Supportive Care in Cancer

, Volume 20, Issue 11, pp 2873–2878

Effect of gamma radiation on bonding to human enamel and dentin

Authors

  • Lucas Z. Naves
    • Department of Restorative Dentistry, Dental Materials DivisionPiracicaba Dental School, University of Campinas—UNICAMP
    • Operative Dentistry and Dental Materials Department, School of Dentistry, Biomechanics Research GroupFederal University of Uberlândia
  • Veridiana R. Novais
    • Operative Dentistry and Dental Materials Department, School of Dentistry, Biomechanics Research GroupFederal University of Uberlândia
  • Steven R. Armstrong
    • Department of Operative Dentistry, College of DentistryUniversity of Iowa
  • Lourenço Correr-Sobrinho
    • Department of Restorative Dentistry, Dental Materials DivisionPiracicaba Dental School, University of Campinas—UNICAMP
    • Operative Dentistry and Dental Materials Department, School of Dentistry, Biomechanics Research GroupFederal University of Uberlândia
Original Article

DOI: 10.1007/s00520-012-1414-y

Cite this article as:
Naves, L.Z., Novais, V.R., Armstrong, S.R. et al. Support Care Cancer (2012) 20: 2873. doi:10.1007/s00520-012-1414-y

Abstract

Purposes

This study evaluated the effect of gamma radiation on the microtensile bond strength of resin-based composite restoration to human enamel and dentin performed either before or after radiotherapy.

Methods

Thirty sound human third molars were sectioned mesio-distally into buccal and lingual halves and then randomly divided into enamel or dentin groups. Enamel and dentin substrates were randomly divided into three sub-groups (n = 10): Control, which received no irradiation; specimens were irradiated before restoration protocol; and specimens were irradiated after restoration protocol. Radiation therapy was defined by application of 60-Gy dose fractionally with daily exposures of 2 Gy, 5 days a week, over 6 weeks. Restorations were carried out using Adper Single Bond adhesive system and Filtek Z250 resin composite. The specimens were sectioned producing 4 sticks per specimen and submitted to microtensile on a testing machine. Data were submitted to two-way ANOVA followed by Tukey test (p < 0.05). Failure modes were examined under optical microscopy and SEM.

Results

Bond strength to enamel was significantly higher than to dentin irrespective radiation therapy. Radiotherapy applied before restoration significantly reduced the bond strength to both substrates. A predominance of adhesive failures was detected for control groups and groups restored before radiotherapy. Cohesive failures in dentin and enamel increased when the specimens were restored after irradiation.

Conclusions

The gamma radiation had a significant detrimental effect on bond strength to human enamel and dentin when the adhesive restorative procedure was carried out after radiotherapy.

Keywords

Bond strengthMicrotensileDentinEnamelGamma radiationRadiotherapy

Introduction

Radiotherapy is a vital therapy for many patients with malignant neoplasm, aiming to exterminate abnormal cells to decrease or eliminate the tumor. Although modern treatments advocates the use of linear accelerators and other high-energy machines, appropriately fitted 60Cobalt units could be fully acceptable in the treatment of a large majority of the patients undergoing radiation treatment for carcinoma of the head–neck region, breast, and sarcomas of soft tissues of the extremities. However, this procedure may cause side-effects to normal cells neighboring affected tissues [1]. The irradiation dosage in the head and neck region usually ranges between 40 and 70 Gy [1], but even lower doses may also indirectly or directly injure dental tissues, potentially leading to problems such as salivary gland alteration [24], changes of oral flora [5] or hypo-vascularization [6]. All these features may pose a risk to caries-producing microorganisms to act in abnormal levels [1, 7].

In addition to predisposing radiation caries, the radiotherapy can cause effects on dental tissues [8]. The severity and extent of radiation effects are dependent on the mineral and organic content of tooth structures. The mineralized portion is susceptible to damage in its crystalline phase [912], decreasing its mechanical properties [11] and wear resistance [13], and reducing its resistance to acid attack [10]. In enamel, radiotherapy may produce physical and chemical alterations [9, 1315], reducing the ultimate tensile strength due to alterations into the protein component of this substrate [8]. Demineralization depth and solubility of enamel is controversial, some studies demonstrated no difference between irradiated and non-irradiated enamel [15, 16], others described increased solubility after irradiation [17, 18], or even a slight decrease in solubility [12]. In addition, it has been reported that irradiated dental tissues might accommodate free radicals within their structure for long periods of time [14]. These radicals could impair the bonding process, acting in a similar way of hydrogen peroxide (O– highly reactive radicals interfering with polymerization) [19], sodium hypochlorite (collagen denaturation by free radical activity) [20], or blood contamination (hemoglobin iron-dependent free radicals) [21, 22].

Moreover, it has been reported that the apatite crystals of dental hard tissues incorporate some sodium, carbonate, and magnesium by entrapment during their formation [23]. When irradiated, these defects could be mobilized from the surface layer of the crystals, removing the entrapped ions and modifying the structure of the crystals, thus potentially interfering with adhesion. The above-mentioned alterations may occur in higher frequency in enamel, since it is formed by 92–96 wt% of inorganic matter [24], as compared to dentin with 70 wt% [25]. On the other hand, other features such as morphologic and compositional alteration in intra- and inter-tubular collagen [26], or metabolic alterations [27], could also have an effect on bond strength to dentin. However, major radiotherapy alterations in dentin are due to damage to collagen fibrils [26], resulting in a significant reduction in; hardness [11], wear resistance [28], ultimate tensile strength [8], and stability of the amelo–dentin junction [29].

Recommendations have been proposed that adhesive restorative techniques are more indicated for patients undergoing radiotherapy treatment, since metallic restorations could increase the incidence of radio-mucositis due to the backscattering of secondary-radiation caused by the high atomic mass number of metal compounds [30]. However, there is no consensus on whether the restorative procedure should be carried out before or after irradiation. Therefore, the aim of this study was to evaluate the effect of gamma irradiation on the microtensile bond strength (μTBS) of adhesive restorations to enamel and dentin performed either before or after the radiation treatment. The null hypothesis tested in this study is that the adhesion of resin bond composites (RBC) to dentin or enamel is unaffected by gamma irradiation either before or after restoration placement.

Materials and methods

Specimen preparation

Thirty sound human third molars were collected and stored in 0.2 % thymol solution for no longer than 2 months after extraction (approved by the Ethical Committee in Research of Federal University of Uberlândia, Brazil, protocol No. 013/05). The teeth were cleaned and the roots were removed approximately 5 mm apically to the cementum–enamel junction using a water-cooled diamond disc (KG Sorensen, Barueri, São Paulo, Brazil). The tooth was vertically serially sectioned into the mesio-distal direction, each half randomly designated for either enamel or dentin substrate group (n = 30). The surfaces were ground to produce flat enamel or superficial dentin surfaces. Specimens for each group were randomly divided into three sub-groups (n = 10): Cont—control group, which received no irradiation, IrB—specimens were irradiated before restoration placement, and IrA—specimens were irradiated after restoration placement.

Radiotherapy was defined by a total application of 60 Gy of gamma radiation in a 60Cobalt irradiation unit (Theratron Phoenix 60Cobalt Radiotherapy Treatment Unit—Theratronics International, Ltd., Atomic Energy of Canada, Ltd., AECL Medical, Ottawa, ON, Canada), with 2 Gy exposure 5 days a week. The placement of the beam and the radiation dose was precisely calculated based on a mean of the most applied doses on head and neck patients reported in the specific literature [1] and used as radiotherapy routine at Cancer Hospital of Federal University of Uberlandia, Minas Gerais, Brazil. Specimens were rotated at different angles aiming to simulate the protocol tumor’s attack from different sides avoiding overexposing healthy tissue. All groups of specimens were stored in distilled water changed daily.

Restoration placement was completed by cleaning the enamel and dentin with a pumice-water slurry, followed by etching with 35 % phosphoric acid (Scotch Bond etchant, 3 M ESPE, Batch No. 7BT, St. Paul, MN, USA) for 15 s, then rinsing with water-air spray for 15 s. Immediately followed by two consecutive coats of a one bottle adhesive system (Adper Single Bond 2, 3M ESPE, Batch/Lot No. 4YE, St, Paul, MN, USA) applied to etched enamel and dentin for 15 s using a fully saturated applicator and gentle agitation. Gentle air thinning for 5 s was performed to evaporate solvents while being careful to avoid excess adhesive on surfaces. Light activation was performed with a halogen light curing unit (XL3000; 3M ESPE) with a 600 mW/cm2 output for 20 s. An RBC block was incrementally built-up in three increments using Filtek Z250 composite (3M ESPE, batch/lot no. 4LU, 4ME, and 4MG) and individually light-cured for 20 s.

After 24-h water storage at 37°C the teeth were vertically serially sectioned into several 1.0-mm slabs, then gently trimmed with an ultra-fine diamond bur under magnification to an hourglass shape with a square-shaped cross-sectional area of approximately 1.0 mm2. Four hourglass-shaped specimens were obtained from each tooth.

Microtensile bond strength test

All faces of each specimen were covered with cyanoacrylate glue (Loctite Super Bonder, Henkel Loctite Co., Munich, Germany) followed by the specimen attachment to a Bencor Multi-T metallic grip and then submitted to μTBS on a mechanical testing machine (DL2000; EMIC, São José dos Pinhais, Paraná, Brazil) at crosshead speed of 0.5 mm/min until failure. After testing, the specimens were removed from the grips with a scalpel blade, and the cross-sectional area at the site of the fracture was measured to the nearest 0.01 mm using a digital caliper (Starret, Itu, São Paulo, Brazil). μTBS values were expressed in MPa, and data submitted to two-way ANOVA followed by Tukey’s test. Statistical significance was set in advance at α = 0.05, considering tooth the statistical unit by averaging specimens obtained from the same tooth.

Failure mode analysis

After bond strength testing, fractographic analysis was performed using stereomicroscopy (Leica Microsystems, Wetzler GmbH Germany) at ×30 to ×100 magnification. For those questionable fractured surfaces, the failure modes were confirmed using a scanning electric microscopy (SEM; LEO 435 VP; LEO Electron Microscopy Ltd., Cambridge, UK).The work distances ranged between 22 and 18 mm, according to specimens height. Then each specimen was classified according to the predominant remaining structure upon its surfaces following the described failure mode: adhesive failure (mode 1); cohesive failure within dental substrate, enamel, or dentin (mode 2); mixed failure involving bonding agent, RBC, and/or tooth structure (mode 3); or cohesive failure within the RBC (mode 4). The results of failure mode classification were submitted to Fisher’s exact test (p < 0.05).

Results

Means and standard deviation of μTBS are shown in Table 1. Two-way ANOVA revealed significance for substrate (p = 0.01), and radiotherapy study factors (p = 0.001); however no significant interaction was found between the two factors. The bond strength to enamel was significantly higher than to dentin (p = 0.03). Compared with the control group, the radiotherapy resulted in a significant reduction in bond strength to both enamel and dentin only when the restorative procedure was carried out after the radiotherapy (p = 0.01), with a reduction in μTBS of 31.3 % for dentin and 20.1 % for enamel.
Table 1

Mean uTBS relative to timing of radiotherapy (means ± SD; MPa; n = 40 per group)

Groups

Enamel

Dentin

Control (no radiotherapy)

39.1 ± 3.0aA

29.1 ± 3.4aB

Restored before radiotherapy

35.9 ± 2.4abA

27.3 ± 3.8abB

Restored after radiotherapy

31.2 ± 3.9bA

20.0 ± 2.4bB

Groups identified with different upper case letter superscripts (analysis in columns) and lower case letters (analysis in rows) represent statistically significant differences (p < 0.05)

Failure mode results are shown in Table 2. The predominance of the adhesive failures was detected for control groups and groups restored before radiotherapy. Otherwise, cohesive failures in dentin and enamel increased when the specimens were restored after irradiation. The Fisher’s Exact Test of the failure modes within each substrate showed a significant association between substrate and restoration timing relative to radiotherapy (p = 0.00000075 for enamel; p = 0.000036 for dentin). The most representative micrographs are shown in Fig. 1.
Table 2

Failure mode distribution for all groups

Group

Modes of failure

Adhesive

Cohesive within substrate

Cohesive within RBC

Mixed

Dentin

Control (no radiotherapy)

85 (34)

2.5 (1)

10 (4)

2.5 (1)

 Fisher’s exact test

Restored before radiotherapy

80 (32)

10 (4)

(0)

10(4)

 (p = 0.000036)

Restored after radiotherapy

52.5 (21)

35 (14)

(0)

12.5 (5)

Enamel

Control (no radiotherapy)

82.5 (33)

5 (2)

7.5 (3)

5 (2)

 Fisher’s exact test

Restored before radiotherapy

82.5 (33)

7.5 (3)

5 (2)

5 (2)

 (p = 0.00000075)

Restored after radiotherapy

37.5 (15)

42.5 (17)

(0)

20 (8)

[%, (n out of 40)]

https://static-content.springer.com/image/art%3A10.1007%2Fs00520-012-1414-y/MediaObjects/520_2012_1414_Fig1_HTML.gif
Fig. 1

a and e Specimen surface showing adhesive failure between dentin and RBC (mode 1); b and f cohesive failure within dentin (mode 2); c and g cohesive failure within enamel (Mode 2); d and h cohesive failure within RBC (Mode 4). Mode 3—mixed failure (not shown)

Discussion

When compared with the control group, radiotherapy significantly reduced μTBS to both human enamel and dentin, but only when the restorative procedure was carried out after irradiation. The null hypothesis was rejected. In addition, the results showed an increase in occurrence of cohesive failures within the dental substrate for groups restored after radiotherapy. When the restorative procedure is carried out before irradiation, the μTBS was similar to non-irradiated group (control). In contrast, when restored after irradiation, the dental substrate might have experienced radiation effects that could compromise bonding ability possible by impairing hybrid layer formation.

In addition to the reduction in bond strength, the increased number of cohesive failures within the substrates (Fig. 1b, c, f, g) after irradiation provides further information to corroborate previous studies on changes in dental tissues due to irradiation [8, 28, 29, 31], and reinforces the need for additional studies on adhesion to irradiated teeth. Notwithstanding, the results also suggest that when hybridization is obtained previously to irradiation, the alterations in the substrate might not be great enough to yield significant differences in bond strength. At least theoretically, the hybridization of the substrate may enhance its structural stability during irradiation, also increasing the stability of the bonding assembly.

Despite the fact that cohesive failures within the restorative material (Fig. 1d, h) were not frequently observed, properties of the resin composite might also be influenced by gamma radiation [32, 33]. There are reports describing chain scission, brittle fracture, and color alteration in matrix polymers [34]. Damage in RBC would also possibly include polymer cracking or fracture, delamination, interphase cracking, and filler dislodgement and debonding. The damage mechanisms operating and the rate of damage accumulation would probably be dose related. In situations where the damage mechanisms are not completely clarified, all possibilities have to be considered, because this kind of alteration could result in reduced restoration service-life.

The results of the present study may have clinical implications, as it was shown that the restoration timing relative to radiotherapy affected bonding ability to enamel and dentin. Therefore, it might be suggested, given that cancer therapy is not unduly delayed, that dental restorative procedures in patients with head and neck cancer should be conducted before irradiation. In addition, since the radiotherapy effects are cumulative and dose dependent to the dentition [1], restorations placed during the time frame of radiotherapy might have improved clinical service compared to restorations placed after completion of radiotherapy.

This was an in vitro study, limiting the ability to predict clinical outcomes due to constraints of study design. For example, the absence of saliva, limits ion exchange within the immersion media, altering the remineralization process [11]. However, two studies carried out by our research group have used different media to store teeth during simulated irradiation protocols, distilled water [8] and artificial saliva [35], both studies found similar results regarding SEM analysis and UTS values. One of these studies [35] found that rinsing with chlorhexidine 0.12 % partially limits mechanical property alterations of irradiated coronal dentin possibly due to inhibition of radiotherapy-activated proteolytic enzymes. Therefore, further studies should be conducted to evaluate the effect of proteolytic inhibitors during the restorative procedure in an attempt to stabilize the bonding interface formed between adhesive systems and dentin.

Conclusions

Gamma radiation had a significant detrimental effect on bond strength to human enamel and dentin when the adhesive restorative procedure was carried out after radiotherapy. When the restoration was carried out before irradiation, no significant alteration in bond strength was detected.

Acknowledgments

The authors are grateful to FAPEMIG for financial support. Authors are indebted to Dr. E.W. Kitajima, Dr. F.A.O. Tanaka and R.B. Salaroli (NAP/MEPA-ESALQ/USP, Brazil) for SEM equipment support.

Conflict of interest

None.

Copyright information

© Springer-Verlag 2012