Rotary Endo Economics

Successful endodontic treatment depends on a series of sequential procedures including shaping and cleaning of the root canal system. Biggest engima for an endodontist is the separation of rotary endodontic instruments .This negates the achievement of efficient bio mechanical preparation , which can result in under-filling of the root canal or the treated case may end in failure. Even with modern NiTi rotary files, which are more sophisticated, stronger and more flexible than stainless steel instruments, fracture is a concern as these instruments can undergo fracture within their elastic limit and without any visible signs of previous permanent deformation.. Preventive procedures related to breakage/ seperation are of utmost importance. Therefore an endodontist should have understanding of the mechanisms and factors contributing to file fracture is essential. It is very essential that both training and adequate skills are imperative for all dental procedures and particularly so in rotary endodontics. The thumb rule for rotary endodontics as with other endodontic hand instruments is examination of the instruments , before and after use, to rule out any stress on the instrument/ stretch marks. Other important rule is that no rotary instruments should not be used in dry canals and or no instrument should be used without any lubrication. Further rotary files should be used according to the manufacturer's instructions and excessive forces should be avoided.

There is a variety of protocols for rotary root canal instrumentation. Frustration of rotary file separation can be avoided by proper training for mastering instrumentation and improving operator competence through learning and experience .Various studies around the globe has reported that the most important factor the rotary instrument seperation was the operator. This is probably attributed to their clinical skills or their decision either to use instruments for a specific number of times or until defects were evident.

Another important factor for rotary instrument seperation is the root canal morphology. M ore complicated the root canal morphology, the greater the risk of endodontic instrument breakage. A higher prevalence of rotary instrument separation has been reported in molars particularly in the mesial roots of mandibular molars.Further , the risk of instrument seperation in the apical third of the canal is higher when compared with coronal and middle thirds.Rotary files undergo greater fatigue as the curvature increases and the contact surface with the dentinal walls is greater since most curved canals are narrow. Clinically, fatigue of an instrument may be related to the degree of instrument flexure when placed in a curved root canal. When the curvature of canals is pronounced, the cyclical fatigue of the instrument is greater and thus its life expectancy is lower. The parameters of the angle and the radius of curvature are independent in such a way that even if two canals have the same angle of curvature they may have different radii of curvature, which indicates that some curves are sharper than others. Therefore , the radius of canal curvature is more important than the curvature angle itself. Further, the rate of file separation increases as the radius of curvature decreases. When several variables, including those related to root canal anatomy, are considered, the radius of curvature is the most significant factor in rotary file seperation.

An thorough understanding of the characteristics of endodontic rotary file metullary is important when seperation is considered. The properties of endodontic rotary instruments differ from each other depending, on their metallurgy composition. The manner and efficiency of cutting dentine are not the same for any rotary file systems. . As a result, their resistance to defect formation and fracture differ.Cross-sectional shape determines the bulk of the file and the contact area between the file and dentinal walls, as well as its cutting efficiency. The greater the area of file contacting the dentinal walls, the greater the quantity of cut dentine. Consequently there is greater friction and a fatigued rotary file has a reduced life span. There are various types of endodontic file cross-sections, some rotary file instruments have active cutting blades, a radial land between the cutting edge and the flute. It is this radial land which contacts the canal wall on its entire surface. In general, some rotary file instruments cut more effectively and aggressively as compared to some rotary files which perform a scraping or burnishing rather than a real cutting action, and remove dentine more slowly. The size of the rotary file determines how many times it should be used. As the diameter of the rotary file increases, the force needed to unwind or fracture also increases. However, it is recommended that clinically large rotary instruments that are used many times should be reused with great care or be discarded. In the use of rotary instruments the contact surface should be considered especially in clinical cases, as some cross-sectional shapes have greater areas of contact than others. Clinically, the greater the file taper, the greater the contact surface with dentine, and the less resistance to fracture. The finite element analysis studies on the mechanical properties of endodontic rotary files have shown that as the area of the inner core of the cross-section increased, the model was more torque-resistant.

There is no agreement in the literature regarding the number of times of use of rotary instruments related to fracture. The seperation of rotary endodontic files is a complex multi-factorial clinical problem, and therfore one can not expect or recommend how many cases or even canals may be prepared by a rotary file. Individual canals are not anatomically the same as well as all other factors that influence file separation. It is difficult to predict when an instrument will separate. Consequently, single use has been recommended for absolute safety.Various studies have proved that even single used rotary files,14% of instruments showed various defects and 0.9 % fractured. Therefore even with with single use, endodontic rotary files still undergo microstructural defects. These defects are stress-concentration points at which microcracks can initiate and later propagate resulting in file separation.However , the question of the clinical usability of rotary NiTi files remains unanswered though it is almost that endodontic rotary files fracture not because of how many times they are used, but rather how they are used.

It is well accepted that thermo-cycling during sterilisation results in metal fatigue. There is no consensus in the literature regarding the effects of the specific type of thermo-cycling during sterilisation of rotary endodontic instruments. Some studies conclude that sterilisation does not adversely affect endodontic rotary instruments while other studies suggest slight or significant adverse effects of sterilisation, further stating that these adverse effects are not of clinical importance. Few studies have reported increased resistance to failure after endodontic files are subjected to sterilisation procedures however, most studies did not simulate the clinical situation. Recently the single use of endodontic rotary files has been recommended. This is due to the concern that effective disinfection is not possible with respect to prion disease.

During instrumentation, rotary files tip or flutes may engage a portion of the canal smaller than their diameter. Hence, instruments tend to lock or screw into the canal walls and the torque rises rapidly leading the file to be subjected to high levels of stress. The torque generated during instrumentation of small canals is higher than that in large canals. Also, as the file diameter increases, the torque (force) needed to begin unwinding or to fracture also increases. If the torque reaches a critical level, the instrument undergoes structural failure resulting in separation. When a high torque is used, the instrument is very active and the incidence of instrument locking and consequent deformation and separation tends to increase. Sometimes instruments become less active and the operator may force the instrument into the canal, leading to deformation and separation. With respect to root canal curvature, smaller files fail at less torque, as do files in more acutely curved canals.Studies have proved that instruments used with low-torque motors (<1 N/cm) are more resistant to fracture than those used with high-torque motors (>3 N/cm). Therefore, the practitioners should use electric motors set at low torque levels during root canal preparation.

Most manufacturers suggest using rotary files at speeds ranging from 150 to 350 rpm. A higher rate of rotary instrument seperation was reported when rotating at high speeds (300-350 rpm). Also the time for rotary instruments to fail significantly decreases as rotation increases. On the other hand, some studies showed no effect of different rotation speeds on the incidence of rotary file fracture. However, it should be noted that file fracture was considered as a main criterion for failure of instruments while deformed instruments should be considered as failed . However, it is always recommended that the operator adhere to the speed recommended by the manufacturer for each rotary system and for each specific file.

Therefore to conclude to minimising the risk of file separation, appropriate training before using files of new design should always be considered.Understanding of root canal anatomy and establishing of glide path before cleaning and shaping is a must –A Fundamental Rule. The new files should be examined as some defects can occur while being manufactured and or examination of files during treatment regularly even with a single use under magnification is a must as some defects can not be seen by naked eye. Under all circumstances the adherence to the manufacturer's instructions is a must .