## 4 Discussion  
The aim of this study was to propose a standardised testing methodology for assessing the accuracy of 3D printed full-arch dental models and the impact of storage on them. The current study utilised the combination of metrology and 3D linear measurements using an industrial coordinate measuring machine and inspection software as a gold standard to reduce the error associated with physical calliper measurements and best-fit superimposition [14; 15; 16]. Previous studies have relied on the best-fit (iterative closest point) algorithm for superimposition, which may compensate positive deviations and negative ones resulting in the under- or over-estimation of errors [16; 17]. On the other hand, physical, digital calliper measurements have been criticised for their poor repeatability, reliance on reference areas that may change over time, and limited access to small areas on the models [18]. The use of varying typodont models in different studies presents significant challenges to the standardisation of testing and meaningful comparison among studies. Henceforth, the current study proposed the use of simple, spherical geometry that can be readily replicated to determine the highest possible accuracy of 3 DP whilst avoiding the use of complex tooth morphology that may introduce a greater risk of variation and measurement error [13; 19]. Indeed, the choice of spherical geometry was based on the accuracy testing methodology adopted by the International Organization for Standardization in several of its standards, which involves the measurement of spheres, including ISO12836:2015 annex C evaluating _digitizing devices for CAD/CAM systems for indirect dental restorations_ and ISO10360:2009/2020 parts 2 and 5 for _acceptance and reverification tests for coordinate measuring systems (CMS)_. The methodology employed in the current study was also previously validated for assessing the dimensional accuracy and stability of Type IV stone dental models, presenting a suitable follow-up in the assessment of 3D printed dental models [13]. The proposed methodology relied on a direct comparison between the 3D printed samples and the STL image of the reference model, subsequently eliminating errors arising from the scanning of a physical reference model such as scanning system error, the dimensional stability of a stone cast, optical properties of the reference model, operator influence, or light conditions [6; 13; 20; 21; 22; 23; 24]. The dimensional trueness identified in the current study with an error of \(<\)120 \(\upmu\)m concurred with the accuracy findings reported in similar studies [25; 26]. Additionally, for full-arch accuracy, the detected anteroposterior and cross-arch dimensional contraction associated with the SLA printer was also identified in previous studies [16; 27; 28]. This cross-arch contraction progressively increased towards the posterior aspect as the model diverged, resulting in a reduction in cross-arch support, which was also reported by [PERSON] et al. [16]. However, the study by [PERSON] et al. [25] reported greater cross-arch trueness for their SLA printer (ZENITH; Dentis, Daegu, Korea) when compared to their DLP printer (M-ONE; MAKEX Technology, Ningbo, Zhejiang, China). Therefore, the cause of the dimensional distortion for the tested Form 2 models is more likely associated with the resin formulation as the investigation from [PERSON] et al. [29] suggested that different compositions of ethoxylated bisphenol A-dimethacrylate, triethylene glycol dimethacrylate, and urethane dimethacrylate may influence the accuracy of 3 DP. Moreover, [PERSON] et al. [30] reported that the choice of post-curing method played a significant role in the degree of conversion of the photo-sensitive resin - with the Otoflash G171 demonstrating the greatest degree of conversion versus the LC-3D Print Box being the lowest, which possibly explained the delayed dimensional changes identified in the study.