The restoration of large posterior cavities is always a challenge, due to the high occlusal forces and functional stresses continuously exerted on the posterior teeth [1]. The widespread use of direct resin composite for posterior restorations is associated with several desirable properties, including the esthetics, conservative preparation, adhesive bonding and ability to reproduce natural tooth anatomy [2,3]. Traditional composite restorations, however, can have the disadvantages of polymerization shrinkage, polymerization stress, marginal adaptation issues, post-restorative sensitivity, and fracture, especially in large cavities exposed to significant occlusal forces [4].

Mechanical properties, polishability, wear resistance and esthetic appearance of conventional nanohybrid composites consist of a combination of nano- and larger filler particles [2,5]. Although these benefits exist, their incremental placement technique can be clinically time-consuming and can be technically challenging in deep and/or large posterior cavities [6]. Further, the polymerization shrinkage and related stresses arising during curing are significant factors, especially in the case of restorations, where the configuration factor is high and the material does not flow during polymerization [4].

In order to make the restorative procedure easier, bulk-fill composite materials were developed so that they can be placed in thicker increments, typically around 4-5 mm, depending on the material and manufacturer guidelines [7].

These materials have been modified in resin chemistry, photoinitiator systems, translucency and filler composition to allow sufficient penetration of light for adequate polymerization [8]. More recently, the development of fiber reinforced bulk-fill composites [9,10] has been introduced. Short reinforcing fibers in the resin matrix could have an effect on stress dispersion, fracture resistance, crack propagation resistance, and resistance to functional loading. These properties might be especially important in the posterior teeth with large cavities and high occlusal loads [11,12].

While laboratory studies have shown good mechanical properties in fiber-reinforced composite materials, there are various factors that affect the clinical performance such as cavity size and shape, bonding techniques, occlusal loading, moisture control, operator skill, and patient variables. Thus, direct clinical comparison to conventional nanohybrid composites should be performed to understand whether the theoretical mechanical benefits of fiber reinforcement are associated with better clinical results. Postoperative sensitivity, marginal integrity, form, surface characteristics, secondary caries, wear and fracture of a restoration may yield clinically relevant data relating to the performance of the restoration over time.

Research Objective

To clinically compare the performance of fiber-reinforced bulk-fill composite materials with conventional nanohybrid composite restorations in high-stress posterior teeth by assessing postoperative sensitivity, marginal integrity, anatomical form, surface characteristics, secondary caries, wear, and restoration fracture during the study follow-up period.

Study Design, Setting and Duration A prospective, parallel group comparative clinical study was carried out in the Department of Dentistry, Pakistan. This study was performed for a 12-month period (May 2025 to April 2026) with an enrollment of participants for about 5 months and a clinical follow up period of 6 months. Study Population and Eligibility Criteria The study population consisted of adult patients with composite restorations indicated in high-stress posterior teeth, and with a medium to large sized class II or MOD cavity, particularly in the premolars and molars. Patients over 18 years old whose vital posterior teeth, oral hygiene was good and they were willing to follow-up were included. Patients excluded included those with uncontrolled systemic disease, poor oral hygiene, active periodontal disease, bruxism, and severe occlusal parafunction, poor moisture control, or a lack of follow-up visits. Sample Size and Sampling Technique The number of restorations included were a total of 132 (66 restorations in each group) with around 10% attrition. Eligible patients attending the department during the study period were retrieved using consecutive sampling method until the sample size was fulfilled. A computer-generated random sequence was used to randomly allocate eligible restorations to either of the two treatment groups (fiber-reinforced bulk-fill composite or conventional nanohybrid composite materials) in 1:1 ratio. Study Materials and Operative Procedures Two restorative materials were tested: a fiber-reinforced bulk-fill composite (BFC) and a standard nanohybrid composite (NC). The cavity preparation, isolation, adhesive application, placement of composite, light curing, finishing and polishing procedures were standardized and conducted by a calibrated operator based on the respective manufacturers' recommendations. The prepared cavities in Group A were filled with the chosen fiber-reinforced bulk-fill composite as indicated in the manufacturer's instructions for placement and curing. In Group B, the prepared cavities were restored using the selected conventional nanohybrid composite with the recommended incremental placement and curing technique. For both groups, where appropriate, the same type of adhesive used, and the same curing unit. Postoperative Instructions and Follow-up Patients were given regular post-operative suggestions for oral care, diet, and complaints of sensitivity, pain, fracture and other restoration complications. Clinical evaluation was conducted at baseline, 1 month, 3 months and 6 months after the restoration was placed. Clinical Outcome Assessment The restorations were clinically assessed according to the following clinical criteria: postoperative sensitivity, marginal integrity, anatomical form, surface characteristics, secondary caries, wear, and restoration fracture. The main outcome was overall clinical performance of the restoration at 6 months, with secondary outcomes being postoperative sensitivity, marginal integrity, anatomical form, surface characteristics, secondary caries, wear, and fracture of the restoration. Examiner Calibration, Data Collection and Management The clinical examiner was calibrated prior to the beginning of the study and a subsample of restorations was re-evaluated to assess intra-examiner reliability. A proforma was used to identify the data which were collected and entered into a password-protected database. The confidentiality of the participants was respected during the study. Statistical Analysis Data were analysed using SPSS version 26.0. Continuous variables were summarized as mean ± SD or median (IQR) as appropriate, and categorical variables were summarized as frequencies and percentages. As the distribution pattern of data was different appropriate parametric or non-parametric tests were used. Categorical outcomes were analysed using a chi-square test or Fisher's exact test; outcomes over time were analyzed using repeated-measures analysis. A p value of < 0.05 was deemed statistically significant.

The two groups were comparable at baseline, with similar mean age (38.7 ± 9.6 vs. 39.4 ± 10.2 years; p=0.683), gender, tooth type, cavity type and extent, arch, and number of restored surfaces (all p>0.05), shown in table 1. Postoperative sensitivity decreased in both groups from baseline to 6 months, with 1.52% of fiber-reinforced and 6.06% of nanohybrid restorations remaining sensitive at 6 months (p=0.169). At 6 months, clinically acceptable marginal adaptation was observed in 92.42% vs. 84.85% (p=0.140), anatomical form in 93.94% vs. 87.88% (p=0.191), surface characteristics in 95.45% vs. 89.39% (p=0.244), and absence of secondary caries in 96.97% vs. 92.42% (p=0.308), respectively. Wear was unacceptable in 6.06% vs. 15.15% (p=0.120), while fracture occurred in 3.03% vs. 9.09% (p=0.121).

Table 1. Baseline Characteristics, Postoperative Sensitivity, and Clinical Performance of Restorations

Note: Values are presented as mean ± SD or frequency (%). Independent-samples t-test was used for normally distributed continuous variables. Chi-square test was used for categorical comparisons, while Fisher's exact test was used where appropriate. A p-value <0.05 was considered statistically significant.

At 6 months, successful restorations were recorded in 61/66 (92.42%) of the fiber-reinforced bulk-fill group and 55/66 (83.33%) of the conventional nanohybrid group, while failures occurred in 5 (7.58%) and 11 (16.67%), respectively, shown in table 2. The difference in overall clinical success was not statistically significant (χ²=2.49, p=0.115).

Table 2. Overall Clinical Success of Restorations at 6 Months

Postoperative sensitivity scores decreased progressively in both groups. Scores were 2.1 ± 1.1 vs. 2.2 ± 1.2 at baseline (p=0.611), 1.2 ± 0.8 vs. 1.4 ± 0.9 at 1 month (p=0.186), 0.6 ± 0.5 vs. 0.9 ± 0.7 at 3 months (p=0.006), and 0.3 ± 0.4 vs. 0.6 ± 0.6 at 6 months (p=0.001) for fiber-reinforced and nanohybrid restorations, respectively. However, the overall reduction from baseline to 6 months was similar between groups (−1.8 ± 1.1 vs. −1.6 ± 1.2; p=0.324) ,shown in table 3.

Table 3. Changes in Clinical Outcomes Over the 6-Month Follow-up Period

At 6 months, clinically successful restorations were 92.42% in the fiber-reinforced group versus 83.33% in the nanohybrid group (p=0.115), shown in table 4. Marginally unacceptable restorations were 7.58% vs. 15.15% (p=0.140), anatomically unacceptable restorations 6.06% vs. 12.12% (p=0.191), unacceptable surface characteristics 4.55% vs. 10.61% (p=0.244), secondary caries 3.03% vs. 7.58% (p=0.308), excessive wear 6.06% vs. 15.15% (p=0.120), and fractures 3.03% vs. 9.09% (p=0.121), respectively. None of these categorical differences reached statistical significance.

Table 4. Comparison of Overall Restoration Performance Between the Two Groups

Note: Categorical variables were compared using Chi-square or Fisher's exact test as appropriate. A p-value <0.05 was considered statistically significant.

In the current study, the clinical performance of the two types of restorations (fiber-reinforced bulk fill and conventional nanohybrid) was broadly comparable at 6 months follow-up. The overall clinical success in the fiber-reinforced group was 92.42%, with 83.33% in the nanohybrid group, but with no statistical significance (p=0.115). The results are similar to the randomized clinical trial, which found no difference between the 3-year survival of bulk-fill and nanohybrid composites, with annual failure rates of 2.4% and 1.8%, respectively (p=0.70). The latter, therefore, should be regarded more as a trend rather than as evidence for superiority and is apparent when fiber reinforcement is used in the present study [13]. Sensitivity decreased gradually after surgery in both groups. There was no significant difference between 6 months sensitivity (1.52% for fiber-reinforced and 6.06% for nanohybrid restorations, respectively, p=0.169). Although the overall reduction from baseline was similar (−1.8 ± 1.1 vs. −1.6 ± 1.2; p=0.324), the continuous sensitivity scores were significantly lower with fiber reinforcement at 3 months (0.6 ± 0.5 vs. 0.9 ± 0.7; p=0.006) and 6 months (0.3 ± 0.4 vs. 0.6 ± 0.6; p=0.001). In previous study, no sensitivity was observed in either bulk-fill and nanohybrid after 12 months, which is in line with the generally positive pulpal response of modern posterior resin composite [14]. There was no significant difference between the two groups when it came to marginal adaptation, since 92.42% of the fiber-reinforced restorations and 84.85% of nanohybrid restorations showed clinically acceptable marginal adaptation (p=0.140). The anatomical form was acceptable in 93.94% and 87.88% (p=0.191) respectively. The results suggest the fiber-reinforced material was better, but neither of the results were statistically significant. This is consistent with the systematic review and meta-analysis by Hamdi et al., (2025) who reported no statistically significant difference between short fiber-reinforced and conventional resin composites for clinical parameters, such as marginal adaptation [15]. Similar results were obtained also for surface quality and secondary caries. Ninety-five percent of the fiber-reinforced and 89.39% of the nanohybrid restorations exhibited clinically acceptable surface characteristics (p=0.244) and secondary caries occurred in 3.03% and 7.58%, respectively (p=0.308). The outcome of the study is similar to that of Sekundo et al. (2022) that found no significant difference between the functional, biological, and esthetic parameters of bulk-fill and nanohybrid restorations 3 years after cavity preparation [13]. Analogously, there was no difference between the bulk-fill and nanohybrid restorations regarding marginal adaptation or secondary caries in 12 months (Colak et al., 2017) [14].Mechanical complications with unacceptable wear were higher in the nanohybrid restorations (15.15%) than the fiber-reinforced restorations (6.06%) (p=0.120), and for fracture, 9.09% versus 3.03%, respectively (p=0.121). While these differences were not significant, the numerical differences in wear and fracture with fibre reinforcement were clinically relevant in the high stress posterior cavities. Recent studies have shown that short fibers could be beneficial in terms of stress distribution and fracture resistance, but clinical reports are varied, and generally are not consistent in their results of superiority over traditional composites [16,17]. The results of the present study are generally favorable for the use of fiber-reinforced bulk-fill composite as a clinically acceptable material for use in high-stress posterior restorations, but the results do not show that fiber-reinforced bulk-fill composite is statistically superior to conventional nanohybrid composite at 6 months. The outcomes are similar to those recently reported in clinical trials, in which fiber-reinforced and conventional restorative materials yielded similar results. Relatively few events and a short follow-up period may have limited the detection of differences in relatively rare events (fracture and secondary caries). Thus, further research with a randomized clinical design is needed to assess the long-term consistency of the significant number change trends seen in sensitivity, marginal adaptation, wear and fracture [18]. Strengths and Limitations The main advantage of this study was its prospective, randomized comparative design, and the application of standardized cavity preparation, adhesive procedures, curing protocols and clinical follow-up to allow direct comparison between fiber-reinforced bulk-fill and conventional nanohybrid composites in high-stress posterior teeth. Multiple clinically relevant outcome measures (postoperative sensitivity, marginal adaptation, anatomical form, surface characteristics, secondary caries, wear, and restoration fracture) allowed for comprehensive assessment of the performance of the restorations. Calibration and assessment by the examiner at several intervals after the initial evaluation further improved the reliability of the results. The study, however, has some shortcomings. The number of follow-ups in the relatively short 6 months limits evaluation of long-term deterioration, fracture, wear, and secondary caries; and the sample size may not have been large enough to identify statistically significant differences in relatively uncommon complications. Additionally, it is a single center study with a limited clinical population and so may not be generalizable. Further multicenter randomized clinical trials with larger numbers of patients and longer follow-up are thus warranted.

The clinical performance of the FRC bulk-fill and conventional nanohybrid composites was acceptable for restorations in high-stress posterior teeth. Fiber-reinforced bulk-fill restorations exhibited positive outcomes in terms of some clinical parameters, with the exception of post-operative sensitivity, which were statistically superior, and marginally adapted, which were statistically comparable, but there was no overall statistical superiority.Positive trends were obtained in several clinical parameters such as postoperative sensitivity, marginal adaptation, wear, and fracture, but these parameters failed to achieve overall statistical superiority. Both restorative methods seem to be clinically viable, with more extended research required to see whether the possible benefits of fibre reinforcement become apparent in the long term.