Abstract. Introduction

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1 ISSN Effect of enamel preparations on fracture resistance of composite resin buildup of fractures involving dentine in anterior bovine teeth: An in vitro study GANDHI K. a, NANDLAL B. b Abstract Sixty bovine teeth with simulated mesio-incisal angle fracture were randomly and equally divided in one untreated (control) group and three experimental groups (Bevel, Chamfer and newly introduced Stair-step Chamfer preparation group) to evaluate the effect of enamel preparations on the fracture resistance of composite resin. Post restoration, fracture mechanics approach was used to quantify the failure of composite resins in testing the samples in Instron testing machine. Mean peak failure load (Newton) of composite amongst experimental groups was observed in the order; Chamfer ( ), Stair-step chamfer ( ) and Bevel ( ). Results of the One-Way ANOVA revealed significant difference in the mean peak failure load values of the four different groups. (P< 0.001) Scheffe s Post-Hoc comparison test (Subset for alpha = 0.05) revealed that there was no significant difference in the mean peak failure load values of the bevel, stair-step chamfer and chamfer preparation when considered together, but the mean peak values of control group ( ) were observed significantly higher. Failure mode evaluation revealed, majority of failures occurred as cohesive and mixed type for all the experimental groups. Adhesive type failure was observed maximum (33%) in the bevel group. Stair-step chamfer preparation showed greatest potential for application and use as it no only demonstrated comparable values to Chamfer preparation [ t value (0.39) (P> 0.05)] but also involved sacrificing less amount of tooth structure adjacent to fractured edge. Key words: Bevel, chamfer, fracture resistance, stair-step chamfer Introduction Fracture resistance of a material is a measure of its ability to retard crack initiation and propagation. [1] High fracture resistance of restorative material is required in clinical situations where high impact stresses are experienced and incisal angle restoration of a fracture in anterior tooth is one such demanding situation. It is estimated that 1 out of every 4 persons under the age of 18 years sustains a traumatic dental injury in the form of an anterior crown fracture. [2] Irrespective of the causes of such injuries; the overall seriousness of traumatic injuries to the anterior teeth often focuses on the esthetic implications of the injury. A fractured or missing anterior tooth can have a negative effect on a person s physical attractiveness that can inadvertently affect his self-esteem, which in itself is a complex issue of his action and correlates with his success. Hence the pain caused by traumatic injury is not only the important primary concern when the trauma affects the appearance of the teeth as well. [3] A fractured anterior tooth in a young child confronts the dentist with a challenge for many reasons such as the large pulp that contraindicates certain restorations, incomplete root-end closure that may prevent root canal therapy and the impact of the psychoa Ex-Post Graduate Student, b Professor and Head, Department of Pedodontics and Preventive Dentistry, JSS Dental College and Hospital, Mysore , Karnataka, India logical well being of the child and parents. [4] Reattachment of fractured incisal portion of crown is a widely accepted and popularly established procedure. But in the commotion of injury, seldom do the parents or child realize the importance of retrieving fractured fragment, which is then occasionally lost. Hence, the choice of treatment procedure mostly narrows down to restoration with composite resins. The number of restorative techniques developed to manage such fractures, attests to the difficulty in placement and frequent failure of these types of restorations. Shortcomings of such procedures are generally retention or esthetics. Methods of enhancing retention of tooth colored restorative materials are sought from various designs of enamel preparations prior to acid etching. Besides efforts to enhance retention and expose reactive enamel, preparation techniques of enamel have also evolved to achieve high degree of esthetics. Recently described technique of masking margins by placing them within natural developmental grooves on enamel of anterior tooth in form of stair-step is an example of such a change. [5] Since the mechanical testing of biomaterials and techniques is a pre-requisite for their safe and effective clinical use, enamel preparation designs namely bevel, chamfer and newly introduced stair-step chamfer were evaluated and compared for their effect on fracture resistance of composite resin restoration of standardized simulated incisal angle fracture in anterior bovine teeth. 69 J Indian Soc Pedod Prev Dent - June 2006

2 Materials and Methods Lower bovine incisors with similar morphological features were atraumatically dissected from intact mandibles removed from animals in dental age groups of approximately 42 months within 12 hours of the slaughter. Dentition was roughly gauged using BSE information guide. [6] No animal was sacrificed for the study and institutional ethical committee approval was obtained prior. After removal of all adherent blood and soft tissue, they were stored in 0.5% Chloramine-T bacteriostatic / bacteriocidal solution for 1 week. Thereafter they were stored in distilled water in a refrigerator at 4 o C. In order to reduce deterioration the storage medium was replaced periodically. [7] 50 visibly near equal sized and shaped bovine teeth were measured for their mesio-distal width at the incisal edge. Average value (13.0 mm) deduced thereafter formed the basis of selection of specimens for testing. All teeth were within 1 mm difference of their mesio-distal incisal widths and teeth with closed root apex were used for the study. Using a mounting jig all specimens were mounted in the same degree of angulation and vertical projection in aluminum casings with roots embedded in self-curing acrylic resin [Figure 1]. marked area of 2 mm from the fractured edge [Figure 2] Lingually all preparations were restricted to a full thickness inclined bevel extending 2 mm cervically from the fractured edge [Figure 2]. With a standard diamond rotary bur (ISO NO. 160/014), an inclined bevel was placed on the cavosurface angle on the labial surface. The bevel included entire thickness of enamel and extended from the dentino-enamel junction to the enamel surface. Similarly with a standard diamond rotary bur (ISO NO. 288/012), a chamfer preparation and stair-step chamfer preparation was placed on labial surface of teeth in their respective groups. The chamfer and stair-step chamfer included approximately half the thickness of the enamel in depth and extended 2 mm cervically beyond the edge of the fractured surface. Stair-step chamfer was produced in accordance to the technique described by Harry F. Albers [5] wherein two gradually merging vertical and horizontal chamfers were placed alternatively to achieve enamel prepara- Oblique Bevel margin Individual custom-made strip crowns were fabricated on die models using positive pressure Biostar machine (Scheu Dental-Germany). Coordinates marking 5mm gingivally and 5mm distally along the incisal edge were joined to form a base of an imaginary triangle with apex corresponding to mesio-incisal line angle. A standardized experimental mesio-incisal fracture was created using a slow-speed diamond saw (Ducom ) with water-cooled 0.5 mm wafering blade at a constant speed of 0 rpm. All the experimental preparations were restricted to the Oblique chamfer margin Figure 2: Enamel preparations Figure 1: Bovine teeth mounted in Jig Figure 3: Instron testing machine with sample J Indian Soc Pedod Prev Dent - June

3 tion design resembling stair steps [Figure 3]. Adper TM Prompt TM (3M ESPE, USA), a self-etch system was applied on the prepared surface according to manufacturer s instructions. All restorations were placed with use of custom strip crown formers in a bulk pack technique. Vent holes were placed at the mesial incisal corners to prevent entrapment of air and allow the escape of excess resin. Using a soft-start polymerization-curing unit (Translux energy, Hareus Kulzer), composite resins were light cured through the strip crowns for one cycle of 40 seconds each from labial and lingual aspect. The tip of the curing light was placed in close similar approximation to all the samples. Strip crowns were gently removed after curing. Gross marginal excess resin was removed with high-speed water-cooled multi-fluted carbide bur. Finishing of the incisal edges was done with superfine aluminum oxide flexible abrasive disks (Soflex- 3M ESPE, USA). Specimens were aged for 24 hrs in distilled water at 37 degrees, celsius in an incubator. After the incubator storage, the samples were thermocycled to further stress the toothcomposite interface. The samples were thermocycled between 5 C and 55 C for 500 cycles with a dwell time in each thermal bath of 1 minute. The specimens were subjected to cantilever bending test in an Instron universal testing machine (Instron Corp. Canton, USA series 4301) [Figure 3]. A loading force was applied along the predetermined standardized spot on lingual aspect of the specimen by 6-inch stainless steel rod with a 2.5 mm radius at a constant crosshead speed of 5 mm per minute until the resin was dislodged or fractured. [4] After testing, the labial surfaces of experimental samples were examined under stereomicroscope at 6.5 x magnification to evaluate the site of failure. chamfer preparation and stair-step chamfer preparations along with Standard deviation values were observed as ± 48.96, ± 67.38, ± and 3.21 ± in Newton respectively [Table 1, Graph 1]. One-way ANOVA revealed a significant difference in the mean peak failure load values of 4 different groups. F value of with 3 and 56 degrees of freedom was found to be highly significant (P< 0.001) [Table 2]. The Scheffe s post-hoc comparison test revealed that only mean of control group was significantly different from all other mean peak failure load values, having the highest value as indicated by the subsets formed. There were no significant differences in the mean peak failure load values of bevel preparation, stair-step chamfer preparation and chamfer preparation, where all the means were grouped in one subset [Table 3]. Independent samples t test when applied between mean peak failure load values of control and the experimental enamel preparations groups individually revealed significant difference (P< 0.001) for all the pairs with t value of 16.34, and 11.79, for bevel, chamfer and stair-step chamfer groups respectively [Table 4]. Independent samples t test when applied for comparison between mean peak failure load values of bevel with chamfer and bevel with stair-step chamfer, revealed a significant difference with t value of 2.82 (P <0.01) and 2.24 (P <0.05) respectively [Table 4]. A non-significant difference was observed between mean peak failure load values of chamfer preparation and stairstep chamfer preparation groups. Independent samples t test revealed a t value (0.39), which was found to be nonsignificant at (P> 0.05) [Table 4]. The actual mode of failure was recorded according to the following criteria: Adhesive (A) : Failure at the tooth resin interface Cohesive (C) : Complete failure within the resin restora tion. Mixed (M) : Partial fractures of resin restoration and par tial adhesive failure at the interface Results The mean peak failure load for control, bevel preparation, Graph 1: Mean peak failure load values of various enamel preparation groups Table 1: Descriptive statistics of peak failure load (Newton) for various groups Groups I II III IV Group description Control Bevel preparation Chamfer preparation Stair-step chamfer preparation Total N 60 Mean S.D S.E J Indian Soc Pedod Prev Dent - June 2006

4 Table 2: Results of one-way ANOVA for peak failure load (Newton) for various groups Source of variation Sum of squares df Mean square F Sig. (P) Between groups Within groups Total df-degrees of freedom; F-Fisher s value; P-probability Stereomicroscopic evaluation of the fracture site at 6.5 X revealed the failure types as shown in the Table 5 and Graph 2. Discussion Fracture resistance along with fracture toughness, flexural strength and modulus of elasticity are important mechanical properties of restorative materials when they are used in situations where biting stress can propagate internal defects or initiate fracture. [1] Fracture resistance reflects the ability of a material to resist crack initiation and unstable propagation. When applied to an adhesive interface, it should account for both interfacial bond strength and inherent defects within the resin at or near the interface. [1,8] It is a recognized fact that interfacial bond strength alone cannot be regarded as a material property (like fracture strength). [9] In other terms, bond strength estimation in Megapascals (MPa) using similar experimental design would not be a true indicator of interfacial bond strength as the failure load values would take into account other factors like material properties and internal defects along with interfacial bond strength. [9] The introduction of acid-etch concept [10] and composite resins as restorative material, has laid foundation for most of the present day techniques for the restoration of fractured teeth. Composite resin is the material of choice for most of the dentist world wide when considering options to restore esthetically demanding fractures in anterior teeth. The methodology design employed in the present study was Table 3: Results of scheffe s post Hoc test Groups N Subset for alpha = Bevel preparation Stair-step chamfer preparation 3.21 Chamfer preparation Control based on that of previous studies by Stephen R. Nelson and colleagues, [4] J B Black and colleagues, [11] Bagheri J and Denehy G, [12] H Eid and G E White, [13] but differed to their studies in number of aspects. Sixty bovine anterior lower incisors were used for the study. The most ideal substrate for adhesion study like this would have been human maxillary central incisors since the prevalence studies have revealed the extent of involvement of maxillary central incisor in uncomplicated fractures of anterior teeth resulting from direct trauma. [14-19] However with the continual increase in dental health, more conservative dental therapeutics and limits on access to human materials, it is increasingly difficult to get sound human incisors for studies. It is also difficult to get non-carious sound human incisors of similar size for the studies. It is furthermore intricate to obtain non-carious sound human incisors from the same age group of patients in order to eliminate the bias. Sound bovine teeth of near equal sizes can be procured from the sacrificed animals of similar age group. Many authors have recommended various preparations on enamel, which have ranged from no undercut shoulder, [20] Saw tooth edges, [21] cross hatching, [22] butt joint margins, 45 degree bevels, threaded pin and retentive slot preparations, chamfer preparation, [23] short bevel, [12] long bevel and modified bevel. [24] The use of acid-etch technique to bond composite resin to fractured enamel is amply documented in these studies. It s established that the strength of the bond and degree of retention varies directly with the enamel surface available for acid etching. [21,22,25] Various conservative designs of enamel preparation have evolved over time taking advances in material and adhesive technology into consideration. One of the popular approaches to restore fractured anterior tooth was a No cavity preparation in order to prevent further insult to the fractured tooth. [23,25,26] The resin was featheredged over acid etched uncut enamel immediately adjacent to the fracture site. Such an approach has been recognized as basis of Table 4: Comparison between various groups using independent samples t test Between groups t value P value Inference Control - Bevel <0.001 (HS) Control - Chamfer <0.001 (HS) Control - Stair-step chamfer <0.001 (HS) Bevel - Chamfer <0.01 (S) Bevel - Stair-step chamfer <0.05 (S) Chamfer - Stair-step chamfer >0.05 (NS) HS-Highly significant: S-Significant: NS-Non-significant J Indian Soc Pedod Prev Dent - June

5 Table 5: Percentage wise distribution of failure mode at fracture site in various enamel preparation groups Group Adhesive Cohesive Mixed Total Bevel percentage Chamfer percentage Stair-step chamfer percentage Total percentage Percentage Graph 2: Percentage wise distribution of failure mode in the enamel preparation groups potential failure and currently virtually discarded for several reasons. Firstly, unprepared enamel surfaces may be highly resistant to acid conditioning because of the presence of fluorosed or prism-less enamel in the superficial layers. Secondly, this technique resulted in over-contoured restoration, which may be esthetically objectionable. Thirdly, the featheredged margins, which are far from ideal, would be weak, distort or fracture easily, thereby increasing the leakage pattern. Other reasons cited for definitive preparation of enamel are; provision of strength to the bond between enamel and the resin because there is more enamel surface to bond, exposing the enamel prisms in the ideal end-on relationship for etching and bonding, [27] adding to the bulk of the restoration [22] and reducing the polymerization shrinkage. [28] Most clinicians prefer to use bevel because of conservative approach especially towards traumatized teeth and to gradient the color change from tooth matter to restoring material. Chamfer preparation is also popular as it, effectively removes the acid resistant superficial enamel surface and exposes the more reactive subsurface enamel to the effect of acid etching, provides a well-defined marginal periphery to which the resin material may be exactly finished, provides a resin lap-joint, which affords marginal bulk of restorative material, which effectively makes the abrupt resinenamel interface of a butt joint and enhances the effects of acid conditioning by ensuring that the ends of enamel prisms rather than their longitudinal axes are exposed to the effects of acid. Chamfer however lacks on the count of esthetics when compared to gradually merging bevel. It is a universal observation by most of the clinicians that the bulk of composite resin at the chamfered margin does not blend with the natural tooth as well as the beveled margin does. To address to this problem Donly KJ and Browning Randall [29] have even recommended beveling of the cavosurface margin finally after placing the chamfer. In order to address to the same concern and achieve high esthetical result with chamfer technique, stair-step chamfer technique of enamel preparation was introduced by Albers. [5] The technique involved masking of chamfer margins within the natural anatomic horizontal and vertical lines on the tooth. In other terms the chamfered margin could be stair-stepped to correspond to the anatomic contours of the tooth for better masking effect [Figure 3]. The bulk pack technique of composite resin using preformed strip crowns employed for the present study is a desired procedure in pediatric dental practice set-up to reduce the treatment time especially when dealing with patients with multiple carious lesions or an uncooperative child. Soft start polymerization was used to cure the composite resin using Translux energy (Hareus Kulzer) in order to address the concern of stress buildup at the interface when using bulk pack technique, which could result in polymerization shrinkage of composite during curing. Lingual side was chosen for the application of loading force to simulate oral conditions where trauma from biting of hard objects contributes maximum towards failure of such restorations. Cantilever bending test was employed to determine the fracture resistance of composite buildup. In this test setup, loading force was applied on the restoration in a lingual to labial direction perpendicular to the longitudinal axis of the tooth with the specimen firmly held in the lower jaw of the testing machine. In a simple mechanical model as suggested by Black and others in 1981, [11] if neutral axis were to remain located within the tooth, the cantilever bending would expose lingual surface to tension, whereas the labial surface would be exposed to compression. Therefore the fracture will begin at the lingual side and on initiation of crack it would expose the labial surface to a complex bending load thereby putting to test the enamel preparation design for its contribution to the retention of resin restoration. One of the assumptions necessary to make valid conclusions from the numerical data is that a normal distribution of data exists and that standard deviation among experimental groups is to a degree similar. As observed from the data, the control group of untreated teeth comparatively showed substantially higher mean failure load values compared to the experimental groups [Table 1, Graph 1]. 73 J Indian Soc Pedod Prev Dent - June 2006

6 Higher values of chamfer preparation compared to restoration with a bevel preparation margin might be expected to have greater fracture resistance due to the larger volume of composite resin available at the restoration margin. The chamfer preparation also provides for a resin lap joint, which could probably explain for increased resistance to fracture. The findings of this study were in accordance with the study of Donly KJ and Browning. [29] The difference between the mean peak failure load of chamfer group and stair-step chamfer group was found non-significant (P > 0.05) Stair-step chamfer preparation showed slightly less failure load values compared to chamfer group. The possible rationalization for such an observation could be less exposure of enamel rods for bonding since stairstep chamfer technique involved sacrificing less amount of natural enamel around the fractured edge. The findings of this study were in contentious disagreement with the findings of Hani Eid and White GE [30] where slightly higher mean shear bond values were recorded for stair-step chamfer group (. MPa) compared to those of chamfer group (14.93 MPa). The essential observation to make here is that the fracture resistance values recorded in the present study (in Newton) are not influenced by the estimated surface area unlike shear bond strength estimation. The complex compressive forces generated during the loading are likely to cause fracture through the restorative material before the shear components could operate at the interface between resin and enamel causing adhesive failures. Similar results were noticed in the present study where in total only 8/45 =18% failures were recorded as adhesive (A) compared to 19/45=42% cohesive (C) and 18/45= 40% as mixed (combination) (M) failure [Table 5, Graph 2]. Compared to chamfer and stair-step chamfer group, notably higher number of failures were recorded as adhesive in bevel group [Table 5, Graph 2]. This observation could possibly be an indicative of limited contribution of bevel preparation design on the interfacial bond strength when compared to chamfer and stair-step chamfer preparations. In notably high number of cases material strength possibly exceeded the interfacial bond strength thereby resulting in adhesive failures. In chamfer and stair-step chamfer group majority of the failures occurred within the resin thereby indicating high resistance to interfacial failure (adhesive) offered by these designs. It could be possibly inferred that the chamfer and stair-step chamfer preparation had positive influence on the fracture resistance of the composite resin. In general it is established that, as the retentive values for the restorative material increased, progressively larger proportions of materials remained bonded to the enamel surface. This is an indication that a large proportion of the failure occurred within the restorative material. As the retentive values decreased, less restorative material remained on the enamel surface. These observations in the present study was in support of findings observed by the study of Nelson, Till and Hinding. [4] Composite restorations showed maximum resistance to fracture when chamfer preparation was used. Originally an esthetic technique, stair-step chamfer on the other hand showed greatest acceptability since it involved removal of less amount of natural enamel, had similar effect on fracture resistance of composite resistance and showed similar failure modes with occurrence of high percentage of cohesive failures in comparison to chamfer preparation. However, the ease of performing the clinical procedures by the clinician is a major consideration when executing a specific operative procedure. In the present study, every effort was taken to duplicate the oral situations however; the in vivo responses to the enamel preparation design might differ from the results of this study. Nonetheless, clinical implications of this research project are significant. The conclusion of this in-vitro investigation must be extrapolated to the clinical situation with care and further in vivo trials with these materials and surface treatments are indicated to confirm the validity of these recommendations. References 1. Shortall AC, Uctasli S, Marquis PM. Fracture resistance of anterior, posterior and universal light activated composite restoratives. Oper Dent 2001;26: Worthington RB, Murchison DF, Vandewalle KS. Incisal edge reattachment: The effect of preparation utilization and design. Quintessence Int 1999;30: Strassler HE. 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