Invasive cervical root resorption(ICR) is an insidious and aggressive form of external cervical root resorption that damages periodontium. The predisposing factors of ICR are orthodontics, dental trauma, intracoronal bleaching, surgery involving cement enamel junction(CEJ), bruxism, delayed eruption, developmental defect and deep root scaling. According to Heithersay, ICR is classified as Class1: A small invasive resorption lesion near the cervical area with shallow penetration into the dentin. Class2: A well defined invasive resorptive lesion that has penetrated close to the coronal pulpal chamber but shows little or no extension into the radicular dentin. Class3: A deeper invasion of resorbing tissue in dentin, not only involving the coronal dentin but also extending atleast to the coronal third of the root. Class4: A large invasive resorptive process that has extended beyond the coronal third of the root canal.
Clinically ICR presents with a pink spot next to cervical margin and radiographically as a single resorption cavity which penetrates the tooth through small denuded areas. It spreads within the radicular dentin and does not directly penetrate the pulp due to protective predentin, but rather spread around the root canal. With time, it may involve pulp. The basic aim of the treatment of ICR is to inactivate all active resorbing tissue by nonsurgical or surgical approach and restoration with suitable adhesive restorative material. Various adhesive materials can be used for restoration of these ICR cavities such as CGIC(conventional glass ionomer cement) , RMGIC(resin modified glass ionomer cement), FC(flowable resin composite), amalgam, MTA, giomere. Endodontically treated teeth are weaker than sound teeth due to loss of eihter coronal or radicular tooth structure as a result of endodontic treatment that may result in fracture on functional loading. The objectiveof the study was to compare the fracture resistance of endodontically treated teeth with simulated ICR cavities when restored with different adhesive restorative materials namely CGIC, RMGIC, FC and Giomer. The null hypothesis tested is there is no difference in the fracture resistance of endodontically treated teeth with simulated ICR cavities restored with different adhesive restorative materials.

Sixty recently extracted (<3 months) human maxillary central incisors with single root and single canal similar in dimension, were obtained from mixed population. Teeth were examined under the dental operating microscope for pre-existing craze lines or cracks and immersed in 5% sodium hypochlorite (NaOCl) for removal of organic debris. The teeth were stored in saline solution at room temperature to prevent dehydration before and during experimental procedures. The selected teeth were randomly assigned to six groups (n=10).

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Group 1: Intact teeth (negative control)
Group 2: The samples were endodontically treated with simulated ICR cavity which was left unrestored (positive control)
The samples in groups 3,4,5,6 were endodontically treated with simulated ICR cavity restored with CGIC, RMGIC, FC and Giomer respectively.

The teeth in all the groups except the negative control group were endodontically treated. Access cavity was prepared and working length was determined by introducing a 10 no. K file inside the root canal until its tip was visible in the apical foramen and the file was then retracted 1mm short of the apex. To simulate the clinical condition apex was sealed with sticky wax. The canal was enlarged to MAF size 40 K file followed by step back preparation to size 80 Kfile. Root canals were irrigated with 10 ml of 5% NaOCl ((Prime dental products Pvt Ltd, India) during instrumentation. Smear layer was removed with 1 ml of 17% ethylenediamineteteraacetic acid(EDTA) (Dentwash, Prime Dental Products, India) for one minute followed by 5ml of 5% NaOCl. The irrigation was done 5 mm from the working length agitated with irrisafe (Satelec Acteon group, India) at a power setting of 5 for 20 seconds. Canals were irrigated with 3ml of sterile saline (Althea Pharma Private Ltd, India) following the final rinse with NaOCl.
Teeth in groups 3 to 6 were obturated with 2% gutta percha cones (Dentsply, India Pvt Ltd) and AH plus sealer (Dentsply, India Pvt Ltd) by lateral compaction technique and the access cavity were sealed with Cavit(3M ESPE dental products,US). Teeth were stored for 72 hours at 370C and 100% relative humidity in an incubator (Lab hosp, India) to allow the sealer to set.

Preparation of simulated ICR cavity
Resorptive cavity was prepared on teeth belonging to groups 2 to 6. The point of intersection of long axis of the tooth and CEJ was marked using a surveyor (Bego, Paraflex, US). A circle of 3mm diameter and 2mm depth was drawn so that half the circle was occlusal to the CEJ on the crown and other half on the root surface cervical to the CEJ( Fig 1) using a customized template with a round diamond point (Mani, India).

Restoration of simulated ICR cavity
The cervical cavity of samples of Groups 3,4,5 and 6 was rinsed with saline. Further, the internal wall of the cavity was gently rubbed with 90% aqueous solution of trichloroacetic acid (Merck Specialties Private limited, Mumbai) for 2 minutes. The cavity was rinsed, dried and restored with the respective restorative material as assigned to the groups in accordance to the manufacturers’ instructons.

Group 3 : Conditioning with GC cavity conditioner (GC Corporation, Tokyo, Japan) followed by placement of CGIC which was adapted with mylar matrix till it attained initial set. The finished restoration was protected by a layer of light cured dentin bonding agent (3M ESPE Adper Single Bond 2).
Group 4 : Conditioning with GC cavity conditioner (GC Corporation, Tokyo, Japan) followed by placement of RMGIC (GC India Dental Pvt Ltd) which was adapted with mylar matrix till it attained initial set. The finished restoration was protected by a layer of light cured dentin bonding agent (3M ESPE Adper Single Bond 2).
Group 5: Dentin and enamel walls were etched with 37% phosphoric acid (Scotchbond Etchant, 3M ESPE) thereafter rinsed for 10 seconds. After blotting drying, 2-3 consecutive coats of adhesive was applied to etched enamel and dentin for 15 seconds and light cured for 20 seconds. Flowable composite(3M ESPE Filtek Z350 XT) was expressed into the simulated cavity, contoured and cured.

Group 6 : Application of 2-3 consecutive coats of adhesive was applied to enamel and dentin for 15 seconds and light cured for 20 seconds. Incremental placement of giomer restoration (Shofu Inc. Kyoto Japan) done followed by Light curing of each increment was done for 20 sec each increment.

All samples were then allowed to set for 24 hours at 37?C and 100% humidity in light proof containers. The teeth were mounted in acrylic blocks measuring 30mm in height and 25mm in diameter perpendicularly in the center of acrylic resin using a surveyor with CEJ 2mm coronal to the resin surface. The samples thus prepared were stored in distilled water at room temperature for 24 hours before subjecting to fracture testing. The specimens were submitted to a compressive load in a universal testing machine with the jig that allowed loading at an angle of 1350 to long axis of tooth. The load was applied 3mm from the incisal edge on the palatal surface at a constant crosshead speed 0.5mm/min with a 45o beveled metallic rod until the first sharp drop of the load was observed on the displacement-load curve indicating a sudden decrease in specimen’s resistance to compressive loading (failure load) and was recorded in Newtons (N).

Statistical Analysis
The data obtained was normally distributed and parametric tests were used to compare the means between different groups. Further pairwise comparison for the significance for any two groups was done applying Tukey’s multiple comparison test. The level of significance was set at 5% and p value ; 0.05 was considered statistically significant.

Table 1 shows the mean, standard deviation, minimum and maximum load to failure of all the 6 groups. The highest mean fracture resistance was shown by group1 followed by groups 6,5,4,3and2 respectively. The ANOVA test depicted significant difference between the mean fracture load of six groups when compared(table no. 2{a}). Further pair wise comparison using PostĀ HocĀ Tests(table no.2{b}) showed statistically significant difference with higher fracture resistance of group1 when compared to groups 2,3,4. Samples of group2 showed significantly lower load to failure when compared with group 3,4,5,6. The fracture resistance of groups 3,4,5,6 were statistically insignificant in the pair wise test. Also, when comparing mean values of group 1 with group 5,6 (table no.2{b}) it was statistically not significant. All specimens underwent oblique horizontal crown-root fracture through cervical portion of the tooth with (38.3%) favorable (restorable) and (61.7%) catastrophic (non-restorable) fracture.
Maxillary incisors are the most commonly affected teeth by ICR hence recently extracted maxillary central incisors were selected for the study. Moreover, we have considered Heithersay’s class 3 resorption, therefore the simulated ICR cavity was prepared in the cervical third of the tooth. Often times the teeth with class 3 lesions may also be necrosed necessitating root canal treatment . Hence sample teeth were endodontically treated. treatment of ICR may involve topical application of a 90% aqueous solution of trichloroacetic acid (TCA)to the resorptive tissue to enable curettage, endodontic treatment where necessary and restoration with suitable restorative material.Hence to simulate the same situation the sample cavities too were rubbed with TCA.

Large variety of materials glass ionomer and resin composite based are available for restoring cervical cavities especially non carious cervical lesions as they shows satisfactory retention in cervical dentin (sclerosed dentin), provides fluoride release, inhibits secondary caries and optimum abrasion resistance, color match, marginal discoloration and adaptation. Tyas et al, recommended RMGIC over CGIC and composite resins. It is the first preference for restoration of non-carious cervical lesions and second option is RMGIC/GIC liner base laminated with resin composite. According to Van Meerbeek et al, the low modulus of these materials allows deformation and absorbion of induced stresses. Thus helping to resist polymerization shrinkage stress and favorably dissipate the stresses produced by thermal variations, water absorption and occlusal loads across the interface. Also it allows the restoration to flex with tooth rather than debond. Therefore the simulated ICR cavities were restored with CGIC, RMGIC, FC and Giomer.
CGIC bond chemically to tooth structure and their clinical performance seem to be superior in terms of retention rates where bonding to cervical sclerosed dentin is required. The RMGIC are hybrid materials that retain a significant acid/base reaction as part of their overall curing process with improved setting characteristics over CGIC. They show rapid development of early strength and are less sensitive to the effect of moisture.
Flowable composites are low-viscosity resin based restorative materials that differ from conventional composites in filler load and resin content. The reduction in filler concentration has direct influence on modulus of elasticity which causes dissipation of contraction stresses during polymerization and coefficient of thermal expansion close to the tooth structure, which further increases marginal adaptation. The fluoride releasing resin materials with “Pre Reacted Glass” or PRG named as Giomer have properties of both glass-ionomers (fluoride release, fluoride recharge) and resin composites (excellent esthetics, easy polishability, superior surface finish and biocompatibility). Giomer has a cross linked polymer matrices, the compressive strength and toughness of the material is higher than the gel network formed by acid-base reaction in GIC.
Following restoration placement specially designed jig that oriented the long axis of the resin block and tooth at 45 degree to the horizontal plane was prepared. This jig allowed the force to be applied at 135 degree angulation to the long axis of the tooth on the palatal surface which is an approximate value of the interincisal angle formed between maxillary and mandibular incisors as in Class I occlusions.

In this study, the intact teeth had the highest resistance to failure (table no. 1), which shows that endodontic treatment and the ICR cavity probably weakened the tooth. Moreover, among the experimental groups higher resistance to failure was shown by giomer group. The unique SPRG filler technology, cross linked polymer matrices and its modulus of elasticity could have contributed to obtain superior results in the teeth restored with giomer material. However there was no statistically significant difference in the load to failure when the ICR cavities where restored with different restorative materials (table no.2{b}). Hence the null hypothesis was not rejected.
Majority of samples showed catastrophic oblique fractures as fracture line apical to the CEJ (unfavorable- 62%). The maximum tensile and compressive stresses are noticed at the CEJ level, both on the buccal and palatal aspects of the root . Such behavior suggests that the incisors bend in palatal to buccal direction and that the increased stress state at CEJ is due to the differential rigidity between the crown structure (higher rigidity) and the root structure (lower rigidity.). Stress progressively decreases from the outer to the inner part of the root and from the CEJ towards the incisal margin of the crown as well. In group 3 samples GIC has modulus of elasticity closer to dentin as compared to that of enamel due to which the restoration flexes with dentin, better absorbing the induced stresses at dentinal end rather than the enamel end. This makes the enamel end weaker and prone site for the fracture line to pass through obtaining higher favorable fractures (60%) in group 3. Also it was observed that none of the restorative materials debonded spontaneously from the test samples during fracture testing as observed in the study by Xie H et al.
Root factures is one of the common causes of failure for teeth with ICR because of the weakened cervical coronal and radicular dentin as a result of resorption. Hence, the tooth has to be adequately reinforced. In many instances it is not possible to obtain a ferrule in the labial aspect , there by making it difficult o place a full coverage crown. As per the results of this study the specimens restored with all the four adhesive restorative materials showed comparable fracture resistance values which are higher than the fracture loads subjected to in normal physiological state intraorally. Moreover, specimens restored with Giomer showed fracture resistance values closest to that of intact teeth. The experiments were carried out in invitro conditions and circular cavities were prepared in order to standardize the preparations which may not confirm to clinical presentation. Moreover periodontal simulation was not done in the study due to the direction of load application. As there were no previous studies carried out in simulated invasive resorption cavities there could have no comparisons made. These results need to be further confirmed by in-vitro studies and long term clinical observations in order to establish a suitable material to be used in ICR cavities.

Within the limitations of this invitro study, following conclusions were drawn:
Preparation of ICR cavities weakened the teeth.
Adhesive restorative materials placed in the simulated ICR cavities enhanced the fracture resistance.

The mean fracture resistance of endodontically treated teeth with simulated ICR cavities restored with Giomer was found to be the highest among the experimental groups followed by FC group, by RMGIC and CGIC groups.