Roughness analysis on porcelain sectional surface of porcelain fused to Co-Cr alloy endocrowns

Abstract

This study aims to compare the roughness between the central and edge points on the porcelain sectional surface of porcelain fused to Co-Cr alloy endocrowns. Utilizing anatomical data from average molar dimensions, a simplified model for the endocrowns was created. Eight porcelain fused to Co-Cr alloy endocrowns were fabricated with an edge thickness of 0.3 mm. Following casting, firing, cutting, and polishing procedures, the roughness on porcelain sectional surface at both the central and edge points of the inner crown was assessed using an atomic force microscope (AFM). The roughness measurement (Sq value) for the central point on porcelain sectional surface was (10.46 ± 3.37 nm), and for the edge point, it was (10.50 ± 1.99 nm). There was no statistically significant distinction between the central and edge points in terms of roughness. Despite the uneven thickness of the inner crown in porcelain fused to Co-Cr alloy endocrowns, it was observed that this disparity had negligible impact on the internal microstructure of the porcelain. Therefore, its application in dental clinical settings could be deemed viable.

Introduction

Patients presenting with limited occlusal gingival distance in posterior residual crowns are a common occurrence in clinical practice. These individuals often exhibit extensive dental defects, increased dental wear, inadequate space for restoration, and compromised prosthesis retention. While periodontal crown lengthening has been employed to address these issues, it is noteworthy that a substantial number of clinic patients are elderly and afflicted with systemic ailments. As a consequence, this approach assumes a surgical procedure of heightened risk. Furthermore, the recovery period following periodontal crown lengthening is prolonged, occasionally lacking full stabilization even after a six-month interval ¹. For multi-rooted teeth, embracing such lengthening entails more bone removal, impacting the root bifurcation zone and engendering root bifurcation lesions². Additionally, periodontal crown lengthening perturbs the crown-root ratio, thereby diminishing occlusal functionality³.

Historically, post-core crowns were conventionally employed to address such dental defects. Nevertheless, in the backdrop of contemporary advances in dentistry and prosthodontics, there is a growing consensus among experts and scholars that the necessity for posts is gradually diminishing^4,5. Mechanistically speaking, posts were originally tasked with force transmission, augmenting core retention and fortifying residual tooth strength. Cores, in turn, were responsible for rectifying dental defects and contributing to crown retention. The retentive capacity of crowns was predicated upon cores and the residual tooth structure.

Facilitated by advancements in bonding technology, prosthetic retention has been markedly enhanced, thereby substantially reducing the imperative for precise retention shape in clinical restorations. The need for substantial tooth tissue removal to attain optimal retention contours has been mitigated, allowing a more discerning focus on enhancing resistance. Consequently, the possibility of achieving both resistance and retention has been actualized through the utilization of the pulp chamber and residual tooth structure, culminating in the innovation of endocrowns.

In recent years, the field of endocrowns has undergone rapid evolution, particularly with the advent of CAD/CAM chairside manufacturing systems for all-ceramic restorations^6,7. Notably, these systems offer the advantages of reduced clinical time and enhanced aesthetics; however, their cost remains substantial. Another approach involves the creation of metal-porcelain restorations using conventional embedding and casting techniques. This approach, to a certain extent, balances considerations of cost and aesthetics. However, when considering the application of endocrowns made from metal, it becomes evident that the inner crown’s shape tends to be irregular. The retention core within the chamber is relatively broad, while the edge points are comparatively thin. Consequently, the traditional casting process yields inconsistent shrinkage rates across the entire inner crown, thereby potentially giving rise to flawed microstructures like micropores and cracks during porcelain application^8,9. This may subsequently lead to issues such as porcelain delamination and fractures (or complete porcelain breakage) following restoration. Such micropores, cracks, and other flawed microstructures within the porcelain can be identified as uneven surfaces via atomic force microscopy (AFM) post-cutting^10,11. Hence, the primary objective of this study was to utilize AFM in comparing the roughness at both the central and edge points on porcelain sectional surface of porcelain fused to a Co-Cr alloy endocrowns. This investigation aimed to shed light on the microstructure of the internal porcelain composition of said endocrowns, thereby offering valuable insights for the clinical implementation of porcelain fused to Co-Cr alloy endocrowns.

Materials and methods

Utilizing average anatomical measurements of molars, a simplified stainless-steel model for the endocrowns was meticulously fabricated (Fig. 1). This simplified model comprised two essential components: the upper cover (A) and the base (B). Through the amalgamation (Arrow direction) of A and B, a representative specimen of the inner crown (C), possessing an edge thickness of 0.3 mm and a chamber retention core (D), was successfully produced. Subsequently, a layer of porcelain was meticulously applied to the surface of core D.

Figure 1.

Simplified study model for endocrowns.

Molten wax was introduced into the chamber of the base after being liquefied within the wax melting apparatus. Subsequently, a dental red wax piece measuring 0.3 mm in thickness was carefully positioned onto the plane of the base (B). Excess wax beyond the model’s edge was subsequently trimmed off. This meticulous procedure yielded a total of eight wax specimens, each representing the inner crown with an edge thickness of 0.3 mm.

The wax specimens were meticulously examined to ensure their absence of deformation and defects. These wax specimens were subsequently embedded in phosphate and cast using Co-Cr alloy (Bego, Bremen, Germany) at 1485 °C. Routine treatment procedures were performed on the cast specimens, encompassing integrity assessment and blasting with 100 μm aluminum oxide particles at a distance of approximately 2.0 cm, exerting a pressure of 0.5 MPa for 20 s. After undergoing this process, all casting specimens were subjected to thorough washing with distilled water, followed by drying. They were then stored without any handling until the porcelain application phase.

Preoxidation heat treatment (1000 °C) was applied to casting specimens before porcelain bonding. Porcelain was applied to casting specimens in the form of a creamy paste resulting from the matching liquid mixed with porcelain powder. The surface of the cast specimens was initially coated with a layer of opaque porcelain and subsequently fired, then coated a second layer. After firing, dentin porcelain powder (VITA Zahnfabrik, Bad Sackingen, Germany) was applied, also two layers in total. Post-firing dentin porcelain, all porcelain specimens were ground using a grinder, with careful attention to controlling the thickness of the porcelain layer under the help of a micrometer (accuracy 0.01 mm). The overall thickness of the porcelain layer was regulated to 2.0 mm, as illustrated at point E in Fig. 2. The permissible error was maintained within ± 0.05 mm. The final step encompassed glazing all porcelain specimens. The firing procedures of porcelain were shown in Table 1. The porcelain application adhered to the manufacturer’s guidelines and was carried out by an experienced dental technician.

Figure 2.

Analysis sites on porcelain sectional surface.

Table 1.

The firing cycle of VITA porcelain.

Program	Pre-Heating Temp. (◦C)	Drying Time (min)	Heating Time (min)	Final Temp. (°C)	Holding Time (min)
Opaque 1	600	4	4	950	1
Opaque 2	600	4	4	930	1
Dentin 1	600	6	6	930	1
Dentin 2	600	6	6	920	1
Glaze	600	4	4	930	1

All porcelain specimens were fully embedded in epoxy resin and subsequently sectioned along the mesial-distal axis using a slow-cutting machine (Kejing, Shenyang, P.R. China). This procedure exposed the metal-porcelain interface. The interface was polished using a metallographic polisher (Yonghui, Shanghai, P.R. China) with #280–2500 water-resistant sandpaper. Subsequent in order to obtain a mirror-like appearance for detection, polishing was performed using soft flannelette and specialized polishing paste. The final step involved cleansing with acetone and ultrasonic distilled water to remove contaminants.

The roughness (Sq) on porcelain sectional surface, corresponding to both the central and edge points of the inner crown, was quantified using an Atomic Force Microscope (AFM) (MI, California, USA). The analysis sites for this study were identified in the central point (F) and edge point (G) of the porcelain sectional surface, with a longitudinal distance of 1.0 mm, the edge measurement point was located 1.0 mm inside the outer wall of the porcelain, as depicted in Fig. 2.

Statistical analysis of the roughness values for the central and edge points on porcelain sectional surface was executed using SPSS 22 statistical analysis software. The data from the two experimental groups were tested and verified to meet the criteria of normality and homogeneity of variance (F = 0.438, P = 0.519). The normally distributed data were expressed as mean ± standard deviation (M ± SD). The two data groups underwent a t-test for two independent samples, and statistical significance was defined by P-values less than 0.05.

Results

Roughness values (Sq) at the central and edge points of eight specimens were shown in Table 2.

Table 2.

Roughness values at the central and edge points of eight specimens (nm).

Specimens No	Central point	Edge point
1	6.39	10.5
2	9.38	6.78
3	18	9.72
4	10.3	9.55
5	9.43	11.8
6	8.88	10.1
7	11.4	12.7
8	9.91	12.9

The roughness value at the central point on porcelain sectional surface measured (10.46 ± 3.37 nm), while the value at the edge point was (10.50 ± 1.99 nm). Notably, no significant distinction existed between the central and edge points in terms of roughness.

Atomic Force Microscope (AFM) images captured the central point on porcelain sectional surface (Fig. 3), illustrating a relatively even surface appearance. Wheel-like rolling traces were discernible, and occasional small dark pores were observed. Conversely, the edge points (Fig. 4) exhibited somewhat lesser surface integrity compared to the central point. Similar wheel-like rolling traces were evident, accompanied by a slightly higher incidence of small dark pores.

Figure 3.

Central point.

Figure 4.

Edge point.

Discussion

Endocrowns represents an innovative prosthesis utilizing the dental pulp chamber as the basis for restoration and retention following root canal therapy. It involves creating a circular butt-joint interface and a core retainer within the pulp chamber¹². The restoration and retention mechanisms of the endocrowns are grounded in the substantial retention shape within the pulp chamber, coupled with robust bond strength. This design amplifies the contact friction area between the prosthesis and the tooth, forging a complex prosthesis-adhesive-dentin bond through clinical bonding. This complex enhances the crown’s resistance against horizontal movement and occlusal dislocation. Remarkably, the endocrowns forgoes the conventional post-core system within the root canal, refraining from tissue preparation therein. This strategy augments tooth resistance. The endocrowns presents numerous advantages, including maximal preservation of residual tooth tissue and adherence to prosthetic, biological, and biomechanical principles^13,14. Additionally, it effectively restores the physiological and anatomical tooth contours, catering to aesthetic demands¹⁵. Recent years have seen scholars further integrating the endocrowns into the restoration of teeth with limited gingival distance, encompassing cases of severe wear and residual crowns with restricted prosthetic space.

Presently, there exist three primary categories of restorative materials for endocrowns: all-ceramic, resin, and metal-porcelain. All-ceramic restorations offer numerous merits, yet they are often accompanied by a substantial price tag. Resin restorations, distinguished by their straightforward processing technology and commendable aesthetic outcomes, are susceptible to wear over time. Meanwhile, metal-porcelain restorations encompass precious and non-precious metal variants. Precious metal endocrowns can prove financially prohibitive, prompting the exploration of non-precious metal alternatives such as cobalt chromium alloy. The latter, acknowledged for its affordable cost, favorable biocompatibility, and aesthetic appeal, continues to be a prevalent choice in clinical settings^16,17.

Metal-porcelain restoration imposes specific prerequisites, encompassing both a requisite inner crown thickness and a uniform distribution of this thickness. Typically, non-precious metal-porcelain restorations mandate an inner crown thickness ranging between 0.2 mm to 0.3 mm⁹. Homogeneity in inner crown thickness engenders minimal deformation during the firing process, thereby ensuring stability within the porcelain’s internal microstructure and mitigating the risk of failure. Conversely, uneven inner crown thickness contributes to disparate cooling durations post-firing, inducing incongruent shrinkage rates across different sections. This irregularity perturbs the porcelain’s internal microstructure, predisposing it to the formation of micro pores and cracks. Such microstructural imperfections, while initially inconspicuous, interact with saliva and the cyclic forces of oral function to generate protracted static fatigue stress. Over time, this phenomenon precipitates porcelain failure—a pivotal contributor to the malfunction of metal-porcelain restorations (34%). This failure mode encompasses porcelain detachment and fractures¹⁸.

Surface roughness pertains to the minute variations in surface topography, encompassing the close proximity and irregularity of minor peaks and valleys, where the spatial separation between two adjacent peaks or valleys is diminutive, typically measuring less than 1 mm. Research underscores that post-polishing surface roughness disparities may stem from porcelain powder particle size, albeit a more pronounced association exists with the abundance of micro pores and cracks inherent in the porcelain structure. Alas, polishing fails to entirely eradicate these microstructural imperfections^19–21. Notably, this study employed exclusively Vita VMK 95 porcelain powder for firing, thereby negating the first potential causal factor. Consequently, a uniform foundation for the second proposition’s viability was established, rendering the study’s comparability robust.

This investigation discerned no statistically significant disparity between the roughness values recorded for the central and edge points on porcelain sectional surface within the porcelain fused to Co-Cr alloy endocrowns. Several factors potentially account for these results: Firstly, the elevated strength, hardness, and melting point of Co-Cr alloy in comparison to precious metals bestows resilience to deformation during the relatively lower-temperature porcelain application phase, following the higher-temperature casting phase. Secondly, the endocrowns’s abbreviated periphery confines its coverage to the occlusal defect, distinguishing it from comprehensive crown restorations that extend over broader axial walls. This unique configuration of a slender, delicate periphery minimizes the likelihood of firing-induced deformation, thereby preserving the integrity of the porcelain’s internal microstructure. Thirdly, the involvement of a seasoned professional technician in the porcelain application process constitutes a pivotal consideration. Meticulous execution of technical procedures such as precise porcelain layer deposition, optimal water absorption, and vigilant avoidance of repeated firing constitute crucial measures in mitigating the emergence of micro pores and cracks within the internal porcelain structure.

In a prior investigation by the same research group, shear strength testing was employed in conjunction with scanning electron microscopy and energy dispersive spectrometry to scrutinize the impact of inner crown thickness on the bond strength of porcelain fused to Co-Cr alloy endocrowns. The outcomes of this research segment revealed that porcelain bond strength within Co-Cr alloy endocrowns, possessing thicknesses of 0.3 mm, 0.5 mm, 0.8 mm, and 1.0 mm, all satisfactorily met the clinical requisites²². In the present study, the research scope concentrated on inner crowns characterized by an edge thickness of 0.3 mm. Notably, a substantial variance in thickness exists between the chamber retention core and the aforementioned edge. Intriguingly, the microstructural development of internal porcelain after firing exhibited no discernible disparity between these two components. Thus, this investigation provides a degree of corroboration for the assertion that porcelain within Co-Cr alloy endocrowns, featuring an edge thickness of 0.3 mm, fulfils the clinical criterion for metal-porcelain bond strength.

While the findings of this investigation indicate the absence of a significant disparity in the roughness between the central and edge points on porcelain sectional surface within Co-Cr alloy endocrowns, it is cautioned that relying solely on roughness as the exclusive metric for assessing the internal porcelain microstructure may not be judicious. Consequently, the results presented in this study serve as a form of observational reference, offering insight into the internal microstructure of Co-Cr alloy endocrowns porcelain. However, these findings do not conclusively establish its holistic suitability for clinical application, particularly within the context of porcelain components.

In consideration of the limitations inherent in this study, the ensuing conclusions have been drawn: notwithstanding the uneven thickness of the inner crown within porcelain fused to Co-Cr alloy endocrowns, the discernible impact on the internal porcelain microstructure remains minimal. As such, the adoption of such endocrowns can be contingent on the specific clinical context, thereby accommodating variations in individual circumstances.

Author contributions

Independent author, responsible for all.

Data availability

The datasets used and analysed during the current study are available from the corresponding author on reasonable request.

Competing interests

The author declares no competing interests.