High and low torque handpieces: cutting dynamics, enamel cracking and tooth temperature

Objective The aim of these experiments was to compare the cutting dynamics of high-speed high-torque (speed increasing) and high-speed low-torque (air -turbine) handpieces and evaluate the effect of handpiece torque and bur ty pe on sub-surface enamel cracking. Temperature changes were also recorded i n teeth during cavity preparation with high and low torque handpieces with diamond and tungsten carbide (TC) burs. The null hypothesis of this study w as that high torque handpieces cause more damage to tooth structure during cutting and lead to a rise in temperature within the pulp-chamber Materials and methods Images of the dynamic interactions between burs and e namel were recorded at video rate using a confocal microscope. Central inci sors were mounted on a specially made servomotor driven stage for cutting w ith a type 57 TC bur. The two handpiece types were used with simultaneous r ecording of cutting load and rate. Sub-surface enamel cracking caused by th e use of diamond and TC burs with high and low torque was also examined. Lo wer third molars were sectioned horizontally to remove the cusp tips and th en the two remaining crowns cemented together with cyanoacrylate adhesive, by their flat surfaces. Axial surfaces of the crowns were then prepared wit h the burs and handpieces. The teeth were then separated and the original s ectioned surface examined for any cracks using a confocal microscope. Heat generation was measured using thermocouples placed into the pulp chambers o f extracted premolars, with diamond and TC burs /high-low torque handpiece variables, when cutting occlusal and cervical cavities. Results When lightly loaded the two handpiece types performed similarly. Ho wever, marked differences in cutting mechanisms were noted when increased f orces were applied to the handpieces with, generally, an increase in cuttin g rate. The air turbine could not cope with steady heavy loads, tending to stall. 'Rippling' was seen in the interface as this stall developed, coinci ding with the bur 'clearing' itself. No differences were noted between diff erent handpieces and burs, in terms of sub-surface enamel cracking. Similar ly no differences were recorded for temperature rise during cavity preparat ion. Conclusions Differences in cutting mechanisms were seen between handpieces with high and low torque, especially when the loads and cutting rates were increased. The speed increasing handpiece was better able to cope with incr eased loading. Nevertheless, there was no evidence of increased tooth crack ing or heating with this type handpiece, indicating that these do not have any deleterious effects on the tooth.