On comparison of groups in which different variations of loss of continuity of PCD in the form of removal of walls were studied, it was observed that research group 3 presented with lowest values followed by groups 7,9 and 5 indicating that mesial wall removal had more deleterious effect on strength of the ETT followed by buccal and lingual wall removal. The least impact was observed for removal of distal wall amongst all these groups. On statistical analysis, there was statistically significant difference observed between groups 3 & 5. No statistically significant difference was observed on comparison of all other groups. This means removal of any of the walls of PCD except distal has same delitirious effect on loss of fracture resistance.
On comparison of groups 3,5,7 and 9 with group 1, there was statistically significant difference observed indicating that loss of any of the walls of PCD leads to a significant loss of fracture resistance. There was a decrease of fracture resistance by 40-50 percent with loss of the walls.
Our results are in agreement with the research done by Arunpraditkul S et al (2009) who evaluated the fracture resistance of ETT between those with 4 walls and those with 3 walls of remaining tooth structure of crown and the role of site of missing wall coronally. Teeth with 4 walls of remaining coronal dentine had higher fracture resistance than samples with 3 walls. The site of missing wall did not affect the resistance of ETT. There was a decrease in fracture resistance by 35-50 percent depending on the loss of wall. Ng CC et al (2006) investigated the fracture resistance of ETT when remaining tooth structure axially was limited to ½ the perimeter of tooth preparation. For restored ETT which do not have enough tooth structure all around between the core & the finish line, location of residual crown affected their resistance to fracture.
Bassir M M et al (2013) evaluated the fracture resistance and mode of fracture of ETT with varied amount of residual tooth structure. MO preparation decreased the fracture resistance of teeth. Soares PV et al (2008) evaluated effect of endodontic-restorative treatment on the resistance against fracture. In ETT with access and proximal cavities, the fracture resistance was decreased by approx. 55 percent. Shafiei F et al (2011) observed that preparation on both the surfaces reduced resistance to fracture by 75 percent as compared to normal teeth. Cobankara FK et al (2008) compared the fracture resistance of ETT with proximal cavities restored with varied techniques. Although all groups were stronger than the preparation only group, none of techniques were able to re-restore the fracture resistance loss from proximal tooth preparation.
On comparison of groups in which different variations of loss of tooth structure in the form of removal of internal and external dentine from proximal walls was studied, it was observed that research group 11 presented with lowest values followed by groups 17, 13 and 15 indicating that removal of external PCD had more deleterious effect on strength of the ETT followed by inside mesial and distal walls removal. This was followed by mesial wall removal. The least impact was observed for removal of inside distal wall amongst all these groups. On statistical analysis, there was no statistically significant difference observed between the groups. This means removal of structure from any of the inside dentine walls of PCD and external dentine walls in the form of over preparation for crown has same effect on loss of fracture resistance. On comparison of groups 11,13,15 and 17 with group 1,there was statistically significant difference observed indicating that internal loss from any of the walls of PCD leads to a significant loss of fracture resistance. There was a decrease of fracture resistance by 20-30 percent with loss of external or internal PCD which was less as compared to the loss of walls. There was also a statistically significant difference between groups 3 and 13; groups 5 and 15 indicating that every small fraction of loss of PCD leads to decreased fracture resistance with loss of wall causing the main drop in fracture resistance.
Though there are no direct studies published in the literature to compare loss of external or internal PCD. Our results for the drop in fracture resistance values with loss of tooth structure (PCD) are in agreement with studies done by various researchers as mentioned below.Seow L L et al (2005) suggested a correlation between the residual tooth structure and the fracture resistance following varied tooth preparations used in the restoration of the ETT. Beuer F et al (2008) evaluated the effects of varied preparations on the fracture resistance of ETT. A shoulder preparation reduced the fracture resistance more than the chamfer preparation. Ohlmann B et al (2008) in a study concluded that minimal preparations lead to an acceptable level of fracture resistance for posterior composite crowns. Nissan J et al (2008) assessed the resistance to fracture of crowned ETT under simulated mastication load, while saving varied amount of residual tooth structure. Residual coronal structure most influenced the resistance of crowned ETT.
Taha N A et al (2011) compared fracture mechanics of ETT with varied preparation designs and composite restorations. Unrestored teeth were weaker with extensive preparation designs. Access preparation in the occlusal floor did not significantly reduced strength in comparison to a proximal preparation. Axial wall loss weakened ETT considerably. Scotti N et al (2013) in their study observed that the remaining wall thickness could be a good parameter in the choice of restorations for ETT. In samples with preparation wall thickness >2 mm, direct composite restorations along with a fibre post lead to a comparable resistance to fracture. In cases with remaining wall thickness < 2 mm, cusp coverage with or without a fibre post led to a fracture resistance which was satisfactory.
Quality and quantity of the residual tooth structure are the most important factors which affect fracture resistance; however, sound marginal ridges form a continuous circle of tooth tissue in unaltered teeth which prevents fracture of cusps. Dentine is a suitable base for restorations, and its strength depends on quality and continuity of tooth form. This means decreasing the amount of residual sound dentine decreases the support needed for restoration.
Previous researchers have shown that fracture resistance of a posterior ETT with access preparation is less than unaltered tooth. After proximal preparation, deflection & strain of cusps are about 3 times more than that of unaltered teeth, and stiffness decreases by 20 percent. As suggested by Soares P V et al (2008) who studied strategies for ETT to decrease strains and improvise stress distribution under masticatory forces with analytic software based on finite elements revealed that tooth tissue removal and type of restorative altered the stress patterns with higher concentration of stress within the residual tooth tissue. Further, they suggested that greater the loss of tooth tissue, more is the stress development in the residual tooth tissue left. Taha NA et al (2012) also stated that access preparation and proximally located preparations increase cusp deflection pattens leading to reduced fracture resistance of ETT.
In our study, the most probable reasons could be overall loss of tooth structure, loss in continuity of surrounding walls and the access preparation being located mesially potentiated any loss located near to this. This could be the reason that mesial, buccal and lingual walls loss caused a little more decrease in fracture resistance as compared to distal wall as they were located more near to the access preparation with lesser amount of PCD present in that area.
On comparison of groups with reinforcement, it was observed that research group 1 samples presented with more mean fracture resistance values than research group 2 and control group 1 indicating that statistically significant reinforcement of PCD was observed with the use of nanoionomer. Further, on comparison of groups with loss of walls (group 2,4,6,8,10); group 4 (mesial wall missing) presented with slightly lesser values than other groups. All other groups presented with similar values but statistically no significant difference was observed amongst all groups (2,4,6,8,10) indicating that reinforcement was more or less independent of nature of loss of walls of PCD.
On comparing groups 11 and 12, it was found that though there was significant reinforcement but the values of group 12 were lower than groups (4,6,8,10) indicating that the loss of external PCD due to overpreparation of samples cannot be replaced back as this was a nonrestorable loss. This could have led to lower values after reinforcement for these groups as compared to other groups where the lost structure was replaced back with the nano-ionomer. In this group, though the intraorifice space and access preparation were restored back but PCD lost due to over preparation was non restorable leading to compromised reinforcement.
On comparison of groups (2,12,14,16,18),it was observed that groups 2,14,16,18 presented with almost similar values and there was no statistically significant difference observed between those groups. Group 12 presented with lesser values and the comparison was statistically significant as well to other groups when compared. Thus, in this comparison as well, it was clear that the reinforcement was independent of site of loss of internal PCD.
Further, on comparison of groups 4 with 14;groups 6 with 16, it was found that the reinforcement of PCD was even comparable in groups with minimal loss and substantial loss (loss of wall) indicating a substantial potential of this reinforcement with nano-ionomer.
The results clearly indicated that the use of nano-ionomer for reinforcement of the 8 mm zone of PCD was effective in increasing fracture resistance of samples. The role of GIC in the reinforcement of ETT is not new. Taleghani M and Morgan RW (1987) in a review of the literature on restoration of ETT discussed the role of GIC as an excellent re-constructive material. Homsy F et al (2015) discussed the positive role of filling the pulp chamber with GIC until the floor of pulp for reinforcement before placing final restorations. Taha NA et al (2011) in a similar study found that GIC core significantly reinforced ETT. Endodontic access preparations confined within the occlusal surface did not affect strength in comparison to MOD preparation. Loss of walls axially weakened teeth considerably. Restorations increased the strength of prepared ETT without axial walls. ETT with a GIC core were not significantly weaker than sound teeth.
Shah P et al (2012) evaluated the effect of RCT on the fracture resistance and reinforcing ability of 3 different core materials - IRM, amalgam and GIC. Results showed a reduction in the fracture resistance on access preparation (1/3) and out of 3 core materials GIC was shown to be the best giving the highest fracture resistance followed by amalgam and IRM. Ferrier S et al (2008) compared the fracture resistance of corono-radicular restorations with different restorative materials: amalgam, composite and RMGIC. The results demonstrated that RMGIC restored ETT were stronger than un-restored teeth and failed at loads in excess of forces as seen in normal mastication. RMGIC tended to fail in a less catastrophic manner than other materials.
Yardley RM et al (1990) suggested a method for constructing cores at one visit using GICs. Taha NA et al (2012) evaluated the open lamination technique using GICs with a low shrink composite for restoring ETT in extensive MOD cavities plus access preparations. The two lamination groups displayed better marginal sealing than composite alone, but sealing with traditional GIC deteriorated a lot after thermocycling. Lamination of restorations had extra beneficial effect in terms of decreasing cuspal deflection & improved seal, with better strength. Foley J et al (1997) compared the strength of a reinforced GIC and a LC GIC used as an alternative to amalgam in core fabrication to restore ETT. The teeth were restored with any of these materials, a cermet, RMGIC or amalgam.
Extension of core material into canal orifice increased the fracture resistance. Shafiei F et al (2014) observed that using nanoionomer base under composite had a positive effect on fracture resistance and pattern of ETT. Cabrales Salgado R et al (2012) studied the role of the adhesive restoratives to improve the fracture resistance of ETT with incomplete root formation. Nanoionomer was quite instrumental in improving fracture resistance. Aboobaker S et al (2015) evaluated and compared the fracture resistance of ETT obturated with guttapercha using amalgam, LCGIC and flowable composite as orifice barriers (coronal 3 millimeter).Composite and LCGIC can be used as intra-orifice barrier to improve fracture resistance in ETT. Nagas E et al (2010) investigated & compared the reinforcing potential of 3 different intra-orifice barriers 3 mm mineral trioxide aggregate, RMGIC and FRC placed over canals restored with guttapercha or Resilon.The use of GIC/FRC improvised the fracture resistance. MTA did not display any reinforcement as an intra-orifice barrier.
Although the role of dentine as a suitable solid base for dental restorations is undebatable, its structural strength depends on the quality and integrity of tooth anatomical form. It means that, reducing the amount of remaining sound dentine reduces the support provided for the restoration. The advent of newer adhesive materials with improved properties have revolutionised the concept of the reinforcement potential of ETT. Recently, a material with adhesion of GIC and strength of composite has emerged as a promising material. This nanotechnology based product is a revolutionary material which led to a significant reinforcement in our study. The concept of intraorifice barrier monoblock reinforcement along with intracoronal reinforcement led to a significant improvement in fracture mechanics of PCD. The most probable reasons for this reinforcement can be attributed to increased bulk of material (8 mm), monoblock formation, chemical bonding and the role of heavy loading of nano particles. The formation of a dual monoblock in our study that is primary monoblock with nano-ionomer in reinforced zone and secondary monoblock with AHplus in obturated root must have played a major role in our study. Nano-ionomer has better chemical bonding to dentine as evidenced by Abd El Halim et al(2011).This chemical bonding of nano-ionomer must have further potentiated the monoblock reinforcement. and the nano filler loading provided an edge due to the strength.
Modulus of elasticity of nanoionomer matches to that of dentine. Secondly, the filler loading (69 percent by weight) with nanofilled particles must have contributed to increased strength values (Abd El Halim S and Zaki D, 2011). Similar results were obtained by Gupta SK et al (2012) who reported that due to higher filler loading in nRMGIC resulted in lower polymerisation shrinkage and lower coefficient of thermal expansion, thus improved bonding to tooth tissue. Kolahduzan AR et al (2014) demonstrated formation of a physical barrier by GIC and also chemical bonding with dentinal walls. Darvell B et al (2008) also demonstrated increased bond strength of nRMGICs in comparison to conventional GICs to tooth tissue.
In some more recent studies conducted by Arora V et al (2015),the reinforcement of PCD was quite significant with the use of nano-ionomer. On comparison with silorane composites, the results were insignificant indicating that nano-ionomers behave like composite in causing reinforcement of ETT retaining the chemical adhesive potential as well.
Though there are some of the limitations that cannot be avoided in in vitro studies eg. compositional and structural differences of radicular and coronal dentine which varies amongst individuals, age group and region. In our study, we standardized the access and biomechanical preparations. The sample was a homogenous unit of PCD consisting of root and crown dentine unlike most of the studies in literature which are based on testing of root specimens after removal of coronal portion of teeth for standardization. The conditions of this study are not completely same to the real intra-oral conditions, although a committed attempt was made to best simulate the oral environment, thus, results are difficult to apply directly as such in the clinical practice. The application of load was only in one direction and one specific point, which could not mimick varied ways of occlusal force exertion, as well as para-functional forces. Therefore, further studies on longevity of restorative techniques simulating the clinical conditions and the possible inclusion of para-functional forces should be conducted. Since, only very few studies have been conducted.
Regarding the clinical importance of the study, this study highlights the role of tooth tissue conservation as a major criteria for long term prognosis and sets few clinical guidelines. Based on the results of this study, even small loss of tooth tissue especially PCD can lead to a major loss in terms of fracture resistance. The clinician has to take into account the nature of loss due to carious lesion or trauma before preparation of an access so that the size and shape of access preparation can be modified as per the tooth tissue present. The loss of tooth tissue on mesial side or loss of mesial wall be supplemented with more structure on distal side and vica versa. Similarly, the buccal loss be supplemented with preservation of tooth tissue on lingual and vica versa. As every mm of tooth tissue lost leads a big drop in resistance to fracture, a conservative approach should be followed while preparing an access.
The preparation of root canal should be done as conservatively as practical. Access preparation should be based on conservative concept (CEC) and should be as practical as possible. The preparation of canal should be done with those rotary instruments which generate less cracks in root canal dentine. The taper should not be more and each case should be evaluated separately. The chemicals used for irrigation such as sodium hypochlorite, EDTA etc. should be used with caution and scientifically recommended protocol be followed for time and concentration. Newer Technologies such as lasers and PIPS etc should be used for canal disinfection. Obturation should be done in such a manner to avoid excessive forces in the canal. Post endodontic restoration should be done immediately if possible. Adhesive sealers and obturating materials should be used. The use of drills should be done with caution and post-core option should be carefully weighed before advocating. The preparation for crown should be conservative as practical and adhesives should be used for luting for added reinforcement. Since, external removal of PCD is more or less not replacable, so in cases where the preparation of teeth involving external PCD is inavoidable, internal reinforcement should be done.
Lastly, the reinforcement protocol as discussed in this study should be used as a standard protocol for all ETT. The access preparation design should take into account before hand the residual tooth tissue and possible removal of tooth tissue for post-endodontic restoration.
Since, till date there are only two studies published regarding PCD, this study can be a milestone in this direction. The multiple parameters used in this study also make this study more clinically relevant. This study has given a new method for measurement of PCD for in vitro specimens which could be a useful parameter for further invitro studies to follow for standardisation. This is the first study to take into consideration PCD rather than the root as used in various studies. This study is also one of its kind for the use of nanoionomer for intraradicular reinforcement of samples. Till date, no other study has mentioned about the role of this new material for radicular reinforcement.
Regarding the future scope, studies involving more materials for reinforcement and their comparative assessments are suggested. Clinical studies taking quantitative and qualitative aspects of PCD into consideration need to be conducted. Long term clinical studies of reinforced teeth will also help to increase understanding regarding this concept of reinforcement with nanoionomers. Finite element analysis studies can also help to understand the concentration of forces before and after reinforcement.
Further, variations with conservative endodontic access preparation can also be studied in reference to PCD. The effect of biomimetic materials for reinforcement can also be studied in future. The role of PCD in anterior teeth also needs some more elaborate research to be conducted. Further, the loss in height of PCD, loss of multiple walls, post space preparation and role of crown placement also needs further elaborate research.
Under the limitations of the study, it has been concluded that fracture resistance decreases after access cavity and biomechanical preparation due to loss of PCD but the effect is not that significant. The loss of PCD in the form of any of the walls of the preparation leads to a significant decrease in the fracture resistance. In mandibular molars, any loss of PCD in mesial wall causes more decrease in fracture resistance whereas opposite is true for distal wall. Nanoionomer has got significant reinforcement potential. The placement of nanoionomer reinforces PCD irrespective of the location and nature of the loss.