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Implantology with “zirconium oxide” implants

Dental implantology has established itself as an important treatment method in dentistry and is based on the biological and functional stabilization of the implant in the surrounding bone tissue, called osseointegration. Screw-shaped, metallic implants made of titanium or a special titanium-zirconium metal alloy have established themselves as the “gold standard”. Numerous experimental and clinical studies prove the excellent osseointegrative capacity and clinical reliability of titanium implants with microrough surface topography.

The development of high-performance ceramics opened up new, metal-free treatment options for both patients and practitioners. Due to its superior biomechanical and biocompatible properties, zirconium dioxide (zirconium oxide, ZrO2) has prevailed over other oxide ceramics and has been used in dentistry for about 25 years. In recent years, zirconium oxide has also established itself on the market as an alternative to titanium in dental implantology. In order to establish zirconium oxide permanently as an alternative to titanium for implant manufacture, ceramic implants must be developed that can grow reliably into bone tissue. Therefore, ZrO2 implants must also have a similarly microrough surface topography as modern titanium implants. However, due to the material properties, it is very difficult to create a microrough surface on ZrO2 implants without weakening the biomechanical strength of the ceramic. The ZrO2 implant systems initially established on the market since about 2004 had a 1-part implant design and already had a “roughened” surface, but there were still differences to the microrough surface topography of modern titanium implants. In addition, implant fractures were also reported in isolated cases due to manufacturing processes that were not optimized for specific materials. Therefore, the early 1-part ZrO2 implants of the “first generation” still showed deficits in terms of clinical reliability. Parallel to the optimization of the surfaces and the manufacturing processes, the macro design of the implants was also adapted and the first 2-part ZrO2 implants were developed and established on the market. This process was not least influenced by the wishes of many users and also confirms the trend towards “two-part” ceramic implantology. A variety of different ZrO2 implant systems now allow the treatment of partially edentulous and edentulous patients, but many users are still very sceptical about the clinical application of commercially available products.

Clinical data on ceramic implants

ZrO2 implants have been available on the market since the beginning of the 2000 years. Over the following years, the industrial implant manufacturing and production process was and is being adapted and optimized. Besides the development of new surface structures, attention was paid to different macroscopic designs of the implants. While the first ZrO2 implants still had a 1-part design, 2-part ZrO2 implant systems have now become available on the market. This allows the fabrication of reversibly screw-retained prosthetic reconstructions. In recent years, numerous clinical studies have been published. However, these differ from each other in the diversity of the implant systems studied and in the survival rates reported. When interpreting the results, it must be taken into account that recent publications in recent years have listed results of ZrO2 implants that are no longer commercially available. In several meta-analyses it could be shown that the different implant systems have significantly different survival rates.

In a systematic review, all clinical studies were integrated that examined ZrO2 implants over a period of at least 12 months in at least 10 treated patients. Studies published between January 2004 and March 2017 were taken into account. The 1-year meta-analysis showed a significantly higher survival rate for commercially available ZrO2 implants (98.3%) compared to ZrO2 implants that were scientifically studied but no longer commercially available on the market (91.2%). Interestingly, the differences in marginal bone loss after 1 year between both study groups were not statistically significant (commercially available: 0.7mm; commercially not available: 1.0mm). In addition, a 2-year survival rate of 97.2% could be calculated for the commercially available implants. Cofactors such as implant design, loading protocol, simultaneous bone augmentation and type of prosthetic reconstruction did not significantly influence the survival rate. These results demonstrated for the first time that the survival rates of ZrO2 implants improved significantly between 2004 and 2017.1 In this context it must be added that not all ZrO2 implants currently on the market have been scientifically investigated.

According to the currently available scientific literature, the clinical follow-up period of commercially available ZrO2 implants is limited to a maximum of 5 years.

Haro Adanez and colleagues also conducted a systematic review with meta-analysis of the results of zirconium oxide implants. They included a total of 17 investigations in their survey. These studies included 1002 patients with 1704 implants (1521 1-part implants 183 2-part implants). The observation period ranged from 1 to 7 years and the mean survival rate was calculated at 95%. 1-piece implants showed a survival rate of 95%, 2-piece implants 94%. The meta-analysis regarding bone loss included 11 examinations with a mean loss of 0.98 mm. The authors assessed the heterogeneity of implant survival and bone loss as statistically significant. The results for 1-part implants were considered good based on the available evidence, whereas the evidence for 2-part implants did not justify their clinical use.

The meta-analyses mentioned above naturally only include the scientific literature available at a given time.

The literature for commercially available ZrO2 implants, in the time frame of the meta-analyses and afterwards, is currently limited to a maximum of 5 years.

The “zirconium dioxide” material

Zirconium oxide is a material composed of zirconium, oxygen and other components, whereby the individual elements are firmly connected to each other in a crystal lattice, i.e. the oxygen is an integral part of the material structure. In contrast, metallic titanium implants only form a stable but very thin oxide layer on the metallic surface when exposed to air. This “protective layer” does not give the metal any physical ceramic properties, but it ensures that there are no undesirable interactions between titanium and adjacent biological material. Therefore, titanium is not a bioinert material per se, but receives its bioinert properties from the stable oxide layer. Furthermore, it should be noted that colloquially ZrO2 ceramics are often wrongly called zirconium or zirconium. Zirconium is the pure metal whose element, identical to titanium, is found in the 4th group of the periodic table. Zircon is the silicate sand zirconium silicate (ZrSiO4), which can be converted into zirconium dioxide in various other technical processes. The ceramic zirconium oxide compounds must be strictly distinguished from the metal zirconium and from zirconium metal alloys. In contrast to metal alloys (e.g. titanium-zirconium alloy), the individual elements in oxide ceramics are not firmly connected to each other by a metallic bond but by a so-called ionic bond. This ionic bond is responsible for the fact that there are only localized electrons in oxide ceramics. This means that, unlike metals or metal alloys, no electrons can be released from the material structure and cause undesirable interactions – such as corrosion.

Ceramic implants and fractures

Compared to other oxide ceramics (such as aluminium oxide), zirconium oxide shows clearly superior biomechanical properties (high bending strength and fracture toughness, low modulus of elasticity). These improved mechanical properties are responsible for the fact that ZrO2 implants can withstand the chewing forces in the oral cavity.

An important factor regarding the fracture frequency is the production process of the implants, especially the method of creating the microrough surface topography. Scientific studies have shown that uncontrolled manufacturing processes to create micro-rough surfaces can reduce the breaking strength of ZrO2 implants.

For the reasons just mentioned, the manufacturing processes for creating micro-rough surfaces are therefore of decisive importance and must be adapted to the material properties of ZrO2. In addition, standardized quality controls must be carried out at the end of the manufacturing process to ensure that the material structure of the oxide ceramic ZrO2 has not been damaged by the manufacturing process. By means of these manufacturing processes adapted to the material properties, it is now possible to produce ZrO2 implants which show a similar fracture rate to titanium implants: the fracture susceptibility of ZrO2 implants was investigated as a co-factor in a meta-analysis. All clinical studies on ZrO2 implants published between 2004 and 2017, which have examined at least 10 patients for a period of 12 months, were integrated. The authors were able to show that the fracture rate improved from 3.4% to 0.2% between 2004 and 2017.

Phase transformation

An important term in the context of fracture susceptibility is the phase transformation of zirconia. This describes the transition from a phase that is unbreakable (tetragonal phase) to a phase that is more susceptible to breakage (monoclinic phase). This transformation is associated with a volume expansion and can stop the propagation of mechanically induced microcracks in the material structure. However, if the ceramic material structure is incorrectly treated or processed (e.g. uncontrolled grinding, manufacturing processes not optimised for the material structure), this transformation can be triggered at an early stage. This means that any microcracks that may occur later can only be compensated for to a limited extent. Processing technologies such as those used for metals cannot and must not be applied to ceramic materials for the same reasons, as the structure of the material can be damaged if it is handled incorrectly. This fact must be taken into account by both manufacturers and users.

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Ceramic implants – clinically relevant advantages over titanium implants?

Through the development of microrough surfaces, titanium implants have become an extremely reliable treatment option. Clinical studies report survival and success rates of over 95% for follow-up periods of up to 10 years.

Therefore, the main reason for the establishment of an alternative implant material – such as zirconium oxide – is not primarily to be found in a further improvement of osseointegrated properties or healing rates. Rather, the question is whether clinically relevant advantages can arise from a new implant material. Pathological tissue alterations on implants – such as peri-implant infections – are among the main reasons for early and late loss of titanium implants and can, therefore, harm clinical reliability. In this context, a reversible inflammation limited to the peri-implant soft tissue, called mucositis, must be distinguished from an inflammation that affects both the peri-implant soft and hard tissue, called peri-implantitis. Scientific studies describe an incidence of 43% for mucositis and 22% for peri-implantitis. A clinically highly relevant question is whether the use of zirconium oxide as implant material can reduce the risk and extent of peri-implant infections compared to titanium.

The development of peri-implantitis can be regarded as multifactorial. An important point here is the microbial colonization on implant surfaces. In an experimental in vitro study, the formation of biofilm was therefore first investigated. For this purpose, an in vitro 3-species biofilm (consisting of Streptococcus sanguinis, Fusobacterium nucleatum, and Porphyromonas gingivalis) or a biofilm obtained from human plaque samples was applied to smooth and micro rough titanium and zirconium oxide surfaces. Subsequently, incubation for 72h in an anaerobic flow chamber followed. The raster-electronic investigation showed a structured, organized biofilm only on the micro rough titanium surfaces, whereas on the other surfaces only non-structured bacterial accumulations were found. The determination of the thickness of the formed biofilm showed a significantly lower biofilm thickness of zirconium oxide compared to titanium for both the 3-species and the human plaque biofilm. In contrast, the determination of the biofilm mass formed only in the human plaque samples showed significantly less biofilm on zirconium oxide compared to titanium, whereas the analysis of the 3-species biofilm did not show significant differences between the two materials.

Independent of these results, however, it is unclear whether material properties – zirconium oxide compared to titanium – also have an influence on inflammation-related peri-implant bone loss. To answer this question, an animal study was conducted, which for the first time investigated artificially caused peri-implantitis with ZrO2- compared to titanium implants in vivo. In the dog model, both implant types were placed in the mandible 18 weeks after initial tooth extraction. After 6 weeks of unloaded healing, the prosthetic restoration of the implants with single crowns was carried out. After a functional loading of 4 weeks, oral hygiene measures were stopped and peri-implant infections caused by subgingival placement of cotton threads in the area of the implant shoulders (active inflammatory phase). After 8 weeks the cotton threads were removed and a spontaneous inflammatory phase of 16 weeks followed, during which no oral hygiene measures were carried out. During the active and spontaneous inflammatory phase, standardized radiographs were taken every 2 weeks to determine peri-implant bone resorption. At the end of the study, the ZrO2 implants showed statistically significantly less peri-implant bone resorption than the titanium implants. Interestingly, a titanium implant was lost during the spontaneous inflammatory phase, whereas no implant loss occurred with ZrO2 implants.

Based on these pre-clinical in vitro and in vivo data, a trend can be seen that ZrO2 implants may have an advantage over titanium implants in the development and course of peri-implant inflammation. However, there is an urgent need for long-term clinical studies to confirm these experimental results.

Hard tissue integration

For zirconium oxide implants to be considered successful, they must heal (osseointegrate) into the bone in the same way as titanium implants. The bone-implant contact is a measure of the biocompatibility of a material. The last 5 years have produced numerous publications dealing with the biocompatibility of zirconium oxide implants, using bone-to-implant contact as well as biomechanical stability as surrogate parameters.

Pieralli and colleagues investigated the extent of osseointegration of zirconium oxide implants in animal experiments. In the end, they were able to include 54 studies in their review that met their inclusion criteria. They were interested in the bone-to-implant contact (KIK; in %), the removal torque (RTQ; in Ncm), and the push-in force (in N). In their analyses of bone-implant contact averaged over all animal models, the titanium implants showed an average KIK of 61% and the zirconium oxide implants an average KIK of 57% – 63%. The difference was found not to be statistically significant.

Interested readers will find in the meta-analysis differentiated results according to influencing variables (surface topography, animal model, etc.). Concerning the torque out, the authors found no significant difference between titanium (103 Ncm) and zirconium oxide implants (95 Ncm). In the rat model, no unscrewing attempts were possible due to the size of the implants. For this reason, push-in tests were carried out: again, there was no significant difference between titanium (52 N) and zirconium oxide (54 N). “Smooth” surfaces generally showed lower removal torques and push-in values than “structured” surfaces. The authors concluded from the results found that there are apparently no significant differences between titanium and zirconium oxide in the ingrowth into the bone.

In a second systematic review with meta-analysis of 37 preclinical studies, the hard and soft tissue integration of zirconia implants was investigated. Again, the study variables were the KIK, the removal torque and the push-in values. The KIK for titanium implants was 59% and for zirconium oxide implants 56% and therefore showed no significant differences. The removal torque for titanium was 103 Ncm and 72 Ncm for zirconia. In contrast to the KIK, this difference was significant. Titanium showed push-in values of 25 N and zirconium oxide of 22 N. In their review, Roehling and colleagues observed significantly lower values for the removal torque for zirconium oxide implants and for the push-in for both materials. However, the authors were able to show that the reported significant differences were not due to the material properties – titanium versus ZrO2 – but to the different surface properties. In this context, the meta-analysis could also show that a surface finish or an increased surface microroughness was associated with an increased osseous integration of the ZrO2 implants. Furthermore, these differences could also be attributed to the different inclusion and exclusion criteria and the different study protocols or animal models (54 studies vs. 37 studies). Therefore, Roehling et al. concluded from their results that an identical hard and soft tissue integration exists in titanium and zirconium oxide implants.

Statement of the ESCI

When selecting ceramic dental implants, it makes sense to ask for scientifically evidence-based data that can be used as a basis for the respective medical device and define its probability of success. From the preclinical investigations it can be concluded that microrough surfaces of ceramic implants lead to a positive influence on the KIK.

Soft tissue integration

preclinical

For soft tissue aesthetics, healthy and constant vertical dimensions of the peri-implant soft tissue are crucial. On teeth and implants, the soft tissue dimensions include the sulcus depth, the marginal epithelium and the connective tissue attachment, which together as a vertical “unit” form the so-called biological width/the so-called dentogingival complex.

From a periodontal perspective, peri-implant soft tissue has a similar barrier function to dentogingival tissue and helps prevent bacterially caused peri-implant infections.

In einer Literaturübersicht stellten Nishihara et al. fünf vorklinische Untersuchungen vor, die sich mit der Weichgewebsantwort gegenüber Zirkonoxidimplantaten beschäftigten. Die Autoren stellten fest, dass die Mehrzahl der Untersuchungen keinen Unterschied in der Weichgewebsmorphologie zwischen Titan und Zirkonoxid zeigten. Der Weichgewebsring um den Implantathals bestand bei beiden Materialien aus einer ähnlich dicken Epithelschicht mit darunterliegender Bindegewebsschicht. Eine der vorgestellten Untersuchung zeigte eine Weichgewebshöhe von 4,5 mm für Zirkonoxidimplantate und 5,2 mm für die Titanimplantate. Die Ausdehnung des Epithels bei den beiden Materialien war vergleichbar (2,9 mm). Die Ausdehnung des Bindegewebes zeigte einen – statistisch nicht signifikant – Unterschied (Zirkonoxid: 1,5 mm; Titan: 2,4 mm). Die anderen Untersuchungen zeigten insgesamt eine geringere Weichgewebshöhe in der Größenordnung von 3 bis 4 mm, abhängig vom angewandten Tiermodell.

Materialeigenschaften – Zirkonoxid im Vergleich zu Titan – scheinen sich nicht signifikant auf die peri-implantäre Weichgewebsintegration auszuwirken. Daher zeigen beide Materialien ähnliche physiologische Prozesse bei der Entwicklung des peri-implantären Weichgewebes.

In further experimental studies an equivalent qualitative and quantitative soft tissue integration and identical dimensions of biological width could be demonstrated for ZrO2- compared to titanium implants. For both materials, the biological width and the peri-implant papilla height obviously do not depend on the loading and surgical protocol but on the implant design and the position of the microgap between implant shoulder and prosthetic supra-construction.

Interestingly, an experimental study reported faster maturation of peri-implant epithelial and connective tissue for ZrO2 implants.

clinical

Apart from function, the aesthetic result is decisive for patient satisfaction. The aesthetics are influenced by tooth crowns and the surrounding soft tissue. Of particular importance are non-irritating peri-implant soft tissue conditions, characterized for example by the gingival margin or peri-implant papilla formation (pink aesthetics).

Clinical data that allow an objective assessment of peri-implant mucosal conditions are rare. With regard to the Jemt Papilla Index, a significant increase in peri-implant papilla formation was observed. The authors were able to show that there was a significant increase in the Papilla Index between the time of functional loading and the 3-year follow-up.

In addition, the peri-implant mucosal conditions were evaluated using the “Pink Esthetic Score (PES)” according to Fürhauser. In clinical studies a constant increase of PES values could be observed within the first 2 years after implantation for 1- and 2-part implant designs. Interestingly, in one of these studies even significantly higher PES values were found for ceramic implants (prosthetically restored with ceramic crowns, PES 6.9 – 11.2) compared to titanium implants (prosthetically restored with titanium abutments and ceramic crowns, PES 2.4 – 10.8).

For 1-part ceramic implant designs, clinical studies have also shown that a significant increase in peri-implant disc height is achieved with increasing time of functional loading. The distance between the alveolar ridge on the adjacent tooth and the lowest point of contact of the adjacent crown was an important factor for peri-implant papilla formation.

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