1.

Introduction

The strength of bonding between the tooth structure and restorative materials can have a significant influence on the long-term clinical performance of a dental treatment. Continuous attempts have, therefore, been made to obtain a permanent adhesion between the tooth and the restorative materials. Generally, three main factors ensure the formation and permanency of an adhesive bond: (a) establishing intimate contact between the liquid adhesive and the solid adherend (i.e., good wetting); (b) minimizing the concentration of stress at the interface; and (c) minimizing the attack on the interface by environmental factors,^1,2 although studies have shown that the wettability of dental hard tissues is the most important of these factors.³

Three main factors influence the wetting of a solid surface by a liquid.¹ The first is the relative surface energy of the solid and the surface tension of the liquid. This relationship is a function of the chemical composition and heterogeneity of the solid and liquid surfaces to be bonded. The second factor is the surface topography of the solid surfaces to be bonded. It is presumed that surface rugosity and irregularities increase wettability by increasing the surface area available for bonding.⁴ The third factor is the viscosity of the liquid, which is a measure of its resistance to flow.⁵

In recent years, lasers have been used for dental treatment.^6–16 In 1990s, the ultrashort pulsed laser was proven to be superior to both conventional rotary instruments and long-pulse laser systems when ablating dental hard tissues.¹⁷ With the appropriate parameters, use of the ultrashort pulsed laser, which employs a plasma-mediated ablation mechanism,^18–21 reduced levels of microcracking, carbonation, melting and resolidification, and smear layer formation while leaving dentinal tubules mostly open.^19,22–24

Since surface roughness and wettability greatly influence bond strength between dental hard tissues and restorative materials, the effects of ultrashort pulsed laser ablation of dentin on these factors should be explored. In the current literature, only a few studies have reported on these specific relationships.

2.

Materials and Methods

3.

Results

3.1.

Surface Roughness

The effects of ultrashort pulsed laser ablation on dentin surface roughness were studied, and the results are shown in Table 2 and Fig. 1.

Table 2

Mean surface roughness values of ultrashort pulsed laser-ablated dentin. Mean surface roughness or arithmetical mean deviation of the profile (Ra), root mean square of the profile (Rq), and maximum peak-to-valley height of the profile (Rz).

Group	Mean±SD
	Ra (μm)	Rq (μm)	Rz (μm)
G1	$4.99\pm 0.54$	$5.85\pm 0.65$	$36.86\pm 5.77$
G2	$6.43\pm 0.35$	$7.79\pm 0.41$	$46.61\pm 4.23$
G3	$5.58\pm 0.51$	$6.77\pm 0.73$	$43.57\pm 9.93$
G4	$4.57\pm 0.25$	$5.74\pm 0.33$	$45.23\pm 3.22$
G5	$3.45\pm 0.29$	$4.33\pm 0.36$	$43.08\pm 13.29$
G6	$3.42\pm 0.25$	$4.28\pm 0.31$	$44.50\pm 8.19$
G7	$3.47\pm 0.52$	$4.35\pm 0.63$	$41.67\pm 6.58$
G8	$3.64\pm 0.32$	$4.57\pm 0.39$	$40.77\pm 6.68$
G9	$4.59\pm 1.18$	$5.64\pm 1.44$	$34.65\pm 7.50$

Fig. 1

Mean surface roughness of ultrashort pulsed laser-ablated dentin: groups (G1 to G8) are differentiated based on the absence or presence of pulse overlap (PO; 0% or 50%, respectively) for each level of fluence used.

Statistical analysis of Ra was carried out among groups G1 to G4 (no PO) and G5 to G8 (50% PO) to evaluate the effect of laser fluence at the same PO. Least significant difference post-hoc comparison tests revealed that there was a significant difference between each group from G1 to G4 ($P<0.05$) and that there was no significant difference from G5 to G8 ($P>0.05$). The influence of PO on Ra was also evaluated, albeit among group pairs (G1/G5, G2/G6, G3/G7, and G4/G8) ablated with the same laser fluence but different PO. Independent-samples $T$ tests revealed that the PO significantly influenced Ra in each pairing, regardless of fluence level ($P<0.05$), which meant that there were significant differences between members of each group pair. Further independent-samples $T$ tests revealed that the Ra of all test groups, except for G1 and G4, was significantly different than that of control G9 ($P<0.05$).

3.3.

Scanning Electron Microscopy Evaluation

As shown in the micrographs of Fig. 2, following ablation, laser fluence, and PO had noticeable effects on the integrity of the dentin structure. Specifically, in G1 to G3, some of the dentin structure remained intact; clear ridges and valleys were observed, and the dentin surfaces were rougher. At the highest laser fluence (G4) or at the higher PO (G5 and G6), the dentin surfaces were entirely ablated without any remaining intact dentin structure. No smear layer was observed in any of the micrographs, and most of the dentinal tubules remained open. However, with highest laser fluence and higher PO (G8), areas of melting and resolidification could be observed. Microcracks could be observed in all groups.

Fig. 2

Representative micrographs of ultrashort pulsed laser-ablated dentin surfaces: (a) G1, (b) G2, (c) G3, (d) G4, (e) G5, and (f) G6.

Topographic images of laser-ablated dentin were also obtained using a 3-D profile measurement laser microscope (Fig. 3). G1 topography [Fig. 3(a)] was consistent with the SEM micrograph; clear ridges and valleys were observed. G8 topography [Fig. 3(b)], which represents use of the highest laser fluence and higher PO, was also consistent with the SEM micrograph; the surface was flatter, without ridges and valleys.

Fig. 3

Topographical images of ultrashort pulsed laser-ablated dentin obtained from a three-dimensional profile measurement laser microscope: (a) G1 and (b) G8.

4.

Discussion

In this study, different parameters of an ultrashort pulsed laser were analyzed quantitatively for their effects on the surface roughness and wettability (critical factors of bond strength) of dentin following ablation and compared to the effects of a common rotary grinding instrument.

With pulse duration of 20 ps and the laser fluence, the peak power (PP) used in this study was about ${10}^6$ to ${10}^7\mathrm{W}$. With the high PP, the effect of wavelength on dentin would be less of an issue. By reviewing the literatures, the laser fluence is considered as an important factor that influences the outcome after ablation of dental tissues,^25–27 and the laser fluence also has a relationship with $\mathrm{PP}(=F·\pi {\phi }^2/4\tau )$. The effect of PO could guide the determination of scanning speed and scanning line spacing when choosing laser parameters. It was also noted that when the highest laser fluence and higher PO were used, melting and resolidification was observed. One reason for this effect could be heat accumulation; Rego et al. reported that thermal damage could be present when higher laser energies are used or when the tooth is significantly dehydrated.²⁸ In almost all groups, microcracks were observed. It seems that the microcracks could be induced during sample preparation for SEM evaluation.

Surface roughness is an important factor that influences the bond strength between tooth structure and restorative materials. It can also have an effect on the quality of a preparation as it directly affects the accuracy of cast and prosthesis retention.^29–32 The Ra value of surface roughness is the most important parameter used to describe the surface roughness of a substrate, so the current study focused on the analysis of Ra following ablation. In this study, when a PO of 0 was used, laser fluence significantly influenced dentin surface roughness, and it seems that from G2 onward, the dentin surface roughness decreased with increasing laser fluence. However, when a PO of 50% was used, laser fluence did not significantly influence dentin surface roughness. These effects can be easily reasoned. As fluence is increased, the surface is ablated with more power, thus removing more of the surface and causing it to become flatter, or less rough. When a 50% PO is added, however, each focal point is effectively ablated twice, which in this case appeared to significantly reduce the influence of laser fluence (at least at the levels tested here) on surface roughness. Notably, surface roughness of the control group (G9) appeared to be generally lower than that of some groups without PO (G1 to G3) and higher than that of groups with 50% PO (G5 to G8), albeit without statistical significance.

Wettability is an important factor affecting the bond between the dental substrate and restorative materials.^33,34 For having been used as a means of characterizing the wetting of a substrate by the fluid phase, contact angle measurement is considered to be a useful indicator of interfacial tension.^4,35 Although it was hypothesized that surface roughness may improve wettability by reducing the contact angle,³⁶ the mean contact angle of all laser-ablated groups in this study was approximately 0 deg, regardless of surface roughness, while that of the control group was significantly larger.

Vorobyev and Guo³⁷ reported on using a femtosecond pulse laser with a unique surface texturing approach to make human dentin superwetting. In their studies, superwetting is strongly related to the presence of open capillary systems, such as surface grooves, which depend on surface geometry.^38–40 By varying the geometry of the microgrooves, wetting could be controllable. However, the femtosecond laser used in their study operated at a 1-kHz repetition rate, which was not enough to allow for efficient ablation. Some researchers have voiced concern over the low ablation efficiency of ultrashort pulsed lasers in dentistry.^26,41 In recent years, however, some studies reported that, with the use of a laser scanning system, it was possible to ablate a large area with an ultrashort pulsed laser.^42–44

In the current study, with the use of such a scanning system and operating at a 100-kHz repetition rate, the ablation efficiency of an ultrashort pulsed laser was indeed proven. When a lower PO was used, microgrooves were observed, possibly contributing to the superwettability of the dentin surface. When a higher PO was used, dentin surfaces were flatter and had a lower surface roughness than control group; however, superwettability still existed, which meant that microgrooves were not essential to improve the wettability of the dentin surface.

The smear layer produced by mechanical rotary instruments could have a more prominent role than surface roughness in determining wettability.^29,45 Indeed, it tends to act as a physical barrier to the diffusion of oral fluids and restorative materials,⁴⁶ occluding dentinal tubules and reducing dentin permeability.^47,48 In this study, no smear layer was observed in any of the laser-ablated dentin samples. Moreover, the dentinal tubules remained mostly open, which may have contributed to the superwettability of dentin surfaces, even of surfaces ablated with a 50% PO where the surface roughness was lower than that of the control. Achieving superwettability through ultrashort pulsed laser ablation should enhance bond strength. This effect will be explored in future studies.

5.

Conclusion

Surface roughness of ultrashort pulsed laser-ablated dentin decreased with higher laser fluence and PO. However, a 50% PO appeared to trump the effect of the fluence levels used in this study. Furthermore, wettability was significantly increased in all laser-ablated dentin samples, which each showed a contact angle of approximately 0 deg, compared to the manually ground control samples.