Fracture strength of methacrylate resin based versus resin free composite restorations supporting occlusal rest of removable partial denture: In Vitro Study

Aim: compare fracture strength of composite restorations (methacrylate resin-based, methacrylate free) restoring abutments having occlusal rest seat in them and applying vertical stresses through the rest. Materials and Methods: 28 extracted first maxillary premolars were used, class II cavities were prepared. According to composite restoration, specimens randomly into two equal groups; first group methacrylate resin-based composite Tetric N-ceram® while in second group Ormocer-based composite (methacrylate-free) Admira fusion®. Saucer rest seats were prepared, then samples were stored in distilled water for 2weeks and exposed to thermo-cycling. Metallic tool milled with dimensions of RPD rest was used in universal testing machine applying vertical load where fracture resistance test used. In-addition, site of fracture and fracture mode were inspected under optical light microscope. All records were tabulated and analyzed. Results: Results showed, Tetric N-ceram samples had higher strength values (1189.00±27 N) than those of Admira fusion (1012.40±25 N), yet difference wasn't statistically significant (p=0.06). There was equal distribution of fracture site in both groups, commonly mixed type (100%). While fracture mode, Tetric N-ceram group, 28.5% showed non-catastrophic mode and 57.1% in Admira fusion group. Besides, Tetric N-ceram showed 71.43% catastrophic mode while 42.86% in Admira fusion group. Conclusion: There was no significant difference in fracture resistances of resin composite restorative materials compared to resin free. Tetric N-Ceram and Admira can be considered suitable restorative material used under rests of RPDs and safely support them


Introduction
2][3] RPD is a mechanical structure related and functioning with biological structures.From the biomechanical aspect, its success will rely on the design and construction of the metallic framework.Planning and designing of RPD should provide components' simplicity, occlusal stability, good oral hygiene, patient's comfort and satisfactory esthetics.These factors altogether are essential in maintaining the remaining structures, that is the principal objective of oral rehabilitation. 4,5 properly designed RPD considering the appropriate distribution of the masticatory loads on the abutment teeth can extend the permanence of RPD and the surrounding oral structures. 6,7Occlusal rest is one of RPD components that establish vertical support.It permits the transmission of stresses in an axial direction on the abutment teeth, keeps the prosthesis in the planned position not moving during functional loading and avoids irritation to the nearby soft tissues. 2,3,8,9In addition to its main role in RPD support, it assists in directing the exerted forces down the long axis of abutment teeth, thus has the advantage in preserving periodontal health of the abutments. 10Where teeth and their surrounding tissues are most fitting for resisting forces that are guided axially. 11nhancing the axial load on the supporting abutment is accomplished by occlusal rest seats prepared in its supporting, definite surfaces. 5Where preparing and remove in the enamel to attain depressions with proper dimensions necessitated to reach smooth and saucer-shaped occlusal rest-seat layout. 12This modification confirms that the rest would be of sufficient thickness and strength to preclude its distortion or fracture under masticatory load; that it does not interfere with the occlusion, and that the abutment is loaded axially and to prevent the existence of lateral forces on the supporting structures. 1,13,14he rest seat material is considered an essential concern to preserve a positive supportive position for the rest placement and precludes the entire prosthesis from moving during function. 9,15Rest seats are preferably done in the enamel of abutment teeth, in addition can be comprised in indirect cast restorations, in the surveyed crown, resin or amalgam restoration. 3,8est seat preparations on direct restorations necessitate adequate depth and width to prevent exposure of the underlying dentin or surpassing the restoration or its lining. 13ecently in restoring posterior teeth, resin-based composites are widely used.They offer tooth-like appearance as well as overcoming the problems of amalgam restoration.8 Besides, in restoring damaged teeth, composites can be used as replacement to ceramics. 19Other advantages of using composite; more conservative tooth preparation for composite, inexpensive cost, reduced chair-side time, easily placed and can be repaired intraorally. 20ince the 1960s, resin based composite (RBCs) have been used to restore posterior teeth.The fracture resistance of restored teeth had been significantly improved by the use of resin composites since it can strengthen the remaining tooth structure owing to bonding to it. 21The intra oral performance of the newly presented composites has been greatly enhanced to gain sufficient strength and resistance so as to endure the masticatory forces and reduce the polymerization

ASDJ Ain Shams Dental Journal
shrinkage and improved curing depth.However, in stress-bearing posterior restorations, the relatively increased brittleness and decreased fracture strength of the latest composites still considered a main problem. 22On the other hand, mechanical properties of composite revealed higher compressive than tensile strength.This makes significance to disregard the likelihood of failure when it is applied in stress bearing areas like under the RPD rests.

23,24
Technological advancements in development of RBCs have resulted in improved filler and organic matrices.In recent years, alterations in fillers, particularly; particle size, distribution and type, have enhanced mechanical and physical properties of composite materials. 25urthermore, progression of monomer enhances both the reactivity of polymerization and mechanical characteristics of the generated adhesive layer.Bis-GMA and UDMA dimethacrylates are now the most common monomers used in dental composites.For its high viscosity, low molecular weight monomers must be introduced to get the desired viscosity for clinical formulation.These diluent monomers cause the composite resin to absorb more water, shrink during polymerization, and discolor.New monomers have been explored, trying to improve properties of composite restorative materials. 26rmocer is an abbreviation aimed at organically modified ceramic.Its synthesis is depends on hydrolysis and polycondensation reactions (sol-gel processing), which result in molecule having long-chain inorganic silica backbone and lateral organic chains. 27omposites containing ormocer are known for having greater degree of conversion, less polymerization shrinkage, beside enhanced surface hardness owing to development of the more strongly network of cross-linked polymer.
Furthermore, ormocer has improved biocompatibility since the increased amount of chemical interactions between methacrylate groups reduces the quantity of unreacted free monomers inside the polymer network. 28ne of principal characteristics of dental materials is fracture resistance, which based on the materials' resistance to propagation of cracks from their internal defects.The cracks can result in microscopic fractures of filling margins or bulk fracture of the restoration itself. 29n selecting the rest seat material, mechanical properties of dental restoration as flexural strength, fracture resistance, modulus of elasticity should be considered, as they are elements of the durability of the restoration. 30Masticatory load applied by patients wearing RPDs range between 65N-235N. 3,16,31,32urthermore, dental restorations during function are exposed to cyclic loading and aging effects by being subjected to various kinds of fluids at altered temperatures. 30,33Consequently, dental restorations should have adequate strength thus tolerate masticatory forces besides aging processes. 34nfortunately, merely limited information and comparison are presented on the fracture strength of this newly introduced composite materials depending on the nature differences of their resin matrix, besides their mode of fractures that occur following exposure to altered oral environmental factors.Thus, there is an increased demand for other studies to assess their mechanical behavior.The study hypothesis estimates that both types of composite restorations that are restoring a prepared occlusal rest seat would have comparable fracture strength.

Clinical Relevance
Selecting high strength restoration to restore the main abutment teeth of RPD will increase the longevity of prosthesis and fulfill

ASDJ Ain Shams Dental Journal
patient's satisfaction with the removable partial denture.

Materials and Methods: 1.Laboratory steps for specimens' preparation: Teeth selection
Altogether, 28 recently extracted sound maxillary first premolars were selected from Oral Surgery Clinic, Faculty of Dentistry, Ain Shams University for this study.These teeth were extracted since orthodontic or periodontal indications where extracted after the patients had signed (approval) written consent form.They were inspected under a stereomicroscope (10× magnifications) to exclude the presence of any erosive or abrasive lesions, fissures, fractures, carious lesions or restorations.

Sample Size Calculation:
Power analysis was proposed to have acceptable power for applying a two-sided statistical test of the null hypothesis that there isn't any difference among the tested groups regarding fracture resistance.Assuming alpha (α) level of 0.05, beta (β) level of 0.2 (i.e.power =80%) and effect size (d) of 1.14 was calculated depending on results of a preceding study. 35Sample size calculation was made using R statistical analysis software version 4.3.2 for Windows. 36roposed sample size (n) was overall 28 specimens (14 samples per group).
Each premolar was vertically placed in premade wax mould packed with autopolymerized acrylic resin (Vertex, Netherlands), such that cemento-enamel junction (CEJ) was 1 mm above the acrylic resin base.To ensure parallel alignment of the samples in their mould, the proximal surfaces were aligned with the analyzing rod of the dental surveyor (Ney surveyor, NeyTech, USA) such that the long axis of the abutment tooth would be parallel to the vertical plane.
Standard class II restorative cavities (disto-occlusal cavities) were prepared in the selected teeth by same operator using a highspeed hand-piece and H245 a pear-shaped, operative carbide bur (Komet USA) under continuous air-water coolant.The cavity preparation dimensions were nearly the same in all; where the buccolingual width of the occlusal cavity was one third the inter-cuspal distance, the pulpal floor depth was 1.5 mm and gingival margin was positioned 1 mm occlusal to CEJ.Cavo-surface margins were 90 • , having rounded internal line angles with parallel walls.The same periodontal probe was used to measure the dimensions to check there were standardized dimensions in all preparations. 35,37,38n each cavity, selective enamel etching was carried out by applying an etchant (phosphoric acid gel, 37%) for 30 sec, then copious rinsing with water for 30 sec followed by drying gently by oil-free air stream for 2 sec.Afterwards the bonding agent was applied (Universal Adhesive Tetric ® N-bond), used according to instructions of manufacturer and light cured for 10 sec by a LED light curing unit (LED, Woodpecker, China) (1400 mw/cm 2 ) for 10 sec.

2.Grouping of the Specimens:
After completing all cavity preparations, the specimens were divided randomly into two equal groups (n=14) using the function of random numbers presented in the Microsoft Excel sheet.For their allocation according to the type of composite material used in the restoration of the selected abutment teeth, a randomization sequence with 1:1 allocation ratio using 28 small papers written in first 14 letter "T" and second 14 letter "A" and were put in similar sealed envelops.At the time of the restoration insertion, one of these papers was drawn blindly to enroll this specimen on the selected Both composite materials were used to restore the prepared cavities by oblique incremental layering technique, each was light cured for 20 sec then final finishing and polishing of the restoration was carried out.

Preparation of the occlusal rest seats:
All restored teeth were stored in distilled water at 37 • C for 24 hours, then 28 standard saucer shaped rest seats were prepared in the restoration by same operator.The prepared rest seats were rounded triangle having saucer or spoon shaped floor with dimensions (2 mm buccolingual width, 2.5 mm mesiodistal length and 1.5 mm depth). 13,14ig. 1 (A,B,C) the dimensions were checked using the customized metallic tool as checker and to ensure its adaption to it too.
Specimens were collected in distilled water at room temperature for 2 weeks before fracture resistance testing.

4.Thermocycling Procedure:
Specimens were subjected to 5000 thermo cycling aging cycles at temperatures ranging 5 • C± 2 and 55 • C ± 2 inside a thermocycler (Thermo Fisher Scientific, Nalgene, USA) where soaked for 30 sec in water bath at each relevant temperature then 5 sec transfers between the temperature baths, by this simulated short-term aging of clinical service for 6 month. 34,40 racture resistance test: a-Preparing the metallic tool.
A customized steel tool similar to the required dimensions of the rest of RPD was made.The tip part was made using wax pattern.A preheated and softened rode of wax was inserted into the prepared rest seat to take its dimensions and form then it was casted into metal.While the upper part of the tool was milled similar to the universal machine tools.Then the two parts were assembled by soldering.The customized tool was used for applying the vertical load applied by universal testing machine (Instron 8871 Universal Testing Machine, Shakopee, USA).To ensure its adaptation to the rest seats during testing, this metallic tool was held in the upper movable part of the testing machine for applying compressive load with a crosshead speed at 1 mm/min.Fig. 2 (A, B

ASDJ Ain Shams Dental Journal
b-Fracture strength testing Fracture resistance test was performed within 24 hours following thermal cycling.A static compressive axial loading was applied on the specimens centralized on the rest seat through the specially milled tool.The load was increasing until specimen fracture takes place.Maximum load reached for fracture in each specimen were recorded in Newton (N).
The crosshead speed was 1 mm/min with load cell capacity 50 kN.Load was continuously applied till the specimens were fractured and the force was documented in Newton.Peak forces needed to cause fracture of the specimens were recorded as fracture resistance (FR).Following a standardized method, the same operator performed the tests to keep consistency during testing procedures.c-Fracture and Failure Analysis.
Failure modes of specimens; the site of fracture was identified and recognized by cohesive, adhesive and mixed mode of failure.Cohesive type of failure denoted fracture that arose in the bulk of tooth structure or restoration (without any adhesive layer exposure), while adhesive type denoted fracture occurring at the boundary among the restoration and tooth.Mixed failure comprised combination of cohesive and adhesive type of failure. 41,42In addition, the fracture mode was categorized based on the fracture extent to catastrophic where the fracture extends beyond CEJ apically and non-catastrophic when the extent is occlusal to CEJ. 43 Three different operators identified the fractured specimens under an optical microscope (Nikon, H550L, Tokyo, Japan) to assess the failure mode.In each group, number of types of specimen failure was collected, calculated their percentage and tabulated.

Statistical Analysis
The collected numerical data was presented by mean, standard deviation (SD), median and interquartile (IQR) values.They were tested for normality by viewing the distribution and using Shapiro-Wilk's test.They were found to be non-parametric and analyzed using Mann-Whitney U test.The significance level was determined at p<0.05 in all tests.Statistical analysis was made with R statistical analysis software version 4.3.2 for Windows. 36

Results
Summary statistics and intergroup comparisons results for fracture resistance are presented in table (1) and in fig.(4).Results revealed that Tetric N-ceram samples had higher strength values in fracture resistance with mean 1189.00 ± 278.66 N than those of Admira fusion that showed 1012.40 ± 255.34 N, yet there was not statistically significant difference (p=0.060).

Failure mode analysis
Regarding the fracture site, the results showed in both composite materials equal distribution of fracture site, where fracture was mainly happened in mixed type (100% of the samples) within the restorative materials and at the junction between the restoration and the tooth in both tested groups.Regarding the fracture mode, in Tetric N-ceram group, 4 specimens (28.57%) showed noncatastrophic mode while 8 specimens (57.14%) were non-catastrophic fracture in

ASDJ Ain Shams Dental Journal
Admira fusion group.On the other hand, in Tetric N ceram group, 10 specimens (71.43%) showed catastrophic mode while 6 specimens (42.86%) were catastrophic fracture in Admira fusion group.as presented in table (2).

Discussion
The current in vitro study assessed fracture strength of two composite restorative materials having prepared occlusal rest seat as done in abutment teeth for RPDs for rest support.The null hypothesis of this study was that no difference would exist in fracture strength of the composite restorations used under the rest of RPD.The hypothesis was accepted as the results obtained showed there was no significant difference found in fracture resistance of Tetric N-ceram and Admira fusion composite materials restoring class II cavities with occlusal rest seat.
RPD must have adequate support for appropriate rehabilitation of occlusion.Improper fit between the rest and its rest seat, not appropriate size and shape of the occlusal rest, and inappropriate placement of the rest seat could lead to reduced support. 44Thus the occlusal rest should provide positive vertical support to transmit properly the occlusal forces from the artificial teeth to the abutment teeth along their long axis. 15,45,46 eth and their supporting structures are best tolerating forces directed axially. 11hough when the occlusal rest assists in directing the load applied at the long axis of abutment tooth that will aid in the maintenance of the abutments and their periodontal health. 10Since Rest seat form and inclination greatly affect the appropriate rest function, same operator did the rest seat in all the specimens with the same dimensions.In addition the customized metallic tool was used as a checker after each preparation to ensure its proper adaptation in the seat thus ensure accurate positioning of the applied load.
Alterations or damage in the tooth structure because of trauma, caries or endodontic and restorative measures have an adverse outcome on its fracture strength in addition increase the risk of cracks in the cusp furthermore its fracture.8][49][50] Besides, premolars have similar size, form, shape, and frequently are most common sound teeth extracted aimed at orthodontic purpose. 51For standardization of the specimens, teeth with similar dimensions were collected and same operator who did the cavity preparation and application of the composite restorations according to the

ASDJ Ain Shams Dental Journal
instructions of manufacturer in both groups and another operator performed the fracture strength test for all the specimens.Class-II cavity in maxillary premolars produces a particular challenge for the restorative materials to be used where there is a concern about the resistance to tooth fracture and durability.Hence, the damaged teeth required being restored using restorative material that is efficient in resisting fracture upon receiving excessive occlusal load. 52Furthermore, the rest seat material is considered a critical requisite to preserve positive supportive foundation that would ultimately preserve the abutment teeth and their supporting periodontium. 15omposite resin restorative material are applicable options to replace amalgam at stress-bearing areas in posterior teeth and increase fracture strength of the restored teeth structure significantly owing to their capability to reinforce the tooth via bonding to it furthermore can stabilize the tooth by acting as an interior splint. 34,52,53In addition owing to the resiliency of composite restorations, it acts like a cushion under the rest of RPD and can absorb the masticatory load. 5,32anohybrid composite resin was used to restore class II cavity model since nowadays it is the first choice of the restorative materials because of their ability to imitate the lost dental structures, ease of handling, adequate adhesion, acceptable mechanical resistance. 54In addition, nanohybrid composite resins encompass 40 nm to 1 μm particles and provide good aesthetic with acceptable physical, chemical and biological properties that are considered essential for improved permanence of the restoration. 55ong-term successes of composite restorations are determined by resistance of the components of this complex system to stress and deformation.56  Ahmed et al. 50in an in vitro study that evaluated using different cavity designs and restored them with different restorative protocols using four different types of resin based composites and subjected them to thermocyclying.They found that restoring weakened maxillary premolars with new restorative materials formed by different new technologies had enhanced their fracture resistance and distributed better the stresses in the restored teeth.
Model materials and testing conditions were chosen carefully to imitate clinical conditions as feasible accordingly the simulation can resemble a clinical situation.According to the standard protocol, design of cavity preparation and dimensions of occlusal rest seats were carried out.Consequently, the results became more reliable and applicable for clinical cases.
Specimens were stored in distilled water (37°C) for 14 days till testing, an adequate time for composite resin restoration to get a state of equilibrium of water sorption.Specimens were exposed to thermal aging replicating temperature fluctuations and wet environment that occur in oral conditions.It is significant that fatigue resistance and durability of restorations having rest seat preparation be assessed following artificial aging like thermal changes.The temperature of thermal cycling applied between 5°C-55°C for imitating the minimum and maximum temperatures of food and drinks taken by patients, this can give an indication about the longevity of the restoration inside the oral cavity.44, 57 Masticatory load in human shown to be approximately 40 N, while the mean maximum masticatory forces of posterior teeth ranges from 200 N-540 N. 58 Average recorded fracture forces in both composite materials under investigation used in this study showed comparable results and no significant differences between them.They were reported of higher values than that average masticatory load; 1189.00 ±278 N in Tetric N ceram group while in Admira fusion The results showed that the fracture site in all the specimens in the two groups were (100%) mixed type of fracture this may be explained that the teeth restored by the composite were less vulnerable to masticatory loads because of the cusp reinforcement achieved by applying enamel and dentin bonding that distribute the stress through the bonding junction.Subsequently, increased the fracture resistance and the occlusal forces had a reduced impact on the composite resin filling material.50, 72,73 In addition, composite resin was incremental packed that permitted adequate micromechanical adhesion to enamel and dentin, hence enabling conservative cavity preparations and strengthens the persisting breakable dental structure.74 The layering

ASDJ Ain Shams Dental Journal
technique for composite resin application aids in decreasing polymerization shrinkage with accompanied stresses by letting sufficient light penetration to achieve suitable polymerization so as to avoid the problem of deficient curing beyond certain depth.75 Fouda et al. 34 in an in vitro study compared the fracture strength of amalgam and composite restorations with rest seat preparation after being exposed to thermomechanical aging then cycling loading.They found the fracture strength recorded for Tetric N-Ceram, Filtek Z250, Megalloy EZ were comparable to that of amalgam restorations and that Tetric N-Ceram composite presented equal distribution of fracture sites in the restorative material and tooth and unlike other groups, showed the highest number of noncatastrophic fractures as they presented more the fracture within restorations.
Non-catastrophic mode of fracture (above CEJ) was more evident in Admira Fusion composite (57.14%), than in composite Tetric-N Ceram (28.57%).This increased prevalence of non-catastrophic fracture mode within Admira Fusion composite could be subsequent to the lower stresses of polymerization shrinkage created by inclusion of stress relievers in its composition, accordingly, maintaining the adhesive bond between tooth surface, adhesive and restorative material.76  One of the study limitations is the small number of samples utilized, and the aging process used is merely thermo-cycling, which in future studies can be supplemented by chewing simulation using cyclic loading.From the outcomes of this study, a recommendation to conduct further in vivo research for an extended period of time.

Conclusion
Based on the outcomes of this in vitro study, it could be concluded that, both types of composite materials either Tetric N-Ceram or Admira can be considered a suitable restorative material to be used under the rests of RPDs and safely support the occlusal rests.
Resin based composite material Tetric N-ceram® (methacrylate-based composite) a universal Nano-Hybrid Composite Ivoclar Vivadent, Switzerland was used for restoring the prepared cavity Group 2: Ormocer based composite material Admira fusion® (non-methacrylatebased composite) a universal Nano-ORMOCER restorative material Voco GmbH, cuxhaven, Germany was used for restoring the prepared cavity.

Fig. 1 .
Fig.1.A sample showing triangular rest seat preparation.A. Proximal view showing the depth at its apex.B. Occlusal view showing rest seat buccolingual dimension.C. Occlusal view showing rest seat mesiodistal dimension.

Fig 2 .
Fig 2. A. The wax fitted to the seat preparation.B. The tool tip (wax), C. the tool upper part (wax) D. Shape of the final customized tool (metal), E. The tool tip (metal) F. The tool fitted to the preparation. Fig.3

Fig 3 .
Fig 3. Sample secured to the universal testing machine for FR testing.

Fracture
strength of methacrylate resin based versus resin free composite restorations supporting occlusal rest of removable partial denture: In Vitro Study| Mona M. Aboelnagga et al.JUNE2024.ASDJ Ain Shams Dental Journal group were 1012.40 ±255 N. Besides, they are surpassing the highest occlusal forces applied by the users of RPDs which are in the range of 100-230 N in the premolar and molar regions.3, 59,60 These high fracture strength readings may be attributed to composition of the materials used in this study, where Tetric N Ceram composite contains dimethacrylates (19-20 wt.%) and the fillers include barium glass, ytterbium trifluoride, mixed oxide and copolymers (80-81 wt.%) while Admira Fusion composite comprises three main components organic polymers, inorganic components and poly-siloxanes.Organic polymers have influence on the polarity and the network formation and the inorganic components impact thermal expansion and chemical stability.Poly-siloxanes provide the elasticity and interface characteristics and nano filler Al, Ti, Zr alkoxides are (84 wt.%).61,62 So generally, the resin composites depending on nanotechnology have developed remarkable improvements in wear resistance and significant advancement in mechanical properties.63 Though the mechanical properties of composite resins are critical for the durability of the restorations, mechanics could not be the single factor to be contemplated.Considerations in the composite polymerization and the resin matrix were taken to decrease the damaging effects of polymerization shrinkage.64 Accordingly, more attention was paid on the changes in the resin matrices formulations.63 In a three year study, Bottenberg et al. 65 reported that in occlusal stress-bearing cavities, the Ormocerbased composites performed similar to the conventional BisGMA-based composite.An in vitro study assessed the fracture resistance of maxillary premolars having large cavities prepared and restored by resin filling materials have three types of novel bulk-fill composite resin materials and found non significant difference between the ones restored by bulk-fill types and Tetric Evoceram with conventional incremental-fill technique.66 Fracture strength of composite Tetric N-Ceram and Admira Fusion were comparable in the current study that could be owing to their structural composition.The increased filler weight fraction of composite provides improved packing efficiency; thus mechanical properties are enhanced within the stress transfer and even distribution among the filler particles in composite restoration.67 Furthermore, the presence of pre-polymerized fillers in composite Tetric N Ceram could enhance the fracture resistance, since advocated by Ramdas et al. 67 and Blackham et al. 68 These results were in accordance with former studies that showed comparable results of similar strength in different types of composite resin.37,69,70 Moreover, Mesallum et al. 71 stated that the fracture behavior, under functional loading, of different composite resins receiving occlusal rest seat for RPD was similar.