ENDODONTOLOGY ABSTRACT INTRODUCTION

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1 To Analyze the Distribution of Root Canal Stresses after Simulated Canal Preparation of Different Canal Taper in Mandibular First Premolar by Finite Element Study An In Vitro Study. DHANYA KUMAR N. M. * ABHISHEK SINGHANIA ** VASUNDHARA SHIVANNA *** ABSTRACT Was to Investigate stress distribution patterns in simulated biomechanically prepared mandibular first premolars with four different tapers at two different compaction forces and an occlusal load with finite element analysis. Six recently extracted, intact, non-carious, undestroyed mandibular premolars similar in-straight root canals were selected. Four finite element models were designed on the software varying only in canal taper of mandibular first premolars. Gutta-percha was compacted by vertical condensation technique in three separate vertical increments under two different vertical compaction forces that are 10N and 15N. Finite element meshes were generated with this model by using soft ware to know the pattern of distribution of radicular stresses during obturation. At last access opening will be filled by using simulated restorative material (composite). A masticatory load of 50N was applied; again Finite element meshes were generated. The highest circumferential and radial stresses were found during compaction of first gutta percha increment, while an increase in taper reduced the stress level for the same compaction force. During obturation, higher stresses were found at the canal surface, using the smallest taper, in apical third, during the first gutta percha increment and gradually decreased along the canal length. Root stresses during occlusal load application generates the highest stresses at external root surface and concentrate at cervical third, an increase taper size caused only slight lower root stresses. With increasing taper root stresses decreased during root canal obturation. Root fracture at the apical third is likely initiated during obturation. Root fracture at the cervical third is likely initiated during occlusal load. KEYWORDS: canal taper, compaction force, finite element analysis, occlusal load, root stresses, vertical compaction, vertical root fracture. INTRODUCTION After endodontic therapy, teeth are more prone to vertical root fracture because of loss of moisture (9%) and become more brittle when compared to vital tooth. Vertical root fracture can occur in teeth during or subsequent to endodontic therapy. The causative factors for vertical root fractures are the compaction of gutta percha, placement of intraradicular posts, masticatory load, trauma, and traumatic injuries. Vertical root fractures are more * PROFESSOR, ** POST GRADUATE, *** PROFESSOR & HEAD, DEPARTMENT OF CONSERVATIVE DENTISTRY AND ENDODONTICS, COLLEGE OF DENTAL SCIENCES (C.O.D.S.), DAVANGERE , KARNATAKA. 12

2 DHANYA KUMAR N. M., ABHISHEK SINGHANIA, VASUNDHARA SHIVANNA common during the vertical condensation technique of obturation and often complicate or prevent subsequent restorative procedures. These fractures a count for the most serious complication of root canal treatment and often result in tooth extraction because of poor prognosis 6. It is generally accepted that the strength of an endodontically treated tooth is directly related to the amount of remaining tooth structure. Several treatment procedures such as caries removal, access preparation, instrumentation of root canal, irrigation of root canal with sodium hypochlorite, and long term intracanal dressing with calcium hydroxide lead to loss of tooth structure or weaken the root dentine. The prevalence of Vertical root fracture is not equally distributed over the different tooth types. Maxillary and mandibular premolars have both recorded a high prevalence 6. Stresses distribution in endodontically treated teeth can be measured by photoelastic method, strain gauge and instron testing machine. But the major disadvantage of all these methods in stresses can measured at selected sites only and not inside the root canal 7. Finite element analysis is an engineering method for the numerical analysis of complex structures based on their material properties (Young s modulus and Poisson ratio) to determine the distribution of stresses and strain pattern induced in internal structure of tooth / bone / implants / any living tissue 5. The purpose of the present study is to investigate stress distribution patterns in simulated biomechanically prepared mandibular first premolars with four different tapers at two different compaction forces and an occlusal load with finite element analysis. METHODOLOGY Twenty four samples have been derived from six recently extracted, intact, non-carious, undestroyed mandibular first premolars. Six x-ray films were used to know the canal curvature of six mandibular first premolars. Optical scanner was used to digitalize the external surface morphology of six extracted mandibular first premolars on computer software that has been designed for Finite Element analysis. GROUPING Four finite element models were designed on the software varying only in canal taper of each mandibular first premolar. Each model carried six specimens. These models were assigned as: Group 1 with taper 2%, Group 2 with taper 4%, Group 3 with taper 6% and Group 4 with taper 12%. All other aspects of the models were held constant including boundary conditions, material properties, compaction forces during filling and magnitude / direction of applied occlusal load. The tooth model was created by digitizing the external surface of extracted human mandibular first premolar with an optical scanner in combination with Finite element analysis computer software (NISA). A straight root was chosen for this study to eliminate effects due to canal curvature. Guttapercha were compacted by vertical condensation technique in three separate vertical increments (apical 1/3, middle 1/3, cervical 1/3) under two different vertical compaction forces that are 10 Newton and 15 Newton for each increment. 200µm thick periodontal ligament layer and a surrounding bone volume to support the root were created on 13

3 TO ANALYZE THE DISTRIBUTION OF ROOT CANAL STRESSES AFTER SIMULATED CANAL PREPARATION OF DIFFERENT CANAL TAPER IN MANDIBULAR FIRST PREMOLAR BY FINITE ELEMENT STUDY AN IN VITRO STUDY. finite element model. Subsequently, a simulated standard access opening was made in the crown, and root canals were created that represented 2%, 4%, 6% and 12%. The 4% and 6% tapers were chosen for clinical relevancy, as these are incorporated into commonly used nickel titanium rotary files and are representative of clinically imparted tapers on the canal space. The drastic 12% tapered canal preparation was chosen arbitrarily to simulate the effects of excessive canal preparation. All models were created with a final apical preparation of 0.35 mm at the point of constriction, 0.5 mm from what would be clinically perceived as the radiographic apex. All canal preparations were straight. Isotropic properties were applied for the dentine, periodontal ligament, supporting bone volume, gutta-percha and restorative composite. The periodontal ligament was modeled as a soft incompressible connective layer. An arbitrary range of friction coefficients ( ) were evaluated to account for the friction between the gutta-percha and the root canal wall. The development of radicular stresses was analyzed during three consecutive filling steps as well as for an occlusal load after the root filling using finite element analysis. Warm gutta-percha was compacted in three separate vertical increments until the canal was filled. The gutta-percha temperature at the start of compaction was 60 o C o and was gradually cooled down during the filling procedure until it reached 37 o C o. In this analysis, two vertical compaction forces were tested at 10 and 15 N for each increment. The forces were applied by means of a simulated plugger. The plugger surface had slightly rounded edges and a tip-diameter that was 0.5 mm smaller than the canal diameter at each compaction increment. After complete simulated obturation, the simulated access space was closed using a simulated bonded restorative composite. The composite was filled; a 50 N occlusal load was applied in the buccolingual plane to the triangular ridge of the buccal cusp (functional cusp) at an angle of 60 0 with the vertical axis. The value of the occlusal force was chosen to represent a relatively high biting force and buccal cusp was selected because it is the functional cusp and lingual cusp is rudimentary in mandibular first premolar. During the analysis, the root was supported by the surrounding bone volume via the soft periodontal ligament layer, which was given incompressible properties to approximate fluid behavior. The mean value, standard deviation and one way ANOVA was used to evaluate the site of maximum stress concentration. Statistical analysis (ANOVA) was used for multiple comparison and correlation analysis to assess the relationship between different taper and radicular stresses. RESULTS Four finite element models were designed on the software varying only in canal taper as- Group 1 with taper 2%, Group 2 with taper 4%, Group 3 with taper 6% and Group 4 with taper 12%. In group 1 with taper 2%, the highest mean value and standard deviation was at apical third followed by middle and cervical third under compaction force of 10 newton (Table-1). From group 1 to group 4, the highest mean value and standard deviation was at apical third of 14

4 DHANYA KUMAR N. M., ABHISHEK SINGHANIA, VASUNDHARA SHIVANNA 2% taper (group-1) followed by middle and cervical third of 2% taper under compaction force of 10 Newton (Table-1). Similar results were obtained in different groups under compaction force of 15 Newton (Table-2). During occlusion load application (50N) in different group, highest mean was recorded in group 1 with 2 % taper and least in group 4 with 12% taper. Highly significant pairs were group 1 & 2, group 1 & 3 and group 1 & 4 (Table 3). On comparing the different tapers with each increment under compaction force of 10N shows the significant pairs apical third and coronal third for taper 2% ( P<0.05), apical third and middle third, apical third and coronal third for taper 6% and 12% taper (P<0.001) (Table 4). Also on comparing the different tapers with each increment under compaction force of 15N shows the significant pairs apical third and middle third, apical third and coronal third for taper 4%, 6% and 12% taper (P<0.001) (Table 5) Results: Highest circumferential and radial stresses were found during compaction of first gutta percha increment, while an increase in taper reduced the stress level for the same compaction force. During obturation, higher stresses were found at the canal surface and in apical third with smallest taper and during the first gutta percha increment and gradually decreased along the canal length. Root stresses distribution during occlusal load application generated the highest stresses at external root surface that concentrated at the tooth surface of the cervical third, an increase taper size caused only slight lower root stresses. DISCUSSION The prognosis of endodontically treated teeth depends not only on the success of the endodontic treatment but also on the amount of remaining dentine tissue, and the nature of final restoration. Fractures of restored endodontically treated teeth are a common occurrence in clinical practice and it is the second most frequent identifiable reason for loss of endodontically treated teeth 4. The increased susceptibility of fracture in endodontically treated teeth had been attributed due to the increased brittleness of dentine, due to loss of moisture - Helfer et al reported that the moisture content of dentine from endodontically treated teeth was about 9% less than teeth with vital pulp. However, Papa et al emphasized the importance of conserving the bulk of dentine to maintain the structural integrity of post-endodontically restored teeth. Other studies have also emphasized that the loss of tooth structure is the key reason for the increase in fracture predilection of endodontically treated teeth 1. The fracture resistance of the restored endodontically treated tooth is a function of the strength of the root (taper of prepared canal) and remaining coronal tooth structure. 34 Tooth fracture has been described as a major problem in dentistry, and is the third most common cause of tooth loss after dental caries and periodontal disease. Generally, an endodontically treated tooth undergoes coronal and radicular tissue loss due to prior pathology, endodontic treatment, and/or restorative procedures. The loss of dentine tissue will compromise the mechanical integrity of the remaining tooth structure 1. Mandibular premolars and maxillary premolars 15

5 TO ANALYZE THE DISTRIBUTION OF ROOT CANAL STRESSES AFTER SIMULATED CANAL PREPARATION OF DIFFERENT CANAL TAPER IN MANDIBULAR FIRST PREMOLAR BY FINITE ELEMENT STUDY AN IN VITRO STUDY. have both recorded a high prevalence of vertical root fracture in endodontically treated teeth 21. Vertical root fracture seems to be a more common reason for extraction of endodontically treated teeth. 2 Root canal biomechanical preparation can be done by a hand file or rotary file. Canal preparation involves dentin removal and may compromise the fracture strength of the roots 25. The development of new design features such as varying tapers, noncutting safety tips and varying length of cutting blades in combination with the metallurgic properties of alloy nickel-titanium have resulted in a new generation of instruments and concepts 33. Although no significant difference in the fracture load of hand and rotary nickel titanium canal preparation could be demonstrated 3. Given increasing acceptance of rotary instrumentation as a technique for cleaning and shaping the canal space, it is important to examine the effect of specific tapers imparted by rotary instrumentation of the canal wall as it relates to vertical root fracture. The clinician must make a decision to use instruments which have an inherently larger or smaller taper based on the architecture present in a given canal. Choosing a smaller taper may reduce the risk of procedural accidents and untoward events during cleaning and shaping, but it may compromise the cleanliness of the canal system and placement of filling material. Choosing too large a taper may increase canal cleanliness (especially in the coronal and mid-root areas), but may also increase the potential for strip perforations, other procedural accidents, and may predispose the root to vertical fracture if, indeed, greater reduction of root structure increases stress in the canal wall. 7 Assessment of stress levels patterns in root canal can be measured by a number of ways that includes Instron universal machine, photoelastic method, strain gauges and most recent one is finite element analysis 10. Assessment of stress levels by measuring deformation patterns inside the root canal is extremely difficult, leaving investigators with indirect external observations at best extremely difficult. Finite element analyses have been utilized to address these difficulties and gain insight into internal stress distributions 7,2. Finite element analysis which is an engineering method for the numerical analysis of structure based on their material properties has been used for stress analysis. Material properties such as the Young s modulus and Poisson Ratio can be utilized by computer generated analyses to describe the mechanical behavior of a structure 5. CONCLUSION With in the limitation of this finite element analysis, the following conclusions were drawn. During simulated obturation, root stresses decreased as the root canal taper increases and stresses were greatest at the apical third and along the canal wall. After simulated root canal obturation was completed and occlusal force was applied, the generated stresses were greatest at the cervical portion of the root surface, and decreased as taper increases. It was likely that vertical root fractures initiated at the apical third as result of compaction forces, whereas vertical root fractures initiated cervically were a manifestation of subsequent masticatory load on the root canal obturated teeth. However additional in-vivo and in-vitro tests and clinical trial are desirable in order to elucidate the accuracy of finite element analysis. 16

6 DHANYA KUMAR N. M., ABHISHEK SINGHANIA, VASUNDHARA SHIVANNA 10 NEWTON OF COMPACTION FORCE Apical 1/3 Middle 1/3 Cervical 1/3 Tapers Mean SD Mean SD Mean SD 2% % % % Table-1: Compaction Force of 10N on Apical, Middle and Cervical Third in Different Tapers. 15 NEWTON OF COMPACTION FORCE Apical 1/3 Middle 1/3 Cervical 1/3 Tapers Mean SD Mean SD Mean SD 2% % % % Table-2: Compaction Force of 15N on Apical, Middle and Cervical Third in Different Tapers. Tapers Mean 2% % % % P* Value, Sig P<0.001 HS Significant Pairs** I&II, I&III,I&IV Table-3: Comparision of Mean Occlusion Load of 50N in Different Tapers. Tapers Apical 1/3 Middle 1/3 Cervical 1/3 P* Value, Sig Significant Pairs** 2% P<0.05 S At & Ct 4% P>0.05 NS - 6% P<0.001 HS At &Mt, At & Ct 12% P<0.001 HS At &Mt, At & Ct Table-4: Comparison Of Different Tapers With Each Increment To Evaluate Significant Pair Under Compaction Force Of 10N. 17

7 TO ANALYZE THE DISTRIBUTION OF ROOT CANAL STRESSES AFTER SIMULATED CANAL PREPARATION OF DIFFERENT CANAL TAPER IN MANDIBULAR FIRST PREMOLAR BY FINITE ELEMENT STUDY AN IN VITRO STUDY. Tapers Apical 1/3 Middle 1/3 Cervical 1/3 P* Value, Sig Significant Pairs** 2% P>0.05 NS - 4% P<0.001 HS At &Mt, At & Ct 6% P<0.001 HS At &Mt, At & Ct 12% P<0.001 HS At &Mt, At & Ct Table-5: Comparison Of Different Tapers With Each Increment To Evaluate Significant Pair Under Compaction Force Of 15N Fig 1 apical third g.p filling on model with taper 2% under compaction forces (10n) fig. 2 (15n). Fig 3.middle third g.p filling on model with taper 2% under compaction forces (10n).fig 4 (15n). Fig 5cervical third g.p filling on model with taper 2% under compaction forces (10n). Fig 6 (15n). Fig 7 occlusal load of 50n applied on model (2%), after filling the access cavity with composite (stress graph ).fig 8 occlusal load of 50n applied on model (2%), after filling the access cavity with composite (displacement graph). Fig 9 apical third g.p filling on model with taper 4% under compaction forces (15n). Fig 10 (10n). Fig 11middle third g.p filling on model with taper 4% under compaction forces ( 15n). Fig 12 (10n). 18

8 DHANYA KUMAR N. M., ABHISHEK SINGHANIA, VASUNDHARA SHIVANNA Fig 13 cervical third g.p filling on model with taper 4% compaction forces (15n) fig 14 (10n) fig 15 occlusal load of 50n applied on model (4%), after filling the access cavity with composite(stress graph ) fig 16 (displacement graph) fig 17 apical third g.p filling on model with taper 6% compaction forces ( 15n) fig 18 (10n) fig 19 middle third g.p filling on model with taper 6% under compaction forces ( 15n). Fig 20 (10n). Fig 21 cervical third g.p filling on model with taper 6% compaction forces (15n) fig n fig 23 cervical third g.p filling on model with taper 6% compaction forces (10n) occlusal load of 50n applied on model (6%), after fiiling the access cavity with composite (stress graph ) fig 24occlusal load of 50n applied on model (6%), after fiiling the access cavity with composite (displacement graph)

9 TO ANALYZE THE DISTRIBUTION OF ROOT CANAL STRESSES AFTER SIMULATED CANAL PREPARATION OF DIFFERENT CANAL TAPER IN MANDIBULAR FIRST PREMOLAR BY FINITE ELEMENT STUDY AN IN VITRO STUDY Fig 25 apical third g.p filling on model with taper 12% under compaction forces (10n). Fig 26 (15n). Fig 27 middle third g.p filling on model with taper 12% under compaction forces (10n) fig 28 (15n). Fig 29 cervical third g.p filling on model with taper 12% under compaction forces (10n). Fig 30 (15n). Fig 31 occlusal load of 50n applied on model (12 %), after fiiling the access cavity with composite (stress graph ) fig 32 occlusal load of 50n applied on model (12 %), after fiiling the access cavity with composit e (displacement graph) MATERIAL PROPERTIES APPLIED IN THE STRESS ANALYSIS (FEA) Material Elastic Reference Poisson s Reference Modulus (GPa) Ratio Reference Enamel 84 Craig & Powers 0.33 Farah et al. (1989) (principal direction) (2002) Enamel 42 Craig & Powers 0.31 Farah et al. (1989) (transverse plane) (2002) Dentine 14.7 Sano et al. (1994) 0.50 Farah et al. (1989) Periodontal ligament Dyment and Synge 0.30 (1935) Bone 0.49 Moroi et al. (1993) 0.30 Farah et al. (1989) Gutta-percha Temperature 0.30 (0 C 0 ) 0.35 Dependent (30 C 0 ) 0.40 (60 C 0 ) Restorative composite 14 Willems et al. (1992) 0.24 Craig & Powers 0.24 Craig & Powers (2002) (2002) GRAPHS 20

10 DHANYA KUMAR N. M., ABHISHEK SINGHANIA, VASUNDHARA SHIVANNA BIBLIOGRAPHY 1) Aviad Tamse. Vertical root fractures in endodontically treated teeth: diagnostic signs and clinical management. Endodontic Topics 2006;13: ) B. D. Rundquist & A. Versluis. How does canal taper affect root stresses? International Endodontic Journal. 2006;39: ) Chankhrit Sathorn, Joseph E.A. Palamara, and Harold H. Messer. Effect of root canal size and external root surface morphology on fracture susceptibility and pattern: A Finite Element Analysis. J Endod 2005;31: ) Chankhrit Sathorn, Joseph E.A. Palamara, and Harold H. Messer. A comparison of the effect of two canal preparation technique on root fracture susceptibility and pattern, J Endod 2005;31: ) Linda J.-William, Peter G. Fotos, Vijay K. Goel, James D. Spivey, Eric M. Rivera and Satish C. Khera. A-three dimensional finite element stress analysis of an endodontic prepared maxillary central incisor. J Endod 1995; 21: ) Tannaz Zandbiglari et al. Influence of instrument taper on the resistance to fracture of endodontically treated roots. Oral surg oral med oral path oral radio endo 2006;101: ) Yeera Lertchirakarm et al. Finite element analysis and strain gauge studies of vertical root fracture.j Endod 2003;29: