Zirconia Based Implants: The Future Is Now!

INTRODUCTION

Titanium and titanium alloys have been the materials of choice for fabrication of dental implants. However, the principal disadvantage of titanium is its dark grayish color, which often is visible through the peri-implant mucosa, therefore impairing esthetic outcomes in the presence of a thin mucosal biotype. This is of great concern especially when the maxillary incisors are involved.¹ Galvanic effects are also associated with titanium in the presence of saliva.² Although allergic reactions to titanium are very rare, cellular sensitization has been demonstrated.^3,4 Since titanium implants have shown potential immunologic and possible esthetic compromises, newer implant technologies are being developed. Zirconia was introduced as an esthetic solution to metal and metal ceramic restorations. Now zirconia implants are introduced into dental implantology as an alternative to titanium implants because of its toothlike colour, mechanical properties, biocompatibility, and low plaque affinity.

THE ERA OF ZIRCONIA

The first type of ceramic implants to be used were made of aluminum oxide.^5,6 Aluminum oxide implants can be osseointegrated but their biomechanical properties, as reflected by fracture toughness, are unsatisfactory. Clinical studies on these implants have shown long term survival rates of between 65% and 92%.^7,8,9 The discrepancy in these results has led to the diminished application of this material for routine use. Consequently, aluminum oxide implants were withdrawn from the market in the early 1990s.

Zirconium dioxide (zirconia) ceramics with improved properties were then introduced as an alternative to aluminium oxide. They were first used for the fabrication of crowns and implant abutments. Currently, tetragonal zirconia polycrystal i.e. yttria stabilized zirconia, is the ceramic of choice for dental implants. The white, opaque color of zirconia, along with its proven biocompatibility and low affinity to bacterial plaque, makes it a material of interest in biomedical sciences. Zirconia also exhibits several promising physical and mechanical properties such as low thermal conductivity, high flexural strength (900 – 1,200 MPa), favourable fracture resistance, as well as wear and corrosion resistance.

OSSEOINTEGRATION

The success of an implant treatment depends on its osseointegration. Several studies were conducted to compare the osseointegration of zirconia implants with that of titanium implants. Most of the studies^10,11,12 revealed no significant difference between the osseointegration of zirconia implants and that of titanium implants. Depprich et al.¹³ found similar attachment of both titanium and zirconia implants to bone, with similar features ultrastructurally. The proliferation of osteoblasts was found to be more around zirconia implants as compared to titanium implants, though the attachment and adhesion strength of primary cells was more with titanium. Mosgau et al.¹⁴ and Dubruille et al.¹⁵ reported higher bone-to-implant contact (BIC) with zirconia than with titanium. According to Gahlert et al.¹², submerged zirconia implants exhibited superior peri-implant bone volume density as compared to titanium implants.

SOFTTISSUE INTEGRATION

The soft tissue to implant interface is a complex structure that plays a major role in the maintenance of health in the peri implant region. The quality of this mucosal barrier seemed to also depend on implant surface characteristics¹⁶. An in vitro study¹⁶ was conducted to relate the behaviour of human gingival fibroblasts to the characteristics of the surfaces on which they were grown. After 48 and 72 hours of incubation, the proliferation of human gingival fibroblasts was significantly faster on smooth zirconia disks than on rough zirconia and on both smooth and rough titanium, with the fibroblasts spreading more evenly on smooth zirconia. Irrespective of the material, smooth surfaces also showed better cell alignment.

Comparison of the mucosal seal around zirconia and titanium implants with machined necks in five adult pigs found that collagen fibers in the connective tissue had a similar orientation (parallel and parallel oblique) on both implant surfaces.¹⁷ Another experiment was carried out in dogs, where the soft tissue healing around abutments made of titanium and zirconia¹⁸ was studied. It was observed that the dimensions of the peri implant mucosa were similar around titanium and zirconia abutments, and that they remained stable over a period of 5 months. The length of the epithelium was 1.83 and 1.75 mm for titanium and zirconia specimens, respectively. A smaller proportion of leukocytes was detected in the barrier epithelium around zirconia abutments compared with the barrier epithelium around titanium abutments. It was suggested that zirconia could enhance epithelial attachment during the healing phase. These findings were in agreement with the results of a previous study¹⁹ in which the soft tissue conditions were analyzed around one piece custom made zirconia and titanium implants in monkeys. The biological width was 5 mm around the titanium implants and 4.5 mm around the zirconia implants. The length of the epithelium was similar in both groups (2.9 mm). It was hypothesized that the longer junctional epithelium and the higher density of collagen fibers could improve the soft tissue seal and reduce the inflammatory infiltration around zirconia implants. Therefore, zirconia implants could result in a relatively more mature soft tissue integration.

RECENT ADVANCES IN ZIRCONIA BASED IMPLANTS

In order to overcome the inherent drawbacks of zirconia, a lot of surface modifications and additives have been under the research banner.Recently, ceramic blocks called as TZP-A were produced by adding small quantity of alumina to 3Y-TZP. Alumina traces improved stability and durability under humid environments and high temperatures. But this was achieved at the compromise of reduction in translucency of ceramic blocks and hence it is considered unaesthetic.¹

Minimizing LTD in 3Y-TZP systems was attempted by adding small quantities of silica, using Yttria-coating instead of co-precipitated powder, reducing grain size and increasing stabilizer content and formation of composites with Aluminum Oxide (Al₂O₃). The composite material processed with tetragonal zirconia polycrystals (ZrO2-TZP) and Alumina at 20% (Al₂O₃) was claimed to show excellent mechanical and biological characteristics. Adding alumina to Zirconium reduces aging or in the least, diminishes its kinetics as it alters from grain boundary chemistry and limits tetragonal grain growth during process of sintering, therefore resulting in more stable structure.

There are also advancements made with Zirconia in terms of enhanced surface topography and the modifications providing improved osseointegration.

Several studies and experiments were conducted to achieve surface modified zirconia-like sand blasted, sand-blasted light grit and plasma anodized, acid etched and ceramic coated zirconia. The tests have shown stronger bone response to sand blasted and acid etched Zirconia implant surface^21,22. The coated or surface modified Zirconia implants also showed higher value of removal torque compared to machined implants of zirconia. With various surface enhancement methods to roughen zirconia surface, it was found that the surface roughness was comparable to that of titanium implants. Even though it is difficult to achieve surface modification of zirconia, but with the help of procedures like CO2 lasers, it is possible to produce distinctive surface changes to zirconia. Ceria stabilized Zirconia or Alumina nano-composites for dental applications have shown to have high flexural strength, reliability and high resistance to low temperature degradation.

At present, the evidence for a final verdict is still insufficient, as these studies were mostly in-vitro and were not backed by adequate clinical data. More clinical investigations need to be carried out to identify all relevant technical and biological factors with impact on success and patient satisfaction.

CONCLUSION

Titanium, being a biomaterial of choice, has been and is still largely employed in the fabrication of dental implants. However, its disadvantages related to aesthetics and immunology are still looming. Zirconia is proving to be a viable alternative in replacing titanium. The need for substantial back-up in the form of long-term clinical trials is of paramount importance for zirconia implants, following which, zirconia will have the potency to stand as a flag-bearer in the era of metal-free implants.

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