As evident from the published research in the literature, loss of tooth structure due to any reason has been suggested as the main precipitating factor for fracture of ETT. There is an appreciable loss of tooth structure while removing dental caries and preparing an access for endodontics with cumulative effect of loss of dentine during biomechanical preparation resulting in significant weakening of the tooth. In fact, it is well accepted that the removal of excessive dentine compromises the long term survival of root canaled teeth and that the fracture resistance of ETT is directly related to the residual unaltered tooth structure (Faria M I et al, 2013).Besides this, endodontic procedures involving use of sodium hypochlorite, EDTA, calcium hydroxide, bleaching agents and obturation technique are also responsible for reduction in flexure strength of ETT.
As reported in a study by Reeh et al (1989), the remaining tooth tissue of crown, particularly the marginal ridge continuity, seemed to be more prudent for resistance of ETT. The largest loss in strength was linked to break in marginal ridge continuity followed by endodontic access preparation. This observation is complemented by studies done by various authors. Cobankara F Ket al (2008) compared the resistance against fracture of ETT with MODs restored using varied restorative techniques. Within the limitations of this research, restorative techniques were unable to restore theresistancelost from MOD preparation. MOD tooth preparation caused a loss of 80 percent of fracture resistance of ETT when compared with the intact teeth. Arunpraditkul S et al (2009) also observed a drop of about 50 percent in fracture resistance in ETT with one wall of missing. Luthria Aet al (2012) evaluated the fracture resistance of ETT with wide MODs restored with either composite or composite reinforced with varied types of fibres. RCT and MOD preparation reduced the fracture resistance. Bassir M Met al (2013) evaluated the fracture resistance and the fracture mode of ETT with varied amount of tooth structure left. MO (40 percent) and MOD (50 percent) preparations significantly reduced the fracture resistance of teeth. Khan S Iet al (2013) compared the resistance to fracture of ETT with MODs restored using two types of fibres. The fracture resistance loss of 75 percent was observed with MOD preparation in ETT. Polyethylene ribbon and glass fibre under MOD composite restorations enhanced fracture strength.
Pradeep P et al (2013) also reported a decrease in fracture resistance of ETT by 50 percent with MOD preparation and RCT with minimal effect of 10 percent decrease by RCT. Kalburge V et al (2013) reported that fracture resistance decreases by 65 percent in ETT after preparation of MOD preparation and 35 percent after DO preparation. Zelic Ket al (2014) in a finite analytic study found that a 2 surface composite preparation reduced the strength of theETTby 23.25 percent. The failure pattern showed ETT were unlikely tofail under high masticatory stress (710 Newtons). However, afterpreparation of access, the Failure stress reached a point under high masticatory force as found in the posterior region, fractureoccurred. The combined effect of both tooth preparation and RCT led to decrease in resistance of sixty two percent under load (710 Newtons) finally leading to fracture. Access preparation led to more decrease intoothresistance while canal preparation did not lead to this effect.
Based on the above research background, various clinical situations were studied in this study – inside mesial and distal walls removed with 1 mm tooth structure to simulate overcutting during endodontic procedures and removal of caries in that portion of tooth. Similarly, mesial, distal, buccal and lingual walls were removed to simulate clinical variations in loss of tooth structure. It was presumed that temporary restorative material does not lead to any reinforcement and was used in samples to compare with the reinforcing material. The effect of over preparation of teeth due to overzealous crown preparation was also simulated and studied. In all these situations, effect of reinforcement of PCD was also studied with the use of an adhesive restorative material.
After a careful study of forces responsible for stress development in ETT, it was observed that the cervical portion of the tooth is the most prone for concentration of stress and dentine in this portion undergoes maximum episodes of stress and strain cycles. As evident from the studies done by various authors, stress concentration in the cervical portion is the most critical for the failure of ETT. Goel VKet al (1991) investigated stress variations in the enamel and dentine near the DEJ. The effect of variations in the contour of the DEJ on the stress pattern for enamel and dentine was also researched. The normal (compressive/tensile) and shear forces in the dentine and enamel portion of the DEJ were calculated. The stresses were maximum on the occlusal and vanished along the buccal and lingual surface of DEJ. The results proved that the mechanical inter-locking between enamel and dentine in cervical portion is weaker than in other areas of DEJ.
Pierrisnard Let al (2002) compared the effect of different coronol and radicular reconstructive techniques on stress distribution to tooth tissues. The greatest stress developed in the cervical region. The post was shown to be beneficial in conditions with sufficient remaining dentine. In the presence of post, cervical stress was lower than when no post was there. Moreover, the higher the elastic moduli, the lower the stress was observed.
Palamara J Eet al (2002) in a study investigated strains in enamel of lower premolars in a finite analytic study. Strain in areas near to contacts and marginal ridges was less than near the CEJ and on buccal/lingual surfaces. The amount of strain increased with oblique forces on cusp inclines of cusp. Dejak Bet al (2003) analyzed the stress formed in a lower molar during clenching and chewing of morsel with different elastic moduli. The study revealed that during mastication, maximum stress occurred in the cervical portion of the lingual walls of the lower first molar. Zeng Yand Wang JD (2005) developed a 3D analytic model of the lower first molar and studied stress distribution under various loads. The stress was maximum and chiefly distributed at cervical region, furcation and apical portion of the canal.
Marchi G Met al (2008) evaluated the effect of remaining dentine around post-core restoration and stress on the fracture resistance of ETT. The remaining dentine thickness influenced ETT restored with customised cast post-core; weak roots presented lower fracture resistance.
Zogheib LV et al (2011) in their study evaluated the resistance to fracture of ETT reinforced with different reconstructive protocols. The radicular dentine in cervical portion was the most important to enhance fracture resistance than the reconstruction technique used. Yang WL et al (2012) studied the distribution of stress in teeth with defect located in the cervical region. The stresses were concentrated in dentine at the EDJ in situations of different load simulations. Afroz S et al (2013) analyzed the effect of varied combination of post-core materials on stress distribution in dentine of ETT. The cervical portion of the teeth was subjected to the most stresses ir-respective of the material for fabrication. The difference in elastic moduli of the restoratives can lead to non-uniform stress development and stresses in different areas. The materials that have an elastic moduli close to dentine should be used. Veríssimo Cet al (2014) studied the effect of remaining dentine coronally on the bio-mechanics of ETT. The presence of 2 mm of dentine coronally improved the mechanical behavior of ETT.
Based on this scientific evidence, it is clear that the cervical portion of tooth is the most susceptible area for concentration of forces and dentine in this area is most critical for the strength of the tooth.
This dentine in this cervical portion of the tooth which plays a critical role in fracture resistance of tooth was termed Peri-Cervical Dentine (PCD). Peri-cervical dentine is an area 4 mm coronal and apical to crestal bone. It acts like the neck portion of the tooth and transfers occlusal forces to the root.(Clark D et al, 2010; Clark D J, 2010; Clark DJ, 2007). PCD is a critical landmark in relation to the strength of ETT.
In line with Minimal Invasive Dentistry(MID) and with this scientific evidence, the focus has shifted towards Conservative Endodontic Cavities (CECs) and Biomimetic endodontics which rely on the principles of tooth conservation and reinforcement with tooth like materials (Clark D J, 2010; Clark DJ, 2007; Malterud M, 2013; Krishan R et al, 2014).
Biomimeticsor bio-emulation allows for the amalgamation of 2 fundamentals at the helm of modern restorative treatment protocols: Tooth preservation & adhesion. This concept allows the retention and continuity of maximum tooth tissue possible, while ensuing clinical longevity and esthetics. It allows the preservation of biological, esthetic, bio-mechanical & functional properties of tooth tissues. Now -a -days, it is possible to prepare access having conservation of enamel and dentine to allow bonding of restoratives (Clark DJ, 2007).The main principle guiding bio-mimetics is to return tooth tissues to function by a bond that transfers masticatory stresses allowing the tooth to its function. (McMahon and Evron, 2011).
Krishan Ret al (2014) suggested Conservative endodontic cavities (CEC) to improve fracture resistance without compromising the instrumentation of canals. Their study assessed the impact of CEC on both variables in 3 tooth types: upper incisors, lower premolars, and molars. Although, CEC was associated with compromised instrumentation of canals only in the distal canal of molar, it conserved cervical dentine in all tooth types and conveyed an advantage of improved fracture resistance.
In the past as well, utilising the bio-mimetic principles, reinforcement protocols have been used as the Guttapercha, the oldest obturating material alongwith ZOE leads to no reinforcement as reported by several authors. In current years, obturation materials & sealers have been formulated based on dentine adhesive technology from reinforcement based restorative dentistry to prevent microleakage in the root canals and to reinforce the tooth effectively. With increasing success and predictability of adhesive strategies used for intra-coronal adhesive sealing, likewise, potential improvements in apical and coronal seals and strengthening of ETT may be expected between the intra-radicular dentine and adhesive canal fillings.
Adhesive dentistry has been introduced in Endodontics by introduction of obturation systems with a focus on ‘monoblock’ in which the core material, sealer and dentine forms a single cohesive unit. Shafer et al (2007) and Teixeira et al (2004) reported that resin sealers increase the fracture strength of ETT. To achieve monoblock, resin materials were proposed to reinforce ETT through the use of adhesion based sealers.
Monoblock concept was introduced in 1970s but became popular with advent of adhesive materials for obturation and rekindled in 2004 (Tay FR and Pashley DH, 2007). To serve this concept, resin materials have been proposed for use to reinforce ETT with the use of adhesion based sealers in the canals. Replacement monoblock formed in the root canals are defined as primary/ secondary and tertiary based on the number of interface between the adhering substrate and the core. A primary monoblock has 1 interface which circumferentially extends in between material and the walls of the canal e.g. Hydron, MTA etc.
Secondary monoblocks comprise 2 circumferential interface, 1 between the cement & dentine and the other between cement & the core. This is traditionally achieved in the restorative-endodontic literature e.g. Resilon, iRoot SP etc. Tertiary monoblocks have a 3rd circumferential interface b/w substrate and for bonding & the restoring material e.g. EndoREZ, ActiV GP etc. (Tay FR and Pashley DH, 2007;Darrag AM and Fayyad DM, 2011). The creation of a ‘monoblock’ that adheres to the canal to strengthen ETT is convincing. However, a ‘monoblock’ of Guttapercha has not been possible due to lack of chemical union between guttapercha (a poly-isoprene) and various sealers such as ZOE, epoxy resin and GIC based sealers (Tay FR et al ,2005).
Tay FRand Pashley DH (2007) defined a wider meaning of monoblock to be applicable for restorative which have been used
Fig.53 : Diagram representing Classification of Endodontic monoblocks
The creation of a ‘monoblock’ that adheres to the canal to strengthen ETT is convincing. However, a ‘monoblock’ of Guttapercha has not been possible due to lack of chemical union between guttapercha (a poly-isoprene) and various sealers such as ZOE, epoxy resin and GIC based sealers (Tay FR et al ,2005).
Tay FRand Pashley DH (2007) defined a wider meaning of monoblock to be applicable for restorative which have been used in past & present for re-habilitation of root canals. The potential of current bonding materials to achieve homo-genous mechanical units with radicular dentine is defined in relation to traditional concept in which monoblock was 1st used in restorative and later in endodontic speciality.
Belli Set al (2011) investigated using analytic study: primary/secondary/tertiary monoblock was formed either by an adhesive sealer or by a varied adhesive post-core and checked the effects of interface on transfer of stress. Stresses in roots enhanced with an enhancement in number of interface. Creation of primary monoblock in canal by sealer or with adhesion based post-core system reduced the stress that forms inside tooth tissues. Mattos C Met al (2012) also supported the monoblock adhesive re-construction for root reinforcement for long term results to be attained.
Though the different adhesive resinous materials have been tried and tested, there was not any significant difference reported in comparison with guttapercha and AHplus sealer. Even guttapercha & AH plus performed better than other combinations in improving fracture resistance of teeth has been reported by some authors.
Karapinar Kazandag M(2009) observed that system meant to achieve monoblock were not better than the traditionall AHPlus & guttapercha in terms of resistance to fracture. The resistance of ETT using ActiVGP & lateral condensation was less in comparison to AHplus & guttapercha. Lertchirakarn Vet al (2011) evaluated the VRF resistance of upper incisors restored with varied obturation materials and sealers. Fracture resistance of ETT restored with guttapercha and AHplus was greater than the RealSeal system.
Bhat S Set al (2012) concluded that resin sealers Roekoseal, AH plus and pulpdent used in this study were similar in effectiveness in improving fracture resistance. Sağsen Bet al (2012)stated that AHplus, calcium silicate sealer and MTA Fillapex increased the fracture resistance of prepared canals with AH plus better than other. Topçuoğlu H Set al (2013) reported that Tech Bio-seal Endo, Endosequence BC and AHplus Jet sealers increased the fracture resistance in ETT. Sandikci T and Kaptan RF(2014) stated that lateral compaction performed with AHplus and guttapercha and Thermafil technique were more successful in enhancing fracture resistance of ETT. Mandava Jet al (2014) in their study observed that AH Plus displayed better fracture resistance in comparison with the sealers: MetaSEAL and Fillapex.
This led us to choose this combination of sealer AH Plus and guttapercha which is the most preferred, time tested, researched, recommended and used combination clinically to best simulate clinical conditions. Morever, the above mentioned scientific evidence clearly indicates a positive impact of AH Plus on fracture resistance of ETT comparable to other adhesive materials and sealers used for obturation.
Intra-coronal strengthening and intra-radicular reinforcement of teeth play an important role to protect ETT against fracture. With recent advances in adhesive technology, new and stronger adhesive restorative materials are available that bond directly to the tooth structure and strengthen it by creating conservative and highly aesthetic restorations. Based on the ‘monoblock’ concept of root canal reinforcement coupled with the scientific evidence supporting the use of intraorifice barriers of RMGIC used for reinforcement, an adhesive restorative material – nanoionomer was selected to reinforce this critical area of PCD.A non-adhesive and non-reinforcing temporary restorative material was also used as post obturation restoration to study and compare the effect of post obturation restorative material on the reinforcement of tooth structure.
It was assumed that this highly reinforced chemically and micromechanically adhering GIC will form a primary monoblock inside the area of root canal meant for reinforcement of PCD. Since, this material is indicated for core build up as well, so it was used for coronal reinforcement as well. This continuum of reinforcement starting from apical end of intra-orifice preparation till coronal end of PCD was a major reason for selection of this material which was capable of improving coronal seal as well. This led us to select Ketac N 100 nanoionomer for the study.
Nanoionomers are the most popularly used materials as they have the setting, physical and mechanical properties like composites and chemical adhesion like glass ionomer components. Further to enhance their mechanical properties, nanofillers were added in recently introduced formulations. Nanofilled resin modified GIC (Ketac N100) or nano ionomer has the dual benefits of resin-modified light cured GIC and bonded nano-filler technology. nRMGIC contains fluoroaluminosilicate glass together with nano-fillers (5-25nm) and nanoclusters (1-1.6µm).They have the dual advantages of acid base reaction of traditional GIC and the polymerisation reaction of resins leading to formation of 2 separate matrix systems i.e. metal poly-acrylate matrix and polyHEMA matrix. The filler loading is 69 percent by weight which is quite near and comparable to dental composites. In recent studies, it has been shown that RMGIC increases the fracture resistance of ETT as evidenced by Prathibha RS (2011).The nanoionomer has major advantage of providing a good seal, chemical adhesion, fluoride release and increased fracture resistance of teeth as evidenced by Croll TP and Berg JH (2007),Coutinho Eet al (2009),Killian CMand Croll TP (2010), El-Askary FSand Nassif MS (2011), Neelakantan P(2011), El-Askary Fand Nassif M(2011), Upadhyay Sand Rao A (2011), Konde S et al (2012), de Fúcio SB(2012), Upadhyay Set al (2013), Falsafi A et al (2014), Eronat Net al (2014), Sungurtekin-Ekci E et al (2014), Babannavar Rand Shenoy A (2014) , Shafiei F and Akbarian S et al (2014).
This strong scientific background and specific properties of this material led us to bank upon this material for reinforcement.
In the present study, recently extracted mandibular molars were selected because these teeth show a high incidence of fractures in the clinical settings as reported in the various studies and surveys. Tamse A et al(1999) in a survey evaluated 92 VRF cases of ETT. The lower molars (24 percent) were the most fractured teeth. Zadik Yet al (2008) in a study found that of the 547 ETT, lower (44.6 percent) and upper (20.5 percent) first molars were the most common involved teeth in VRF. The most common extracted ETT were lower first molars. Touré B et al (2011) also found that the lower first molar was the most frequently removed tooth. Yoshino Kand Ito K (2014) observed that the ETT type with the highest percentage of extraction due to VRF amongst molars were mandibular first molar (50 percent) in males and the lower first molar (54 percent) in females.
The teeth were stored following the ISO recommended protocol (ISO ⁄ TS 11405) for use of distilled water or chloramine-T with periodic replacement (Naumann M et al,2009).The teeth were stored in this solution to avoid getting dry and later becoming brittle as suggested in a study by Helfer et al (1972).This storage solution prevented microbial activity.(Sparrius and Grossman, 1989).
The teeth were ground flat and enamel was carefully removed to expose dentine. The enamel removal was confirmed with the help of a stereomicroscope. This methodology used was same as used by Arora V et al (2015) for standardisation of specimens. The volume of PCD was also measured for all the specimens in a customised apparatus not mentioned in the literature. The volumetric measurement was helpful in distribution of specimens across all the groups so that the variations could be reduced. All the groups received similar pattern for distribution of samples with higher, average and lower values of measured PCD. Though the literature mentions the use of spiral computed tomography (SCT) for measurement of tooth volume. These methods are costly and require a costly equipment. Our technique is new, simple, absolutely cost-effective and does not require any complex instruments at all.
Endodontic access cavities with minimal dimensions were prepared to prevent excessive removal of tooth structure and to prevent its impact on fracture resistance as the removal of tooth structure has a direct impact on fracture resistance of ETT. Working length was calculated by deducting one millimeter from the canal length, which had been measured by the insertion of 15k file into canal till the tip of file was seen at the apical foramina. This is the most commonly used, simple and predictable method in the literature for working length determination for an in vitro study.
Crown down technique, which is the most advocated technique for biomechanical preparation was used as it minimizes coronal interferences, eases penetration of instrument, increases tactile sensitivity apically, minimises change in working length during instrumentation, allows irrigant penetration to working length, reduces the contact area of instruments (reducing torque and increasing efficiency and safety), and reduces the instrument tip contact and thereby the procedural errors. Biomechanical preparation was done in a standardized manner using Protaper rotary file system sequence upto F3 size for mesial canals and distal canals as per manufacturer’s instructions.
Scientific researchers have recommended different minimum instrumentation sizes needed for effective penetration of irrigants to the apical third of the root canal. Khademi Aet al (2006) conducted a study to find the minimum size of preparation for the predictable penetration of irrigants & removal of debris and smear layer from apical one third of root canals in the mesio-buccal canals of lower molars prepared using the crown down technique. It was stated that the minimum preparation size as required for penetration of the irrigants to the apical one third of the canal was a normal taper #30 file. Akhlaghi NM et al (2014) evaluated the effect of MAF size/ taper on penetration of the irrigants in the apical 1/3rd of curved mesio-buccal canals of lower molars. Based on this study, 30/.06 was recommended as the minimum MAF size for acceptable debridement Mohammadzadeh Akhlaghi Net al (2014) evaluated the effect of size and taper of master apical file in decreasing bacteria from the apical one third of the curved canals. MAF #25/0.04 had similar effect as compared to other groups with bigger apical size and taper in eliminating bacteria.
The protaper universal system was chosen as it is the most used rotary system across the globe inspite of the fact that it produces more microcracks in dentine decreasing the fracture resistance of the tooth. The clinical situations can be best simulated using a system which is most commonly used across the globe. Liu Ret al (2013) compared the incidence of cracks seen in radicular dentine after canal preparation with Protaper system. Cracks were found in 50 percent of teeth after RCT with the Protaper files. Hin E S(2013) reported that RCT with ProTapers caused significant damage to root canal dentine in form of cracks. Abou El NHM & Abd El K KG(2014) also got same results that Protaper files were quite aggressive in causing micro-dentinal fractures in root.
The irrigation protocol was standardized for all experimental groups. The canals were irrigated with two ml of 5.25 percent NaOCl after every instrumentation followed by final rinse with 5 ml 17 percent EDTA as it is documented in the literature as a preferred method to be used clinically. Teixeira CS et al (2005) verified the effect of irrigation time with EDTA and sodium hypochlorite on removal of smear layer. In this laboratory study, irrigation with EDTA and sodium hypochlorite for 1, 3 and 5 min. were same in effectiveness in elimination of smear layer from root canal wall. Mello Iet al (2008) analyzed the effect of varied amounts of 17 percent EDTA as final rinse on removal of smear layer. Final rinse of 5 ml EDTA led to good smear layer removal, and opened tubules as well. Mello Iet al (2010) verified that a final rinse with 5 mL of EDTA can efficiently eliminate the smear layer. Guo X et al (2014) compared the efficacy of 4 irrigation protocols to conclude that the combination of 60 degrees C, 3 percent NaOCl and 17 percent EDTA could eliminate smear layer efficiently. The final irrigation was done with distilled water in our study to neutralize the effects of the irrigating solutions used.
Single cone obturation of the prepared canals was done with protaper matched manufacturer recommended cones and AH Plus sealer. Morever, single cone technique excluded both the wedging forces of the spreaders during lateral compaction and the excessive dentine removal required to facilitate the plugger’s insertion during vertical compaction. Morever, this is the recommended technique to follow with rotary protaper systems. Studies have shown that the single cone technique would not decrease the fracture resistance of teeth more than other obturation techniques. Ersev Het al (2012) assessed the influence of single cone or lateral compaction technique on fracture resistance of ETT. They concluded that when used with single cone technique, Metaseal & AHPlus had the potential for reinforcing ETT. Vishwanathan PKet al (2012) compared the fracture resistance of ETT instrumented with rotary systems and obturated with single cone guttapercha. The group obturated with single cone guttapercha and AH Plus sealer was more fracture resistant than lateral condensation groups.
The manipulation, application of sealer and obturation was done with the help of matched cones as per manufacturer’s instructions to standardise the working protocol. Ketac N100 and temporary cement were placed in the intraorifice space and access cavities in respective samples after obturation according to the manufacturer’s instruction as previously described in methodology using a manufacturer recommended standardised technique.
Thermocycling was done to simulate oral temperature changes as well as the effect of saliva on the restorations. Thermocycling was then performed between 5 and 55˚C. This procedure was performed in accordance with ISO (ISO/TS 11405/2003). Thermocycling was considered to simulate aging of materials (Heydecke et al., 2001, 2002) Medina Tirado JIet al (2001) determined the effect of thermocycling on resistance against fracture and hardnes of 5 core materials. The thermocycling process reduced the fracture resistance and hardnes of core restoratives. Sampaio PC et al (2011) analyzed the effect of GIC as a liner on the dentin-resin interface after thermocycling. Lining with RMGIC resulted in lesser gap observation at the dentin-resin interface after thermocycling.
Tabatabaei M Het al (2013) evaluated the effect of simulating liquids and thermo-cycling on the elution of resin monomer from nano-filled resins in varied immersion time. Thermal shock and storage time are factors that also increased the release of monomer from the resin composites. Shanthala GSand Xavier MK (2013) reported the negative effect of thermocycling on the resistance and hardnes of the core restoratives. Korkmaz Yet al (2014) evaluated the effects of adhesives and thermo-cycling on bond strength of a nanocomposite to coronal and radicular dentine. Thermocycling did not affect the bonding of coronal dentine but affected root dentine.
To simulate clinical conditions, periodontal ligament simulation with polysiloxane impression material was done so that it can absorb masticatory loads and resist the compressive and tangential forces like a normal tooth. Previous researchers have stated high values without periodontal ligament simulation as they mimic ankylosed tooth rather than a normal tooth. Sterzenbach G et al (2011) stated that a polysiloxane combined with an acrylic resin is suitable for simulation of tooth mobility for invitro experiments. This prevents stress accumulation in a particular portion, and transfers the stress generated by loads uniformly along the root. Kumagae N et al (2012) reported that vinyl-polysiloxane in thin layers could simulate the periodontal ligament, and provides a cushioning effect mimicking the clinical situation, and avoids the re-inforcement of root externally by rigid acrylic. Thus, artificial ligament modifies mode of fracture, and has a role to play in fracture resistance studies.
Soares CJ et al(2005) evaluated the role of an embedment material and ligament simulation on fracture resistance of ETT. Root embedment material and ligament simulation has an effect on fracture resistance. In the study by Kern et al. (1993), a layer of gum resin applied on roots of teeth mimicked physiologic mobility during mastication and fracture testing experiment. When using these methods, the tooth mobility was 100 µm in horizontal frames and 65 µm in vertical frames. These values matched to physiologic mobility values mentioned by Mühlemann et al (1951).It is a proven fact that mobility of teeth is a main factor in the testing of resistance, against fracture and if a small amount of rotation permitted, failures are more relevant clinically (Kelly et al, 1995).
Point of application of load has also been mentioned as one of the major factors to achieve dependable lab results (Newman et al.,2003; Fokkinga et al, 2005; Hannig et al.,2005; Hayashi et al, 2005; Nagasiri and Chitmongkol suk, 2005).Furthermore, for application of load, area changes related to tooth anatomy & type. When anteriors are loaded, especially upper incisors, the load is applied on palatal surface in an area extending 2 to 3 millimetres from incisal edge. This was due to gnathological role of upper anterior teeth, designed to undergo non-axial forces and not axial loads.
The area of force coincides with the incisal guidance during functional movements. (Akkayan & Gulmez, 2002; Fokkinga et al., 2005; Nagasiri & Chitmongkolsuk, 2005). When the fracture resistance of ETT is tested mainly upper premolars, the force is applied in portions varying from center of occlusal region to supporting cusp (Hayashi et al., 2005).Such a choice was based on varied gnathological considerations to achieve the best simulation of occlusal forces during function. The load under compression was applied on occlusal surface of molar teeth at 90° to the long axis of the samples as used and recommended by authors.
Many researchers advocated loading perpendicular to cusp slopes, noticed fractures of teeth than restoratives (Hannig et al., 2005; Hayashi et al., 2005). Most fracture tests reported in studies are characterized by loading on supporting cusp at 130°-150° to longitudinal axis. (Hayashi et al., 2005).Such load application generated compression loads perpendicular to cusps. On the contrary, during mastication, the occlusion generated nonaxial forces resolved into their vector along side of cusps following the parallelogram of loads. Such a phenomenon was explained by different studies in which it was finally decided to load the samples in a parallel direction to longitudinal axis to distribute stresses more evenly between the remaining tooth tissues and the restorative material to mimick a normal physiological occlusion (Sorrentino et al., 2006; Salameh et al., 2006).
The mounted specimens were adjusted on the lower portion of a high accuracy universal testing machine. A 4 mm steel bar, mounted in the crossheads of the machine was used and compression load was applied parallel to the long axis of the sample at a cross-head speed of 1 mm/min until failure occurred. The 4 mm bar was applied at the restoration & occlusal surface of buccal & lingual surface for standardisation of loading points to all samples.
During fracture testing, the contact point of load bar & the occlusal surface of teeth might differ in samples. This could be the cause of large standard deviation in fracture testing. It was seen that the best technique to measure resistance against fracture was use of a cyclinder of defined diameter. Therefore, a cylinder bar of 4 millmeter in diameter was used to load samples and was able to achieve contact with filling and cuspal inclines. The failure load was noted as the maximum load during experiment.
Different authors have quoted different crosshead speeds ranging from 0.75 mm/min to 5 mm/min. The crosshead speed is an important parameter for static load application. Speed of application of load should mimick tooth functions just like mastication. High speed leads to non-homogenous stress in both tooth and restorative materials, whereas low speed is not representative of normal oral functions. On such research background, specimens have been loaded at a speeds ranging from 0.5 to 2 mm/min (Heydecke et al., 2002; Newman et al., 2003; Hannig et al., 2005).Authors have stated that at lower speed, more time is there for the stored energy to lead to crack growth and slower propagation of cracks. At a higher loading speed, a restorative needs more energy to propogate the crack growth. In simple words, a high speed leads to over-estimation. So, the present study involved load application a cross-head speed of 1 mm/min using a testing machine to simulate the occlusal loading in clinical conditions. The relatively slow crosshead speed was selected in order to produce an even force.
Specimens have been loaded with varied jigs, whose width is chosen according to anatomy of teeth, occlusal surface morphology and type of restoration, to reproduce the average width of opposing teeth. The jigs mentioned in research studies were different in shape and materials (Akkayan & Gulmez, 2002; Schwartz & Fransman, 2005).Usually round tips are chosen for homogeneous application of loads. Sharp tips develop concentration of stress areas. Most researchers have used ball tips whose diameter varied from 2.5 to 6 mm based on shape and anatomy (Heydecke et al., 2002; Hannig et al., 2005).Only a few researchers used cylindrical jigs (Newman et al., 2003).Furthermore, different materials have been used for load application – ceramics, hardened steel etc. (Akkayan & Gulmez, 2002).Based on the above literature support, a hardened steel load jig of spherical shape and 4 millimeter in diameter was selected in this study.
The data so obtained was tabulated and was subjected to analysis of variance (ANOVA) and unpaired student‘t’ test. The tests used in the study were applied at 95 percent level of confidence using statistical software (SPSS version 21).
According to our findings amongst non-reinforced samples, samples with access preparation only and no biomechanical preparation of root canal filled with temporary in pulp chamber space (control group 1) had significantly higher mean fracture resistance(1183 N) than all other samples. On comparison with research group 1(1083 N), the values were less than control group 1 but the difference was not statistically significant (P< 0.05) indicating that root canal treatment leads to decrease in fracture resistance of teeth but the effect is not that significant. Similar results were obtained in studies done by Schäfer Eet al (2007), Ribeiro F Cet al (2008), Karapinar Kazandag M et al (2009), Vishwanathan P K et al (2012),Bhat SS et al (2012), Nagpal A et al (2012) , Zamin Cet al (2012), Topçuoğlu HSet al (2013), Sandikci T and Kaptan RF et al (2014) and Mandava Jet al (2014);the intact samples without RCT were stronger than the groups with instrumentation and an obturation with AH plus and Guttapercha increased the fracture resistance of instrumented roots to some extent but not as much as the non-instrumented samples.
In the above mentioned studies as well, the effect of loss in fracture resistance of ETT was not very significant but definitely the strength decreases due to cumulative effect of factors such as the access preparation, slight loss of moisture, use of irrigants such as NaOCl, chelating agents such as EDTA, loss of dentine due to rotary root canal preparation to name a few as detailed in introduction. Though in the research group 1, root canal space was obturated and then obturation material was removed till the depth in the canal to be reinforced leaving practically less depth of obturated canal. This could be the reason that the effect of resin sealer reinforcement was not that significant due to a significant portion of obturation removed for intraradicular reinforcement and the effect of tooth loss and effect of other factors responsible for decrease in fracture resistance was comparatively more significant.