Background
Dental implants have become a well-established treatment method for oral rehabilitation after tooth loss. Pure titanium is still the material of choice when it comes to dental intraosseous implants and has been used for decades. However, titanium implants have esthetic limitations, especially in the front aspect of the maxillary jaw. The recession of the gingiva can lead to visible implant necks. Furthermore, titanium may cause immunological reactions with early local infection and possible risk for implant loss [
1]. Ceramic implants are proclaimed as a new alternative to titanium implants. The first tooth-colored ceramic implants were inferior to titanium-based implants due to their biomechanical characteristics such as low fracture toughness [
2]. In the 1980s, the Tübinger immediate implant was introduced, fully made of aluminum oxide (AL
2O
3), but was withdrawn from the market because of high fracture rates [
3]. Other investigations on different AL
2O
3 implants found less bone-implant contact compared to titanium [
4] as well as reduced survival rates [
2,
5]. Since the introduction of yttrium-stabilized tetragonal zirconia polycrystalline (Y-TZP)-based implants, it could be shown that these implants show high similarity in osseointegration compared to titanium implants [
2].
Titanium implants with smooth or roughened surfaces have shown high success rates in various indications [
2,
6,
7]. Surface characteristics of dental implants, as a new development over the last decades, are seen as an important factor that affects osseointegration, especially in compromised patients (e.g., following radiation therapy, bone augmentation, class D4 bone) [
8]. By improving the implant design, implant material, and implant surface characteristics as well as surgical techniques and implant loading conditions, osseointegration can be affected [
9]. Several new techniques are performed nowadays to speed up the osseointegration process by altering the surface of the implant chemically (incorporating inorganic phases onto the titanium oxide layer) or physically (increasing the level of roughness) [
10,
11]. Advantages of surface-modified implants include (a) establishing a greater contact area followed by better primary stability, (b) providing surface-retaining blood clots, and (c) stimulating bone formation [
10,
12]. In vitro tests of surface roughness showed higher proliferation, cytokine, and growth factor production of osteoblast-like cells. Those factors are known to affect proliferation, differentiation, and matrix synthesis of chondrocytes [
13‐
16]. Many studies on surface characteristics of titanium implants were performed over the last years. Due to the renaissance and new development of zirconia implants, it is now necessary to study their behavior and surface characteristics and to compare them to titanium implants. However, data regarding the surface characteristics of these zirconia implants are very rare. Therefore, the aim of this study was to examine the surface characteristics, element composition, and surface roughness of the five different commercially available dental zirconia implants.
Discussion
Implant surface characteristics are of ongoing scientific interest. Implants made from titanium are still the most common to be used. Titanium implants are made from alpha-beta alloy which consists of 6% aluminum and 4% vanadium (Ti-6Al-4V). These materials have low density, high strength, and resistance to fatigue and corrosion, and their modulus of elasticity is closer to the bone than any other implant material [
18]. However, titanium implants are discussed to trigger hypersensitivity reactions due to surface corrosion [
1,
19]. Titanium implant surfaces are machined, etched, sandblasted, and sometimes coated with special (company-specific) coatings. For titanium implants, roughness values (Ra) around 1.5 μm are known to provide successful osseointegration [
20].
Ceramic implants experienced a renaissance since their reentry into the market. New ceramic implants with yttria (Y
2O
3)-stabilized tetragonal zirconium polycrystalline (Y-TZP) material have superior corrosion and wear resistance in comparison to titanium implants as well as high flexural strength (800 to 1000 MPa) [
18]. However, due to manufacturing imperfections or flaws created during zirconia implant fabrication and because of special surface treatments, their strength can be compromised [
18,
21]. Due to their brittle nature, ceramic implants tend to fracture. Especially sharp, deep, and thin threads can easily lead to implant failures [
18,
21]. The surface treatment on ceramic is developed due to a process of sandblasting, etching, and heat treatment [
22]. Sandblasting is usually done with alumina particles that lead to sharp edges and scratches on the surface. The treatment with hydrofluoric acid as the following procedure may smoothen the surface again [
22‐
24]. However, in zirconia implants, due to stress caused by sandblasting, a tetragonal to monoclinic phase transition may be caused [
22,
25]. This monoclinic volume fraction can be seen in 10–15% of the cases [
26] and initially leads to a surface compression of the zirconia material [
22]. According to Fischer et al., the long-term effects and the implant stability after this procedure are not yet proven [
22]. However, it can be reversed by a thermal treatment that is higher than the transition temperature [
22,
27].
The surface shape (droplet-like surface), which was observed in the SEM samples, can be caused due to the sintering process in which ceramic powder was melted and then formed. Different particle, immersion, and droplet sizes can also change due to possible reasons like usage of various types and dosages of acid for the etching process and change of exposure time to acid effect. A longer exposure time to etching process could also be responsible for lowering aluminum corundum from sandblasting processes. However, despite a very fine surface microstructure, implant 4 shows the highest amount of aluminum on the outer surface. This could be explained by sandblasting with aluminum-containing corundum particles followed by a shorter etching process. The higher amount of aluminum in implants 1, 4, and 5 might be due to the individual material composition while sintering the material mixture or to corundum particles of the machining and sandblasting process. Implants with aluminum under the detection limit could be caused by a final etching process. Implants 1 and 5 are not advertised with a special etching process. However, implant 4 is supposed to be etched. The etching could have happened before sandblasting, or the acid used was not strong enough to eliminate all aluminum particles.
All implants excluding the Ceralog Monobloc (implant 5) show typical parallel grooves of the machining process in the confocal laser scan and rougher surfaces in the treated areas. Ceralog is the only implant with a rough surface that can also be found in the machined area. Zirconia implants which are treated with a process of sandblasting, etching, and heat treatment are showing a micro-structured surface resulting in a surface roughness in the range of 1.2 μm [
22]. In this study, implants 2 and 5 showed roughness values in the range of 1.2 μm. The other implants showed different roughness values. The surface porosity of titanium implants after sandblasting and etching processes is much more rigorous than that of the ceramic implants that were investigated. In this study, implants 2 and 5 can approximately be compared to titanium surface characteristics in the SEM samples. However, implant 5 was not sandblasted and etched because of a special “injection molding technique” and shows a wide distribution of roughness values. A similarity to the surface structure of titanium implants cannot be proven yet.
The semi-quantitative energy-dispersive X-ray spectroscopy (EDX) can be used to further analyze the components of the implant surface. None of the implants showed any impurity or unexpected results. Implants 4 and 5 showed yttrium under the detection limit in the EDX analysis. This could be caused by the lower dosage of yttrium endowment in the stabilization processing in comparison to other implants [
17].
This investigation shows results on a sample basis with one implant tested and shall not be used for generalization.