ROEN0210

I 25 opinion _ instrumentation I roots2_2010 to the reciprocating handpiece for what is generally a smooth and rapid negotiation to the apex. As is clear, fewer more vertically oriented flutes increase tactile perception. Fewer flutes also make the instru- ment less work-hardened, which in turn makes the instrument more flexible, another feature that en- hances tactile perception. Placingaflatalongtheentireworkinglengthfur- ther improves tactile perception by further reducing engagement along length, while making the instru- ment even more flexible. Those 64 contact points are now reduced to 48 (Fig. 3). A cutting tip is an addi- tional feature that can be added to improve tactile perception. Unlike a non-cutting tip that has the po- tential to impact pulp tissue, a cutting tip tends to pierce it. There is no concern about a cutting tip cre- ating its own pathway because the degree of motion islimitedtoeitheratightwatch-windingstrokeorthe 30° arc generated by the reciprocating handpiece. If a system of instruments with these design fea- tures is used according to the prescribed method for the entire shaping proce- dure, tactile perception will not be compromisedatanypointduring the instrumentation procedure. Compare this approach to the use of K-files and the subse- quentuseofrotaryNiTifiles.The K-files are poorly designed to en- hance tactile perception because they engage excessive amounts of tooth structure along length. Their horizon- tally oriented flutes are designed to en- gage,notcut,andthegreatnumberof flutes resulting from twisting the wire more times produces a stiffer instrument incompatible withsuperiortactileperception. Rotary NiTi is now used in a crown-down fashion, where the goal is to determine when exces- sive resistance is encountered along length, not at the tip. In fact, the tips of these rotary NiTi instruments do not engage apically until the shaping procedure is almost com- pleted and then rarely exceed a diameter of apical preparation beyond what was established by the K-files.WhenrotaryNiTifilesprepareapicalprepara- tionsbeyondthedimensionsproducedwithK-filesin curved roots, the likelihood of separation due to ex- cessive torsional stress and cyclic fatigue increases. Relieved reamers not only supply more accurate information regarding differentiating a solid imped- imentfromatightcanal,butcanalsodifferentiatebe- tween a round and oval canal. Some advocates of ro- tary NiTi have gone to great lengths to explain the extent to which the apical end of a canal should be prepared, using such terms as tuning and gauging, where the apical preparation is determined by the presence of clean dentine filings on the flutes of the rotaryinstruments.Tuningistofirstseefilings.Gaug- ing is to take the diameter up to the point where the filingsareclean.Ifcleanfilingsarepresent,rotaryNiTi advocates take this as clear evidence that the canals have been shaped adequately to assure clean walls circumferentially. However, two factors make mehesitateinaccepting‘tun- ing’ and ‘gauging’ as effec- tive and predictable proce- dures. First, the literature clearly demonstrates a high incidence of canals that are oval in their apical anatomy rather than round.1–3 Second, a symmetrical instrument, like all rotary NiTiinstruments,cannotdifferentiate between a round and oval canal. Only an asymmetrical instru- ment,onewithaflatalongits length can make that de- termination (Fig. 4). When a symmetrical instrument produces dentinal filings at the tip of the instrument, it may only mean that the filings havebeenremovedfromthesmaller diameterofanovalcanal,producingnoin- formationaboutthewiderdiameteroftheovalcanal. Theliteraturehasreportedthatthewiderportionofan ovalcanalmaybethreetofivetimesthatofthesmaller diameter.3 ThoseusingrotaryNiTiinstrumentswillnot knowthisandwillnothavetakenappropriatestepsto adapt to this situation. GiventheincreasedvulnerabilityofrotaryNiTifiles tobreakageasthetipsizeandtaperoftheinstruments increase, it is comforting to consider small prepara- tions as adequate for cleansing and irrigational pur- poses,eventhoughthereismuchevidencetocounter Fig. 3 Fig. 4