Evaluation of effect of different disposable infection control barriers on light intensity of light-curing unit and microhardness of composite - An in vitro study

Abstract

Aims:

This study evaluated effect of infection control barriers on light intensity (LI) of light-curing unit (LCU) and microhardness of composite.

Materials and Methods:

Four different disposable barriers (n = 30) were tested against the control. LI for each barrier was measured with Lux meter. One hundred and fifty Teflon molds were equally divided into five groups of thirty each. Composite was filled in bulk in these molds and cured without and with barrier. Microhardness was evaluated on top and bottom surface of composite specimen with microhardness testing machine and hardness ratio (HR) was derived.

Statistical Analysis Used:

One-way analysis of variance, Tukey's honestly significant difference test, and paired t-test using SPSS version 18 software.

Results:

All barriers had significantly reduced the baseline LI of LCU (P < 0.0001), but only Cure Elastic Steri-Shield and latex cut glove pieces (LCGP) significantly reduced the microhardness of the composite (P < 0.05). However, HR determined inadequate curing only with LCGP.

Conclusions:

Although entire tested barrier significantly reduced the LI; none, except LCGP markedly affected the degree of cure of the composite.

INTRODUCTION

“To do no harm” is one of the prime ethical considerations that every dental practitioner should be concerned. Failure to sterilize the instrument can cause cross-infection leading to iatrogenic diseases. The exposure of dentist, auxiliaries, and patients to a variety of infectious agents is a potent danger which was recognized but generally overlooked.[1] Cross-infection with the agents causing diseases such as hepatitis B and acquired immunodeficiency syndrome has diverted clinician's attention to the importance of sterilization and thus led to the development of variety of infection control procedures.[1]

Visible light-curing units (LCUs) are indispensable in the practice of clinical dentistry today.[2] They are commonly used to cure composite and other light-activated resin-based restorative materials.[2] A survey published in 1998 showed that 27% of dentists used resin composites almost exclusively for posterior restorations.[3]

The light guide (LG) of LCU has been put under “semi-critical category” in the Centers for Disease Control and Prevention (CDC) guidelines because they are used within the oral cavity, have the potential for saliva or blood contamination, and can cause transmission of infection among the patients.[1,4] Hence, various attempts to sterilize or disinfect the LGs were considered.

Literature suggests four most common methods of maintaining sterility of the LG: (a) wiping the guide with a suitable disinfectant after use in each patient;[1] (b) using autoclavable guides;[5] (c) using presterilized, single-use plastic guides;[6] and (d) using disposable barriers to cover the guide.[7,8]

The first two methods promised excellent infection control but proved to be time consuming and caused irreversible damage to the structures in the LG.[1,5,9] Single-use plastic LG causes a significant reduction in the light intensity (LI) if the sides of clear LG come into contact with the oral tissues.[6]

Disposable infection control barriers (ICB) such as light tip sleeves, plastic wrap, and finger cots may be a cost-effective alternative to avoid contamination of the LG.[8] It provides a convenient, noninvasive method of preventing contact between the oral tissues and the guide[8] and eliminates the risk of damaging.

However, these types of barriers may have its ill effects on the light output of the LCU, inadequate curing, and thus affecting the properties and quality of the light-cured material. Thus, the present study evaluated the LI of LCU after placement of various disposable ICB on LG, its effect on the Vicker's microhardness (HV) of the composite which indirectly reflects the degree of polymerization of composites. The null hypothesis was made that disposable ICB do not affect the LI of LCU and microhardness of composite.

MATERIALS AND METHODS

Bluephase (Ivoclar Vivadent, Liechtenstein), light-emitting diode (LED) LCU with 10 mm standard LG was used throughout this study. A custom jig was prepared to standardize the position of tip of LCU over photodetector sensor and surface of composite.

Evaluation of light intensity

LCU with fully charged battery was mounted on custom-made jig in such a way that the light-curing tip should descend perpendicular to the photodetector sensor of the Lux meter (TES Digital Light Meter, Taiwan). Based on the type of disposable ICB, the distribution of samples (n = 30) was as follows:

• Group A: Control group (CG): No barrier was placed

• Group B: Cling Wrap PVC (CWPVC) (Ezee, Mumbai, India)

• Group C: Complete LED Curing Light Sleeve (CLCLS) (Pinnacle, Kerr Total Care, Romulus, MI, USA)

• Group D: Cure Elastic Steri-Shield (CESS) (Steri-Shield, Santa Barbara, California)

• Group E: Latex-cut glove pieces (LCGP) (Nulife, Mumbai, India).

In CG, LI was measured on thirty separate occasions to achieve mean baseline LI of the LCU. Before proceeding to the every next group, each time battery was fully charged. In study (barrier) groups (i.e., Group B to E), ICB was placed on LG according to manufacturer's instructions, and the LI was measured on ten separate occasions for each barrier and its mean was taken as a LI for that barrier. Before placing every next barrier, the tip of the guide was cleaned with absorbent gauze. The values were recorded in terms of Lux. These values were converted into mW/cm² by the equation mentioned by McCabe and Carrick[10] and Harrington and Wilson.[11]

1 Lux = 0.005 mW/cm² Eq. (1)

Evaluation of Vicker's microhardness of composite

One hundred and fifty circular molds (5 mm diameter × 2 mm thickness) were prepared from 150 Teflon disks (Maxwell, India) of 2 mm thickness. The resultant molds were randomly and equally divided into above-mentioned five groups (n = 30). Teflon mold was placed on a Mylar strip (Samit products, New Delhi, India) and glass slab. Bulk of composite resin (Filtek Z 350 × T, 3M ESPE, St. Paul, MN USA.) was inserted into the Teflon mold. Firm pressure for 30 s was applied with the glass slide (Blue Star, Mumbai, India) to extrude the excess of material.

These composite samples were irradiated through the disposable ICB of respective groups; with same Bluephase LED LCU which was mounted on the jig, for 40 s with the tip kept in close contact and perpendicular to the surface of the composite specimen. In CG, the samples were cured without placing any barrier on the LG. The top surface of each Teflon mold was marked with an indelible marker.

Specimens in each group were stored in a light proof container at 37°C and 100% relative humidity for 24 h. Both surfaces of composite restorations were polished with composite finishing and polishing kit (Sof-Lex, 3M ESPE, St. Paul, MN, USA).

HV was measured on top and bottom surface of each composite sample using a Mitutoyo microhardness testing machine (Instrument No. 810-117E, Mitutoyo, New Delhi, India). The indenter was applied at three predetermined points on the central part of each specimen surface with the load of 50 g for 15 s. The indentations in the sample surface were measured under 100 times magnification. The mean of three was taken as a microhardness of each surface of specimen.

A hardness ratio (HR) was also obtained as suggested by Hwang et al.[12]

The thickness of each barrier in the study group was measured with the digital micrometer (Mitutoyo, Japan).

Data were analyzed with one-way analysis of variance (ANOVA), Tukey's honestly significant difference test, and paired t-test using SPSS version 18 software (SPSS Inc. Released 2009. PASW Statistics for Windows, Version 18.0. Chicago, Ill., USA: SPSS Inc.).

RESULTS

Entire tested barrier had significantly reduced the baseline LI of LCU (P < 0.0001). CWPVC (6.57%) caused the least whereas LCGP (47.19%) caused the greatest percentage reduction [Table 1 and Graph 1]. Except for the CG, all the groups showed the significant difference between mean microhardness of top and bottom surface of composite (P < 0.05). Among the top and bottom surfaces of different groups, HV was significantly reduced in case of CESS and LCGP (P < 0.05) [Table 2 and Graphs 2, 3]. Whereas HR had shown that composite resin in all groups except LCGP was adequately polymerized [Table 3].

Table 1.

Light intensity (mW/cm²) among different types of barriers

Graph 1.

Mean intensity (mw/cm2) for control and different types of barriers. Dissimilar color shows statistically significant difference among different groups (P < 0.0001; one-way analysis of variance)

Table 2.

Vicker's microhardness of composite specimens in different groups

Graph 2.

Mean microhardness of top surface of composite specimens in different groups. Dissimilar color shows statistically significant difference among different groups (P < 0.05; Tukey's honestly significant difference test)

Graph 3.

Mean microhardness of bottom surface of composite specimens in different groups. Dissimilar color shows statistically significant difference among different groups (P < 0.05; Tukey's honestly significant difference test)

Table 3.

Hardness ratio for composite resin belonging to different groups

DISCUSSION

Visible light-activated materials have been used in dentistry since 40 years and have revolutionized the field of clinical dentistry. According to the CDC Guidelines for infection control, LGs of LCU should be sterile before using it in every next patient.[4,12]

Use of disposable ICB that covers the LG found to be most acceptable, more practical, and convenient method. However, it apparently causes the reduction in the LI of the LCU.[7,8] The present study evaluated the effect of four commercially available disposable ICB on LI of LCU and its subsequent effect on the microhardness of composite.

Currently, Dental Clinicians are preferring LED LCU over other types because of it's many advantages like: it has longer bulb life with fewer maintenance related problems, it emits radiation only in blue segment of visible spectrum of light, it consumes lesser energy, it can be powered by rechargeable battery source and thus can be cordless and convenient for operator to handle, since it does not generate any heat; cooling fan is not required and hence these are silent, it produces higher power intensity; thus reduces polymerization time.[13]

Precision of commercially available dental radiometers which measures the power output of LCU is proven to be questionable as compared to laboratory grade power meter.[2,14] TES Digital Light Meter is one such laboratory grade light meter which measures LI in terms of Lux.[15] It is analogous to the radiant intensity unit, i.e., watts per square meter.[16]

In the present study, since all the tested barriers have significantly reduced the baseline LI of LCU [Table 1 and Graph 1], the null hypothesis was rejected. These results are in agreement with those of Chong et al.,[7] Scott et al.,[8] Chang et al.,[17] and McAndrew et al.[18]

In the present study, CWPVC was found most efficient in transmitting the light whereas LCGP was least [Table 1 and Graph 1]. However, none of the tested barriers reduced the LI below the critical value 300 mw/cm². This may be due to high-intensity LCU (675 mW/cm²) was used in this study. However, according to clinical survey by Hegde et al., majority of tested LED LCU were having inherent LI in between 200 and 400 mw/cm².[19] Therefore, we discourage the use of LCGP as a disposable ICB for LCU with lower inherent power output.

Differences in the mean intensities among different ICB can be attributed to its transparency,[20] thickness of the tested barriers,[17] presence of folds,[21,22] and formation of air pouch between light tip and barrier.[8,23]

Two of the tested barriers, i.e., CWPVC and CLCLS found to be more transparent than CESS and LCGP which were translucent. Thus, CWPVC and CLCLS allowed to pass significantly more light than two other barriers.

Further, there was a significant difference in LI in between transparent and in between translucent barriers which can be explained on account of their thicknesses. The mean thickness was as follows:

• LCGP - 0.219 mm

• CESS - 0.126 mm

• CLCLS - 0.055 mm

• CWPVC - 0.028 mm.

One-way ANOVA showed a statistically significant difference in mean thickness across the barriers (P < 0.0001). Thus, LCGP due to its greatest thickness caused maximum reduction in the LI.

de Moraes Porto et al.[21] and Coutinho et al.[22] revealed that folds and wrinkles in the barriers at the tip of LG can cause light deviation and energy loss and thus decrease in LI. In this study, the unavoidable fold at the tip in CLCLS might have caused more intensity reduction than CWPVC which was without folds.

Pollington et al.[24] have mentioned that entrapped air between barriers and LG may increase the distance between LG and restoration and decrease the curing effectiveness. Formation of air pouches was avoided in this study by careful placement of barriers.

To confirm whether there is any effect of reduced LI due to various disposable ICB, on the polymerization of composite resin, a HV test was performed on top and bottom surface of composite specimen. LCGP caused the maximum percentage reduction in HV on both the surfaces paralleling its deleterious effect on LI [Table 2].

When compared to CG, CWPVC, and CLCLSs (i.e., transparent barriers) have not significantly affected the mean HV of top and bottom surfaces of composite specimens, whereas CESS and LCGP (i.e., translucent barriers) have significantly decreased it. Thus; in case of microhardness, null hypothesis was accepted for CWPVC and CLCLS whereas it was rejected for CESS and LCGP [Table 2 and Graphs 2, 3]. These results are in partial agreement with those of Chong et al.[7] Al-Marzouk reported the use of nonopaque transparent barriers did not cause any significant difference in the microhardness values of composite resin.[20] In Contrast, Coutinho et al.[22] have concluded that the use of translucent protective barriers on the LGs can impair the percentage degree of conversion (DC) of composite.

According to Hwang et al., the DC of the composite resin specimen is usually assessed by the HR (Eq. 2).[12] The HR should be more than 80% for composite resins that are adequately cured.[12] In the present study, all the barriers except LCGP showed HR more than 80% [Table 3]. These results are in agreement with Sword et al., who revealed only latex-based barriers significantly reduced curing of composite.[25]

CONCLUSIONS

Although different disposable ICB have significantly reduced the LI of the tested LCU, it did not affect the DC of the composite resin to be clinically significant, except for LCGP. Thus, the present study recommends the use of disposable ICB over the LG of the LCU except LCGP.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.