Effect of Tooth Position on Molar Zirconia Crown Restoration with a Modified Dental Measurement Software

Summary

This study investigates a modified digital technique for the individualized zirconia crown restoration of posterior teeth.

Abstract

This study employed a modified digital technique to evaluate parameters of molar preparations for monolithic zirconia crown restoration, investigating whether clinical preparations with different tooth positions influence restorative outcomes. A total of 238 prepared posterior abutments were analyzed using an intraoral scanner, with parameters including total occlusal convergence (TOC) angle, margin perimeter, and mean abutment height assessed for statistical analysis. The results revealed that the average TOC angle of each posterior tooth exceeded 6°, with the maximum average observed in the mandibular left second molar (35.96 ± 20.21°) and the minimum in the maxillary right first premolar (10.97 ± 6.84°). Statistically significant differences were found in the TOC angle of homonymous second premolars (p < 0.05), where the mandibular left second molar was significantly larger than the other homonymous teeth. Additionally, significant differences existed between teeth in the same quadrant with different positions (p < 0.05), showing a linear increase in TOC angle as the tooth position moved backward. A positive correlation was observed between TOC angle and margin perimeter, while a negative correlation existed between TOC angle and mean abutment height. The findings indicate that clinical zirconia crown preparations often deviate from theoretical TOC recommendations, particularly requiring tailored criteria for mandibular teeth. The developed software integrates digital acquisition with clinical analysis, demonstrating its relevance in prosthodontic practice and education.

Introduction

High-quality zirconia-crown preparation is crucial for the long-term success of dental restoration^1,2,3. It has been observed that the total occlusal convergence (TOC) angle, diameter, and abutment height are correlated^1,4,5. Several in vitro studies have indicated that these factors substantially influence the fit, retention, resistance, and longevity of restoration^5,6,7. The TOC angle in crown preparation is defined as the angle formed by the convergence of two opposing axial walls in the same plane. Inadequate tooth preparation may result in both mechanical and biological complications. Mechanical failures can manifest as restoration loosening, debonding, or fracture, as well as tooth structure fracture. Biological complications may include periodontal inflammation and mucosal soft tissue infections⁸. The TOC angle is often influenced by manual operation, unlike abutment height and diameter, which are determined by anatomical variables⁹. Due to its variations, the TOC angle is essential for determining the retention and resistance quality of the preparation. During tooth preparation, the angulation and taper of the bur determine the tooth preparation angle at each location on the tooth¹⁰.

Pioneers like Ward were the first to support the TOC angle measurement for preparations, proposing a convergence angle between 3° and 12°¹¹. Subsequent in vitro studies by Jorgensen¹² and Kaufman¹³ revealed that retention force decreases with increased convergence angle, indicating a higher TOC beyond 5°. Furthermore, Ohm and Silness preliminarily measured the TOC angle on clinically prepared teeth and revealed significantly larger values than the recommended range¹⁴. A systematic review (1978-2013) showed that the ideal TOC angle of 2°-5° was practically unachievable and suggested that a realistic TOC angle of 10-22°¹⁵. Moreover, it has been suggested that qualified dentists typically achieve a TOC angle between 15° and 25°^{16,17,18,19,20,21,22,23}. Shillingburg HT proposed specific convergence angles for different tooth positions, ranging from 10 to 24°²⁴. Nordlander et al. examined data from 10 dentists comprising 208 cases and proposed a minimum angle of 17.3° in the anterior region and a maximum of 27.3° in the posterior region²⁵. The literature also suggests that preparation's axial surfaces should be parallel to each other, or with a convergence angle of <6° ²⁶. However, teeth are complex and unique, and those with different positions should be treated with a clinically recommended value tailored to their individual needs. The statistical analysis by Janine Tiu on >100 stone dies prepared for glass-ceramic crown restorations showed that the greatest mean TOC angle for the maxillary left second molar was 74.49° (n = 4)²⁷. However, the low strength of glass-ceramic crown materials limited their application in the molar region²⁸. Therefore, it is crucial to comprehensively analyze statistics on the posterior restoration based on zirconia crowns.

Recent advancements in ceramic materials and digital dentistry have made monolithic zirconia ceramic crowns a favored option for posterior fixed restorations using intraoral optical scanning (IOS) systems for tooth defect restoration, particularly because of their high strength, biocompatibility, and aesthetic qualities²⁹. Conventional digital techniques capture only limited geometric parameters and, when combined with traditional 3D scanning methods that are unable to directly assess internal preparation features, demonstrate significant limitations³⁰. This study introduces an individualized modified digital technique to evaluate zirconia crown restorations for posterior teeth, offering a clinically applicable method to optimize fit and longevity. The proposed technique can be specifically employed for customized crown adaptation, such as for teeth with reduced abutment height, uneven marginal configurations, or non-ideal taper. This method systematically analyzes TOC angle variations across different posterior tooth positions, helping clinicians achieve optimal preparation guidelines and reducing the risk of mechanical failure or cementation issues. Furthermore, comparing TOC angles with recommended values offers practical insight for dental practitioners during tooth preparation, ensuring better clinical outcomes. Moreover, the correlation analysis between TOC angle, margin line length, and average abutment height provides valuable insights for restorative planning. Clinicians can use these findings to adjust preparation techniques or select alternative restorative solutions in cases of short clinical crowns or excessive taper. This technique's digital workflow enhances accuracy and reduces chairside time. This approach supports more predictable and durable zirconia crown restorations in posterior dentition by bridging the gap between digital design and real-world restorative challenges.

Protocol

All experiments were conducted in accordance with a protocol approved by the Institutional Review Board (IRB) of Beijing Shijitan Hospital, Capital Medical University. The ethical approval reference number was IIT2023-021-001.

1. Experiment preparation

1. Software preparation
   1. Employ the IOS, 3D inspection software, and dental measurement software.
2. Personnel preparation
   1. Perform measurement procedures with three board-certified prosthodontists (mean clinical experience: 12.4 ± 2.1 years).
   2. Conduct sample preparation with a team of nineteen qualified prosthodontists, each possessing a minimum of 5 years of specialized clinical experience. Ensure all operators receive standardized training prior to study initiation to maintain procedural consistency.

2. Data acquisition

1. Inclusion and exclusion criteria
   1. Apply the following inclusion criteria: (1) patients aged 18 years or older, (2) patients able to provide informed consent, and (3) patients requiring a single-unit crown on a natural tooth.
   2. Apply the following exclusion criteria: (1) individuals requiring retainers for fixed bridges, (2) individuals needing multiple single-unit crowns in a single appointment, and (3) individuals with severely tilted teeth.
2. Data source
   1. Obtain 238 Polygon File Format (PLY) comprehensive datasets on posterior tooth preparations for crowns made of monolithic zirconia ceramic from the Department of Stomatology. Ensure all tooth preparations are derived from clinically indicated cases requiring full-coverage restorations for posterior teeth with structural defects (Figure 1).
   2. Include additional digital models from 19 dentists who voluntarily contributed to the study. Calibrate the scanner (Supplementary Figure 1) according to the manufacturer's protocol. Capture all preparations using the following standardized scanning protocol: (1) maintain optimal illumination conditions, (2) ensure proper moisture control, and (3) acquire partial arch scans with adjacent teeth.

3. Data preprocessing

1. Initial data screening
   1. Process all 238 PLY datasets using 3D inspection software. Verify mesh integrity by checking for non-manifold edges and holes.
   2. Confirm that scan resolution achieves a minimum of 20 µm point spacing. Assess surface texture quality against reference standards.
2. Preprocessing phase
   1. Import the scan data into the 3D inspection software and set the unit specification to millimeters. Use the Mesh Doctor command to analyze and repair the polygonal mesh, then enter the editing interface.
   2. From the menu tools, select the Lasso Tool and enable the "Through" function to isolate the preparation model from surrounding teeth. Execute the Mesh Doctor algorithm to automatically fill holes, smooth surfaces, and optimize mesh edges (Supplementary Figure 2).
3. Output generation
   1. Export all final datasets in dual formats: high-resolution PLY files (retaining original color data) and production-ready Stereolithography (STL) files (binary format, 0.01 mm chord height tolerance).

4. Measurement procedure

1. TOC angle measurement
   1. Operate the measurement software and select Import Preparation Model to display the tooth preparation on the screen. Choose the Insertion Path option to project a guiding line indicating the crown insertion direction onto the preparation surface.
   2. Maintain the insertion path perpendicular to the occlusal plane and rotate the preparation model. Select measurement points at both the preparation margin and 2 mm above it in the mesial, middle, and distal thirds along the mesiodistal (MD) axis^16,31.
   3. Execute the TOC Angle Measurement command to display the angular data beside the screen. Repeat the procedure to measure the TOC angles at the buccal, middle, and lingual thirds along the buccolingual (BL) axis^12,32.
2. Margin perimeter analysis
   1. Execute the Extract Margin Line command and select a starting point on the peripheral edge of the preparation finish line. Trace around the entire circumference of the preparation until returning to the starting point.
   2. Allow the system to automatically delineate the hard-soft tissue boundary, generating a margin perimeter along the cervical outer edge of the preparation. Click on Split to separate the portion of the preparation within the margin line³³.
3. Abutment height measurement
   1. Activate the Height Initialization function and select measurement points at the mesial, distal, buccal, and lingual positions of the preparation's occlusal surface, ensuring all points align with the insertion path axis.
   2. Execute the Calculate command to determine the mean height of the tooth preparation³¹. Figure 2 displays the workflow diagram encompassing procedural Steps 3 to 4.

5. Quality control

1. Evaluate measurement reproducibility by instructing three calibrated dentists to independently apply the method to a single tooth, with each operator performing 15 repeated measurements prior to the main study.
2. Confirm inter-operator reproducibility by verifying that standard deviation (SD) values meet predetermined acceptance thresholds: ≤0.39° for TOC angle variation within a single plane, ≤0.14 mm for margin perimeter consistency, and ≤0.11 mm for abutment height measurement precision.
3. Pool the results across all three operators and calculate mean SDs, which should approximate 0.35° (TOC angle), 0.10 mm (margin perimeter), and 0.08 mm (abutment height). Ensure that individual operator SDs remain below the maximum allowable thresholds (Table 1).
4. Conclude that the evaluation method provides clinically acceptable reproducibility when performed by trained operators.

6. Statistical analysis

1. Categorize the data based on tooth positions into 16 groups (Table 2 and Table 3). Subject the datasets to TOC angle, margin perimeter, and mean abutment height analyses.
2. Present normally distributed data as mean and SD (X ± S), and express non-normal data as median and quartiles [M, (P25~P75)]. Compare differences in TOC angles between teeth in identical positions (14-24-34-44, 15-25-35-45, 16-26-36-46, and 17-27-37-47) using analysis of variance (ANOVA), and perform post-hoc LSD-t tests for pairwise comparisons.
3. Carry out pairwise t-tests to analyze TOC angle variations between teeth in the same quadrant but in different positions (14-15-16-17, 24-25-26-27, 34-35-36-37, and 44-45-46-47) in both BL and MD directions.
4. Perform a linear trend test to assess whether the TOC angle increases with posterior tooth positions in the same quadrant.
5. Conduct Pearson's correlation analysis to determine the relationship between TOC angle, margin perimeter, and mean abutment height. Perform all analyses using SPSS 26.0 software, and set the significance level at α = 0.05.

Results

General characteristics
The number of maxillary specimens (n = 132) was greater than that of mandibular specimens (n = 106), with the maxillary right first molar being the most frequently prepared tooth (n = 24). The angles exhibiting a negative value were deemed invalid and omitted from statistical analysis. Table 2 delineates the quantity and classification of invalid TOC angle specimens. Table 3 presents the mean TOC angle for each posterior tooth. Furthermore, clinical TOC angles are compared wit...

Discussion

This study aimed to evaluate the total occlusal convergence (TOC) angle of monolithic zirconia crowns using digital technology and analyze its relationship with tooth position, an area still lacking in research. Additionally, the application of digital techniques in the preparation and assessment of these crowns remains underexplored. Clinical samples were collected based on tooth location, and for each posterior tooth, the TOC angle, margin perimeter, and mean abutment height were measured. The findings revealed that mo...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
DentalEngineerV1.0	SuZhou, China	Dental measurement software
3Shape TRIOS3	3Shape, Danmark	Intraoral scanner
Geomagic Studio12.0	3D Systems, USA	Intraoral scanning data processing