Evaluation of marginal accuracy of polyetheretherketone and zirconia copings fabricated using computer-aided design/computer-aided manufacturing technique: An in vitro study

Abstract

Aim:

The aim of this study was to compare the marginal accuracy of polyetheretherketone (PEEK) and zirconia copings fabricated using computer-aided design/computer-aided manufacturing (CAD/CAM) technology, and to assess the impact of their material properties on accuracy when produced with a 4-axis milling system under controlled conditions.

Settings and Design:

The study employed an in vitro design with a stainless steel die model featuring a 6 mm axial wall height, a 6-degree total occlusal convergence, and a radial shoulder finish line.

Materials and Methods:

Thirty stone dies were created from silicone impressions of the metal die and poured using type-IV dental stone. The dies were divided into two groups: Group-A (PEEK) copings and Group-B (zirconia) copings. All copings were fabricated using a CAD/CAM system. Vertical marginal accuracy was assessed with a stereomicroscope and image analysis software at ×20 magnification. The marginal gaP values were subjected to a student (independent) t-test for statistical analysis.

Statistical Analysis Used:

The statistical analysis involved a student (independent) t-test to compare the marginal gaP values between Group A (PEEK) and Group B (zirconia).

Results:

The mean marginal discrepancy for Group A (PEEK) and Group B (zirconia) was 92.84 μm ± 3.48 μm and 63.12 μm ± 31.47 μm, respectively. A statistically significant variation (t = 3.635, P = 0.001) between the groups was observed, indicating better marginal accuracy with zirconia copings compared to PEEK copings.

Conclusion:

Both PEEK and zirconia copings demonstrated vertical marginal discrepancies within the clinically acceptable limit of <120 μm. However, zirconia copings exhibited superior marginal accuracy in this in vitro study.

INTRODUCTION

Computer-aided design/computer-aided manufacturing (CAD/CAM) technologies have transformed the field of dentistry by offering a consistent and repeatable approach to fabricating dental restorations from a wide range of materials, including ceramic, zirconia, composite, and acrylic resins, and can replace traditional casting methods.[1,2,3,4] Among these materials, yttria-stabilized zirconia ceramic has gained popularity due to its remarkable strength and esthetic properties.[5] However, processing fully sintered zirconia poses a challenge due to its exceptional strength, which requires prolonged processing times and causes tool wear. At present, presintered zirconia is predominantly utilized, and the final sintering is conducted after fabrication.

As a consequence of this manufacturing approach, linear shrinkage of 20%–25% may occur, resulting in potential concerns regarding the fit of the zirconia prostheses after the completion of firing.[6,7] In contrast, the introduction of a new dental CAD/CAM material, polyetheretherketone (PEEK), has provided a solution to these issues. This material is chemically stable and highly durable in mechanical resistance conditions such as tension, fatigue, and bending.[8] PEEK materials, along with polyetherketoneketone (PEKK) materials,[9] belong to the polyaryletherketones (PAEKs group, which exhibit high biostability and contain both keto and ether groups. Compared to PEKK materials, PEEK materials have a higher ratio of ether groups. This results in stronger solidification of the glass and polymer chains, leading to a higher melting temperature and greater compressive strength.[8,9] In recent years, CAD/CAM technology has transformed dental prosthetics by enabling high-precision fabrication of restorations. However, variations in the equipment used for coping fabrication can significantly impact marginal accuracy, a critical factor in the long-term success of dental restorations. While previous studies have independently evaluated PEEK and zirconia copings, direct comparisons under standardized in vitro conditions remain limited. Our study distinguishes itself by employing a 4-axis CAD/CAM milling system, which offers enhanced multi-axial control during the milling process.[10,11,12,13] This precision system enables a more accurate reproduction of both marginal and internal surfaces of the copings, providing a level of detail that may not have been achieved in studies utilizing conventional milling systems. The use of this advanced technology allowed us to assess the comparative marginal accuracy of PEEK and zirconia in a way that more closely reflects clinical reality, and this methodological distinction may explain the variation in findings compared to earlier research. Specifically, the 4-axis system contributed to zirconia exhibiting superior marginal fit in our study, contrary to results seen in previous investigations where PEEK was often found to have better marginal accuracy. Poor fit can lead to various problems, including plaque deposition, periodontal destruction, and restoration failure. Previous studies used the silicon replica method, which has limitations in validity and accuracy.[10,11] The advancement of digital technology has given rise to a new technique for three-dimensional analysis, which enables the creation of highly precise and detailed representations of objects and surfaces.[14,15,16] This method involves gathering a vast amount of three-dimensional image data, which is then processed by connecting individual data points to form a triangular mesh.[15,17] This process results in a comprehensive and intricate three-dimensional image that can provide an in-depth understanding of the object or surface under analysis. This study aimed to compare the marginal and internal fit of PEEK and zirconia coping in different types of abutment teeth using this three-dimensional analysis. The null hypothesis was that there would be no significant difference in the fit of the coping based on the type of abutment tooth or CAD/CAM materials.

MATERIALS AND METHODS

The sample size for this study was determined based on a power analysis to ensure adequate statistical power to detect differences in marginal accuracy between PEEK and zirconia copings. Approval for the study protocol was obtained from the Institutional Ethics Committee (IEC) of Chhattisgarh Dental College and Hospital, under the reference number Min No. CDCH/ERC/2018/32. Cohen’s d, a measure of effect size, was calculated using the mean marginal discrepancies and standard deviations observed in both groups.[18] Using a significance level (α) of 0.05 and a desired statistical power of 80%, the degrees of freedom were estimated from the variances and sample sizes of the groups. The power analysis yielded a required sample size of approximately 9 participants per group to achieve 80% power. Considering the complexity of the statistical analysis and potential for data variability, we chose to include 15 participants in each group, exceeding the calculated sample size requirement. This sample size was deemed sufficient to detect the observed differences in marginal accuracy between PEEK and zirconia copings with a power of 80% at a significance level of 0.05.

Schematic Representation: [Figure 1].

Figure 1.

Schematic Representation of the Step-by-Step Procedure

Master die preparation

A stainless-steel die [Figure 2] was employed as the master die. The die was modified with a 1-mm-thick 360 degree rounded shoulder margin for ceramic crown preparation. Reduction of the occlusal and incisal surfaces by 1.5–2.0 mm was performed. The die was machined to have a 6 mm height of axial wall and 6 degrees of total occlusal convergence. Four small notches were created approximately 2 mm below the die margins for reference points. A metal special tray, leaving a 2-mm space between the tray and metal die, was utilized for impression making.

Figure 2.

Stainless steel master die for precision manufacturing

Impression taking and stone replica fabrication

Medium-viscosity impression material (Hongium AM-MONO, Dental Avenue India Pvt. Ltd.) was used to obtain impression bodies of the master dies. Surfactant was applied and dried on the internal surface of the impression bodies to reduce surface tension. The metal special tray was positioned over the metal die, secured with screws over vertical rods, and left for a setting time of 3 min and 30 s. Type IV gypsum (Ultra Rock, Kalabhai Karson Pvt Ltd Mumbai) was mixed and poured into the impression bodies. Fifteen impressions were made.

Making of specimens with type IV gypsum

Excess impression material was trimmed, and the impression was boxed with boxing wax. Type IV gypsum (CAM-Stone N, Siladent, Gloslar, Germany) was mixed in a water/powder ratio of 20 ml/100 g using a vacuum mixing machine. The mixture was poured into the impression and left to set for 40 min. Thirty dies were created in total.

Fabrication of polyetheretherketone and zirconia copings

The 30 dies were divided into two groups, Group A and Group B, and scanned using a 3D DS-X scanner (SHINING 3D Technology GmbH, Breitwiesenstra, Stuttgart, Germany). Frameworks were designed using SHINING 3D Software. Fifteen copings [Figure 3] were fabricated from PEEK disk in a 4-axis milling unit. A 4-axis milling unit enhances machining capabilities by introducing an additional rotational axis, known as the A-axis, which allows the workpiece to rotate around the X-axis. This configuration enables the machine to perform complex operations that would be challenging or impossible with a standard 3-axis machine. Typically, 4-axis machines are designed for vertical machining, where the spindle rotates around the Z-axis, and the workpiece can be positioned along the X-axis while being rotated. This setup allows for machining on multiple sides of a part without the need for repositioning, significantly improving efficiency and precision in producing intricate geometries and complex profiles. Similarly, 15 copings were milled from a CAD/CAM zirconia block [Figure 4] and sintered in a ZENOTEC FIRE M2 sintering furnace.

Figure 3.

Sample group of polyetheretherketone copings

Figure 4.

Sample group for zirconia copings

Evaluation of copings

All 30 copings (15 PEEK and 15 zirconia) were examined for nodules or marginal irregularities using a magnifying glass. Absolute marginal discrepancy was assessed with respect to the die using a stereomicroscope [Figures 5 and 6] and image analysis software at ×20 magnification. A total of 120 readings were taken on 30 copings by capturing four digital images of the margins at four predetermined measuring locations on the die along the cervical region of each specimen. The specimens were securely placed on a holder during measurements.

Figure 5.

Measurement of marginal accuracy using stereomicroscope

Figure 6.

Employing Image analysis software for scoring marginal discrepancy

Statistical analysis

The marginal discrepancies in this study were evaluated using Geomagic Verify (Version 5.2, Geomagic GmbH, Stuttgart, Germany) a three-dimensional inspection software designed for precise measurement and comparison of CAD data. The software’s advanced image analysis capabilities allowed for detailed assessments of the marginal accuracy of the copings. In addition, a G*Power (Version 3.1, Heinrich-Heine-University, Düsseldorf, Germany) power analysis was performed to calculate the required sample size based on expected effect size and study parameters, ensuring an 80% statistical power with a significance level (α) of 0.05. Statistical analysis of the marginal discrepancies between PEEK and zirconia copings was performed using a Student’s t-test with SPSS for Windows (Version 26.0, IBM Corp., Armonk, NY, USA) to evaluate the significance of the results at a significance level of 0.05.

To assess the marginal discrepancies between the PEEK and zirconia copings, a Student’s-t-test was selected as the appropriate statistical method. This decision was based on several factors. First, the marginal discrepancy data were assumed to follow a normal distribution, an essential requirement for parametric tests like the t-test. The assumption of normality was confirmed through the Shapiro–Wilk test and visual inspection of histograms, showing that the data adhered to a normal distribution.

Moreover, the t-test is designed for comparing the means of two independent groups—in this case, the PEEK and zirconia copings—which involve continuous, quantitative measurements. Homogeneity of variances, another assumption of the t-test, was tested using Levene’s test, and the variances between the two groups were found to be sufficiently equal. Although the sample size (n = 15 per group) was relatively modest, the t-test is robust for smaller sample sizes when the assumptions of normality and homogeneity are met. A power analysis, performed using G*Power 3.1, confirmed that the sample size was adequate to provide a statistical power of 80%, ensuring that the study could detect significant differences between the two groups. The Student’s t-test was ultimately selected due to its simplicity, wide acceptance, and ability to provide clear, interpretable results, making it well-suited for comparing group means in the context of this study. Statistical significance was set at a threshold of α = 0.05.

RESULTS

The mean and standard deviation for Group A (PEEK) are 92.84 μm and 48 μm, respectively. In Group B (zirconia), the mean values for the lowest and highest marginal discrepancies are 18.93 μm and 117.35 μm, respectively. The mean and standard deviation are 63.12 μm ± 31.47 μm [Table 1]. Values for marginal discrepancy of PEEK and zirconia copings have been compared using student (Independent) t-test. There was a statistically significant variation between the groups (t = 3.635, P = 0.001).

Table 1.

Comparison of mean marginal discrepancy of polyetheretherketone and zirconia copings using student (independent) t-test

Group	n	Mean	SD	SEM	Mean difference	t	P
PEEK	15	92.840	3.48109	0.89881	29.72	3.635	0.001*
Zirconia	15	63.120	31.47761	8.12748

*The mean difference is significant at the 0.05 level. SD: Standard deviation, SEM: Standard error of mean, PEEK: Polyetheretherketone

Comparison of mean marginal discrepancy of the entire tested specimen for Group A (PEEK) and Group B (Zirconia) is represented in Graph 1, where X-axis represent group (A and B) and Y-axis represent mean marginal discrepancy in μm.

Graph 1.

Bar graph representing mean marginal discrepancy of Group: A (polyetheretherketone) and Group: B (Zirconia)

DISCUSSION

Precise seating and fitting of prostheses are crucial for successful restorations. Both marginal fit and internal fit rely on clinical and laboratory steps. Improper marginal fit can lead to increased plaque accumulation, gingival sulcular fluid flow, and bone loss, resulting in micro-leakage, recurrent caries, periodontal disease, and a decrease in the longevity of prosthetic restorations.[19,20,21] To address these challenges, CAD/CAM technologies have been developed as alternatives to conventional casting methods.[22] CAD/CAM systems offer automation of the fabrication process, which improves the quality, accuracy, and efficiency of the restoration. By eliminating processing errors, CAD/CAM systems contribute to better postoperative clinical treatment and delivery.[22,23]

In this study, the mean marginal discrepancy for Group A (PEEK) was reported as 92.84 μm ± 3.48 μm, whereas Group B (zirconia) had a mean marginal discrepancy of 63.12 μm ± 31.47 μm. The marginal accuracy of Group A (PEEK) and Group B (zirconia) showed statistically significant variation (t = 3.635, P = 0.001) between the two groups. This study used a 4-axis milling unit, which represents a significant advancement over 3-axis machines, particularly in terms of efficiency and versatility. By allowing for simultaneous rotation and linear movement, 4-axis machines can achieve higher precision and better surface finishes on complex parts. They reduce the need for multiple setups, which minimizes the risk of human error and improves consistency across machined features. In addition, the ability to machine multiple sides of a workpiece in a single setup not only saves time but also enhances the overall quality of the finished product. While 5-axis milling units may offer even greater capabilities, the 4-axis machine strikes an optimal balance between complexity, cost-effectiveness, and performance, making it an excellent choice for a wide range of applications, especially in the production of multiunit restorations where accuracy is paramount.[24,25] Michael Emad conducted a study on the vertical marginal gap distance of veneered PEEK and zirconia coping and found that PEEK material can be used intraorally for single crown restoration.[26] Similarly, Somayeh Zeighami compared the marginal adaptation of different implant-supported frameworks before and after cementation and found that zirconia frameworks exhibited clinically acceptable marginal adaptations, while PEEK frameworks were considered borderline. These findings support the better marginal fit of zirconia, as observed in the present study.[27]

Contrary to previous studies by Bae et al. and Hossam M, which reported better marginal fit of coping fabricated with PEEK compared to Zirconia, our study demonstrated that zirconia copings had less marginal discrepancy than PEEK copings.[12,13] This difference in findings could be attributed to various factors, such as the use of different scanners, the size of milling bur, and the number of milling axes. The higher marginal gaP values observed in PEEK copings compared to zirconia copings in this study can be explained by the semi-crystalline structure of PEEK material, which contains a smaller amount of ceramic filler embedded in the resin matrix. This can result in more shrinkage and consequently a higher marginal gap during fabrication compared to the polycrystalline structure of zirconia. In addition, potential stress concentration in curvature regions, like the finish line during the milling process, may contribute to marginal fit defects. Although zirconia copings demonstrated better marginal fit in this study, it is important to note that all marginal gap values fell within the acceptable range of 40–120 μm. Hence, both PEEK and zirconia materials can be considered suitable for intraoral use in single crown restorations based on their acceptable marginal measurements.

It is essential to recognize the constraints of our study. While we used standardized stainless steel master dies for measurements, they may not perfectly simulate clinical conditions. Human teeth would provide a more accurate representation. In addition, our three-dimensional evaluation method, while minimizing distance errors, might introduce inaccuracies during merging. Furthermore, our study environment differed from intraoral conditions, using acrylic resin dies with properties distinct from natural teeth. Furthermore, our design only allowed for measuring marginal gaps, leaving internal gaps unexplored due to cementation requirements. We acknowledge the absence of an aging simulation, which is crucial for mimicking oral conditions. Future research should expand to include both marginal and internal fits, conduct trials in broader contexts like bridges, and explore the impact of aging on margin distortion.

In the medical field’s quest for biocompatible bone replacements, plastics like PAEKs, particularly PEEK, have emerged as promising alternatives to metals. These materials offer robustness against various stresses and have found applications in dentistry, including as abutments and frameworks for dental prostheses. The integration of CAD/CAM systems has revolutionized productivity, design quality, and communication in dentistry. By eliminating traditional procedures, CAD/CAM reduces errors and enables early detection and correction of design flaws, ultimately leading to precisely fitting restorations and improved treatment outcomes.

This study not only showcases the current applications of PAEK materials in dentistry but also anticipates their growing significance. With ongoing advancements in material science and manufacturing technologies, PAEK-based materials are poised to play a vital role in dental prosthetics. The integration of CAD/CAM systems with PAEK materials promises even greater precision, efficiency, and patient-specific customization in restorations. Moreover, exploring the long-term performance and biocompatibility of these materials holds immense promise for enhancing clinical outcomes and patient satisfaction in the future.

CONCLUSION

This study provides evidence that zirconia copings offer superior marginal fit compared to PEEK copings. The use of CAD/CAM technology in the fabrication process contributes to the improved accuracy and quality of restorations. However, further research is needed to explore the factors affecting marginal fit and to optimize the manufacturing process for both materials.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.