## 1 Introduction  
With the introduction of the computer-aided design and computer-aided manufacturing (CAD-CAM) system, fixed dental prostheses (FDP) can be produced using a digital workflow through three-dimensional (3D) data acquisition, CAD, and CAM processing, as opposed to the conventional workflow involving a manual process using the lost wax technique [1; 2; 3]. Prior to the introduction of intraoral scanners, the digital workflow was used to fabricate a gypsum model through impression-taking in the patient's mouth, and to obtain a virtual model using a desktop optical scanner [2; 4]. On the other hand, the intraoral scanner can acquire a virtual model through direct optical scanning of the patient's mouth. The introduction and development of intraoral scanners have led to a significant increase in the demand for digital workflows [5; 6; 7].  
The digital workflow for FDP can be divided into a partially and fully digital workflow [8]. The partially digital workflow is used in combination with a conventional workflow for impression-taking and gypsum model-making [8]. On the other hand, a fully digital workflow does not require the production of a gypsum model because it directly scans the patient's mouth in the chairside environment [8]. The application of a fully digital workflow is increasing [9]. However, due to the cost burden of intraoral scanners and the need to fabricate the FDP in dental laboratories, a partially digital workflow may be preferred [10].  
The compatible equipment used in digital workflows differs according to each production process (scan acquisition process, CAD process, and CAM process) and the manufacturer [11]. As the demand for a digital workflow increases, some manufacturers support accessibility to other manufacturers, while other manufacturers highly support self-compatibility. Equipment allowing accessibility with other manufacturers have the advantage of being usable in combination with a wider variety of equipment desired by users in each manufacturing step. On the other hand, equipment with closed accessibility has the advantage that they can be easy to control in each manufacturing step.  
Many previous studies have validated FDP by evaluating their marginal and internal fit [12; 13; 14; 15; 16; 17; 18]. Poor marginal fit is a known cause of periodontal disease, cement dissolution, and limits long-term healthy use [12; 16]. The clinically acceptable range of marginal fit proposed by many studies was reported to be within 120 \(\upmu\)m [12; 16; 17; 18]. An ideal internal fit increases the retention of the FDP [12; 16]. Thus, many studies have examined the marginal and internal fit.  
In the digital workflow, virtual FDP designed in the CAD software is produced by actual FDP through CAM processing. Many studies have been carried out to analyze the machining accuracy of the CAM process by comparing the designed virtual FDP with the actual FDP [19; 20; 21]. The ceramic FDPs are typically fabricated through the CAM process by a subtractive (milling) process using zirconia and lithium disilicate materials [22; 23; 24]. Poor trueness of the milling unit makes it difficult to reproduce the designed FDP, which can affect the marginal and internal fit. Although studies have examined the trueness and fitness, studies on the relationship between the trueness of the milling unit and the FDP fitness are lacking.  
According to a systematic review by [PERSON] [25], multi-unit FDP fabricated using the digital workflows have been reported to require further studies for clinical reliability. Therefore, the purpose of this study was to evaluate and correlate the fitness and trueness of a 3-unit FDP fabricated from two digital workflows. The null hypothesis is as follows: (1) The 3-unit FDPs fabricated from two digital workflows have no difference in their trueness and fitness; (2) the fitness and trueness evaluated in the 3-unit FDP fabricated from two digital workflows are not correlated.