The Effects of Peri-implantitis Decontamination Treatments on the Surface Roughness and Chemistry of a Titanium Alloy Used for Dental Implants: Implications for Bone Reintegration

Background

The long-term success of Ti alloy dental implants largely depends on rapid healing with safe integration into the jaw bone (osseointegration) [1]. The surface topography of the Ti alloy is crucial for the long term success of dental implants. Treatments for Ti implant materials have been developed in the last decade in a concentrated effort to improve the osseointegration process [2]. Recently, it has been shown that changes in the physicochemical properties of Ti alloy implants results in significant modulation of cell recruitment, adhesion, inflammation and bone remodeling activities, in addition to regulation of the bone formation response [3]. The use of dental implants has been in general highly successful with around 96 % of them remaining in the patient more than 10 years. Figure 1 shows a basic description of how dental implants, with the abutement and crown, are typically placed into the jawbone of the mouth.

In recent years, several treatment strategies (mechanical, chemical, physicochemical, etc.) have been proposed for the infection of the dental tissue around an implant (peri-implantitis) [4]. Peri-implantitis is an "inflammatory process around an implant, characterized by soft tissue inflammation and loss of supporting bone" [5]. After a healing time is allowed for re-osseointegration, the dental implant is loaded with a prosthetic abutment and crown in order to replace the missing teeth. However, bone loss is a potential complication for dental implants once inflammation of the gum and dental tissue around the implant sets in due to bacterial infection. The advantageous dental implant surface properties, i.e. the high reactivity of the oxidized titanium that allows cellular adhesion, is altered by the presence of bacteria and the residues of their metabolic activity. Thus, the contaminated surface acts as a foreign body and can lead to more inflammation of the soft tissue and bone loss surrounding the implant. The treatment of peri-implantitis involves surface decontamination and cleaning. These different treatments for peri-implantitis could possibly be used in a strategic way to modify the Ti alloy surface and cause an improvement in the host-to-implant response [6]. This report discusses the effect of the different dental treatments on the Ti alloy implant material’s surface properties and whether these may accelerate the re-osseointegration of dental implants during healing after peri-implantitis. The osseointegration of dental implants is relatively long (3–6 months), therefore, surface modifications that can accelerate this phenomenon will lead to shorter healing times, lower failure rates, and minimization of discomfort for the patient [7].

Enhanced titanium implant surface topography improves the bone-to-implant contact and the mechanical properties of the enhanced interface [8–11]. In the absence of controlled comparative clinical trials, the aggregate experimental evidence supports the use of titanium implants with enhanced surface topography [12].

Surface roughness has been identified as an important parameter concerning the capacity of implant materials to be anchored into bone tissue [13]. There are a variety of methods to increase the surface roughness of the implant material, where the most commonly used are: machining, sandblasting, acid etching, anodic oxidation, laser modification, or a combination of these techniques. Furthermore, commercially available implants have been categorized according to their average surface roughness value (S_a), the mean height of the peaks and mean depth of the valleys on the surface, into 4 groups: smooth (S_a < 0.5 μm), minimally rough (0.5 μm < S_a < 1.0 μm), moderately rough (1.0 μm < S_a < 2.0 μm) and rough (S_a > 2.0 μm) [14]. Another important parameter is S_dar which represents the developed surface area of a rough surface in comparison to a perfectly flat, smooth surface. According to Teughels et al. [15] roughness of the implant surface, as well as its chemical composition, has a significant impact on the amount and quality of plaque formation. Finally, there is no existing evidence that implant surfaces exposed to the oral cavity show the development of biofilms having compositions which depend on the surface roughness [16–17]. How can implant topography be represented and categorized? Wennerberg concisely reviewed topographic measurement methods that are applicable to dental implants. Three-dimensional (3D) measurements are needed to account for the isotropic deviations of topographic elements from the mean surface plane [18]. Contact instruments, such as profilometers, can underestimate the surface feature dimensions. Optical instruments are preferred for the evaluation of screw shaped implants of soft materials, such as titanium alloys [19]. The limitations of 2D surface measurement and characterization have motivated the development of efficient and practical 3D surface measurement and characterization. Three dimensional techniques give a better understanding of the surface in its functional state.

Titanium alloy modification processes

A total of 25 cylindrical implants (5 per sample, control sample + 4 treatment processes) made from grade 5 CP (commercially pure) titanium (Ti) alloy (PM international suppliers, LLC, EEUU), i.e. Ti-6Al-4V (TAV) with a composition of 90 % Ti, 6 % Al (aluminum), and 4 % V (vanadium), were analyzed. The samples were 10 mm in diameter and 5 mm in length. The implant samples were surface-treated by several different processes typically used in dentistry:

• chemical: tetracycline and photosensitizer exposure;
• physical: ultrasonication and
• physicochemical: bicarbonate jet polishing;

which will be explained in more detail below.

The following types of microscopy were used:

1. Confocal Microscopy (CM): CM provides a convenient means of acquiring 3D images of an object. The Leica DCM 3D dual system was used to measure the surface topography and calculate the surface roughness parameters. Images were taken with a confocal objective with a magnification of 20x and a numerical aperture (NA) of 0.50.
2. Atomic Force Microscopy (AFM): AFM studies were conducted in air at room temperature using a Dimension 3100 AFM head with a Nanoscope IVa controller (Bruker). All AFM images were recorded in tapping mode with rectangular silicon cantilevers (nominal tip apex radius of 10 nm, spring constant of 15 N/m, and resonance frequency of 145 kHz) at a scan rate of 1 Hz and 512 × 512 data points per image.
3. Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS): A JSM-7100F SEM (JEOL) and an INCA 250 EDS (Oxford Instruments) were used to study the surface morphology and composition of the Ti alloy. The operating conditions were 15 kV acceleration voltage at 200x magnification and a working distance of 10 mm.

The effect of the following types of typical dental treatments on the Ti alloy surface roughness and chemistry were investigated:

1. Bicarbonate jet polishing
   A jet of fine bicarbonate powder (around 150 μm average particle diameter) can clean teeth rapidly and effectively (Turbodent by Mectron). The particles of sodium bicarbonate are combined with warm water and then accelerated at a high rate which enables an extremely fine and regular jet to be obtained. The Ti alloy implant samples were jet polished for 1 minute with physiological saline being used as an irrigant, followed by rinsing with plenty of physiological saline.
2. Tetracycline
   Tetracycline hydrochloride (Sigma-Aldrich) is an antibiotic which acts primarily as a bacteriostat, but can act as a bactericide at certain concentrations. Tetracycline hydrochloride in the form of powder is highly caustic and disinfects the implant surface. For this study, the Ti alloy was exposed to a tetracycline/physiological saline solution (tetracycline concentration = 50 mg/ml) for 1 minute, followed by rinsing with plenty of clean physiological saline.
3. Ultrasound
   Ultrasonic cleaning is a common technique of modern dentistry. It is used for both periodontal and peri-implant treatments. The ultrasonic tip is made of very thin hardened steel (Sirosonic by Sirona). The vibration induces a phenomenon called cavitation which is the formation of cavities or bubbles in a liquid medium containing therein a gas or vapor. In addition, the vibratory motion allows debridement which is the breakdown of microorganisms attached to the surface of the tooth or implant. The Ti alloy implant samples were ultrasonicated for 1 minute (30 kHz) with physiological saline being used as an irrigant.
4. Photodynamic therapy
   Photodynamic therapy (PDT) involves the use of light-activated dyes (photosensitizers). When the photosensitizers are activated in the presence of oxygen they produce cytotoxic species which are known to be effective against viruses, bacteria, and fungi, therefore, PDT can be used as a therapy for localized infections. Photodynamic therapy in dentistry involves the application of a photosensitizer gel. For this investigation, Toluidine Blue (Sigma-Aldrich) was exploited which is activated by light with a wavelength of 570 nm. The photosensitizing gel produces free oxygen radicals which are highly reactive with the cell walls of microorganisms and, thus, toxic to them. This therapy is often used for peri-implantitis treatment. In this study, the Ti alloy implant samples were covered with a Toluidine Blue gel, then illuminated (Application FotoSan Lamp at 570 nm), left to stand for 1 minute (100 µg/mL), then illuminated a second time (application of a  Soft laser 906 nm), and finally rinsed with plenty of physiological saline.

Surface roughness was measured with a confocal 3D optical surface metrology system (Leica DCM 3D) using blue light illumination for higher resolution. The different surface area roughness parameters (Table 1) were obtained from five different Ti alloy implant samples for each treatment process.

Tab. 1: 3D roughness parameters analyzed for this study.

Confocal Microscopy (CM)

CM 2D and 3D image data were obtained for the control Ti alloy implant sample plus each treated sample. Some examples are shown below in Figure 2.

AFM 3D image data were obtained for the control Ti alloy implant sample only. Examples from 2 different areas of the sample are shown below in Figure 3.

As already mentioned above, the max z-range for the area in Figure 3A is 1,219 nm (1.22 µm) and that for the area in Figure 3B is 1,628 nm (1.63 µm) indicating a large variation of 25 % – 35 % between the 2 areas.

SEM and EDS data showed that the bicarbonate jet polished, tetracycline treated, and photodynamic therapy treated Ti alloy samples showed the greatest amount of contamination on the surface when compared to the control sample (unmodified TiO₂ surface). However, the ultrasonicated Ti sample was as clean as, if not cleaner than, the control sample.

SEM and EDS image data were obtained for the control Ti alloy implant sample plus each treated sample. Some examples of SEM images are shown below in Figure 4 and the EDS data showing local area composition are shown in Table 2.

A: Control

B: Bicarbonate jet

C: Tetracycline

D: Ultrasonicated

E: Photodynamic therapy

Tab. 2A–E: Data from EDS analysis of the local area composition at the surface of the Ti alloy samples, where K_α is the X-Ray line measured and σ is the standard deviation value for the % wt concentration data: A) control sample; B) bicarbonate jet polished sample; C) tetracycline treated sample; D) ultrasonicated sample; and E) photodynamic therapy treated sample.

The roughness analysis focusses mainly on the data for the Ti alloy samples obtained with CM which allowed the values of three important 3D roughness parameters to be determined: S_a, S_q, and S_dar. These data are shown in Table 3 below.

Tab. 3: Surface roughness data for the Ti alloy samples obtained with confocal microscopy. The analysis focusses on 3 roughness values: S_a (arithmetic mean height); S_q (root mean square [rms] height); and S_dar (developed area/projected area).

First of all, the S_a and S_q roughness values of the photodynamic therapy and tetracycline treated and bicarbonate jet polished samples are practically the same as the control sample. On the other hand, the S_a and S_q roughness values of the ultrasonicated sample are about 30 % to 50 % lower than those of the control sample. The S_dar roughness values show a similar trend, although the jet polished and tetracycline treated samples have noticeably lower S_dar values than the control sample (refer to Figure 5 below).

A preliminary analysis of the AFM images for the control sample (without modification) taken at two different places show that there is a high potential for a large variation in roughness values between smaller areas of a very rough surface, so the AFM is not a practical technique for measurements over large areas, which are needed for this study, on this kind of sample.

Tab. 4: Surface roughness data for the 2 areas of the Ti alloy implant control sample obtained with atomic force microscopy. The analysis focusses on 3 roughness values: S_a (arithmetic mean height); S_q (root mean square [rms] height); and S_dar (developed area/projected area).

As seen in Table 4 above, the S_dar values for the 2 areas differ by about 10 %, the S_q values differ by about 35 %, and the S_a values differ by about 30 %. On the other hand, the AFM images shows peaks and mountains with a z-range difference of more than 1 µm, a large difference for AFM analysis, which can lead to the presence of artifacts in the images, i.e. "streaks" where probably the tip touched the surface as it was scanning the sample. In general, the AFM image results will depend upon the sample topography and mechanical properties, the gain of the feedback loop, the scan rate, etc.

Currently used clinical Ti alloy dental implants show a wide variety of surface characteristics, both in terms of structural and chemical properties. The surface modifications outlined above retain the key physical properties of the implants and modify only their outermost surfaces with the ultimate goal of achieving the desired biological response. The advantages and disadvantages of different physicochemical, physical, and chemical surface modifications were presented. These methods will help us in understanding better how implant material surface modification affects the bone-implant interface and the development of a method which optimizes the implant’s re-osseointegration properties during healing after the successful treatment of peri-implantitis, the infection of the dental tissue surrounding the implant. It is not completely clear the degree of influence of the surface roughness and chemistry on osseointegration. The ideal degree of roughness for an optimal clinical performance still remains unknown [20–23].

For samples modified with tetracycline and photodynamic therapy, the morphology is affected by the chemical attack of precipitate intermetallic particles in the titanium alloy. For samples modified with jet polishing and ultrasonication, the mechanism is more mechanical. The surface roughness of both jet polished and photodynamic treated samples was comparable to that of the control sample (based upon the S_a, S_q, and S_dar values). The surface roughness of both the ultrasonicated and tetracycline treated samples was less than that of the control sample. In fact, ultrasonication significantly flattens the surface.

The contamination of the samples was greatest for the bicarbonate jet polished samples (salt residues), followed by the tetracycline and photodynamic therapy treated samples, and finally the ultrasonicated samples. It is not surprising that the jet polished and tetracycline and photodynamic therapy treated Ti alloy samples were the most contaminated, as they were exposed to salt (bicarbonate) or a chemical (tetracycline or toluidine blue) compound. The ultrasonicated samples were as clean as, if not cleaner, than the control samples, except for the presence of a small quantity of iron (Fe) probably coming from the steel tip which was used for ultrasonication.

It may be possible to propose a strategic use of these dental treatments to optimize the chances of re-osseointegration of an Ti alloy implant after peri-implantitis. Perhaps there would be a first step of applying the photodynamic therapy or bicarbonate jet polishing (with a lower frequency, e.g. 20 kHz, and power) to maintain the surface roughness of the implant, followed by a light, brief ultrasonication which would be effective for surface decontamination.

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The authors would like to express their gratitude to James DeRose of Leica Microsystems for useful discussion of the results and extensive review and proof-reading of the manuscript and to Aranzazu Villuendas (CCiTUB) for the useful discussion of the SEM results.