Department of Fixed Prosthodontics, Osaka University Graduate School of Dentistry, Osaka, Japan.
J Prosthodont. 2019 Feb;28(2):e477-e484. doi: 10.1111/jopr.12715. Epub 2017 Nov 30.
Intraoral digital scanning can accurately record single abutment tooth preparations despite their geometry, and the algorithms of the CAD software can be set to manage different abutment forms. The purpose of this in vitro study was to evaluate the marginal and internal fit of CAD/CAM zirconia crowns fabricated over conventional and reverse-tapered preparations.
Crown preparations with known total occlusal convergence (TOC) angles (-8°, -4°, 0°, 8°, 12°, 16°, and 22°) were digitally created from a maxillary left central incisor and printed in acrylic resin. Next, casts were scanned with a TRIOS intraoral scanner, and crowns were designed with KaVo multiCAD software using default parameters (50 μm cement space) on abutments with positive TOC angles, whereas reverse-tapered abutments (negative TOC angles) were digitally blocked out at 0° and had an extra mesiodistal gap set to 50 μm. Then, zirconia crowns were fabricated, and their marginal and internal discrepancies were recorded with the silicone replica technique. All replicas were examined under a stereomicroscope at 50× magnification. Collected data were analyzed with one-way ANOVA and post hoc Tukey test for marginal fit. For the axial and incisal fit, measured values did not follow a normal distribution; therefore, the Kruskal-Wallis and the Dunn/Bonferroni multiple comparison tests were applied (p = 0.05).
The mean marginal fit of -8° crowns (58.2 ± 6.0 μm) was statistically different (p < 0.0001) from all the remaining crowns (range 42.1-47.3 μm). Also, the internal fit was statistically significant when comparing crowns fabricated over abutments with positive and negative TOC angles (p < 0.0001). The largest median axial discrepancies were found in the -8° (165.5 μm) and -4° (130.8 μm) groups; however, when evaluating the incisal fit, they showed the smallest discrepancies (67.3 and 81.8 μm, respectively).
Under the conditions of this study, the marginal and internal fit of zirconia crowns fabricated over inverse-tapered preparations is within clinically accepted values.
尽管单个基牙牙体预备的几何形状存在差异，但口腔内数字化扫描可以准确记录，并且 CAD 软件的算法可以设置为管理不同的基台形态。本体外研究的目的是评估 CAD/CAM 氧化锆冠在常规和反向锥度预备体上的边缘和内部适合性。
从上颌左侧中切牙创建具有已知总牙合（TOC）角度（-8°、-4°、0°、8°、12°、16°和 22°）的冠预备体，并在丙烯酸树脂中打印。接下来，使用 TRIOS 口内扫描仪扫描铸型，使用 KaVo multiCAD 软件在具有正 TOC 角度的基台上设计牙冠，而反向锥度基台（负 TOC 角度）则在 0°处进行数字化截除，并设置 50μm 的额外近远中间隙。然后，制作氧化锆牙冠，并使用硅橡胶复制技术记录其边缘和内部不贴合情况。所有复制体均在 50×放大倍率的立体显微镜下检查。收集的数据采用单向方差分析和边际拟合的事后 Tukey 检验进行分析。对于轴面和切端拟合，测量值不符合正态分布；因此，应用了 Kruskal-Wallis 和 Dunn/Bonferroni 多重比较检验（p = 0.05）。
-8°牙冠的平均边缘拟合度（58.2±6.0μm）与所有其他牙冠（42.1-47.3μm）存在统计学差异（p<0.0001）。此外，当比较具有正 TOC 角度和负 TOC 角度的基台制备的牙冠时，内部拟合具有统计学意义（p<0.0001）。在 -8°（165.5μm）和-4°（130.8μm）组中发现最大的中位数轴向差异，但在评估切端拟合时，它们显示出最小的差异（分别为 67.3μm 和 81.8μm）。
在本研究条件下，反向锥度预备体上制备的氧化锆冠的边缘和内部适合性在临床可接受范围内。
