Zirconia is a bioinert nonresorbable metal oxide that offers mechanical properties which are superior over other ceramic biomaterials, e.g. high fracture toughness and bending strength [14
]. Because of its good chemical and material stability, high strength and resilience it seems to be a suitable material for dental application [7
]. Its successful application in dentistry for fabricating endodontic posts and for crown and bridge restorations has been reported in several studies [15
]. Especially because of its tooth-like colour, zirconia was suggested to be a desirable alternative material to titanium for the fabrication of dental implants. The results of the present study have shown that zirconia implants fabricated with a modified surface seem to be integrated into bone in a similar fashion as titanium.
After one week of healing, distinct gaps between the implant and the bone filled with matrix-rich regeneration tissue were observed in a few locations. After 4 weeks, woven bone, and after 12 weeks, lamellar bone, was visible in intimate contact with the implant surfaces. A loose connective tissue layer separating bone tissue and the zirconia surface as described by Sennerby et al. [6
] previously, was not found in our samples.
Osseointegration of threaded zirconia implants has been recently investigated by Rothamel et al. [18
]. They compared the osseous healing of zirconia implants with modified (machined and sand blasted) implant surfaces from polished commercially pure titanium. After 4 days of healing time, a distinct gap between bone tissue and the implant surface filled with remodelling blood clot was noticed. Two weeks after implantation, woven bone growing in the direction of the implant was observed, followed by the formation of lamellar bone after 28 days. When the healing period was nearly completed after 8 weeks, intimate contact of lamellar bone to the implant surface was evident. However, the barrier resulting from the original gap was still visible with many osteoblasts bridging the gap, which indicates a high biocompatibility of the used implant materials.
The results of the present study also showed an increasing BIC over the healing period. However, there were no statistically significant histomorphometrical differences observed between zirconia and titanium implants. This finding is in accordance with other animal studies which also failed to demonstrate differences between structured zirconia and titanium implant surfaces [2
], likely secondary to the fact that zirconia is highly biocompatible. An average BIC > 60%, which was achieved after 4 weeks following implantation, had been reported by several authors [2
]. The reported differences in BIC seem to be attributable to different animal models (dogs, monkeys, rabbits and minipigs) used for the experiments [2
]. In order to establish standardized conditions for the histomorphometric analysis, implants were placed in the tibia since this bone has constant bone geometries over a longer distance. Therefore, the BIC only depends on the implant osseointegration and not on the bone features at the implantation site. In contrast to the results from a similar study [21
], there were no detachment or separation of bone tissue and the zirconia surface with loose connective tissue detectable at any time.
The BIC measured in our study (45.3% after 4 weeks) showed similar results as demonstrated by Sennerby et al. [6
]. The authors demonstrated a BIC of 36% for the non-modified zirconia implants and BICs of more than 45% for the zirconia implants with surface modification after 6 weeks of healing in the tibia of rabbits.
Scarano et al. [10
] observed 68% BIC of the untreated zirconia implants after 4 weeks in the tibia of rabbits. After 6 months of unloaded healing in the mandibles of dogs, Dubruille et al. [9
] measured a BIC of 65% for the zirconia implants compared with 68% of alumina implants and 54% of the titanium implants. The surface topography of the implants in these studies was not investigated. Kohal and coworkers [2
] determined slightly higher BIC values after implant insertion into the maxillae of monkeys followed by 5 months of loaded healing (68% for sandblasted zirconia implants and 73% for sandblasted and acid-etched titanium implants). However, the surface topography was not measured or described. In the present study, a BIC of 71% for the acid-etched zirconia and 83% for acid-etched titanium implants were measured after 3 months of implant insertion.
It is well known that surface modifications can enhance bone integration of titanium implants in diverse animal models [22
]. According to the results of several earlier experimental studies, surface roughness and topography influence osseointegration of zirconia implants to a greater extend [11
]. Sennerby et al. [6
] used a coating technique to receive porous surface modifications of the zirconia implants (nonmodified implants: Sa = 0.75 μm; modified implants: Sa = 0.93 μm, Sa = 1.24 μm, respectively). In spite of evident differences in surface roughness, there were no significant differences observed in the osseointegration (BIC or bone area filling in the threads) in the investigated implants. Only removal torque test values were significantly lower of the nonmodified zirconia implants compared with all other implant types. These results and the results of Scarano et al. [10
], who used unmodified zirconia implants, indicate a considerable biocompatibility of zirconia implants, even without surface treatment.
In contrast to the study of Sennerby et al. [6
], an acid-etching technique was used in this study to receive structured surface modification of zirconia implants. Surface modification by acid-etching is assumed to affect not only the microtopography, but also submicrometric and nanometric topography of implant materials. Sa or Ra values only refer to the average surface roughness. These values do not provide much information about the submicrometric and nanometric surface topography (Ra is the two-dimensional (2D) counterpart of the three-dimensional (3D) descriptor Sa. Both Ra and Sa reflect the arithmetic mean of the absolute values of the surface point departures from the mean plane within the sampling area [24
Submicrometric and nanometric topography determine cell reactions including cell orientation, changes in cell motility, cell adhesion and cell shape. Therefore these topographic features play an important role in the early state of osseointegration of dental implants [25
]. In addition, differences in the physico-chemical properties of the material also affect cell responses [26
The successful integration of zirconia implants into native bone tissue and comparable BIC was demonstrated in this study, however the used modified zirconia implants exhibited a considerable lower Ra value when compared to the titanium implants. Furthermore, the process of osseointegration of zirconia implants showed similarities to that known for titanium implants. This may be due to the fact that surface topography is not the only controlling factor when studying the biologic response to an implant material.
The results of earlier described studies implicate a good biocompatibilty even of unmodified zirconia implants. The submicrometric and nanometric topography of the zirconia surfaces produced by the acid-etched modification may have an additional synergistic effect on biocompatibilty and osseointegration of zirconia implants [27
]. Further studies are needed to examine the influence of submicrometric and nanometric surface topography of zirconia implants to the osseointegration process.