**3. Mechanical properties of zirconia implants from experimental and clinical data**

Considering the difficulty of analyzing the mechanical outcome of implants in clinical scenarios, preclinical studies are fundamental to accomplish this issue. Different in vitro studies evaluated the biomechanical behavior of zirconia implants with prosthetic reconstruc‐ tions. The fracture strength of zirconia crowns on zirconia implants was compared to that of metal-ceramic crowns on titanium implants, in an upper central incisor model. No difference was found between implants, with and without cyclic loading before fracture test [16]. The same authors also showed that preparation of zirconia implants to receive prosthetic crown may negatively affect the fracture strength, even if it was still in an acceptable clinical range [17]. Another in vitro study evaluated the fracture strength of zirconia implants in comparison with that of titanium implants under a 130° angled load, simulating that of an upper central incisor. Despite the high dispersion of fracture loads (typical of ceramic materials), the mean fracture strength ranged within the limits of clinical acceptance [18].

With caution, it is possible to affirm that from experimental preclinical studies, the biome‐ chanical behavior of zirconia implants does not differ from that of titanium implants. So, no biomechanical contraindications are present for clinical use of zirconia implants [12].

The majority of clinical studies focused on achieving and maintaining osseointegration in time. In these studies, the main cause of failure is represented by marginal bone loss and/or the loss of osseointegration (see below). However, one clinical study considered just implant fracture as cause of failure: the survival rate was 92.5% after about 5 years, the loss of osseointegration has not been taken into account [19].
