Original Article
Influence of different convergence angles of abutment teeth and cement spaces on internal adaptation of anterior CAD-CAM fixed dental prostheses
2024 Volume 66 Issue 1 Pages 55-59
Details
2024 Volume 66 Issue 1 Pages 55-59
Purpose: The aim of this study was to investigate the influence of different convergence angles of abutment teeth and different cement spaces on internal adaptation of anterior fixed dental prostheses (FDPs) fabricated with a computer-aided design-computer-aided manufacturing (CAD-CAM) system.
Methods: Composite resin FDPs for 99 standardized maxillary central incisors were fabricated according to nine parameters: three total convergence angles (4 [DG4], 12 [DG12], and 20 degrees [DG20]) and three cement space settings (10 [CS10], 50 [CS50], and 90 µm [CS90]). Internal space values were measured with a cement space replica technique. The Kruskal-Wallis and Steel-Dwass tests were used to evaluate differences in the total convergence angles and luting agent spaces, respectively (α = 0.05).
Results: For all three cement spaces tested, the median marginal gap values between abutment teeth and FDPs decreased significantly as the total convergence angle increased (P < 0.05). For the CS10 and CS50 groups, the internal space values at the axial area increased significantly as the total convergence angles increased (P < 0.05).
Conclusion: Total convergence angles of the abutment teeth and cement spaces affected the marginal and internal adaptation of anterior FDPs fabricated with a CAD-CAM system.
Due to recent developments in digital dentistry, various dental prostheses can be fabricated with dental computer-aided design-computer-aided manufacturing (CAD-CAM) systems that include intra-oral scanning for digital impressions and milling by machines [1,2]. In comparison with conventional methods, CAD-CAM systems are characterized by better quality and reproducibility of dental prostheses, lower costs, and shorter fabrication periods [3]. Further clinical applications of the system are expected in the future. Polymethyl methacrylate–based materials, composite resins, silica-based ceramics, and zirconia ceramics are currently used as CAD-CAM materials for fixed dental prostheses (FDPs) [4].
To achieve long-term stability, FDPs must be adapted appropriately to abutment teeth, have superior fracture resistance, and meet excellent biological requirements. Good adaptation of FDPs can prevent secondary caries and periodontal disease, which adversely affect the prognosis of the FDPs [5]. According to previous studies, all processes of FDP fabrication by CAD-CAM systems, including scanning, designing, and manufacturing, affect FDP adaptation [6]. Furthermore, the convergence angle of an abutment tooth and the cement space setting affect the adaptation between ceramic restorations fabricated with CAD-CAM systems and abutment teeth [7,8,9].
CAD-CAM composite resin blocks have excellent physical properties and can be used to fabricate FDPs in both premolar and molar areas [10,11]. Furthermore, CAD-CAM composite resin blocks are currently used in anterior FDPs. However, in terms of adaptation, there is little information about the optimal convergence angles of abutment teeth and optimal cement space setting for the clinical use of CAD-CAM composite resin in anterior FDPs.
The aim of this study was to investigate the influence of different convergence angles of abutment teeth and different cement spaces on internal adaptation of anterior FDPs fabricated with CAD-CAM systems. The null hypothesis was that the convergence angle of an abutment tooth and the cement space would not affect internal adaptation of an anterior FDP fabricated with CAD-CAM system.
To simulate the preparation of a machine-milled, full-coverage composite resin restoration for a maxillary right central incisor, titanium-based abutment teeth (Tokyo Giken, Tokyo, Japan) were manufactured to create a circumferential deep chamfer finish line of 1.2 mm in width and 8 mm in height (Fig. 1). These teeth were prepared with three total convergence angles: 4 degrees (DG4 specimen), 12 degrees (DG12 specimen), and 20 degrees (DG20 specimen) (Fig. 1).
The machine-milled restorations were designed and produced following a CAD-CAM workflow (Katana system, Kuraray Noritake Dental, Tokyo, Japan). Each restoration was designed as a maxillary right central incisor (8 mm wide, 10 mm in height) (Fig. 2). Cement spaces had three settings: 10 µm (CS10 group), 50 µm (CS50 group), and 90 µm (CS90 group). In accordance with STL data, machine-milled restorations were manufactured from a composite resin CAD-CAM block (Katana Avencia N, Kuraray Noritake Dental). The machine-milled restorations were classified according to combinations of convergence angles (DG4, DG12, and DG20 specimens) and cement spaces (CS10, CS50, and CS90 groups); for each of the 9 combinations, 11 restorations were fabricated.
A cement space replica technique was used to measure internal spaces between the restorations and abutment teeth [12]. A silicone-based fitting check material (Fit Checker Advanced, GC, Tokyo, Japan) was applied to the inner surface of each restoration, and then a customized jig and a compression gauge (Ohba Keiki, Tokyo, Japan) were used to mount each restoration on an abutment tooth with a static loading pressure of 5 N (Fig. 3a, b). After polymerization, the restoration and silicone-based fitting check material complex was removed from the abutment. An autopolymerizing resin (Fixpeed, GC) was injected into the inner surface of the complex (Fig. 3c). When the resin had polymerized, the silicone-based fitting check material and the autopolymerizing resin were carefully removed from the inner surface of the restorations (Fig. 3d). The removed specimens were covered with acrylic resin (Tray Resin II, Shofu, Kyoto, Japan) (Fig. 3e), and a low-speed precision cutting machine (IsoMet low-speed saw, Buehler, Lake Bluff, IL, USA) was used to segment the specimens in the sagittal plane (Fig. 3f).
The width of the silicone-based fitting check material was measured with a laser microscope (1LM21W, Lasertec, Yokohama, Japan) at ×100 magnification. Nine measuring points at three areas were used: two measuring points for the center of the deep chamfer at the marginal area (A and I), six evenly divided measuring points for the axial area (B-D, F-H), and one measuring point for the center at the incisal area (E) (Fig. 4). The gap values at each of the nine measuring points were determined by averaging the values of 10 optional points around each measuring point. For each specimen, a total of 90 measurements were made on the nine standardized points (A to I). The marginal gap values were calculated from the averages of the two points (A and I). The internal space values at the axial and incisal areas were calculated from the averages of the six points (B-D, F-H) and the one point (E), respectively.
Statistical software (IBM SPSS Statistics, version 27.0, IBM, Armonk, NY, USA) was used for statistical analysis. The Shapiro-Wilk test was used to evaluate normality, and the Levene test was used to evaluate equal variance. Because normality and equivariance were not obtained, a nonparametric test was performed. In addition, the Kruskal-Wallis test and the Steel-Dwass test (Kyplot 6.0, KyensLab, Tokyo, Japan) were used to evaluate differences in the total convergence angle and luting agent space, respectively. The significance level was set at α = 0.05 for all tests.
Table 1 lists the marginal gap values between the restorations and abutment teeth. For the three different cement space settings (CS10, CS50, and CS90 groups), the median marginal gap values decreased significantly as the total convergence angle increased (P < 0.05), and for the three total convergence angles (DG4, DG12, and DG20 specimens), values decreased significantly as the cement space increased (P < 0.05).
The internal space values at the axial and incisal areas are displayed in Table 2. For the CS10 and CS50 groups, the median internal space values at the axial area significantly increased as the total convergence angle increased (P < 0.05). For the three total convergence angles, the internal space values at the axial area increased significantly as the cement space increased (P < 0.05). For all specimens at the incisal areas, the values exceeded 200 µm.
Representative laser microscopic images of the marginal and axial spaces are shown in Figs. 5 and 6. Marginal spaces declined in width as the total convergence angle and the cement space increased (Fig. 5). In contrast, the axial spaces tended to be wider as the total convergence angle and the cement space increased (Fig. 6).
| Total convergence angles | Cement space settings | ||
|---|---|---|---|
| CS10 | CS50 | CS90 | |
| median (IQR) | median (IQR) | median (IQR) | |
| DG4 | 154.0A, a (148.0-169.7) |
129.3B, a (122.9-133.4) |
84.3C, a (78.8-92.9) |
| DG12 | 147.4A, b (140.0-165.6) |
122.8B, b (111.0-130.9) |
78.6C, b (74.7-87.3) |
| DG20 | 141.1A, c (136.3-148.6) |
114.4B, c (102.6-124.2) |
75.2C, c (72.2-78.8) |
Data are listed as median and IQR (interquartile range) values. Within a row, median values with different uppercase superscript letters differed significantly (P < 0.05). Within a column, median values with different lowercase superscript letters differed significantly (P < 0.05). Total convergence angles: DG4, 4 degrees; DG12, 12 degrees; and DG20, 20 degrees. Cement space settings: CS10, 10 µm; CS50, 50 µm; and CS90, 90 µm
| Total convergence angles | Cement space settings | ||
|---|---|---|---|
| CS10 | CS50 | CS90 | |
| median (IQR) | median (IQR) | median (IQR) | |
| Axial area | |||
| DG4 | 56.3A, a (51.2-63.0) |
80.2B, a (64.1-95.0) |
101.2C, a (97.6-108.4) |
| DG12 | 58.5A, a (46.6-69.8) |
84.0B, a (69.8-99.8) |
98.5C, b (95.8-103.4) |
| DG20 | 85.0A, b (66.4-94.3) |
96.1B, b (76.6-109.4) |
103.3C, a (97.3-108.2) |
| Incisal area | |||
| DG4 | 236.2A, a (217.5-266.4) |
225.9A, a (206.2-247.5) |
260.1A, a (196.4-279.3) |
| DG12 | 273.6A, b (260.3-290.0) |
207.7B, a (183.2-254.6) |
231.2B, a (196.1-257.3) |
| DG20 | 259.2A, a, b (242.9-276.7) |
242.9A, B, a (215.2-257.9) |
224.2B, a (206.7-246.8) |
Data are listed as median and IQR (interquartile range) values. Within a row, median values with different uppercase superscript letters differed significantly (P < 0.05). Within a column, median values with different lowercase superscript letters differed significantly (P < 0.05). Total convergence angles: DG4, 4 degrees; DG12, 12 degrees; and DG20, 20 degrees. Cement space settings: CS10, 10 µm; CS50, 50 µm; and CS90, 90 µm
This study investigated the influence of different total convergence angles of the abutment teeth and different cement spaces on the internal adaptation of anterior single crowns fabricated with a CAD-CAM system. The results showed that the DG20 group had significantly smaller marginal gap values than the DG4 and DG12 groups for the same cement space setting. Furthermore, for the same convergence angles, the marginal gap values of the CS90 group were significantly smaller than those of the CS10 and CS50 groups. Therefore, the null hypothesis (that the convergence angle of an abutment tooth and the cement space would not affect internal adaptation of an anterior FDP fabricated with CAD-CAM system) could be rejected.
For all cement space settings (CS10, CS50, and CS90 groups), the marginal gap values decreased significantly as the total convergence angle increased. This finding indicates that total convergence angles influence the marginal adaptation of anterior single crowns fabricated with a CAD-CAM system. These results are consistent with those of previous studies, which highlighted the fact that increases in the convergence angle enhance the marginal adaptation of the restorations through CAD-CAM technology, owing to improved scanning accuracy [7,13].
It has been previously noted that the scanning process converts imported images into digital data that produce a blurred image, known as a “point cloud”, which may affect the adaptation of the marginal areas and occlusal surfaces [14,15]. In this study, the scanning accuracy was enhanced by increased convergence angles of the abutment teeth, which may have mitigated the occurrence of the “point cloud” phenomenon. As the cement space setting increased, a significant decrease in marginal gap values was observed in this study. The findings are consistent with those of a previous report [8]. It can be assumed that when restorations are placed in narrow cement spaces, the resin luting agents are subjected to a higher internal pressure, and this pressure results in heightened resistance and reduced outflow of excess luting material [16].
For the three total convergence angles, the internal space values at the axial areas increased significantly as the cement spaces increased. In this study, the differences between the cement space setting in the CAD software and the corresponding internal space values obtained were approximately 50 µm for the CS10 group, 30 µm for the CS50 group, and 10 µm for the CS90 group. These differences were smaller when the cement space setting was larger; that is, increases in the cement space setting led to a closer approximation of the intended internal space values. Al Hamad et al. [17] pointed out that the causes of internal space values that were larger than the cement space settings include the diameter and shape of the milling tools, tooth preparation designs, and errors during digital data processing. For the CS90 group, the likelihood of such errors is diminished, and thus the difference between the cement space setting and the internal space values were smaller.
The internal space values at the incisal area were approximately twice as large as marginal gap values under certain conditions. These findings are consistent with those of previous studies, in which the internal space values at occlusal and incisal area were wider than those at marginal and axial areas [14,18]. This phenomenon might be attributable to the diameter of the milling tools, which resulted in unintended material removal [18]. Further studies of appropriate milling tools and the development of new machining tools are warranted.
In the CS90 group, the marginal gap values were less than 120 µm, which is within the clinically acceptable range of marginal adaptation [19]. Although direct comparisons with the results of previous studies are challenging, the 90 µm cement space setting appears to be clinically appropriate for all tested total convergence angles in this study. Molin et al. [20] noted that internal space values at the axial area of approximately 50-100 µm are considered favorable for clinical applications when resin luting agents are used. In addition, Bhaskaran et al. [21] showed that the range of 81-136 µm was clinically acceptable for axial spaces. For the CS10 and CS50 groups in this study, the internal space values of the axial area varied from 56.3 to 96.1 µm for all convergence angles. Therefore, the CS90 group had a clinically desirable level of internal adaptation, irrespective of the tested convergence angles.
Previous studies have suggested the use of digital methods such as micro-CT and triple scan techniques to quantify these gaps [22,23,24]. In this study, the silicone replica technique used is a non-destructive method for measuring marginal and internal gaps. The silicone replica technique is well documented and has several advantages, including repeatability and the ability to achieve results equivalent to luting procedures clinically [24,25].
The influence of cement spaces and convergence angles of abutment teeth on fracture resistance and long-term durability of FDPs fabricated with CAD-CAM systems is still unknown; thus, further investigation is warranted. Moreover, the choice of luting agents might influence the adaptation of FDPs [9]. To ensure the proper clinical application of CAD-CAM for anterior FDPs, comprehensive and multifaceted studies are essential.
In conclusion, the total convergence angles of abutment teeth and cement space settings affected the marginal and internal adaptation of anterior FDPs fabricated with a CAD-CAM system. A cement space setting of 90 µm can be recommended for such FDPs to enhance marginal adaptation.
No potential conflict of interest was reported by the authors.
This study was supported in part by JSPS KAKENHI Grant Number (22K0109 and 22K17191), and a Grant from the Sato Fund, Nihon University School of Dentistry (2022 and SATO-2023-17).
