Advertisement
Journal of Prosthetic Dentistry

Evaluation of diamond rotary instruments marketed for removing zirconia restorations

Open AccessPublished:September 21, 2022DOI:https://doi.org/10.1016/j.prosdent.2022.08.009

      Abstract

      Statement of problem

      The high strength of zirconia makes the removal of zirconia restorations challenging and time consuming. Whether diamond rotary instruments marketed for removing zirconia restorations are more efficient is unclear.

      Purpose

      The purpose of this in vitro study was to compare the efficiency of diamond rotary instruments specifically marketed to cut zirconia with the efficiency of a conventional diamond rotary instrument.

      Material and methods

      Two diamond rotary instruments marketed to cut zirconia (JOTA Zirkon Cut Z838L [JOT] and Intensiv ZirconCut Zr02/10 [IZC]) and a conventional diamond rotary instrument (Intensiv FG 334/6 [IFG]) were tested on 2 zirconia materials: 3Y-TZP (IPS ZirCAD LT) and a multilayered 4Y-TZP (IPS ZirCAD MT Multi). Zirconia specimens (2 mm) were cut under water cooling using a force of 2 N or 6 N. Cutting times and maximum temperatures at the tip of the diamond rotary instruments were recorded. The surface roughness before and after use was measured, and the elemental composition was analyzed.

      Results

      Overall, cutting times were shorter for IFG (85 seconds) and IZC (100 seconds) than for the JOT (182 seconds). Cutting times were shorter for MT zirconia than for LT zirconia. Higher temperatures (2 N: 24.6 °C, 6 N: 36.7 °C) and lower surface roughness occurred with higher cutting loads. Impurities of diamond particles were seen for JOT. The diamond particle embedding materials were either nickel alloys (IFG and JOT) or a resin material (IZC).

      Conclusions

      Diamond rotary instruments marketed for cutting zirconia did not perform better or generate less heat compared with a conventional diamond rotary instrument. A load of 2 N with sufficient water cooling is recommended for cutting zirconia to avoid an extensive temperature increase.
      Clinical Implications
      Based on this in vitro study, the following clinical aspect should be considered. Diamond rotary instruments marketed specifically for cutting zirconia did not perform better or generate less heat compared with a conventional diamond rotary instrument.
      Veneered single crowns made of zirconia have an estimated 5-year survival rate of 91.2%.
      • Sailer I.
      • Makarov N.A.
      • Thoma D.S.
      • Zwahlen M.
      • Pjetursson B.E.
      All-ceramic or metal-ceramic tooth-supported fixed dental prostheses (FDPs)? A systematic review of the survival and complication rates. Part I: Single crowns (SCs).
      Chipping was reported to be the most common technical problem, with a cumulative 5-year complication rate of 2.1%.
      • Sailer I.
      • Makarov N.A.
      • Thoma D.S.
      • Zwahlen M.
      • Pjetursson B.E.
      All-ceramic or metal-ceramic tooth-supported fixed dental prostheses (FDPs)? A systematic review of the survival and complication rates. Part I: Single crowns (SCs).
      Approximately 3% to 6.5% of crowned abutment teeth have been reported to require endodontic treatment within 7 years of crown insertion.
      • Peters O.A.
      • Du D.
      • Ho M.Y.
      • Chu R.
      • Moule A.
      Assessing the cutting efficiency of different burs on zirconia substrate.
      Consequently, removal or manipulation of zirconia restorations during the maintenance period may become necessary. Failed ceramic restorations are mainly removed by sectioning the buccal and lingual surfaces of the restoration with diamond rotary instruments.
      • Spath A.
      • Smith C.
      Removal of Modern Ceramics.
      Some authors have also suggested removing silicate ceramics restorations using laser technology.
      • Ghazanfari R.
      • Azimi N.
      • Nokhbatolfoghahaei H.
      • Alikhasi M.
      Laser Aided Ceramic Restoration Removal: A Comprehensive Review.
      ,
      • Gurney M.L.
      • Sharples S.D.
      • Phillips W.B.
      • Lee D.J.
      Using an Er,Cr:YSGG laser to remove lithium disilicate restorations: A pilot study.
      Because of their high strength, the removal of zirconia restorations is challenging and time consuming, taking up to 1.5 to 7 times longer than for lithium disilicate ceramic or leucite ceramic.
      • Nakamura K.
      • Katsuda Y.
      • Ankyu S.
      • Harada A.
      • Tenkumo T.
      • Kanno T.
      • et al.
      Cutting efficiency of diamond burs operated with electric high-speed dental handpiece on zirconia.
      An increase in the applied load during cutting causes higher temperatures and can damage pulp tissues.
      • Schuchard A.
      A histologic assessment of low-torque, ultrahigh-speed cutting technique.
      • Cavalcanti B.N.
      • Otani C.
      • Rode S.M.
      High-speed cavity preparation techniques with different water flows.
      • Kwon S.-J.
      • Park Y.-J.
      • Jun S.-H.
      • Ahn J.-S.
      • Lee I.-B.
      • Cho B.-H.
      • et al.
      Thermal irritation of teeth during dental treatment procedures.
      Heat development is particularly influenced by the load applied by the dentist, revolutions per minute (rpm), diamond rotary instrument design, and type of coolant.
      • Öztürk B.
      • Üşümez A.
      • Öztürk A.N.
      • Ozer F.
      In vitro assessment of temperature change in the pulp chamber during cavity preparation.
      The applied force when preparing teeth for fixed restorations has been reported to vary between 0.5 and 1.5 N.
      • Siegel S.C.
      • von Fraunhofer J.A.
      Dental cutting with diamond burs: heavy-handed or light-touch?.
      ,
      • Elias K.
      • Amis A.A.
      • Setchell D.J.
      The magnitude of cutting forces at high speed.
      When cutting harder materials such as zirconia, dentists tend to increase the applied load to improve cutting efficiency.
      • Nakamura K.
      • Katsuda Y.
      • Ankyu S.
      • Harada A.
      • Tenkumo T.
      • Kanno T.
      • et al.
      Cutting efficiency of diamond burs operated with electric high-speed dental handpiece on zirconia.
      Manufacturers have marketed diamond rotary instruments specifically for cutting zirconia, but their cutting efficiency is similar to or less than that of conventional diamond rotary instruments.
      • Kim J.-S.
      • Bae J.-H.
      • Yun M.-J.
      • Huh J.-B.
      In vitro assessment of cutting efficiency and durability of zirconia removal diamond rotary instruments.
      ,
      • Keeling F.L.
      • Taft R.M.
      • Haney S.J.
      Bur Choice When Removing Zirconia Restorations.
      However, studies on different types of zirconia and the increase in temperature are lacking. Diamond rotary instruments marketed for cutting zirconia have been tested using high-speed handpieces operated at 150 000 to 200 000 rpm
      • Nakamura K.
      • Katsuda Y.
      • Ankyu S.
      • Harada A.
      • Tenkumo T.
      • Kanno T.
      • et al.
      Cutting efficiency of diamond burs operated with electric high-speed dental handpiece on zirconia.
      ,
      • Kim J.-S.
      • Bae J.-H.
      • Yun M.-J.
      • Huh J.-B.
      In vitro assessment of cutting efficiency and durability of zirconia removal diamond rotary instruments.
      ,
      • Keeling F.L.
      • Taft R.M.
      • Haney S.J.
      Bur Choice When Removing Zirconia Restorations.
      but not at 40 000 rpm as is often used in Western Europe. The aim of this study was to compare the performance of diamond rotary instruments marketed for cutting zirconia with conventional diamond rotary instruments on specimens made of 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) and 4 mol% yttria-stabilized zirconia polycrystal (4Y-PSZ). Cutting speeds using 2 different load applications were examined, along with the maximum temperatures reached at the tip of the diamond rotary instruments, change in surface roughness parameters of the diamond rotary instruments, and composition. The research hypotheses were that the cutting speeds of diamond rotary instruments marketed for cutting zirconia would increase compared with those of conventional diamond rotary instruments and that a higher load application would improve cutting speed significantly.

      Material and methods

      Three different dental diamond rotary instruments were placed in a handpiece fixed in a custom-made holder attached to a universal testing machine (Z2.0; Zwick/Roell). The test used was to have the diamond rotary instruments cut through a 2-mm-thick zirconia plate during which the cutting time and heat generation were measured. Two different diamond rotary instruments, JOT (JOTA Zirkon Cut Z838L; JOTA AG) and IZC (Intensiv ZirconCut Zr02/10; Intensiv SA), marketed to cut zirconia were selected and compared with a conventional course-grit diamond rotary instrument IFG (Intensiv FG 334/6; Intensiv SA). All diamond rotary instruments had a coated length of 8.0 mm and a diameter of 1.4 mm. The cutting efficiency of all diamond rotary instruments was tested on 2 different types of zirconia, an opaque 3Y-TZP (IPS ZirCAD LT; Ivoclar AG) with a flexural strength of 1200 MPa and a lower strength (850 MPa) monochromatic 4Y-PSZ (IPS ZirCAD MT Multi; Ivoclar AG) (Table 1).
      Table 1Overview of zirconia blocks and diamond rotary instruments
      Material TypeNameDimensionManufacturerAbbreviation
      3Y-TZPIPS ZirCAD LTB 55: 15.5×19.0×55.0 mmIvoclar AGLT
      4Y-PSZIPS ZirCAD MT MultiB 45: 17.0×19.0×45.0 mmIvoclar AGMT
      Diamond rotary instrument for cutting zirconiaJOTA Zirkon Cut Z838L8 mm×1.4 mmJOTA AGJOT
      Diamond rotary instrument for cutting zirconiaIntensiv ZirconCut Zr02/108 mm×1.4 mmIntensiv SAIZC
      Conventional diamond rotary instrument (control)Intensiv FG 334/68 mm×1.4 mmIntensiv SAIFG
      An automatic cutoff machine (Accutom 100; Struers GmbH) was used to cut 10 slices from each zirconia block. Each disk was trimmed to a thickness of 2.45 mm using a polishing machine (Minitest 363; Presi) with silicon carbide paper (Type M Reflec NAC P2500; Presi). The slices were sintered (VITA ZYRCOMAT 6100 MS; VITA) according to the recommendations of the manufacturer, resulting in a final thickness of 2.0 mm ±0.02 mm. For the measurement of the diamond rotary instrument’s cutting efficiency, the 2.0-mm-thick zirconia specimens were fixed from 2 sides in a custom-made holder attached to a universal testing machine (Z2.0; Zwick/Roell). The diamond rotary instruments were inserted in a handpiece (INTRAmatic 23 ES; KaVo) and positioned parallel to the zirconia specimens. The handpiece was placed in a holding device and then attached to the force sensor of the universal testing machine (Fig. 1). During the cutting process, a constant load of either 2 N or 6 N was applied on the zirconia specimen. The handpiece was operated at a speed of 40 000 rpm and water cooled at a rate of 90 mL/min (Implantmed SI-923; W&H Dentalwerk Bürmoos GmbH). The temperature during the cutting process was recorded with infrared sensors (Optris CTlaser; Optris GmbH) pointing at the tip of the rotary instrument. The room temperature was 22 °C and the water temperature was 22.5 °C. Each diamond rotary instrument of the 3 types (JOT, IZC, IFG) was used only once on one of the zirconia specimens (LT, MT) with a load of 2 N or 6 N, respectively. Eight diamond rotary instruments were tested per group, resulting in a total of 96 measurements. The sample size was chosen by analyzing the outcome of a previous study
      • Kim J.-S.
      • Bae J.-H.
      • Yun M.-J.
      • Huh J.-B.
      In vitro assessment of cutting efficiency and durability of zirconia removal diamond rotary instruments.
      with 5 specimens per group and considering a power of 80% and a level of significance of 5% as relevant. The sample size was raised to 8 because of the unknown effect of the varying cutting distance.
      Figure thumbnail gr1
      Figure 1Custom-made holder connected with universal testing machine. Handpiece with water cooling positioned parallel to zirconia specimen.
      The cutting efficiency was recorded as the time required to cut through the plate thickness of 2.0 mm and measured in seconds. Additionally, the time was recorded after a cutting distance of 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm. A long machining time reflected poor cutting performance. The maximum temperature reached for each specimen was measured.
      The surface roughness of the diamond rotary instruments was evaluated by scanning the surface of the diamond rotary instruments with a 3-dimensional scanning laser microscope (×10 objective, VK-X; Keyence). Three selected diamond rotary instruments from each group were analyzed before (control) and after cutting. The average roughness parameters Ra (arithmetic average of all deviations of the profile from the centerline) and Rz (mean roughness depth) were obtained from 11 parallel measurements per diamond rotary instrument. Images were filtered using λs=8.0 μm. A scanning electron microscope (SEM) (ESEM XL30; Philips) was used to visualize the surface wear of the diamond rotary instruments. The image contrast was adjusted by using a software program (GNU Image Manipulation Program). Further, the elemental composition of the diamond rotary instruments (embedding material and diamond particles) was obtained using energy-dispersive X-ray spectroscopy (EDAX).
      Each cutting time per 0.5 mm of material was tested for the effects of the factor diamond rotary instrument, zirconia type, and load by using a 3-way ANOVA. Additionally, the time per distance was compared by using 1-way ANOVA. For differences among the subgroups, the Fisher least significant difference (LSD) post hoc test was used (α=.05). Surface roughness values for Ra and Rz were compared for the effects of diamond rotary instrument type, zirconia type, and cutting force by using 3-way ANOVA followed by the Fisher LSD post hoc test.

      Results

      The mean time required to cut through the respective distances of the zirconia specimens with the different diamond rotary instruments, the total time, and the maximum temperature [tmax] reached are displayed in Table 2 and Figure 2. A significant effect of the diamond rotary instrument type was observed for all distances (P<.001), with significantly shorter cutting times for the IFG (85 seconds) and IZC (100 seconds) than for the JOT diamond rotary instruments (182 seconds). Cutting times for IFG were slightly shorter than for IZC, but without statistically significant differences. Cutting times for the MT zirconia tended to be shorter than for LT; this was significant in the first 0.5 mm (P=.005) and last 1.5 to 2.0 mm (P=.030). With a higher load of 6 N, the cutting time decreased significantly (P<.001) at all distances compared with the 2 N load.
      Table 2Mean ±standard deviation cutting times in seconds on zirconia materials LT and MT for 2 N and 6 N loads after respective distance and maximum temperature reached (n=8 per group). Results of 3-way ANOVA given with F (degrees of freedom between and within groups) and P values of tested effects for each distance
      Diamond Rotary Instrument, Zirconia, Load0–0.5 mm0.5–1.0 mm1.0–1.5 mm1.5–2.0 mmTotal Timetmax (°C)
      JOT LT 2N36 ±267 ±8115 ±3122 ±10340 ±527.9 ±1.3
      JOT LT 6N6 ±216 ±717 ±319 ±358 ±333.1 ±3.0
      JOT MT 2N28 ±463 ±2489 ±26114 ±36293 ±2325.5 ±0.4
      JOT MT 6N4 ±27 ±812 ±413 ±435 ±537.2 ±4.0
      IZC LT 2N20 ±1435 ±3547 ±6779 ±57182 ±4324.6 ±0.3
      IZC LT 6N5 ±314 ±824 ±630 ±2173 ±935.7 ±2.7
      IZC MT 2N14 ±410 ±1439 ±1631 ±51114 ±2127.0 ±0.4
      IZC MT 6N5 ±37 ±59 ±212 ±233 ±336.7 ±2.8
      IFG LT 2N16 ±232 ±844 ±358 ±10150 ±525.2 ±0.2
      IFG LT 6N4 ±16 ±1312 ±312 ±435 ±527.9 ±1.7
      IFG MT 2N14 ±226 ±638 ±1150 ±15127 ±825.0 ±0.3
      IFG MT 6N3 ±27 ±29 ±29 ±428 ±232.0 ±2.5
      ANOVA
       Diamond rotary instrumentF(2,95)=27.20,

      P<.001
      F(2,95)=16.28,

      P<.001
      F(2,95)=19.07,

      P<.001
      F(2,95)=12.88,

      P<.001
       ZirconiaF(1,95)=8.24,

      P =.005
      F(1,95)=1.51,

      P=.223
      F(1,95)=3.49,

      P=.065
      F(1,95)=4.91,

      P=.030
       LoadF(1,95)=262.18,

      P<.001
      F(1,95)=118.39,

      P<.001
      F(1,95)=114.60,

      P<.001
      F(1,95)=102.91,

      P<.001
      Figure thumbnail gr2
      Figure 2Cutting time of diamond rotary instruments JOT, IZC, and IFG with loads of 2 N and 6 N on zirconia materials MT and LT over total distance of 2.0 mm measured in seconds.
      Overall, the best performance was observed with the IFG conventional diamond rotary instrument. At a 6 N load, the overall mean cutting time was 35 seconds through LT zirconia and 28 seconds through MT zirconia. The slowest performance was for JOT, with mean cutting times of 340 seconds through LT zirconia and 293 seconds through MT zirconia at a 2 N load. All diamond rotary instruments showed higher temperature development with a 6 N load compared with a 2 N load. Overall, the highest maximum temperature was observed with JOT at a 6 N load for MT (37.3 °C) and the lowest with IZC on LT with a 2 N load (24.6 °C).
      Surface roughness parameters Ra and Rz of the diamond rotary instruments before and after cutting either LT or MT are displayed in Table 3 and in Figure 3. Before use, IFG had the highest mean Ra value with 17.3 μm and Rz with 130.9 μm. Ra and Rz were significantly affected by the type of diamond rotary instrument and load (P<.001), but not by the type of zirconia (LT or MT). After use, JOT showed significantly lower Ra values than IFG and IZC. Rz values were significantly higher for IFG than for JOT and IZC (P<.001), between which no difference was observed (P=.760). An applied load of 2 N or 6 N, respectively, significantly decreased Ra and Rz values compared with the control group (both P<.001). The decrease of Ra and Rz values was significantly higher at 6 N than at 2 N (both P<.001). Overall, IZC had the highest postcutting mean Ra value with 14.5 μm after testing on LT at 2 N. The highest postcutting mean Rz value of 111.8 μm was seen for IFG after cutting MT zirconia at a 2 N load. JOT displayed the lowest mean Ra value of 9.4 μm on MT at a 6 N load and the lowest average Rz value of 82.0 μm on LT at a 6 N load among the tested diamond rotary instruments.
      Table 3Surface roughness parameters Ra and Rz (mean ±standard deviation) of diamond rotary instruments before use and after cutting materials LT and MT with 2 N and 6 N loads. Results of 3-way ANOVA given with F (degrees of freedom between and within groups) and P values for tested effects for each parameter
      Diamond Rotary Instrument, Zirconia, LoadRa (μm)Rz (μm)
      JOT (before use)14.7 ±1.4112.1 ±10.3
      JOT LT 2N10.5 ±1.688.4 ±16.0
      JOT LT 6N9.7 ±1.382.0 ±15.1
      JOT MT 2N11.8 ±1.592.2 ±10.5
      JOT MT 6N9.4 ±1.487.2 ±13.5
      IZC (before use)16.5 ±3.3109.7 ±15.6
      IZC LT 2N14.5 ±1.799.5 ±16.5
      IZC LT 6N10.9 ±1.2101.0 ±18.1
      IZC MT 2N12.9 ±2.287.7 ±12.0
      IZC MT 6N12.0 ±1.387.7 ±11.0
      IFG (before use)17.3 ±2.7130.9 ±20.8
      IFG LT 2N12.1 ±1.099.6 ±12.1
      IFG LT 6N11.5 ±1.8102.7 ±22.0
      IFG MT 2N12.8 ±1.1111.8 ±18.5
      IFG MT 6N10.7 ±1.495.6 ±20.6
      ANOVA
       Diamond rotary instrumentsF(2,593)=69.62, P<.001F(2,593)=73.99, P<.001
       ZirconiaF(1,593)=2.46, P=.117F(1,593)=3.25, P=.072
       LoadF(2,593)=447.23, P<.001F(2,593)=179.90, P<.001
      Figure thumbnail gr3
      Figure 3Surface roughness values of diamond rotary instruments JOT, IZC, and IFG before and after use. A, Ra. B, Rz.
      Scanning electron microscopic images of the diamond rotary instruments before and after use at 2 N and 6 N loads are displayed in Figures 4 and 5. As no difference in roughness values and visible surface inspection was found after testing both zirconia materials, only images for LT are shown. JOT appeared to have smaller diamond particles with wider spaces in between and less protruding from the embedding mass compared with IZC and IFG. After use with a 2 N or 6 N load, the surface prominence of the diamond particles was reduced in all diamond rotary instruments. With higher load, the topography of the diamond rotary instruments appeared smoother because of wear and/or loss of diamond particles. At a ×200 magnification, distinct grinding facets were observed at the tip of the diamond rotary instruments when a load of 6 N was applied (Fig. 5).
      Figure thumbnail gr4
      Figure 4Scanning electron microscope images of surface of diamond rotary instruments before use (upper row), after use at 2 N load (middle), and at 6 N load (lower row). Original magnification ×50.
      Figure thumbnail gr5
      Figure 5Scanning electron microscope images of surface of diamond rotary instruments before use (upper row), after use at 2 N load (middle), and at 6 N load (lower row). Original magnification ×200.
      The elemental composition of diamond particles and embedding material determined by energy-dispersive X-ray spectroscopy are listed in Table 4. The diamond particles of IFG displayed the highest carbon content with 96.2 wt%. The lowest amount of carbon was found in JOT (81.6 wt%); the diamonds in these rotary instruments displayed impurities of hafnium (4.7 wt%) and nickel (possibly background noise). A low amount of oxygen was found in all diamond particles of all rotary instruments. IZC revealed a low amount of zirconium (3.3 wt%) in the diamond particles. The composition of the embedding materials varied considerably among the diamond rotary instruments: Nickel was predominantly used for JOT (95.5%), a polymer material reinforced with metal particles (Ni, Cr, Ti) was found in IZC, and a nickel alloy containing chromium and manganese was used in IFG.
      Table 4Element composition of diamond particles (D) and embedding materials (EB) of unused diamond rotary instruments (JOT, IZC, IFG) obtained with energy-dispersive X-ray spectroscopy analysis (percentage by weight [Wt%])
      CONiHfCrMnZrTi
      JOT D81.623.6910.014.68----
      JOT EB4.53-95.47-----
      IZC D91.724.97----3.31-
      IZC EB94.41-1.22-2.96--1.41
      IFG D96.172.980.85-----
      IFG EB32.814.2451.55-11.200.19--
      C, carbon; Cr, chromium; Hf, hafnium; Mn, manganese; Ni, nickel; O, oxygen; Ti, titanium; Zr, zirconium.

      Discussion

      The aim of this study was to investigate the cutting efficiency of different types of diamond rotary instruments on zirconia specimens. Diamond rotary instruments marketed to remove zirconia restorations did not perform better than a conventional course-grit diamond rotary instrument; therefore, the first research hypothesis was rejected. As increasing the load application from 2 N to 6 N reduced the cutting time significantly, the second research hypothesis was confirmed.
      A previous study in which diamond rotary instruments marketed to cut zirconia were placed in a custom-made high-speed handpiece to cut zirconia blocks with a load of 2 N reported comparable results for the cutting performance.
      • Kim J.-S.
      • Bae J.-H.
      • Yun M.-J.
      • Huh J.-B.
      In vitro assessment of cutting efficiency and durability of zirconia removal diamond rotary instruments.
      In the present study, a diamond rotary instrument marketed to cut zirconia performed worse than the conventional diamond rotary instrument with regard to cutting efficiency and heat development on both the tested zirconia materials. The cutting efficiency of all diamond rotary instruments tended to be higher on zirconia MT compared with LT. This difference was likely because of the higher flexural strength of LT (1200 MPa) compared with the more translucent MT (850 MPa).
      • Zhang Y.
      • Lawn B.R.
      Novel zirconia materials in dentistry.
      The applied load of 2 N and 6 N were chosen based on recommendations of the diamond rotary instrument manufacturers. A load of 2 N is recommended by the manufacturer of JOT, while a load application of 6 N is recommended for IZC. Increasing the load from 2 N to 6 N significantly increased the cutting efficiency but also resulted in a mean temperature rise from 24.6 °C with 2 N to 36.7 °C. A water cooling rate of 90 mL/min was used, higher than the manufacturer’s recommendation of 50 mL/min (Intensiv SA), and therefore a limitation of the study. If a temperature increase causes pulp damage highly depends on the residual dentin thickness.
      • Kwon S.-J.
      • Park Y.-J.
      • Jun S.-H.
      • Ahn J.-S.
      • Lee I.-B.
      • Cho B.-H.
      • et al.
      Thermal irritation of teeth during dental treatment procedures.
      Based on the findings of the present study, a load application of 6 N should be avoided to prevent pulp damage. In the present in vitro study, a constant load was used. However, during clinical use, an intermittent load of the diamond rotary instruments along the surface may be applied, which might improve performance. A force of 2 N or 6 N can be visualized by moving a 200 g or 600 g object with the fingertips.
      A higher cutting efficiency also resulted in increased surface wear of the diamond rotary instruments, expressed in lower surface roughness values. Before use, IZC and IFG had similar Ra and Rz values, which were higher than for JOT. After use, especially with the 6 N load, diamond particles were worn off and even fractured from the embedding material. Hence, after cutting through 2 mm of zirconia, the reuse of diamond rotary instruments is not recommended. The varying cutting performance of the diamond rotary instruments may be because of the composition of the diamond particles and the embedding material. Diamond particles of IZC and IFG displayed a high carbon content, while diamonds of JOT demonstrated impurities of hafnium and nickel. The impurities may have reduced the hardness of the diamonds and, consequently, the cutting performance. Nickel alloys were used as embedding material for JOT and IFG. Nickel is the leading contact allergen in most industrialized countries worldwide.
      • Ahlström M.G.
      • Thyssen J.P.
      • Wennervaldt M.
      • Menné T.
      • Johansen J.D.
      Nickel allergy and allergic contact dermatitis: A clinical review of immunology, epidemiology, exposure, and treatment.
      The actual risk to patients from diamond rotary instruments containing nickel has yet to be determined. Nevertheless, exposing patients with high allergy potential to diamond rotary instruments with nickel alloys is not recommended. A resin-based embedding material was used for IZC, which may be a valuable alternative to nickel alloys in the future.
      The clinical relevance of the present in vitro data is that zirconia restorations can be removed with conventional course-grit diamond rotary instruments, although further differences may be found among products of different manufacturers not tested in this study. The cutting time of 3Y-TZP zirconia such as LT is higher than for more translucent 4Y-PSZ materials such as MT. An applied load of 2 N is recommended with sufficient water cooling to prevent pulp damage due to temperature increase. With this procedure (IFG, 2 N) a cutting time of 2 to 2.5 minutes can be expected for 2.0-mm-thick zirconia, and the diamond rotary instrument should not be reused. Removing material from zirconia restorations is a time- and material-consuming procedure that will become more commonplace in future with the increasing trend of using zirconia as a restorative material. Hence, alternative approaches to the removal of restorations are needed.

      Conclusions

      Based on the results of this in vitro study, the following conclusions were drawn:
      • 1.
        Diamond rotary instruments marketed for cutting zirconia did not perform better or generate less heat than a conventional diamond rotary instrument.
      • 2.
        The time required to cut 3Y-TZP zirconia tended to be higher than for 4Y-PSZ zirconia, particularly in the first 0.5-mm and the 1.5- to 2.0-mm layers of the specimens.
      • 3.
        Although a load of 6 N significantly decreased cutting time, a load of 2 N should be used to avoid excessive heating.

      CRediT authorship contribution statement

      Severin Hunziker: Investigation, Methodology, Writing – original draft. Lea Thorpe: Conceptualization, Funding acquisition, Writing – review & editing. Nicola U. Zitzmann: Conceptualization, Supervision, Writing – review & editing. Nadja Rohr: Conceptualization, Formal analysis, Methodology, Supervision, Validation, Visualization, Writing – review & editing.

      Acknowledgments

      The authors thank Ms Sabrina Karlin for the laboratory support.

      References

        • Sailer I.
        • Makarov N.A.
        • Thoma D.S.
        • Zwahlen M.
        • Pjetursson B.E.
        All-ceramic or metal-ceramic tooth-supported fixed dental prostheses (FDPs)? A systematic review of the survival and complication rates. Part I: Single crowns (SCs).
        Dent Mater. 2015; 31: 603-623
        • Peters O.A.
        • Du D.
        • Ho M.Y.
        • Chu R.
        • Moule A.
        Assessing the cutting efficiency of different burs on zirconia substrate.
        Aust Endod J. 2019; 45: 289-297
        • Spath A.
        • Smith C.
        Removal of Modern Ceramics.
        Compend Contin Educ Dent. 2017; 38: 326-333
        • Ghazanfari R.
        • Azimi N.
        • Nokhbatolfoghahaei H.
        • Alikhasi M.
        Laser Aided Ceramic Restoration Removal: A Comprehensive Review.
        J Lasers Med Sci. 2019; 10: 86-91
        • Gurney M.L.
        • Sharples S.D.
        • Phillips W.B.
        • Lee D.J.
        Using an Er,Cr:YSGG laser to remove lithium disilicate restorations: A pilot study.
        J Prosthet Dent. 2016; 115: 90-94
        • Nakamura K.
        • Katsuda Y.
        • Ankyu S.
        • Harada A.
        • Tenkumo T.
        • Kanno T.
        • et al.
        Cutting efficiency of diamond burs operated with electric high-speed dental handpiece on zirconia.
        Eur J Oral Sci. 2015; 123: 375-380
        • Schuchard A.
        A histologic assessment of low-torque, ultrahigh-speed cutting technique.
        J Prosthet Dent. 1975; 34: 644-651
        • Cavalcanti B.N.
        • Otani C.
        • Rode S.M.
        High-speed cavity preparation techniques with different water flows.
        J Prosthet Dent. 2002; 87: 158-161
        • Kwon S.-J.
        • Park Y.-J.
        • Jun S.-H.
        • Ahn J.-S.
        • Lee I.-B.
        • Cho B.-H.
        • et al.
        Thermal irritation of teeth during dental treatment procedures.
        Restor Dent Endod. 2013; 38: 105-112
        • Öztürk B.
        • Üşümez A.
        • Öztürk A.N.
        • Ozer F.
        In vitro assessment of temperature change in the pulp chamber during cavity preparation.
        J Prosthet Dent. 2004; 91: 436-440
        • Siegel S.C.
        • von Fraunhofer J.A.
        Dental cutting with diamond burs: heavy-handed or light-touch?.
        J Prosthodont. 1999; 8: 3-9
        • Elias K.
        • Amis A.A.
        • Setchell D.J.
        The magnitude of cutting forces at high speed.
        J Prosthet Dent. 2003; 89: 286-291
        • Kim J.-S.
        • Bae J.-H.
        • Yun M.-J.
        • Huh J.-B.
        In vitro assessment of cutting efficiency and durability of zirconia removal diamond rotary instruments.
        J Prosthet Dent. 2017; 117: 775-783
        • Keeling F.L.
        • Taft R.M.
        • Haney S.J.
        Bur Choice When Removing Zirconia Restorations.
        J Prosthodont. 2022; https://doi.org/10.1111/jopr.13564
        • Zhang Y.
        • Lawn B.R.
        Novel zirconia materials in dentistry.
        J Dent Res. 2018; 97: 140-147
        • Ahlström M.G.
        • Thyssen J.P.
        • Wennervaldt M.
        • Menné T.
        • Johansen J.D.
        Nickel allergy and allergic contact dermatitis: A clinical review of immunology, epidemiology, exposure, and treatment.
        Contact Dermatitis. 2019; 81: 227-241