A key component of endodontic instrumentation is irrigation, solutions used to remove the smear layer and open the dentinal tubules (17% EDTA) to digest remnants of pulp tissue and to kill bacteria (2-6% NaOCl). The effectiveness of the irrigants is proportional to the time in the canal as well as the volume introduced. The volume of irrigant that can be placed at any one time is determined by the space remaining in the canal when the instrument is also present. It is here we have a dilemma. Greater tapered instruments are generally larger than the original canal space in the mesio-distal plane leaving no room for irrigants in this dimension. Research has also documented the tendency of greater tapered rotary instruments to impact debris into the thinner buccal and lingual extensions creating a conical shape where the buccal and lingual walls are created by that debris, remnants of tissue laced with bacteria. This is not a formula for effective three-dimensional debridement.
One might say that effective irrigation is not necessary during the shaping procedure, that it will be handled with some type of rapid oscillating device after the shaping is done. However, after the use of rotary NiTi instrumentation, research documents impacted debris in the extensions of oval canals preventing the irrigants at this stage gaining access to these impacted extensions. The problem of impacted debris is a function of rotary instruments of greater taper. To avoid this problem the initial instruments used in a canal should first provide enough space for an effective volume of irrigant to be present. The most common sense way to provide enough space is to use very thin 02 tapered relieved reamers. Furthermore, the relieved reamers must not be used in rotation. If the arc of motion is limited to 30-45º there is churning of the irrigants and the elimination of debris impaction buccally and lingually.
The 30-45º reciprocating motion has the distinct advantage of virtually eliminating instrument separation. Once the dentist is confident that the instruments will remain intact, dentists will be far more willing to address the tissue in the buccal and lingual extension. We all know it should be removed, but when weighed against the increased chances of instrument separation, we sacrifice more thorough cleansing for what we consider increased safety. This dilemma no longer exists when instruments are used with short arcs of motion in a 30-45º reciprocating handpiece oscillating at 3000-4000 cycles per minute. Consequently, the thinnest 02 tapered relieved stainless steel reamers can be vigorously applied to the buccal and lingual extensions giving us our best chances of mechanically removing tissue and bacteria lodged in the buccal and lingual extensions of oval isthmus-like canals.
Irrigation, when shaping the canals in this fashion, is increasingly effective as the buccal and lingual preparation is widened creating an ever greater space for an increasing volume of irrigant that is being churned at 3000-4000 cycles per minute. The result is a conservative preparation in the mesio-distal plane and highly effective tissue removal and bacterial reduction in the bucco-lingual plane. These goals are consistent with the reduction in the causes of endodontic infection as well as the preservation of dentin required for long term success.
Greater tapered rotary shaping introduced the mechanization of endodontic canal preparation reducing hand fatigue and speeding up instrumentation. It also introduced unpredictable separation that led to conservative centered conical shaping that has limited application in the cleansing of the majority of the canals we treat. It further compromised the essential function of irrigation. 30-45º engine-driven reciprocation using predominantly 02 tapered stainless steel relieved reamers virtually eliminates instrument separation, provides an environment that augments the use of irrigants, allows the practioner to vigorously remove tissue in the bucco-lingual plane and preserves more tooth structure in the mesio-distal plane.