**4. Osseointegration**

The capacity to achieve osseointegration is the most investigated aspect regarding zirconia implants. To evaluate implant osseointegration, the following parameters are widely used:


The BIC value is usually studied using histomorphometry on histological sections. The torque removal force is considered a biomechanical measure of osseointegration: the greater the force is required to remove implants, the greater the strength of osseointegration. CBL is a clinical parameter related to the maintenance of osseointegration in time, and so it is related to survival and success rate of implant therapy (see Section 5).

One of the first animal studies investigating the osseointegration of zirconia implants was conducted in a rabbit model [20]. After 1 month from the insertion, the histological analysis showed newly formed bone close to the implant surface, affirming the osteoconductive property of zirconia. Titanium and zirconia implants were inserted in monkeys and after 3 months were functionally loaded for 5 months. The histological analysis performed later revealed no difference in osseointegration [21]. Titanium, machined zirconia, and surfacemodified zirconia implants were inserted into rabbit. No difference in the removal torque was found between titanium and surface-modified zirconia, but machined surface zirconia implants performed badly. Such results seem to suggest that a modification of the zirconia surface is recommended to increase the bone tissue response [22]. Titanium, machined zirconia, and sandblasted (rough) zirconia were inserted into the maxillae of miniature pigs, and then removed. The removal torque test revealed that rough zirconia implants can achieve a higher stability than machined implants [23]. A detailed analysis performed using scanning electron microscopy (SEM) [24] and histomorphometry [25] revealed no difference of osseoin‐ tegration between titanium and zirconia implants inserted into minipigs. A study compared the osseointegration of zirconia and titanium implants in dogs, indicating no difference in BIC values between the two types of implants [26]. Another study performed a similar analysis in pigs. After 4 weeks from the insertion, no difference in terms of BIC was found between zirconia and titanium implants (**Figure 2**) [27]. Another histomorphometric study conducted on dogs found no difference in osseointegration and tissue response between titanium, and coated and noncoated zirconia implants [28]. Different implants (titanium and zirconia) used in pigs showed no significance difference in BIC values [29]. Calvo-Guirado et al. [30] found no difference in BIC values between zirconia and titanium implants in an animal model, and they concluded that both implant types produce good osseointegration.

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

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

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

The capacity to achieve osseointegration is the most investigated aspect regarding zirconia implants. To evaluate implant osseointegration, the following parameters are widely used:

The BIC value is usually studied using histomorphometry on histological sections. The torque removal force is considered a biomechanical measure of osseointegration: the greater the force is required to remove implants, the greater the strength of osseointegration. CBL is a clinical parameter related to the maintenance of osseointegration in time, and so it is related to survival

One of the first animal studies investigating the osseointegration of zirconia implants was conducted in a rabbit model [20]. After 1 month from the insertion, the histological analysis showed newly formed bone close to the implant surface, affirming the osteoconductive property of zirconia. Titanium and zirconia implants were inserted in monkeys and after 3 months were functionally loaded for 5 months. The histological analysis performed later revealed no difference in osseointegration [21]. Titanium, machined zirconia, and surfacemodified zirconia implants were inserted into rabbit. No difference in the removal torque was found between titanium and surface-modified zirconia, but machined surface zirconia implants performed badly. Such results seem to suggest that a modification of the zirconia surface is recommended to increase the bone tissue response [22]. Titanium, machined

biomechanical contraindications are present for clinical use of zirconia implants [12].

fracture strength ranged within the limits of clinical acceptance [18].

has not been taken into account [19].

**•** bone-to-implant contact (BIC) value;

and success rate of implant therapy (see Section 5).

**4. Osseointegration**

94 Dental Implantology and Biomaterial

**•** torque removal force; **•** crestal bone loss (CBL).

**Figure 2.** Histological section of zirconia implants inserted into a minipig. 1: neo-osteogenesis; 2: osteoblasts on the im‐ plant surface (courtesy of Dr. Mai, University of Dresden).

From the totality of animal studies, it is possible to conclude that zirconia is an osseoconductive material [14], and therefore it can be utilized as a material for dental implants [31].
