Surface Modifications Of Endosseous Dental Implants By Incorporation Of Roughness And Hydroxyapatite Coatings

Fahd Ahmed¹                                       BDS, MSc

Haroon Rashid^{2                                             BDS, MDSc}

Sadaf Farookhi^{3                                            BDS, MSc}

Vivek Verma^{4                                                   BDS, MSc}

Yuliya Mulyar^{5                                                BA}

Murai Khalifa^{6                                                BDS, DDS}

Zeeshan Sheikh7                                           Dip, Dh, BDS, MSc, PhD                 

INTRODUCTION TO SURFACE TREATMENTS OF IMPLANTS

In the past few decades, there has been a great increase in the number of dental implants being placed. It has been well established already that the success of dental implants is dependent on early as well as late osseointegration.¹Â The surface topography and geometry of an implant are important for their long and short term clinical success. For implant fabrication, titanium is the material of choice^2,3 as it has a low density giving it a high-strength-to-weight ratio. For that reason, it can be effectively alloyed with other metals like vanadium (4%) and aluminum (6%) particularly.⁴ The strength of the alloy is increased by incorporating aluminum and vanadium, while acting as a scavenger, prevents corrosion.⁵

Surface modifications like passivation, anodization, ion-implantation and texturing are essential methods for making dental implants perform better in terms of physical and biological standpoint after implantation.^6,7 It is believed that surface treatments improve the bioactivity and the osseointegrative properties of Titanium.^8-10 Surface texturing can increase implant surface area by up to six times, thereby improving osseointegration10. Implants can also be coated with a variety of materials that can aid implantbone bonding.¹¹ Calcium phosphate-coated implants have been described in the literature which demonstrate early direct bone attachment to the hydroxyapatite surface and this  occursas quickly as one month post-operatively.¹² The aim of the current review is to discuss various implant surfaces and methods that could accelerate the osseointegration process of dental implants.

METHODS TO IMPROVE SURFACE ROUGHNESS OF IMPLANTS

Numerous reports in the literature have suggested that implant surface characterization affects the rate by which bio-mechanical fixation and osseointegration occurs.^13,14 There are three different levels of surface roughness which depend on macro-, micro- and nano-sized topographies. Reports have suggested that the early fixation rates and long-term mechanical stabilization of the prosthesis may be enhanced by a high degree of surface roughness as  compared to decreased roughness or smooth surfaces.^8,15-17 The surface roughness of dental implants can actually be created by subjecting the surfaces to various treatment methods. The methods of incorporating surface roughness are described in subsequent sections.

Figure 1: Various dental implant surface modification techniques.

PLASMA SPRAYING

This is a method in which powdered form of titanium is injected into a plasma torch at very high temperatures. These powdered particles that get condensed upon the surface of the substrate ultimately fuse together to form a thin film over the implant surface.⁸ Usually, the plasma sprayed film is of approximately ~30 µm thickness. The film provides roughness of about ~3-7 µm which also increases the surface area.⁸ The 3D topography results in an increase of the tensile strength at the implant-bone interface.¹⁸ Preclinical studies on mini pigs have shown faster bone formation around the roughened implant surfaces as compared to the implant surfaces that are smooth with an average roughness of about 0.2 µm.¹⁹ Prospective case series studying treatment outcomes of titanium plasma sprayed implants reported that 92% of implants showed less than 1mm of peri-implant bone loss over a period of 20 years.⁸

GRIT BLASTING

This is the process in which bombarding of the titanium surfaces is done using hard ceramic particles which are forced through nozzle by using compressed air at very high velocities.²⁰ This results in surfaces with varying degrees of roughness and the variance is dependent upon the sizes of ceramic particles used on the implant surfaces, the air pressure and hardness values of the actual implant surface itself .²¹ Commonest materials used as grit include alumina, calcium phosphate and titanium oxide. They are biocompatible and are believed to cause no interference with the osseointegration of titanium implants post operatively.^22,23

Titanium implant surfaces blasted with 25 µm titanium oxide particles result in a moderately rough surface of about 1-2 µm.^18,24 Studies have shown that those implants which have been grit blasted using titanium oxide show greater contact with the bone during osseointegration²⁴ and this has been further proven by comparing their levels of osseointegration with machined surfaces.^8,25 Clinical trials have also shown that these implants have a success rate of 97.6% at 5 years and 91.1% at 6 years after implantation.^24,26 A study model demonstrated by Wennerbergand Albrektssonet al. showed implants which have been grit blasted using titanium oxide or alumina have similar bone-implant contact as compared to smooth titanium implants, but with an increased biomechanical fixation.¹⁸ Also, the ability of an implant to withstand torque force increases with an increase in its surface roughness.⁸ Calcium phosphates i.e. hydroxyapatite and β-tricalcium phosphate and their mixtures have also been used to grit blast titanium implant surfaces.²³ Calcium phosphates are osteoconductive and biocompatible and have shown to be resorbable with time.^27-31 Calcium phosphate grit blasted implant surfaces have also been shown to achieve greater implant to bone contact as compared to those surfaces that have only been machined.³²

ACID ETCHING

Hydrochloric acid, hydrosulphuric acid, nitric acid and hydrofluoric acid are strong acids which have been used for roughening of titanium implant surfaces.^33,34 Micropits are with sizes ranging from 0.5 to 2 µm in diameter are produced over the surfaces and this results in an increase in the surface area.⁸ This also causes superior bone adhesion and thus, increases the rate of osseointegration.³⁵ The process of acid Etching is done by immersing the implant into a mixture of concentrated acids for a period of several minutes and then subjecting the implant surface to the process of heating above 100 °C³⁶ The process carried out at high temperatures produces micro porosities which are homogenous in nature that helps in better wettability, superior contact with the bone,  good fibrin attachment and enhanced osseointegration subsequently.³⁶ Other advantages include less contamination since no surface particles are produced, and better osteoblastic retention. The osseo-conductive process has been shown to get enhanced by the use of dual acid etching process that promotes  attachment of fibrin and osteogenic cells resulting in formation of the bone directly over the implant surface.³⁷ However; the use of acids also negatively affect the mechanical properties and thus, creating micro-cracks over the surfaces causing the reduction of fatigue resistance of the implant.⁸ It has been shown that commercially pure titanium with concentration of hydrofluoric acid 48% at 90 °C for 15 min to 8 hours results in high values of surface roughness and increases sub-surface hydrogen.³⁸ Sulphuric acid has been shown to produce more surface roughness than other acids like hydrochloric acid and nitric acid.³⁸ A study conducted in dogs found that implants having dual acid etched surfaces had higher torque to interface failure compared to implants with as-machined surfaces.³⁹

ANODIZATION

Micro or nano-porous surfaces can be created by the potentiostatic or galvanostatic anodization of titanium in acids such as hydrosulphuric acid, hydrophosphoric acid, nitric acid and hydrofluoric acid.⁴⁰ The process of anodization also causes an increase in the oxide layer thickness to more than 1000 nm over titanium surface .^41,42 Because the acids used in the electrolyte solution are strong, this oxide layer is dissolved and is also thickened in other areas. The dissolution is also responsible for generating micro or nano-pores on the titanium surface⁴³, creating a moderately rough surface that facilitates osteoblast cell adhesion.⁴⁴

Anodized surfaces display better bone reinforcement and the osseointegration taking place on such surfaces can be explained either because of the mechanical interlocking in the pores by bone growth or due to biochemical bonding.⁴² Certain chemicals like calcium, magnesium and phosphorous have been incorporated in surface oxide layer.⁴³ Another process called anodic spark deposition has been investigated for its role in enhancement of titanium biocompatibility. A comparison has been done between Anodized alkali treated (AAT) titanium with chemically etched surfaces (bio-rough) and machined implant surfaces.⁴³ The surfaces which were chemically etched showed improved surface morphologies and caused no chemical modifications over the titanium implants. However; those surfaces which were subjected to AAT, showed bioactive micro- and nano-porous Ca and PO₄ enriched titanium oxide layer having a crystalline anatase structure.⁴³ This structure has shown to encourage hydroxyapatite layer nucleation and enhances the deposition of Ca and PO₄. It has been shown in experiments that the AAT surfaces encouraged proliferation and adhesion of cells over a period of 2 weeks and may be used in achieving stable and faster osseointegration of endosseous dental implants.⁴⁶

CERAMIC COATINGS ON DENTAL IMPLANTS

Materials like bioceramics (calcium phosphates) can be used to modify surface of a dental implant.⁴⁷ Bioactive ceramic implants are inorganic materials and depending upon on the physico-chemical nature of coatings, this can accelerate the formation rate of bone implant interface and also increases its stability and strength. The use of ceramics for coating metallic implants that produce an ionic ceramic surface has been documented.⁴⁸ The materials can be plasma sprayed or may be coated on to an implant surface. However it is worth mentioning that ceramics are brittle in nature, having low tensile and compressive strength.¹⁰

HYDROXYAPATITE (HA)

Hydroxyapatite (HA) is a compound of calcium phosphate with a composition of Ca₁₀ (PO₄)₆ (OH)₂ and has a crystallographic structure. It has a space group of p63/m and has a hexagonal atomic arrangement. This space group shows a six-fold c-axis perpendicular to the three equivalent a-axes at angles of 120 degree to one another. Every building cell or unit contains Ca, PO₄ and OH groups with are densely packed with each other.¹⁰ The appearance of Ca ions is in a form of a triangle while the OH groups are at the corner positions and the phosphates tetrahedral show a helical arrangement. Fluoride and chloride ions can be used to substitute the OH group which may cause a change in properties like the morphology, solubility and lattice parameters without the change in hexagonal parameters. For example, substituting  the OH group with fluoride ions causes the contraction of the cell which in turn increases crystallinity and the size of the crystal.⁴⁹ This substitution results in less soluble and more stable structure.⁵⁰

METHODS OF SYNTHESIZING HYDROXYAPATITE

There are many ways by which hydroxyapatite can be synthesized. Following are the common methods [52]:

1. Wet Methods which include precipitation method, hydrothermal techniques and hydrolysis of calcium phosphate.
2. Solid state reactions.
3. Ultrasonic irradiation
4. Sol-gel method
5. Microwave irradiation
6. Chemical precipitation
7. Micro-emulsion.

Depending on the technique used, differing morphological, stoichiometrical and crystallized products be obtained and authors feel that it is beyond the scope of this article to review all preparation methods.

HYDROXYAPATITE COATINGS

Since hydroxyapatite has a porous structure and is similar to the inorganic structure of teeth and bones, is has been widely researched upon for its use as a coating over metallic implants.²¹ However, its bulk is quite brittle if compared with ceramics zirconia oxide and aluminium.⁴⁴ HA coatings help to provide early stabilization of the dental implant in the surrounding bone and thus significantly reducing the healing time so that the prosthetic implant component can be placed earlier. It increases the functional life of the prosthesis over the implant and also produces optimum tissue response⁵¹ so that when the breakdown of the coatings occur, there is no adverse reaction in tissues surrounding the implant. The dissolution rate of HA in both alkaline and neutral aqueous solutions is quite low.^31,51 It has been speculated that even thought there is disintegration of the released PO₄ and Ca ions around the implant area, negative effects on bone formation are minimum. However; HA could cause disruption of the passive oxide layer present on the titanium implant surface. This disruption in turn causes the release of metallic ions which is undesirable.⁵²

It is generally believed that larger the crystallinity of the HA coating, the greater would be the resistance to dissolution in vivo.⁵³ On the other hand, HA surface coatings which are amorphous in nature would demonstrate a substantially higher rates of dissolution.⁵⁴ 50% crytallinity of HA is thought to be ideal for dental implant coatings.⁵⁵ Literature has shown that HA coated implants demonstrate better implant to bone integration. The values have been compared and for HA coated implants, these range between 17.1% for 7 days to 75.9% for 3 months as compared to values for metallic titanium implants which range from 1.2% in 7 days to 45.7%in 3 months^19,56 Although HA coated implants show better torque resistance is short term, it has also been shown that after a period of 6 months, no significant difference in the integration between ceramic coated metallic implants than non-coated metallic implants is seen.⁵⁷ Also, it is worth mentioning that the coating of HA helps in creating an additional interface between the bone and implant surface  and this could be a potential reason for the failure.⁵⁸ The use of nano-crystalline HA powders for coating has been described in the literature having a number of beneficial properties i.e. improved sintering ability, increased surface area and better densification which helps to reduce the sintering temperature.^59,60

METHODS TO COAT IMPLANT SUBSTRATE WITH HYDROXYAPATITE

There are a variety of methods that can be employed to coat implants with HA. However, the minimal requirements of HA being used as dental implant coating are that the crystallinity should be 62 %, phase purity around 95% , density should be 2.98 g/cm³, Ca/P ratio should be 1.67-1.76 , shear strength should be more than 22MPa and tensile strength more than 50.8 MPa.⁶¹ Methods used to coat the titanium dental implants with calcium hydroxyapatite with their merits and demerits are presented in Table 1 and discussed in the following sections.

PLASMA SPRAYING

Plasma spraying has been used to create rough surfaces and can also be used to coat dental implants with HA. This technique has similarities to the procedure in which surface porosities are created. It involves the injection of the ceramic HA into the plasma torch at very high temperatures which is then projected on to the surfaces

of the implant.⁶³ The particles will then condense and fuse resulting in formation of a thin layer on the titanium surface. There is a variation in thickness of this layer and it ranges from a few microns to a few millimetres.⁸ Plasma spraying has an advantage that it is rapidly deposited and has low cost⁵⁸, leading to faster osteointegration and decreased healing time.²¹ As plasma spraying leads to mechanical bonding of Ca/P with the substrate, roughening of the substrate gives good mechanical interlocking⁶⁴, which allows for an extended functional life of the implant.⁸ Disadvantages related to plasma spraying include weak adhesion^9,21 the formation of the residual stresses which may be created at the interface between the substrate and the coating. Plasma spraying may not be considered a technique for smaller dental implants with complex designs⁶⁵as the coating may disintegrate if the implant is insertion into bone with increased density.⁸

BIOMIMETIC DEPOSITION

Biomimetic deposition method is done under lower temperaturesand the risk of the coating flaking off is minimized. It also allows bone like HA crystals with greater bioactivity to be formed.⁶⁶ Firstly, oxidization of the substrate is carried out in a furnace at 800°C for a period of 1 hour. The procedure is started at lower temperatures and is increased at a rate of 5 °C per min until a temperature of 800 °C is achieved. This is held constant for 1 hour and then allowed to cool down slowly. These plates are cleaned using an alkaline medium in an ultrasonic bath and are the substrates are then inserted in plastic tubes containing phosphate buffered saline (PBS) solution, for 7 days in an incubator at a temperature of 37 °C. This procedure will cause the HA particle deposition. It has been shown that those implants having layers of HA placed by the process of biomimetic deposition show better bone bonding and thus, enhanced osseointegration.^66-68

SPUTTERING TECHNIQUE

Sputtered coatings have been gaining popularity recently because of the problems associated with plasma spraying .^63,69 The sputtering of calcium phosphates and HA leads to better bone strength and superior rates of initial osseointegration.⁶³ This process results in the formation of a coating of uniform thickness on flat substrates and this forms a dense coating of 0.5-3 µm thick. The ratio of Ca/P at the surface material is between the range of 1.6-2.6 and this is because of the fact that phosphorous ions are weakly bound onto the surfaces.⁷⁰ In vivo studies have proven that calcium phosphate coatings which are sputtered showed superior  implant fixation and healing response as compared to those surfaces which are grit blasted.^63,71 Another study carried out on rat bone marrow stromal cells showed better osteoconductivity and biodegradability on titanium plates with sputtered HA coatings, as compared to surface-modified titanium.²²

ELECTROPHORETIC TECHNIQUE (EPD)

The technique has been conventionally used to deposit ceramics, glass, polymers and can also be used to deposit composites materials like chitosan and HA.⁷² The cell for the process of electrophoretic deposition includes a cathodic substrate which is centred between two parallel platinum counter electrodes. The voltage for electrophoretic deposition is usually 10 to 30 V⁷³ and the average length of the formed HA is 200 nm which is essentially needle shaped.⁷³ The main advantage of this process is that it allows the deposition of coatings that contain HA-chitosan composites andsilica-chitosan.⁷⁴ This process may be carried out at room temperature that thus, may allow codeposition of various materials.⁷⁵ Studies have found that EPD-HA coatings have higher corrosion resistance as compared to uncoated titanium and also greater adhesion strength than plasma sprayed HA coatings.⁷⁶

DEPOSITION BY ELECTROCHEMICAL PROCESS

Depositing of HA using the electrochemical ways has gained much popularity as it can be used to coat those metal substrates which are highly irregular. The process is done quickly and at low temperatures. The chemical composition and the thickness of the coatings can be controlled provided that there are adequate conditions present during the process.⁷⁷ The specimens are first wetground using an appropriate abrasive paper, polished and rinsed using acetone and then lastly washed with distilled water. The deposition is then carried out using a potentiostat/galvanostatic cathodic polarization. The substrate during the process is used as cathode and platinum plate is used as the counter electrode.⁶⁶ The polarization during the process is conducted from an open circuit voltage (-3.0V) at the rate of -0.6 V per hour. The specimen is rinsed in distilled water so that the residual electrolytes are removed and then allowed to dry for 24 hours. This technique results in greater amount of hydroxyl from reduction of water and forms HA coatings which are more crystalline in nature. Thecoatings  formed by the process describe above are resistant to high scratch load of about 20N.77,78

CONCLUSION

This paper has discussed surface modification of dental implants by incorporation of roughness and coatings. Clinical and preclinical investigations have demonstrated that the bone response to a dental implant is greatly influenced by the surface topography. However, increased surface roughness of titanium implants could have contributed towards peri-implant disease as well as an increase in ionic leakage. The various techniques described above may lead to different in vivo responses after implantation and could affect clinical success rates. It is not easy to do a comparison particularly because of variation in the techniques used for implant surface characterization. Further in-depth research, especially long-term studies are required before anything definitive can be determined.

Author Contribution:

FA performed original design of the manuscript and writing, HR did peer review of the manuscript, correction of typographical errors and critical analysis, YM, MK, VV and SF critically analysed the manuscript and gave final approval, ZS drafted images and tables and reviewed the manuscript. Disclosure: None disclosed

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1. Faculty of Dentistry, Division of Oral Health and Society, McGill University, Montreal, Quebec, Canada.
2. Faculty of Dentistry, Division of Prosthodontics, Ziauddin University, Karachi, Pakistan.
3. Schulich School of Medicine & Dentistry, Western University, London, Ontario, Canada.
4. Family Dental Clinic, Strathmore, Alberta, Canada.
5. Faculty of Dentistry, University of Toronto, Ontario, Canada.

Corresponding author: “Dr. Haroon Rashid” < drh.rashid@hotmail.com  >