The structural Integrity of Instruments and the Roots They S

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The structural Integrity of Instruments and the Roots They S

Postby BarryMusikant on Fri Jun 12, 2015 2:25 pm

The Structural Integrity of Endodontic Instruments and the Roots They Shape

Endodontic instrumentation as it is practiced today confuses the means with the ends. The end is supposed to be a canal cleansed as optimally as possible with the smallest amount of tooth structure removed in the process. To that end, we create large conical preparations. From a clinical point of view, we can justify such shaping when applied to maxillary central and lateral incisors. Their original anatomy is most often in the form of a conically shaped canal widening circumferentially in the coronal direction. Aside from these two teeth, anatomical justification for conical shaping does not exist. The majority of all other teeth tend to display an oval shape in cross-section being wider in the bucco-lingual plane than the mesio-distal plane. The sole exception to that is the palatal root of maxillary molars that tend to be wider in the mesio-distal plane. For all these other teeth, the justification for conical shaping is not the pulpal anatomy, but the safety of the instruments.

The introduction of greater tapered rotary NiTi instrumentation subjects the instruments to increased amounts of torsional stress and cyclic fatigue, the two factors that are responsible for the separation of instruments. These two factors are most prominent when the full working length of the instrument is in contact with the canal walls, particularly in curved canals. One way to decrease the impact of the canal walls is to take steps that reduce the degree of contact along length. By using a crown-down technique instruments of greater taper have reduced contact along length. The widest of the greater tapered instruments are first used, but only to a relatively shallow depth. Instruments of reduced taper are then used to sequentially negotiate deeper into the canal. As the tapers decrease, the increased apical contact gained from greater depth is counterbalanced by the reduced coronal contact that lesser tapered instruments have. In this way, any given instrument has reduced contact along length decreasing the stresses it is subject to as it rotates within the confines of the canal. The obvious goal here is to adopt procedures that have the best chances of keeping the instruments intact.

This technique decreases the incidence of instrument separation and has been widely adapted. When this technique was introduced little if any consideration was given to the weakening of the roots as a result of excessive tooth structure removed in the mesio-distal plane. If weakening of the roots does occur, it is not readily obvious. Any concerns about root weakening are further diluted by widespread approval of the appearance of the canals after they are obturated: a continuous taper along the entire length showing smooth wall preparations often times negotiating curved canals that show close adaptation between the original canal anatomy and the final shaping as observed in the typical two-dimensional periapical x-ray. The smooth continuous taper resulting from greater tapered NiTi rotation surpasses esthetically the far less tapered narrow and off-centered preparations that too often resulted from the use of K-files alone.

For greater tapered shaping to be accepted as a new superior norm, the obturation it produces had to be accepted as not only creating a superior seal, but better for the long term prognosis of the tooth. While the esthetic fills are now used as proof of excellence, there is no data on increased success rates that actually supports this conclusion. On the contrary, increasing amounts of research are documenting that the techniques used to increase the safety of the instruments are being purchased at the expense of the roots’ structural integrity. For example let’s examine the crown-down technique. It creates a large conical shape with the most tooth structure removed coronally. From the perspective of tissue removal, the pulpal space may be so narrow that an .06 tipped 02 tapered reamer may encounter resistance a few mm into the canal. This resistance is most likely coming from the mesial and distal walls. Given such early resistance, one might question just how wide the canal should be to thoroughly cleanse the mesial and distal walls. The likely answer is not very wide. Yet, bucco-lingually the canal may be several times wider than the mesio-distal dimension. Rotary NiTi’s answer is to produce a conical shape generally no less than an 06 taper and cautiously brush against the buccal and lingual walls in the hope of extending the preparation laterally to the tissue often ensconced there. It is a strategy based on excessive removal of tooth structure mesio-distally in the hope of extending it wide enough bucco-lingually to remove the tissue found in this plane. And this is not really the truth either. The greater tapered crown-down technique was devised to protect the instruments. With this as the first given, the greater taper that is confined to removing excessive dentin in one conically shaped location is now suggested to be used to extend that excessive taper to more of the bucco-lingual width further weakening the root. It is fortunate that those using rotary NiTi are unlikely to pursue this goal aggressively knowing that lateral movements increase the incidence of instrument separation.

Going back to the research it is now documented that greater tapered rotary instrumentation produces dentinal defects in two ways, by thinning out the dentin excessively in the mesio-distal plane and via the stresses produced during rotation be it continuous or interrupted. Once dentinal defects are present, any form of active obturation will cause those defects to coalesce and propagate. Other studies have quantified a reduction in resistance to vertical fracture the greater the taper of the preparations. The tooth is paying a high price to increase the safety of the instruments.

By employing instruments of predominantly lesser taper (02) and confining them to a short arc of motion(30º-45º) crown-down preparations are no longer a requirement. The meso-distal taper need be no greater than an 04 and often less than that. Dentinal defects do not result from short amplitudes of motion even when oscillating at 3000-4000 cycles per minute. Confined to short arcs of rotation, the instruments are virtually invulnerable to separation giving the dentist the confidence to extend their shaping potential buccally and lingually into the pulpal spaces that are often present in this plane.  When combined with passive obturation techniques, increased safety is added. No defects are present to start with and there is neither coalescence nor propagation of defects that are not present to begin with. Finally, there is minimal reduction in resistance to vertical fracture because so much tooth structure has been retained.

It is the amount of remaining tooth structure combined with thorough cleansing that is most important in attaining a high success rate. There is a general realization that vertical fractures are becoming an increasing source of failure over the past 25 years. While one cannot say definitively that it is caused by greater tapered preparations, the case against greater tapered rotary canal preparations is strong and growing stronger.

Regards, Barry
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