Abstract
Objectives
The aim of this randomized controlled trial was to see if the minimally invasive approach (reduced restoration thickness) would result in good clinical success of monolithic ceramic crowns compared to conventional layered all-ceramic crowns, and thus be an alternative to conventional tooth preparation.
Materials and methods
The ceramic that was investigated was IPS e.max lithium-disilicate ceramic produced using two different processing methods. A comparison was made between monolithic crowns with reduced thickness and standard layered crowns. Fifty-two patients, who had undergone endodontic treatment on either a premolar or molar, were randomly assigned into two groups. The teeth intended for layered crowns underwent to a 2 mm occlusal reduction with a 1 mm rounded shoulder, whereas the teeth intended for monolithic crowns underwent to a 1 mm reduction in the occlusal area with a 0.6 mm rounded shoulder. The clinical success was evaluated in eight categories using modified United States Public Health Service (USPHS) criteria. The observation period was 36 months, with control appointments every 6 months.
Results
There was no significant difference in clinical success between monolithic and conventional layered crowns after 3 years. One monolithic crown fractured while all other crowns were intact and the survival rate was 96%. All layered crowns were intact and the survival rate was 100%.
Conclusion
The results of this study indicate that the minimally invasive approach can be a good alternative to conventional tooth preparation. IPS e.max lithium-disilicate ceramic demonstrated an exceptional three-year survival rate independently of the thickness of the material.
Key words: Monolithic crown, Layered crown, IPS e.max, Lithium-disilicate, Reduced thickness, Survival rate
Keywords: MeSH Terms: Prosthodontic Tooth Preparation, Crowns, Lithium Compounds
Introduction
In order to prepare teeth for an indirect aesthetic restoration, a significant amount of tooth tissue must be removed, often more than 60% (1). Things get considerably worse when it comes to teeth that underwent root canal treatment because they are already structurally weakened (2). Given that many endodontically treated teeth have already experienced dental structure loss it is crucial to restore them in order to provide functional stability and ensure their long clinical survival. This is especially true for molars which need to withstand high occlusal stresses during mastication (3).
The amount of tooth structure that remains after endodontic treatment is an essential factor in determining the strength and lifetime of the endodontically treated teeth (2, 4–8). The loss of dental structure increases the cuspal deflection, particularly in the posterior teeth which are more susceptible to vertical stresses (6). Occlusal preparation results in a 20% reduction in stiffness, whereas MOD preparation leads to a stiffness loss of about 60% (6, 9, 10). In addition, the removal of the pulp leads to a decrease in the sensory feedback mechanism (3). The force required for the proprioceptor response in endodontically treated teeth is 2.5 times higher than in vital ones (3). This can lead to greater forces during mastication on already structurally weakened teeth, and as a result bigger probability of fracture or other complications (11). Considering all the above, as the endodontically treated teeth are already structurally weakened due to the previous loss of tooth tissue, ensuring long-term function was the main reason why they were included in the study.
Crown coverage increases the clinical survival of endodontically treated teeth (12, 13). Furthermore, studies have shown that root canal treated teeth fitted with a crown have a comparable chance of experiencing clinical failure as teeth that are vital (3, 12, 14, 15). In addition to providing cuspal coverage, it is crucial to have a well-designed occlusal surface of the restoration in order to provide even distribution of axial stresses and reduce the impact of non-axial forces (16).
Given that endodontically treated teeth already have compromised structural integrity resulting from factors such as access preparations, caries, fractures, or previous restorations, it is preferable to minimize any additional tooth preparation in order to retain as much of the remaining tooth structure as possible. Preparing a tooth for layered all-ceramic crown involves the removal of a significant quantity of hard tooth tissue. Due to this, excellent aesthetics can be achieved, but often more than 70% of hard dental tissue is removed (1). This results in a structurally compromised tooth, and often less than 1.5 mm of dentine thickness remains (17). Even with such a substantial reduction, it frequently happens that teeth are overprepared, thus worsening the structural integrity (18). When it comes to endodontically treated teeth, where part of the hard dental tissue has already been removed, this percentage is even higher.
Lithium-disilicate ceramics were introduced in 1998 by Ivoclar, and were named IPS Empress 2 (19). The composition comprised of a glass matrix containing lithium-disilicate crystals of micron size, with sub-micron-sized lithium-orthophosphate crystals scattered within it (20). Approximately 70% of the volume consisted of crystals of lithium disilicate (21, 22). Further development led to introduction of IPS e.max, which had better translucency and mechanical properties (23, 24). Thanks to better optical properties, IPS e.max allows production of fully contoured, monolithic restorations (25–27).
IPS e.max is available as CAD blocks or Press ingots, both industrially produced, resulting in a solid, high-quality material free of pores (28–30). The fully crystallized ceramic is composed of 70% rod-like crystals of lithium-disilicate evenly distributed and cross-linked in a glass matrix, with a flexural strength of 360 MPa for CAD blocks and 400 MPa for Press ingots (25, 26). Apart from good mechanical and optical characteristics, lithium-disilicate ceramics also have good biological characteristics that are manifested in contact with soft tissues and their reaction (31–33). Analyzing the concentration of inflammation indicators in the gingival sulcus fluid in contact with lithium disilicate ceramics, no significant difference was found between a healthy tooth and the aforementioned ceramic (33).
Layered crowns are made from the lithium-disilicate base (core) on which the aesthetic, but weaker, veneering ceramic is applied. The technique is called the layering technique. Due to the similar coefficient of thermal expansion (CTE), fluorapatite aesthetic ceramics are used for layering lithium-disilicate base constructions. Thanks to the multi-layering technique, the unique combination of opalescence, brightness and translucency ensures the superb aesthetic, and a natural-looking appearance can be achieved. On the other hand, a monolithic crown is fully anatomic and is fully made from the same material. As the whole crown is made from lithium-disilicate ceramic, the use of weaker veneer ceramic is excluded, thus minimizing the possibility of chipping or delamination.
This randomized controlled trial aimed to investigate if a thinner all-ceramic crown, monolithic IPS e.max CAD (Ivoclar Vivadent, Schaan, Lichtenstein) lithium-disilicate crown with a wall thickness less than the manufacturer's recommendation can offer a comparable survival rate to a traditional layered all-ceramic crown composed of a lithium-disilicate core (IPS e.max CAD, Ivoclar Vivadent, Schaan, Lichtenstein) and layered with aesthetic ceramic (IPS e.max Ceram, Ivoclar Vivadent, Schaan, Lichtenstein).
The null hypothesis stated for this study was that there exists no statistically significant difference in the clinical survival rate between thinner monolithic IPS e.max CAD and layered IPS e.max CAD single posterior crowns.
Materials and Methods
The Ethical Committee of the School of Dentistry, University of Zagreb, Croatia, approved the study. This research was double-blind with participants being divided into two parallel groups. An equal number of male and female participants were assigned to the control group and intervention groups. The age of the patients was between 18 and 65 years. Patients with signs of parafunctions were not included in the research; hence the presence of parafunctions did not affect the research results.
Patients who visited general dentistry practices (GDPs) in Bjelovar and Zagreb, Croatia, were chosen as study participants. The number of patients that took part in the study was 52, 26 in each group, which was by 20% more than usual, to ensure that the results are unaffected by potential withdrawals. The number of patients was calculated using computer software.
All patients attending the specified GDP underwent a screening process based on specific inclusion and exclusion criteria (Table 1 and 2). The study ran for a total of 45 months, during which the evaluation of the crowns lasted 36 months, with scheduled follow-up appointments every six months. Patients were enrolled in the trial upon meeting the inclusion criteria, rather than upon completion of recruitment of all study participants. The participants were randomly allocated into two groups using computer software, and each participant got a unique identity number. Each patient received only one crown.
Table 1. Inclusion criteria.
| Inclusion Criteria |
|---|
| Men and women with endodontically treated premolar or molar teeth |
| Aged between 18 and 65 |
| Successful endodontic treatment assessed by the post-endodontic radiograph |
| Presence of opposite tooth (to have occlusal contact after placement of the crown) |
| Good oral hygiene and no active caries lesions on selected tooth |
| Patient’s ability to attend regular follow-up appointments according to agreed schedule |
Table 2. Exclusion criteria.
| Exclusion Criteria |
|---|
| The tooth is restored with amalgam restoration, in this case, the restoration will be replaced with a composite one |
| Previous indirect restoration on selected teeth |
| Pain or any other discomfort from selected tooth |
| General uncontrolled periodontal disease |
| Acute gingivitis on selected tooth |
| Sulcus depth more than 3.5 mm |
| Grade 2 or more mobility of selected tooth |
| Furcation involvement on selected tooth |
| Signs of parafunctions (cheek ridging, tongue scalloping, history of tooth/restoration fracture) |
Prior to preparation, two silicon impressions were obtained using putty material (EXA'lence; GC, Tokyo, Japan). The first one was divided into vertical sections and employed as a silicon guide to facilitate precise axial and occlusal tooth preparation. The other impression was used for making immediate provisional crowns. Teeth used for layered crowns were prepared with either an equigingival or supragingival gingival finish line. The preparation included a 1 mm wide rounded shoulder, 2 mm occlusal reduction, and an axial inclination of about 10° (Figure 1) (34). All angles were rounded. The preparation of teeth for monolithic crowns followed the same procedure as for layered crowns, with the exception that for monolithic crowns, a 0.6 mm wide rounded shoulder and 1 mm occlusal reduction were performed (Figure 2) (34). The impression was taken using vinyl polyether silicone material (EXA'lence; GC, Tokyo, Japan) and the working models made using type 4 dental stone (Fujirock EP; GC, Tokyo, Japan). The tooth shade was assessed with the VITA Easyshade V (Vita Zahnfabrik, Bad Säckingen, Germany).
Figure 1.
_123-135-f1.jpg)
Tooth preparation for layered crown
Figure 2.
_123-135-f2.jpg)
Tooth preparation for monolithic crown
All crowns were manufactured in the same laboratory by the same technician. The restorations were fabricated with the CAD/CAM system (Dentsply Sirona, Charlotte, USA). The prepared teeth were scanned with an Omnicam scanner (Dentsply Sirona, Charlotte, USA). The cores for the layered crowns were designed using computer software (in-Lab SW 4.4; Dentsply Sirona, Charlotte, USA) with a consistent thickness of 0.6 mm (34). The entire monolithic crowns were designed using the same program, with a marginal thickness of 0.6 mm and an occlusal thickness of 1 mm. Partially crystallized blue blocks were milled using a CAD/CAM milling unit (inLab MC XL; Dentsply Sirona, Charlotte, USA).
Partially crystallized ceramic cores and monolithic crowns were examined in the mouth. Following the try-in procedure, the monolithic crowns were returned to the laboratory and finalized. The ceramic cores were layered with fluorapatite glass-ceramic (IPS e.max Ceram, Ivoclar Vivadent, Schaan, Lichtenstein) and finalized.
The finished crowns were tried in the mouth. The occlusal contacts were assessed with 8μm articulation foil (Interdent, Celje, Slovenia). Final refinements were made concerning occlusion or proximal contacts. The crowns that underwent adjustments were returned to the laboratory and finished. The crowns were cemented using a clear dual-cure adhesive resin cement (Variolink Esthetic DC, Ivoclar Vivadent, Schaan, Lichtenstein), following manufacturer’s recommendations.
The initial evaluation was conducted one week after the cementation. Following the initial baseline assessment, subsequent follow-up appointments were scheduled every six months. During each appointment, the identical assessment protocol was utilized to evaluate the clinical survival of the crowns.
Clinical survival of tested crowns was evaluated in eight categories using modified United States Public Health Service (USPHS) criteria (35). Tested categories were ceramic fracture, marginal adaptation, color, caries, marginal discoloration, occlusal contact, proximal contact and retention. The assessment was double blind as both the participants and the examiner did not know which type of crown was provided. Since the intervention was equal for all participants (tooth preparation), they could not observe any differences among them. The crowns were clinically examined using standardized diagnostic dental instruments under 2.5 × magnifications. According to these criteria, restorations that had an Alfa or Beta rating were considered successful, while those rated Charlie or Delta were considered failures.
Differences between the initial and the final values within each group were assessed by the Mc Nemar test (for categorical characteristics of crowns). Differences between categorical variables between the studied groups were analyzed using the Fisher-Freeman-Halton test. All P values less than 0.05 were considered significant. The analysis used licensed program support IBM SPSS for Windows, version 25.0.
Results
Differences between study groups in clinical characteristics of crowns immediately after the cementation are shown in Table 3. The only significant difference was observed in color, with monolithic crowns exhibiting greater gradation B compared to veneered crowns: 9 (34.6%) versus 0 (0.0%); P=0.002.
Table 3. Differences in the clinical characteristics of the crowns between the examined groups at the beginning of the study (immediately after cementation).
| Group | P | |||||
|---|---|---|---|---|---|---|
|
Monolithic
N=26 |
Layered
N=26 |
|||||
| N | % | N | % | |||
| Ceramic fracture after cementation | A | 26 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Marginal adaptation after cementation | A | 25 | 96,2% | 26 | 100,0% | 1,000 |
| B | 1 | 3,8% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Color after cementation | A | 17 | 65,4% | 26 | 100,0% | 0,002 |
| B | 9 | 34,6% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Caries after cementation | A | 26 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Marginal discoloration after cementation | A | 26 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Occlusal contact after cementation | A | 26 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Approximal contact after cementation | A | 26 | 100,0% | 26 | 100,0% | |
| B | 0 | 0,0% | 0 | 0,0% | na | |
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Retention after cementation | A | 26 | 100,0% | 26 | 100,0% | |
| B | 0 | 0,0% | 0 | 0,0% | na | |
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
Differences between study groups in clinical characteristics of crowns 36 months after the cementation are shown in Table 4. After 36 months, a significant difference in color remained (P<0.001) - differences were related to the group of monolithic crowns and to the more frequent classification B, while in the case of layered crowns, in 100% of cases with the above characteristics, classification A was noted.
Table 4. Differences in the clinical characteristics of the crowns between the studied groups 36 months after cementation.
| Group | P | |||||
|---|---|---|---|---|---|---|
|
Monolithic
N=25 |
Layered
N=26 |
|||||
| N | % | N | % | |||
| Ceramic fracture after 36 months | A | 25 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Marginal adaptation after 36 months | A | 22 | 88,0% | 26 | 100,0% | 0,110 |
| B | 3 | 12,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Color after 36 months | A | 15 | 60,0% | 26 | 100,0% | <0,001 |
| B | 10 | 40,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Caries after 36 months | A | 25 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Marginal discoloration after 36 months | A | 20 | 80,0% | 25 | 96,2% | 0,099 |
| B | 5 | 20,0% | 1 | 3,8% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Occlusal contact after 36 months | A | 25 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Approximal contact after 36 months | A | 25 | 100,0% | 24 | 92,3% | 0,490 |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 2 | 7,7% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Retention after 36 months | A | 25 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
The differences between the final values and the initial ones within the group of monolithic crowns are shown in Table 5. Significant difference in marginal discoloration was noted, where the proportion of poorly rated crowns (from grade A to grade B) increased over 36 months to 20.0% (P= 0.023).
Table 5. Differences in final values (after 36 months) compared to initial values within the group of monolithic crowns.
| Monolithic group | P | |||||
|---|---|---|---|---|---|---|
| After cementation | Final assessment | |||||
| N | % | N | % | |||
| Ceramic fracture | A | 26 | 100,0% | 25 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Marginal adaptation | A | 25 | 96,2% | 22 | 88,0% | 0,350 |
| B | 1 | 3,8% | 3 | 12,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Color | A | 17 | 65,4% | 15 | 60,0% | 0,776 |
| B | 9 | 34,6% | 10 | 40,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Caries | A | 26 | 100,0% | 25 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Marginal discoloration | A | 26 | 100,0% | 20 | 80,0% | 0,023 |
| B | 0 | 0,0% | 5 | 20,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Occlusal contact | A | 26 | 100,0% | 25 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Approximal contact | A | 26 | 100,0% | 25 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Retention | A | 26 | 100,0% | 25 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
The differences between the final values and the initial values within the group of layered crowns did not show significant differences (Table 6).
Table 6. Differences in final values (after 36 months) compared to initial ones within the group of layered crowns.
| Layered group | P | |||||
|---|---|---|---|---|---|---|
| After cementation | Final assessment | |||||
| N | % | N | % | |||
| Ceramic fracture | A | 26 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Marginal adaptation | A | 26 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Color | A | 26 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Caries | A | 26 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Marginal discoloration | A | 26 | 100,0% | 25 | 96,2% | 1,000 |
| B | 0 | 0,0% | 1 | 3,8% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Occlusal contact | A | 26 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Approximal contact | A | 26 | 100,0% | 24 | 92,3% | 0,490 |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 2 | 7,7% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
| Retention | A | 26 | 100,0% | 26 | 100,0% | na |
| B | 0 | 0,0% | 0 | 0,0% | ||
| C | 0 | 0,0% | 0 | 0,0% | ||
| D | 0 | 0,0% | 0 | 0,0% | ||
Discussion
After the clinical survival of crowns had been considered, the null hypothesis was accepted. Regarding the ceramic fracture, there was no statistically significant difference in the clinical survival between monolithic IPS e.max lithium-disilicate crowns with reduced wall thickness and classic layered IPS e.max lithium-disilicate crowns. One monolithic crown fractured and was characterized as a clinical failure. On all other crowns, there was no damage on the ceramic, and the clinical survival of the monolithic crowns after 36 months was 96%. In the group of layered crowns, there was no damage on any of the crowns, and after 36 months the clinical survival rate was 100%. In all other categories regarding clinical performance of the crowns (marginal adaptation, caries, marginal discoloration, occlusal contact, proximal contact, retention) there was also no statistical difference between monolithic and layered crowns. The only category in which the statistical significant difference was noticed was color, but it is not related to clinical survival.
Modern dental medicine is no longer based only on the replacement of lost dental tissue. Biological factors are becoming more and more important and the preservation of hard dental tissue is becoming the main focus of modern dental therapy. At the same time, new techniques and new materials are being developed that enable modifications of standard procedures in order to preserve as many healthy tooth as possible. Taking all this into consideration, the main focus of this clinical research was to preserve as much hard dental tissue as possible while ensuring good clinical survival.
Lithium-disilicate ceramic was introduced in 1998 and since then it has been one of the most commonly used material in fixed prosthodontics (20, 21). Many years of clinical application, as well as numerous clinical studies, have proven high reliability and long-term clinical success of monolithic, and layered crowns, with success rate of more than 96% after two and four years, and more than 80% after 15 years for monolithic crowns and between 94.8% and 100% over periods of 4 to 11 years for layered crowns (34, 36–48). As the reported success rate of monolithic crowns in one study after 15 years is much lower than for layered crowns, it must be mentioned that both the technical and biological failures were included, that affect the outcome. In most other studies the success rate of monolithic crowns was similar to those of layered crowns.
Currently, there are no documented clinical studies regarding the clinical longevity of monolithic lithium-disilicate crowns with thickness reduced below the guidelines of the manufacturers, whether used on teeth that have undergone root canal treatment or on healthy teeth. Furthermore, no clinical trials have been conducted to directly evaluate the reliability of reduced-thickness monolithic lithium-disilicate crowns with layered lithium-disilicate crowns or other types of all-ceramic or metal-ceramic crowns.
Apart from adequate cleaning and filling of the root canals, to ensure good endodontic treatment, restoration of hard tooth tissue is also necessary (2, 3, 5). Although these two therapies are separate, they are a whole that makes up the entire endodontic therapy. Adequate postendodonthic therapy ensures the structural integrity of treated tooth and good clinical survival. The amount of remaining hard tooth structure represents the most important factor in the structural integrity and longevity of endodontically treated teeth (2, 5, 6). Since the structural integrity of teeth treated by root canal therapy had already been compromised due to caries, access preparation, fracture or previous restorations, this becomes even more pronounced. Since the amount of remaining hard dental tissue directly affects the clinical survival of pulpless teeth, and the provision of a crown represents the best therapy, any preservation of dental tissue during tooth preparation can significantly affect its clinical success.
Modern ceramic systems enable the elimination of metal and the use of only ceramic for almost all indications. In addition, the increasing demands for restorations without metal and with exceptional aesthetics led to the development of many new all-ceramic systems (49). These systems allow the production of all-ceramic restorations made entirely from solid materials, while maintaining high aesthetics.
In parallel with the development of ceramic materials, adhesive techniques are also improving, thus leading to modifications of the classic principles of preparations in fixed prosthodontics (50). Modern dental medicine is increasingly turning to minimally invasive procedures and preservation of as many tooth as possible, leading to rejection of conventional retention preparations (4, 49, 51, 52).
Despite the lack of clinical studies, several authors have performed laboratory studies to evaluate the fracture resistance of monolithic lithium-disilicate crowns with various wall thicknesses (53–56). The results of this study align with the results of the aforementioned laboratory studies, which demonstrated that monolithic crowns with a wall thickness of 1 mm exhibited great resistance to fractures.
Regarding the clinical survival of tested crowns, there were no statistically significant differences in most categories after 36 months. Ceramic fracture occurred on one monolithic crown, whereas on all other crowns whether they were monolithic or layered no fractures, delamination or surface chipping of ceramic occurred. In the group of monolithic crowns slight probe catch without gap and visible dentine was noticed on the margin of three crowns, and the crowns were graded B. In the group of layered crowns, there was no marginal gap, and all the crowns were graded with A. With regard to the marginal adaptation, there was also no significant difference between both groups. None of the crowns in both groups had caries at the margin of the crown and the tooth, and all crowns were graded A. Also, there was no loss of occlusal contact on any crown in both groups, and all crowns were graded A. Considering these two categories, there was no significant difference between the groups. Proximal contact loss was observed on two layered crowns, which is not necessarily related to the observed tooth and is not the result of a complication of the ceramic itself, and the crowns were graded C. In the group of monolithic crowns, no proximal contact loss was observed in any of the crowns. There was no significant difference in this category. Not a single crown in both groups lost retention; hence there were no significant differences between monolithic and layered crowns in this category either.
The only significant difference between the examined groups was in the color of the crowns. In the group of monolithic crowns, 15 crowns were graded A, and on 10 crowns a slight discrepancy in color was observed and they were graded B. All layered crowns were graded A. The slight mismatch in color noted in monolithic crowns is probably the result of thinner ceramics and differences in translucency, which was noticed by clinicians, but in most cases not by patients.
Marginal discoloration was noted in five monolithic and one layered crowns, but only as a slight discoloration at the restoration-tooth margin without spreading under the restoration, and these crowns were graded B. Although there were no significant differences in marginal discoloration between groups, a significant difference was noted within the monolithic group after 36 months. A higher incidence of marginal discoloration may be due to slight color change of composite cement. Several laboratory studies have proven that composite cements change color at the edge of the restoration yet after a year, and that this color change can affect the aesthetic appearance of the restoration (57–61). The ceramic thickness at the margin of the restorations also affects the visibility of marginal discoloration. As a greater number of marginal discolorations were observed on monolithic crowns, the thickness of the ceramics at the margin of the crown (0.6 mm) was probably the reason for such an occurrence. In addition, dual-curing composite cement was used for crown cementation, which is an additional possible reason for the increased number of marginal discolorations on monolithic crowns.
This clinical study showed excellent clinical survival of IPS e.max lithium-disilicate monolithic crowns with reduced wall thickness, comparable to conventional layered IPS e.max lithium-disilicate crowns. As this is the only clinical study that examined the durability of a lithium disilicate crown with reduced wall thickness, further research is required to make this clinical approach standard. Besides, the duration of the study is partially a limiting factor, and further assessment is needed to determine long-term clinical survival.
Conclusions
Considering the limitations of this investigation, it can be concluded that the three-year survival rate of reduced-thickness IPS e.max lithium-disilicate posterior crowns is comparable to that of layered IPS e.max lithium-disilicate crowns.
Acknowledgments
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Institutional Review Board Statement: The research protocol was approved by the Ethical Committee of the School of Dentistry, University of Zagreb, Croatia (Number: 05-PA-26-1/2017; date of approval 13 January 2017).
Footnotes
Conflicts of Interest: The authors declare no conflict of interest.
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