Clinical Performance of Lithium Disilicate Glass-Ceramic Onlays For The Treatment of Tooth Defects


 BackgroundThe aim of this study was to assess the clinical performance of lithium disilicate glass-ceramic onlays for the treatment of tooth defects and to evaluate the clinical performance and whether they are worth more extensive use as that of the full crown. MethodsPatients who received treatment by lithium disilicate glass-ceramic onlays at the Western China Hospital of Stomatology were recalled after 1~4 years. The clinical performance and patients’ satisfaction of onlays for various tooth defects, cracked or uncracked teeth and endodontically treated or vital teeth were retrospectively evaluated with a combination of modified United States Public Health Service Criteria and questionnaire survey. Statistical analysis was performed by using the chi-squared test, Kaplan–Meier analysis and Log-rank test (a = 0.05) where appropriate. ResultsA total of 154 patients with 166 onlays were recalled for clinical examination. Of the 166 onlays examined, 65 (39.19%) were occlusal onlays, 92 (55.42%) were proximal-occlusive onlays and 9 (5.42%) were buccal-or lingual-occlusal onlays. The clinical performance of O and PO onlays was not significantly different (P > 0.05), according to USPHS Criteria. Kaplan-Meier analysis showed that the 4-year survival rate of O and PO onlays was 95.4% and 97.8%, respectively while there was no failure happened in buccal-or lingual-occlusal onlays. Log- rank test showed that the kinds of defects/onlays, tooth vitality and tooth crack had no influence on the survival rate (P > 0.05). The overall satisfaction rate was more than 98%.ConclusionsThis medium-term analysis indicated that lithium disilicate glass-ceramic onlays achieved satisfactory clinical performance for the restoration of different tooth defects. The survival rate of onlays was comparable to that of full crown. Different kinds of tooth defects, tooth vitality and tooth crack did not influence performance.Trial registrationThe study was approved by the Medical Ethics Committee of West China Hospital of Stomatology of Sichuan University with the approval number: WCHSIRB-D-2021-300. Consent to participate was not applicable.

The most common restorative method for tooth defects of endodontically treated teeth and vital pulp teeth with poor lling treatment effect is full crown [1,2]. However, full crown restoration has its own disadvantages, and some complications may occur after restoration [3]. First of all, the occlusal surface and the axis walls need to be prepared for full crown restoration, which can reduce 71% tooth structure, and the fracture resistance of tooth was decreased [4]. After preparation, the thickness of tooth wall may be less than 2 mm, which may not be su cient to support the full crown. Basaran et al. reported that tooth wall with 2 mm was adequate, and support of teeth with 1 mm thick was completely insu cient [5].
In addition, if the margin of full crown restoration close to gingiva is rough, without marginal adaptation or has protrusion, it is probably di cult to conduct oral hygiene measures and stimulate gingival tissue, leading to gingival in ammation, periodontitis, caries, debonding, etc. [6].
In recent years, with the signi cant developments of adhesive and cementation agents, there have been advances in the bonding technology for glass-ceramic [7,8,9]. And using glass-ceramic onlays in the restoration of posterior tooth defect is becoming more popular [10,11]. Compared to zirconia ceramic material, glass-ceramic [12,13] has better adhesive property and sound wear resistance which causes less wear of the clutch tooth. In addition, the horizontal stress of the whole occlusal cusp covered by onlay is less than that of the partial occlusal cusp [14]. Moreover, the tooth preparation of onlay is only related to occlusal surface, 45% less than the full crown [4], which can preserve more tooth structure.
Sometimes, its preparation [15] will not refer to mesiodistal adjacent areas, preventing food impaction (FI) caused by poor adjacency recovery after repair. And lastly the margin of onlays locates in the occlusal 1/3 of tooth, keeping away from gingival margin which can avoid stimulation, facilitate selfcleaning, and be bene cial to the health of periodontal tissue.
There are few studies on clinical performance of lithium disilicate glass-ceramic onlays compared to that of full crown. The purpose of this study was to evaluate the clinical performance of lithium disilicate glass-ceramic onlays for different kinds of tooth defects. 154 patients referred to and treated at the Department of Prosthetics, West China Hospital of Stomatology Sichuan University, China, from 2017 to 2019, were recalled and 166 onlays were examined for onlay survival and clinical performance.

Methods
According to the digital records, patients referred for prosthetic treatment and treated by a professional team with lithium disilicate glass-ceramic onlays between 2017 and 2019 (Department I of Prosthetics, West China Hospital of Stomatology Sichuan University, China) were contacted via phone calls and proposed an appointment for a free clinical examination and satisfaction questionnaire survey.
According to the lithium disilicate glass-ceramic onlays used for different kinds of defects, onlays were also divided into different kinds: occlusal (O) onlays, proximal-occlusive (PO) onlays and buccal-/ lingual-occlusal (BO/ LIO) onlays. The recalled patients were divided into vital teeth (VT) group and endodontically treated (ET) group according to pulp vitality, and cracked teeth (CT) group and uncracked teeth (UT) group according to tooth crack. Clinical examinations were performed by 2 of the authors, who did not participate in the treatment and did not know who restored the teeth that they were evaluating. Before examination, the patients signed an agreement to participate in the study, and all data were kept con dential.
Survive was de ned as presence of onlays without biological and/or technical complications during the entire follow-up period. All onlays were assessed according to modi ed United States Public Health Service (USPHS) criteria (Table 1) [15,16,17], and the examined onlays were divided into 3 ratings, Alpha(A), Bravo (B) and Charlie (C). If there are two or more results for one onlay evaluated at the same time, the highest rating will be chosen.

The Results Of Clinical Examination
The results of 166 onlays examined by modi ed USPHS Criteria are shown in Table 2. According to the digital records, there were 3 tooth fractures happened and nally extracted before clinical examination. The three cases of tooth fracture failures were O group, all of which were cracked teeth; one was the premolar with vital pulp, and the rest were ET molars. One onlay was completely fractured and needed refabrication, and one was debonded and need rebonding, all of which were ET molars with PO onlays.
The above 5 failure cases were rated C. In addition, food impaction was found in 7 cases, and these cases were also rated as grade C. The grade B cases had following problems: slight fracture that did not affect the appearance and function, slight margin un t, marginal discoloration, and mild gingival in ammation. 95.78% onlays reached grade A in proximal contact, and more than 98% onlays reached A in the other observation criteria. The clinical evaluation results showed that 150 onlays were grade A (90.36%), 4 were grade B (3.01%), and 12 onlays were C (7.83%). All the BO/LIO onlays were A; there are 89.23% O onlays and 90.22% PO onlays reached grade A (Table 3).  The result of Log-Rank test analysis showed that the kinds of onlays (according to the kinds of tooth defects), tooth vitality and tooth crack had no in uence on the survival rate of restorations ( Fig. 1, 2, 3).
The result of Log-Rank test analysis also showed that tooth vitality and tooth crack had no in uence on the survival rate of O onlays (Fig. 4, 5).

Results of questionnaire on satisfactory
According to the results of questionnaire (Table 4), no patients were unsatis ed with the appearance of onlays. Overall satisfaction (very satis ed + satis ed) rate was more than 98% in comfort, and more than 90% were satis ed with the functional recovery. ). There were 5 onlays (7.69%) with grade C, including 2 cases with food impaction and 3 cases with the tooth fracture, while there were 7 onlays (7.61%) with grade C in PO group.
The kaplan-Meier survival rate analysis showed that the survival rate of O group and PO group decreased at the second year, but remained at a high level till the fourth year (exceeding 95%), which illustrated satisfactory survival rate in line with other studies [19,20]. Although Georgia et al. [3] demonstrated that the mean survival rate of onlays and crowns were 93.50% and 95.38% respectively, some medium-term studies (2-5 years) indicated a survival rate of 91-100% of onlays [21]. In this study, the survival rate of onlays is 96.99%, which is comparable to that of full crown.
Log-rank test analysis showed that there was no signi cant difference in the survival rate among different kinds of onlays (O, PO and BO / LIO). This means that occlusal, proximal-occlusive and buccal-/ lingual-defects can all be repaired by lithium disilicate glass-ceramic onlay with ideal survival rate. Tavarez et al. [22] concluded that the preservation of the residual tooth tissue is the decisive factor for the fracture strength of the teeth. Compared with full crown restoration, the residual tooth tissue can be preserved to the greater extent by using onlay, which means onlays are more suitable for tooth defects. Interestingly, some previous studies even reported a better clinical performance of onlays than full crown, especially in survival rates [23,24].
Two technical complications were observed from this study. One is onlay fracture. Ceramic fractures represented the most common technical complication [3]. Fracture of restoration can be caused by insu cient thickness of onlay. Murgueitio et al. [25] discovered that the thinner the thickness, the higher the risk of restoration fracture, when the thickness was less than 2mm. Della et al. [26] showed that the microcracks generated during polishing may expand under the excessive bite force, which will eventually lead to the fracture of the prosthesis. This fractured onlay happened in PO group in this study. The patient was a middle-aged man, and the tooth is a mandibular molar, thus excessive bite force may be the reason for this failure. When compared to full crowns, onlays showed better fracture resistance. Beier et al. [24] found that 64.86% of failures of crowns were attributed to ceramic fracture, and only 20.00% were of onlays' failures. Another case failed for onlay debonded after being repaired for 1 year in PO group. When the margin of tooth defects was subgingival, gingival crevicular uid or saliva can contaminate the adhesive interface during the bonding process, which lead to decreased bond strength and nally restoration debonded. Thus, using rubber dam to prevent saliva contamination, and cleaning lithium disilicate glass ceramic surface with orthophosphoric acid or re-etching with hydro uoric acid can reach reliable bond strength [27].
Biological complications observed in this study were three teeth fracture failures, which are all happened in cracked teeth in O group. One is vital premolar, and the rest two are ET molars. But Log-rank test analysis showed that there was no statistical signi cance in survival rate of between the vital teeth and ET teeth, as well as cracked and uncracked teeth. The traditional viewpoint is that after endodontical treatment, the resistant strength of the tooth will decrease and easy to crack due to poor vascular nourishment. Although the repair is successfully completed, the risk of fracture is still higher than that of vital teeth [28]. Manhart J et al. [29] suggested that pulp vitality is not necessarily the main factor leading to restoration failure. The survival rate was signi cantly higher (97%) for cracked teeth receiving a full crown after endodontic treatment [30]. Nevertheless few studies reported the survival rate of cracked teeth receiving ceramic onlays. More studies are needed to reveal the long-term survival rate of cracked teeth with ceramic onlays. Fabbri et al. [31] showed tooth fracture accounted for 7.14% for crowns' failure, and 25.00% in the study of Barnes et al. [32] Reich et al. [33] showed that tooth fracture accounted 4.54% for onlays' failure. These studies demonstrated that less tooth fracture occurred in onlays than in full crown, which can be attributed to that onlay restoration preserved more tissue and provided teeth with better fracture resistance.
Food impaction was observed in 7 cases, in which there are two O onlays and ve PO onlays. The two O cases and three of ve PO cases were horizontal food impaction. The proximal contacts were of suitable tightness when checked by dental oss. The impaction may be related to the atrophy of gingival papilla between teeth. Only two cases were shown to have loose contact to the adjacent teeth in PO group, thus leading to vertical food impaction. When tooth defects were restored by onlays, most often onlays can preserve original adjacent relationship to the largest extent for the margin of onlays was located in the occlusal 1/3 of tooth. But during the tooth preparation of full crown, the original mesio-and distaladjacency of the tooth were all destroyed. Food impaction can also result in secondary caries and caries accounted for 13.50% of the total failures of crowns [24]. The margin of onlays is visible, and easier to keep hygiene. Thus, secondary caries occurs rarely following onlay restoration.
In the survey of patient satisfaction, no patients were unsatis ed with the appearance of onlays. Except the 5 failure cases, only two patients were unsatis ed in comfort, one for severe food impaction and another for the pain during chewing. And 7 cases showed that the food impaction has in uence on functional recovery, but the satisfactory rate was still over 90%. These all showed satisfactory performance of onlays in appearance restoration, comfort and function recovery. Furthermore, when introducing the restoration methods to patients, most of them tend to choose onlay which is more minimally invasive, less tooth tissue preparation.
In addition, the most often used material for posterior full crown is monolithic zirconia [34,35]. Previous study suggested that enamel antagonized against monolithic zirconia resulting in more height loss when compared to lithium disilicate glass ceramic [36]. Thus, lithium disilicate glass-ceramic onlays also showed better performance in antagonist-friendly aspect.

Conclusions
Different kinds of tooth defects can be repaired by lithium disilicate glass-ceramic onlays and achieve ideal outcomes and patient satisfaction. In this study, the lithium disilicate glass-ceramic onlays showed great clinical performance and survival rate, which suggested that onlays are worth more extensive use as full crown for the treatment of tooth defects, with high survival rate and patient's satisfaction.

Declarations
Availability of data and materials The datasets used and analysed during the current study are available from the corresponding author on reasonable request.