Wear of nanohybrid and microfilled composite resin in occlusal restoration of first permanent molar tooth

Abstract

This study compared the wear of nanohybrid and microfilled composite restorative material in occlusal restoration of first permanent molar tooth. In total, 60 first permanent molar teeth having carious lesion without any clinical and radiological indication of pulpal involvement, removal of carious dental hard tissues was performed using round carbide bur and a class I cavity was prepared, rinsed with water and then dried with gentle air. These cavities were filled with either nanohybrid or microfilled composite resin by simple random sampling by lottery method. All teeth were subjected to clinical qualitative and quantitative wear test at baseline, 3, 6 and 9 months observation period. The quantitative wear was analyzed by profile meter. The results showed that the wear depth of microfilled was significant than the nanofilled composite resin. It can be concluded that quantitative wear of microfilled had greater wear than that of nanohybrid composite restorations.

Introduction

The progressive loss of restorative materials from contacting surfaces relative in motion as a consequence of the interaction between surfaces moving in contact, causing gradual removal of material which is usually termed as wear.¹ In the oral cavity, many components are responsible for the wear of enamel and dentin as well as restorative material by chewing on food items (attrition), tooth brushing with toothpaste (abrasion), fatigue and corrosive effects or by acid attacks due to consumption of acidic fruits and beverages (erosion).² Wear of restorative material also results in crack formation, increase surface roughness and fracture.²

The wear resistance is an important property to be evaluated in materials indicated for posterior tooth. The restoration should not only be satisfactory at the time of placement but it should also remain this way over time. In many cases, the abrasive agent abrades the resin matrix and expose fillers, which may then increase the surface roughness of composite resin restoration. The increase of surface roughness causes accumulation of bacterial biofilm, pigments and food debris, which reduces the longevity of restoration.³ Therefore, wear of composite restorative material is known to depend on filler particle-related features, particularly on the content and size of the filler reinforcement,⁴ and resin formulation. ⁵ Finer filler particles decreased interparticle spacing and thereby reduced wear.^6-7 Regarding filler content, some in vitro studies have revealed that increased the filler content may enhance the wear resistance of dental composite.^8-10 On the other hand, increasing the resin content generally lowers the wear resistance.¹¹

First-generation macrofilled composite resin has inferior wear characteristics. The surface becomes rougher as the resin matrix being less hard and wears at a faster rate. After some time, the particles were lost, probably due to either insufficient support of the filler particles from the resin matrix or due to differences in thermal coefficient between fillers and resin.^12-13

The introduction of microfilled composite resin improved the occlusal wear resistance.¹⁴ Because of less filler content, some of their physical properties are inferior. They have high polishability, low fracture toughness and increased marginal breakdown. However, relatively low tensile strength and modulus of elasticity than those of microfilled material that may lead to the development of fatigue cracks in the resin matrix.¹⁵

Recently, several new generation restorative materials such as minifilled, hybrid and nanohybrid composite resin have been introduced based on the philosophy that a high content of small filler particles improves the wear resistance,¹⁶ and their use in Class I, II, III restorations has been expected in the dental clinic.¹⁷ Furthermore, it is highly polishable and has high wear resistance too.¹⁶ However, previous studies did not analyze the clinical quantitative wear of nanohybrid composite. Therefore, this study was performed to compare the clinical qualitative and quantitative wear of nanohybrid and microfilled composite material in occlusal restoration of first permanent molar tooth.

Materials and Methods

The study population was comprised of participants having a shallow carious lesion in vital permanent teeth irrespective of sex. The inclusion criteria were as follows: The first permanent molar tooth having shallow carious lesion (Figure 1A) ranged between 1.5 to 2.5 mm in depth and the age of the participant fixed to 18 to 40 years. The total 60 teeth were divided into two groups by randomized sampling technique (lottery method): Group I: 30 teeth for nanohybrid composite restorative material and Group II: 30 teeth for microfilled composite restorative material.

Figure 1: Representative photographs of nanohybrid composite resin. Initial (A); Cavity preparation (B); Application of nanfilled (C); One year follow-up (D)

Treatment procedure

Class I cavity was prepared following removal of carious dentin by a round diamond bur attached with a low-speed turbine handpiece for removal of carious tissue. The depth of the lesion was dictated by the size of the lesion (Figure 1B). The cavity was restored with one of the material according to manufacturer instructions as follows: All the prepared enamel surface and cavosurface margins of the prepared cavity were acid-etched with 37.5% phosphoric acid for 15 sec, washed with normal saline and dried by oil-free airflow. The primer was applied and wait for 10 sec followed by a bonding agent was applied to the internal surface of cavity utilizing a fine sable brush. According to the manufacturer's instructions, the bonding agent was left in the cavity for 30 sec, then air-dried and light-polymerized for 20 sec with the tungsten halogen light. The resin composite (ivoclar vivadent’s Heliomolar) was placed in the cavity layer by layer and polymerized (Figure 1C). The resin composite was polymerized in maximum layers of 2 mm for 40 sec. Finishing of the restoration was carried out by adapting occlusion and articulation using fine-grit diamonds. Finally, the composite restoration was polished using Super snaps polishing kit.

Evaluation

An impression was taken by alginate impression material and a model was made from the impre-ssion by hard plaster at baseline and the patient was advised to maintain good oral hygiene. Each patient was recalled at 3, 6 and 12 months (Figure 1D). The visual examination of the restoration was done by proper exposure of light using mirror, caries probe, magnification of loupe and the condition was noted.¹⁸

Anatomical form of the restoration was assessed by visual inspection as follows: Restoration’s contour is continuous with existing anatomical form and margin (alpha), restoration is slightly over-contoured or under contoured (Bravo), marginal overhang or tooth structure (Charlei). The surface staining was performed as: A: stain present, B: stain absent. An impression followed by model prepara-tion was done in each observation period. Anatomical form was assessed by visual inspection. Restoration’s contour is continuous with existing anatomical form and margin. The profilometer was used for quantitative analysis of wear in each observation period. After completion of the restoration, the data were collected at baseline, 3, 6 and 9 months interval by a technician who did not know the true nature of the research. Thus, the evaluation was a blinded method.

Statistical analysis

The collected data was analyzed by computer based statistical software, Statistical Package of Social Science (SPSS) version 19 (SPSS Inc. USA). The result was expressed as mean ± SD (Standard deviation). Student’s t-test was applied to assess the difference between wear of nanohybrid and microfilled composite restoration and 95% confident interval (p value <0.05) was followed for the testing level of significance

Results

Table I shows the result of anatomical form that 29 (96.7%) nanohybrid and 27 (90%) microfilled revealed alpha rating at the end of 6 and 9 months, which was not statistically significant. Furthermore, the results of surface staining of 30 nanohybrid and 28 microfilled composite resin restoration demonstrated alpha rating (no stain) at the end of 6 and 9 months. The differences between the two groups were not statistically significant. It was found that 29 nanohybrid and 27 microfilled revealed alpha rating at the end of 9 months, which was not statistically significant. Furthermore, the results of surface staining of 30 nanohybrid and 28 microfilled composite resin restoration demonstrated alpha rating (no stain) at the end of 9 months and the differences between two groups were not statistically significant.

Table II shows the quantitative analysis of wear. It was found that vertical loss of nanohybrid composite resin versus enamel was 42.7 ± 1.2 µm at 3 months followed by 53.8 ± 1.0 µm at 6 months and 66.5 ± 0.9 µm. On the other hand, vertical loss of microfilled composite was 44.8 ± 1.6, 61.4 ± 1.5, 82.1 ± 1.8 µm at 3, 6 and 9 months, respectively. Furthermore, there were significant differences between nanohybrid and microfilled with respect to vertical loss (wear).

Discussion

In the present study, academic related factors were identified as maximum stress provoking (95% confidence interval of the mean scores 1.9 to 3.1) than group activity, drive, teaching-learning or social related factors and was identified as more than moderate in form. Of that group ‘falling behind reading schedule’, ‘heavy workload’, ‘not enough scope of medical skill practice’ and ‘large amount of content to be learned’ were identified as high stressors. The examination was identified as close to the heavy stressor and was the severe stressor in 42.4% male of the study population. But the fraction of female was marked less (16.7%). While opining on the activity related factors, 25.4% respondent expressed ‘feeling of incompetence’ as a severe stressor. More than one-fourth of the respondents identified teaching related factors, ‘inappropriate assignments’, ‘lack of guidance from the teacher’, ‘not enough feedback from teacher’ uncertainty of what is expected of students’ and ‘lack of recognition for work done by student’ as severe stressors. About 30% of respondents expressed that ‘verbal or physical abuse by teacher and ‘conflict with the teacher’ were a severe stressor for them. Male students of this study expressed more dissatisfaction (p=0.011) regarding teaching-learning related factors.

Various scales of measuring the stress were evaluated during the preparation of the protocol of present study^8-11 and Medical student stressor questionnaire (MSSQ)⁸ was observed to be more suitable for the target population because of the nature of questions. In a study conducted by the same group of the researcher on the students of phase I and phase II of the same institution observed that in addition to academic related factors respectively intra- and inter-related factors and social related factors were prominent.¹² Probably with the time senior students could adjust with the environment, but could not come out of academic stressors.

Behzadnia et al. (2018) in a study on preclinical as well as on clinical medical students in Malaysia using MSSQ observed ARS as the major stressor and the mean score observed was 2.6 which is little more than the present study. The “tests and examinations” produced severe stress in the maximum population (72.3%)¹³ and is also different from finding of the present study. This may be because of the variation of style of examination and inclusion of preclinical students in the study. Examination as a stressor is different from other academic stressor factors as it rises to its maximum level before examination decreases automatically after examination.¹⁴

After reviewing 23 studies of different countries Lyndon et al. (2014) opined that in spite of the degree of heterogeneity between these studies ‘assessment’ was observed to evoke stress or anxiety and was more so for female medical students.¹⁵ In this study, male students were observed to be in more stress. Of course, the sample size of this study was very small. On the other hand, the high workloads and frequent work shift schedules were also observed as stress provoking in physicians and other health personnel by Beser and Cevik (2018).¹⁶

Students’ self-perception of feeling incompetent and feeling of need to do well were observed to be significant sources of stress in Malaysian study.¹³ An interesting finding came out of the study of Brooks et al. (2018) on primary care residents training at outpatient clinics in the US that they were in stress because of a feeling of inadequately trained as primary care physicians.¹⁷

A study conducted at Karolinska Institute Medical University, Stockholm, Sweden revealed ‘non-supportive climate’ as the stressor.¹⁸ Inadequate feedback, the uncertainty of teachers’ expectation, the inadequate guideline from the teacher and lack of recognition of work done are the usual non-supportive environment.

Verbal abuse or bullying is an existing problem in medical education. The study has proposed an adaption of policy against bullying and harassment in all of the medical colleges to support students.¹⁹ This research revealed that more than one quarter (28.0%) of the surveyed students reported exposure to some sort of bullying during their clinical rotations and among those reported insults, 90% were verbal abuse. A cross-sectional questionnaire survey on final year medical students in six medical colleges of Pakistan showed that 52% of respondents had faced bullying or harassment during their medical education and about 28% of them experiencing it once a month or even more frequently.²⁰The matter also came out as an observation in the present study.

Various social factors are encouraging institutions to impose various rules and restriction. For total academic engagement various facilities, like accommodation, reading space & materials, food and overall feeling of safety is required which is possible in a campus environment, though this study revealed that campus environment was stress provoking especially in the male. Turley and Wodtke (2010) conducted a study on 2,011 students (age ranged from 18 to 25 years) of postsecondary institutions in the United States, of them, 54% lived on the campus and rest outside campus with or without family. The study observed that those who live on the campus had significantly higher GPAs than similar students at the same institution lived out of the campus with family.²¹

Examination related anxiety is the most suffered problem perceived by a student. This is more in medical education because of the peculiarity of the nature of its curriculum. Soon after entering the course with great interest, a fraction of the team losses interest, become backbencher and develops stress related disorders. Persons experiencing emotional suffering usually manifest as feelings of stress, anxiety, depressed mood, and burnout. Alarming issue is that Dahlin et al. (2005) conducted a study in Sweden on 309 medical students of different years and observed that 28.8% of them thought for suicide ever and 8 (2.7%) attempted for suicide because of stress.¹⁸ Many a time stress is not reflected in performance and remains unfocused. Poor correlation was noted between psychological distress and academic performance by study too.²² Sullivan recommendations for conscious efforts to regulate emotion, behavior, physiology and the environment in response to stressful events or circumstances may be an effective preventive measure.²³

Conclusion

Nanohybrid composite resin is superior to microfilled composite resin in respect to quantitative analysis of wear in class I restorations of permanent molar teeth.

Ethical Issue

The research protocol was approved by the committee and permission for the study was taken from the Institutional Review Board of Bangabandhu Sheikh Mujib Medical University (BSMMU/2017/1998).

Acknowledgement

This work was supported by the Research Grant for Student, Bangabandhu Sheikh Mujib Medical Univer-sity.

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