ORIGINAL RESEARCH Assessing the cutting efficiency of different burs on zirconia substrate Ove A. Peters, DMD MS PhD 1,2 ; Daniel Du, BDSc (Hons) 1 ; Mei Ying Ho, BDSc (Hons) 1 ; Rick Chu, BDSc (Hons) 1 ; and Alex Moule, BDSc (Hons) PhD 1 1 School of Dentistry, University of Queensland, Brisbane, Queensland, Australia 2 University of the Pacific Arthur A. Dugoni School of Dentistry, San Francisco, California, USA Keywords cutting efficacy, diamond bur, scanning electron microscopy, tungsten carbide bur, zirconia. Correspondence Professor Ove A. Peters, Department of Endodontics, University of the Pacific Arthur A. Dugoni School of Dentistry, 155 5th St, San Francisco, CA 94103, USA. Email: opeters@pacific.edu doi: 10.1111/aej.12350 (Accepted for publication 22 March 2019.) Abstract Cutting dental zirconia for endodontic access preparation is difficult. Therefore, this study aimed to determine cutting efficiency of various burs when cutting this material. An air turbine handpiece was used in a customised test rig to cut sin- tered zirconia specimens, using a conventional blue band diamond, two different zirconia-cutting diamond and a zirconia-cutting tungsten carbide bur. Position and speed of the bur were continuously determined using wireless data acquisi- tion over two successive five-minute runs. Differences in cutting efficiency were statistically analysed. Burs were examined using light and scanning electron microscopy (SEM). All diamond burs cut zirconia more efficiently than the tung- sten carbide bur. Overall, all burs showed decreasing cutting efficiency over time. SEM images showed discernible wear and damage to the cutting portion of each bur head. It is concluded that zirconia-cutting burs are advantageous regarding durability, and carbide burs are rather ineffective against carbide substrate. Introduction Restorative dentistry involves appropriate replacement of lost or decayed tooth structure to maintain oral health and function (1). For compromised teeth, fixed dental prostheses such as crowns are viable long-term treat- ment modalities (2,3) and the provision of crowns as a part of routine general dentistry is commonplace (4). An ageing population, coupled with an increased retention of teeth, has resulted in an increased prevalence of these restorations. Between 2000 and 2005, more than one million crowns annually were completed for patients in the UK treated under the General Dental Services (GDS) (5). Porcelain-fused-to-metal crown restorations have been used to great success with reported clinical performance of 15 years (6). However, aesthetic demands and advances in material sciences (e.g. CAD/CAM fabrication) have led to the introduction of other materials, particularly all- ceramic crowns. Recently, zirconia was introduced as an alternative material for fixed dental prostheses (7,8), primarily due to its increased strength compared to other crown materials. However, zirconia does present certain complications, most notably low-temperature degradation and phase transformation that increases sus- ceptibility to fractures and cracking (9,10). Under clinical conditions, dental restorations appear to have a limited lifespan. One meta-analysis reported that fixed dental prostheses have a 10-year survival rate of just under 90% (11), whilst another study looking into single crowns reported a mean lifespan of 9 years (3). Crowns may fail due to porcelain fracture, aesthetic issues and recurrent decay (3,12–16). Depending on the clinical situation, failures may require the removal of the crown for re-assessment and re-treatment (13,15). Under these circumstances, it may be desirable to cut and remove a crown in pieces in order to minimise tooth damage and discomfort to the patient. If restoration failure involves pulpal pathosis, endodontic treatment is often required (4). The literature (4,15,17) reports that approximately 3–6.5% of crowned teeth ultimately require endodontic therapy in the 7-year period after crown insertion. With the increased number of crowns being placed every year (5), there are a corre- sponding number of cases that require endodontic treat- ment. To perform endodontic treatment, it is frequently necessary to cut an access cavity through the crown. However, preparing access cavities in zirconia is both © 2019 Australian Society of Endodontology Inc 1 Aust Endod J 2019