Zirconia implant: close to the natural root?
Fabrice Baudot, Giancarlo Bianca et Pascal Eppe
Titanium implants are generally used to restore function and aesthetics following tooth loss (1). Numerous studies have demonstrated their excellent biocompatibility (2) and high success rates (3). However, the prevalence of peri-implantitis (PI) around titanium implants (4), should be a daily clinical concern due to their high incidence (Fig 1a,b,c).
Recent studies have reported the presence of titanium particles around implants with PI compared to a healthy peri-implant environment (5,6 ). Galvanic corrosion phenomena in the oral cavity could be related to the physiopathogenesis of Peri-implantitis (7). This issue takes up a large part in most of our conferences, and questions the reliability of our long-term implant treatments (8).
The qualities of zirconia ceramics as a prosthetic restorative material show us in daily use an extremely low bacterial colonization (9), and allow soft tissues to act as a barrier to the underlying infection.
This material is not a thermal nor an electric conductor, and, thanks to its high inertia (10), it has excellent chemical stability with almost no ionic release : this greatly contributes to its biocompatibility observed with periodontal cells, and could explain the absence of allergy or hypersensitivity to ceramics (11).
Zirconia implants may be considered as a real alternative to titanium for our patients, especially those with allergies, autoimmune diseases, periodontal risk factors and metal intolerances (12). We must also consider the current trend in dentistry towards metal-free restorations, and the long-term aesthetic outcome of our restorations.
Are the new zirconia ceramic implants resistant to fracture ?
The first generation of alumina-based ceramic implants developed by Prof. Sammy Sandhaus (Cerasand®, 1968) had to be abandoned due to a high fracture rate despite their excellent tissue biocompatibility (Fig 2a,b).
The new generation of zirconia-based ceramic implants corrects this weakness, and offers a fracture resistance compatible with the mechanical stress of mastication. These are hard, rigid, penetration-resistant and bending-resistant materials in which fractures are initiated by cracks from structural defects.
The factor which characterizes this property of resistance to sudden crack propagation and toughness, is called the K1C modulus which is linearly correlated with the bending strength (fig2c). Its high value, approximately 10, allows the clinician, according to the selected implant system, to perform intraoral ajustments, if it is a one-piece implant (Duchatelard 13).
Zirconium is a chemical element, symbol Zr, with an atomic number 40 . Zirconium is a metal, however, and during its oxidation it becomes a ceramic called zirconium oxide or zirconia. Zirconia has a polycrystalline structure and an opaque appearance. In order to strengthen this crystalline network, it is stabilized by the incorporation of Yttrium oxide (yttria) or alumina. Two types of zirconia are used in ceramic implants and available on the market : the main one is Y-TZP (Yttria Stabilized Tetragonal Zirconia Polycrystals), the other one is ATZ (Alumina Toughened Zirconia), which is also used in the manufacture of drills. (13).
The shaping of Y-TZP or ATZ zirconia dental implants is usually performed by machining a zirconia cylinder in the same way as other parts used in prosthetic restorations. This high fracture resistance is achieved by the HIP process “Hot Isostatic Post Compaction” e.g. Z systems®, White Sky Bredent® (Y-TZP) and Nobel Pearl®, Zeramex® (ATZ). (fig 3, fig 5)
At this stage, the material is densified by sintering in a tunnel furnace during three days at 2,000 bars to significantly improve the physical properties of the basic) material. Please note that not all the zirconia implants available on the market undergo this HIP process, allowing a very high resistance to fracture. (14)
There is another process for making Y-TZP zirconia implants (e.g. Ceralog®, Zibone®) : the shaping is performed using an injection-moulding technique in which zirconia powder is mixed with a thermoplastic polymeric binder. (Fig 4)
The mixture is then injected under pressure into a mould in order to obtain a blank with the morphology of an implant, but with larger dimensions. This technique has advantages from several standpoints. First, it allows the rapid and mass production of ceramic parts with complex shapes, which, if this technique is generalized, would allow to reduce the cost of all-ceramic implant therapy. An additional advantage and not the least of this technique is the ability to create rough surfaces, using a chemical or physical treatment on the inner face of the mould (14).
The large majority of these implants have been designed as “monobloc” implants, i.e. single- piece or one-piece implants, the abutment being combined with the threaded endosseous part. e.g. Z systems®, White Sky Bredent®, Ceralog®, Zibone®, etc. This geometry offers both a mechanical and biological advantage for the stability of peri-implant tissues. (15)
Some systems now offer 2-piece implants. This shape facilitates a similar implant protocol as for conventional titanium implant systems. However, the reliability of the connecting ceramic parts, and the machining of an internal thread on ceramics both represent quite a challenge. In 2016, Jank and Hochgatterer, in a study based on 347 two-piece implants at 4 years, reported a cumulative success rate of 96.7%(16).
Ex : Z-Systems® , Ceralog®, Nobel Pearl® (Fig 3, 4 ,5, 6)
Osseointegration of zirconia compared to titanium
Successful integration of implants is based on osseointegration (in the hard tissue), and the formation of a peri-implant mucosal seal (in the soft tissues) (17).
On the hard tissue level
The key parameters to evaluate osseointegration include the measurements of the bone-implant contact (BIC) and the implant removal torque values (18). Most of the studies show no significant differences between titanium and zirconia implants (19, 20). Fig 6
Several reviews of the literature mention the osseointegration capacity of zirconia (21,22), including that of Hashim. D et al. in 2016, which selected 14 articles out of 1519 publications, with a cumulative success rate at 1 year representing 92 %. In all these reviews, the authors concluded that zirconia implants do represent an alternative to titanium, but that further long-term studies are needed to confirm this. Different surface treatments have been proposed. For example, a laser-machined surface allows to achieve an increase of the BIC surface due to an increased micro- and macro-roughness. (Fig 7)
This technique considerably reduces the time for osseointegration. The survival rate currently exceeds 98%, and is comparable to that of new generation titanium implants.
On the soft tissue level
The qualitative and quantitative dimensions of the peri-implant mucosa around zirconia implants are similar to those of titanium implants (22 ,23) fig 8
Fig. 8 : Clinical case of a Z systems® one-piece implant in site 11 showing perfect soft tissue integration. On the day of impression taking. Under these conditions, immediate implantation protocols can also be implemented as for titanium implantology. (Fig. 9 a, b, c, d, e, f, g, h, i)
Soft tissue behaviour towards zirconia : A protective anti-microbial and anti-inflammatory barrier
Our implant restorations are inserted on the long-term, in a very septic and very aggressive environment. The interface between this environment and the underlying structures (bone, vascular network) is provided by the peri-implant soft tissues. There is a real difference in terms of quality of this interface between a tooth and an implant. Berglundh and Lindhe (1991, 1994) described a fiber-free epithelial junction attachment around transmucosal titanium compared to the Sharpey fibers present around teeth, and concluded that the peri-implant soft tissues are more fragile. They offer less mechanical strength, but are also less vascularized and more immune-sensitive (Degidi 2012, Piatelli 1997, Shierano 2002).
The long-term stability of peri-implant soft tissues is a key issue both in the fight against peri-implantitis, and in the aesthetic and functional outcome of implant-supported prosthetic restorations. The quality of the mucosal seal around the transmucosal part of the implant restoration is crucial.
In 2006, E. Rompen et al. published a list of the important and influential soft tissue factors for implant integration. Moreover, this study revealed, among other things, the following points:
– Tissue-level implants behave better than bone-level implants.
– Titanium and zirconia are preferable to gold or feldspathic ceramics for the transmucosal components.
– Smooth surfaces are preferable to rough surfaces.
– In the case of bone-level implants, disconnection and reconnection of the prosthetic abutments should be avoided as much as possible.
Based on this review, it appears logically that tissue-level implantology is better with respect to soft tissue integration of implants (Fig 10a, b) : issues such as gap problems, hermeticity of the subgingival prosthetic parts, the platform switching concept to reinforce the soft tissue seal no longer exist in “tissue-level” implantology. (Fig. 10a, b) : issues such as gap problems, hermeticity of the subgingival prosthetic parts, the platform switching concept to reinforce the soft tissue seal no longer exist in “tissue-level” implantology.
Titanium and zirconia seem to be the best materials for the transmucosal integration of our restorations.
Integrating biological and aesthetic parameters, what is the best material to choose between zirconia and titanium at the soft tissue level ? this is a legitimate issue to question.
Integrating biological and aesthetic parameters, what is the best material to choose between zirconia and titanium at the soft tissue level ? this is a legitimate issue to question.
The peri-implant mucosal seal acts as a protective barrier towards the underlying structures. There are three fundamental aspects to be considered:
– The microbiological aspect
– The biomechanical aspect related to cell adhesion and proliferation around transmucosal implant structures.
– The potential release of metal ions that disrupt local immunity (31).
Microbiological behaviour of zirconia
In an in vitro and in vivo comparative study on titanium, Rimondi et al. (2002) described the transmucosal zirconia interface as an anti-microbial shield. This observation was confirmed by further studies.
Nacimento in 2014, conducted an in vivo study based on the use of split casts worn for 24 hours, comparing zirconia, smooth titanium and rough titanium. Analyses of the pathogenic and non-pathogenic flora revealed less microbial adhesion on zirconia than on the other two titanium surfaces. Consequently, bacterial colonization is lower on zirconia than on titanium. This was confirmed again in 2016 by Nacimento, and later on by De Freitas in 2018 in a 6-month follow-up study comparing transmucosal zirconia and titanium abutments they observed more pathogenic bacteria on titanium.
This difference in microbial behaviour towards titanium compared to zirconia exposes titanium to an increased risk of peri-implantitis, as Iglhaut pointed out very well in his review article in 2014. Microbial colonization causes an inflammatory infiltrate within the tissues in response to this microbial presence. The tissue defence barriers are weakened and more permeable to biofilms. A chronic inflammatory wound then develops within the peri-implant soft tissues which distubs bone metabolism, thus constituting a risk factor for peri-implantitis : a phenomenon very similar to periodontitis. Around transmucosal zirconia, Nothdurft et al. showed in 2015 that inflammation level decreased compared to titanium. The risk of alteration of the soft tissue barrier effect therefore lower with zirconia than with titanium.
The recent study published by Negahdari et al. (2017) confirms the positive influence of transmucosal zirconia on the level of pro-inflammatory cytokines present in the peri-implant sulcus. In this publication, the authors compared the behaviour of the transmucosal zirconia and titanium abutments using a split-mouth in vivo study. They showed that the pro-inflammatory cytokine levels were significantly higher around titanium compared to zirconia.
As often reported in the medical literature, some controversies exist. Linkevicius and Vaitelis in 2015 published, in the very serious Journal of Clinical Oral Implant Research, a meta-analysis comparing the effect of titanium and zirconia on the soft tissues. The inclusion criteria are were strict : 11 studies were selected including only prospective randomized controlled studies on the same patient. They concluded that titanium and zirconia behaved similarly. The only difference in favour of zirconia was aesthetics.
However, very recently in 2018, M. Sanz et al. published in the same Journal a review article and meta-analysis on the effect of transmucosal abutment characteristics on peri-implant soft tissue health: they concluded that the risk of peri-implantitis is increased with titanium compared to zirconia.
Tissue and cell behaviour towards zirconia
In addition to the “antimicrobial” effect of zirconia mentioned above, the literature describes a favourable behaviour of zirconia on the peri-implant soft tissues. The interaction with soft tissues and trans-gingival zirconia generates a mechanical antimicrobial barrier effect that protects the underlying structures.
Bianchi et al. in 2004 conducted a study on the cell behaviour around transmucosal zirconia implant necks compared to titanium : they observed better fibroblast adhesion and cell proliferation around zirconia. Tetè et al. in 2009 (23) in a study on animal histological sections showed collagen fibers orientated perpendicular to the zirconia surface as opposed to titanium where they were parallel. This fiber orientation reinforces the peri-implant mucosal joint and may partly explain the “creeping attachment” phenomenon which is clinically observed around zirconia necks.
Very recently in July 2019, a Korean team (Dong-Joon Lee et al.) carried out a comparative in vivo and in vitro animal study on the behaviour of peri-implant tissues with respect to zirconia, titanium and hydroxyapatite. In particular, they evaluated the quality of the mucosal joints around these 3 implant surfaces. Zirconia obtains the best results on the histological sections and in vitro cell levels; zirconia also promotes better proliferation of human fibroblasts (HPLF and HGF) and extracellular matrix cells (IhCEM) compared to titanium and hydroxyapatite.
Due to its properties, transmucosal zirconia appears to behave more like natural teeth with respect to soft tissues. This can be illustrated by the study of Kajiwara et al. in 2015 which shows that blood flows around transmucosal zirconia abutments are similar to those around natural teeth.
Zirconia appears to be a biomimetic material around which the quality of the peri-implant mucosal joint is better than around titanium. The reduction to biofilm proliferation and the quality of soft tissue integrity which have been demonstrated around zirconia provide a double protective barrier for the underlying tissues to chronic inflammatory infiltration and microbial invasion which is probably the main risk factor for peri-implantitis. Thus, the use of the zirconia implant to establish a high quality peri-implant mucosal seal can be considered a preventive approach in the strategy to control peri-implantitis. (Fig.14 a, b- résolution du cas clinique Fig. 1)
Zirconia dental implants have been around for 20 years, and if at first glance their interest may seem limited to pure aesthetics due to their colour, today we realize that, thanks to their exceptional mechanical properties and optimal biocompatibility and immuno-compatibility, they certainly represent the future of implantology.
Placing zirconia implants in our patients is part of a preventive approach to peri-implantitis, because the quality of the peri-implant tissues achieved around these implants is an anti-microbial barrier which protects the underlying structures.
The absence of oxidation reactions around the zirconia implants and the reduction of bacterial plaque are real assets for their long-term stability in the particularly aggressive environment of the oral cavity. The latest currently available generations of zirconia implants offer mechanical, biological and aesthetic qualities close to those of natural teeth.
Fabrice Baudot, Giancarlo Bianca et Pascal Eppe
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