Application Effect of New Material after Surface Modification of Zirconia Ceramics and Analysis of Patient Evaluation.

Objective: To explore the application effect of new material after surface modification of zirconia ceramics and patient evaluation.
Methods: A total of 60 patients with tooth defect treated in our hospital from April 2020 to April 2021 were selected as the study subjects and randomly divided into the control group and experimental group, with 30 cases each. The patients in the control group were treated with glass-ceramics, and those in the experimental group received LiSi surface treatment, so as to compare the application effect and patients' evaluation between the two groups.
Results: Between the two groups, no obvious differences in surface loss, adhesive strength, and transmittance at 3 months, 6 months, and 1 year were not observed (P > 0.05); and after intervention, the score on dental aesthetics, hardness value, and occlusal force were obviously higher in the experimental group than in the control group (P < 0.001).
Conclusion: The new material enables forming an acid etchable coating on the zirconia surface, increases the adhesive strength, and achieves an aesthetic degree that is welcomed by the patients; meanwhile, after grinding, the edge is defect free and the tightness is higher. Further research will help to establish a better solution for patients.

1. Introduction

Dental defects are common diseases in the Department of Stomatology, which refer to the damages to dental tissue due to many reasons and often have adverse effects on growth, facial appearance, mastication, periodontal tissues, and systemic health [1, 2]. In addition, caries is a common cause of dental defects, followed by trauma, wear, wedge-shaped defects, acid etching, and developmental dental malformations. The related literature has pointed out that caries is a common etiology leading to dental defects, and to prevent them, patients should pay attention in their daily life to avoid the occurrence of caries. If a dental defect is found, it is necessary to go to the hospital in time, and pay much attention to oral hygiene, cleaning up the oral food residue in a timely manner, and changing the bad use habits of the teeth at ordinary times. Fernanda et al. [3] stated that the accompanying symptoms of dental defects include pulpal inflammation, dentin sensitization and periodontal inflammation, which can further affect the quality of life of patients if not promptly treated with good treatment measures. At present, restoring the function and contour of teeth is the main treatment purpose for dental defects, and with the increase of living standards, people have become more demanding on the aesthetics of dental restorations, so porcelain restorations have been widely accepted [4]. All ceramic restorations and metallo-ceramic restorations are relatively common in clinic, and the former has high biocompatibility and stable aesthetic performance, which is significantly effective in oral restorations [5]. Zirconia ceramics and bioglass ceramics are common restorations among all ceramic restorations, among which zirconia ceramics are characterized by wear-resistance, high strength, high toughness and excellent biocompatibility, and are increasingly used in clinic [6]. Bioglass ceramics have been favored due to their good biocompatibility and biomechanical compatibility.

Yang et al. [7] reported that porcelain cracking was relatively common in zirconia all ceramic restorations, with a 3-year cracking rate of 1.95% for posterior single crowns and a 2-year cracking rate of 2.5%–14.9% for 3-4 units of fixed bridges. To avoid phenomena such as porcelain cracking in all ceramic restorations, choosing a reasonable zirconia surface treatment becomes a common focus among scholars [8]. Although glass-ceramics treatment has a good repair effect in dental defects, it is difficult to meet the clinical needs because it fails to reach the expected shade covering, aesthetics effect and restoration effect. The beauty of the affected teeth can be effectively restored by using LiSi surface treatment of zirconia, which is simple to operate and can effectively increase the adhesive strength of zirconia without affecting the placement of the false tooth [9]. Currently, there are fewer reports on the application effect of LiSi surface treatment on zirconia ceramic after surface modification. Therefore, the scheme was adopted in the study and combined clinical intervention was conducted to the subjects, in the hope of providing more clinical evident-based basis for the patients.

2. Materials and Methods

2.1. General Data

A total of 60 patients with tooth defect treated in our hospital from April 2020 to April 2021 were selected as the study subjects and randomly divided into the control group and experimental group, with 30 cases each. The study met the World Medical Association Declaration of Helsinki [10].

2.2. Enrollment of Study Subjects

Inclusion criteria. (1) The patients aged over 18 years; (2) the patients had crown surface discoloration; (3) the dentition was relatively complete, with an essentially normal occlusal relationship present; (4) the patients had good compliance; and (5) the patients had good oral hygiene and periodontal health.

Exclusion criteria for the patients. (1) Systematic diseases; (2) severe tooth wearing; (3) active periodontitis; (4) pregnant or lactating women; (5) cognitive disorder or mental illness; (6) red, swollen and bleeding gums; and (7) participants of other trials.

2.3. Methods

2.3.1. Control Group

Glass-ceramics restoration was conducted in the control group. First, root canal therapy (RCT) of the affected teeth was completed, and within 3 weeks after RCT, the affected teeth were under observation and examined by X-ray film with medical dental X-ray machine (manufacturer: Ningbo Runyes Medical Instrument Co., Ltd.; model: RAY98 (M)), so as to ensure that there were no lesions in the apex of root of teeth for preparing the abutment according to the requirements of glass-ceramics. The preparation amount of abutment met the specifications, and a continuous and smooth 0.5–1 mm right-angle shoulder was formed in the tooth cervix. Each point, plane, line and angle of the abutment body were maintained smooth, and refinement was performed finally. After gingival retraction, the impression was made by the DMG silicone rubber impression material two-step method, color matching was conducted with the Vita-3D shade guide, the clinical crown was fabricated and adhered, and then the restoration was made by the laboratory in combination with the impression.

2.3.2. Experimental Group

(1) Selection of Resin Cements. The resin cement based on light curing was selected. If the fabricated zirconia veneer was thicker and there was certain impact of light illumination, dual-cured resin cement was used. Before adhesion of the zirconia veneer, the color was tested by the try-in paste to simulate the color after adhesion, so as to make sure that the cement used can achieve the color effect expected by the patients or has sufficient masking ability. However, it should be noted that after light curing, the resin cement might have a certain color difference with the try-in paste, and therefore after using the try-in paste, the abutment and the adhesive surface of zirconia veneer were washed with clean water to prevent the residual try-in paste affecting adhesion.

(2) LiSi Connect Spray. First, the side for adhesion of the restoration was cleaned using steam to ensure that the surface of the restoration was completely clean, and the crown was completely dried by air blowing off the surface water or by other tools. Then the Biomis LiSi connect (eznner, Enamel/Modifier, Aidite (Qinhuangdao) Technology Co., Ltd.) was shaken evenly and sprayed onto the adhesive surface in one direction from 10 to 15 cm away, and the spraying was performed twice. The connect must be shaken up and down repeatedly to make sure that the material in the bottle was mixed evenly, and at the same time, the spraying distance must not be less than 10 cm or over 15 cm, in case of uneven spray, the restoration should be cleaned and sprayed again, but excessive spraying was not allowed, otherwise the closeness of restoration would be affected. The tissue surface of the restoration should be sprayed with LiSi connect, and the non-tissue surface was glazed. If the sintering temperature of glaze paste was lower than that of LiSi connect, the sintering of LiSi connect should be completed first before glaze sintering; and if the sintering temperature of glaze paste was higher than that of LiSi connect, LiSi connect sintering should be conducted. The sintering curve was as follows: initial temperature 450°C, dry time 1 min, heating rate 80°C/min, maximum temperatures 890/895/900°C, heat preservation period 1.5 min, vacuum rate 100%, and furnace temperature 300°C.

(3) Hydrofluoric Acid Etching. Hydrofluoric acid of 4.5% (acid etched for 90 s) or 9.5% (acid etched for 45 s) was used to treat the tissue surfaces of zirconia veneers extraorally. It should be noted that safety measures should be taken during acid etching, and the whole tissue surface should be covered with hydrofluoric acid (regardless of the type). In addition, due to the strong corrosion property, the hydrofluoric acid should be neutralized with the neutralizing agent after treatment and washed clean with water.

(4) Tissue Surface Cleaning. After the porcelain veneer was acid etched by hydrofluoric acid, it was cleaned by ultrasonic shock using 95% ethanol, acetone or distilled water for more than 5 min. Before adhesion, the tissue surface of zirconia veneer should be cleaned by 32% phosphoric acid etchant to ensure that the residual hydrofluoric acid was completely removed.

(5) Silanization Treatment. Silanization treatment of zirconia veneer tissue surfaces was carried out using silane coupling agent or silane containing processor, and brushing was conducted strictly according to the instructions.

(6) Resin Adhesive Treatment. The adhesive was uniformly applied on all tissue surfaces of zirconia veneers to be adhered and then blown thin. After application, light curing treatment was not allowed and contact with light sources was avoided.

(7) Abutment Surface Treatment. It is important to note that rubber dam should be used to isolate from other wet sections before adhesion, and if the marginal site of abutment preparation was under or parallel to the gingiva, gingival retraction should be performed before adhesion to avoid being influenced by gingival crevicular fluid. Meanwhile, the abutments were cleaned in advance and the surfaces of the abutments were acid etched with phosphoric acid.

(8) Use of Resin Cements. Based on the color numbers selected by using the try-in pastes, resin cement of the same color was selected and applied on the abutments and tissue surfaces of zirconia veneers, gently pressed down along the direction of placement, with care taken not to create gaps inside, and curing should be done according to the light curing time of the different cements.

2.4. Observation Indicators

The surface loss status of patients in the two groups 3 months, 6 months and 1 year after restoration was recorded by telephone follow-up.

Specimens from each group were placed on a universal material testing machine (manufacturer: Guangdong Hengbang Detection Instrument & Equipment Co., Ltd.), the direction of the shear force was parallel to that of the glass-ceramic and the Lisi to zirconia adhesive surface at a loading rate of 0.5 mm/min, the force was loaded vertically until the adhered porcelain pieces on the specimens fell off, and the adhesive strength was recorded.

The degrees of beauty of the two groups after treatment were evaluated by the Dental Aesthetics Evaluation Scale [11] proposed by the department. The total score was 100 points, with higher scores indicating higher degrees of beauty.

The hardness values of the two groups were evaluated by microscopic Vickers hardness test, 6 specimens were removed from each group, 12 points were taken for each specimen for the test, the measurement parameters in HV2 mode were used, the objective lens was enlarged by 45x, the loading force was 19.5 N, and the loading time was 4 s.

The occlusal force of the two groups was compared by a T-scan III ultrathin inductor. The patients were instructed to bite the inductor once, and the occlusal force data were analyzed by using a computer.

The occlusal force was compared between the two groups.

The transmittance was compared between the two groups. The transmittance tests were performed on white and black backgrounds, and glycerol was dropped between the background and the specimen at the time of testing to ensure optical continuity.

2.5. Statistical Processing

In this study, the data processing software was SPSS20.0, the picture drawing software was GraphPad Prism 7 (GraphPad Software, San Diego, USA), the items included were enumeration data and measurement data, the methods used were X2 test, t-test and normality test, and differences were considered statistically significant at P < 0.05.

3. Results

3.1. Between-Group Comparison of Baseline Data

Table 1 showed that no significant between-group differences in the gender, age, BMI, course of disease, occupation, educational degree, religious faith, family income, smoking, drinking and place of residence were observed (P > 0.05).

Table 1

Between-group comparison of baseline data.

Item	Experimental group (n = 30)	Control group (n = 30)	x 2/t	P
Gender			0.069	0.793
Male	17 (56.67%)	18 (60.00%)
Female	13 (43.33%)	12 (40.00%)
Age (x¯ ± s, years)	34.53 ± 9.46	34.10 ± 10.79	0.164	0.870
BMI (x¯ ± s, kg/m2)	20.11 ± 0.54	19.96 ± 0.54	1.076	0.287
Course of disease (x¯ ± s, months)	2.50 ± 1.07	2.90 ± 1.49	1.194	0.237
Occupation
Civil servant	8 (26.67%)	9 (30.00%)	0.082	0.774
Teacher	5 (16.67%)	6 (20.00%)	0.111	0.739
Accountant	5 (16.67%)	5 (16.67%)	0.000	1.000
Worker	8 (26.67%)	6 (20.00%)	0.373	0.542
Others	4 (13.33%)	4 (13.33%)	0.000	1.000
Educational degree
Junior high school	2 (6.67%)	1 (3.33%)	0.351	0.554
Senior high school and above	10 (33.33%)	11 (36.67%)	0.073	0.787
College and above	18 (60.00%)	18 (60.00%)	0.000	1.000
Religious faith			0.073	0.787
Yes	20 (66.67%)	19 (63.33%)
No	10 (33.33%)	11 (36.67%)
Family income			0.077	0.781
≥5,000 yuan/(month·person)	20 (66.67%)	21 (70.00%)
<5,000 yuan/(month·person)	10 (33.33%)	9 (30.00%)
Smoking			0.077	0.781
Yes	20 (66.67%)	21 (70.00%)
No	10 (33.33%)	9 (30.00%)
Drinking			0.082	0.774
Yes	21 (70.00%)	22 (73.33%)
No	9 (30.00%)	8 (26.67%)
Place of residence			0.089	0.766
Urban area	22 (73.33%)	23 (76.67%)
Rural area	8 (26.67%)	7 (23.33%)

3.2. Between-Group Comparison of Surface Loss Status

Between the two groups, the surface loss status 3 months, 6 months and 1 year after restoration was not obviously different (P > 0.05). See Table 2.

Table 2

Between-group comparison of surface loss status [n(%)].

Group	n	3 months	6 months	1 year
Experimental	30	0 (0.00%)	1 (3.33%)	3 (10.00%)
Control	30	1 (3.33%)	2 (6.67%)	4 (13.33%)
x 2		1.017	0.351	0.162
P		0.313	0.554	0.688

3.3. Between-Group Comparison of Adhesive Strength

Table 3 showed that no obvious difference in adhesive strength between the two groups was observed (P > 0.05).

Table 3

Between-group comparison of adhesive strength ( ± s).

Group	n	Adhesive strength (MPa)
Experimental	30	10.03 ± 1.54
Control	30	9.54 ± 1.44
t		1.273
P		0.208

3.4. Between-Group Comparison of Dental Aesthetics Scores

After intervention, the dental aesthetics score was obviously higher in the experimental group than in the control group (P < 0.001). See Figure 1.

Figure 1

Between-group comparison of dental aesthetics scores ( ± s). Note: the horizontal axis indicated the experimental group and the control group, and the vertical axis indicated the dental aesthetics score (points); after intervention, the dental aesthetics scores of patients in the experimental group and the control group were, respectively, (84.03 ± 3.16) and (55.33 ± 3.51); and indicated significant difference in the dental aesthetics scores after intervention between the two groups (t = 33.284, P < 0.001).

3.5. Between-Group Comparison of Hardness Values

Figure 2 showed that after intervention, the hardness value was obviously higher in the experimental group than in the control group (P < 0.001).

Figure 2

Between-group comparison of hardness values ( ± s). Note: the horizontal axis indicated the experimental group and the control group, and the vertical axis indicated the hardness value index (MPa); after intervention, the patients' hardness value indexes of the experimental group and the control group were, respectively, (1,950.47 ± 29.88) and (348.13 ± 29.08); and indicated significant between-group difference in the hardness value indexes after intervention (t = 210.491, P < 0.001).

3.6. Between-Group Comparison of Occlusal Force

Table 4 showed that after intervention, the occlusal force was obviously higher in the experimental group than in the control group (P < 0.001).

Table 4

Between-group comparison of occlusal force.

Group	n	Occlusal force (N)
Experimental	30	458.99 ± 37.17
Control	30	376.67 ± 14.32
t		11.319
P		<0.001

3.7. Between-Group Comparison of Transmittance

Table 5 showed no obvious difference in transmittance between the two groups (P > 0.05).

Table 5

Between-group comparison of transmittance ( ± s).

Group	n	Transmittance
Experimental	30	30.20 ± 0.65
Control	30	30.66 ± 1.11
t		1.958
P		0.055

4. Discussion

With the high rate of economic development in China, people's dietary habits and structure are more and more diversified, hence a growing number of people suffer from conditions such as dentition defects or tooth defects [12]. Currently, for patients with tooth defects, the main clinical treatment options are glass-ceramics restoration or dental crown treatment [13]. Recently, the technique of dental all ceramic restorations has developed rapidly, and there are more and more new ceramic materials, of which zirconia has become an increasingly used all ceramic restoration because of its high strength, toughness, and stress-induced transformation toughening effect [14, 15]. However, related published works have pointed out that successful restoration depends not only on the excellent performance of materials, but also has a close relationship with the strong adhesion between the restoration and dental tissues [16]. Unlike glass-ceramics, zirconia does not contain glass matrix components and is difficult to form a rough surface to provide mechanical retention forces through acid etch of phosphoric acid or hydrofluoric acid, and without surface treatment, although the adhesive interface of zirconia and resin cement is close, there is almost no mechanical chimeric retention force and chemical bonding retention force between the two, and the bonding strength is low, making it difficult to function in the oral cavity [17]. Most previous studies on zirconia focused on increasing the adhesive strength between ceramic and resin cements, and to date there is no consensus on the optimal surface treatment method for zirconia restorations [18]. However, the adhesive performance between zirconia and surface veneer porcelain is the main factor determining whether it can succeed [19]. Reports pointed out that most scholars recommended treating the zirconia surface by appropriate sand blasting, aiming to partially improve the wettability, increase the roughness of the zirconia surface, and even form small undercuts to increase the adhesive strength of zirconia [20]. However, Ueda et al. [21] pointed out that sandblasting is not popularized in clinic at present, due to the relatively obvious disadvantages such as leading to the creation of defects on the zirconia surface and occurrence of phase transition, thus affecting the long-term stability and mechanical performance of the restorations. With the advent of the LiSi product, it is found that the shedding situation and adhesive strength of zirconia are comparable to glass-ceramics and superior to sandblasting. In this study, the surface loss status and adhesive strength 3 months, 6 months and 1 year after restoration were not obviously different between the experimental group and the control group (P > 0.05), indicating that both surface treatment modalities had significant effect.

Fontolliet et al. [22] stated that aesthetic degree was greatly reduced when discoloration occurred in the late stage of glass-ceramics for patients whose dental nerves had been removed or who had discolored tooth. And with LiSi surface treatment, tooth discoloration is less frequent, so doctors and patients are more likely to choose LiSi products for zirconia treatment. In this study, self-proposed tooth beauty scores were used to evaluate the tooth aesthetic degrees of the two groups, which were obviously higher in the experimental group (LiSi surface treatment) than in the control group (glass-ceramic treatment), and the reason was that evenly spraying LiSi on the adhesive surface of zirconia veneer formed a lithium disilicate coating on the surface, which, without increasing the thickness, had anti-corrosion effect and allowed the veneer to remain harmonious and uniform in appearance and color, further improving tooth aesthetics. Moreover, Alamar et al. [23] stated that the hardness value of zirconia was higher compared to that of glass-ceramics, and in this study, compared with the control group, the hardness value of the experimental group was obviously higher, confirming their result. Zirconia is a high-tech biomaterial, which has better biocompatibility, stable molecular structure and hard texture, so it can provide the affected teeth with stronger retention, adhesion and support, effectively avoid adverse events such as crack propagation and fracture, and further improve the occlusal force of patients [24, 25]. After LiSi treatment, the performance of restorative material can be effectively improved, the careful margin treatment allows the restorative material to better fit the gingival margin, thus reducing gingival bleeding events and helping patients relieve post restorative pain and discomfort. In addition, many properties of zirconia are inferior to those of glass-ceramics, but with LiSi treatment, it is found that zirconia has close properties to glass-ceramics, and there was no obvious difference in transmittance between the two, thus further indicating the effect of LiSi. Shortcomings of the study: First, the cases in the study were patients treated in the local hospital, so the source of cases lacked diversity; second, limited by the observation time, not sufficient sample amount was included in this clinical trial, causing bias in the study results; and finally, the long-term follow-up and observation on patients' intervention effect were lacked, and patients were still evaluated clinically using the form of scale, which could not avoid certain subjectivity and intention when they were answering the questions. Therefore, in the future studies, the design should be improved, and the follow-up time should be prolonged to deeply and carefully explore the application effect of new materials after surface modification of zirconia ceramics and patient evaluation from multiple different perspectives and angles. In conclusion, the initial conclusion obtained in the study needs to be improved by more subsequent research.