Laser technology has been extensively used for dental treatment since the 1990s [1]. In particular hard tissue  ablation  is  performed  by  Er: YAG  lasers  due  to  their  affinity  with  water  and  hydroxipa tite,  thus  avoiding  the   formation of micro- and macro-cracks, and thermal damaged areas[2 - 5]. On the other hand Er: YAG lasers are  affected  by  cumbersome  instrumentation,  and  operation  durations  can  be  higher  than  the  ones  of  traditional   instruments. Records of dental surface treatment with ultra-short pulsed lasers, 150 - 500 fs, are also present, at  such pulse duration remarkable results have been obtained, regardless to the laser emission wavelength [6,7].   In this work human extracted teeth have been exposed to 1064 nm, 500 ps, laser radiation (Helios IR 5W  – Innolight  GmbH).  During  exposures  the  temperature  has  been  continuously  monitored  inside  the  tooth  with  a   thermocouple,  in  this  way  it  was  possible  to  monitor  the  thermal  increase  ( ' T)  where  the  dental  pulp  is.  The   exposed samples have been cooled by continuously flowing water on them.  Fig. 1 represents optical microscope images of drilled holes. It is possible to observe the complete absence of  cracks, and any thermal damage. The  ' T has been measured as 7.1°C on average, when room temperature water  was  used  for  cooling,  above  the  5.5°C  accepted,  after  which  irreversible  damage  to  the  dental  pulp  is  caused.   When  cold  water  (10°C)  was  used  for  cooling,  an  average   ' T  of  4.3°C  has  been  measured,  with  a  maximum   measured temperature of 5°C, in this way it was possible to keep the  ' T under 5.5°C.


Fig. 1
Images of two different holes drilled, in two different teeth, are here reported. It is possible to observe the absence of cracks and smears. Exposures have been performed at the repetition rate of 30 kHz, and pulse duration  of 500 ps.
The results demonstrate that the ablation of dental hard tissue by means of a solid state picoseconds laser, working in the near infrared region, is effective without damaging the tooth vitality, and for this it is fundamental to keep the 'T under the maximum value accepted. The use of this kind of laser can result as a good alternative to Er:YAG lasers, and it is also a cheaper solution compared to the costly fs lasers systems.
Acknowledgment
The authors would like to acknowledge the NEXPRESSO programme for purchasing the Helios laser on behalf of the working group.