M. MANJAIAH, R. F. LAUBSCHER: A REVIEW OF THE SURFACE MODIFICATIONS OF TITANIUM ALLOYS ...
181–193
A REVIEW OF THE SURFACE MODIFICATIONS OF TITANIUM
ALLOYS FOR BIOMEDICAL APPLICATIONS
PREGLED MODIFIKACIJ POVR[INE TITANOVIH ZLITIN ZA
BIOMEDICINSKO UPORABO
Mallaiah Manjaiah, Rudolph Frans Laubscher
University of Johannesburg, Department of Mechanical Engineering Science, Kingsway Campus, Auckland Park,
2006 Johannesburg, South Africa
manjaiahgalpuji@gmail.com
Prejem rokopisa – received: 2015-12-18; sprejem za objavo – accepted for publication: 2016-03-02
doi:10.17222/mit.2015.348
Dental implants are mechanical components that are used to restore the mastication (chewing) function and/or aesthetic appeal
because of tooth loss or degradation. They are affixed (screwed) into the upper or lower jaw and act as a base for single or
bridge-type tooth replacements. They are mostly manufactured from titanium alloys. The surface integrity of the manufactured
implant may have a significant effect on the functioning and success of the implant. A systematic review is described on the
effect of engineered surface integrity on the performance of titanium dental implants as regards the implant fixation, mechanical
performance, bone growth and cell response. The need for surface engineering of the implant is introduced first. This includes
the mechanical, surface-integrity and biocompatibility-required properties. This is followed by introducing and discussing the
dedicated surface-modification processes currently employed. These include: abrasive blasting, electrochemical processes,
hybrid processes and laser modification. The mechanical and biocompatible properties of an implant are the most crucial factor
for their application in biomedical use. Hence, the present review article focused on the latest improvements to dental implant
design based on the mechanical and biocompatible properties. The physical contact of an implant with respect to its surface
roughness is an important factor in dental implant design. The surface roughness of an implant on the macro, micro and nano
scales have specific effects on the implant’s contact with the surrounding tissue of the bone. In addition, the biocompitability of
titanium material is very important to develop a sustainable dental implant. Looking into the importance of the above
mechanical and biocompatible properties of bone implants, the authors review elaborately the development of titanium-based
implants with reference to the above properties.
Keywords: implants, titanium, surface roughness, systematic review, surface engineering process, hybrid process, mechanical
machining, chemical treatment
Zobni vsadki so mehanske komponente, ki se jih uporablja za obnovo funkcije `ve~enja in/ali iz estetskih razlogov zaradi
izgube ali poslab{anja zoba. Pritrjeni (privija~eni) so v zgornjo ali v spodnjo ~eljust in slu`ijo kot osnova za nadomestni zob ali
za mosti~ek. Ve~inoma so izdelani iz titanovih zlitin. Integriteta povr{ine izdelanih vsadkov lahko pomembno vpliva na
delovanje in uspe{nost vsadka. Opisan je sistemati~en pregled o vplivu in`enirske celovitosti povr{ine na zmogljivost titanovega
dentalnega vsadka glede na pritrditev vsadka, mehanske zmogljivosti, vra{~anja kosti in odziva celic. Najprej je predstavljena
potreba po obdelavi povr{ine vsadka. To vklju~uje zahtevane mehanske lastnosti, celovitost povr{ine in biokompatibilnost.
Temu sledi predstavitev in razlaga trenutno uporabljanih postopkov za namensko spremembo povr{ine. To vklju~uje: abrazivno
peskanje, elektrokemijske procese, hibridne procese in modifikacijo z laserjem. Mehanske in biokompatibilne lastnosti vsadka
so najbolj pomemben faktor pri njihovi uporabi v biomedicini. Zato ta ~lanek predstavlja najnovej{i razvoj za izbolj{anje
na~rtovanja dentalnega vsadka na podlagi mehanskih in biokompatibilnih lastnostih. Fizi~ni stik vsadka, glede na hrapavost
povr{ine, je pomemben faktor pri na~rtovanju vsadka. Hrapavost povr{ine vsadka ima na makro-, mikro- in nanonivoju
specifi~en vpliv na stik vsadka z obkro`ujo~im tkivom kosti. Poleg tega je biokompatibilnost titanovega materiala pomembna za
razvoj trajnostnih dentalnih vsadkov. Zaradi pomembnosti mehanskih in biokompatibilnih lastnosti kostnih vsadkov avtorji
prikazujejo, glede na te lastnosti, razvoj vsadkov na osnovi titana.
Klju~ne besede: vsadki, titan, hrapavost povr{ine, sistemati~en pregled, postopki spreminjanja povr{ine, hibridni proces,
mehanska, kemijska obdelava
1 INTRODUCTION
A significant portion of the population may have a
need for implant dentistry to restore the mastication
(chewing) function or aesthetic appeal after losing teeth
because of disease or mechanical trauma. Dental im-
plants are mechanical components that are affixed
(screwed) into the jawbone and act as the base for single
or bridge-type tooth replacement. They are mostly
manufactured from titanium alloys. Dentists and dental
specialists encounter various challenges regarding the
placement of implants into the jawbone structure. They
are continuously seeking new methods and alternative
materials to solve the many challenges at hand while
reducing the risks involved. Dental implants are the
nearest equivalent replacement to a natural tooth that
may be compromised by disease or trauma. A dental
implant is a metal part and therefore a foreign body as
far as the physiology of the patient is concerned. This
may lead to several difficulties such as compatibility
with the rest of the body. Successful application implies
that the dental specialist industry considers all the rele-
vant factors as related to dental reconstruction and resto-
ration. These factors may include the implant geometric
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 621.9.015:669.295:606:61 ISSN 1580-2949
Review article/Pregledni ~lanek MTAEC9, 51(2)181(2017)
design, mechanical performance, aesthetic appeal,
biocompatibility and osseointegration of the implant
with the bone and surrounding tissues. Dental implants
can be categorized into three widely used designs. These
are subperoisteal, transosteal and the more recent and
most popular of the three the endosseous implants. The
endosseous types are placed deep within the mandible or
maxilla, the lower and upper jawbone, respectively. Once
the implant is placed (usually screwed) within the jaw
and left to heal, the jawbone osseointegrates with the
implant to create a secure interface between jaw and
implant. Successful osseointegration, as far as mechani-
cal performance is concerned, is assessed by the implant
withstanding a certain minimum loosening torque,
usually applied with a torque wrench.
Prosthetic and implant manufacture include various
different types of designs, shapes and surface-engineered
components made from various materials. Biocompatible
materials that have been and are used include stainless
steel, carbon, platinum, titanium, silver, cobalt chrome
alloys, alumina, magnesium, sapphire, acrylic, porcelain,
calcium phosphate compounds and zirconia.
Dental and orthopedic practitioners have previously
used stainless steel and cobalt-chrome alloys for their
implants. These materials have good mechanical proper-
ties such as high strength and good corrosion resistance.
Furthermore, they have proven to be compatible with the
human body. They demonstrated clinical success in
many implant cases.1 However, titanium and its alloys
have largely superseded them because of similar and
enhanced properties. Titanium is inert and has good
biocompatibility. It resists a wide range of corrosive
agents and has a superior strength-to-weight ratio when
compared to that of steel. Today, dental practitioners
mainly use commercially pure titanium as their material
of choice. The clinical success of any biomedical ortho-
pedic/dental implant depends on the surface interaction
between the bone tissue and the implant (osseointe-
gration). Hence, the dentists/orthopaedists must make
use of an implant manufactured from a suitable material
that also has a suitable surface integrity. In general, Ti
alloys are more corrosion resistant and less toxic when
used in the human body compared to steel, Co-Cr and
tantalum.2 The fact that titanium alloys also have a lower
elastic modulus, more comparable with that of bone,
helps with the load transfer and subsequent stress profile
at the interface. Titanium-based alloys have therefore
become the material of choice for many implant
applications because of their outstanding characteristics,
such as high tensile strength, corrosion resistance, lower
modulus of elasticity, lower density and enhanced bio-
compatibility (osseointegration ability). Apart from these
applications, titanium alloys have a wide range of appli-
cations in other commercial and aerospace industries.
1.1 Required mechanical and biocompatible properties
of implant materials
The selection of a biomaterial for the intended
application of bio parts is important. The material should
have high durability without immunological rejection in
the human body’s environment and a good response with
tissue cells. The mechanical properties of the materials
should concurrently match with human bone properties
like density, tensile strength, fatigue resistance, hardness,
and a low modulus of elasticity, elongation wear resis-
tance and corrosion resistance. It is difficult to get all the
feasible properties in one material. Corrosion is the
disintegration of an implant alloy that will spoil the im-
plant material and surrounding tissues. For this reason, a
material with a greater corrosion resistance and high
strength for biomedical applications is preferred. These
materials have replaced some of the parts of the human
body, shoulders, knee, hips, elbows, and oro-dental
structure.3 Few materials are used in a very active role,
like actuators, vascular stents, heart vertebras, ortho-
dontic arch wires, etc. The authors reported that
expedient materials for biomedical implants such as
stainless steel AISI 316L, cobalt-based alloys, CoCrMo
alloys, titanium alloys, TiNi shape-memory alloys and
special alloys.2
The materials that are used for surgical implants in
biomedical are listed in Table 1. In addition, all these
biomaterials posses a higher modulus of elasticity than
the bone. Among these materials, the titanium-based ma-
terial is feasible and most appropriate for implantation.
Due to the combination of outstanding characteristics
compared to other materials such as enhanced biocom-
patibility, low modulus, high strength and good osseo-
integration.
These materials are highly non-toxic and do not
cause any allergic reaction with the human body.
Ti6Al4V is the long-term main medical alloy for implan-
tation. However, these alloys have a possible toxic effect
on the body, caused by the vanadium and aluminium.
Due to this reason, vanadium- and aluminium-free tita-
nium alloys are preferred for implant applications.4 The
surface properties of a metallic material play a role in the
spontaneous build up of a stable and inert oxide layer to
make it highly biocompatible. The responses induced by
the material in the human body and degradation of the
material are the two main factors in biocompatibility.
Commercially pure Ti materials are preferred as they
give bio-integration with the surrounding tissues, cells of
the bone and healing, bone growth, etc. Material with a
highly appropriate surface is required for the implant to
assimilate with adjacent bone. Hence, surface engineer-
ing plays a major role in the development of good
osseointegration. The success of a dental implant is
highly dependent on the chemical, physical, mechanical
and surface topography characteristics of the implants.
Surface topography plays a vital role in osseointegration
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and shorter healing time from implant placement to
restoration.2
1.2 The reason for surface modification
The biomedical applications of titanium alloys in-
clude replacement parts such as hip, knee, shoulder,
screws, nuts, plates, nails, housing devises for pace-
makers and artificial heart valves, surgical instruments
and so on.5–7 The goal of dentistry is to restore the
normal function of a patient’s speech, oral health and
aesthetics, regardless of a weakened, diseased, or other-
wise injured oral system. Although titanium alloys are
extensively used in dental-implant manufacture, failure
of the fixation may still occur because of insufficient
early osseointegration, infection, surgical trauma, or pre-
mature overloading, improper surgical placement,
fatigue and inadequate quality of the bone surrounding
the implants.8 Successful dental implantation is highly
dependent on the biochemical, physical, mechanical and
surface topography characteristics of the implant surface.
The biocompatibility of the material is important and
needless to say it must be non-toxic and should not cause
any allergic reaction with the human body. Ti6Al4V has
been extensively used for implant manufacture, but
concerns have been raised about their long-term effects
because of their vanadium and aluminium content. Due
to this reason, commercially pure titanium alloys are pre-
ferred for implant applications.4 The surface properties
of metallic materials play a significant role in the
spontaneous build up of a stable and inert oxide layer,
which is usually highly biocompatible. The response
induced by the implant material on the human body and
the degradation thereof are the two main factors that
contribute to biocompatibility. These commercially pure
titanium alloys are preferred as they are bio-compatible
with the surrounding tissue and bone cells and do not
inhibit healing and bone growth. It does, however, also
imply that a material with an appropriate surface is
required for effective osseointegration with adjacent
bone. Hence, surface engineering plays a significant role
in the improvement of the implantation process. Surface
topography has a significant effect on osseointegration
and a shorter healing time from implant placement to
restoration.2 Hence, many studies tried to optimize dental
and orthopedic implants by the modification of surface
chemistry and surface topography by using many me-
thods such as sandblasting, acid etching, electrochemical
machining and anodizing to improve the aesthetic
appearance.9 However, the surface engineering of tita-
nium improves the aesthetic appearance of the implant,
the biocompatibility of the implant, the corrosion resis-
tance, the fatigue life of the implant and to reduce the
friction between the implant and abutments. The surface
modification is also used to help osseointegration, a
faster healing time, improved bone implantation contact
and the life expectancy of titanium implants. Titanium
dentures become dimmer, weakening its aesthetic aspect
when being used for a long time in the oral environment.
With the higher demand of dental implants not only for
the restoration of oral function, such as chewing,
pronunciation and durability, but also dental aesthetics, it
is necessary to improve the titanium dental aesthetic for
practical clinical uses.
The self-colouring of the anodisation of titanium has
been patented1 and the anodisation improves the aesthe-
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Table 1: Bio-material and their mechanical properties1,4
Tabela 1: Biomateriali in njihove mehanske lastnosti1,4
Material Yield strength(Mpa)
Ultimate tensile
strength (MPa)
Modulus
(GPa)
Elongation
(%)
Density
(g/cc)
316L steel 290 580 210 50 7.99
CoCrMo 275-1585 600-1795 200-230 8 8.3
CoCrNiMo 241 793 232 50 8.43
TiNi 195-690 895 80 25-50 6.45
CP Ti grade I 170 240 102 24 4.5
CP Ti grade II 275 345 102 20 4.5
CP Ti grade III 380 450 102 18 4.5
CP Ti grade IV 483 550 104 15 4.5
Ti-6Al-4V-ELI 795 860 113 10 4.4
Ti-6Al-4V 860 930 113 10 4.4
Ti-6Al-7Nb 880-950 900-1050 114 8-15 4.4
Ti-5Al-2.5Fe 895 1020 112 15 4.4
Ti-15Zr-4Nb-2Ta-0.2Pd 693-806 715-919 94-99 18-28 4.4
Ti-29Nb-13Ta-4.6Zr 864 911 80 13.2 4.4
Ti-13Nb-13Zr 900 973-1037 19-84 15 4.99
Ti-12Mo-6Zr-2Fe 1000-1060 1060-1100 74-85 18-22 5.0
Ti-35Nb-7Zr-5Ta 742-806 596 55 11-22 5.0
Ti-29Nb-13Ta-4.6Zr 715 911 65 22 5.0
Ti-35Nb-5Ta-7Zr-0.4O 590-1074 1010 66 21-27 5.0
Ti-15Mo-5Zr-3Al 1475 724-900 82 14 4.95
tic appearance. The uniform colours of the interference
pattern can be obtained for any large surface area. From
the practical point of view the surface treatment leads to
producing uniform colours. This surface treatment
improves the aesthetic appearance in dental implant
application and other industrial uses such as jewellery
and architectural purposes.1 The surface treatment of
titanium also improves the corrosion behaviour of the
alloy. A. Karambakhsh et al.10 have studied the effect of
anodisation on the corrosion behaviour of a commer-
cially pure (CP) titanium alloy. Anodising in sulphuric
acid greatly reduced the corrosion resistance of the
samples, which is due to the formation of a resistant
anodic film. The greater film thickness increased the
corrosion resistance. The corrosion resistance of the
implant needs to be good because specific metal ions
released from the implant can induce inflammation
reaction with the surrounding tissues. Moreover, in the
long term, it may be harmful to the human body. The
porous oxide films formed by the anodic spark depo-
sition, the porous oxide, predominantly consist of the
TiO2 phase. The crystalline structure of the film consists
of anatase and rutile. The anodic oxidation improves the
corrosion resistance of the CP titanium alloy.11
Titanium has excellent corrosion resistance, good
fatigue strength and acceptable fracture toughness, but it
has poor sliding characteristics. These alloys fail by
galling and often exhibit high and unstable friction
coefficients. To improve these properties, surface engi-
neering techniques are required, such as hard coating,
soft coating, diffusion treatment, and shot-peening.
Diffusion treatments include oxygen diffusion, nitriding
and carburizing.12 The surface modification of titanium is
also necessary to prevent the release of toxic elements
from titanium such as aluminium and vanadium, which
are known to cause toxicity.13 Porous implants have an
effect on the fatigue strength in a highly loaded appli-
cation, such as the hip joint. These alloys experienced a
drastic reduction in strength due to the porosity, and due
to the stress intensity the pores are the major sources of
weakness in the fatigue strength. To achieve a func-
tionally strong implant, a porous implant design needs to
account for these losses in metal strength. Hence, the
surface engineering of these alloys is essential.
The bulk properties of biomaterials, such as non-toxi-
city, corrosion resistance or controlled degradability,
modulus of elasticity, and fatigue strength have long
been recognized as being highly relevant in terms of the
selection of the right biomaterials for a specific biome-
dical application. The events after implantation include
interactions between the biological environment and
artificial material surfaces, the onset of biological
reactions, as well as the particular response paths chosen
by the body. The material surface plays an extremely
important role in the response of the biological
environment to artificial medical devices. In implants
made of titanium, the normal manufacturing steps
usually lead to an oxidized, contaminated surface layer
that is often stressed and plastically deformed, non-uni-
form and rather poorly defined. Such native surfaces are
clearly not appropriate for biomedical applications and
some surface treatment must be performed. Another
important reason for conducting surface a modification
of titanium medical devices is that specific surface
properties that are different from those in the bulk are
often required. For example, in order to accomplish
biological integration, it is necessary to have good bone
formability. In blood-contacting devices, such as
artificial heart valves, blood compatibility is crucial. In
other applications, good wear and corrosion resistance
are also required. The proper surface modification tech-
niques not only retain the excellent bulk attributes of
titanium and its alloys, such as a relatively low modulus,
good fatigue strength, formability and machinability, but
also improve specific surface properties required by
different clinical applications. According to the different
clinical needs, various surface modification schemes
have been proposed and are shown in Table 2.
In the following sections, the surface modification of
titanium implants to improve the bioactivity, biocom-
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Table 2: Summary of surface-modification methods for titanium implants15
Tabela 2: Pregled metod za modifikacijo povr{ine titanovih vsadkov15
Surface modification methods Modified layer Objectives
Mechanical methods:
machining, grinding, polishing, blasting
Rough or smooth surface formed by
subtraction process
Produce specific surface topographies;
clean and roughen surface; improve
adhesive in bonding
Chemical methods
acid treatment
<10 nm of surface oxide layer Remove oxide scales and contamination
Alkaline treatment ~1μm sodium titanate gel Improve biocompatibility, bioactivity or
bone conductivity
Hydrogen peroxide treatment ~5 nm of dense inner oxide and porous
outer layer
Improve biocompatibility, bioactivity or
bone conductivity
Sol-gel ~10 nm of thin film, such as calcium
phosphate, TiO2 and silica
Improve biocompatibility, bioactivity or
bone conductivity
Anodic oxidation ~10 nm to 40 μm of TiO2 adsorption and
incorporation of electrolyte anions
Produce specific surface topographies;
improved corrosion resistance; improve
biocompatibility, bioactivity or bone
conductivity
patibility, wear and corrosion resistance by the various
surface modification technologies are discussed. These
methods are classified into mechanical, electrochemical
and hybrid processes according to the formation
mechanism of the modified layer on the material surface.
2 ABRASIVE BLASTING
Abrasive blasting is one of the mechanical surface
modification methods involving plastic treatment,
shaping or the removal of materials from the surface.
The objective of this mechanical modification process is
to obtain a surface roughness, topography, removal of
surface contamination and improve its surface-adhesion
properties. The surface of titanium is abrasively sand
blasted with hard ceramic particles to increase the sur-
face roughness. Depending on the particle size to which
the surface roughness can be produced, the surface
roughness depends on the bulk material properties,
ceramic particle material, particle size, particle shape,
particle impact speed and the density of the particles.
The surface may consist of craters, ridges and particles
embedded on the surface. The surface roughness
increases with an increase in ceramic particles of size 25
μm to 250 μm of TiO2 or Al2O3. The blasted surface with
a particle size of 25 μm has a higher surface roughness
compared to machined surfaces, but smoother than 75
μm and 250 μm particles on blasted surfaces.14 The
authors also made comparisons between different
particle sizes (25μm and 75μm) of Al2O3 blasted on the
surface of titanium implants on the torque and surface
topography. They concluded that more torque is required
to remove an implant with the surface blasted with 75μm
Al2O3 particles compared to 25μm particles. It is ob-
served that the surface was blasted by different particle
sizes, such as 25 μm and 75μm of Al2O3. It was charac-
terized that two surfaces having different irregularities
and different degrees of surface roughness have a greater
surface roughness when blasted with a particle size of 75
μm (Sa-1.45 μm) compared to a surface blasted with 25
μm (Sa-1.11 μm), as shown in Figure 1. Titanium oxide
particles can be used for the grit blasting of dental
implants, which produces an average surface roughness
of 1 μm to 2 μm. Many researchers reported that the
torque force increases with surface roughness.14,15 This
indicates an improvement in the biocompatibility, cell
activity and osseointegration of the titanium implant
using the sandblasting method. The roughening of
implants by titanium plasma spraying used to produce a
rough surface of the implant can be obtained by a
process known as grit blasting, which makes use of hard
ceramic particles. The hard particle collides with the
surface of the implant at a high velocity using com-
pressed air. Different surface roughness can be obtained
from the size of the ceramic particle and the type of
particle. After blasting with a ceramic particle, cleaning
the surface of the titanium is very important due to some
of the residues of the alumina being embedded on the
surface of the implant. The alumina ceramic particle is
insoluble in acid. This does not, however, completely
remove the osseointegration difficulties of the implant. A
residue of particles may react with the surrounding tissue
cells and cause failure in the implant fixation. The
blasted surface has greater bone implant contact (BIC)
compared to a machined implant. Titanium oxide
particles can also be used for blasting the implant, which
shows an improvement in the BIC compared to a
machined surface.16 The experimental demonstration of
A. Abron et al.17, shows a higher bone implant contact in
the blasted surface implants compared to machined
surface implants. These studies confirm that roughening
of the titanium dental implants increases their
mechanical fixation to the bone, but not their biological
fixation.18–20 A. Karacs et al.21 investigated the
morphology of machined, blasted and laser-treated
surfaces of titanium. The Al2O3 blasted surface has a
unique surface morphological characteristic that
enhances the osseointegration process. Research
conducted on animals indicates a 50 % improvement in
the removal torque of an implant can be expected. The
aforementioned indicates that the sand-blasting
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: Sand-blasted titanium surface with different particle sizes:
a) 25 μm and b) 75 μm16
Slika 1: Povr{ina titana, obdelana s peskanjem z delci razli~ne veli-
kosti: a) 25 μm in b) 75 μm16
preparation method is a promising technique for
preparing titanium dental implant surfaces.
3 ELECTROCHEMICAL PROCESS
An electrochemical process of surface modification
includes electro polishing, anodic oxidation, acid etching
and electrochemical machining. Electro polishing is a
controlled electrochemical dissolution of the surface.
The process was carried out to obtain a mirror-like
smooth surface finish that removes plastically deformed
amorphous surface layer residues due to machining.22
3.1 Anodizing
Anodizing is an electrolytic chemical oxidation
process whereby the oxide layer thickness is engineered
to the aesthetic appearance (colour) of the titanium. A
thin passive oxide layer is formed, which is usually more
stable and thicker than the natural oxide layer that is
formed when it first made contact with the air. Anodic
titanium oxide has been used in various fields of
advanced technologies and industries, e.g., an electrical
component, resistive material for friction and wear,
decorative coating, resistance to corrosion, as a reflective
material and recently as photo electrode material and as
well as to improve the aesthetic appearance of im-
plants.17,23 The anodic oxidation of the titanium surface
for implant applications is relatively inexpensive and
may produce a uniform thickness throughout the surface
area.24 The anodic oxidation is a simple and novel
method for colouring titanium to improve the aesthetic
appeal due to the high reactivity of titanium with oxy-
gen. The anodization of titanium has been patented.1
Anodization is a surface-modification technique that
has been proposed to minimize the rate of ion release
from titanium alloys. There is an alternative method to
produce surface modification that includes ion implan-
tation, chemical passivation, and plasma spraying. All
the common methods used to perform surface modifi-
cations lack the required layer thickness. However, the
plasma spraying method can produce a thick coating of
oxide layer. but is a difficult method for generating a
uniform layer on the surface when applied to porous and
non-regular substrates.25
Anodizing is an electrochemical oxidation process of
thickening the oxide layer on the surface of titanium
metals. Electrochemical anodizing is the most common
method to control the colouring of titanium.26 This
process is a surface-modification process that is efficient
in forming an uniform and stable oxide layer on the
implant surface compared to other surface-modification
processes. e.g., electrochemical, ion implantation and
heat treatment, etc.27 Titanium anodizing improves the
surface properties which increases the lubricity,
anti-galling and fatigue properties of the alloy. Because
of the aforementioned properties, anodizing is becoming
rapidly popular in treating components used in the
medical industry, especially on orthopaedic implants.
This process endows with an extensive formation of
oxide coating under controlled conditions to offer the
desired result. Due to this, it is biocompatible and non-
toxic, resulting in a drastic improvement in performance
purposes used for biomedical dental implants. The
anodizing process will be carried out with either constant
current (Galvonostat) or constant voltage (Potentiostat).
Among the other surface-modification processes, the
anodic oxidation process is easy to deposit the oxide film
on the titanium surface by means of an electrochemical
process. This process has various controlled variables
such as the type of electrolyte used, the voltage applied
across the electrodes and the current used for the pro-
cess. A change in any of these variables can affect the
surface morphology, chemical composition and film
thickness of the titanium implant.28 The thick oxide film
formed during anodic oxidation at higher applied volt-
ages leads to a high surface roughness, which provides a
high bonding strength between the oxide and the tita-
nium substrate. Moreover, the surface hardness is
improved near the oxide layer due to anodic oxidation,
which is caused by the incorporation of the oxide into
the titanium alloy.29,30 It is reported that the bioactivity of
titanium can be improved by oxide film formation on the
titanium surface by anodic oxidation. This oxide layer is
either anatase or a mixture of anatase and the rutile
crystal structure. Many researchers have reported that the
bioactivity of titanium can be improved by varying the
thickness of the oxide layer and the crystal structure. The
anodization takes place in either galvanostatic or poten-
tiostatic conditions with an increase in the voltage or
current density leading to an increase in the oxide layer
thickness. The manipulation of the oxide layer also
affects the crystalline structure surface of the implant.
The anodic oxidation process creates a porous surface
structure on the titanium surface as shown in Figure 2.
Figure 2 shows how the oxidized dental implant mor-
phology has volcano-shaped saliencies as the oxide form
on the surface as a function of the anodic voltage,
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Figure 2: Anodized dental implant surface, resembling small
volcanoes34
Slika 2: Anodizirana povr{ina vsadka, podobna majhnim vulkanom34
current, concentration of electrolyte and a change in the
temperature. The titanium oxide (TiOX), where (1<X<2)
in the general form, depending on the X values 5 crys-
talline oxides are formed such as cubic (TiO, ao = 0.424
nm), hexagonal (Ti2O3, ao = 0.537 nm), tetragonal (TiO2,
Anatase, ao = 0.378 nm, rutile, ao = 0.458 nm), and
orthorhombic (Brookite, ao = 0.917 nm). There are also
nonstoichiometric oxides and amorphous oxides. It is
commonly understood that among these oxides, only the
rutile and anatase phases are stable under normal con-
ditions. The rutile and anatase oxide layer has different
physical properties in terms of surface tension, the rutile
condition is hydrophobic, and the anatase condition is
hydrophilic. These oxide layers are the most important
structure for the osseointegration of implants. K. Kim et
al.31 studied the surface properties and biological res-
ponses of anodized samples. The anodized surface has a
porous and thick oxide layer of TiO2. This exhibits better
corrosion resistance and shows a significantly lower
water contact angle compared to machined surfaces. The
anodized surface showed enhanced alkaline phosphate
activity compared to machined surfaces. The higher
hydrophilic property and wettability is generally
favourable for biocompatibility. Increased wettability
promoted the interaction between the impact surface and
biological environment. Cell activation was more rapid
on hydrophilic surfaces. The hydrophilic surface pro-
motes the adhesion of the relevant proteins.
The anodized implant surfaces were characterized by
X-ray diffraction, as shown in Figure 3.32 It indicates
that the predominant anatase phase exists on the ano-
dized surface. When compared with the machined,
sandblasted and acid-etched surface, the main oxide
layer is rutile. With respect to surface morphology, hyd-
rophobicity was observed in the machined surface, but
the SLA and acid etched had different surface morpho-
logies. The anodized implant has volcano-like surface
porous structures, having both rutile and anatase on the
surface, as shown in Figure 3. The tissue healing process
and bone growth is quick on the anodized and acid-
etched implant surfaces compared to machined implants.
B. C. de V. Gurgel et al.33 studied the efficiency of
anodized implants on animals. Dogs were used to
perform implant testing. After 3 months the initial
trauma, each wound was inspected and defects were
observed, which measured 5 mm in height and 4 mm in
width. Anodized implants were planted inside the root
canal of the wound and were allowed to heal for an
additional 3 months. The percentages of bone-to-implant
contact and the bone density of the anodized implants
were observed to be 57.03±21.86 % and 40.86±22.73 %,
whereas the machined implants were 37.39±23.33 % and
3.52±4.87 %, respectively. Burgos et al.34 also conducted
experiments on rabbits. They observed an osseointe-
gration rate for the anodized implants of 20 % (after 7 d),
23 % (14 d) and 46 % (28 d), compared to 15 % (in 7 d),
11 % (14 d), and 26 % (28 d) for the machined surface.
Despite titanium’s biological compatibility, it is con-
sidered to be an inorganic material. This means that the
metal does not form part of the human body. It has some
properties that may require modifications to improve its
integration into the jawbone, patient comfort, satisfaction
and confidence. The potential problem in clinical appli-
cation is that titanium dentures become dim, which
impairs its aesthetic appearance after a long period of
time. There is a higher demand for implant prostheses.
This is not only for oral functions such as mastication,
pronunciation and durability, but also dental aesthetics is
also important. Several patients who have received tita-
nium dental implants have complained about the decay
of its aesthetic appearance. In most cases the gum
covering the implant is thick enough to prevent implant
visibility. But since implants have been used to replace
or insert teeth, it was found that not all patients have the
same gum thickness. Several cases have been reported
where the dark metallic colour of titanium shown
through the patient’s gums. In cases where dissatisfac-
tion was present of its aesthetic appeal, colour appear-
ance was changed by an electrochemical anodic
oxidation process which produces the interference of
colours.
Titanium has an iron-dark appearance and is visible
when it becomes exposed. In other cases the patient has
thin gums and the titanium would appear beneath the
skin. Many alternative measures have been sought ought
to improve on its aesthetics. Some materials have been
developed to replace titanium. Dentists would also use
techniques to veneer the titanium with a white material
or graft tissue over the visible part. However, mechanical
methods are unmanageable, inexact and form contami-
nant particles on the titanium implant surfaces. Similarly,
acid etching has no ability produce controllable surface
topographies and it has the potential to form residual
surface acids, which is harmful to bone growth. Implant
surfaces formed through this process are nonuniform on
the microscale or macroscale. However, osteoblast may
accustom to a nanoscale topology rather than a micro-
scale environment. As a result, more desirable methods
to modify Ti surfaces are needed to promote tissue cells
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Figure 3: XRD pattern of anodized sample33
Slika 3: Rentgenogram (XRD) anodiziranega vzorca33
and bone growth.35-37 The surface integrity of the
Ti6Al4V alloy was studied by applying different surface
treatment processes. The study focused on pickling and
anodization.38 An investigation of these processing
techniques revealed no significant changes in the micro-
structure of the implant and surface oxygen and hydro-
gen superficial content were found to be unchanged. The
roughness characteristics remained mostly unchanged.
This surface-treatment technique revealed that com-
pressive residual stresses significantly decreased and that
internal stresses were mainly located on the oxide
interface. Moreover, the fatigue resistance decreased
after the application of either pickling or anodization on
the Ti64 alloy. The effect of anodization time on the sur-
face morphology, surface roughness, and crystal struc-
ture was studied by L. Wu et al.39 The surface roughness
increased with anodization time up to 20 min, and later
dropped down, which is due to localized rapture of the
compact inner layer and nucleation of the secondary
oxide particles. The uniformity of the surface and the
relative intensity of the anatase and rutile tended to in-
crease with longer anodization times. Y. T. Sul40 studied
the electrochemical growth behaviour and surface beha-
viour of TiO2 nanotubes fabricated on TiO2 grit-blasted
screw-shaped rough titanium implants. The potenti-
ostatic anodization of blasted screw-shaped implants at
20 V in 1-M H3PO4 + 0.4 % of mass fractions HF for
30 min, 1 h and 3 h resulted in highly nanopore struc-
tures and vertical aligned nanotubes. The surface rough-
ness value decreased with the reaction time and the
results of the animal study provided significant evidence
that the nature of nanotubes have superior bone
responses compared to blasted implants. This indicates
that TiO2 nanotubes have the potential to be used in the
field of bone implant and bone tissue engineering.
3.2 Acid etching
Acid etching is a chemical process used to modify
the surface of titanium implants. It increases the reten-
tion between the implant surface and the bone by
enhancing the surface properties of the implant such as
osteoblast activity and quick bone growth. Strong acids
used for acid etching include H2SO4, HNO3, HCl and
hydrofluoric acid. These acids produce micro pits on the
titanium implant surface, as shown in Figure 4. The
acid-etching process has been used to promote the
osseointegration process beyond 3 years. There is no
need to use any external reagents that contaminates the
implant surface. The biological response of this surface
in terms of bone apposition and bone-implant contact
ratio is comparatively high. Compared to the machined
surfaces, acid-etched surfaces have a higher bone im-
plant contact.16 A significantly higher torque is required
to remove the acid-etched samples compared to the
machined implants, but a lower torque compared to
plasma-sprayed implants. The disadvantage of acid
etching is that it reduces the mechanical properties of
titanium implants, which is caused by hydrogen em-
brittlement. The presence of micro cracks is also
observed on the surface of the implants, leading to a
reduction in fatigue resistance. This hydrogen embrittle-
ment forms the brittle phase in the titanium, leading to a
reduction in ductility. The reduction in ductility leads to
the occurrence of fracture in implants.41 The influence of
the implant surface on the primary stability is
imperative.42 The surface topography and roughness
positively influences the healing process of the bone by
favouring the cellular response and cell-surface
interaction. Rough surfaces are considered to enhance
the primary stability and allow firm mechanical fixation
to the surrounding tissues.43 Figure 5 shows the primary
stability of dental implants due to different surface-
finishing techniques. The surface treatment process
significantly alters the surface-roughness parameters,
leading to a change in the cell-surface interaction, as
indicated in Figure 5. The anodized surface (Ra-1.11 μm)
has a greater surface roughness compared to the
machined surface (Ra-0.74 μm) and acid etched surface
(Ra-0.91 μm), which supports the healing process. It is
clear that the improved surface roughness of the
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Figure 5: Effect of surface treatment on torque removal34
Slika 5: Vpliv obdelave povr{ine na moment pri odstranitvi34
Figure 4: Acid etched surface of titanium35
Slika 4: S kislino jedkana povr{ina titana35
acid-etched surfaces has a positive effect on removal
torque compared to machined samples. The anodized
sample has remarkable improvement in the torque
removal force over the machined and acid etched. The
average surface roughness is highest for anodized sam-
ples, compared to other surface-treatment techniques.
M. V. D. Santos et al.43 studied the implant surface
finish and geometry on the primary stability of dental
implants. Their results showed that the maximum torque
insertion depends on the coefficient of friction between
the implant surface and the placement of the wall, the
implant design thread geometry and the
surface-treatment technique. It was also found that the
insertion torque varies as the geometry of the implants
varies. As seen from Figure 6, the torque insertion is
higher in a conical geometry compared to a cylindrical
implant. This is due to the different thread geometries.
The surface area of contact with host tissues is increased
in a conical implant. As the surface area increases, the
friction between the implant surface and the bone wall
increases, leading to a higher insertion torque. The
implants of the anodized surface have a higher roughness
and a larger coefficient of friction than the machined
one. They reported that the rough surface has greater
significant success rates compared to the smoother
surface implants. The surface treatment improves the
primary stability of the implants. The anodized implants
have a higher primary stability compared to the
acid-etched and machined implants.44
4 HYBRID PROCESS
4.1 Sand blasted and acid etched (SLA)
This consists of dual processes to obtain both the
surface roughness, as well as the removal of particles
from the implant surface. Sand blasting is beneficial for
removing the surface contaminants, roughening surfaces
to increase effective surface area and can produce
beneficial compressive residual stresses. The acid
etching chemical process changes the surface structure
and leads to the creation of a hydride layer thickness of
1–2 μm on the intermediate oxide layer and implant
surface. Titanium implants are blasted with ceramic
particles and then subsequently etched by acids. In the
sand-blasting process, peaks and craters/pits are more
commonly found. These peaks and pits can be reduced
by acid etching and small pits will be formed, which
reduces the surface roughness.45 The different etching
processes may also form the hydrides on the surface and
the replacement of oxygen by the titanium hydrides,
resulting in a slow transformation of the implant surface.
This results in a nano-meter surface roughness, and helps
in protein adhesion immediately after the implant is
placed in the human body.46 SLA increases the bone
formation and amount of growth. The dual modified
implants have improved torque removal force compared
to the single modified process, like acid etching,
machined and plasma sprayed surface.47 Several studies
have shown that SLA active implants have a better
bone-contact ratio, stability and this reduces the healing
time duration. J. K. Lee9 has studied the bone implant
contact ratio (BIC) of modified surfaces in titanium
implants. In an vivo study of dental implants they were
classified into machined surface implants, sand blasted
with large grit sizes and acid etched (SLA) surface
implants, TiO2 nano tube array surface implants and TiO2
nano tube surface implants with rhBMP-2. Histo-
morphometric analysis studies were performed and the
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Figure 7: Bone-to-implant contact ratios of machined surface, SLA
surface, TiO2 nanotube array surface, and TiO2 nanotube array surface
with rhBMP-2, (I1) machined surface implants, (I2) SLA surface
implants, (I3) TiO2 nanotube array surface implants, and (I4) TiO2
nanotube array surface implant with rhBMP-2
Abbreviations: BIC – bone-to-implant contact ratio, SLA – sandblasted
large-grit and acid-etched, rhBMP-2 – recombinant human bone morphogenetic
protein-29
Slika 7: Razmerje stika kost-vsadek pri strojno obdelani povr{ini,
SLA-povr{ina, TiO2 matrika povr{ine z nanocevkami in TiO2 matrika
povr{ine z nanocevkami z rhBMP-2; (I1) strojno obdelana povr{ina
vsadka, (I2) SLA-povr{ina vsadka, (I3) TiO2 matrika povr{ine vsadka
z nano cevkami in (I4) TiO2 matrika povr{ine vsadka z nanocevkami
in z rhBMP-2
Okraj{ave: BIC – razmerje stika kost-vsadek, SLA – peskano z debelim peskom
in jedkano s kislino, rhBMP-2 – rekombinantni morfogeni protein-2 ~love{ke
kosti9
Figure 6: Effect of implant design and surface treatment on torque
insertion46
Slika 6: Vpliv zgradbe vsadka in obdelave povr{ine na moment pri
obdelavi46
highest BIC of 29.5 % was obtained I4 followed by I3
(16.3 %), I2 (14.7 %) and I1(11.1 %) groups, as shown in
Figure 7. The bone-volume ratio was also measured
around the implant threads, which was found to be
highest in the I4 group (77.3 %) followed by I3, I2 and I1
groups (67.2 %, 53.7 %, and 66.9 % respectively). The
authors suggest that the nano tube array surfaces have
improved osseointegration properties compared to
machined and SLA-implant surfaces. TiO2 nano tube
implant surfaces have an enhanced bone formation, bone
strength and cell adhesion compared to other modified
surfaces.
4.2 Electro polished and anodized implants
The bone response in the early healing period and the
bone responses to different types of titanium surfaces
were studied. The responses of different surfaces were
analysed; the electro polished implant surface has a low
degree of bone-to-implant contact compared to the
machined implant surface after 1 week. After 3 weeks
post-implantation there were no major differences bet-
ween the machined, machined plus anodized, electro
polished and electro polished and anodized implant
surfaces, as can be seen in Figure 8. As observed from
Figure 8, after 6 weeks the electro polished implant
surfaces had less bone contact compared to the other
three groups. The electro polished plus anodized implant
shows higher bone implant contact, but has less BIC in
comparison to the machined and machined plus anodized
surfaces. The reason is that the electro polishing removes
a significant amount of material from the surface, which
decreased the diameter of the implant. The reduction in
diameter of implants caused a lower BIC ratio during the
initial healing. After 6 weeks, the electro polished sur-
faces had less bone contact, which may be due to a
slower rate of mineralization around the surfaces. This is
because the electro polished surface is smooth and the
amount of bone attachment to the implant is less
compared to the machined implants. The bone response
depends on two different types of surface roughness; the
smooth surface expresses a lower degree of bone for-
mation compared to the rough surface finish. The degree
of a rough surface is adequate to furnish an overall
response of the bone. However, the thick oxide on the
bone formation was not practical when compared to two
groups of machined implants. Thus, it seems that a com-
bination of surface topography and thick oxide present
on the electro polished plus anodized implant surfaces
are the most encouraging type of surface roughness for
bone growth and the rapid healing of interfacial tissues.48
5 LASER MODIFICATION
Laser is an emerging field in the manufacturing
micro-components of complex surfaces of micro and
nano levels. Laser surface engineering is advanced
enough to modify the surface. This method becomes
more popular method for resolving peri-implantitis. This
technology claims a noncontact, no media and conta-
mination-free method. The laser modification technique
is very suitable for selective modification of surfaces and
allows the generation of complex microstructures, this
technique makes important in geometrically complex
biomedical implants. The laser micromachining changes
the micro and nano structured surface roughness of im-
plant threads. The inner part of thread is more important
than the outer part for bone formation and growth. The
laser technique has advantage to treat only the inner part
of the thread and leave the outer part machined. The
pulsed laser melting of the titanium implant surface in a
vacuum chamber is one of the surface-modification
processes. The laser-treated surface morphology consists
of more or less periodic wavelength of peaks with
50–100 μm between the elements.16
S. Cho et al.49 studied laser treated, commercially
pure titanium screws and inserted in right tibia
metaphysics of white rabbits for 8 weeks. It was reported
that the SEM of laser treated implants demonstrated a
deep and regular honeycomb pattern with small pores
and the removal torque was 23.58 N-cm for the con-
trolled machined implants and 62.58 N-cm for the laser-
treated implants. A. Gaggl et al.50 made an comparative
study on four different dental implant surfaces treated
with machined roughness, titanium spray coating, treated
by aluminium oxide and treated by laser. It is reported
that the laser-treated surface has high purity and showed
enough surface roughness for good osteointegration and
had a regular pattern of micro pores with an interval of
10-12 μm, a diameter of 25 μm and a depth of 20 μm.
Gill et al.51 studied the influence of laser surface
modifications on the mechanical and electrochemical
behaviour of Ti and Ti6Al4V implants. When laser
modification was carried out for each metal, the
microstructural changes were observed:
a) melting zone with small grain size and martensitic
structures in aforementioned metal and
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Figure 8: Total bone contact (%): after 1 week, 3 weeks and 6 weeks51
Slika 8: Celoten stik s kostjo (%): po 1 tednu, 3 tednih in 6 tednih51
b) the heat-affected zone (HAZ) with -phase in CP Ti
with a higher grain size and Widmanstatten structure
in Ti6Al4V.
Positive tensile residual stress was determined by
means of X-ray analysis in the zones marked by laser.
Furthermore, corrosion behaviour was studied in a
simulated body fluid at 37 °C. It was found that pitting
was observed in different zones near the HAZ, and
results showed that the corrosion resistance decreased in
the laser-treated samples, and residual stresses and the
martensitic microstructures favoured the decrease of the
corrosion-fatigue life by around 20 % in both metals
under physiological conditions. G. Romanos et al.52
studied the osteoblast attachment on titanium surfaces
after laser irradiation. It is reported that osteoblasts could
be grown in all of the surfaces. The cell density is higher
in the laser-irradiated surface than in the non-irradiated
specimens because of the cleaner effect on superficial
layer by the lasers. The laser irradiation on the titanium
surface may promote the osteoblast attachment and
further bone formation. Palmquist et al.53 made an
in-vivo study of laser modified titanium and nano-scale
surface topographic features. The authors concluded that
the torque removal significantly increased in the laser-
modified implant and of clinical importance, the nano-
structured surfaces supported long-term bone bonding
and interface strength between the titanium implant and
the bone. The removal torque is also substantially in-
creased. S. S.-Y. L. Kang et al.54 studied the biomecha-
nical properties and inter-phase responses of machined
and laser-treated stainless steel (SS) implant screws. The
laser-treated implants have a higher surface roughness
and no compromise in fracture resistance compared to
the machined one. The surface roughnesses were in-
creased due to repeated melting and solidification of the
material. The bone implant’s contact ratio was deter-
mined and there was no significant difference between
laser-treated and machined micro-screw implants.
6 CONCLUSIONS
This article discussed the surface modification
methods for titanium alloys in improving the biological
properties and friction properties of implants. Based on
the above survey, the research spotlight is especially
focused on electrochemical anodizing. It can be used for
the formation of an oxide layer on a commercially pure
Ti surface for dental implants due to the high expecta-
tions regarding their applications. The anodization pro-
cess is a simple and fast surface-modification technique
to create a nano-structured TiO2 on the Ti surface. It will
improve the cell growth on the Ti surface. The degree of
optimised geometry and surface roughness for implant
fixation is still unknown. There is a contradiction in lthe
iterature relating in-vivo and in-vitro studies with
moderate surface roughness. Some of the researchers
mentioned not only the degree of roughness is important;
the textured surface of implant is also. There is scope for
an evaluation of optimum surface roughness and surface
morphology for dental implant applications. This is
because the surface roughness plays a major role in
quality, the coefficient friction and the rate of osseointe-
gration in titanium dental implants. Aesthetics in this
regard refers to the engineering of the surface colour for
both marking and implant aesthetic reasons. There is no
literature found which investigates the sliding friction
behaviours of titanium on titanium with specific refe-
rence to the required torque for the axial preloading of
titanium screws. Very little research has been conducted
on laser surface-modification techniques that investigate
its effect on biocompatibility.
7 FUTURE SCOPE
From this research gap one can perform research on
the aesthetical appearance of dental implants by per-
forming the anodizing process. However, there is no
evidence on the statistical relationship between bone
contact and surface roughness. Also there is a lack of
appropriate measurement on the sliding friction between
the titanium on titanium implants with specific reference
to the required torque for the axial preloading of titanium
screws. Hence, more studies are required to gain knowl-
edge about the aesthetic appearance, surface topography
on the bone growth and friction between the bone and
titanium implants. The performance of titanium and its
alloys can be improved intensely by developing a
suitable surface-modification procedure that will lead to
increased aesthetic appearance and wear, and frictional
properties.
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