The exponential relationship between the mass loss and the Hydrofluoric acid concentration may indicate a phenomenon of auto-catalysis as ithas been observed for instance during the HF etching of siliconoxide. On one hand, the reaction products could increase theetch rate. On the other hand, the Sdrincrease over time implies anincrease of the area available for reacting and thus a faster etching.

Since a high value of Sais crucial for osseointegration,  Hydrofluoric acid 40% seems to be the most suitable etching solution because it leadsto the most substantial increase of this parameter in the minimumamount of time. Moreover HF 40% likely leads to a more homoge-neous etching of the surface: the value of the relation Sz/Sais lowcompared to other concentrations, which indicates probably lessexceptional events such as very deep valleys (for instance pits)/highpeaks. On the other hand, Sdr, which is another important parame-ter for osseointegration decreased after two hours of etching in Hydrofluoric acid 40% whereas the mass loss kept increasing, potentially inducingmore damage to the material. It appears thus that within the lim-its of this study a concentration of 40% and an etching time below two hours are the most appropriate conditions for the etching ofzirconia dental implants.

The results of ESI-FTMS highlighted that the etching of 3Y-TZPwith Hydrofluoric acid leads to the formation of soluble zirconium complexes. Twocompounds could not be identified however their isotopic profilesevidenced that they contained zirconium. Consequently the pres-ence of compounds containing yttrium was unlikely although itcannot be discarded. Literature regarding the ESI-FTMS of zirco-nium complexes is scarce, nevertheless the existence of hydroxidessuch as [Zr(OH)3]+ and of fluorides such as [ZrF5]?and [Zr2F9]?is in good agreement with other studies. Doubly chargedspecies [ZrF6]2?were not observed, but solid salts such as K2ZrF6or (NH4)2ZrF6, and hexafluorozirconic acid (H2ZrF6) are known tobe stable. Therefore, taking into account that structural changesmay have occurred during the drying/redissolution process, theirpresence in the original etching solution should not be excluded. productsTo the best of the knowledge of the authors, this is the first timethat the presence of adhered reaction products on the surface of zir-conia is reported during HF etching. For short etching times, onlyoctahedrons were present. As evidenced by 

the SAED and the EDSanalysis they are crystalline and composed of Yttrium and Fluorine.Besides, XPS showed the presence of YF3bonds at the surface of “asetched” zirconia, which suggest that the octahedrons could be YF3crystals. This hypothesis tends to be confirmed by their morphol-ogy which is consistent with what can be found in the literatureregarding this kind of crystals.