Effect of deep cryogenic treatment on cyclic fatigue resistance of controlled memory wire nickel-titanium rotary instruments

Abstract

Background: Cryogenic methods have been used to increase the wear, abrasion, corrosion resistance and to improve the strength of metals. The aim of this study was to evaluate the effect of cryogenic treatment on the cyclic fatigue resistance of rotary nickel-titanium (NiTi) instruments. Materials and Methods: In this in vitro study, 20 Hyflex (Coltene, Altstätten, Switzerland) nickel-titanium instruments, size 25, 0.06 taper, were randomly divided into two groups as follows: untreated NiTi rotary files (Group A) and cryotreated NiTi rotary files (Group B). The instruments of Group A were completely immersed in a cryocan containing liquid nitrogen (–196°C) for 24 h. After 24 h, the instruments were removed from the bath and were allowed to return to room temperature gradually. All files were used (at a speed of 500 rpm and a torque of 2.5 Ncm) in an artificial canal with a 60° curvature until fracture. Time to failure was recorded with a stopwatch in seconds and subsequently converted to number of cycles to fracture (NCF). Groups were compared using the independent-samples t -test. The level of statistical significance was set at 0.05. Results: Mean NFCs in Group A and B were 1576 and 1395, respectively. However, statistical analysis showed that there was no significant difference between the groups ( P = 0.2). Conclusion: Deep cryotherapy of NiTi endodontic files cannot improve the cyclic fatigue resistance of HyFlex files, but further studies are required to evaluate these cryogenically treated NiTi files clinically.

Advancing Nitinol: From heat treatment to surface functionalization for nickel–titanium (NiTi) instruments in endodontics

“…Similarly, Vinothkumar et al [ 147 ] demonstrated that dry CryoT drastically increased the cyclic fatigue resistance by 13%. This contrasts with more recent studies that found no improvement in cyclic fatigue resistance in NiTi rotary instruments after cryogenic treatment [ 148 , 149 ]. Differences in methodology likely explains the contrasting results.…”

“…Differences in methodology likely explains the contrasting results. Yazdizadeh et al [ 149 ] and Sabet et al [ 148 ] used a wet CryoT technique and measured cyclic fatigue using a static model which is not reflective of clinical practice.…”

Analysis of cyclic fatigue resistance of ProTaper Universal and ProTaper Next rotary instruments

“…Glavne uzroke preloma instrumenata predstavljaju torziono opterećenje i ciklični zamor [7,8,9]. U slučaju kada je vrh turpije zaglavljen u kanalu tokom rotacije instrumenta, kada se deo instrumenta uvije u zid kanala, kada je vrh instrumenta veći od promera kanala korena, ili kada se vrši prekomerni pritisak na instrument u toku rada, dolazi do preloma instrumenta usled torzionog opterećenja [10,11]. Mnogo češći uzrok preloma instumenata je ciklični zamor koji nastaje usled neprekidnog smenjivanja kompresije i istezanja u svakoj tački savijenog instrumenta u predelu krivine kanala.…”

“…The main causes of instrument breakage are torsional load and cyclic fatigue [7,8,9]. In the case when the tip of the file is stuck in the canal during the rotation of the instrument or part of the instrument is twisted into the wall of the canal, the tip of the instrument is larger than the diameter of the root canal or when excessive pressure is applied to the instrument during operation, the instrument breaks due to torsion loads [10,11]. A much more common cause of broken instruments is cyclic fatigue that occurs due to the continuous alternating compression and stretching at each point of bent instrument in the region of the canal curvature.…”

“…A much more common cause of broken instruments is cyclic fatigue that occurs due to the continuous alternating compression and stretching at each point of bent instrument in the region of the canal curvature. This mechanism is responsible in 90% of instrument breakage cases [10,11]. There are a number of additional factors that can affect the occurrence of fractures of NiTi instruments, such as the size, conicity and cross-sectional shape of instruments, as well as the training of the therapist [12].…”