## 4 Discussion
The similarities between the results of the current study and literature supports the ability of the simple spherical geometry to assess the dimensional distortion of 3D printed models. The present study also investigated the dimensional stability of 3D printed models using two printing systems over a period of six weeks. Currently, there are limited data in the literature investigating the dimensional stability of 3D printed models. The dimensional stability of 3D printed models is a clinically important parameter. The study by [PERSON] et al. [31] showed that, although the fit of crowns produced on 3D printed models was acceptable, it remains inferior to conventional dies. This might be attributed to the detected dimensional errors of 3 DP that need to be accounted for during the fabrication process of extra-coronal restoration to achieve better fitting restoration with minimal internal and marginal discrepancies. In the current study, the diameter of the seven spheres represents the short-span accuracy, as well as the twenty-one vectors extending between the centres of the seven spheres, representing the full-arch parameter of 3 DP, was investigated to elucidate the pattern of dimensional changes exhibited for SLA and DLP manufactured dental models. No significant difference was detected for the diameter of the spheres between 2 weeks and 6 weeks indicating that for short-span application, any errors arising from model storage are expected to be within the reported clinically acceptable thresholds of 120 \(\upmu\)m [32; 33] and supporting their suitability, from an accuracy perceptive, for single crown and short-span application irrespective of storage time. Furthermore, for both SLA and DLP printers, the magnitude of the detected dimensional changes was similar to those reported for type IV die stone over 8 weeks of storage [34], albeit stone models exhibited expansion, as opposed to the contraction exhibited by 3 DP. In contrast, the full-arch findings indicate significant and delayed contraction for both printing systems after storage and in agreement with similar in vitro studies [11; 17]. The SLA models in this study exhibited significant contraction in an anteroposterior and cross-arch direction, in contrast to DLP which contracted evenly. Such contraction may impact the fit of full-arch appliances and restorations fabricated using these 3 DP models due to their localised and skewed error pattern of \(>\)200 \(\upmu\)m. On the other hand, the dimensional stability results of the DLP printer were within the clinically acceptable error margin of 59-150 \(\upmu\)m [16] required for accuracy-demanding prosthodontic application such as implant-retained fixed prosthesis even after six weeks of storage. These findings do support the notion that a clear understanding of the performance and limitation of the 3D printing system is cardinal for determining their most suitable dental application and the timing of the manufacturing workflow. The main limitation of this study is its in vitro nature and the reliance on simple object geometry for a reference model. Hence, whilst the methodology facilitates reproduction and standardisation of testing, the derived results represent ideal testing conditions that do not account for other clinical factors such as complex dental morphology and the presence of orthodontic crowding. Moreover, a limitation shared with similar studies is the applicability of the results to the resins used, which, albeit recommended by the manufacturers of the tested SLA and DLP printers for full-arch models, may not fully represent the array of printing resins currently in the market. Future research should be aimed at establishing the effect of different variables such as other resins and alternative post-processing methods on the accuracy of 3D printed full-arch models.