Evaluation Of Fracture Strength Of Tooth Roots Following Hand And Rotary Instrumentation – An Invitro Study

INTRODUCTION

       Resistance to fracture is an important consideration in endodontics for subsequent restoration and successful function of endodontically treated teeth. Vertical root fracture of endodontically treated teeth is an important clinical problem and often results in tooth extraction, because the prognosis of a vertical root fracture is very poor.

       Endodontic and restorative procedures such as caries removal, access preparation, instrumentation of the root canal, post placement, forces during lateral condensation lead to loss of tooth structure and may weaken the dentin. But numerous experimental studies have challenged this conclusion. They have revealed that dentin of endodontically treated teeth does not exhibit significant difference in mechanical properties from those of vital teeth; that is dentin does not appear to become brittle.

       It has been shown that ideal access cavity preparation has no significant effects on tooth stiffness and the load generated during lateral condensation is also far lower than the load required to fracture the roots¹. Thus obturation also should not be regarded as a major cause of vertical root fracture.

       It is generally accepted that the strength of an endodontically treated tooth is directly related to the amount of remaining sound tooth structure. Root canal preparation involves dentin removal and may compromise the fracture strength of the roots². This is an important factor to be considered as a potential cause of vertical root fracture.

       With regard to the instrumentation of the root canal during endodontic treatment, several nickel – titanium instruments have been developed to be used in a rotary technique. The introduction of rotary nickel-titanium instruments for root canal preparation has changed canal shape, size and taper compared to hand instrumentation with stainless steel K-files. Canals prepared by Ni-Ti instruments have shown increased canal cleanliness, less straightening, apical canal transportation and perforations. Canals also have a rounded or oval shape and remain more centered. Studies have also shown that hand instrumentation with stainless steel K-files is more conservative than the rotary techniques which may influence root strength³. Therefore the aim of this in vitro study was to determine whether these canal preparation techniques with hand and various rotary instruments would influence the susceptibility of the root to fracture.

MATERIALS AND METHOD

       Extracted human mandibular first molars were stored in normal saline until they were used for study. The teeth were thoroughly cleaned of tissues and calculus with curettes and ultrasonic scaler and examined by the naked eye. Teeth with immature apices, cracks on the root surface, gross caries involving the root and exceptionally short, thin or curved roots were discarded.

       Forty teeth with slight to moderate mesial root curvature were randomly allocated into four groups for canal preparation by following methods

  Group 1 - Step  backtechnique using conventional stainless steel K-files

  Group 2 - Rotary NiTiProtaper instruments

  Group 3 - Rotary NiTiRaCe instruments

  Group 4 - Rotary NiTiProFile instruments

       The crown of each tooth was cut off 2mm coronal to the cementoenamel junction with a high speed diamond disc to facilitate straight line access for instrumentation and obturation. Access was prepared and the working length of the mesiobuccal canal was determined by placing a No. 8 size K-file to the apex. This length was measured and the working length was set 0.5mm short of this distance. The flat surface which is 2mm above the cementoenamel junction was used as the reference point.

       All canals were then prepared by hand until a size 15 K file bound at the working length. The four groups were then prepared according to the following techniques.

Group 1 Step back techniques using Stainless steel K files

       The mesiobuccal canals were prepared by hand instrumentation with size 25 as the master apical file (MAF). Gates Glidden drills (size 2 & 3) were used initially to preflare the canal. This was followed by hand filing to the MAF and then stepback in 1mm increments for three additional file sizes.

       Recapitulation with the MAF at the working length was carried out after each step back size file.

Group 2 Rotary NiTiProTaper instruments

       All the canals were instrumented to the working length 0.5mm short of apex, using a gear reduction handpiece powered by electric motor. First step is to achieve a straight line access to reduce all overlapping dentin areas, by using Sx file. The pulp chamber was filled with irrigating solution sodium hypochlorite during the whole shaping procedure. In addition, viscous EDTA chelator was used to minimize force on the instrument.

       Now S1 is moved to working length in a brushing motion. After the successful insertion of S1, S2 is used in one or two strokes to working length. Next step, the apical diameter of the root canal is gauged with a No. 20 K-file. F1 which has the same diameter at the instrument tip of S2 is used till working length. Following this, preparation was finished with F2 which has a taper of 8% and master apical file size of 25.

Group 3 Rotary NiTi RaCe instruments

       The first step is to achieve straight line access and reduce all overlapping dentin areas by two orifice shapers, which have a 8% taper with 35 ISO tip size and 10% taper with 40 ISO tip size.

       The pulp chamber was filled with sodium hypochlorite or EDTA during the whole procedure. A viscous chelator is also used to reduce the force on the instrument. Now the instrument which has 2% taper with 25 ISO tip size is worked up to the working length, followed by 4% taper with 25 ISO tip size. Finally the instrument which has 6% taper and 25 ISO tip size instrument is used to complete the preparation.

Group 4 Rotary NiTi ProFile instruments.

       The first step in the preparation is to prepare the coronal and middle thirds of the canal using 6% taper with 30 ISO tip size and 25 ISO tip size, along with 4% taper with ISO tip size 30 and 25.

       Now the apical preparation is done by 4% taper with ISO tip size 25. The final flaring is done by 6% taper with 25 ISO tip size instrument.

       All canals were irrigated frequently with 5% sodium hypochlorite to remove debris during the instrumentation procedure and after instrumentation was complete. Teeth were stored in water after the instrumentation procedures to prevent dehydration until the testing procedure was started.

       Before obturation, the canals were dried with paper points. Zinc oxide eugenol root canal sealer was mixed manually. A file which is one size smaller than the Master apical file size was selected. A rubber stopper was positioned at the working length in the canal and rotated counterclockwise to spin the sealer in the canal.

       All the samples were obturated with guttapercha by lateral condensation method. The distal root was removed by using diamond disc and the mesial root was wrapped with lead foil backing from an X-ray film to the level of cementoenamel junction and the outer surface was lubricated with Vaseline. Then the specimen was then embedded in a fresh mix of acrylic resin to the level of the cementoenamel junction.

       After the acrylic resin has set, the root was removed along with the lead foil. Addition Silicone putty impression material was mixed according to the manufacturer's instructions and painted onto the surface of the root. The root was repositioned in its created acrylic resin socket and excess silicone impression material was removed. This created an artificial socket which simulated the periodontal membrane found in a tooth socket.

       An Instron testing machine running at a cross head speed of 1mm/min was used to test the fracture load. An ISO 25 tip size stainless steel finger spreader of the same size as master apical file was attached to the machine and was inserted into the root canal to contact gutta-percha.

       Once the test was started, the spreader tip gradually applied force within the canal via the gutta-percha and stopped immediately after fracture was detected. The load at which fracture occurred was recorded in Kilogram force. The results were then statistically evaluated using oneway test of variance (ANOVA).

       The loads at fracture for specimen among the groups 1 to 4 are presented in (Table 1 – 4).

DISCUSSION

       Vertical root fracture of endodontically treated teeth is an important clinical problem that results inevitably in extraction or resection of the affected root. The roots of endodontically treated teeth may be weakened by excessive removal of dentin during canal preparation or post space preparation, increasing susceptibility to root fracture.¹ Endodontic and various restorative procedures have been investigated as precipitating factors for vertical root fracture of endodontically treated teeth. Obturation stresses produced within the tooth due to various obturation techniques and by post preparation and placement have also been investigated as major causes of vertical root fractures.

       In 1999, Veera Letchirakarn et al⁴ studied forces encountered during lateral condensation technique and concluded that lateral condensation alone should not be a direct cause of vertical root fracture, as loads generated during lateral condensation were significantly lower than the load required to fracture the root. This indicates the need for further investigation into factors that predisposes to fracture. One possibility is the weakening effect of excessively large canal preparations.

       Advancements in rotary nickel-titanium instruments over the last decade have led to new design concepts and techniques of canal preparation. Canals prepared by NiTi instruments show increased canal cleanliness and less straightening, less apical transportation and perforations.⁵ Canals also have a rounded or oval shape and remain more centered. These benefits are because of the greater flexibility of specific design features of NiTi instruments allowing the natural canal curvature to be maintained.

       The Step-back technique using conventional stainless steel K-files and rotary techniques with ProTaper, RaCe prepare different canal tapers. Hand preparation with master apical file (MAF) of ISO size 25 creates an apical stop at 0.5 to 1mm from the apex with step back resulting in a taper of approximately 0.05mm/mm to atleast 5mm from the apex. Depending on the extent of the use of gates glidden burs the coronal taper will be variable.

       ProTaper NiTi rotary instruments creates a canal preparation at a distance of 0.05 to 1mm from the apex with Master apical file size (MAF) of 25 with 0.08/mm taper from the apical stop.

       RaCe and ProFile NiTi rotary instruments creates canal preparation with MAF size of 25 and a taper of 0.06mm/mm at a distance of 0.5 to 1mm from the apex.

       The aim of this in vitro study is to determine whether these canal preparation techniques by NiTi rotary instruments when compared to hand instrumentation by conventional stainless steel Kfiles would influence the susceptibility of the root to fracture; or alter the location and pattern of fracture lines.

       Experimental techniques for investigating root fracture have generally involved the generation of force within the canal space by means of a spreader inserted into the obturated canal. ^4,5,6,7

       Loading of the canal walls may occur by the spreader binding directly against the canal wall or via gutta-percha.⁴

       In this study, the canals were obturated by lateral condensation method. The method of obturation was not intended to simulate the recommended obturation techniques, but to fulfill the need of having guttapercha as the medium within which forces could be transmitted throughout the canal.⁹

       The load required to fracture the root provides an indication of fracture susceptibility of the root when subjected to forces encountered during obturation, post placement or subsequent clinical function.⁹

       In order to standardize the apical canal diameter of the enlarged root canals, a size 25 master file was used in all groups. A taper of 6% was used while preparing the canals with NiTi RaCe and ProFile instruments. A taper of 8% was used for ProTaper instrument system as this taper was available for a master apical file size of 25.

       The Universal Instron testing machine was used in this study as it is a highly accurate and versatile testing instrument used for the precise measurement of the properties and behavior of materials like compression, flexion and torsion. The instrument weighing system uses strain gauge load cells for measuring the loads applied to the specimens.

       Specimen in this study were clamped to the stable jaw of the Instron testing machine, the load via spreader was uniformly increased at a cross head speed of 1mm/mm to evaluate the load at which the specimen fractures. Breaking load values were recorded and the values were obtained in Kilogram force (Kgf).

       The Group I specimens in which the root canals prepared by step-back technique using stainless steel K-files showed lower fracture resistance of all the groups. The specimens fractured at the mean load of 11.47Kg. The mean fracture load obtained by Lertchirakarnet al⁴ for mesial roots of mandibular molars was 8.1Kg and according to Lindauer et al⁸ the minimum force required to fracture a mandibular molar was 7Kg of force. These values are found to be closest to the hand preparation group in this study, but considerably lower than the fracture loads required to fracture the specimens in the Rotary NiTi instrumentation groups.

       The Group II specimens prepared by NiTi Rotary ProTaper instruments showed higher fracture load than hand instrumentation but lower than the Rotary NiTi Race and ProFile instruments. The specimens fractured at a mean load of 14.55kg.

       The Group III specimens prepared by Rotary NiTi RaCe instruments showed higher fracture load of all of the groups. The specimens fractured at a mean load of 15.12kg.

       The Group IV specimens prepared by Rotary NiTi ProFile instruments showed higher fracture load than the hand instrumentation group and NiTi rotary ProTaper group. The specimens fractured at a mean load of 15.02kg.

       The results of this in vitro study showed that no statistically significantly differences were observed among groups with regard to fracture loads. But there appears to be some tendency for higher fracture resistance for roots prepared with rotary NiTi instruments. This may be the result of effect of the rounder canal shapes prepared by Rotary instrumentation groups. This result coincided with the studies done by Patsandra P.S. Lam et al⁹ and ChankhritSathorn et al.¹⁰

       The introduction of rotary nickel-titanium instruments for canal preparation has changed canal shape, size and taper compared to hand instrumentation. Canal preparation after preparation with hand files can be quite irregular.⁵ From a fracture mechanics point of view, the presence of structural defects, cracks or canal irregularities are likely to play a major role in determining fracture strength. With rotary NiTi preparation, canal shapes are more likely to be rounder and smoother and canal irregularities are likely to be incorporated into the preparation and eliminated.¹⁰

       Harvey et al¹¹ has showed by photoelastic model system that flared preparation allowed condensation forces to be exerted on the apical third of the root and also allowed greater distribution of stresses.

CONCLUSION

       Based on the results of this study, these NiTi rotary instruments do not weaken roots any more than conventional step-back K-file preparations and probably may increase the fracture resistance. Further investigations into other types of rotary NiTi instruments and in other group of teeth can give further details about the effects of different rotary NiTi instruments on fracture strength of the teeth and susceptibility to vertical root fracture.