**6. Zirconia and surrounding bone**

conclusions. The largest prospective clinical study (831 implants in 378 patients) reported a success rate of 95% after 5 years [32]. The success rate of the acid-etched implants was slightly higher than that of coated and noncoated implants. A 1-year follow-up case series analyzed 56 implants (12 in upper jaws and 44 in mandibles) inserted into 28 patients. A survival rate of 98.2% was found, with an average marginal bone loss of almost 2 mm, which appears quite high, lowering the success rate to 60% [33]. A prospective study with a very small number of cases found a success rate of 100%, with a minimal bone loss after 4 years (0.6 mm) [34]. An in vivo study found a greater bone loss around zirconia implants in respect to titanium implants after 12 months of function. However, no difference in the survival rate was recorded [35]. A recent systematic review of 13 studies (maximum follow-up of 4 years) concluded that the

**Figure 3.** The radiographic control of prosthetic crown cemented on a zirconia implant (courtesy of Prof. Andrea Enri‐

In conclusion, from the available data the osseointegration of zirconia implants seems not to be a problem (**Figure 3**) [37]. Nevertheless, survival and success rates of zirconia implants are inferior to those of titanium ones [13]. For this reason, the majority of authors [12] remain

co Borgonovo, University of Milan).

survival rate of zirconia implants ranges from 67.6 to 100% [36].

96 Dental Implantology and Biomaterial

As the stiffness of zirconia is twice that of titanium, an excessive stress on the trabecular bone around the implant may be expected. Various mathematical studies were performed to analyze the biomechanical behavior of the surrounding bone. One of the first studies in this field compared the response of surrounding bone around titanium and zirconia root-shape implants. No difference emerged from finite element analysis (FEA) [38]. A three-dimensional FEA found no difference in the stress distribution of bone between two versions of the same implant: one made of titanium and the other one made of zirconia [39]. A numeric stress analysis was performed to reproduce the mechanical behavior of the bone around zirconia and titanium implants [40]. The numeric model was also validated from the experimental point

**Figure 4.** The experimental validation of numeric model of a zirconia implant (from Mobilio 2013).

**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).

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 loss.

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].
