**4. Factors influencing biological seal of implant-soft tissue interface**

The existence and function of biologic width around dental implant are well documented in animal and human histological studies. Any factors affecting soft tissue reaction around dental implant might also affect the biologic width, thus the biologic seal of the peri-implant region. As mentioned earlier in the text, the nature and health of soft tissue surrounding an implant may be influenced by many factors. The presence of keratinized mucosa surrounding an implant is thought to influence the dimension of biological seal [54]. Moreover, the attachment of epithelial and connective tissues may also be influenced by material properties and surface modifications of implant abutment materials. Within the context of this chapter, how soft tissue responds to material and surface modification of implant/implant abutment is only discussed briefly.

#### **4.1. Bulk of materials**

Material properties appear to affect the attachment formed by epithelial tissue. Most often, titanium is the material used for dental implants and abutments, and is therefore the most extensive and widely studied material. Commercially pure titanium (Grade 2 and Grade 4) is commonly used in the fabrication of dental implants and implants abutments. Recently, zirconia is gaining more popular and seems to be a suitable implant material because of its excellence aesthetics, mechanical properties and biocompatibility. The presence of zirconia in dentistry is now being embraced, with the manufacturers promoting the esthetic, biomechan‐ ical and biological qualities of the material. Despite the extensive literature in the field of osseointegration of zirconia [39, 55], the response of soft tissue towards zirconia is starting to gain attention from many researchers [9, 11, 40, 41]. In animal experiments, Abrahamsson et al. [11] showed that an epithelial downgrowth occurred and migrated towards the implant neck and associated bone loss which was noted around the abutments of gold and gold alloys fused with dental ceramics, as compared to abutments made of pure titanium and aluminium oxide (Al2O11) ceramics where peri-implant cuff of about 3.5 mm width was noted to be present. Kohal et al. [39] also reported a satisfactory soft-tissue formation on both titanium and zirconium oxide (ZrO2) surfaces, without evidence of perpendicular fibres on the monkey model. Likewise, another study showed that the soft-tissue dimension at Ti and ZrO2 abut‐ ments remained stable after 5 months of healing, meanwhile at gold/platinum alloys abutment sites, an apical shift of the barrier epithelium and marginal bone loss occurred [41]. In contrast, a human clinical study conducted by Vigolo et al. [46] revealed no significant differences regarding peri-implant bone loss and soft-tissue level when abutments of titanium and gold alloy were used with cemented single implant crown. Similarly, Linkevicius and Apse [56] in their systematic review concluded that available data failed to give evidence that titanium abutments are better at maintaining stable peri-implant tissues as compared to gold, alumi‐ nium oxide and zirconium oxide abutments. The performance of zirconia vs titanium abut‐ ments over long term is yet to be available. Recently, Zembic et al. [57] has published a 5-year comparison of the clinical performance of both titanium and zirconia abutments, and they found no statistically and clinically relevant difference between the survival rates, and technical and biological complication of these two abutment types.

#### **4.2. Surface modifications**

**3.2. Analyses of the soft tissue-implant interface**

48 Dental Implantology and Biomaterial

briefly.

**4.1. Bulk of materials**

The soft-tissue implant interface can be investigated through histomorphometric and histo‐ logic analyses. Of both, the preparation for latter analysis is very difficult to carry out especially if the implant is attached to the tissue. The histological studies also allow identification of specific protein markers expressed by any of the tissue or cells in response to dental implants. The histological sections can then be analysed under different types of microscopies. Among the known microscopic analyses for assessing the peri-implant interface are light microscopy (LM), scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), focus ion beam (FIB) and transmission electron microscopy (TEM). Similar to SEM, CLSM allows assessment of peri-implant interface and cell-cell interaction without the need of histological processing as for light microscopy. With these two, direct visualization of implant-soft tissue interface is possible with appropriate preparation for each microscopy. The tissue or specimens can also be fluorescently labelled for the identification of adhesion molecules or cells and examined under CLSM [43]. While the use of 3D oral mucosal model with implant material intact may allow direct examination of the connective tissue attachment, the method to prepare the specimen still remains challenging and technically demanding. More studies in term of optimization of certain promising technique such as FIB for TEM analysis must be explored

in order to obtain the ultrastructural nature of the implant-soft tissue interface.

**4. Factors influencing biological seal of implant-soft tissue interface**

The existence and function of biologic width around dental implant are well documented in animal and human histological studies. Any factors affecting soft tissue reaction around dental implant might also affect the biologic width, thus the biologic seal of the peri-implant region. As mentioned earlier in the text, the nature and health of soft tissue surrounding an implant may be influenced by many factors. The presence of keratinized mucosa surrounding an implant is thought to influence the dimension of biological seal [54]. Moreover, the attachment of epithelial and connective tissues may also be influenced by material properties and surface modifications of implant abutment materials. Within the context of this chapter, how soft tissue responds to material and surface modification of implant/implant abutment is only discussed

Material properties appear to affect the attachment formed by epithelial tissue. Most often, titanium is the material used for dental implants and abutments, and is therefore the most extensive and widely studied material. Commercially pure titanium (Grade 2 and Grade 4) is commonly used in the fabrication of dental implants and implants abutments. Recently, zirconia is gaining more popular and seems to be a suitable implant material because of its excellence aesthetics, mechanical properties and biocompatibility. The presence of zirconia in dentistry is now being embraced, with the manufacturers promoting the esthetic, biomechan‐ ical and biological qualities of the material. Despite the extensive literature in the field of

Surface modifications of titanium dental implants or implant abutment are performed to improve the biological, chemical and mechanical properties of implants. Over the years, specific surface properties such as topography, structure, chemistry, surface charge and wettability have been investigated to help enhance the soft tissue attachment. Commonly, the surface modification can be broadly classified into modification of physical properties of the surface or chemical properties of the surface. In the subsequent paragraphs, the surface modifications of titanium dental implant/abutment are divided into surface topography and surface/chemical composition of the material. The surface topography of the implant can be altered in many ways. However, the methods of surface modifications of dental implant are not discussed since they are not within the scope of this chapter.

#### *4.2.1. Surface topography*

Different materials exhibit different surface energy. The differences in surface free energy may reflect their wettability characteristics. The higher the hydrophilicity of the material, the better adhesion of the cells thus enhancing the attachment formed by these cells [58]. Improving the surface texture with various techniques, thus altering the surface chemistry also enhances the wettability of certain material. Modification of surface texture will create different surface topography of dental implant material including abutment materials. Analysis of surface topography can be obtained from scanning profilometer (**Figure 5**) or SEM (**Figure 6**) in which the surface details can be visualized three dimensionally. The definition of surface roughness of dental implant has been proposed by Albrektsson and Wennerberg [59, 60]. This definition can be used for study of osseointegration or implant-soft tissue interface. Accordingly, the characterization of surface topography is shown in **Table 2**. The values of *S*a were determined by optical interferometry using Gaussian filters. There is a need to emphasize that **Table 2** shows a summary of several studies cited in this chapter.

**Figure 5.** A light interferometry micrograph showing the surface topography of the four types of Ti surfaces. (a) Pol‐ ished, (b) machined, (c) sandblasted, and (d) TiUnite. Scale bar: (a) 21.06–0.95 mm, (b) 21.65–2.15 mm, (c) 211.39–6.70 mm, (d) 23.28–4.82 mm. (Reproduced with permission from [52]).

**Figure 6.** Scanning electron micrographs of the four types of Ti surface topographies.(Reproduced with permission from [52]).


**Table 2.** Implant surface roughness.

the surface details can be visualized three dimensionally. The definition of surface roughness of dental implant has been proposed by Albrektsson and Wennerberg [59, 60]. This definition can be used for study of osseointegration or implant-soft tissue interface. Accordingly, the characterization of surface topography is shown in **Table 2**. The values of *S*a were determined by optical interferometry using Gaussian filters. There is a need to emphasize that **Table 2**

**Figure 5.** A light interferometry micrograph showing the surface topography of the four types of Ti surfaces. (a) Pol‐ ished, (b) machined, (c) sandblasted, and (d) TiUnite. Scale bar: (a) 21.06–0.95 mm, (b) 21.65–2.15 mm, (c) 211.39–6.70

**Figure 6.** Scanning electron micrographs of the four types of Ti surface topographies.(Reproduced with permission

shows a summary of several studies cited in this chapter.

50 Dental Implantology and Biomaterial

mm, (d) 23.28–4.82 mm. (Reproduced with permission from [52]).

from [52]).

Surface texture is known to influence epithelial cells and fibroblast attachment, although there is no complete agreement in the literature on the exact effect. One report found no significant differences concerning soft tissue reactions between roughed or smoothed surface implant [13], whereas Cochran et al. [35] found that smooth surfaces were more favourable for epithelial cell proliferation, as the fibroblasts appear to attach and proliferate better on rough surfaces. Simion et al. [61] reported that epithelial cells adhered and spread better on metallic surfaces than on ceramic surfaces with well-organized focal contacts and pre-hemidesmo‐ somes found on metallic surfaces, but not on porcelain and aluminium oxide.

Brunete and Chehroudi [62] in their review have suggested that the micro-fabricated grooved surfaces are able to inhibit epithelial downgrowth on implants depending on the dimension of the grooves in vitro. Similarly, fibroblasts also exhibit contact guidance on grooved surfaces, although its shape in vitro differs from that found in vivo. Delgado-Ruiz and co-workers [63] noted that micro-grooved surfaces were able to induce transverse collagen fibre formation, thus supporting two studies [26, 64]. It is also important to include a study by Nevins et al. [26] who demonstrated that soft tissue in humans is attached mechanically by perpendicular collagen fibre bundles on a micro-grooved pulsed laser surface.

### *4.2.2. Surface composition*

Over the years, many strategies have been explored to improve the biological seal of periimplant tissue by changing the surface chemistry of dental implants and implant abutments. The surface chemistry of the materials may be altered by biological modification, or by changing the chemical composition of the materials. As for biological modifications, methods of surface modification available include adding or coating with biomimetic/bioactive substances such as fibronection or intergrin onto the surface with the aim of promoting cellular adhesion and controlling cell behaviour. Fibronectin is a glycoprotein present on cell surfaces, found in connective tissues, basement membranes, and extracellular fluids, and is known to play a role in cell-to-cell and cell-to-substrate adhesion and enhances gingival fibroblast attachment. It is interesting to note that epithelial cells and fibroblasts have different affinities for adhesive proteins of the extracellular matrix. Dean et al [65] noted that higher number of fibroblasts bound to fibronection coated implant surface than epithelial cells, while gingival epithelial cell binding on implant surface coated with laminin was higher in number than fibroblasts [66, 67]. Collagen Type 1 was also used to modify surface chemistry as it was found to improve initial fibroblasts attachment [68].

The chemistry of material surfaces can also be altered by using element such as calcium or magnesium coating. Hydrothermal treatment of titanium with CaCl2 or MgCl2 was found to enhance initial attachment of epithelial and fibroblasts cells, and may increase the quality of the soft tissue seal around dental implant [69]. In addition, surface chemistry of materials may also inadvertently altered by the presence of impurities, surface contamination and saliva. A clean surface has a high surface free energy, while a contaminated one has a lower surface energy.
