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Implants have been widely used in
dentistry since along time. Titanium and titanium alloys are a gold standard
for rehabilitation of edentulous spaces. However, with advancement in
technology and potential immunological and esthetic compromises with titanium
implants need for neoteric material was perceived. The use of zirconia as an
alternative to titanium implants for oral rehabilitation is being considered as
zirconia has better tissue acceptance and superior mechanical, biological and
esthetic properties. The following study aims to review clinical and research
articles conducted on zirconia implants and their comparison with titanium
implants and to analyze the creditability of zirconia implants over titanium
implants as an alternative for rehabilitation.

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The rehabilitation of edentulous spaces
in patients with an osseointegrated dental implant is a scientifically accepted
and well-documented treatment modality. Branemark in 1908, first discovered the
concept of osseointegration when blocks of titanium placed into the femur of
rabbit got ankylosed with the surrounding bone and could not be retrieved.
Since then, numerous investigations and clinical studies have established
titanium as a reliable biomaterial for oral rehabilitation and reconstruction.
Various modifications in the structure, composition, and design of titanium
implants have been made since then to enhance its physical, mechanical and
optical properties 1–4.

However, the development of undesirable
allergic reactions, cellular sensitization, galvanic current formation and
aesthetics gray hue have raised demands for more aesthetic and biocompatible
implant material 5–9. However, ceramics are known to be sensitive to shear
and tensile loading, and surface flaws may lead to early failure. These
realities imply a high risk for fracture 11. Recently, high strength zirconia
ceramics are considered as new materials for dental implants. They are
considered to be inert in the body and exhibit minimal ion release compared with
metallic implants. Zirconia, is emerging as a promising
alternative to conventional titanium based implant system for oral
rehabilitation with superior biological, aesthetic, mechanical and optical
properties 10.

However, it is important to understand
the similarities and differences between zirconia and titanium implant system
so as to enable the clinician to provide the best treatment outcomes for their
patients. This review aims to analyze the reliability of zirconia as an
alternative to replace titanium based implant system.

Material and methods

This review started with a PubMed search
from 1975 to 2016. The search was conducted using the following key words:
zirconia or zirconium dioxide, dental, and implant. The full text of articles
was obtained where possible. If it was not possible to obtain a full
text, the electronically available abstracts were collected. Thus, the
inclusion criteria for articles were as follows: (1) Articles were related to
zirconia dental implants, and (2) abstracts were obtained when the full texts
could not be obtained. Articles about zirconia implants for orthopedic usage
were excluded from the review.


The results will be discussed under the
following headings:

composition and structure of zirconia implants

 Properties of zirconia implants

and biocompatibility of zirconia implant

tissue compatibility and soft tissue healing around zirconia implants





a)    Chemical
composition and structure of zirconia implants:

advent of zirconia (ZrO2) as a high-performing ceramic has its
origins in a classic paper by Garvie et al. (1975) and subsequent work of
others in the materials science community. It has evolved into several
variants, depending on powder selection, sintering additives, heat treatment,
and other processing factors. Three crystalline phases occur in zirconia
implants: monoclinic (m), tetragonal (t) and cubic (c). The monoclinic phase of
Zirconia exists at room temperature and is stable for up to 1170C.
Above 1170C, the monoclinic phase changes to tetragonal phase with
4-5% decrease in volume. At 2370C, the cubic phase starts appearing.
Upon cooling, a tetragonal to monoclinic transformation with a 3–4% increase in
volume takes place for about 1000C till 1070C. This
increase in volume and resultant expansion without a mass transfer upon cooling
generates stress and causes it to become unstable at room temperature. To
prevent this phenomenon and to generate a Partially Stabilized Zirconia (PSZ)
with stable tetragonal and/or cubic phases, various stabilizing oxides 16 mol%
magnesia (MgO), 16 mol% of limestone (CaO) or 8 mol% Yttria (Y2O3)
are added to zirconia implants. This martensitic-like phase transformation
toughening significantly increases the crack resistance, fracture toughness,
and longevity of zirconia. (1). Alumina
has also been added to Yttria stabilized-tetragonal Zirconia polycrystal
(Y-TZP) in low quantities (0.25 wt%) to yield tetragonal zirconia polycrystal
with alumina (TZP-A) with significant improvement in the durability and
stability of zirconia crystals under high temperatures and humid environment.
This improves the resistance of implant to low temperature degradation (LTD)
and “ageing”. Studies have shown that implants without alumina when exposed to
the artificial mouth have a survival rate of 50%, whereas implants with alumina
have a survival rate of 87–100%.12




b)    Properties
of zirconia implants:

mechanical and physical properties of zirconia implants depend upon its
composition, nature of crystals, metastable polymorphic structure, ratio of the
monoclinic to tetragonal phase, percentage of stabilizing metal oxide, ageing
process, macro and micro design of the implant, nature of the finish line on
the implant abutment, characteristics of implant abutment, and amount of
occlusal load. Though transformation toughening improves the fracture strength
and toughness of Y-TZP implant, it hampers the phase integrity and makes the
implant susceptible to LTD or ageing. An increase in moisture or stress can
cause transformation of zirconia crystals to a monoclinic phase with micro
crack formation that increases the water penetration, crack propagation,
surface deterioration, phase destabilization and decreased resistance to load. The
macro design of zirconia implants such as the depth of thread, diameter, and
implant neck design of the implant are important criteria’s that should be
evaluated before selecting a zirconia implants system. The thread design of the
implant plays a critical role in crack initiation and propagation. A profound
thread depth should be avoided as it may hinder bone clearance during the
surgical implant placement and generate unnecessary bending forces on the
implant body, especially in the patients with dense bone. Any sharp or pointed
thread design with a narrow diameter, notched edges, minor scratches, and any
surface modifications including grinding, acid etching, sandblasting etc.
should be avoided to prevent local stress concentration, mechanical overloading,
and subsequent implant fracture. Since mechanical overloading is considered as
one of the main reasons for the implant fracture, zirconia implants with a
diameter less than or equal to 3.25 mm are not recommended for clinical use. An
important advantage of zirconia implant over titanium is in relation to its
excellent aesthetics. The optical behavior of zirconia varies with its
composition, crystal size, grain distribution and methods of machining. The
enhanced aesthetics of zirconia is attributed its ability to mask dark
substrates with good opacity in the visible and infrared spectrum and
controlled translucency. The masking ability is due to its grain size being
greater than the length of light, high refractive index, low absorption
coefficient, high density with low residual porosity (<0.05%) even in thin sections, the presence of various additives, stabilizers and pigments.      c)    Osseointegration and biocompatibility of zirconia implants: Zirconia-based ceramics are chemically inert biomaterials with minimal local or systemic adverse reactions; good cell adhesion; excellent tissue response and a high degree of biocompatibility with the surrounding bone and soft tissues. Animal and human clinical studies have evaluated and confirmed the deposition of newly formed mature bone in close proximity to zirconia implant surfaces with few marrow spaces, minimal inflammation and numerous small actively secreting multinucleated osteoblasts 14– 18. Various in vitro and in vivo studies have revealed the osteoconductive nature of zirconia with no cytotoxic, oncogenic or mutagenic effects on the bone and fibroblasts after implantation into muscles or bones 16-22. Scarano et al. demonstrated a good bone response to zirconia implants at four weeks with Bone to Implant contact (BIC) of nearly 68.4% 21. Dubruille et al. compared the BIC in titanium, alumina, and zirconia implants and found no statistically significant difference between the three types of implants. The BIC was found to be 68% for alumina, 64.6% for zirconia, and 54% for titanium. When osseointegration of zirconia implant is compared to titanium implant, minimal difference in the BIC and distribution of stress patterns were observed 22     d)    Implant tissue compatibility and soft tissue healing around zirconia implants: The bio-inert properties of zirconia help in rapid proliferation of the human gingival fibroblasts over the implant surface and formation of a good mucosal barrier 23. However, various factors such as surface characteristics and design, nature of implant material and degree of roughness influence the nature and amount of the mucosal seal around zirconia implants. A smooth implant surface promotes a good soft tissue seal in comparison to a rough implant surface. Various differences have been observed in the peri implant mucosa around zirconia implants as compared to titanium. The expression of chemical mediators such as integrin alpha2, integrin alpha5 and type I collagen are found to be more up-regulated on smooth zirconia implants as compared to titanium. However, the pattern of connective tissue adhesion and transgingival collar around zirconia implants is similar to that seen around machined titanium surface (collagen fiber orientation predominantly in a parallel oblique pattern). The color of the peri implant mucosa, the amount of bleeding on probing and probing depth is similar around zirconia implants as compared to titanium implants. Zirconia implant has also shown to inhibit bacterial adhesion and biofilm formation on its surface because of its hydrophobicity, bio-inert properties, optimal smoothness, reduced surface free energy and surface wettability 24-35. e)     Clinical Studies: Three clinical studies investigated zirconia implants. Blaschke et al reported that dental     implants made from zirconia are a feasible alternative to titanium dental implants. In addition to excellent cosmetic results, zirconia implants allow a degree of osseointegration and soft tissue response that is superior to that of titanium dental implants. Oliva et al47 reported the first clinical evaluation of 100 zirconia implants (CeraRoot, Barcelona, Spain) with 2 different surface roughness in humans after 1 year of follow-up. Two implants failed after15 days. These failed implants were placed in situations where sinus elevation was required. The overall success rate was reported as 98%. Given the sinus elevation requirement, future investigators may exclude patients with less than 5 mm residual bone. Pirker et al48 placed a zirconia implant to the maxillary first premolar region immediately and evaluated the clinical outcome of this implant. At 2-year follow-up, a stable implant and an unchanged peri-implant marginal bone level were observed. No bleeding was detected on probing.   4.  Conclusion: Studies have proved zirconia implants is a potential alternative to titanium having better soft-tissue response, biocompatibility, and aesthetic with comparable osseointegration. The early fracture of zirconia implant, especially in the posterior region is a critical factor to be considered regarding its use and acceptance in all clinical situations. Since most clinical studies on zirconia implants are short-term, substantial evidence supported by long-term clinical trials are required before zirconia based implant systems can completely replace titanium for prosthetic rehabilitation. Further research to prevent the ageing, improving surface characteristics, structure and osseointegration of zirconia implant are needed.                       5. REFRENCES: 1Gahlert M, Röhling S, Wieland M, Eichhorn S, Küchenhoff H, Kniha HA. Biomechanical and histomorphometric comparison between zirconia implants with varying surface textures and a titanium implant in the maxilla of miniature pigs. Clin Oral Implants Res 2007;18:662–8. 2 Depprich R, Zipprich H, Ommerborn M, Naujoks C, Wiesmann HP, Kiattavorncharoen Sirichai, et al. Osseointegration of zirconia implants compared with titanium: an in vivo study. Head Face Med 2008;4:30. 3 Steinemann SG. Titanium—the material of choice? 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