**7. Peri-implant soft tissue response**

of view (**Figure 4**). The results showed that stress states generated in the bone by the two implant types were very similar; therefore, from a mechanical point of view, zirconia is found to be a feasible substitute for titanium. But more interestingly, results showed that the two implants moved differently: titanium implants generate higher stress on the cortical bone, whereas zirconia implants produce stress mainly in the trabecular bone. This different behavior is directly related to different Young's modulus values of the two materials: while titanium leans against the cortical bone and its exterior part is more prone to bending under load, zirconia is too stiff to bend and transmits stresses along its axis down to the trabecular bone, thus moving more as a rigid body (**Figure 5**). This difference in motion between the two implants is important considering crestal bone loss. Bone resorption around implants is a common phenomenon that begins at the cervical level and can progress in the apical direction. No conclusive data are available on contributing factors involved in such a bone loss, but concentration of stresses around the neck of the implant due to functional and nonfunctional loads may be one such factor. In this view, it can be speculated that decreasing the stress concentration at the cervical level may reduce the effect of mechanical factors on crestal bone

**Figure 5.** Deformed shape of the titanium (on right) and zirconia implants (30× magnification): the titanium implant

shows a higher head displacement in the *x*-direction (from Mobilio 2013).

98 Dental Implantology and Biomaterial

Other FEA studies found similar results. The model of a maxillary overdenture on four implants with ball attachments revealed no difference in the stress and strain values in periimplant bone, using titanium or zirconia [41]. A three-dimensional FEA model found no difference between titanium and titanium-zirconium alloy implants, neither for early nor conventional functional loading [42]. A study found difference in bone behavior depending

on the macrogeometry of the zirconia fixture [43].

loss.

Zirconia is advocated to have high biocompatibility and to have no adverse effect on the surrounding tissues (**Figures 6** and **7**) [44]. Many studies evaluated tissue response to zirconia, concluding that zirconia has the ability to interact with peri-implant soft tissues (**Figure 8**) [14]. The low bacterial colonization typical of the zirconia surface maybe plays a role in this high biocompatibility [12]. In a randomized-controlled trial (RCT), both titanium and zirconia onepiece implants supporting overdentures were evaluated [35]. Even if the crestal bone level changed greatly, no difference in clinical parameters (probing depth, bleeding index, plaque index, etc.) was found around the two types of implants after 12 months of function.

**Figure 6.** Clinical aspect of the abutment part of zirconia implant before cementation of crown (courtesy of Prof. An‐ drea Enrico Borgonovo, University of Milan).

**Figure 7.** Clinical aspect after finalization (courtesy of Prof. Andrea Enrico Borgonovo, University of Milan).

**Figure 8.** Histological section of zirconia implant inserted into a minipig. The tight connection of the soft tissue to the implant surface is shown (courtesy of Dr. Mai, University of Dresden).

