## 1 Introduction
Since the introduction of screw-retained implant-supported prostheses, clinical studies have associated mechanical complications with misfits between the superstructure and the ankylosed implant [12]. Currently, a maximum misfit threshold of 150 \(\upmu\)m for complete-arch implant rehabilitations is generally accepted in clinical practice [11,13]. However, this value remains controversial in the scientific literature [2; 13; 14; 15; 16; 17; 18], with some authors suggesting that discrepancies between 30 \(\upmu\)m and 50 \(\upmu\)m may be more appropriate for ensuring long-term treatment success [13; 17; 18]. Digital implant impressions in partially edentulous patients have been reported as a clinically acceptable alternative to conventional impressions [1; 19; 20]. Nevertheless, the digitization of fully edentulous arches has been approached with increased caution. Larger scanning areas elevate the risk of cumulative angulation errors, while the featureless nature of the denture-bearing region can compromise the accuracy of digital impressions [18; 19]. In fact, the edentulous ridge lacks distinct anatomical landmarks required by IOSs for reliable image stitching, thereby increasing the likelihood of stitching errors and inaccuracies in the final digital model. Additionally, the smooth and reflective surface of the mucosa may lead to light reflection artifacts, interfering with the scanner's ability to capture accurate data. The presence of saliva may further compromise scan quality, particularly in the mandibular arch [20; 21], which is also prone to mucosal displacements during mandibular movement. Furthermore, the inherent flexibility of the mandible during function may distort the scanned model over larger spans, potentially resulting in misfits in the final prosthesis. Accuracy is defined by the concepts of \"trueness\" and \"precision\", as outlined in the International Organization for Standardization (ISO) 5725-1:2023 [22]. \"Trueness\" is defined as the \"closeness of agreement between the arithmetic mean of a large number of test results and the true or accepted reference value\", while \"precision\" is defined as \"the closeness of agreement between different test results\" [23; 24]. Assessing the trueness and precision of digital impressions is therefore critical to ensuring the quality of digital models, and achieving consistent, predictable clinical outcomes. The main limitation of current IOS systems lies in their three-dimensional (3D) image reconstruction technology, which relies on a best-fit algorithm for stitching consecutive images. Continuous reference points are essential to enhance the matching accuracy of the acquired 3D datasets [4; 25]. Photogrammetry represents an alternative digital impression technology that simultaneously records 3D objects and their spatial relationship by analyzing points within photographic images captured by two stereo cameras [26]. Dental photogrammetry has been applied to impressions in full-arch implant-supported rehabilitation [4], as the photogrammetry device is capable of capturing the 3D spatial position of implants in the dental arch without being affected by factors such as saliva or patient movement [27]. However, an additional scan of the soft tissues using an IOS is still required. Although promising, the use of photogrammetry technology has produced conflicting results depending on implant distribution [4; 28; 29]. In fact, studies assessing the accuracy of photogrammetry for complete-arch implant impressions have reported controversial findings [6; 30]. In light of these inconsistencies, the aim of this in vitro study is to evaluate the trueness of digital impressions for full-arch implant-supported fixed dental prostheses with varying implant distributions, using IOSs based on different technologies and a photogrammetry device. The first null hypothesis (H\({}_{0}\)1) states that there are no differences in trueness based on the scanner used.