## 4 Discussion
These findings elucidate why CAD-CAM expertise and experience is one of the most critical factors in the success of CAD-CAM rehabilitations, as previously described [9; 10]. This is particularly relevant for the two well-known state-of-the-art laboratory CAD software programs used herein. Despite the relatively similar interfaces, one of the present software programs was created for open systems (DentalCAD,Exocad), and the other (Modellier, Zirkonzahn) is directly integrated to a milling device that is part of an established CAD-CAM system (Zirkonzahn), commonly used in dental laboratories and dental clinics with in-house CAD-CAM laboratory production and has presented high accuracy in previous research [34]. It is also important to mention that the Modellier software can also export the digital designs of dental prostheses as STL files, which enables comparisons and digital assessments used in CAD-CAM dental research [8; 9; 10; 11]. This study used an integrated laboratory CAD-CAM system with a high-end five-axis milling device (Zirkonzahn M1). Laboratory milling is a more complex procedure but has shown superior results compared to chairside milling devices [35]. Nevertheless, the milling machine of the system used herein was found in to be user-friendly for the dentist of this study and has physical structures that withstand any vibrations during the milling process, which leads to increased accuracy and precision as advantages of this system, which uses a proven technology of simultaneous use of 5 + 1 milling axes. This procedure is controlled by a computer and includes eight exchangers of drills and milling burs, as well as three block changers. Another advantage of the system is the possibility of virtually positioning the prostheses and objects to be milled inside the block using an interface of user-friendly software (Zirkonzahn Nesting). This procedure can actually be performed manually or automatically, and then corrected or amended manually if needed. On the other hand, although the software and devices of this system are integrated, there is a tool to enable importing STL files of prostheses designed on other software programs to be milled at the milling machine of the system, as performed herein with the Zirkonzahn M1. The system has also fully automatic and high-resolution optical light scanners, which are also automatically integrated to the CAD software programs and CAM systems. Several methods have been used to measure accuracy and precision of CAD-CAM restorations. Among the main methods are 3D mesh superimposition and digital caliper linear measurement [3; 4]. However, a digital caliper should only be used with a precision of two decimal places and with the caliper tips positioned on marks designed on the crowns on each pair of opposing vertical surfaces. Even these measurements are only valid if performed at least twice by two or more trained observers at intervals of two weeks to eliminate bias from memorized procedures. Three-dimensional mesh superimposition, in turn, requires an extra digitalization procedure of the manufactured crowns, which can be performed with intraoral scanners that have 360 scanning tools. Then, the STL files of the scanned crowns can be digitally aligned with their respective digital wax patterns CAD software programs dedicated for 3D mesh comparisons. Other studies have assessed accuracy of CAD-CAM systems by means of measuring marginal and internal gaps of the dental prostheses on their respective abutments. It is noteworthy that this preliminary study assessed patient-centered variables, which shows the role of CAD-CAM and the technological development in dentistry as an advantage that comes for the benefit of the patient [2].