The effect of different temporary filling materials and pre-endodontic build-up on fracture resistance of upper premolar teeth: an in-vitro study
A vertical root fracture (VRF) is a longitudinal fracture involving the cementum, dentin, and root canal system. Depending on the cause, they may develop in a corona-apical or apico-coronal direction . The presence of pre-existing cracks in the tooth, changes in the biomechanical properties of the root dentin, and unfavorable occlusal arrangement are etiological factors that can cause VRF . Additionally, anatomical factors such as root canal morphology may predispose some teeth to VRFs . The impact of access cavity design and root canal instrumentation protocols should also be considered. Their effect on the fracture resistance (FR) of root-filled teeth has been investigated but not clearly confirmed . The prognosis of VRF treatment is quite low. It is considered one of the most undesirable complications of root canal treatment, which often requires tooth extraction . Therefore, preventing VRFs is critical; conservative endodontic procedures should be performed along with appropriate treatment methods, and temporary filling (TF) materials should be used. Since root canal treatment is sometimes completed in more than one session, a TF material that is easy to process, resistant to chewing forces, and chemically and physically stable should be placed in the endodontic access cavity . In addition, temporary closure of the access cavity plays an important role in maintaining the bactericidal effect of the intracanal medicament . There are TF materials with different contents such as zinc oxide, zinc sulfate, and calcium sulfate. Nowadays, Cavit G is the most frequently used among these materials . Cavit G (3 M ESPE, St Paul, MN, USA): It is a temporary filling material with superior sealing ability compared to Cavit (which does not contain resin) thanks to its various resin content. The excellent edge sealing property of Cavit G can be explained by the fact that it is a hygroscopic material that allows it to adapt to the dentin walls . Today, colored and light-curing TF materials that clarify the preparation boundaries are produced in order to protect the remaining tooth structure during the removal of TF materials. Voco Clip F (VOCO, Germany) is a urethane dimethacrylate-based, lightweight polymerized composite temporary filling material . Thanks to its high elasticity and surface hardness, its adhesion ability is quite good. It provides effective protection against the development of secondary caries with the fluoride mineral it contains . There are many studies investigating the interaction of different variables with crown or crown-root fractures that may occur during and after endodontic treatment . However, there are few studies evaluating the effect of TF materials on the FR of teeth [ – ]. In almost all cases of endodontic treatment, there is no remaining dentin on the level of the gingival margin, because in these cases, the crown is often completely restored. To achieve absolute isolation with a rubber dam, pre-endodontic build-up is often performed. In many cases where root canal treatment is applied, the tooth cannot be permanently restored in the same session after root canal filling. This may cause re-infection and a decrease in the strength of the tooth . Therefore, it is indispensable to place a TF with sufficient properties to prevent marginal microleakage and increase the FR of the tooth. When the literature is examined, there is no study evaluating the effect of pre-endodontic build-up on the FR of teeth. The current study aims to compare the effect of two different TF materials that are placed in the teeth that have undergone endodontic treatment on the FR of the teeth with pre-endodontic build-up. The null hypotheses of this research are as follows: (1) There will be no difference between the FR of the teeth with mesio-occluso-distal (MOD) cavities and the ones that have undergone a pre-endodontic build-up procedure, and (2) the type of TF material will not affect the FR of teeth. 

Teeth selection and preparation of the cavities The study protocol was approved by the institutional ethics committee for non-interventional clinical research at Erciyes University (2023/531). The sample size calculation was based on a previous study , which indicated that the sample size for each group should be a minimum of twelve (power = 0.92, effect size = 0.842, significance level of a = 0.05). Finally, 20 teeth in each experimental group were selected. After selecting 90 upper premolars that had recently been extracted due to periodontal and orthodontic reasons, they were stored in saline solution until used in the study. Calculus and tissue residues on the teeth were cleaned using periodontal curettes. Teeth were selected using an operating microscope (Zeiss, Oberkochen, Germany) at x24 magnification, and teeth with open apex, caries, cracks, or root fractures were excluded from the study. Periapical radiographs were taken before the procedure to confirm whether the examined teeth had undergone root canal treatment and whether there was resorption or calcification in the canals. The dimensions of the teeth in the current study were evaluated using a digital caliper, and teeth with average mesiodistal (7.20 ± 0.88 mm) and buccolingual (9.49 ± 0.62 mm) sizes were selected. Five teeth were randomly selected to form the negative control group (group 1, unprepared). The endodontic access cavities of all remaining teeth were prepared with a carbide bur (Dentsply Sirona, Ballaigues, Switzerland) under cooling water. The buccopalatal width of the occlusal surface of the MOD cavity is two-thirds of the intercuspal width, the buccopalatal width of the proximal part was prepared to be two-thirds of the buccopalatal width of the crown. Pre-endodontic build-up and temporary filling The working length of the teeth, except for the 5 randomly selected teeth that constituted the negative control group (group 1, unprepared), was measured with a #10 K-file (Dentsply Sirona, Ballaigues, Switzerland) until it was seen from the apical opening of the root. The working length was adjusted to be 1 mm shorter than the measured length. The canals were prepared using OneCurve (OC; Coltene Micro Méga, Besançon, France) (25/0.6) files. During preparation, the canals were irrigated with 20 milliliters (ml) of 5.25% sodium hypochlorite. Final irrigation was performed with 20 ml of 17% ethylenediaminetetraacetic acid solution (1 min) and 10 ml of distilled water. For irrigation agitation, Ultra X (Changzhou Sifary Medical Technology Co.Ltd., Changzhou, China) ultrasonic device was used and the canals were dried with paper points (Dentsply Sirona). Five teeth were randomly selected as the positive control group (group 2, prepared, no temporary material placed). After canal preparation of the remaining 80 teeth was made, calcium hydroxide paste (UltraCal XS; Ultradent, South Jordan, UT) was placed to simulate clinical conditions, and the canal orifice was covered with Teflon tape. For the TF application, 80 teeth were randomly divided into 4 groups ( n = 20). In Group 3, TF was performed with Cavit G (3 M ESPE) on the teeth with MOD cavity preparation. Tofflemire matrix band (Kerr GmbH, Rastatt, Germany) was placed on the teeth with MOD cavity in Group 4 and built up with nanohybrid resin composite (Tokuyama Estelite Sigma Quick, Tokuyama, Japan). The TF was performed with Cavit G (3 M ESPE). In Group 5, TF was made with Voco Clip F (VOCO, Germany) on the teeth with MOD cavity preparation, and an LED light device (Valo Cordless, Ultradent, Utah, USA) was used to polymerize the filling material for 40 s to ensure hardening. Tofflemire matrix band (Kerr GmbH, Rastatt, Germany) was placed on the teeth with MOD cavity in Group 6 and built up with nanohybrid resin composite (Tokuyama Estelite Sigma Quick, Tokuyama, Japan). TF was made with Voco Clip F (VOCO) and an LED light device (Valo Cordless) was applied to polymerize the filling for 40 s to ensure hardening. After the matrix band was removed, all teeth were stored at 37 °C and 100% humidity for 1 week. Fracture resistance The prepared samples were embedded vertically in self-hardening acrylic resin 2 mm below the cemento-enamel junction. Fracture strength values were measured using a universal testing machine (Instron Corp, Canton, MA, USA). A stainless-steel ball (6 mm in diameter) placed in the testing machine was centered on the occlusal surface of the restoration and compressive force was applied vertically to the restoration at a rate of 1 mm/min until fracture occurred. The moment of a sudden decrease in force that was seen on the screen of the testing machine was defined and saved as the maximum force. Also, it was calculated in Newtons. Types of fracture: It was classified as “restorable” (Fig. ) above the acrylic level and “unrestorable” (Fig. ) when the fracture extended below the cemento-enamel junction. Statistical analysis The Kolmogorov-Smirnov test was used for normality testing and these values ​​showed a normal distribution. A 2-way analysis of variance with the Tukey post hoc test was used for multiple comparisons. The statistical significance level was determined as p < 0.05. 

The study protocol was approved by the institutional ethics committee for non-interventional clinical research at Erciyes University (2023/531). The sample size calculation was based on a previous study , which indicated that the sample size for each group should be a minimum of twelve (power = 0.92, effect size = 0.842, significance level of a = 0.05). Finally, 20 teeth in each experimental group were selected. After selecting 90 upper premolars that had recently been extracted due to periodontal and orthodontic reasons, they were stored in saline solution until used in the study. Calculus and tissue residues on the teeth were cleaned using periodontal curettes. Teeth were selected using an operating microscope (Zeiss, Oberkochen, Germany) at x24 magnification, and teeth with open apex, caries, cracks, or root fractures were excluded from the study. Periapical radiographs were taken before the procedure to confirm whether the examined teeth had undergone root canal treatment and whether there was resorption or calcification in the canals. The dimensions of the teeth in the current study were evaluated using a digital caliper, and teeth with average mesiodistal (7.20 ± 0.88 mm) and buccolingual (9.49 ± 0.62 mm) sizes were selected. Five teeth were randomly selected to form the negative control group (group 1, unprepared). The endodontic access cavities of all remaining teeth were prepared with a carbide bur (Dentsply Sirona, Ballaigues, Switzerland) under cooling water. The buccopalatal width of the occlusal surface of the MOD cavity is two-thirds of the intercuspal width, the buccopalatal width of the proximal part was prepared to be two-thirds of the buccopalatal width of the crown. 

The working length of the teeth, except for the 5 randomly selected teeth that constituted the negative control group (group 1, unprepared), was measured with a #10 K-file (Dentsply Sirona, Ballaigues, Switzerland) until it was seen from the apical opening of the root. The working length was adjusted to be 1 mm shorter than the measured length. The canals were prepared using OneCurve (OC; Coltene Micro Méga, Besançon, France) (25/0.6) files. During preparation, the canals were irrigated with 20 milliliters (ml) of 5.25% sodium hypochlorite. Final irrigation was performed with 20 ml of 17% ethylenediaminetetraacetic acid solution (1 min) and 10 ml of distilled water. For irrigation agitation, Ultra X (Changzhou Sifary Medical Technology Co.Ltd., Changzhou, China) ultrasonic device was used and the canals were dried with paper points (Dentsply Sirona). Five teeth were randomly selected as the positive control group (group 2, prepared, no temporary material placed). After canal preparation of the remaining 80 teeth was made, calcium hydroxide paste (UltraCal XS; Ultradent, South Jordan, UT) was placed to simulate clinical conditions, and the canal orifice was covered with Teflon tape. For the TF application, 80 teeth were randomly divided into 4 groups ( n = 20). In Group 3, TF was performed with Cavit G (3 M ESPE) on the teeth with MOD cavity preparation. Tofflemire matrix band (Kerr GmbH, Rastatt, Germany) was placed on the teeth with MOD cavity in Group 4 and built up with nanohybrid resin composite (Tokuyama Estelite Sigma Quick, Tokuyama, Japan). The TF was performed with Cavit G (3 M ESPE). In Group 5, TF was made with Voco Clip F (VOCO, Germany) on the teeth with MOD cavity preparation, and an LED light device (Valo Cordless, Ultradent, Utah, USA) was used to polymerize the filling material for 40 s to ensure hardening. Tofflemire matrix band (Kerr GmbH, Rastatt, Germany) was placed on the teeth with MOD cavity in Group 6 and built up with nanohybrid resin composite (Tokuyama Estelite Sigma Quick, Tokuyama, Japan). TF was made with Voco Clip F (VOCO) and an LED light device (Valo Cordless) was applied to polymerize the filling for 40 s to ensure hardening. After the matrix band was removed, all teeth were stored at 37 °C and 100% humidity for 1 week. 

The prepared samples were embedded vertically in self-hardening acrylic resin 2 mm below the cemento-enamel junction. Fracture strength values were measured using a universal testing machine (Instron Corp, Canton, MA, USA). A stainless-steel ball (6 mm in diameter) placed in the testing machine was centered on the occlusal surface of the restoration and compressive force was applied vertically to the restoration at a rate of 1 mm/min until fracture occurred. The moment of a sudden decrease in force that was seen on the screen of the testing machine was defined and saved as the maximum force. Also, it was calculated in Newtons. Types of fracture: It was classified as “restorable” (Fig. ) above the acrylic level and “unrestorable” (Fig. ) when the fracture extended below the cemento-enamel junction. 

The Kolmogorov-Smirnov test was used for normality testing and these values ​​showed a normal distribution. A 2-way analysis of variance with the Tukey post hoc test was used for multiple comparisons. The statistical significance level was determined as p < 0.05. 

The mean FR values ​​of the groups are presented in Table . The FR of group 1 is higher than the other groups ( p < 0.05). The FR of group 2 is lower than all groups ( p < 0.05). The FR of groups 4 and 6 is significantly higher than groups 3 and 5 ( p < 0.05). The distribution of the groups according to fracture types is given in Table . The most common type of fractures were restorable fractures. There is a significant difference in fracture types between experimental groups ( p < 0.05). 

In numerous instances where root canal therapy is performed, immediate permanent restoration is not feasible, increasing the risk of reinfection and compromising tooth integrity. Thus, applying a TF material with properties to reduce microleakage and improve FR is essential . It has been reported that the FR of teeth prepared with MOD cavities is weakened by 63% . TF materials placed on these teeth expand and apply lateral forces, which may cause crown or crown-root fractures . Composite restorations increase the FR of the tooth due to the interaction between tooth tissues and the bonding agent . At the same time, composites are materials that can absorb the forces coming to the tooth with a low modulus of elasticity . In this study, the effect of different TF materials and pre-endodontic build-up on the FR of teeth with excessive material loss was evaluated. Preventing contamination of the root canal by saliva, blood, and microorganisms is crucial during endodontic treatment and between appointments until the final obturation is completed. Rubber dam isolation is commonly employed to achieve contamination control during endodontic procedures. However, securing a rubber dam can be challenging when there is a significant loss of tooth structure, as the clamp may not attach securely to the tooth being treated. In such cases, performing a pre-endodontic build-up before initiating treatment can benefit structurally compromised teeth. This approach simplifies optimal rubber dam isolation for subsequent visits, ensures effective inter-appointment temporization to minimize bacterial microleakage, prevents seepage of intracanal medicaments and gingival ingrowth into the cavity, protects the weakened tooth structure from fractures, preserves repeatable reference points, and facilitates post-endodontic restoration . Crown fractures, especially in the cervical part of the occlusal and palatal parts of the upper premolar teeth, are quite high compared to other tooth groups. Many studies also confirm this situation . Upper premolar teeth were preferred in this study because they are easy to collect (extracted for orthodontic reasons) and have single roots. Additionally, premolars are more likely to be subjected to lateral forces than molars . For this reason, upper premolars were used to evaluate the FR of different variables and were considered suitable for this purpose in several studies . Differences in the structural and anatomical characteristics of teeth and storage conditions make the standardization required for the evaluation of samples difficult . In order to ensure consistency in the current study, the mesiodistal and buccopalatal lengths of the teeth were measured and teeth of similar size were selected . Evaluating the effects of different cavity designs and TF materials on the FR of teeth, Balkaya et al.  reported that groups in which the MOD cavity width was prepared as one-third of the intercuspal width showed higher FR compared to tooth groups prepared with two-thirds of the intercuspal width. In addition, MOD cavity groups prepared with one-third of the intercuspal width showed similar FR to intact tooth groups. Similar studies also confirm these results . Since we aimed to ensure that premolar teeth become more susceptible to fracture in the current study, the width of MOD cavities was prepared to be two-thirds of the intercuspal width. In a study by Karapinar Kazandag et al. , a force was applied vertically at a speed of 1 mm/min to provide a simulation of the mechanical forces acting on the teeth. In the present study, the FR test was applied in the same way. According to Srivastava et al. , being able to predict the outcome and prognosis of endodontic treatment depends on the type and quality of coronal restoration. Atreya et al.  stated that canal-filling techniques, the type, and thickness of TF materials are important factors in the occurrence of tooth cracks and fractures. Cavit G is a TF material containing zinc oxide, calcium sulfate, glycol acetate, polyvinyl acetate resins, polyvinyl chloride acetate, triethanolamine, and pigments. It has good sealing properties thanks to its high expansion coefficient . Resin-based Voco Clip F has minimal polymerization shrinkage and does not affect sealing, but has insufficient bonding ability to prevent marginal leakage . According to the in vivo study of Laustsen et al. , when the fracture frequency of teeth temporarily filled with Coltosol ® F and Zinc oxide eugenol was compared, the Coltosol ® F group exhibited a higher fracture incidence. This result was attributed to the hygroscopic expansion of Coltosol ® F in a cavity that may lead to cusp deflection, infraction development, and fracture. The in vitro study conducted by Srivastava et al. , comparing FRs of different TF materials, showed similar results. The second null hypothesis of the current study was that there would be no difference in FR between types of TF materials. According to the results obtained, the tested null hypothesis is accepted, as there is no significant difference in the FR of different materials. According to a study by Baba et al. , while TFs based on zinc oxide, eugenol, or calcium sulfate protect the root canal system from reinfection for a short time, glass ionomer or resin-reinforced composite restorations can prevent bacterial contamination for a longer time. In addition, the high bonding ability of composite materials and the nanostructured particles that contain can increase the mechanical strength lost during the preparation of the cavity . In the current study, the first null hypothesis of our study was rejected because the ones that had undergone pre-endodontic build-up procedures showed higher FR compared to teeth with MOD cavities. Fractures occurring at the enamel-cement level and above can be easily repaired without the need for any additional treatment, and this type of fracture is more advantageous for both the physician and the patient. However, if the fracture line is below the enamel-cement junction, more complex restorative procedures are required . In the current study, it was observed that a higher rate of restorable tooth fractures occurred in teeth with pre-endodontic build-up compared to other groups. 

According to the results of the current study, it can be said that the rate of restorable tooth fractures is higher in teeth where TF materials are placed after pre-endodontic build-up between endodontic treatment sessions. Further studies are needed to evaluate the effects of different variables on the FR of teeth with extensive material loss that require more than one endodontic session. 

