An evaluation of superhydrophilic surfaces of dental implants - a systematic review and meta-analysis

Because of the current excellent success rate of implant treatment (as high as 99% [1]) and good aesthetics achievable using this method, this is no longer the only criterion that is considered when choosing an implant system. The dental implant industry is constantly seeking the most effective and safest connection between the implant and the prosthetic superstructure to prevent the marginal bone loss, and we also observe the evaluation of implant surfaces, especially those being in contact with the bone. In the search for the ideal implant researchers have considered a number of factors, such as: the position of the implant platform in relation to the height of the alveolar crest (supracrestal, bone level, subcrestal); the type of connection between the implant and the superstructure, e.g. utilizing the platform switching [2] or platform matching, [3]; the type of connection between the abutment and the implant where a Morse taper [4, 5], tri-channel, external-hex or internal-hex [6, 7] connection can be used; as well as, finally, the choice of implant neck, which can be polished or have a modified surface like the remaining part of the implant [8‐10]. The implant systems available on the market combine these concepts in different ways.

The characteristics of the surface of a dental implant have a decisive influence on the process of osseointegration. The physical properties of implants, especially their surface characteristics, are responsible for the progress made in implant treatment in the last few decades. According to the current state of knowledge, modification of the microsurface can not only increase the surface but also affect the morphology of cells, and in this way have a positive impact on osseointegration [11, 12]. Many methods exist for increasing the implant surface roughness. One technique that is widely employed involves combining sand-blasting and etching with acid (SLA), and successful osseointegration using this approach has been widely documented in the literature. Through successive modifications of the surface, such as improving wettability and the hydrophilic properties of the implant, it is possible to further shorten the time needed to achieve secondary stability of the implant and accelerate absorption of proteins on the implant surface, which makes its surface hydrophilic.

Straumann [Straumann Holding AG, Switzerland] has developed an implant with a SLAactive® surface which has hydrophilic properties and is chemically active. Thommen [Thommen Medical AG, Switzerland] has devised an implant called Inicell®, whose hydrophilic surface accelerates the absorption of proteins on implant surfaces. Inicell is the conditioned state of the sandblasted and thermal acid-etched Thommen implant surface. During conditioning the surface chemistry of the microrough surface is slightly modified. Conditioning occurs immediately before implantation through contact with the conditioning agent (patent pending). The result of this process is increased surface energy and improved wettability due to superhydrophilic properties. Both types of implant were developed with the aim of significantly shortening the time required to achieve biological secondary stability (osseointegration) and reduce healing time to 3–4 weeks [13‐15].

The objective of this study was to compare survival rates and marginal bone loss as well as assess the degree of stability of dental implant with superhydrophilic surface - Straumann SLAactive® and Thomenn Incell® implants. The present article systemises the data from available publications and in this way provides a summary of the available information on both the purpose and effectiveness of the above-mentioned implants.

The primary objective of the present study was to assess the basic success parameters for Straumann SLActive® and of Thommen Inicell® implants with a superhydrophilic surface after either functional immediate loading or loading after a short healing period (up to 4 weeks).

Straumann [Straumann Holding AG, Switzerland] has developed an implant with a new kind of surface that has hydrophilic properties and is chemically active. The SLActive® surface differs from the SLA surface due to its hydrophilic properties. Its initial angle of contact with water is 00, compared with 139.90 for SLA. Preparation of such a surface involves sandblasting it with Al₂O₃ abrasive grain with 25–50 μm particle size and then etching it with a mixture of HCl + H₂SO₄. After etching the implants are rinsed in a nitrogen atmosphere and then stored in a NaCl solution. A noble gas atmosphere followed by physiological salt protects the titanium surface from contamination with hydrocarbon and carbon compounds originating from atmospheric air. The aim of such chemical modifications is to maintain the hydrophilic character and the high natural surface energy of titanium dioxide up to the moment of implant placement. The SLActive surface shown in the SEM image is identical to the SLA type surface [35].

According to the authors of the evaluated studies, the most important criterion of success is the Cumulative Implant Survival Rate (CSR%), as it is the only parameter which was assessed and described in each analysed publication. The CSR rate varied between 94% [31] and 100% [11, 17, 19, 22‐25, 29, 33]. Of the 2890 observed implants (1613 Thommen implants and 1367 Straumann implants) a total of 45 were lost, which gives a CSR of 98.5%. This value is fully comparable with the results of studies based on standard procedures [36, 37].

The second parameter most commonly evaluated by researchers was MBL - Marginal Bone Loss. Of the 20 publications analysed in this study 9 provided complete data on marginal bone loss after 6 months (MBL after 6 months), while 13 described marginal bone loss after 12 months (MBL after 12 months) (Tab. 2). Based on the available data average marginal bone loss (MBL) was M = 0.60 mm after 6 months and M = 0.65 mm after 12 months. A comparison of the results obtained with the results achieved with standard procedures shows that they do not deviate from the established norm, according to which MBL should not be greater than 1.5 mm in the first year and then 0.1 mm in each following year [38‐41]. It is important to note that some researchers claim that original remodelling of the alveolar process occurs after implant loading with the aim of restoring biological width, such that MBL should be assessed 12 months after restoration [42].

The third evaluation criterion was the assessment of stability. For the needs of our analysis we exclusively used results obtained using an Osstell measuring device (Osstell AB, Goteborg, Sweden). Osstell measures stability by analysing the RFA resonance frequency (Resonance Frequency Analysis), which provides us with objective information on the degree of implant integration [43] independent of the researchers. The Implant Stability Quotient (ISQ) is read in RFA (Resonance Frequency Analysis), which is measured in kHz. The ISQ scale converts kHz values into a scale of 1 to 100 ISQ units, which is more practical for clinical assessment purposes. The result is given within a range of 1 to 100 ISQ units (Implant Stability Quotient). The higher the ISQ value the greater the stability of the implant. The frequency analysis technique makes it possible to assess implant stability as functions of implant-bone connection rigidity. The ISQ value depends on the bone density, implant healing time, the ratio of the exposed part of the implant to the part embedded in bone as well as the type of implant [44]. The mean primary stability was M = 68.2, the standard deviation SD = 7.4, mean secondary stability M = 74.0 and the standard deviation SD = 5.6. According to the protocol devised by Osstell, such values ensure safe implant loading. Eight of the 20 studies measured stability [11, 18, 19, 22, 25, 27, 29, 33]. Hicklin et al. and Allen et al. [19, 25] did not report any standard deviation values when measuring stability, and as a consequence these results were excluded from the analysis. In the other studies the researchers assessed stability in descriptive terms (e.g. correct, very good, etc.) or used a Periotest device.

Biological complications affected 45 of the 2659 placed implants, which translates into a complication rate of 1.69%. In the majority of cases, early implant loss was a consequence of complications in the osseointegration process, one reason for which may have been overheating of the implant bed, infection or micro movements in the case of immediate loading. More than 50% of cases of implant loss occurred at an early stage, i.e. before functional loading of the implant [45‐47]. Late implant loss, as a result of bone loss caused by infection or peri-implantitis, occurred in approximately 60% of cases in the first year after loading. A study conducted by Snauwaert et al. showed that early implant loss as a result of biological complications occurred in 3.8% of cases, while late implant loss affected 2% of cases [48].

However, it is also important to bear in mind that the majority of researchers use as their measure of success the simple ratio of the total number of placed implants in relation to the total number lost. Such an approach is not entirely correct for it does not consider other factors that have an impact on the survival rate of implants, including [49]:

However, considering the fact that analogous schema is used for conducting research when assessing standard implant procedures we can state that the above-described results fall within the accepted schema for studies addressing implant-prosthetic treatment issues.

In a clear majority of the studies cited here the authors did not assess the satisfaction of patients with the treatment they received. In those manuscripts which did discuss this issue no assessment was made using the Visual Analogue Scale. Hinkle et al. provided information on patient satisfaction in descriptive form: "All patients who participated in the study were completely satisfied [20]. Slotte et al. assessed patient satisfaction on the basis of a non-specific questionnaire, which was filled in by the patients on completion of the treatment [34]. In turn, when describing patient satisfaction following treatment Dard M. et al. observed that in cases of shorter treatment duration due to earlier implant loading patients had a more positive view of their experience than patients whose treatment followed the standard procedure [24].

The assessment revealed certain differences between the two systems in terms of the studied parameters: the risk of implant loss was greater in the case of the Straumann implants compared with the Thommen implants. In turn, marginal bone loss (MBL) was higher after 6 months with the Thommen implants while primary and secondary stability was higher in the case of the Straumann implants. However, these differences were not significant enough to declare with any authority that one implant type has an advantage over the other. Based on the analysis and the implant placement success criteria devised by Buser et al. [52] (the implant – during functional loading – produces no pain or any kind of discomfort, there are no signs of inflammation, infection or implant mobility, and a radiological examination shows no low density bone structure foci around the implant or recurrent bone loss) we can state that both systems, Thommen Inicell and Straumann SLActive, help shorten the time of implant-prosthetic treatment while ensuring a high survival rate, good implant stability and low marginal bone loss.