Timothy A. Svec

SEM Observations of Nickel-Titanium Rotary Endodontic Instruments that Fractured During Clinical Use

Numerous discarded ProFile GT, ProFile, and ProTaper nickel-titanium rotary instruments obtained from two graduate endodontic clinics were examined by scanning electron microscopy. These instruments had an unknown history of clinical use and had fractured or experienced considerable permanent torsional deformation without complete separation. The failure processes generally exhibited substantial ductile character, evidenced by a dimpled rupture fracture surface. Crack propagation at grain boundaries and cleavage surfaces indicative of transgranular fracture were observed for some specimens. It appeared that oxide particles from the manufacturing process served as nucleating sites for the microvoids, leading to dimpled rupture. A previously unreported fracture mode also was observed, in which crack propagation, approximately parallel to the local flute orientation, connected pitted regions on the surface. Combining present and previous scanning electron microscopy observations of clinically failed instruments, suggestions are offered for improving their fracture resistance.

Incidence of pain associated with clinical factors during and after root canal therapy. Part 2. Postobturation pain

Patients who begin endodontic therapy with nosymptoms may experience pain during therapy or after obturation of the canal system. Part 2 of this study analyzed the relationship between postobturation pain and clinical factors or conditions existing before, during, and after root canal therapy. Significant relationships were found between postobturation pain and interappointment pain and between postobturation pain and the type of chemical agents used for irrigation and intracanal medication. Of the 229 patients included in this study, 109 (47.6%) reported postobturation pain at some time during the 60-day postobturation observation period. The highest incidence and degree of postobturation pain occurred during the first 24 h. Sixty days after obturation, 227 (99.1%) patients were free of symptoms.

Chemomechanical removal of pulpal and dentinal debris with sodium hypochlorite and hydrogen peroxide vs normal saline solution

The effectiveness of chemomechanical preparation with normal saline solution and with a combination of sodium hypochlorite and hydrogen peroxide was evaluated microscopically. The results indicated that a combination of NaOCl and H 2 O 2 was significantly more effective in cleansing the canal system at 1 and 3 mm from the apex. At the 5-mm level, normal saline solution was equally effective as an irrigant.

Scanning Electron Microscope Observations of New and Used Nickel-Titanium Rotary Files

The appearances of the tip sections of ProFile 0.04 taper and Lightspeed 25-mm long, ISO size 25, nickel-titanium rotary instruments were compared with a scanning electron microscope in the as-received condition and after one, three, and six simulated clinical uses to prepare mesial canals of extracted mandibular molars. For the used ProFile instruments, there was some flattening of the characteristic material rollover and minor apparent wear at the edges of the flutes, but there was little change in the tip regions of the used Lightspeed instruments. Deposits on the surfaces of the instruments were attributed to the manufacturing processes and the in vitro preparation of root canals in the extracted teeth. The simulated clinical use did not cause substantial changes in the regions of these two brands of rotary instruments that are involved in the clinical preparation of root canals.

Analysis of Ni-Ti versus Stainless Steel Instrumentation in Resin Simulated Canals

This study compared step-back preparations in curved canals of resin blocks using nickel-titanium K-files and stainless steel K-files. Forty canals in resin blocks were cross-sectioned at 3 levels: 1 to 2 mm from the apical foramen, middle of the curve, and coronal. Direct digital computer images were recorded before and after instrumentation. Superimposition of the images combined with digital subtraction computer software allowed direct measurement of area instrumented, distance of transportation, and shape analysis. Time for instrumentation was recorded. Results showed Ni-Ti files to cause significantly less transportation and remain more centered at the apical level (p < 0.05). Area removed by instrumentation was significantly greater for stainless steel files at the middle level (p < 0.05). Cross-sectional shape of the instrumented canal was not significantly different (p < 0.05). It took significantly longer to prepare a canal with Ni-Ti K-files in resin blocks compared to stainless steel (p < 0.05). Resin simulated canals showed similar results compared to canals in extracted roots using an identical methodology.

Incidence of pain associated with clinical factors during and after root canal therapy. Part 1. Interappointment pain

Patients who begin endodontic therapy with no symptoms may experience pain during therapy or after obturation of the canal system. A clinical study was conducted to determine whether any clinical factors or conditions are associated with an increased incidence or degree of interappointment or postobturation pain. Of 229 patients evaluated in part 1 of this study, 28.8% had slight interappointment pain and 15.7% had moderate to severe pain. Statistical analysis showed no significant relationship between interappointment pain and any of the analyzed clinical factors or conditions.

Analysis of clinical toxicity of endodontic irrigants.

A clinical investigation was conducted to determine the effect of the use of various endodontic irrigants on interappointment pain. Of 253 patients who were treated, 65.6% were free of interappointment symptoms, 27.7% had slight pain, and 6.7% had pain that required palliative treatment. Statistical analysis showed no relationship between interappointment pain and the type of irrigant used.

The deterioration of rotary nickel-titanium files under controlled conditions.

It has been shown that rotary nickel-titanium files cannot be used indefinitely. Researchers and clinicians have not been able to agree on how many times a file can be used before being discarded, except if a file has fractured or become visibly distorted. This study used ISO size 20 files of 0.04 taper in the curved canals of extracted mandibular molars. The canals had been previously instrumented to an ISO size 15 with stainless steel hand files. The irrigant used during rotary and hand instrumentation was Glyde. The rotary files were closely examined with scanning electron microscopy before use to detect any defects. They were then reexamined after each of five uses to document deterioration. An electric handpiece was configured to rotate at 150 rpm and secured to the testing device, which also held the extracted tooth. The testing device controlled the load at 8 N, the depth of penetration for each canal and the rate of penetration (12 mm/min). Used instruments demonstrated surface fatigue wear and cracking. Torsional moment at failure was determined on a torsiometer for used and new instruments. Data (n = 10) were analyzed by analysis of variance. The torsional moment for used and new instruments was not affected by use (p = 0.25).

Differential Scanning Calorimetric Studies of Nickel Titanium Rotary Endodontic Instruments

Differential scanning calorimetric (DSC) analyses were performed between -130 degrees and 100 degrees C on specimens prepared from nickel-titanium (NiTi) rotary endodontic instruments: ProFile (n = 5), Lightspeed (n = 4), and Quantec (n = 3). The ProFile and Lightspeed instruments were in the as-received condition, whereas the Quantec instruments were randomly selected from a dental clinic and had unknown history. The DSC plots showed that the ProFile and Lightspeed instruments analyzed had the superelastic NiTi property, with an austenite-finish (Af) temperature of approximately 25 degrees C. Differences in DSC plots for the ProFile instruments and the starting wire blanks (n = 2) were attributed to the manufacturing process. The phase transformation behavior when the specimens were heated and cooled between -130 degrees and 100 degrees C, the temperature ranges for the phase transformations, and the resulting enthalpy changes were similar to those previously reported for nickel-titanium orthodontic wires having superelastic characteristics or shape memory behavior in the oral environment. The experiments demonstrated that DSC is a powerful tool for materials characterization of these rotary instruments, providing direct information not readily available from other analytical techniques about the NiTi phases present, which are fundamentally responsible for their clinical behavior.

Differential Scanning Calorimetric Studies of Nickel- Titanium Rotary Endodontic Instruments after Simulated Clinical Use

Differential scanning calorimetric (DSC) analyses have been performed between -130 degrees and 100 degrees C on single-segment specimens obtained from ProFile and Lightspeed nickel-titanium rotary endodontic instruments in the as-received condition and after one, three, and six periods of simulated clinical use in extracted teeth. The DSC analyses showed that both brands of instruments were always in the superelastic condition, although the enthalpy values for the transformation from martensitic NiTi to austenitic NiTi were much smaller for the Lightspeed instruments. Simulated clinical use had no evident effect upon this transformation for both brands, which is attributed to insufficient mechanical deformation of the instruments. There were substantial differences in the enthalpy change associated with the transformation from martensitic NiTi to austenitic NiTi for test segments from different positions along the shafts of the instruments and for as-received instruments from two different batches that were analyzed in this study and a previous study. These differences are attributed to variations in work hardening along the shaft during instrument fabrication and to processing differences during production of the two batches of each instrument brand.