On the other hand, lack of significant differences between the two groups in the present experiment could be due to the fact that the effect of hydrophilicity, in terms of accelerating bone healing and osseointegration, was unfolded before the first evaluation time-point of 2 weeks, i.e., during the very early healing period. Consistently, pre-clinical investigations show the potential of chemically modified surfaces to rapidly modulate the host-to-implant response upon prompt adsorption of blood proteins [20, 22]. In a recent study reported by Vasak et al. [39], hydrophilic implants (SLActive^a) strengthened the apposition of newly formed bone at the very early healing period between 5 and 10 days in the intra-oral model of minipigs, even though they did not reveal significant statistical differences in comparison to hydrophobic surfaces (SLA). Clinically, a systematic review of human histological studies on molecular aspects of osseointegration [29] has shown that moderately rough implant surfaces with high hydrophilicity enhance molecular processes related to osseointegration during the early stages of wound healing. Similar clinical studies assessed the degree of new bone-to-implant contact (%NBIC) around SLActive implants in comparison to SLA placed in the mandibular retromolar region in man during the early stages of osseointegration [28, 40]. The authors reported a progressive increase in %NBIC around both implants, whereas chemically active surfaces disclosed higher values at 2 and 4 weeks compared with SLA, no longer observed after 6 weeks of osseointegration.

Studies including biomechanical analysis of osseointegration are usually assessing maximum removal torque. Although this parameter is important, use of a single parameter appears not sufficient for a complete biomechanical assessment of osseointegration. Herein, connection stiffness, reflecting the rigidity (i.e., the stability of the implant under load), and removal energy, reflecting the overall energy (workload) necessary to loosen the bone-to-implant connection, were additionally assessed. It is suggested that the use of these three parameters in a complementary manner is essential for complete evaluation of the biomechanical properties of implants. Indeed, two different osseointegrated implants can have the same maximum removal torque, but distinct connection stiffness; similarly, they may show the same removal energy but perform in a very distinct fashion and show distinct connection stiffness and maximum removal torque. To the best of the authors’ knowledge, this is the first original research reporting removal energy as an intrinsic removal torque property in relation to connection stiffness during biomechanical assessment of hydrophilic implants; however, removal energy has already been used assessing mini-implants with hydrophobic surfaces [41].