Journal of Oral Research and Review

: 2021  |  Volume : 13  |  Issue : 1  |  Page : 25--30

Effect of fiber reinforcement on color stability and degree of polymerization of different composite resins

Ozcan Karatas 
 Department of Restorative Dentistry, Nuh Naci Yazgan University, Kayseri, Turkey

Correspondence Address:
Ozcan Karatas
Department of Restorative Dentistry, Nuh Naci Yazgan University, Kayseri


Aim: The aim of this in vitro study was to determine the effect of fiber-reinforcement on the color changes and degree of polymerization of two different composite resins. Materials and Methods: A bulk-fill composite resin and a methacrylate-based composite resin with A2 shade were used in this study. Three groups of specimens (control group with no reinforcements, polyethylene fiber-reinforced composite and glass fiber-reinforced composite groups) were prepared from each composite. The color change of the specimens with polymerization was measured. Then, to determine the degree of polymerization, the hardness ratios were calculated by measuring the bottom and top surface hardness of all specimens. The data were analyzed by analysis of variance, Duncan's multiple range tests, and Independent sample t-test. Results: Statistical analysis of variance presented the significance difference between composite and fiber for color change (P < 0.05). The highest color change by polymerization was seen in the polyethylene fiber-reinforced bulk-fill composite group. It was found that the addition of fiber to composite resins significantly reduced the degree of polymerization (P < 0.05). Conclusion: The addition of fiber may lead to color change and reduce the degree of polymerization of composite resins. The amount of change may differ depending on the structural properties of the composite resins and fiber.

How to cite this article:
Karatas O. Effect of fiber reinforcement on color stability and degree of polymerization of different composite resins.J Oral Res Rev 2021;13:25-30

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Karatas O. Effect of fiber reinforcement on color stability and degree of polymerization of different composite resins. J Oral Res Rev [serial online] 2021 [cited 2021 Jun 24 ];13:25-30
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Research in esthetic dentistry, often conducted by dental materials manufacturers, focuses on new techniques and materials to increase the clinician's ability to provide cosmetic dental treatment. In recent years, a significant increase has been observed in the development and use of dental aesthetic restorative materials.[1] Fiber-reinforced composites have recently been successfully used in dental restorations with appropriate mechanical properties. Compared to metals, fibers offer many advantages such as superior strength-to-weight ratios, ease of repair, low corrosion, similar translucency to the tooth structure and good bonding properties.[2] With these advantages, fibers are used in dentistry in different areas such as periodontal splinting, orthodontic applications, single crowns, postcore systems, adhesive fixed partial prosthesis, and stabilization of traumatic teeth.[3],[4]

Resin composites have provided dentists with a tooth-colored, direct restorative material as an acceptable alternative to metallic restoratives. To achieve the best esthetics, resin composite restorations must match natural teeth in appearance and must maintain that appearance over time. Intrinsic color stability and resistance to staining affect the long-term esthetics of composite restorations.[5],[6] Composite resins are hydrophobic and harden through polymerization reaction. They show successful results in long-term with appropriate color stability as a direct restorative material.[7] However, the composite resin restorations may undergo color changes in the mouth as a result of several factors, including chemical changes caused by free-radical degradation, foods, and retention of plaque caused by surface roughness.[8]

The amount of conversion of monomers into the polymer during the polymerization of composite resins is called the degree of conversion or polymerization. The degree of polymerization affects the physical and mechanical properties of the restoration. In case of insufficient polymerization, problems such as marginal leakage, discoloration, increased wear, increased water absorption, and low mechanical resistance have been reported. In addition, monomer release is increased and monomers can exceed the dentine tubules and cause an irreversible reaction in the pulp.[9] The polymerization degree of composite resins may be affected by various factors such as the organic and inorganic structure of the composite material, the proportion of the filler, the filler type, optical properties of the composite material, the power of the light source, the distance between the tip of the light device and the restoration, and the thickness of the restoration.[10]

Conventional methacrylate-based composite resins should be placed in large and deep cavities in layers due to the limited polymerization degree and increased risk of polymerization shrinkage. The use of this technique, which is called the “Incremental technique,” requires a lot of curing and causes time loss.[11] In order to overcome this negativity, manufacturers have sought ways to place composite resin with larger masses and reduce polymerization shrinkage. As a result, a new generation of composite resins called “Bulk-Fill” was developed. Hydroxyl-free BISGMA, aliphatic urethane dimethacrylate, partial aromatic urethane dimethacrylate or high branched methacrylate were added to the resin matrix structure of Bulk-Fill composites. This structure of Bulk-fill composites has improved polymerization dynamics, allowing them to be applied to deep cavities in one layer.[12]

The aim of this in vitro study was to determine the effect of fiber-reinforcement on the color changes and degree of polymerization of a bulk-fill and a methacrylate-based composite resin. The null hypothesis of the study is that (1) the addition of fiber does not affect the color change, and (2) degree of polymerization of the composite resins.

 Materials and Methods

Specimen preparation

The chemical composition and manufacturer details of the materials are listed in [Table 1]. A bulk-fill resin composite (Filtek bulk-fill; 3M ESPE Dental Products, St. Paul, MN, USA) and a methacrylate-based resin composite (Valux Plus; 3M ESPE Dental Products, St. Paul, MN, USA) of A2 shade were used in this study. The specimen size was determined based on the results of Tuncdemir and Güven[13] aiming to obtain a power of 80%. Twenty-four specimens were prepared from each composite. The specimens were divided into three subgroups (n = 8). The first group was used as control group with no reinforcements, the second group was reinforced with a polyethylene fiber (Ribbond Inc., Seattle Washington, USA) third group was reinforced with a glass fiber (Everstick; Stick Tech, Turku, Finland) for each composite resin. Fibers were cut using fiber cutting scissors supplied by the manufacturer to a length of 6 mm and a width of 2 mm, then were placed center of composite disc specimens with sufficient pressure and light-cured for 20 s. Ribbond fibers are not impregnated with resin, so they were saturated with an adhesive bonding agent (Clearfil SE Bond, Kuraray, Kurashiki, Japon) before used. Composite resin discs with a diameter of 8 mm and a thickness of 2 mm were prepared and covered with celluloid strips on glass plates. After curing with a light-curing unit (Elipar S10, 3M ESPE, St. Paul, Minn, 1200 mW/cm2) for 40 s each from the topsides, the strips were removed. During the polymerization, the light-curing unit device was kept in contact with the glass plate on the specimen surface, and the power of the curing light was checked with a radiometer (Kerr Corp, California, USA) every ten specimens.{Table 1}

Color change measurement

Initial color measurements of specimens were made after the composite resin placement in the mold with a spectrophotometer (Shadepilot; DeguDent Gmbh, Rodenbacher Chaussee 4 MHT, Optic Research AG, 63457 Hanau, Germany). Color measurements were recorded under the D 65 machine, which reflects daylight. L, a, b values of each specimen were recorded. Color stability analyses a spectrophotometer was used to determine CIE Lab tristimulus parameters (Illuminant D65, specular component included): L* (lightness, from 0 = black to 100 = white), a* (from -a = green to +a = red) and b* (from -b = blue to +b = yellow). After polymerization, the specimens were stored in distilled water at 37°C for 24 h. Then, color measurements of all specimens were repeated and the mean color changes were calculated using this formula;

ΔE = ([ΔL*]2 + [Δa*]2 + [Δb*]) ½.

ΔE = 3.3 was determined as the clinically acceptable color change limit.

Evaluation degree of polymerization

The specimens, whose color measurements were completed, were stored in distilled water for 24 h at 37°C. Then, hardness measurements of all specimens were made from randomly 3 points at the top and bottom surfaces with a universal test device (MVK-H1, Akashi Co, Japan), (VH). Hardness ratio (HR) of each specimen was calculated with;

HR = VHbottom/VHtop formula.

Specimens with a HR of 0.80 and above were considered to have a sufficient degree of polymerization.

Statistical analysis was conducted with SPSS software 20 (SPSS Inc., Chicago, IL, USA). Means and standard deviations were calculated for color change and HR values of each groups. Two-way analysis of variance analyzed the obtained data and then Duncan test was performed for comparisons among groups at the 0.05 level of significance.


Variance analysis determined that there were statistically significant differences between the color change amounts of composite resins using different fibers (P < 0.05) [Table 2]. Color change by polymerization in bulk-fill composite specimens was found to be statistically significantly higher than methacrylate-based composite specimens (P < 0.05). There was no statistically significant difference between the mean color change of the glass fiber groups and control groups of the same composite (P > 0.05), while the mean color change of polyethylene fiber groups were significantly higher (P < 0.05) [Table 3].{Table 2}{Table 3}

When the surface hardness of the specimens was examined, the mean surface hardness of the methacrylate-based composite resin specimens was found higher than the bulk-fill composite resin specimens [Table 4]. In bulk-fill composites, the HR in all groups was found above 0.80, whereas in the methacrylate-based composite it was only found in the control group. The addition of fiber to both composite specimens has reduced the HR. The mean HR of the control groups of both composites was found to be statistically significantly higher than the fiber-supported groups (P < 0.05) [Table 5].{Table 4}{Table 5}


In this study, the color change by polymerization, and the degree of polymerization after curing of fiber reinforced composite resins were investigated. Fibers are used for splinting, bridge support or strengthening composite resin restoration in anterior areas where aesthetics are important.[14] However, the effect of fiber addition on the polymerization and color dynamics of composite resin is an important issue. Therefore, in our study, the effect of the addition of composite resin fiber on color change and degree of polymerization was investigated. Discoloration can be assessed using different techniques, such as visual or instrumental techniques. For measurement the color change of composite resin the spectrophotometer which equipped with an integrating sphere can use. This configuration of spectrophotometer permits good reliability of the result better than visual assessment or other instruments.[15] Due to these advantages, we used the spectrophotometer in this study for the color measurement.

It has been reported by Seghi et al.[16] that a ΔE value equal or lower than 1, the color change can be perceived with difficulty whereas a ΔE value >2 can usually be detected clinically. Um and Ruyter[17] argued that the ΔE = 1 value is visually perceptible in their studies. On the other Johnston and Kao[18] investigated color differences by visual evaluation and colorimetry and they found that the threshold value at which the color difference can be distinguished in the oral environment is E = 3.3. A clinically acceptable ΔE value was stated to be 3.3 because a lower ΔE value was visually is not perceived.[18],[19] In our study, mean color change values in the all bulk-fill composite groups were found above the clinically acceptable limit.

It has been reported that color change may be observed by polymerization in composite resins.[20] In particular, the color of polymerization initiator molecules such as camphorquinone may become lighter by the curing reaction.[21] Whereas in our study, it was determined that the specimens showed darker color and less chromatic properties with polymerization in fiber supported groups. In our study, the mean E value in the glass fiber group was found higher than the polyethylene fiber group. These results may be related to the chemical structure of the fibers we use in our study. Glass is an amorphous material consisting of silica tetrahedral combined in a random mesh. This inorganic structure is different from fibers containing organic structures such as polyethylene. Fibers with inorganic content exhibit more hydrophobicity and more resistance to discoloring than organic fibers.[22] The differences in chemical structure may be explained that Ribbond (polyethylene) reinforced composite materials show more color change than EverStick (glass) reinforced composite materials in the present study. According to these results, our first hypothesis that fiber addition does not affect the color of the composite resin has been rejected. Similar to our results, Tuncdemir and Aykent[23] found that glass fiber-reinforced composite specimens exhibited less color change than polyethylene fiber supported specimens in their studies. In addition, Hasani Tabatabaei et al.[24] reported no statistically significant difference between glass fiber and control groups in their study.

The chemical properties, matrix structure, filler type and amount directly affect the color change in resin composites.[25] In our study, the mean color change of bulk-fill composite groups was found statistically significantly higher than methacrylate-based composite. Similar to our results, Mansouri and Zidan[26] found that bulk-fill composite resins showed more color change than methacrylate-based composite resins. Similarly, Shamszadeh et al.[27] found that bulk-fill composites showed statistically significant more color change than conventional methacrylate composites with different staining processes. The improved translucent structure of bulk-fill composites and photoactive groups placed in methacrylate resin may allow better control of the polymerization kinetics of these composites and polymerization to a depth of 4–5 mm.[28] The resin matrix structure of bulk-fill composites is arranged to penetrate the light deeper. This matrix structure, polymerization initiators, and photoactive groups may lead these composites to change color more than methacrylate-based composites during polymerization.[29]

In addition to the chemical structure degree of polymerization may affect the color change values of composite resins.[30] One of the most commonly used methods to measure the degree of polymerization of composite resins is the hardness measurement method.[31] In this method, the top and bottom surface hardness of the composite resin is measured and proportioned. In our study, the surface hardness of the samples was measured with a Vickers hardness tester device. The short tip structure of the Vickers device is effective for measuring the surface hardness of composite resins.[32] When the surface microhardness of composite resins was compared in our study, the mean surface microhardness of methacrylate-based composite resin specimens were found higher than the bulk-fill composite. Since the chemical composition and filler contents of composite resins affect their physical properties, differences between microhardness values are observed.[33] Braem et al.[34] reported that the materials with high filler content have high surface hardness as a result of their study. Similarly, in our study, the mean surface hardness values of Valux plus specimens with a higher filler ratio (85% wt) were found higher than the Filtek Bulk-Fill (76% wt) specimens. In parallel with our study, Abed et al.[35] found that the surface hardness of bulk-fill composite resins was lower than that of methacrylate-based composites in their study.

In our study, a sufficient degree of polymerization was observed in all bulk-fill composite groups. However, the mean HR of the methacrylate-based composite was found above 0.80 only in the control group and the addition of fiber reduced the polymerization degree of composite resins. Given our results, our second hypothesis claiming that “fiber addition would not affect the degree of polymerization of composite resin” was rejected. The different light transmission properties of glass fiber and polyethylene fiber can cause the fiber-reinforced composite to have different degrees of polymerization.[36],[37] Since glass fibers transmit light better than polyethylene fibers, they allow the lower layers of composite resin to be better polymerized. This may allow the degree of polymerization to be better in restorations using glass fiber than in restoration using polyethylene fiber.[38] However, sufficient degree of polymerization was not observed in both fiber groups of the methacrylate composite used in our study. This may be due to the fact that the fibers affect the dispersion of light, preventing the light from reaching the lower layers, and acting as a barrier in the resin which weakens the polymerization reaction. There are studies in the literature that the addition of fiber to methacrylate-based composite resins reduces the degree of polymerization.[39],[40] Similar to our study, Garoushi et al.[41] found that adding fiber to the methacrylate-based composite resin structure reduced the degree of polymerization. However, the researchers reported that this decrease was not statistically significant. In our study, the addition of fiber to both methacrylate-based composite and bulk-fill composite statistically significantly reduced the degree of polymerization (P < 0.05). In our bulk-fill composite specimens, although the degree of polymerization decreased significantly in fiber groups, sufficient polymerization was observed in all specimens. This may due to the monomer structure and light transmittance of bulk-fill composites mentioned above.


Within the limitations of this study, the addition of fiber may lead to color change during polymerization of the composite resin, and affect the degree of polymerization. The degree of this effect may differ depending on the structural properties of the composite resin and fiber. The clinician should take this into consideration before applying the fiber-reinforced composite resin restoration.

Ethical clearance

Since this study was an in-vitro material study, it was stated that there was no need for ethics committee approval by the clinical research ethics committee.

Finncial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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