A review on the wettability of dental implant surfaces II: Biological and clinical aspects

Abstract

Dental and orthopedic implants have been under continuous advancement to improve their interactions with bone and ensure a successful outcome for patients. Surface characteristics such as surface topography and surface chemistry can serve as design tools to enhance the biological response around the implant, with in vitro, in vivo and clinical studies confirming their effects. However, the comprehensive design of implants to promote early and long-term osseointegration requires a better understanding of the role of surface wettability and the mechanisms by which it affects the surrounding biological environment. This review provides a general overview of the available information about the contact angle values of experimental and of marketed implant surfaces, some of the techniques used to modify surface wettability of implants, and results from in vitro and clinical studies. We aim to expand the current understanding on the role of wettability of metallic implants at their interface with blood and the biological milieu, as well as with bacteria, and hard and soft tissues.

Keywords: Hydrophilicity; Osseointegration; Surface conditioning; Surface energy; Titanium implant roughness.

Figure 1

Schematic of the possible interactions with (A) hydrophilic and (B) hydrophobic surfaces at different length scales. (A) Hydrophilic surfaces interact closely with biological fluids, allowing normal protein adsorption to the surface and subsequent interactions with cell receptors. (B) Hydrophobic surfaces are prone to hydrocarbon contamination, leading to entrapment of air bubbles that can interfere with protein adsorption and cell receptor adhesion/activation.

Figure 2

Efficiency of hydrophilization upon UV-A or UV-C treatment, respectively, on three different Ti surface oxides: the native passive layer, a thicker electrochemically manufactured anodic oxide, and a pulse magnetron sputtered layer of polycrystalline pure anatase. The efficiency classification used in the figure is as follows: no hydrophilization means the CA did not change compared to an untreated specimen; moderate hydrophilization reflects CAs that decreased but without reaching superhydrophilicity; high hydrophilization indicates the wetting liquid spread on the surface, forbidding CA measurements and signaling superhydrophilicity. Hydrophilization from UV-A irradiation is mediated by the formation of hydroxyl groups and by cleansing of the surface from organic contaminants due to photocatalytic formation of radical and anionic species at the material/organic contamination interface. Hydrophilization by UV-C irradiation also depends on cleansing of the surface from organic contaminants, but in this case due to direct photolytic decomposition.

Figure 3

Schematic depicting the effects of (A) hydrophilic and (B) hydrophobic surfaces on protein adsorption and conformation. (A) Hydrophilic surfaces in contact with blood and biological fluids promote protein adsorption in a conformation that exposes adhesion motifs and enhances cell adhesion. (B) Hydrophobic surfaces can partially denature proteins, disturbing their tertiary structure and causing cell-binding sites to be less accessible, which results in diminished cell adhesion.

Figure 4

Fibronectin adhesion on a hydrophobic, blasted and acid-etched (BAE) Ti implant surface, compared to adhesion on different hydrophilic modifications (modBAE stored at pH 4–6 or 8–9) of the original surface. Samples were incubated with a physiological concentration (0.4mg/ml) of human plasma fibronectin for 1h. Bound fibronectin was determined by immunological quantification using an antibody directed against the fibronectin cell-binding domain. Data represent means and standard deviations (n = 3) from three independent experiments. All data are expressed as % related to BAE.

Figure 5

Change in response of MG63 cells to differences in surface wettability. MG63 cells were plated on Ti or Ti6Al4V specimens with different surface modifications (machined, nanomodified, or micromodified) and covering a wide range of CAs, and cultured to confluence on TCPS [22, 90, 137]. Data represent the levels of osteocalcin secreted into the conditioned media (n=6 independent samples) normalized to production on tissue culture polystyrene surfaces. The specimen abbreviations from the respective references were maintained: PT = machined, pre-treatment Ti; A = acid-etched Ti; SLA = sandblasted with large grit and acid-etched Ti; modA = hydrophilized A; modSLA = hydrophilized SLA; NM-PT = heat-treated, nanomodified Ti; NM-SLA = heat-treated, nanomodified SLA; sTiAlV = smooth Ti6Al4V; rTiAlV = (micro) rough, double acid-etched Ti6Al4V; NM-sTiAlV = nanomodified sTiAlV; NM-rTiAlV = nanomodified rTiAlV.

Figure 6

Photometric quantification of crystal violet bound to the different experimental surfaces for human oral keratinocyte surface coverage evaluation. Keratinocytes were incubated on (A) control machined (MA) surfaces or (B) MA surfaces hydrophilized by cold plasma treatment or (C) samples hydrophobized by silane coupling to the surface, for 3 to 5 days, stained with crystal violet, and photodocumented. (D) The bound crystal violet was eluted and quantified photometrically to determine surface coverage. Data represent means and standard deviations (n = 3) from three independent experiments. All data are expressed as % related to MA.