Annotated literature review
Literature
Scientific information on ceramic implants
In dental implantology, ceramic implants are becoming increasingly important and are a serious addition to the proven implantological therapy with titanium implants. In particular, intensive research and rapid development in the areas of materials, surface design and restorative care have contributed to this development. Short and medium-term scientific data are already available – further studies must follow. It is important to evaluate these data correctly, interpret them correctly and classify them for a broader application, transport them objectively and put them into practice with background knowledge. Open questions must be discussed and answered on an evidence-based basis in the interest of the patient.
In this section “Specialist Information”, the ESCI Scientific Advisory Board therefore compiles and continuously updates carefully compiled, scientifically sound and evidence-based facts about ceramic implants and their application.
Ceramic Implantats
Ceramic implants... a new way?
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 one-part implant design and already had a roughened surface, but there were still differences compared to the microrough surface topography of modern titanium implants. Additionally, implant fractures were reported in isolated cases due to manufacturing processes that were not optimized for specific materials. Therefore, the early one-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 two-part ZrO2 implants were developed and established on the market. This process was influenced by the wishes of many users and confirms the trend toward 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 skeptical about the clinical application of commercially available products.
Evidence...how reliable are ceramic implants?
Clinical data on ceramic implants
ZrO2 implants have been available on the market since the beginning of the 2000s. Over the following years, the industrial implant manufacturing and production process was 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 one-part design, two-part ZrO2 implant systems have now become available. This allows the fabrication of reversibly screw-retained prosthetic reconstructions.
In recent years, numerous clinical studies have been published. However, these differ in the diversity of the implant systems studied and the survival rates reported. When interpreting the results, it must be considered that some recent publications list results of ZrO2 implants that are no longer commercially available. Several meta-analyses have shown that different implant systems have significantly different survival rates.
In a systematic review, all clinical studies examining ZrO2 implants over at least 12 months in at least 10 treated patients were analyzed. Studies published between January 2004 and March 2017 were included. The one-year meta-analysis showed a significantly higher survival rate for commercially available ZrO2 implants (98.3%) compared to those no longer commercially available (91.2%). Interestingly, the differences in marginal bone loss after one year between both groups were not statistically significant (commercially available: 0.7mm; commercially unavailable: 1.0mm). Additionally, a two-year survival rate of 97.2% was calculated for 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 ZrO2 implant survival rates significantly improved between 2004 and 2017. However, not all ZrO2 implants currently on the market have been scientifically investigated. According to the available literature, the clinical follow-up period of commercially available ZrO2 implants is limited to a maximum of five years.
Haro Adanez and colleagues conducted another systematic review and meta-analysis on zirconium oxide implants. They included 17 studies covering 1002 patients with 1704 implants (1521 one-part implants, 183 two-part implants). The observation period ranged from one to seven years, with a mean survival rate of 95%. One-part implants had a survival rate of 95%, while two-part implants had a survival rate of 94%. The meta-analysis on bone loss included 11 studies with a mean loss of 0.98mm. The authors concluded that the results for one-part implants were favorable, but the evidence for two-part implants was insufficient to justify their clinical use.
The meta-analyses mentioned above only include scientific literature available at a given time. The literature on commercially available ZrO2 implants, within the timeframe of these meta-analyses and beyond, is currently limited to a maximum of five years.
Material Ziconiumdioxide
Zirconium dioxide... what is that material?
The “zirconium dioxide” material
Zirconium oxide is a material composed of zirconium, oxygen, and other components, where the individual elements are firmly connected in a crystal lattice. This means 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 ZrO2 ceramics are often incorrectly referred to as zirconium or zirconium metal. Zirconium is the pure metal that, like titanium, belongs to the 4th group of the periodic table. Zircon is a silicate sand known as zirconium silicate (ZrSiO4), which can be converted into zirconium dioxide through various technical processes. Ceramic zirconium oxide compounds must be strictly distinguished from the metal zirconium and zirconium metal alloys.
Unlike metal alloys (e.g., titanium-zirconium alloy), the elements in oxide ceramics are not connected by a metallic bond but by an ionic bond. This ionic bond ensures that electrons remain localized in oxide ceramics. As a result, unlike metals or metal alloys, no electrons can be released from the material structure, preventing undesirable interactions such as corrosion.
Stability...do ceramic implants break?
Ceramic implants and fractures
Compared to other oxide ceramics (such as aluminum oxide), zirconium oxide shows significantly superior biomechanical properties, including high bending strength, fracture toughness, and a low modulus of elasticity. These improved mechanical properties allow ZrO2 implants to withstand chewing forces in the oral cavity.
An important factor affecting fracture frequency is the production process of the implants, particularly the method used to create the microrough surface topography. Scientific studies have shown that uncontrolled manufacturing processes for micro-rough surfaces can reduce the breaking strength of ZrO2 implants.
Because of these considerations, manufacturing processes for creating micro-rough surfaces must be carefully adapted to the material properties of ZrO2. Additionally, standardized quality controls must be implemented at the end of the manufacturing process to ensure that the material structure of oxide ceramic ZrO2 remains intact.
With manufacturing processes tailored to the material properties, it is now possible to produce ZrO2 implants with a fracture rate comparable to titanium implants. A meta-analysis examined the fracture susceptibility of ZrO2 implants as a co-factor. This study integrated all clinical research on ZrO2 implants published between 2004 and 2017, which evaluated at least 10 patients over a period of 12 months. The authors found that the fracture rate improved from 3.4% in 2004 to just 0.2% in 2017.
Phase transformation...influence on stability?
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 or manufacturing processes not optimized 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 used for metals cannot and must not be applied to ceramic materials for the same reasons, as improper handling can damage the material structure. This fact must be taken into account by both manufacturers and users.
Roehling S, Gahlert M. Keramische Zahnimplantate – wissenschaftliche Grundlagen und klinische Anwendung. Zahnmedizin up2date 2015;5:425-444.
Composite ceramics...what are the differences in material?
Coming soon
Hydrothermal degradation... alternative ceramic implants?
Coming soon
Biological Factors
ceramic implants... are there clinical benefits?
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 improve osseointegration or healing rates but to determine whether it offers clinically relevant advantages. Peri-implant infections, such as mucositis and peri-implantitis, are among the primary causes of early and late loss of titanium implants. Scientific studies report an incidence of 43% for mucositis and 22% for peri-implantitis. A key clinical question is whether zirconium oxide implants can reduce the risk and severity of peri-implant infections compared to titanium.
Peri-implantitis development is multifactorial, with microbial colonization playing a crucial role. An in vitro study examined biofilm formation on titanium and zirconium oxide surfaces. Biofilm was tested using both a three-species bacterial mix and human plaque samples. The results showed that structured, organized biofilm formed only on micro-rough titanium surfaces, while zirconium oxide surfaces had significantly lower biofilm thickness and mass.
Mombelli A, Lang NP. Microbial aspects of implant dentistry. Periodontol 2000 1994;4:74-80.
It remains unclear whether material properties directly influence inflammation-related peri-implant bone loss. An animal study investigated peri-implantitis development in vivo, comparing zirconium oxide and titanium implants. In the study, dogs received both implant types in their mandibles. After six weeks of healing and four weeks of loading, peri-implantitis was induced by subgingival cotton threads. Over a 24-week period, zirconium oxide implants exhibited significantly less peri-implant bone resorption than titanium implants. Additionally, one titanium implant was lost, whereas all zirconium oxide implants remained intact.
Based on these pre-clinical in vitro and in vivo studies, zirconium oxide implants may offer an advantage over titanium implants in peri-implant inflammation and bone loss. However, long-term clinical studies are necessary to confirm these findings.
Osseointegration... do ceramic implants grow in?
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 key measure of biocompatibility. Over the last five years, numerous publications have investigated the biocompatibility of zirconium oxide implants, using bone-to-implant contact and biomechanical stability as key parameters.
Pieralli and colleagues investigated the osseointegration of zirconium oxide implants in animal studies, reviewing 54 studies that met their inclusion criteria. They analyzed bone-to-implant contact (KIK, %), removal torque (RTQ, Ncm), and push-in force (N). The results showed that titanium implants had an average KIK of 61%, while zirconium oxide implants ranged from 57% to 63%. The difference was not statistically significant.
Interested readers can find further details in the meta-analysis, which differentiates results based on surface topography, animal models, and other factors. Regarding removal torque, the study found no significant difference between titanium (103 Ncm) and zirconium oxide implants (95 Ncm). In the rat model, unscrewing attempts were not possible due to implant size, so push-in tests were conducted instead. Again, no significant difference was found between titanium (52 N) and zirconium oxide (54 N). In general, “smooth” surfaces showed lower removal torques and push-in values than “structured” surfaces. The authors concluded that there are no significant differences between titanium and zirconium oxide in terms of bone ingrowth.
A second systematic review with meta-analysis of 37 preclinical studies investigated the hard and soft tissue integration of zirconia implants. The study examined bone-implant contact (KIK), removal torque, and push-in values. The KIK for titanium implants was 59%, while zirconium oxide implants had 56%, showing no significant difference. However, removal torque values for titanium (103 Ncm) were significantly higher than for zirconia (72 Ncm). Titanium implants also showed higher push-in values (25 N) compared to zirconium oxide (22 N).
The meta-analysis by Roehling and colleagues observed lower removal torque and push-in values for zirconium oxide implants. However, these differences were not due to material properties but rather differences in surface characteristics. The study found that increased surface microroughness was associated with better osseous integration of zirconium oxide implants. Differences in study protocols, inclusion/exclusion criteria, and animal models (54 studies vs. 37 studies) also influenced the results. Based on these findings, the authors concluded that titanium and zirconium oxide implants exhibit comparable hard and soft tissue integration.
Statement of the ESCI
When selecting ceramic dental implants, it is important to rely on scientifically validated data that define the expected success rate of a given medical device. Preclinical investigations suggest that microrough surfaces on ceramic implants positively influence bone-to-implant contact (KIK).
Soft tissue integration... Special features of ceramic implants?
Soft tissue integration
Preclinical
For soft tissue aesthetics, maintaining healthy and stable peri-implant soft tissue dimensions is crucial. On both teeth and implants, soft tissue consists of sulcus depth, marginal epithelium, and connective tissue attachment, forming the so-called biological width or dentogingival complex.
From a periodontal perspective, peri-implant soft tissue acts as a barrier similar to dentogingival tissue, helping to prevent bacterial peri-implant infections.
A literature review by Nishihara et al. summarized five preclinical studies on soft tissue response to zirconium oxide implants. Most studies found no significant differences in soft tissue morphology between titanium and zirconium oxide implants. Both materials exhibited a peri-implant soft tissue structure consisting of a similarly thick epithelial layer with underlying connective tissue.
One study reported a soft tissue height of 4.5 mm for zirconium oxide implants and 5.2 mm for titanium implants. The epithelial extension was similar for both materials (2.9 mm), while connective tissue extension showed a non-significant difference (zirconium oxide: 1.5 mm; titanium: 2.4 mm). Other studies found slightly lower soft tissue heights (3–4 mm), depending on the animal model used.
Material properties – zirconium oxide versus titanium – do not seem to significantly affect peri-implant soft tissue integration. Both materials demonstrate similar physiological processes in soft tissue development.
Further experimental studies confirmed equivalent soft tissue integration and similar biological width dimensions for zirconium oxide and titanium implants. The biological width and peri-implant papilla height were found to be influenced not by loading or surgical protocol but by implant design and the position of the microgap between the implant shoulder and prosthetic restoration.
Roehling S, Cochran D. Soft Tissue Integration of Zirconia Implants. Forum Implantologicum 2018;14.
Interestingly, one experimental study reported faster maturation of peri-implant epithelial and connective tissue for zirconium oxide implants.
Clinical
Beyond function, aesthetics play a crucial role in patient satisfaction. Aesthetic outcomes depend on both tooth crowns and the surrounding soft tissue. Important factors include non-irritating peri-implant soft tissue conditions, such as gingival margin positioning and peri-implant papilla formation (pink aesthetics).
Clinical data evaluating peri-implant mucosal conditions objectively are limited. However, studies using the Jemt Papilla Index observed a significant increase in peri-implant papilla formation between the time of functional loading and the three-year follow-up.
Additionally, peri-implant mucosal conditions were assessed using the “Pink Esthetic Score (PES)” by Fürhauser. Clinical studies reported a steady increase in PES values within the first two years after implantation for both one- and two-part implant designs. Interestingly, one study found significantly higher PES values for ceramic implants (prosthetically restored with ceramic crowns, PES 6.9–11.2) compared to titanium implants (restored with titanium abutments and ceramic crowns, PES 2.4–10.8).
For one-part ceramic implant designs, clinical studies found a significant increase in peri-implant disc height over time with functional loading. The distance between the alveolar ridge of adjacent teeth and the lowest point of contact with the adjacent crown was a key factor for peri-implant papilla formation.
Roehling S, Cochran D. Soft Tissue Integration of Zirconia Implants. Forum Implantologicum 2018;14.
Clinical Aspects
Prosthetic concepts... one-piece and two-piece systems?
Coming soon
Aesthetics... an advantage of ceramic implants?
Coming soon
Augmentative procedures... what is there to consider with ceramic implants?
Coming soon
