The effect of cleaning and sterilization on the torsional properties of protaper rotary Nickel Titanium endodontic instrument

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1 The effect of clening nd on the torsionl properties of protper rotry Nickel Titnium endodontic instrument Yhy A. Ad B.D.S. (1) Mjid K.W. AL-Hshimi B.D.S., M. Sc (2) ABSTRACT Bckground: The torsionl properties of rotry nickel titnium endodontic instrument my e ffected y clening nd. This study ws conducted to evlute the effect of the numer of clening procedures involving immersion in NOCl with two different concentrtions (1% nd 5.25%) nd procedures (utoclve nd dry het) on the torsionl properties of protper rotry nickel titnium instrument when sujected to clockwise torsionl lod). The types of defects for the frctured instruments lso were studied. Mteril nd methods: A totl of ninety rotry protper finisher F3 instruments were divided into 3 groups. Group 1 comprises of ten instruments were sujected to neither clening nor cycles nd ssigned s the control group. Group 2 nd group 3 were sujected to five nd ten clening nd cycles respectively. Ech group comprises of forty instruments, twenty of them soked within 1% NOCl nd the other twenty were soked within 5.25% NOCl, ten of ech these twenty instruments, were sterilized with utoclve while, the other ten instruments were sterilized with dry het oven. Then, ll instruments were sujected to clockwise torsionl lod until frcture, the torsionl moment nd ngulr deflection t frcture were clculted. Then the frctured instruments exmined to detect the signs of distortion. Results: The results show sttisticlly significnt reduction in the torsionl strength fter clening nd cycles regrdless the concentrtion of NOCl, type nd numer of cycles. For the ngulr deflection, there ws highly significnt reduction, where, the numer of cycles ws the most influencing fctor for reduction of the ngulr deflection wheres, the concentrtion of NOCl nd the type of were show no significnt influence on the ngulr deflection. The exmintion of the frctured instruments show signs of plstic deformtion of the spirls, unwinding, reverse winding, reverse winding with tightening of the flutes of the instruments or comintion of ll these signs. Key words: Torsionl strength, ngulr deflection, NOCl, nickel titnium. (J Bgh Coll Dentistry 2009; 21(4):53-59) INTRODUCTION Rotry NiTi endodontic instruments re gining populrity sed on their superior preprtion of cnls reltive to hnd instruments. (1). Becuse of cost, the instruments re frequently reused, mking it essentil tht cross infection control procedures should e strictly followed. Cross infection etween ptients could involve cteri nd viruses. To remove the orgnic mtter nd deris completely from the files, effective clening procedures must e crried out efore proceeding to the process (2). Presoking the endodontic files in NOCl is proven method for complete removl of orgnic nd necrotic deris efore plcing them in n utoclve or dry het sterilizer. The strength of NOCl solution must e lnced ginst potentil dmge to instruments y corrosion s it is highly corrosive to metls (3). (1) MSc student, Deprtment of conservtive Dentistry, College of Dentistry, University of Bghdd. (2) Professor of Endodontics, Deprtment of conservtive Dentistry, College of Dentistry, University of Bghdd. Unfortuntely, NOCl could cuse corrosion for endodontic instrument in form of pitting which potentilly wekening the structure of the instruments (4-6). Reuse involves repeted exposure to etween uses (utoclve or dry het ). The physicl properties of these instruments (flexiility, resistnce to frcture y twisting nd cutting efficiency) should not e significntly ltered y procedures. The hypothesis which stted tht multiple s of endodontic nickel titnium instruments will led to continuous decrese in the resistnce of the instruments to seprtion ws tested y mny investigtors. Some studies showed tht multiple s do not determintely ffects mechnicl properties of NiTi lloy (7-9), while other studies showed tht multiple s could influences NiTi lloy properties (5,6,10). Sttpn et l documented two min types of file frcture: torsionl frcture nd flexurl ftigue. Torsionl frcture occurs when the tip or ny prt of the instrument is locked in cnl while the shft continues to rotte; the instrument exceeds the elstic limit of the metl nd shows Restortive Dentistry 53

2 plstic deformtion followed y frcture, while flexurl or cyclic frcture occurs t the point of mximum flexure when the instrument is freely rotting in curved cnl (11) A few studies were mde to evlute the effect of NOCl clening nd procedures (utoclve nd dry het) concomitntly on the torsionl properties of rotry NiTi instrument s these two fctors could influences the mechnicl properties of NiTi instrument nd led to undesirle nd unexpected file frcture during root cnl instrumenttion. MATERIAL AND METHOD A totl of 90 protper rotry NiTi files (finisher files F3) (Dentsply Millefer, Switzerlnd) were withdrwn from seled oxes, nd then rndomly divided into three experimentl groups (Figure 1): Group 1: consisted of ten instruments. This group ws ssigned s the control group which ws not exposed to neither clening nor procedures. Group 2 nd group 3 were sujected to five nd ten clening nd cycles respectively ech group ws divided into two sugroups: sugroup A: 20 instruments soked within 1% NOCl solution. Agin sugroup A ws divided into: -ten instruments were sterilized using utoclve sterilizer nd - Ten instruments were sterilized using dry het sterilizer- Sugroup B: consisted of 20 instruments which were soked within 5.25% NOCl solution. This sugroup ws divided into: - ten instruments were sterilized using utoclve sterilizer nd - Ten instruments were sterilized using dry het sterilizer. procedure. Ech clening cycle involved immersion of the instruments in NOCl solution. Instruments were soked for 10 minutes for ech clening cycle (12). Then they were thoroughly rinsed under distilled wter immeditely fter ech cycle nd llowed to dry 30 minutes efore proceeding the cycle. Steriliztion procedure. Steriliztion ws crried out ccording to the Interntionl Orgniztion for Stndrdiztion specifiction 3630 (Interntionl Orgniztion for Stndrdiztion 1992) (13) nd ANSI/ADA specifiction 28 (Americn Ntionl Stndrds Institute 1988) (14) with the use of n utoclve nd dry het sterilizers. Steriliztion prmeters were pressure of 30 PSI t 136 ± 2 C mintined for 30 ± 0.5 minutes for the utoclve nd temperture of 180 ± 5 C mintined for 30± 0.5 minutes for the dry het. After the instruments were llowed to cool down to room temperture for 30 minutes efore strting the next clening nd cycle. Group 1 Control group A 1% NOCL Group 2 5 cycles 40 smples Protper F3 90 smples B 5.25 % NOCL A 1% NOCL Group 3 10 cycles 40 smples B 5.25 % NOCL 2A 2A 2B 2B 3A 3A 3B 3B Figure 1: Flowchrt for the smples of protper instrument Torsionl strength nd ngulr deflection tests. The torque testing device (Figures 2) which ws custom designed for this study ws previously descried y Sttpn et l, pltform ws custom designed for this purpose, t one end, the gerox unit ws mounted, nd t the other end, the torque meter ws plced (15). A driving chuck ws presented t ech end. A chuck ws ttched firmly to the output shft of mnully driven reversile gerox unit. The reduction rtio for the gerox unit ws 60:1 nd hd revolution counter on its top nd input hndwheel (ech revolution for the input hndwheel produces 6 deflection for driving chuck). At the other end torque meter ws plced, which is metl shft, which hd support nd ws freely rotting smll chuck ws ttched to the shft of torque meter. The lever rm, 5 cm rdius (R), ws ttched perpendiculrly to the long xis of the shft. At one end of the lever rm, it ws rested on 500 g cpcity within 0.1 g ccurcy lod cell (Dimond, model 500, Chin). Figures 2: The torque testing mchine. Testing method Resistnce to frcture y twisting (torsionl moment) nd resistnce to ngulr deflection were Restortive Dentistry 54

3 determined ccording to ANSI/ADA specifiction 28 (Americn Ntionl Stndrds Institute 1988) (14) nd ISO specifiction 3630 (Interntionl Orgniztion for Stndrdiztion 1992) (13). The torsionl moment (in g.cm) represented the moment when instrument filure occurred during clockwise rottion nd ngulr deflection (in degrees) represented the twist ngle for rekge. The methods used for mesuring these prmeters hve een descried in previous study (6). Briefly, the hndles were removed with wire cutters t the point where the hndle is ttched to the instrument shft. One millimeter of the instrument shft ws secured in driving chuck tht ws ttched to the gerox unit, while the lst 3 mm of the working prt of the instrument were held in smll chuck ttched to the shft of the torque meter. A jig ws constructed to ensure reproducile positioning of the tip of the instrument in the chuck (Figures 3). The results were otined from this formul represented the torsionl moment. Angulr deflection ws otined y multipliction of the numer t the revolution counter t the time of the frcture y six (degree of deflection otined y single revolution of the input hndwheel). Frctugrphic exmintion The frcture instruments were evluted for signs of distortion using the microscope t 40 mgnifiction. Microphotogrphs were tken for the seprted instruments using cmer otined from the sme mnufcturer of the microscope, inserted into to the eyepiece tue of the microscope. RESULTS Torque t frcture The results of this study were collected nd nlyzed sttisticlly. The men, stndrd devition, minimum nd mximum vlues of the torque t frcture in g.cm of ll groups re presented in figure 5 nd tle 1. Tle 1: Descriptive sttistics results of Torque (g.cm) for ll groups Figure 3: Jig used to determine the clmping point in reproducile mnner (3 mm from tip of the instrument). N Men SD Min M A A Figure 4: Grsping of two ends of the instrument. The lst 3 mm of the working end of ech tested instrument ws grsped first nd then the hndle ws grsped in the chuck ttched to the gerox unit chuck. This llowed the investigtor tht ws loding the files to ccurtely lod 3 mm of working end into the chuck (Figure 4). Torsionl loding ws performed mnully through driving the input hndwheel in mnner tht reduces clockwise rottion for the output shft of the gerox unit until the instrument frctured. During test the redings of the lod cell were recorded continuously. The lrgest reding (pek) of the lod cell in grms (G) t the moment of the frcture ws recorded. The mximum torque t the frcture ws clculted through the formul: (16) Mximum Lod(G.cm)=Rdius(cm)xMximum Torque(G) 2B Torque 2B A A B B In comprison mong the groups, sttisticl nlysis of the torque vlues of ll groups, using nlysis of vrince (ANOVA) test, ws performed nd the results re shown in Tle 2, it is ovious tht there ws sttisticlly significnt difference mong the tested groups, regrding their torque vlues. Restortive Dentistry 55

4 Men Torque (g.cm.) Men Angulr deflection (in degrees) A 2A 2B 2B 3A 3A 3B 3B Figure 5: Br chrt of men Torque vlues (in g.cm) of ll groups. Tle 2: Anlysis of vrince (ANOVA) of the Torque of ll groups Torque (g.cm) Between Within SS df MS F Sig S Totl Further sttisticl nlysis ws performed to exmine the differences etween the pired groups. Student s t-test ws used for the comprison etween the group 1 (control group) nd the other groups, in regrd to their torque vlues (Tle 3). Tle 3: Student s t-test results of group 1 (control group) regrding the torque (in g.cm). Compred groups t-test P-vlue Sig. 1 Vs. 2A NS* 1 Vs. 2A S** 1 Vs. 2B S 1 Vs. 2B S 1 Vs. 3A S 1 Vs. 3A S 1 Vs. 3B S 1 Vs.3B S *P<0.05 Significnt **P>0.05 Non significnt The ngulr deflection The descriptive sttistic of the results regrding the ngulr deflection (in degrees), which included the men, stndrd devition, minimum nd mximum vlues of ll groups re presented in Figure 6 nd Tle A 2A 2B 2B 3A 3A 3B 3B Figure 6: Br chrt of men Angulr deflection vlues (in degrees) of ll groups. Tle 4: Descriptive sttistics results of Angulr deflection for ll groups. Angulr deflection N M SD Min Mx A A B B A A B B Regrding the degree of the ngulr deflection, nother nlysis of vrince (ANOVA) test, ws performed, in comprison mong the groups, nd the results of this second (ANOVA) test showed very highly sttisticlly significnt difference mong the tested groups s shown in Tle 5. Tle 5: Anlysis 0f vrince (ANOVA) of the Angulr deflection of ll groups. Angulr deflection (in degrees Between Within SS df MS F Sig HS Totl Since the (ANOVA) test showed very highly sttisticlly significnt difference, Student s t-test ws crried out for the comprison etween the group 1 (control group) nd the other experimentl groups, regrding to their ngulr deflection (in degrees) (Tle 6). Restortive Dentistry 56

5 Tle 6: Student s t-test results of group 1 (control group) regrding the ngulr deflection (in degrees). Compred groups t-test P-vlue Sig. 1 Vs. 2A HS*** 1 Vs. 2A S** 1 Vs. 2B NS* 1 Vs. 2B S 1 Vs. 3A HS 1 Vs. 3A HS 1 Vs. 3B HS 1 Vs. 3B HS *P<0.05 Significnt **P>0.05 Non significnt ***P<0.001 High significnt Frcture chrcteristics The frctured instruments showed plstic deformtion of spirls, unwinding, reverse winding for the originl direction of spirls, reverse winding with tightening of the flutes djcent to the frctured site or comintion of them (Figures 7). Most of the frctured instruments which showed unwinding nd reverse winding for the originl direction of spirl were elong to the group 2 nd group 3 while most of the frctured instruments which showed reverse winding with tightening of the flutes were elong to the control group. Figures 7: Top left new instrument. Top right, distorted with unwinding. Bottom left, distorted instrument with reverse winding. Bottom right, with reverse winding with tightening of the flutes x40 DISCUSSION More instruments were filed in torsion thn from flexurl ftigue (11). Mny studies showed tht Protper files hs higher frcture rte thn other rotry NiTi systems (17,18). Finisher files were tested s they used to shpe the picl third of the cnl, they undergo firly high ending nd pure torsionl stress in their thinner nd weker prt (picl 3 mm) s the life expectncy of finisher files less thn shpers s the ltter ccumultes more torsionl stress, (19). The ngulr deflection is sign of the instrument flexiility in contrst to the torsionl moment which is sign of strength of mteril (20). The mximum ngulr deflection is very importnt prmeter, s it cts s sfety fctor in reltion to the frcture of the instrument. Lrger the ngulr deflection of n endodontic instrument, the lrger will e its toughness nd plstic deformtion efore strting the frcture (16). Clinicl recycling exposes endodontic files to chnges in their physicl properties, when the files re sujected to repeted cycles with dry het or n utoclve (6). In the present study the effect of comined effect of oth corrosive NOCl nd on the torsionl properties of NiTi files ws studied. A frctugrphic nlysis ws mde lso for the frctured instruments. Torque t frcture Torque t frcture limit ws defined s the mximum force tht n instrument cn withstnd efore frcture, when twisted (15). The rnge of verge torque t filure for the new nd treted instruments ws etween (185.5 nd g.cm ws higher thn tht recommended y the mnufcturer who dvise to preset torque levels for the motor t Ncm for the protper finisher files (21). These results showed tht these instruments could e used sfely t this rnge of torque without fering from frcture regrding to torsionl strength The results of our study showed significnt reduction in the torsionl strength of rotry NiTi instruments fter exposure to NOCl nd. This my come in greement with the findings of other studies (6,22-24), which ws the results of comined dverse effects of corrosive NOCl nd on the mechnicl properties of NiTi instruments. s upon torsion test, smples sujected to comined tension compression sher stress, frcture cused y ftigue filure mechnism occurs due to crck initition t the cutting surfces nd propgtion towrd the file s xil centre (25-27). It is likely tht pitting or crevice corrosion might occur first which serves s wek points nd sites for the crcks initition nd susequently propgtion (28). Corrosion mechnism might e ctivted during Restortive Dentistry 57

6 chemomechnicl preprtion, chemicl disinfection or. There re no significnt differences etween different experimentl groups regrding the numer of cycles when other vriles were fixed (e.g. NOCl concentrtion nd type of the ). This ws consistent with the results of nother study (29). The present study showed no significnt differences etween different experimentl groups regrding the type of the (dry het or utoclve), when other vriles such s numer of the clening nd cycles nd NOCl concentrtion were fixed. This finding greed with the findings of other studies (6,30). The explntion for this sitution is tht torsionl strength minly ffected s result of exposure to NOCl rther thn to the het, which ws confirmed y the results of Hoy et l, who showed lower mens of torsionl strength of NiTi instrument fter prolong exposure to 1% NOCl lone (23). The torsionl strength for the treted instruments not severely deteriorted regrding to the torque t frcture vlues, even fter 10 cycles. The possile explntion for this phenomenon ws not cler, ut my e relted to the eneficil effect of the het tretment on the lloy strength. However, considering propgtion of ftigue crcks is directly ssocited with the mechnicl strength of the mteril, the increse of instrument ftigue resistnce ws found in mny studies fter het procedures which my e direct consequence of the oserved increse in hrdness. Serene et l suggested n improvement in the lloy strength following het tretment (5). Angulr deflection For instnce, the present study results showed tht vlues of ngulr deflection t filure ws drmticlly decresed fter 5 nd 10 cycles s trend towrds higher ngle of rottion t frcture ws oserved with the new instruments compred to the sterilized ones. These findings re in full greement with the results of other studies (6,8,31,32). This my e explined y fct tht het tretment of NiTi rotry endodontic instrument will mke them stiffer nd less flexile (5). These results lso ws confirmed y study found tht 5 times sterilized nd used profile instruments were less flexile thn new instruments when exposing them to the ending test (33). Under the prmeter of this study, there re no significnt effects regrding NOCl concentrtion nd type of on the ngulr deflection vlues, when compring different experimentl groups t fixed numer of cycles, s these two prmeters hving little effect on the instrument flexiility. The most influencing fctor ws the numer of the cycles nd it is ovious tht the ngulr deflection ws more ffected thn the torque t filure regrding the numer of the cycles. The type of procedure does not exhiit significnt influence despite the deference in the temperture, used for the utoclve nd dry het sterilizers. This finding is of generl greement with the results of other studies (6,8). The explntion of the deteriortion is physicl, s n excessive numers of cycles my hve cumultive effect on nneling of the instrument, this confirmed y study showed tht het tretment for orthodontic NiTi wire t 500 C for 10 minutes cused miniml effect on cntilever ending force, ut 2 hours of het tretment t the sme temperture incresed drmticlly the superelstic ending moment (34). It ws cler tht there is no liner reltionship etween the torsion nd the ngulr deflection vlues for the control group nd the experimentl groups. This finding is in consistence with the findings of the other studies (35,36). The est explntion for this sitution is tht NiTi lloy of rotry instrument cn posses significnt ductility in torsion without experiencing seprtion to certin limit (37). According to the results of our study this limit my e reduced (ductility) s het tretment cusing mteril stiffer. Frctugrphic nlysis The frctured instruments showed the typicl signs of torsionl filure such s plstic deformtion of spirls, unwinding, reverse winding for the originl direction of spirls, reverse winding with tightening of the flutes djcent to the frctured site or comintion of them. This hs een in close greement with the results of other studies (11,25,38-40). This might e explined y the fct tht the direction of the pplied force (clockwise) ws n pposite to the spirl direction of the flutes so the flutes will tend to e widened (opened). Most of the frctured instruments which showed unwinding nd reverse winding for the originl direction of spirl were elong to the group 2 nd group 3 while most of the frctured instruments which showed reverse winding with tightening of the flutes were elong to the control group. This my e explined s the control group is more flexile, nd so on it ws sujected to higher degree of distortion efore it is rekge, while the other experimentl groups where stiffer so, they were frctured with little distortion s they re stiffer thn control group. Restortive Dentistry 58

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