Clinical Outcomes and Predictors of Satisfaction in Patients with Improved Lithium Disilicate All-Ceramic Crowns

• In this study, lithium disilicate (LD) crowns had a long survival rate at 3 years and provided good aesthetic replacement for the lost tooth structure. Clinical oral hygiene habits including brushing, flossing, and regular dental visits were predictors of the clinical survival of LD crowns.

Lithium disilicate (LD) ceramics have revolutionized all-ceramic restorations by enhancing the mechanical and aesthetic properties of glass-based ceramics in dentistry [1]. LD ceramics consist of a glassy matrix of silica through which lithium oxide crystals are dispersed. The crystals are oriented in an interlocking manner that inhibits the propagation of cracks and provides flexural strengths of up to 440 MPa [2]. In addition, LD ceramics can be bonded adhesively to the tooth structure through surface treatments (hydrofluoric acid) and chemical interactions (silanes), thereby allowing them to be used as conservative aesthetic restorations with improved mechanical strength [3]. The LD restorations are chemically stable and show excellent compatibility with surrounding periodontal tissues [4]. In addition, due to their excellent optical properties, LD-based aesthetic rehabilitations also enhance [5] the patient's self-esteem.

LD crowns are widely used to restore anterior teeth due to their excellent aesthetic properties [6]. Nevertheless, the survival rate depends on a number of factors such as marginal adaptation, anatomic form, and retention [7]. In a study by Yu et al. [8], the cumulative failure rate of LD crowns was 3.3% involving ceramic chipping and fracture. Fracture of the core ceramic has also been reported [9] as an important reason of failure.

The clinical success of management with LD restorations is related to the quality of the prosthodontic work, aesthetic colour matching, restorative fit, functional ability of restorations, cleansability of the crowns, and maintenance of oral hygiene [10,11]. The most common oral diseases such as dental caries and periodontal disease are considered to be behavioural diseases, as healthy oral habits are critical for controlling oral infections [11]. Traditionally, good oral health practice consists of self-care habits such as dental hygiene, restriction of sugar intake, use of fluoride products, and utilization of dental services like oral health education and professionally applied preventive measures [12]. Maintenance of optimum oral hygiene ensures good health of soft and hard tissues associated with restorations, in turn improving their clinical success and prognosis [9].

An important aspect of clinical success in patients receiving LD restorations is patient satisfaction. Assessment of satisfaction outcomes allows for a direct appraisal of patients' opinions and feelings towards different aspects of the prosthodontic rehabilitation. Patient satisfaction with LD ceramic treatments is effected by the improvement in their oral health and aspects of their quality of life (such as function, comfort, and aesthetics) [13]. Previous studies [13,14] have assessed and reported patient satisfaction for all-ceramic restorations with respect to oral hygiene and satisfaction of treatment. However, there are no studies reporting predictors of patient satisfaction with LD ceramic restorations and their clinical outcomes in a Malaysian population. Therefore, the aim of this study was to investigate the clinical performance of LD single crowns and to determine the predictors of satisfaction in patients restored with LD all-ceramic crowns among a Malaysian population.

The study protocol was approved by the Medical Ethics Committee of the Faculty of Dentistry, University of Malaya, Kuala Lumpur, Malaysia. This cross-sectional survey was conducted from January to June 2016, among patients who had been provided with LD-based core, IPS e.max Press crowns at the Postgraduate Clinic, Faculty of Dentistry, University of Malaya.

The sample size was calculated through calibration data obtained from 6 patients with 11 crowns before the commencement of the study. The sample size calculation showed that 26, 31, 41, and 94 pairs of subjects were needed to reject the null hypothesis of dental plaque, pocket depth, gingival recession, and bleeding, respectively, with 80% power. The alpha level was set at 0.05.

The inclusion criterion was LD ceramic (IPS e.max Press) crowns from graduate students in medically fit patients. Exclusion criteria were crowns made from other material, severe periodontal disease, parafunctional habits, and temporomandibular joint disorders.

Forty-seven patients completed the questionnaire (Appendix A) [15]. The questionnaire assessed sociodemographic characteristics, oral health habits, characteristics of the restorations, and overall satisfaction. Written informed consent was obtained from all the participants.

A single calibrated examiner (M.S.S.) examined the crowns. Intra-examiner calibration was done using the modified United States Public Health Service (USPHS) evaluation criteria (Table 1) [16]. The kappa value of all the parameters that had been examined on the participants was greater than 0.8 (0.89). All the crowns were evaluated for biological and technical complications. Pulpal and periapical conditions were clinically examined and investigated using digital periapical radiographs. Retention and fit of the crowns were detected by the rating criteria (fit: 0, unfit/mobile: 1). Crown colour match was determined using the VITAPAN classical shade guide.

Forty-seven patients (31 females and 16 males) with 88 LD crowns (79 anterior and 9 posterior) were included in the study. The age of the participants ranged from 18 to 64 years.

A total of 88 teeth were crowned due to the following clinical indications: aesthetic inadequacy (n = 24), tooth crown fracture (n = 19), secondary caries (n = 17), defective restoration (n = 9), primary caries/pain (n = 9), crown replacement (n = 6), and diastema (n = 4). These crowns had been cemented with self-adhesive resin cement (RelyX™ U200). In addition, among all (n = 88) crowned teeth, 19 were vital and 69 were non-vital. Sixty-eight non-vital teeth were restored with prefabricated fibre posts (RelyX™, 3M ESPE) and composite cores (Filtek™ Z350 XT, 3M ESPE), and 1 tooth was restored with glass ionomer cement core (Fuji IX, GC, Tokyo Japan). Overall patient characteristics are presented in Table 2.

Kaplan-Meier survival analysis was performed. Differences between crowns and controls were estimated using the Wilcoxon signed-rank test. The Student t test and analysis of variance (ANOVA) were used to compare means across variables. Correlation analysis was used to assess the association between continuous variables. A multiple linear regression model was used for multivariate analysis.

Of the 88 LD crowns assessed, the survival rate was 96.6% (n = 85) after a mean evaluation period of 35.9 ± 9.2 months. Fractures (failures) were recorded in 2 (2.2%) crowns (major chipping) on the palatal surface of a non-vital maxillary incisor 32 months after insertion and on the occlusal surface of a non-vital maxillary second premolar, which occurred 40 months after insertion. One crown (1.13%) exhibited minor chipping on the incisal edge of a root-treated maxillary central incisor. Of the 88 crowns, 20 (22.7%) and 13 (14.8%) exhibited explorer catches with no caries on the labial and palatal margins, respectively; 16 (18.2%), 12 (13.6%), and 3 (3.4%) crowns exhibited minor colour mismatch, slight over-contour and minor fractures, respectively. One (1.1%) of the fractured crowns exhibited a delta rating: “obvious crevice at margin, dentine exposed.” Postoperative sensitivity, retention, and secondary caries exhibited a 100% alpha rating in this group of subjects (Table 3). The clinical survival rate was 100% at 24 months (Fig. 1). The location of the crown had no significant effect on the crown survival rates (p = 0.17) (log-rank test).

The mean age of crowns was 34.7 ± 9.7 months. Significant associations between success variables and sample characteristics are shown in Table 4. There was a significant association between chewing ability satisfaction and tooth brushing frequency (p˂ 0.001), dental visit regularity (p = 0.006) and flossing (p = 0.009). A strong negative correlation was observed between aesthetic satisfaction and age of crowns (r = -0.717, p˂ 0.001).

In multivariate analysis, for cleansability satisfaction, the significant predictor was flossing frequency (p = 0.006). For all 6 satisfaction domains, the significant predictors were tooth brushing frequency (p <0.001), dental visit regularity (p = 0.021), and age of crowns (p = 0.006). There was no multi-collinearity between the variables (Table 5). In multivariate analysis, there were no significant predictors for overall satisfaction, and the total model was also not significant (p = 0.403).

In this study the LD crowns showed overall satisfactory clinical performance (low risk of failure). In addition, oral hygiene habits showed significant influence on patient satisfaction with LD crowns.

The survival rate of 96.6% at 35.9 ± 9.2 months in the present study was similar to those reported in previous studies (95.4 and 97.8%) [17,18]. A possible explanation for the high survival rates in the present study was the location of crowns, as 89.7% of all LD crowns were on anterior teeth. A high failure rate of up to 8.2% for posterior LD single crowns, in comparison to 3.2% for anterior teeth, has been reported [19]. In the present study, 2 crowns showed major chipping and 1 crown showed minor chipping. The major chipping observed in non-vital teeth at 32 and 40 months, respectively, could be due to lack of proprioception [20]. Only 1 crown exhibited minor chipping on the incisal edge of a root-treated maxillary central incisor. Major and minor chipping is the most common form of failure in layered LD restorations. In a study by Yang et al. [19,] 41.2% of failures among LD restorations was due to veneer ceramic chipping. In the present study all crowns were layered, therefore the fabrication technique of LD ceramics (layered) could possibly have caused the failures observed in the study. As a consequence, monolithic LD ceramic restorations are increasingly investigated for their mechanical properties and are introduced clinically to avoid veneer fracture, hence improving clinical outcomes [21].

In the present study, patients were satisfied with the aesthetics and functional performance of the LD restorations provided. In addition, a self-adhesive resin luting cement designed to be light-cured was employed, making the cement more colour stable. It has been reported that although all-ceramic restorations reproduce highly aesthetic outcomes [22,23], it depends on an adequate application of techniques and selection of cement type [24,25]. Moreover, patient satisfaction with LD crown treatment was also found to be associated with level of education and age of crowns along with oral hygiene predictors. The aesthetic and functional satisfaction findings of the present research are in agreement with previous findings [26,27].

This was a retrospective study, and therefore it limited the ability of the investigators in controlling clinical techniques, which vary among different operators. In addition, postgraduate students, and not experts, operated on all the included patients, possibly influencing the clinical outcomes of the LD crowns. In addition, the subject numbers at recall visits were low. However, low response rates do not necessarily compromise the results of population surveys unless systematic differences between participants and non-participants are observed [28]. An important finding was the excellent aesthetics outcome with the use of light-cured resin cement and layered LD restorations. In addition, oral health habits were significantly associated with aesthetic satisfaction. In a general perspective, the tradition of regular self-care practices in the Malaysian community was high; around two-thirds of the respondents brushed their teeth more than once a day and another one-third claimed tooth brushing less than once a day. In contrast with studies carried out in Scandinavia [29] and Latvia [30], oral hygiene habits of Malaysian people were not influenced by gender and level of education. It is worth noting that self-care practices in relation to aesthetic appearance and functional satisfaction in oral health tend to be more frequent in dental attenders than non-attenders. This study recommends that dentists should educate patients on oral hygiene habits associated with treatment success with LD crowns. Furthermore, clinical outcomes of monolithic LD restorations should be assessed by undergoing randomized controlled trials.

The LD crowns provided satisfactory clinical performance (low risk of fracture) and had a survival rate of 96.6% for a follow-up period of up to 55 months. Moreover, oral health habits such as brushing, flossing, and regular dental visits influenced patient satisfaction with LD crowns.

The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through the research group (RGP-1438-024). The authors also appreciate the work of the technical laboratory staff at the Faculty of Dentistry, University of Malaya.