P > 0.1) [Table 2].
|Table 1: Mean fracture resistance in Newton (n) in all four groups as seen in the study|
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| Discussion|| |
Restoration of root filled teeth, despite posing a major challenge to the clinician, is an essential final step of root canal treatment. The purpose of the restoration is not only to repair, reinforce and strengthen the tooth, but also to provide an effective seal between the canal system and the mouth. The gold standard of post endodontic restorations is the full crown; however, there are situations where the clinician may choose to delay the delivery of the full crown. In such cases, an efficient, direct, high strength, low cost and quick intermediary restoration may be desirable. Adhesive restorations allow a more efficient transfer and distribution of functional stresses across the bonding interface to the tooth structure. Nanocomposites display improved performance due to their dense filler loading. The smaller size of the fillers allows them to have excellent optical properties along with good mechanical properties. ,
Despite the advances in material sciences, composite resins are not commonly used for extensive restorations due to their brittle nature. Reinforcement with fibres has been shown to impart strength and toughness to composite resins. , The fibres used in this study had different arrangements and surface treatments. Original Ribbond™ (Group C) consists of cold plasma treated, Leno Weave Ultra High Modulus (LWUHMW) polyethylene fibres, which are arranged in a lock-stitch design. Interlig™ (Group B) consists of glass fibres pre-impregnated with light curable composite resin arranged in a braided design. Fibres were placed occlusal to the restoration following a technique given by Oskoee et al., and Belli et al.. Placing fibres on the occlusal surface keeps buccal and lingual cusps together and protects the natural cusps resulting in higher fracture resistance. In addition, fracture resistance increases when fibres are placed close to the point where the force is exerted because it leads to a shorter working arm according to levers principle. , The results of the present study revealed that the positive control group had highest mean fracture resistance (811.90 N). This is in accordance with the study by Beli et al.,  and also denotes that endodontic and restorative treatment have a detrimental effect on fracture resistance of teeth. 
Among the experimental groups, Group B had the highest mean fracture resistance (600.49 N) followed by Group A (516.96 N), and Group C (514.64 N) [Table 1] [Figure 2]. The One Way ANOVA test done in the present study revealed a statistically significant difference (P = 0.001) between all the groups [Table 1]. Post-hoc Tukey test revealed a moderately significant difference between Control and Group B (P = 0.034) and a strongly significant difference between Control and Group A (P = 0.002), and between Control and Group C (P = 0.001). No significant difference was found between any of the experimental groups (Group A, B and C) (P > 0.1) [Table 2]. Group A showed an acceptable fracture resistance due to the use of nanocomposites in the present study. The high filler loading enables nanocomposites to demonstrate good physical and mechanical properties and reinforce the tooth structure well. Single bottle total etch system gave evidence of better bond strength than the newer self- etch systems.  The presence of the adhesive layer under the restoration acts as a stress buffer. A Study by Ausiello et al., has shown that an optimal adhesive layer thickness leads to maximum stress release while preserving interface integrity. The fracture resistance of Group A can be attributed to the increase in strain capacity of the adhesive resin and the improved physical and mechanical properties of nanocomposites. 
This study recorded that Group B had the highest fracture resistance amongst the experimental groups. The superior performance of Group B may be because of the pre-fabricated resin impregnation of these fibres by the manufacturers. Also, glass fibres have very high tensile strength, density and percentage of elongation allowing them to withstand high stresses without fracturing.  Results of this study were in agreement with the study by Kolbeck et al.,  who reported the glass fibres performed better than polyethylene fibres due to pre-impregnation with light cured composite which ensures a good bond with the composite resin. Group C also showed reasonably good fracture resistance. Polyethylene fibres have shown high fracture resistance in previous studies. , According to the manufacturer Original RibbondÔ fibres are woven using the lock-stitch leno weave which prevents slipping of fibres within resin matrix, prevents micro-cracks from propagating to form larger cracks and reinforces the restoration in multiple directions. Beli et al., found that insertion of polyethylene fibre in the occlusal third of a composite restoration increased the fracture resistance of root filled teeth compared to teeth restored with composite resin alone. However, the present study showed that insertion of polyethylene fibre did not significantly increase the fracture resistance of a root filled tooth as compared to teeth restored with composite resin alone (Group A) (P > 0.1).
The causes for lower fracture resistance of Group C may include the following:
Though it was not among the objectives of the study it was observed that the majority of failures in the fibre reinforced groups were favourable in nature. The fracture in 80% cases occurred superior to the CEJ. This can prove immensely beneficial to clinicians as despite fracturing, the tooth remains repairable with techniques such as post and core followed by full crown. Additionally, the study done by Reeh et al.,  showed a relative loss of stiffness of 5% due to endodontic procedures and 63% in MOD cavity preparation whereas the present study found a relatively smaller decrease of 25-35% in fracture resistance of root filled teeth with MOD cavity preparation restored with nanocomposites or nanocomposites restored with different fibres. Despite this decrease in fracture resistance all the experimental groups demonstrated results much higher than the average normal biting force of human maxillary premolars (100-300N).  Many differences exist between fractures occurring clinically and those induced by a machine. Forces generated intraorally during function vary in magnitude, speed of application and direction, whereas the forces applied to the teeth in this study were at a constant direction and speed and they increased continuously until fracture occurred. Further in vivo studies should be done to test the reinforcement effect of fibres in clinical situations.
- Non uniform wetting of fibre with unfilled resin - Manual wetting of Original RibbondÔ (Group C) with unfilled adhesive was done as suggested by the manufacturer. This may lead to areas of non-uniform wetting in the fibre which will affect the adhesion of fibre to resin matrix. This in turn, may cause several problems in the future of the restoration. The poorly impregnated regions may become areas of increased water sorption, hence causing deterioration of mechanical properties of the composite. Discolouration due to penetration of oral microbes into these voids may also occur. Such voids may also act as oxygen reserves hence preventing complete polymerisation of composite resin. 
- The LWUHMW polyethylene fibre is treated with cold gas plasma to convert the material from hydrophobic to hydrophilic. This treatment is meant to make the fibre surface more receptive to bonding with the resin however; it makes the fibre very technique sensitive. Any contamination to the fibre surface may affect its adhesive properties. Handling of the polyethylene fibres was found to be tougher in the present study. The fibres frayed on cutting, and became very stiff after the wetting procedure.
- An additional reason for its poor performance may be lower tensile strength, density and elongation compared to glass fibres.
| Conclusions|| |
- Root canal treatment and MOD cavity preparation significantly reduced fracture resistance of root filled maxillary premolars (P = 0.001).
- Even though there was no statistically significant difference between the test groups, the fracture resistance of the composite impregnated glass fibre reinforced group was much higher.
| Acknowledgment|| |
Dr. R. Gopalan, Composite technology park, Kingeri
Dr. KP Suresh (statistician)
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Department of Conservative Dentistry and Endodontics, The Oxford Dental College, Hospital and Research Centre, Bommanhalli, Hosur Road, Bangalore - 560 068, Karnataka
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]
[Table 1], [Table 2]