Sodium hypochlorite is a major irrigant in endodontics, and nickel-titanium instruments are gaining in popularity. This paper investigated the corrosion of nickel-titanium Lightspeed instruments in 1% and 5% NaOCl solutions. The instruments were immersed in ultrasonicated NaOCl solutions for varying times up to 1 h. Corrosion was determined by electrothermal absorption spectrometry in 100 microL aliquots of NaOCl. Background contamination of nickel in the 1% and 5% NaOCl solutions used was low, but high enough to interfere in detecting any increases in nickel after immersing the instruments. The amounts of titanium recorded in the 1% NaOCl solutions were insignificant. However, a statistically significant amount of titanium was detected from the Lightspeed instruments after immersion times of 30 and 60 min in 5% NaOCl. Clinically such instruments do not have an 'in situ' time of 30 min, and this corrosion may be considered irrelevant clinically.

The cutting heads of the first 18 instruments in six sets of clinically used Lightspeed instruments (sizes 20-65) were examined in a scanning electron microscope (SEM). Three sets had been used by full-time faculty and discarded as follows: instrument sizes 20-32.5 after 18 treated root canals; instrument sizes 35-47.5 after 36 treated root canals; and instrument sizes 50-65 after 54 treated root canals. Three other sets had been used by a private clinician who discarded all Lightspeed instruments after preparing 20 root canals. The cutting heads of the 108 instruments were examined for the presence of surface debris, metal strips, metal flash, disrupted cutting edges, microfractures, tears, pitting and fretting and fatigue cracks. Fewer instruments ultrasonically cleaned before examination in the SEM had debris than those not cleaned before examination. The prevalence of metal strips and metal flash was similar for both groups. In contrast, microfractures and tears were more prevalent in instruments used by the full-time faculty group, who used their instruments more frequently before discarding them. Pitting and fretting occurred equally frequently in both groups whilst no fatigue cracks were noted in any of the 108 instruments. Overusage of Lightspeed instruments may predispose the flutes of the cutting heads to microfractures and clinicians should be aware of this possibility.

The diameters of six different sized Lightspeed cutting heads were measured using a modified optical micrometer. Six specimens of instrument sizes 20, 22.5, 27.7, 30, 32.5 and 45 were examined. Except for one cutting head of size 30, all the recorded diameters were oversized compared to the expected diameter and the permitted tolerance (+/- 0.005 mm). Perfectly machined instruments would increase by 12.5%, 9.09% and 8.3% from instrument sizes 20-22.5, 27.5-30 and 30-32.5, respectively. Under the conditions of this study, the corresponding increases were 7.1%, 6.75% and 8.54%. Instrument size 20 had the widest range and is the smallest instrument in the Lightspeed set. The largest differences between the recorded and expected means were registered for the size 45 group. An earlier study using a different measuring method reported that Lightspeed instruments were mostly undersized.