Doctoral student, Department of Fixed Prosthodontics, Osaka University Graduate School of Dentistry, Osaka, Japan.
Associate Professor, Department of Fixed Prosthodontics, Osaka University Graduate School of Dentistry, Osaka, Japan.
J Prosthet Dent. 2017 Sep;118(3):392-399. doi: 10.1016/j.prosdent.2016.10.021. Epub 2017 Feb 17.
Direct (intraoral) and indirect (desktop) digital scanning can record abutment tooth preparations despite their geometry. However, little peer-reviewed information is available regarding the influence of abutment tooth geometry on the accuracy of digital methods of obtaining dental impressions.
The purpose of this in vitro study was to evaluate the influence of abutment tooth geometry on the accuracy of conventional and digital methods of obtaining dental impressions in terms of trueness and precision.
Crown preparations with known total occlusal convergence (TOC) angles (-8, -6, -4, 0, 4, 8, 12, 16, and 22 degrees) were digitally created from a maxillary left central incisor and printed in acrylic resin. Each of these 9 reference models was scanned with a highly accurate reference scanner and saved in standard tessellation language (STL) format. Then, 5 conventional polyvinyl siloxane (PVS) impressions were made from each reference model, which was poured with Type IV dental stone scanned using both the reference scanner (group PVS) and the desktop scanner and exported as STL files. Additionally, direct digital impressions (intraoral group) of the reference models were made, and the STL files were exported. The STL files from the impressions obtained were compared with the original geometry of the reference model (trueness) and within each test group (precision). Data were analyzed using 2-way ANOVA with the post hoc least significant difference test (α=.05).
Overall trueness values were 19.1 μm (intraoral scanner group), 23.5 μm (desktop group), and 26.2 μm (PVS group), whereas overall precision values were 11.9 μm (intraoral), 18.0 μm (PVS), and 20.7 μm (desktop). Simple main effects analysis showed that impressions made with the intraoral scanner were significantly more accurate than those of the PVS and desktop groups when the TOC angle was less than 8 degrees (P<.05). Also, a statistically significant interaction was found between the effects of the type of impression and the TOC angle on the precision of single-tooth dental impressions (F=2.43, P=.002). Visual analysis revealed that the intraoral scanner group showed a homogeneous deviation pattern across all TOC angles tested, whereas scans from the PVS and desktop scanner groups showed marked local deviations when undercuts (negative angles) were present.
Conventional dental impressions alone or those further digitized with an extraoral digital scanner cannot reliably reproduce abutment tooth preparations when the TOC angle is close to 0 degrees. In contrast, digital impressions made with intraoral scanning can accurately record abutment tooth preparations independently of their geometry.
直接（口内）和间接（桌面）数字扫描可以记录基牙牙体预备，尽管其几何形状可能不同。然而，关于基牙几何形状对获得牙印数字化方法的准确性的影响，几乎没有经过同行评审的信息。
本体外研究的目的是评估基牙几何形状对常规和数字化方法获得牙印的准确性的影响，包括准确性和精密度。
从上颌左中切牙数字化创建具有已知总覆（TOC）角度（-8、-6、-4、0、4、8、12、16 和 22 度）的牙冠预备，并以丙烯酸树脂打印。对这 9 个参考模型中的每一个都用高精度参考扫描仪进行扫描，并以标准曲面细分语言（STL）格式保存。然后，从每个参考模型制作 5 个常规聚硅氧烷（PVS）印模，并用参考扫描仪（PVS 组）和桌面扫描仪扫描并以 STL 文件导出的类型 IV 牙科石膏进行浇注。此外，对参考模型进行直接数字化印模（口内组），并导出 STL 文件。通过比较印模获得的 STL 文件与参考模型的原始几何形状（准确性）和每个测试组内（精密度）来进行比较。使用具有事后最小显著差异检验（α=.05）的 2 因素方差分析对数据进行分析。
总准确性值分别为 19.1μm（口内扫描仪组）、23.5μm（桌面组）和 26.2μm（PVS 组），总精密度值分别为 11.9μm（口内）、18.0μm（PVS）和 20.7μm（桌面）。简单主效应分析表明，当 TOC 角度小于 8 度时，口内扫描仪组的印模比 PVS 和桌面组的印模更准确（P<.05）。此外，还发现了一种类型的印象和 TOC 角度对单牙牙印的精密度的影响之间存在统计学显著的相互作用（F=2.43，P=.002）。视觉分析显示，口内扫描仪组在所有测试的 TOC 角度下显示出均匀的偏差模式，而 PVS 和桌面扫描仪组的扫描在存在倒凹（负角度）时显示出明显的局部偏差。
当 TOC 角度接近 0 度时，单独的传统牙印或使用口外数字扫描仪进一步数字化的牙印都不能可靠地复制基牙牙体预备。相比之下，使用口内扫描制作的数字印模可以独立于其几何形状准确记录基牙牙体预备。
