ROEN0210

24 I I opinion _ instrumentation _Whenitcomestotactileperception,mostden- tists doing root canal therapy would agree—more is better. But, what exactly do we mean when we talk abouttactileperception?Tome, tactile percep- tionisameasureofthedegreetowhichwe can determine what the tip of the negotiating endo- donticinstrumentisencountering.Isitencountering animpedimentlikeasolidwallorisitlodgedinatight canal? Or is the canal that the tip of the instrument is entering round or oval? Superior tactile perception is a direct result of the instrument’s design and the way it is used. Assuming that tactile perception is exactly as I define it, a rea- sonable analytic task is to determine what endodon- ticinstrumentdesignsandtechniquesofuseenhance tactile perception. One basic insight is that the infor- mation conveyed from the tip of the instrument will become increasingly clear as the engagement alonglengthisreduced.Ifthereisagreat deal of engagement along length, exactlywhatthetipofthein- strument is encoun- tering becomes murky. In that light, the typical K-file de- sign consisting of 30 hori- zontally oriented flutes along length (Fig. 1) will engage the walls of the canal significantly more than a reamer with 16 flutes that are more vertically oriented (Fig.2).Toclarifythispoint:ifboththereamerandthe file are made from a square wire, the reamer with 16 fluteswillhaveatotalof64pointsofcontactbecause eachflutealonehas4contactpoints(fabricatedfrom atwistedsquarewire),whilethefilewith30fluteswill have a 120 contact points. The greater the number of contact points, the greater the engagement and the consequent increase in resistance to apical nego- tiation. In short, increasing resistance along length reduces the tactile perception of what the tip of the instrumentisengaging.Anincreaseinthenum- ber of flutes increases engagement and reduces tactile perception, while the more horizontal orientation of the flutes engages the dentine rather than cutting it when used with the recom- mended watch-winding motion. The file design is similartothatofascrewandtactileperceptionatthe tip is secondary to engagement along length. While thegoalofascrewisengagement,thatisnotthegoal of an endodontic shaping instrument and the more horizontally oriented flutes along the length of a file arecounterproductivetothegoalsthedentistwishes to achieve. Ideal tactile perception tells the dentist when a solidwallhasbeenencountered.Thedentistdifferen- tiates this type of engagement from being in a tightcanalbythedegreeoftug-backpresent whenhe/shepullstheinstrumentback.No immediatetug-backmeansthedentistis encountering a solid wall. Immediate tug-backmeansthedentistismostlikelyin a tight canal that will allow him/her to progress to greater depths either using a tight watch-winding motionorviatheinstrument’suseinthe30°recipro- cating handpiece. I emphasise the word immediate because a solid wall continuously being pecked at with an instrument will start to produce tug-back simply because the repeated pecks into a solid wall will start to establish its own man-made pathway, an inaccuracy a dentist wants to avoid from the start. Knowing that a solid wall, as an impediment, has been encountered tells the dentist that he/she must remove the instrument, place a small bend at the tip and attempt to negotiate around the impediment manually. Once around, the dentist leaves the instru- mentatthenewlynegotiateddepthandreattachesit roots2_2010 Tactile perception in endodontics Author_ Dr Barry Lee Musikant, USA Fig. 2 Fig. 1