Evaluation of Tooth Reduction for Prefabricated Zirconia Crowns in Primary First Molars Using 3D Scanner
Article information
Trans Abstract
The aim of this study was to quantitatively evaluate the amount of tooth reduction required for prefabricated zirconia crowns (PZCs) of maxillary and mandibular primary first molars by evaluating different areas of the tooth. A total of 271 intact primary first molars were scanned using a 3D scanner, and PZCs were virtually superimposed to analyze the amount of tooth reduction. The results indicated that in the maxillary primary first molars, more reduction was required on the buccal occlusal surface compared to the lingual occlusal surface, whereas in the mandibular primary first molars, a similar amount of reduction was required on both buccal and lingual occlusal surfaces. For the axial surfaces, the maxillary primary first molars required the most reduction on the lingual surface than buccal and proximal surfaces, whereas the mandibular primary molars required significantly less reduction on the lingual surface. Additionally, a trend was observed where larger crowns required less reduction on the buccal and lingual surfaces. This study provides detailed guidelines on the required amount of reduction for PZC restorations, contributing to more efficient preparation in clinical practice.
Introduction
The use of prefabricated zirconia crowns (PZCs) is becoming widespread with the increasing aesthetic demands of pediatric patients and their parents. Most pediatric patients prefer aesthetically pleasing colored restorations for posterior teeth [1], and surveys of parents have shown that zirconia crowns are highly regarded for their aesthetic qualities [2,3]. Therefore, PZCs restorations are performed not only in the anterior teeth but also in the posterior teeth.
The excellent durability and resistance to occlusal forces of zirconia make it a suitable full-crown restoration for posterior teeth; additionally, its biocompatibility makes it favorable for maintaining gingival health [4-6]. PZCs have shown equivalent performance to stainless steel crowns (SSCs) in terms of gingival health, occlusion, surface roughness, wear of the opposing arch tooth, anatomic form, marginal discoloration, proximal contact area, and secondary caries [7,8].
However, clinical symptoms and loss of restoration are reported as the main failure types associated with zirconia crowns [9,10]. Previous studies have shown that zirconia crowns are difficult to adjust the shape and crimped, they also require a passive fit, and require twice the amount of tooth reduction as SSCs, potentially increasing the risk of pulp exposure and complications [11]. In addition, the occlusal-cervical height after tooth preparation affects crown retention, and excessive reduction can increase the risk of failure [12]. Inexperienced clinicians may increase the taper of the axial wall and reduce the crown height, resulting in reduced retention [13,14]. Therefore, the clinician should prepare the teeth to adapt to the zirconia crowns. A precise and careful tooth preparation is necessary for a successful restoration.
Several manufacturers provide guidelines for tooth reduction, but most provide approximate and broad amounts of reduction on the occlusal and axial surfaces [15-17]. These guidelines are insufficient for clinicians to ensure the accuracy and efficiency of the procedure, and there is a risk of over-reduction of the tooth. Thus, tooth-specific and area-specific guidelines are needed to provide more detailed guidance.
Previous studies on the amount of tooth reduction for PZCs in posterior teeth have been conducted [11,18-20]; however, they did not reflect the detailed differences in the amount of reduction because they predicted the required amount of tooth reduction by measuring the crown thickness or measuring the volume by comparing the weight before and after tooth reduction. Additionally, they have the limitation of using typodonts.
Therefore, this study is aimed to present a clinical guideline for PZCs by measuring the size and shape of the primary first molars using a 3D scanner, virtually superimposing the PZCs, and quantitatively evaluating the required amount of tooth reduction for each area.
Materials and Methods
This study was approved by the Institutional Review Board (IRB) of Chosun University Hospital (IRB file No.: CUDH-2409-002).
1. Study subjects and materials
In this study, study models of 271 children (141 boys and 130 girls) with sound maxillary and mandibular primary first molars without caries, fractures, or restorations were selected. The external and internal surfaces of the PZCs for maxillary and mandibular primary first molars (NuSmile ZR®, Orthodontic Technologies, Houston, TX, USA) were 3D scanned. The crowns were available in various sizes (No. 1 to No. 7), and each size was individually measured and analyzed.
2. Study method
1) 3D scanning and measurement of the primary first molars and zirconia crowns
The study model was scanned using an extraoral laboratory scanner (Freedom HD, DOF lab, Seoul, Korea), and the external and internal surfaces of the first molar PZCs were scanned using an intraoral scanner (PRIMESCAN®, Dentsply Sirona, Bensheim, Germany).
The primary first molars and PZCs measurements were performed using the Geomagic® design X program (3D systems, Rock Hill, SC, USA). The mesiodistal and buccolingual lengths and the occluso-cervical heights in the buccal and lingual areas were measured. The reference points and planes set for the measurements are described in Fig. 1. The reference plane was set as the plane passing through three points: the mesiobuccal cusp, distobuccal cusp, and mesiolingual cusp tip. The mesiodistal length was defined as the distance between two parallel planes passing through the most prominent points of the contour on the mesial and distal surfaces. The buccolingual length was defined as the distance between two parallel planes passing through the most prominent points of the contour on the buccal and lingual surfaces. The occluso-cervical heights in the buccal and lingual areas were defined as the distance from the reference plane to the most inferior point of the cervical margin. The crown shape ratio was calculated as follows: buccolingual length / mesiodistal length.
Reference points and planes of the primary first molar. (A) Setting the reference plane, (B) Measurement of the mesiodistal lengths, buccolingual lengths, (C, D) Measurement of the occlusocervical lengths.
a: mesiobuccal cusp tip, b: distobuccal cusp tip, c: mesiolingual cusp tip, d: mesial height of the contour, e: distal height of the contour, f: buccal height of the contour, g: lingual height of the contour, h: the farthest point from the reference plane in the buccal surface, i: the farthest point from the reference plane in the lingual surface.
All teeth and PZCs on the right side were measured three times by one examiner, and the average value was recorded. For the zirconia crowns, three measurements were taken with different lot numbers for each size.
2) Superimposing of the primary first molar and zirconia crown
Using a 3D design program (Meshmixer, San Rafael, CA, USA), the 3D data of the maxillary and mandibular primary first molars were sectioned and stored as separate stereolithography (STL) files. A crown of appropriate size was selected based on the measured mesiodistal length. The primary first molar and PZC files were superimposed using the Dentbird Studio program (Imagoworks Inc., Seoul, Korea), after manually setting the major anatomical structures (mesiobuccal cusp, mesiolingual cusp, distobuccal cusp, distolingual cusp, mesial marginal ridge, and distal marginal ridge) as reference points. The superimposed files were three-dimensionally placed on the study model using Geomagic® Design X, and the PZC files were manually delicately adjusted, considering the heights of the marginal ridges of the adjacent teeth and the adequacy of the occlusal relationship. According to the manufacturer’s guidelines, the crown margin was positioned subgingivally at a depth of 1 – 2 mm. Subsequently, the existing study model files were removed after the final positioning of the crowns (Fig. 2).
Superimposing of the primary first molar and zirconia crown. (A) Section of the primary first molar and study model, (B) Superimposing with reference to anatomical structures, (C) Considering the marginal ridge height and occlusal relationship.
3) Analysis of the required amount of tooth reduction for the zirconia crown
The required amount of tooth reduction was evaluated by comparing the superimposed primary first molars and the internal surfaces of the PZCs in the Geomagic® Design X program. The tooth was divided into two parts in the mesiodistal direction to calculate the amount of tooth reduction on the proximal surface. Furthermore, it was divided into three parts in the buccolingual direction to calculate the amount of tooth reduction on the occlusal, buccal, and lingual surfaces at the mesial, central, and distal areas. The amount of tooth reduction was evaluated by determining the maximum distance between the tooth and the plane in contact with the internal surface of the PZCs in the divided areas (Fig. 3).
Analysis of the amount of tooth reduction required for zirconia crown. (A) Divide into two parts in the mesio-distal direction and calculate the amount of reduction in the proximal surface, (B) Divide into three parts in the bucco-lingual direction and calculate the amount of reduction in the buccal, lingual, occlusal (buccal, lingual) surfaces. B: buccal; L: lingual; M: mesial, D: distal.
3. Statistical analysis
The collected data were analyzed and statistically processed using SPSS (version 29.0.2.0, SPSS, Chicago, IL, USA). The measurements of the teeth and zirconia crowns were performed three times by one examiner using the same method, and the intraclass correlation coefficient (ICC) was calculated to analyze the reliability. The required amount of tooth reduction for each area was divided into the occlusal surface (buccal and lingual) and axial surface (buccal, lingual, and proximal). One-way analysis of variance (ANOVA) was performed to analyze the differences by area, followed by the post hoc Bonferroni correction test, and the significance level was set at 0.05. In addition, the same method was performed to evaluate the differences in the amounts of tooth reduction according to the crown sizes.
Results
1. Intraexaminer reliability
The ICC value at 0.997 – 1 (p < 0.001) indicated a robust intraexaminer reliability.
2. Morphological measurements of the primary first molars and zirconia crowns
The measurements of the primary first molars and PZCs are presented in Table 1. The average mesiodistal length, buccolingual length, and crown shape ratio of the primary maxillary first molars were 7.276 ± 0.45 mm, 8.695 ± 0.45 mm, and 1.193 ± 0.05, respectively. The average mesiodistal length, buccolingual length, and crown shape ratio of the mandibular primary first molars were 8.21 ± 0.53 mm, 7.27 ± 0.53 mm, and 0.883 ± 0.09, respectively. The crown shape ratio of the zirconia crowns was 1.063 – 1.079 for the maxilla and 0.822 – 0.877 for the mandible. PZCs with sizes ranging between No. 2 and No. 3 were the most similar to the average mesiodistal lengths in the maxilla and mandible.
Morphological measurements of primary first molars and zirconia crowns
3. Distribution of the zirconia crown sizes
The distributions of the zirconia crown sizes selected based on the mesiodistal lengths of the primary first molars are presented in Table 2. In the maxilla, the most frequently selected crown sizes were No. 2 (n = 96, 35.42%), No. 3 (n = 72, 26.57%), and No. 1 (n = 56, 20.66%); the corresponding sizes in the mandible were No. 2 (n = 93, 34.32%), No. 3 (n = 90, 33.21%), and No. 4 (n = 57, 21.01%).
Size distribution of zirconia crowns
4. The mean values of the required tooth reduction
The mean and SD values of the required tooth reduction for each area are presented in Fig. 4.
The mean values of the required tooth reduction. (A) Maxillary primary first molar, (B) Mandibular primary first molar. B: buccal; L: lingual; M: mesial, D: distal.
In the maxilla, the occlusal (buccal) surface required more reduction than the occlusal (lingual) surface. On the buccal surface, the mesial area required more reduction than the central and distal areas, whereas on the lingual surface, there was little difference between the mesial, central, and distal areas. On the proximal surface, the mesial area required more reduction than the distal area.
In the mandible, the central area required the most reduction, followed by the mesial and distal areas on the occlusal surface (buccal and lingual). In addition, the lingual surface required less reduction than the buccal surface. On the proximal surface, the distal area required more reduction than the mesial area.
5. Comparison of the amount of tooth reduction required according to area
The amount of tooth reduction required was assessed by dividing the areas as follows: occlusal surface (buccal and lingual) and axial surface (buccal, lingual, proximal); Table 3, Fig. 5.
Comparison of the amount of tooth reduction according to areas. (A) On occlusal surface, (B) On axial surface. A, B, C, D, E, F: The same character means no statistical difference by the Bonferroni test.
1) Occlusal surface
In the maxillary primary first molar, significant differences were observed in the remaining areas except for the mesial and central areas of the buccal occlusal surface and mesial and central areas of the lingual occlusal surface in the occlusal surface. In the mandibular primary first molar, significant differences were observed in the remaining areas except for the central and distal areas of the buccal and lingual occlusal surfaces.
2) Axial surface
In the maxillary primary first molar, significant differences were observed in the remaining areas, except for the mesial areas of the buccal and lingual surfaces, and central and distal areas of the buccal surface. In the mandibular primary first molar, no significant difference was observed between the buccal and proximal surfaces. In most cases, the lingual surface showed significant differences between the buccal and proximal surfaces.
6. Comparison of the amount of tooth reduction required according to the size of the zirconia crown
1) Maxilla
The comparison of the amount of tooth reduction required according to the size of the zirconia crown in maxilla is presented in Table 4. Significant differences were observed in the mesial, central, and distal areas (p < 0.0001) on the occlusal surface (buccal). Likewise, significant differences were observed in the mesial and distal areas (p < 0.0001, p = 0.005) but not in the central area (p = 0.090) of the occlusal surface (lingual). On the buccal surface, no significant differences were observed in the mesial and distal areas (p = 0.766, p = 0.017); however, significant differences were observed in the central area (p = 0.017). Significant differences were observed in the mesial, central, and distal areas (p < 0.0001) on the lingual surface and mesial and distal areas (p = 0.003, p < 0.0001) on the proximal surface. There was no particular tendency on the occlusal and proximal surfaces; however, there was a tendency for the amount of reduction to decrease as the crown size increased on the buccal and lingual surfaces.
Comparison of the amount of tooth reduction according to zirconia crown size in maxilla
2) Mandible
The comparison of the amount of tooth reduction required according to the size of the zirconia crown in maxilla is presented in Table 5. Significant differences were observed in the mesial, central, and distal areas on the occlusal surface (buccal) (p = 0.003, p < 0.0001, p < 0.0001) and in the mesial, central, and distal areas on the occlusal surface (lingual) (p < 0.0001, p < 0.0001, p < 0.0001). On the buccal surface, a significant difference was observed in the mesial and central areas (p = 0.027, p < 0.0001) but not in the distal area (p = 0.329). On the lingual surface, no significant differences were observed in the mesial and distal areas (p = 0.054, p = 0.324); however, there was a significant difference (p = 0.001) in the central area. On the proximal surface, significant differences were observed in the mesial and distal areas (p < 0.0001). There was no particular tendency on the occlusal and proximal surfaces; alternatively, a tendency for a decrease in the amount of reduction was observed with the increase in the crown size on the buccal and lingual surfaces.
Comparison of the amount of tooth reduction according to zirconia crown size in mandible
Discussion
Studies on the amount of tooth reduction required for PZCs in posterior teeth are limited. In the study by Clark et al. [11] using typodonts, it was reported that zirconia crowns, especially Cheng Crowns®, required more reduction than SSCs based on weight, though it remains unclear which specific areas required more reduction. Lee et al. [18] used 3D scanning to measure crown volume and suggested more reduction was needed on the occlusal surface than on axial surfaces or gingival margins, but their method did not superimpose the crown on the tooth. Sparks et al. [20] analyzed amounts of reduction on the occlusal and mesiobuccal surfaces using typodonts, finding Kinder Krowns® required the most reduction, but noted limitations since actual teeth were not used. In the current study, the primary first molar was 3D scanned, the size of the zirconia crown was selected based on the mesiodistal length, and the crown was virtually superimposed on the tooth on the 3D image. The occlusal surfaces (buccal and lingual), buccal surface, lingual surface, and proximal surface were divided into mesial, central, and distal areas, and the amount of reduction was quantitatively measured for each of the 14 areas.
The teeth were measured to determine the size of the PZCs. The average mesiodistal and buccolingual length of the maxillary primary first molars were 7.276 mm and 8.695 mm, respectively, and those of the mandibular primary first molars were 8.21 mm and 7.27 mm, respectively. In the study conducted by Baek et al. [21] in Korea, the mesiodistal length of the maxillary primary first molar was reported as 7.33 mm in boys and 7.27 mm in girls, and the mesiodistal length of the mandibular primary first molar was 8.13 mm in boys and 7.98 mm in girls, which were similar to the results of the current study. In addition, the buccolingual length of the maxillary primary first molar was reported to be 8.98 mm in boys and 8.79 mm in girls, and that of the mandibular primary first molar was 7.83 mm in boys and 7.73 mm in girls; the values in the mandible were slightly higher than those in the present study.
Several studies have used the crown index, crown module, and crown area to evaluate the size and shape of the crown [21,22]. In this study, the crown shape ratio was calculated using Tsai’s method [23] through the crown index. In the study by Axelsson and Kirveskari [22] comprising Icelandic children, the crown shape ratio of the maxillary primary first molar was 1.238, and that of the mandibular primary first molar was 0.923. The corresponding values in the Korean study by Baek et al. [21] were 1.22 in the maxilla and 0.97 in the mandible. In the current study, the crown shape ratios were 1.193 in the maxilla and 0.883 in the mandible, which were lower than those in the studies mentioned above. These discrepancies may be due to differences in the sample size, measurement method, and research environment.
In this study, the distance between the primary first molar and the inner surface of the zirconia crown was measured to predict the amount of reduction, and statistical analysis was performed by dividing them into the occlusal and axial surfaces. First, in the comparison of the occlusal surface in the maxilla, significant differences were observed in all areas except for the mesial and central areas of the buccal occlusal surface and the mesial and central areas of the lingual occlusal surface. In the mandible, significant differences were observed in all areas except for the central and distal areas of the buccal and lingual occlusal surfaces. Thus, when preparing the occlusal surface, the maxilla requires a greater reduction in the occlusal surface (buccal) than in the occlusal surface (lingual). In addition, on occlusal (buccal and lingual) surfaces of the mandible, a greater amount of reduction was observed in the central areas than in the mesial and distal areas. These findings indicate that sufficient reduction of the mesiobuccal and mesiolingual cusps extending toward the central area of the crown should be considered when preparing the occlusal surface of mandibular primary first molars.
When comparing the axial surface, significant differences were observed in all areas except the mesial area of the lingual and buccal surfaces and the central and distal areas of the buccal surface in the maxilla. No significant difference was observed between the buccal and proximal surfaces in the mandible, and the lingual surface required significantly less reduction. Therefore, the maxillary primary first molar requires a large reduction in the order of the lingual, buccal, and proximal surfaces. The mandibular primary first molar requires a similar amount of reduction on the buccal and proximal surfaces but a smaller amount on the lingual surface. In the maxilla, more reduction was required for the buccal and lingual surfaces than for the proximal surface; this difference may be due to the difference in the crown shape ratio between the PZCs and the tooth. In this study, the average crown shape ratio of the maxillary primary first molars was 1.193, and that of the PZCs was 1.063 – 1.079. However, the average crown shape ratio of the mandibular primary first molars was 0.883, and that of the zirconia crowns was 0.822 – 0.877; the mandible showed relatively more similar values than the maxilla. Thus, the PZCs had a shorter buccolingual length than the maxillary primary first molars, and excessive reduction in the lingual area was required to compensate for this difference. The tendency for the buccal and lingual surfaces to decrease as the size of the crown increases can also be explained by this reason.
The manufacturers of the NuSmile ZR® crown provide tooth preparation guidelines on their website [15], where they recommend a reduction of 1 – 2 mm on the occlusal surface while maintaining the natural contour, a 0.5 – 1.25 mm circumferential reduction, and a feather edge of approximately 1 – 2 mm subgingivally. However, this guideline provides approximate and broad amounts of reduction in the occlusal and axial surfaces. The average amount of reduction required in the present study was 1.23 – 1.362 mm on the occlusal surface (buccal), 1.008 – 1.121 mm on the occlusal surface (lingual), 0.676 – 1.217 mm on the buccal surface, 1.002 – 1.215 mm on the lingual surface, and 0.771 – 0.835 mm on the proximal surface for the maxilla. The corresponding values in the mandible were as follows: 0.885 – 1.359 mm on the occlusal surface (buccal), 0.633 – 1.458 mm on the occlusal surface (lingual), 0.693 – 0.836 mm on the buccal surface, 0.454 – 0.681 mm on the lingual surface, and 0.789 – 0.816 mm on the proximal surface. The maxillary and mandibular primary first molars required less reduction than the guidelines on the occlusal surface. On the axial surface, the buccal and lingual surfaces of the maxillary primary first molar was consistent with those of the guidelines, but the remaining areas showed a smaller amount of reduction than the guideline. The lingual surface of the mandibular primary first molar showed a smaller amount of reduction than that of the guidelines (about 50%).
However, since this study evaluates the minimal tooth reduction by measuring the distance between the tooth and the inner surface of the crown, consideration of the cement space is necessary. The appropriate cement space for zirconia crowns is unclear but may be approximately 0.2 mm. A 0.2 mm internal occlusal gap has been observed in previous studies [24-26]. Therefore, an additional reduction of 0.2 – 0.3 mm is necessary for clinical convenience, which may vary depending on the clinical situation.
This study included only zirconia crowns from a specific manufacturer, so there is a limitation that the amount of reduction may differ when using crowns from other manufacturers. Further studies with different manufacturers are needed. Nevertheless, the significance of this study is as follows; Unlike previous studies using typodonts, this study was performed using actual teeth, providing more clinically applicable results. In addition, the use of a 3D scanner allowed for accurate and reproducible measurements by virtually superimposing the teeth and zirconia crowns, and the teeth were divided into 14 areas to quantitatively measure the required amount of tooth reduction. Therefore, it provides more clinically valuable guidelines on the required amount of tooth reduction of PZCs.
Conclusion
This study analyzed the required amount of tooth reduction for prefabricated zirconia crowns (PZCs) on primary first molars using 3D scanning and virtual superimposition. The results showed that the amount of reduction varied by areas. In maxillary primary first molars, the buccal occlusal surface required the most reduction, while the lingual surface required more reduction on the axial surface. In mandibular primary molars, the central area of the occlusal surface required the most reduction, with less reduction required in the lingual surface on the axial surface. In addition, the crown shape ratio between the tooth and the PZCs also influenced the required reduction, with more reduction required on the buccal and lingual surfaces of the maxilla than mandible due to discrepancies in crown shape ratio.
Overall, the required reduction was smaller than the manufacturer’s guidelines, particularly for the lingual surface of the mandible. This finding implies that generalized and approximate guidelines could lead to excessive reduction and risks such as pulp exposure. This study provides detailed area-specific and tooth-specific guidelines on the required amount of reduction for PZC restorations, contributing to more efficient preparation in clinical practice.
Notes
Conflicts of Interest
The authors have no potential conflicts of interest to disclose.