**4. Dental implant surface treatments**

Currently, the most manufacturers of dental implants are introducing changes in implant surfaces in order to improve the success and quality of osseointegration.

Some studies have noted that with greater surface roughness, the rate of osseointegration, and the biomechanical fixation of Ti implants both increase [45, 46]. The methods used to modify the surface properties can be divided into additive and subtractive. Before certain surface treatments, pretreatment such as grit blasting or polishing may sometimes be indicated to guarantee the absence of contaminations, scratches, and irregularities [47, 48].

#### **4.1. Additive methods**

**3.2. Implant diameter**

112 Dental Implantology and Biomaterial

the size of the implant platform.

and diameter of the implant itself.

resistance to occlusal loads [40].

conical and cylindrical implants [41].

**3.3. Implant shape**

The implant diameter is the distance from the outermost point of the screw to the opposite side. It measures the external dimension of the implant screw and should not be confused with

Implant diameters usually range from 3 to 7 mm to make them compatible with the most sizes of alveolar processes. The choice of diameter depends on both surgical and prosthetic factors. In order to achieve maximum primary stability, the implant should be lodged between the vestibular-lingual/palatal cortical bones. From a biomechanical point of view, wider implants are able to join a larger amount of bone to the implant surface and obtain a higher bicortical anchorage, thus achieving a better distribution of stress in the surrounding bone. Another advantage of large diameter implants is that they can be inserted immediately in failure sites [31–33]. Some authors have found that increasing implant diameter by 1 mm increases the surface of bone–implant contact by 35% [34]. However, another parameter to consider is the crestal bone around the implant. According to Misch [35], this bone has a strong influence on the occlusal load; this author hypothesizes that it may be even more important than the length

The primary stability of dental implants at the time of surgery has been considered an important factor for integration [36]. Langer et al. recommended large diameter implants to improve primary stability in low-density bones. The authors argue that increasing the diameter

Small diameter implants have been introduced for narrow residual alveolar ridges and for edentulous spaces with small interdental distances. These implants do not include miniimplants, which are used to hold temporary dentures and have diameters <2.7 mm [38]. The main indications for narrow implants are the lower incisors, upper lateral incisors, and the restoration of teeth with residual spaces smaller than 5 mm without any possibility of space recovery or bone regeneration [39]. The main limitation of these implants is their reduced

Shape has been one of the most thoroughly studied aspects of implant design. The most current implant systems are solid cylinders with thread; hollow implants are rare today. As for the design of the thread, attempts have been made to increase their self-threading capacity and to reduce heat generation during implantation. These design variations are most often applied in the crestal and apical areas. Some designs have attempted to imitate the natural root with a stepped cylindrical shape in the apical and crestal third of the implant. Some authors note that stepped cylindrical implants achieve better stress distribution and crestal bone load than

Kan et al. [42] reported that threaded implants provide the best immediate retention. Other studies show that the use of a serrated thread can increase primary stability and that thread geometry plays an important role in the biomechanical properties of the implants [43, 44].

increases the bone–implant contact, thereby reducing initial implant mobility [37].

Additive methods supply extra materials to the implant surface, either via coating or via impregnation. Coating involves the addition of a material of variable thickness to the surface of the core material. The techniques used are Ti plasma spraying (TPS), plasma-sprayed HA coating, alumina coating, and biomimetic calcium phosphate (CaP). For its part, impregnation requires the full integration of the chemical material or agent into the Ti core. This is the case of CaP crystals within the TiO2 oxide layer or the incorporation of fluoride ions to the surface [8].

*Plasma Spray Coating*: The coating process includes the spraying of thermally melted materials on the implant substrates [8]. This technique usually involves a fine layer of deposits such as HA and Ti. The combination of HA coating on Ti alloy substrates offers attractive mechanical properties and good biocompatibility [49]. Plasma spray significantly increases the surface area of the implant by increasing its roughness [50]. Thus, many studies have shown that plasma spray is a good additive method for improving the biomechanical behavior [47, 51– 55]. Some studies have even described a possible optimization of scar formation and cell proliferation thanks to HA coating [56, 57].

#### **4.2. Subtractive methods**

Subtractive techniques areprocedures thatremove a layerof corematerialordeformthe surface in order to increase its roughness [58]. These methods can be divided into mechanical, chemi‐ cal, and physical. Removal of surface material using mechanical methods includes shaping/ removing, grinding, machining, or grit blasting using physical force. Chemical treatment of Ti alloys using either alkaline or acid solutions is carried out not only to increase the roughness but also to modify the composition and improve the wettability and surface energy [59]. Complementary physical treatment of the coating surface, such as thermal spray and plasma spray, improves the aesthetic appearance of the materials and their performance [8].

*Grit blasting* is a mechanical subtractive procedure which increases surface roughness by the pressurized projection of particles onto the surface of the implant. The main materials used are sand, HA, alumina, or TiO2 particles. After grit blasting, acid etching is applied to remove the residual particles. Grit blasting is one of the most commonly used surface treatments for increasing the surface roughness of dental implants. However, in itself, it does not accelerate the osseointegration capacity [8, 60] (**Figure 1A**).

**Figure 1.** Environmental scanning electron microscope micrograph (ESEM) of the surface of dental implants: (A) shot‐ blasted; (B) acid etched [66].

Aparicio et al. [61] observed that the increase in the surface roughness of the material induced by blasting in cp Ti was not the only cause of the differences in the electrochemical behavior and corrosion resistance; they also mentioned the compressive residual surface stresses induced by shotblasting.

Commercially, pure Ti is a bioinert material which lacks the ability to establish chemical bonds with surrounding bone. Kokubo et al. [62] demonstrated that the treatment of this Ti with heat and alkali procedures rendered it bioactive. Aparicio et al. [63] observed that the surface of the implant achieved by grit blasting and thermo-chemical treatment improved adhesion and differentiation of human osteoblasts. Gil et al. [64] also observed positive results for this bioactive Ti, although improvements are necessary in order to prevent bacterial colonization. It is important to bear in mind that bacteria have a greater capacity to colonize rough surfaces [8] (**Figure 2**).

**Figure 2.** ESEM micrograph of the sodium titanate surface of the implants treated by shotblasting and thermochemical treatment (two-step treatment) [66].

Some researchers have found that the apatite layers formed on grit-blasted surfaces have a higher adhesion strength to the substrate than plasma-sprayed apatite coatings. They note the potential clinical application of this type of surface treatment in dental implants [65].

The evolution of bioactive surfaces into osseoconductive biomimetic surfaces (Contact Ti) was described by Gil et al. In this process, a CaP layer is obtained on the implant surface by thermochemical treatments. This achieves a structure equal to the CaP formed by the mineral content of the bone (HA). This apatite should not be confused with an additive coating; in this case, there is an extremely strong chemical bond and so it is not dislodged by mechanical action. These bioactive implant surfaces significantly reduce the time of osseointegration. The most important mechanisms involved are the protein adsorption capacity, wettability, and an optimized zeta potential which reduces the electrostatic dispersion between particles. Finally, this procedure also aims to increase the kinetics of adhesion, proliferation, and differentiation of osteoblast cells compared to other current surface treatments in order to facilitate bone formation around the implants [66–68] (**Figure 3**).

increasing the surface roughness of dental implants. However, in itself, it does not accelerate

**Figure 1.** Environmental scanning electron microscope micrograph (ESEM) of the surface of dental implants: (A) shot‐

Aparicio et al. [61] observed that the increase in the surface roughness of the material induced by blasting in cp Ti was not the only cause of the differences in the electrochemical behavior and corrosion resistance; they also mentioned the compressive residual surface stresses

Commercially, pure Ti is a bioinert material which lacks the ability to establish chemical bonds with surrounding bone. Kokubo et al. [62] demonstrated that the treatment of this Ti with heat and alkali procedures rendered it bioactive. Aparicio et al. [63] observed that the surface of the implant achieved by grit blasting and thermo-chemical treatment improved adhesion and differentiation of human osteoblasts. Gil et al. [64] also observed positive results for this bioactive Ti, although improvements are necessary in order to prevent bacterial colonization. It is important to bear in mind that bacteria have a greater capacity to colonize rough surfaces

**Figure 2.** ESEM micrograph of the sodium titanate surface of the implants treated by shotblasting and thermochemical

Some researchers have found that the apatite layers formed on grit-blasted surfaces have a higher adhesion strength to the substrate than plasma-sprayed apatite coatings. They note the

potential clinical application of this type of surface treatment in dental implants [65].

the osseointegration capacity [8, 60] (**Figure 1A**).

blasted; (B) acid etched [66].

114 Dental Implantology and Biomaterial

induced by shotblasting.

treatment (two-step treatment) [66].

[8] (**Figure 2**).

**Figure 3.** ESEM images showing: (A) 2S bioactive surface; (B) *in vitro* nucleation of apatite on 2S bioactive surface; (C) *in vitro* formed apatite layer on 2S bioactive surface; (D) 2S bioactive surface at higher magnification [67].

*Anodic oxidation* is an electrolytic process used to strengthen and increase the thickness of the natural oxide layer. This passivation technique manages to turn a smooth Ti surface into a tubular nanostructure with diameters below 100 nm [69]. Some authors suggest that by modifying the parameters of voltage, current density, and chemistry of electrolytes, it is possible to control the physical and chemical properties of the implant surfaces, the spacing, and the diameter of nanotubes [70]. Anodization forms pillar-like nanostructures with tunable size on the surface of Ti and deposits long nanotube arrays (10 microns), thus improving the cell bioactivity [71].

*Acid treatment* is a chemical subtractive method that cleans the surface of the metal and modifies its roughness. Hydrofluoric acid (HF), nitric acid (HNO3), and sulfuric acid (H2SO4) are commonly used, either alone or in combination [8]. This technique obtains a homogeneous surface roughness for different sizes and shapes. The acid-etched surfaces facilitate the process of osseointegration by increasing the capacity of cell adhesion and bone formation [72–74]. Furthermore, the surface roughness of Ti also determines the stability of the bone formation and resorption at the interface with the implant [75]. The dual acid etching treats the surface by chemical means or by acids applied sequentially or in combination [76, 77]. This technique achieves a surface with micro-roughness, which some authors associate with higher values of reverse removal torque than machined surface implants [78] (**Figure 1**B).

*Alkali treatment* is a procedure in which the Ti implant is immersed in either potassium or sodium hydroxide followed by heat treatment (800°C for 20 min) and subsequent rinsing with distilled water. This technique achieves a nanostructured and bioactive sodium titanate layer on the surface of the dental implant, which provides favorable conditions for bone marrow cell differentiation [69]. The thermal oxidation works by changing the crystal structure of the nanometric oxide layer and thus increases the bioactivity of a biocompatible metal [79].

*Sandblast*, *large grit*, *and acid etching (SLA)* applies a strong acid on the blasted surface for the purposes of abrasion. The procedure starts with large particle blasting, which obtains a rough, irregular surface. Then, the acid etching produces surface uniformity and obtains a macroroughness and micro-pits which are able to improve osseointegration. Kim et al. [80] observed that human osteoblasts grow well on the SLA surface which provides space for cell adhesion and proliferation.

#### **4.3. Other techniques**

Other procedures such as ion implantation, laser treatment, sputtering, and the combination of some of the techniques already mentioned have also been studied in order to improve the surface properties of dental implants [81–84].

Ion implantation causes atomic rearrangement. It permits the injection of any element on a nearby surface with a beam of high-energy ions (10 KeV) which impacts on the surface of the metal in a vacuum chamber. On colliding with the ions of the substrate material, the incident ions lose energy and settle on the surface of the nearby metal. This technique is considered an ultra clean process because the concentration and depth of the impurities are easy to control, allowing the creation of a layer of high purity. Furthermore, the adhesion between the implanted surface and the substrate is excellent; the process does not alter the properties of the core and is highly reproducible and controllable [85]. However, some authors warn that the possible modification of the nanoscale features and the creation of stress on the Ti surface should be taken into consideration [86, 87].

Ultraviolet (UV) photo-functionalization is one of the recent advances in the chemical modi‐ fication of implant surfaces which does not alter the bioactive properties.

Laser technology is an extremely clean, fast, and accurate method which allows nanostructural micromachining at the implant surface [88]. Laser peening involves striking the metal with high-intensity pulses of a laser light beam which produces a deep, regular honeycomb pattern with small pores [2].

The slow rate sputter deposition method achieves a thin layer of Ti oxide (300 pm–6.3 nm). This technique increases the oxygen components without altering the surface topography. These biological activities are correlated with the thickness of the TiO2 coating and the oxygen saturation of the surface. This means that the biological response of Ti can be improved even with picometer super thin coatings [69].
