Abstract:
STATEMENT OF THE PROBLEM :
The use of zirconia in dentistry has increased. However, little attention has been given to the difficulty experienced by clinicians when cutting zirconia restorations intraorally. Evidence for which grit size and type of rotary instrument is best for cutting zirconia intraorally is lacking.
PURPOSE :
The purpose of this in vitro study was to identify the most efficient diamond rotary instrument grit size for cutting zirconia intraorally.
MATERIALS AND METHODS :
Efficiency was measured by comparing the cutting depth of each rotary instrument into zirconia, analyzing zirconia specimens for surface damage after cutting, and measuring instrument deterioration. Thirty zirconia specimens of the same measurements were used as test specimens and cut with 30 diamond rotary instruments with different grit sizes. An electric handpiece was used with constant force (1.7 N), speed (40 000 rpm), time (1 min), and water flow rate (25 mL/min) to produce comparative data. The mean cutting efficiency values were compared by analysis, and the median values were compared by the nonparametric Kruskal-Wallis test (α=.05). Each test was followed up with pair wise comparisons of the mean or median values if significance was indicated.
RESULTS :
The greatest cutting depth was achieved with a fine-grit instrument with a mean cutting depth of 5.79 mm compared with 4.54 mm for the coarse-grit instrument (P=.032). The greatest damage to zirconia was done by the coarse- and supercoarse-grit instruments (both 33%), with no substrate damage by the superfine-, fine-, and medium-grit instruments. The greatest instrument deterioration was found on the supercoarse rotary instruments (9.05%). With only 3 exceptions, the power calculations were all sufficient and above 83%.
CONCLUSIONS : The fine grit rotary instrument (between 40 and 50 µm) was the most efficient, achieving the greatest cutting depth, with no detectable macroscopic damage to the zirconia and minimal instrument deterioration.
CLINICAL IMPLICATIONS :
Contrary to the common perception that more abrasive rotary instruments would be more effective in cutting zirconia, the empirical findings in the present study found that fine-grit diamond rotary instruments were more effective than coarse-grit instruments for cutting zirconia intraorally. The study provided new insights into the efficient cutting of zirconia in clinical practice, ensuring maximum productivity (cutting depth) with minimum wasted time and expense (instrument deterioration and substrate fracture).
