2. Porphyromonas gingivalis Structure and Characteristic
Principally, bacterial surface components and their extracellular compounds, such as fimbriae or pili, LPS, and EPS, combined with environmental conditions and quorum-sensing signals, are critical for biofilm formation
[29][24]. Below,
wresearche
rs will discuss the
P. gingivalis structure and its significance, especially in biofilm formation.
2.1. Fimbriae
Bacteria, as well as
P. gingivalis, commonly express their extracellular polymer known as pili or fimbriae, which are similar terms
[30][25]. Fimbriae are considered significant factors in determining
P. gingivalis virulence, as fimbriae help in bacterial adhesion to the host surface, antibiotic surface, and or in between bacterial cells
[31,32,33][26][27][28]. In an in vitro polymerase chain reaction assay (PCR), it was found that the number of fimbriae in a
P. gingivalis strain is equal to its ability to adhere
[32][27]. A
P. gingivalis mutant 33277 (MPG1) with minimum fimbriae could not adhere to both epithelial cells or the gingival fibroblast
[34][29]. Fimbriae are also classified into several types based on several classification schemes. The most popular one is a classification based on its morphology and function by Brinton in 1965. Brinton classified six types of fimbriae and then, one year later, Duguid et.al. added a seventh (Type 1 to 6 and F)
[33][28]. However, with
P. gingivalis, the widely used classification is based on the nucleotide sequences, in which six genotypes of the
fimA gene have been identified (
fimA I, Ib, II, III, IV, and V)
[35][30].
P. gingivalis, in general, has two types of fimbriae, which are
FimA and
Mfa1 fimbriae. The
FimA fimbriae are composed of
FimA proteins encoded by
FimA genes and called the long fimbriae. Similarly, the
Mfa1 fimbriae are composed of the
Mfa1 protein encoded by
Mfa1 genes, and are called the short fimbriae
[30,36,37][25][31][32]. These fimbriae help bind specifically to and trigger various host cells, such as epithelial, endothelial, and spleen cells, as well as peripheral blood monocytes in humans, resulting in the release of several distinct adhesion molecules and inflammatory cytokines
[30][25]. In addition, there are several accessory proteins which are incorporated into fimbriae; for example,
Mfa4 which are incorporated into
Mfa1 fimbriae.
Mfa4 mediate the formation of
Mfa1 by promoting the maturation of
Mfa3 and stabilizing
Mfa5 within the cell surfaces; thus they are crucial in biofilm formation
[38][33].
2.2. Capsule
The
P. gingivalis strain exhibits significant heterogeneity, in which some strains are encapsulated, whereas others are non-encapsulated
[39][34]. Previous studies have reported that the encapsulated strain of
P. gingivalis has higher virulence than the non-encapsulated one. The capsule plays a major role in evading host immune system activation, reducing phagocytosis, increasing bacterial activity survival ability within the host cells, and boosting its virulence
[39,40,41][34][35][36].
2.3. Cell-Wall
In the Gram-negative bacteria, such as
P. gingivalis, the cell wall is formed from a single layer of peptidoglycan covered by a membranous structure called the outer membrane vesicles (OMVs).
P. gingivalis expresses protease activity which can be extruded with the OMVs
[42][37]. OMVs enable bacteria and host communication as they can carry molecules involved in immune modulation
[43][38].
P. gingivalis OMVs are adherent and small, with the ratio of cells to OMVs at approximately 1:2000
[44][39].
3. Biofilm Formation
Biofilm is a microbial community attached to the interface enclosed in an EPS that exhibits a distinct phenotype correlated to its gene transcription and growth rate. It is known that the biofilm has been shown to have a specific mechanism for initial attachment to a surface, development, and detachment
[45][40]. Overall, it is believed that biofilm formation begins with bacterial attachment on the surface, which transforms from reversible to irreversible. Adhesive components of bacteria aid this transformation. This attachment then advanced through EPS production, which later entrapped the whole structure. Finally, some bacterial cells escape from the mature biofilm to form new colonies
[46][41]. Once the biofilm is developed, killing the bacteria inside or removing the biofilm from the surface becomes difficult. Bacteria inside the biofilm are packed and resistant to the adverse environment, for example, antibiotics
[47][42]. Hence, interspersing initial bacteria attachment, including their sensing mechanism, is crucial to preventing biofilm formation and related problems
[48][43].
Biofilm formation on the subperiosteal implant is affected by several factors such as (1) oral environment, (2) bacterial properties, and (3) material surface characteristics, including chemical composition, surface free-energy, hydrophilicity, and surface topography (roughness)
[17,49][17][44]. Higher surface free energy has shown significant correlation to bacterial adherence. Higher surface free energy favours bacterial attachment
[50][45]. In addition, the combination of surface free energy and surface roughness is the major factor and proportional to surface hydrophilicity with low surface energy and smoother-surface-producing higher hydrophobicity
[51][46]. Almost all in vivo studies suggest that a smooth surface reduces the amount of biofilm compared to a rough one. An increase in surface roughness of more than 0.2 μm and or an increase in surface energy promotes biofilm formation, with surface roughness being more dominant
[49][44].
4. Surface Topography
Physical modification of surfaces can provide long-term effectiveness and is environmentally friendly. Thus, the physical modification is believed to be a more promising alternative compared to the chemical modification of surfaces
[52][47]. One of the important parameters in the identification of physical surface properties is surface topography, which refers to both the profile shape and the surface roughness, including the waviness and the asperity or the finish
[53][48].
Furthermore, the most frequently used parameters for characterizing surface topography are average surface roughness (R
a) and root-mean-square surface roughness (R
rms), that stands for the average and root-mean-square deviation of height values from the mean line, respectively. However, both R
a and R
rms provide no information on the spatial distribution or shape of the surface features. Some researchers have offered new parameters for a more comprehensive characterization of the surface topography, such as summit density (S
ds) and developed area ratio (S
dr)
[52][47].
In the next paragraph, the surface roughness is presented in R
a or S
a. The average roughness, R
a, provides a general measure of the height of the texture across a surface. It is the average of how far each point on the surface deviates in height from the mean height, while S
a is an absolute value that expresses the difference in height of each point to the arithmetical mean of the surface
[54][49]. In general, surface energy (often presented as water contact angle) changes as the surface roughness changes
[55][50]. However, one should keep in mind that surface chemistry also plays key roles in the changes in surface energy
[56,57,58][51][52][53].
5. Subperiosteal Implant Materials and Surface Modification
Material selection in subperiosteal implant placements plays a key role in implant success
[59][54]. In general, like the endosseous implant, subperiosteal implant material is divided into three categories which are metal, ceramic, polymer, and composite
[60][55]. To improve materials properties, surface treatment is commonly applied. The addition of surface treatment improves cell attachment and bacterial debridement. Surface treatment is arguably the most studied topic regarding implant design alteration. There are various types of surface treatment; however, they can be simplified into two types which are chemical and physical. Both of these types showed efficacy in increasing bone attachment and or bacterial debridement
[61][56]. In this section,
wresearche
rs will discuss the materials used for subperiosteal implants and the various surface modification methods applied.
One of the widely used materials for subperiosteal implants is titanium and its alloys
[59,62][54][57]. Titanium and its alloys are still a material of choice for dental implants, as they have a high success rate, are durable, and display adequate osseointegration
[63][58]. There are several methods used for titanium surface modification such as sandblasting, acid etching, a combination of both sandblasting and acid etching (SLA), fluoride treatment, calcium phosphate coating, and anodic oxidation
[64,65][59][60]. Among these methods, sandblasting is one of the most popular. Sandblasting or acid etching or a combination of both can increase surface roughness, increasing the surface area for osteoblast attachments. Hence, it increases bone healing, interfacial stress distribution, and bonding strength
[66][61]. In addition, Alagatu et al.
[59][54], mentioned sandblasting as the best method for titanium and zirconia.
Recently, the popularity of zirconia as an alternative for implant materials has increased. In their review, Alagatu et al.
[59][54] showed that some clinical studies demonstrated that zirconia has better anti-inflammatory properties than titanium. In addition, zirconia is less prone to peri-implantitis than titanium. Zirconia also can be combined with titanium to improve both properties. The addition of zirconia increases implant biocompatibility compared to titanium alone
[67][62]. Several attempts have been made to improve the properties of zirconia such as the addition of hydroxyapatite
[68][63] or calcium phosphate
[69][64], sandblasting
[70][65], acid etching
[71][66], laser treatment
[72][67], and ultraviolet photo-functionalization
[73][68].