Keywords: Femtosecond laser; Surface processing; Mathematical model; Dental hard
tissues; Physiochemical properties; Ablation profile; Ablation parameters.
The conventional dental treatment causes a lot of pain and discomfort to patients with
excessive tissue damage. The laser-based tooth preparation is an emerging non-invasive
a technique in dentistry, reasoning that it can overcome the drawbacks of conventional tools.
Recently developed ultrashort pulse laser is capable of delivering very high-intensity laser
pulses within the thermal relaxation time. As a result, the high precision and efficient laser
ablation can be achieved on dental hard tissues with minimal thermal damage by reducing the
laser-tissue interaction time.
The present research aims at developing a reliable mathematical model to predict the laser ablation
profile concerning the teeth heterogeneity for safer laser surgery. Concurrently, the research
work also explores ways to enhance femtosecond (fs) laser ablation rate and ablation efficiency
using physiochemical surface modification in view of reducing the treatment time. Since not
much work has been done on the complete prediction of laser ablation profile on dental hard
tissues, a simple mathematical model was constructed based on Gaussian beam distribution of
laser intensity. The laser ablation experiments were carried out within the well-ablated region
on dental hard tissues to obtain laser ablation parameters (effective Gaussian beam radius,
ablation threshold fluence, and effective energy penetration depth). The laser ablation profile,
ablation rate and ablation efficiency were predicted from the mathematical model. It was found
that the predicted ablation profile agrees with the experimental profile for both enamel and
dentin except a slight deviation at higher fluence for dentin. The calculated ablation rate was
comparable to that of experimental results whereas for ablation efficiency appreciable
deviation was observed in the case of dentin. Further, the study has also been focused on extracting
the laser ablation parameters from the physicochemical characteristics of teeth.
The physicochemical properties of teeth (surface morphology and chemical composition) play
a vital role in deciding the value of laser ablation parameters; therefore, the surface morphology
(dentin tubules area) and elemental composition (Ca, P and C) of different dentin samples
obtained by scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy
(EDS) were correlated with their corresponding laser ablation parameters. The surface
morphology and chemical compositional analysis indicate a strong correlation between
physicochemical characteristics and laser ablation parameters. From the knowledge of linearity
analysis, the laser ablation parameters of a random dentin sample were obtained. A complete
theoretical ablation profile was constructed and validated with an experimental ablation profile. Furthermore, the laser ablation rate and ablation efficiency were also predicted by this method
so as to optimize the laser processing conditions for any specific teeth overcoming the problem
of teeth heterogeneity. However, for obtaining physiochemical characteristics, the SEM and
EDS used are generally destructive methods.
Consequently, a similar study has been carried out using nondestructive methods such as
Fourier transform infrared spectroscopy (FTIR) and Raman. It is well known that the teeth
heterogeneity arises out of variation in chemical composition such as phosphate and amide
group. The heterogeneity of the teeth has been established by a chemical composition which was
measured in terms of the peak intensity of the spectra (FTIR and Raman) on different enamel and
dentin samples. An empirical relation was established between chemical composition and laser
ablation parameters. A strong correlation has been observed between laser ablation parameters
and peak intensity of the phosphate group obtained from both FTIR and Raman spectroscopy.
Using the correlation, the laser ablation parameters for the random dentin and enamel samples
were calculated and the ablation profile was predicted. The predicted ablation profile based on
both FTIR and Raman probes well-matched in dentin whereas they show a slight deviation in
enamel. Thus, FTIR and Raman probes will enable to predict ablation profile non-destructively
in real-time clinical use and obviates the need for trial runs. Furthermore, the present study has
predicted the laser ablation rate and efficiency for performing laser surgery in optimum laser
processing conditions by incorporating teeth heterogeneity.
On the other front, the employability of non-invasive fs laser ablation technique for dental
treatment is severely limited by its low ablation rate. The conventional method of increasing
the laser ablation rate by increasing the laser power also aggravates tissue damage. The present
work explores the physiochemical surface modification as a novel way to improve fs laser
ablation rate and efficiency. Surface modification of dental hard tissues has been carried out
by pretreatment (food graded orthophosphoric acid and carie-care gel) and the laser ablation
characteristics were studied. It was found that the ablation threshold fluence decreased by
almost one-third whereas ablation rate and ablation efficiency nearly tripled upon pretreatment.
The results were discussed in terms of the heat accumulation model and defect model. The
microstructural and compositional analysis clearly reveals that the surface modification and
demineralization reduce the threshold fluence and increase the ablation rate by effective
utilization of the laser beam energy. Hence, the physiochemical surface modification can be an
efficient method to improve the laser ablation rate and ablation efficiency.
In essence, the present study has led in the development of a highly reliable method for predicting
ablation profile and augmented the ablation rate. Thus, high-precision dental surgery can be
done in real-time with limited supervision, which can be readily employed in clinical settings.