*The Accuracy of Printing and milling Zirconia Crowns*
*Background Information: *Zirconia crowns are the most durable dental crowns today. They are made of Zirconium, which is a compound that was invented by Klaproth in 1789. The crowns should be fitted appropriately to ensure the maximum benefits, which include extreme durability and 100% biocompatibility. A perfect dental prosthesis needs to be stable, esthetic, and has to fit perfectly. A perfect fit is crucial. In cases where it doesn’t fit the individual is prone to recurrent dental carries, microleakage and in some cases cement dissolution
*Purpose: *The study focuses on the evaluation of the accuracy of printed and milled zirconia crowns
*Methodology: *The study involved 20 models of mandibular teeth. Each opposing wall inclined 3 degrees to the path of insertion, which ensured that the needed 6-degree taper was produced. The teeth were also digitized using a laboratory scanner. The master file of restoration was used to mill and print the zirconia crowns using the DWX 52D 5 Axis Dental Milling Machine and the Stratasys 3D printers, respectively. Dental Designer Software was used for a full-coverage restoration. The superimposition of the digitized file over the master design file was done using the Geomagic software. The OraCheck software was used to analyze the accuracy of the milled and printed crowns in relation to the master design.
*Results: *There were significant differences between the milled and the printed crowns
*Conclusion: *The printing of Zirconia crowns improves the accuracy of a model and ensures a perfect fit and marginal alignment. Printing has several benefits such as low predisposition to fractures, better shade, easily manufactured in a short period, better color, and there is no limit on the cutting instrument to be used.
*Keywords: *3D printed and milled teeth, Zirconia crowns, Printing Zirconia Crowns, Milling Zirconia crowns
Zirconia is a crystalline dioxide of Zirconium. It was first used for medical purposes in 1969 for orthopedic purposes. However, the ever growing interest in esthetics, and the increasing concern and adverse events such as allergies to certain alloys led dentists to Zirconia (Madfa et al. 2014). Patients and dentists were searching for metal-free tooth-colored restorations. They were looking for a material that could be developed into high strength dental ceramics, with less adverse effects, less brittle, and is one that could withstand time-dependent stress (Madfa et al. 2014). Zirconia was identified as the solution to the problem and has since been used for dental ceramics (Madfa et al. 2014).
There are many types of zirconia containing ceramics that are available today, but only three are being used in dentistry. They are yttrium cation-doped tetragonal zirconia polycrystals (3Y-TZP), magnesium cation-doped partially stabilized zirconia (Mg-PSZ) and zirconia-toughened alumina (ZTA) (Madfa et al. 2014). Zirconia has multiple purposes in dentistry. It is used to make dental posts, bridges, and orthodontic brackets. It is also used to make crowns, an aspect which is the main focus of this research study.
Zirconia crowns can be milled or printed. The process starts at the dentist’s office, who makes the impression of the patient’s tooth or teeth and sends the impression to the laboratory. The mode is cut into sections resembling the impression, and it is scanned using laser, red lights, or blue-light technology. The scan is then uploaded into a computer that has the CAD/CAM software that is effective in designing the crown (Daou et al. 2014). Two types can be derived from the software. Hard milling is when the crown is milled out of a sintered monoblock, and soft milling is when the restoration is derived from a white monoblock with subsequent sintering (Ebert et al. 2009). Printing of the crown is slightly different from the milling. The material is made into a suspension that is injected into standard printing cartilage as decided by the technician. A printing output resembling the impression made by the dentist is made, and the suspension is sent to a printing device which prints the crown (Ebert et al. 2009).
*Materials and Methods*
The study was done in a dentistry laboratory of a large healthcare facility with the equipment to do both the milling and printing of zirconia crowns. The impression (Master file) taken by the dentists were sent to the laboratory physically or scanned impressions that were directly sent to the laboratory computer. For each impression of a crown sent to the laboratory, two restorations were made, one printed and the other milled. In total, there were 20 models of mandibular teeth that required restoration of crowns. The milling of crowns was made when the sections were cut from the model and scanned using the laboratory scanner (D2000, 3Shape; Blue LED Multi-line; 5 – 8 μm accuracy). For the printed crowns, the impression was made in the print output after the cartridges were filled in the printing cartilages. The models were all made using the Dental Designer Software (3Shape). Milling was done using the Axis Dental Milling Machine, while printing was done using the Stratasys 3D printers.
Geomagic software was used for comparing the master file with the milled or printed files through superimposition. After the milling and printing processes were complete, the researchers went to the dentist’s clinic and observed as each of the models was fitted into the patient and analyzed the accuracy of each model using the Oracheck software.

*Evaluation of Accuracy of Ceramic Crowns*
The accuracy of the Zirconia crowns will be evaluated by comparing the printed models of the printed and milled first molar. The models will both be prepared using high standards to ensure the recommended 6-degree taper and 1.5mm chamfer finish line are produced. The digitization will be done using a laboratory scanner.
The full-coverage restoration will be done using the digitized file of the preparation, and it will be done using the Dental Designer Software. The fabrication of the printed zirconia will be done using the master file of the designed restoration. Once the process is done, the laboratory scanner will be sued to scan each crown. The Geomagic software will be used for the superimposition of the digitized files of the fabricated crowns over the master design file. The comparison with the master design file will lead to the determination of positive and negative deviations. The information will be used to determine the efficacy of the fabricated crowns, which will be expressed in terms of trueness.
The same master design file will be used for the fabrication of the milled zirconia crowns using the DWX 52D 5 Axis Dental Milling Machine. The described methods will also be used for the assessment of the accuracy of the milled zirconia crowns. The results will be compared to those obtained in the accuracy assessment of the printed zirconia crowns.
The information acquired from both groups will be analyzed using the two-way analysis of variance. The evaluated aspects will include the crown-type, fabrication method, and the interaction between the two. Post-hoc analysis will be done using Dunnett’s multiple comparison test. P <0.5 has been set.References Daou, E. E. (2014). The zirconia ceramic: strengths and weaknesses. *The open dentistry journal,* 8, 33. Ebert, J., Özkol, E., Zeichner, A., Uibel, K., Weiss, Ö., Koops, U., & Fischer, H. (2009). Direct inkjet printing of dental prostheses made of zirconia. *Journal of dental research*, 88(7), 673-676. Madfa, A. A., Al-Sanabani, F. A., Al-Qudami, N. H., Al-Sanabani, J. S., & Amran, A. G. (2014). Use of zirconia in dentistry: An overview. *The Open Biomaterials Journal*, 5(1).

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