Trueness evaluation of digital impression: The impact of the selection of reference and test object

Objectives: This study evaluated the importance of defining the reference and the test object during 3D surface comparisons to assess the trueness of an intraoral scanner. Materials and methods: A maxillary complete-arch cast with interdental spaces was digitized with a high resolution scanner to obtain the ground truth dataset [GT]. Fifteen intraoral scanning datasets [IOS] were obtained with an intraoral scanner. The trueness of the [IOS] datasets were evaluated by two different comparison procedures using a 3D analysis software: In the first comparison [REF-GT], the [GT] dataset was set as reference object and the [IOS] dataset was defined as test object. In the second comparison [REF-IOS], the [IOS] dataset were set as reference object and the [GT] dataset was defined as test object. The mean trueness of both comparisons was calculated with absolute mean deviation, (90-10)/2 percentile, and root-mean-squared (RMS) error method. Statistical significance was analyzed using the t-test (alpha=0.05). Results: The mean trueness values of [REF-GT] were 31.4(+/- 6.1) mu m for (90-10)/2 percentile, 77.0(+/- 5.3) mu m for absolute mean deviation, and 203.1(+/- 4.8) mu m for RMS error method. [REF-IOS] revealed 23.9(+/- 4.8) mu m, 28.3 (+/- 6.3) mu m, and 39.6(+/- 9.5) mu m, respectively. The results differed significantly. Conclusion: The datasets obtained from the intraoral scanner captured more adequately interproximal spaces in comparison to the [GT] dataset. Therefore, the [GT] dataset defined as reference object in the analysis software for 3D comparisons revealed misleading results. Clinical significance: The selection of the reference object and of the areas to be compared have to be defined carefully regarding complete arch scanning accuracy analysis.