Comparative study between laser and conventional techniques for class V cavity preparation in gamma-irradiated teeth (in vitro study)

Abstract Objective: The purpose of this study was to compare laser with conventional techniques in class V cavity preparation in gamma-irradiated teeth. Methods: Forty extracted human teeth with no carious lesions were used for this study and were divided into two main groups: Group I (n = 20) was not subjected to gamma radiation (control) and Group II (n=20) was subjected to gamma radiation of 60 Gray. Standard class V preparation was performed in buccal and lingual sides of each tooth in both groups. Buccal surfaces were prepared by the Er,Cr:YSGG laser (Waterlase iPlus) 2780 nm, using the gold handpiece with MZ10 Tip in non-contact and the “H” mode, following parameters of cavity preparation – power 6 W, frequency 50 Hz, 90% water and 70% air, then shifting to surface treatment laser parameters – power 4.5 W, frequency 50 Hz, 80% water and 50% air. Lingual surfaces were prepared by the conventional high-speed turbine using round diamond bur. Teeth were then sectioned mesio-distally, resulting in 80 specimens: 40 of which were buccal laser-treated (20 control and 20 gamma-irradiated specimens) and 40 were lingual conventional high-speed bur specimens (20 control and 20 gamma-irradiated specimens). Results: Microleakage analysis revealed higher scores in both gamma groups compared with control groups. Chi-square test revealed no significant difference between both control groups and gamma groups (p=1, 0.819, respectively). A significant difference was revealed between all 4 groups (p=0.00018). Conclusion: Both laser and conventional high-speed turbine bur show good bond strength in control (non-gamma) group, while microleakage is evident in gamma group, indicating that gamma radiation had a dramatic negative effect on the bond strength in both laser and bur-treated teeth.


Introduction
Radiotherapy is a common therapeutic modality for malignancies of head and neck. It is usually associated with some possible complications such as radiation caries, xerostomia, osteonecrosis, loss of taste and trismus 17 .
The symptoms of microleakage range from postoperative hypersensitivity or loss of the restoration due to bond failure to damage to vital dentin and pulp tissue, which in some cases may be irreversible. Effects Naves, et al. 16 (2012) 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. The authors concluded that gamma strength to human enamel and dentin when the adhesive restorative procedure was carried out after radiotherapy.
Since previous studies evaluated either the effect of laser cavity preparation or gamma irradiation on surface roughness and microleakage, the combined effect of both variables still needs to be investigated. Therefore, this study was carried out to detect the surface morphology and presence of microleakage in composite resin restoration following etched bur cavity preparation and laser cavity preparation in irradiated teeth.

Material and methods
In this study, we used 40 molar teeth with no carious lesions, extracted from males of age 40-50 years from the Surgery Department of the Faculty of Dentistry, Cairo University, due to periodontal diseases or in preparation to receive a full denture. Teeth were stored in a 0.1% thymol solution until the study was carried out 26 .
They were divided into two main groups: Group I (n=20) was not subjected to gamma radiation (control).
Group II (n=20) was subjected to gamma radiation of 60 Gray, which is the therapeutic dose for head and

Laser application
Buccal surfaces of teeth were prepared by the Er,Cr:YSGG laser (Waterlase iplus) of wavelength 2780 nm ("H" mode), using the gold handpiece with MZ10 tip in non-contact. We used the following parameters during cavity preparation: power 6 W, frequency 50 Hz, 90% water and 70% air, shifting to surface treatment laser parameters of power 4.5 W, frequency 50 Hz, 80% water and 50% air.
Comparative study between laser and conventional techniques for class V cavity preparation in gamma-irradiated teeth (in vitro study) From each group 10 specimens were subjected to scanning electron microscope (SEM) to study their surface morphology and 10 specimens were used to measure the microleakage, using stereomicroscope.

Scanning electron microscope (SEM)
To study surface morphology using scanning electron microscopy, specimens were prepared as follows: Each specimen was dehydrated in graded alcohol

Microleakage analysis
Results of microleakage analysis revealed higher scores in both gamma and control groups. The Chiboth control groups, neither between both gamma groups (p=1, 0.819, respectively). A significant difference was revealed in all four groups (p=0.00018) (  Figures 2-6).
Scanning electron microscope revealed structural  (Figures 8, 9). In the gamma laser (GL) group, the inter-tubular dentine was ablated more than the peritubular dentine, giving the appearance of irregularity and protrusion of the dentinal tubules ( Figure 10). was noted in some areas, while in others the tubular structure was maintained and revealed variation in tubular diameter with irregular dentine surface ( Figures 11, 12).
On the other hand, in the cavity surface prepared with the conventional high-speed turbine bur in the relatively closed ( Figure 13); however, after treatment with 37% phosphoric acid, the smear layer was removed and the dentinal tubules became obvious.  Comparative study between laser and conventional techniques for class V cavity preparation in gamma-irradiated teeth (in vitro study)   The inter-tubular dentine was more ablated than the peritubular one ( Figure 14).
Moreover, specimens prepared with the conventional high-speed turbine bur in gamma (GB) group (before smear layer, with many cracks and debris (Figure 15).
Similarly to the control group, after treatment with 37% phosphoric acid, the smear layer was removed and the dentinal tubules became obvious, where the inter-tubular dentine appeared more ablated than the peritubular one ( Figure 16). areas, while in other areas, the tubular structure is maintained and reveals variation in tubular diameter (blue arrow), (x500) Figure 13-Electron-micrograph in control (CB) group subjected to conventional high speed turbine bur before acid etching showing Figure 14-Electron-micrograph in control (CB) group subjected to conventional high speed turbine bur after acid etching where the smear layer was removed and the dentinal tubules were obvious. The intertubular dentine appears more ablated than the peritubular one (x2,000) Figure 15-Electron-micrograph in gamma (GB) group subjected to conventional high speed turbine bur after cavity preparation revealing cracks and debris on the dentine surface (x750) Comparative study between laser and conventional techniques for class V cavity preparation in gamma-irradiated teeth (in vitro study) Laser parameters used in this study revealed that the surface mean temperature did not exceed 4ºC that is believed to be safe for pulp vitality 13 .

Discussion
After performing the microleakage test, some dye may have penetrated into the resin in a muchdiffused way such that score 0 means low-level dye penetration.
Previous morphological studies of enamel surface showed protruding prism sheaths without erosion, while the dentine surface showed exposed dentinal tubules orifices. This is believed to be due to microexplosion effects caused by the hard tissue ablation with the Er,Cr:YSSG laser of wavelength 2780 appearance 1 . However, in case of acid etching during normal restorative procedures, chemical changes may lead to increasing dentine permeability and wetness inorganic part of the hard tooth structures 4,19 .
According to a study conducted by Yamada, et al. 28 Figure 16-Electron-micrograph in gamma (GB) group subjected to conventional high speed turbine bur after acid etching, the dentinal tubules became more obvious, and the intertubular dentine appears more ablated than the peritubular one and the smear layer was removed (x2,000) The bonding process could be impaired due to the presence of free radicals within the structure of dental tissues previously exposed to ionizing radiation 15 .
These free radicals act in a similar way to hydrogen peroxide (Ohighly reactive radicals interfere with polymerization) 3,20 , sodium hypochlorite (free radicals act on collagen denaturation) 8 , or blood contamination (hemoglobin iron-dependent radicals) 22 .
Moreover, the hydroxylapatite crystals of dental hard tissues contain some sodium, magnesium and carbonate by entrapment during their formation 16 , in which sodium and magnesium may substitute calcium and carbonate can substitute phosphate and hydroxyl group; those substitutions distort the dental structure and make it more soluble 24 . After irradiation, these defects could be mobilized from the surface layer of crystals, removing entrapped ions and modifying the dental crystal structure, thus interfering with adhesion to restoration. This occurs more in enamel, which contains higher inorganic matter compared with dentine 14,16,24,25 . The morphologic, metabolic and compositional alteration in intra-and inter-tubular collagen might have an effect on bond strength to dentin 10,16 .
Finally, the high degree of microleakage in gamma group denotes that gamma radiation had a dramatic negative effect on bond strength in both laser and burtreated teeth. This might be attributed to alteration in the crystalline structure and the chemical composition of both enamel and dentine surface.

Conclusion
Both laser and conventional high-speed turbine bur may have good bond strength in control (non-gamma) group. However, both groups showed a high degree of microleakage in the gamma group, suggesting that gamma radiation had a dramatic negative effect on bond strength in both laser and bur-treated teeth.