What Different Surface Treatments Are Available for Dental Implants?

Implant restoration has become the current optimal method for restoring the aesthetics and chewing function of patients with missing teeth. The ability of an implant to form stable osseointegration is crucial, making surface treatment a key focus in current research within the field of oral implantology. After undergoing physical, chemical, and biological modifications, the surface morphology, chemical composition, and biological activity of titanium implants are altered, leading to the successful and stable formation of osseointegration.

The following is a comprehensive review of surface modification methods for implants from three aspects: physical, chemical, and biological.

Ⅰ What is Physical Modification?

Physical modification involves altering the surface morphology and roughness of implants through physical methods such as plasma spraying, sandblasting, laser treatment, etc., to provide a better foundation for osseointegration.

1. Plasma Treatment

Plasma spraying technology utilizes a plasma arc driven by direct current as a heat source to heat materials such as ceramics, alloys, metals, etc., to a molten or semi-molten state. These materials are then rapidly sprayed onto the pre-treated surface of the implant, forming a firmly attached surface layer. Currently, hydroxyapatite coatings are commonly used in clinical applications. Hydroxyapatite particles are sprayed onto the implant surface at high temperatures, and after rapid cooling, a crack-coated layer is formed. Research indicates that atmospheric plasma-sprayed porous titanium coatings can enhance the osseointegration ability of pure titanium implants.

2. Ion Implantation

The fundamental principle of ion implantation technology involves directing an ion beam into the implant, where the ions undergo a series of physical and chemical interactions with the atoms or molecules in the implant. The incident ions gradually lose energy and eventually come to rest in the implant, causing changes in the surface composition, structure, and properties of the implant. Utilizing ion implantation technology, elements such as magnesium, zinc, calcium, and silver can be implanted into the surface of the implant to optimize its surface properties. Implanting silver ions into acid-etched titanium implants can reduce the mobility of silver nanoparticles, resulting in excellent antibacterial activity and good compatibility with mammalian cells.

Injecting magnesium ions into the surface of implants can enhance surface cell adhesion, beneficial for improving the osseointegration ability of the implant. Using plasma immersion ion implantation and deposition technology to inject zinc ions into the smooth surface of pure titanium can increase its biocompatibility. Ion implantation technology is versatile, robust, and exhibits good controllability and repeatability. However, the implanted layer is thin, ions can only travel in straight lines, and the equipment cost is high. While the application prospects are broad, widespread adoption poses challenges.

3. Sandblasting Treatment

Sandblasting utilizes compressed air as power to generate a high-speed jet to spray abrasives onto the surface of implants, providing them with a certain degree of cleanliness and roughness. Sandblasting increases the surface area, improving cell adhesion and proliferation, thereby enhancing osseointegration.

4. Treatment with Absorbable Abrasive Media

Surface treatment with absorbable abrasive media involves spraying calcium phosphate ceramics onto the surface of the implant. The titanium implant treated with absorbable abrasive media can enhance the absorption effect after chlorine disinfection, showing higher antibacterial activity. Studies have found that implants treated with absorbable abrasive media exhibit a higher overall success rate in implantation, with no significant loss of alveolar bone. Using this method to achieve a certain surface roughness, residual calcium phosphate particles on the surface can be removed with a weak acid, reducing the retention of foreign substances. This is considered advantageous.

5. Physical Vapor Deposition

Physical Vapor Deposition (PVD) is a technology that operates under vacuum conditions, transforming the material source-solid or liquid-into gaseous atoms, molecules, or partially ionized ions. This is achieved by low-pressure gas, allowing deposition on the surface of the implant to create functional thin films. In recent years, large-scale magnetron sputtering coating has been developed. It exhibits a high deposition rate, good process repeatability, and is easily automated. Applied in the modification of implant surfaces, it enhances bone-implant contact, showing significant potential for development.

6. Electrical Discharge Machining

Electrical Discharge Machining (EDM) is a specialized processing method that utilizes the erosive effect of pulsed discharge between two electrodes immersed in a working fluid to remove conductive materials. The treated surface of the implant develops a micro-rough and biologically active layer of titanium dioxide, enhancing the strength of the bone-implant interface and reducing the sensitivity of the surrounding bone to stress shielding. Experiments have shown that placing EDM-treated implant screws in the femurs of large white rabbits results in a significant increase in new bone volume after one week. Therefore, Electrical Discharge Machining is likely a surface modification method that supports enhanced osseointegration.

7. Heat Treatment

Heat treatment is a comprehensive process in which the implant is heated, maintained at a certain temperature, and then cooled in a specific medium to control its performance by altering the structure of the surface or interior. As the temperature rises, the surface characteristics and biocompatibility of the implant also increase. Heating titanium in the atmosphere or peroxide can form a dense oxide film on its surface, enhancing biocompatibility. Experiments have shown that preparing a titanium nitride coating on the implant surface using nitrogen gas, followed by heat treatment with a calcium chloride solution, determines 120°C as the critical temperature for hydrothermal treatment. Heat treatment below 120°C can bind calcium to the surface of titanium nitride, maintaining its morphology and hardness, and improving wettability.

After heat treatment, the adhesion and proliferation of fibroblast cells on the surface significantly improve, suggesting that hydrothermal treatment has the potential to provide titanium coatings with good wear resistance and soft tissue biocompatibility. Additionally, through hydrothermal treatment, magnesium can be fixed on the titanium surface, increasing surface protein absorption, enhancing cell adhesion and spreading, indicating that using a magnesium chloride solution for heat treatment is an effective method to increase the biocompatibility of titanium implant surfaces.

8. Laser Treatment

Laser treatment involves rapidly heating the implant using a laser beam in environments such as the atmosphere or vacuum. This process achieves localized rapid heating or cooling, inducing changes in tissue structure or introducing other materials to improve surface performance. Research has shown that implants with surfaces treated by laser surface melting can promote cell adhesion and proliferation. Experiments indicate that hydroxyapatite coatings can be deposited on titanium surfaces through pulse laser deposition, resulting in higher mechanical adhesion forces. However, significant changes in surface morphology and roughness occur after laser ablation treatment, with less noticeable adhesion and proliferation of osteoblast-like cells on the treated surface.

9. Ultraviolet Treatment

Short-wave ultraviolet (UV) irradiation can enhance the osseointegration ability of titanium implants. Spiral-shaped pure titanium implants, acid-etched, were implanted into the femoral shafts of rabbits after being exposed to UV light for 48 hours and compared to implants not subjected to UV treatment. The results indicate that UV treatment can increase the cortical bone volume of the coronal position of titanium implants without reducing bone density. Additionally, UV-treated implant surfaces exhibit positive effects on the behavior of human gingival fibroblasts, including adhesion, proliferation, and collagen release, 24 hours after treatment.

10. Nanoscale Surface Roughening Treatment

Through nanotechnology, implants can achieve enhanced surface microstructure, promoting osseointegration and reducing healing time. Micro/submicro-scale roughness, along with nanoscale features, increases osteogenic cell differentiation and the production of local factors, suggesting an improved potential for implant osseointegration in the body. The diverse dimensions and distribution of the modified implant surface determine unique nanoscale structures, allowing the modulation of relevant bone responses within the body. The uncertainty associated with current technological developments makes it a focal point for further research.

Ⅱ What is Chemical Modification?

Surface chemical modification refers to the alteration of the surface structure and state through the chemical adsorption or reaction between the implant surface and surface modifiers. It is currently the most commonly used surface modification method, including anodization, acid-base treatment, sol-gel technique, etc.

1. Anodization

Anodization is the process of forming an oxide film on the metal anode surface in the presence of an electrolyte and specific process conditions under the action of an applied electric current. The porous surface created by anodization of titanium implants enhances osteogenic cell responses, strengthens osteogenic gene expression, and improves the nanomechanical properties of mineralized tissue.

Studies have shown that anodization of titanium and titanium-zirconium alloy in an electrolyte containing DL-α-glycerophosphate and calcium acetate increases oxygen, calcium, and phosphorus content on the implant surface. The average roughness increases, the contact angle significantly decreases, and cell proliferation, alkaline phosphatase activity, and calcium deposition around the implant significantly increase, contributing to enhanced osseointegration. The development of titanium-zirconium alloys has also provided new opportunities for anodization.

2. Microarc Oxidation

Microarc oxidation relies on the transient high-temperature and high-pressure effects generated by arc discharge on the surface of titanium and its alloys in combination with electrolyte and specific electrical parameters to form a ceramic film layer based on metal oxides. Microarc oxidation can create nano-bioactive titanium oxide layers, enhance implant adhesiveness, and increase cell adhesion strength. Using microarc oxidation, porous hydroxyapatite coatings can be prepared on titanium alloy surfaces, significantly improving bone-implant contact and mechanical properties at the contact interface, thereby promoting bone growth.

3. Plasma Electrolytic Oxidation

Plasma electrolytic oxidation is a technique that uses high voltage and large current to generate instantaneous plasma micro-arc discharge on the electrode surface immersed in an electrolyte, breaking through the passivation layer and sintering to form a ceramic oxide film. Experimental results show that plasma-treated coatings exhibit enhanced hydrophilicity, noticeable cell adsorption behavior, and increased collagen synthesis. Bilayer hydroxyapatite-titanium dioxide coatings prepared by plasma electrolytic oxidation possess the bioactivity of hydroxyapatite and the improved morphology of titanium dioxide, effectively promoting osseointegration. This method holds great promise in biomedical applications.

4. Electrodeposition

Using electrochemical deposition techniques to deposit strontium-doped calcium phosphate coatings on implant surfaces enhances osteoblast proliferation, suggesting that strontium-doped calcium phosphate coatings may contribute to increased bone formation around the implant. Initially, titanium surfaces undergo anodization to form titanium dioxide nanotubes, and then carbon nanotubes are electrophoretically deposited onto the titanium dioxide nanotubes. The results indicate that coating carbon nanotubes on titanium dioxide nanotubes helps improve their bioactivity, making this type of surface modification suitable for biomedical applications.

5. Acid-Base Treatment

Acid-base treatment involves using chemical processes to generate a stable compound or change the surface morphology of the implant through chemical or electrochemical interactions with acidic media. Acid etching is a phenomenon where the implant surface undergoes a change in morphology due to chemical or electrochemical interactions with acidic media, significantly affecting bone-implant contact. It is a reliable surface modification method. Surfaces treated with alkali thermal treatment exhibit characteristics such as a large number of ordered nanospikes and a sponge-like uniform porous inner network. This enhances collagen synthesis in gingival fibroblasts, resulting in good resistance to attachment of periodontal-like connective tissue. Alkali thermal treatment can increase bone-implant contact, effectively applied to thermally sprayed titanium metal to enhance osseointegration strength.

6. Sol-Gel Method

The sol-gel method involves using compounds with high chemical activity as precursors to create a uniform, transparent sol system under liquid conditions. These materials are then uniformly mixed, undergo hydrolysis and condensation reactions, form stable transparent sol systems, and eventually develop into gels through aging. After drying, sintering solidifies the gel, resulting in molecular and even nanostructured materials. Organic polymers can be applied through the sol-gel method to impart biological activity to the surface of titanium dioxide. For example, surface modification using poly(ethylene glycol terephthalate) enhances the mechanical stability, biocompatibility, excellent osseointegration ability, and bone conduction ability of the bone-implant contact surface.

Combining soft lithography and sol-gel is a method to create micro-patterns of biologically active nanoparticles on implant surfaces, guiding tissue regeneration. Using this method to produce a thin film of zirconium oxide with micro-sized silicon, it was found to be biocompatible and able to induce osteoblast adhesion, proliferation, and spreading. It can be used to guide periodontal tissue regeneration, thereby promoting the deposition of dense tissue and preventing gingival recession and peri-implantitis.

Ⅲ What is Biological Modification?

In recent years, surface bio-modification of implants has become a research hotspot, aiming to impart bioactive functions to the coating of implants with high mechanical stability. Simultaneously, the goal is to mimic the structure and compositional characteristics of natural bone to better promote bone growth. Here are several introductions to different methods.

1. Self-Assembly Technology

Self-assembly refers to a technique where basic structural units spontaneously form ordered structures. The self-assembly monolayer technology on titanium surfaces utilizes several different functional groups to better immobilize biochemical agents, providing surface chemical design with certain controllability. Layer-by-layer self-assembly involves the alternate deposition of polyelectrolyte multilayers on a charged substrate in a solution with an oppositely charged polyelectrolyte.

Using the layer-by-layer method with chitosan/siRNA functionalizing titanium surfaces, the surface wettability, morphology, and roughness alternate during the film formation process, significantly promoting osteogenic cell differentiation.

Establishing lysozyme multilayer coatings using silver nanoparticles, chitosan, and hyaluronic acid as raw materials is a strategy for preparing long-lasting antibacterial multilayer coatings, effectively preventing early implant infections. Self-assembly technology is straightforward, requires no special equipment, and offers molecular-level control of membrane structure, gaining widespread attention in recent years.

2. Biomolecule Adsorption

Adsorption refers to the phenomenon where molecules or ions in the surrounding medium are attracted to the surface of the implant. Physical adsorption is mainly governed by intermolecular forces, while chemical adsorption relies on chemical bonds between molecules. Utilizing anchoring peptides to add biological functionality to the implant surface, specific surface-binding peptides in the cell culture medium exhibit stable adsorption, enhancing the implant's biocompatibility.

Chitosan, sodium alginate cross-linked with chitosan, and pectin cross-linked with chitosan covalently bonded to titanium surfaces can improve wettability and provide chitosan-coated surfaces with swelling and drug release properties.

Covalently fixing a polydopamine coating with collagen protein may also be a method to enhance surface performance. Compared to physical adsorption, chemical adsorption, utilizing the forces of chemical bonds, offers greater adsorption energy and stability.

3. Bioactive Material Coatings

Bioactive materials generally refer to non-living materials used for medical purposes, intended to come into contact with living tissues and realize their bioactive functions. Bioactive glass is a material capable of repairing, replacing, and regenerating tissues within the body, forming bonds between tissues and materials. Its degradation products can promote the generation of growth factors and cell proliferation, enhance osteoblast gene expression, and facilitate bone tissue growth. Coating it on implants can safely and effectively achieve osseointegration, representing the development direction of dental implant coating materials.

Moreover, numerous bioactive drugs can be used for surface modification of implants. For example, coating titanium implants with a polyethylene terephthalate/1α,25-dihydroxyvitamin D3 solution using electrospinning technology can produce submicron-sized particles that stimulate bone formation. Applying resveratrol, an anti-inflammatory molecule, for surface modification or coating on implants may accelerate bone formation. Combining broad-spectrum antibiotics, such as streptomycin, on the surface of implants can improve bone formation and reduce the risk of infections around the implants.

4. Biomimetic Deposition

Biomimetic deposition mimics the mineralization mechanism of hydroxyapatite in the body. Hydroxyapatite coatings naturally deposit on the substrate surface in a water solution under conditions similar to the body environment. Using bone marrow mesenchymal stem cells induced in vitro on biomimetically modified titanium alloy surfaces, their adhesion ability improves, and proliferation rates increase. Biomimetic deposition of hydroxyapatite on titanium surfaces after mixed acid treatment can induce biological activity in titanium. When studying the effect of simulated body fluid immersion time on the osteogenic cell activity of anodized titanium nanotubes, it was found that a 3-hour immersion was optimal.

Immersing in 10 times the simulated body fluid is a rapid and economical technology to improve anodic titanium implant osseointegration. However, the bioactivity potential of implants may be limited by excessive and uncontrolled hydroxyapatite coatings.

The above surface modifications of implants have been a research hotspot in recent years. The methods are diverse, and significant progress has been made in promoting bone formation, osseointegration, and infection resistance. Combined applications may lead to more comprehensive performance and could be a trend in future research development.