D. DOLINAR et al.: BIOMATERIALS IN ENDOPROSTHETICS
89–98
BIOMATERIALS IN ENDOPROSTHETICS
BIOMATERIALI V ENDOPROTETIKI
Drago Dolinar1, Matev` Goren{ek2, Monika Jenko3, Matja` Godec3, Barbara [etina Bati~3,
^rtomir Donik3, Aleksandra Kocijan3, Mojca Debeljak4, Bo{tjan Kocjan~i~1
1Department of Orthopaedic Surgery, University Medical Centre Ljubljana, Zalo{ka 9, 1000 Ljubljana, Slovenia,
2MD-Medicine, Sanatorium Ljubljana, Bohori~eva 8, 1000 Ljubljana, Slovenia,
3Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia
4University Rehabilitation Institute, Republic of Slovenia, Linhartova 51, 1000 Ljubljana, Slovenia
monika.jenko@imt.si
Prejem rokopisa – received: 2017-11-20; sprejem za objavo – accepted for publication: 2017-12-22
doi:10.17222/mit.2017.196
The use of endoprosthetics for hip- and knee-joint replacements is currently the most common and successful method in
orthopaedic surgery to treat degenerative joint disease, for relieving pain and for correcting deformities. Cobalt-chromium-
molybdenum alloys, titanium alloys, trabecular tantalum, Biolox ceramics, UHMWPE polyethylene and PMMA bone cement
are the most common biomaterials used in endoprosthetics. The published results of long-term investigations demonstrate
excellent clinical results for at least 15 years after TJR implantation surgeries. Using new, improved surgical methods as well as
new, improved implants made of advanced biomaterials, better clinical results are expected. While these surgeries have positive
outcomes, approximately 10 % of implants fail prematurely. Aseptic loosening and periprosthetic joint infection are the main
causes of failure for joint arthroplasty. The Orthopedic Clinic Ljubljana performs between 80 and 100 revision surgeries of knee
and hip endoprostheses per year. The most common causes for revision surgeries are aseptic loosening and implant infection.
For all treated patients the clinical course of treatment including X-ray documentation is precisely followed. The retrieved
endoprostheses are sent for bacteriological analysis, and afterwards are preserved for further investigations. The surface and
microstructure analyses of retrieved hip and knee endoprostheses were performed in cooperation with IMT Ljubljana using
advanced analytical and integrated electron spectroscopy techniques. Two new and two retrieved endoprostheses were studied.
The surface chemistry and microstructures of both the new and used titanium alloys and CoCrMo alloys used for hip and knee
endoprostheses were determined using SEM (morphology), EBSD (phase analysis), and AES and XPS (surface chemistry). The
SEM SE and BE images revealed their microstructures, while the EBSD provided the phases of the materials. During the
production of hip and knee endoprostheses, these materials are subject to severe thermomechanical treatments and
physicochemical processes that are decisive for CoCrMo alloys. The AES and XPS results showed that thin oxide films on a)
Ti6Al4V are a mixture of primarily TiO2 with a small amount of Al2O3, while the V is depleted, b) Ti6Al7Nb is a mixture of
primarily TiO2 with a small amount of Al2O3 and Nb2O5, and c) the CoCrMo alloy is a mixture of primarily Cr2O3 with small
amounts of Co and Mo oxides.
Keywords: biomaterials, knee and hip endoprosthesis, microstructure, surface chemistry, SEM, EDS, EBSD, AES and XPS
Endoprotetika kol~nega in kolenskega sklepa (TEP) je dandanes uveljavljena in uspe{na metoda zdravljenja napredovanih
degenerativnih sprememb teh sklepov za laj{anje bole~in in za odpravljanje deformacij. Prav v zadnjih letih se uveljavljajo nove
operativne metode, izbolj{ani implantati iz novej{ih, vse bolj{ih materialov. Hitrej{a je tudi pooperativna rehabilitacija teh
bolnikov. Najpogostej{i biomateriali, ki se uporabljajo za izdelavo kol~nih in kolenskih endoprotez, so titanove zlitine,
kobalt-krom-molibdenove zlitine, trabekularni tantal, keramika in UHMWPE polietilen in PMMA kostni cement. Kljub dobrim
fizikalnim lastnostim, se pri uporabi teh materialov pojavljajo te`ave, povezane z obrabo in korozijo, kar dolgoro~no privede do
omajanja vgrajenih implantatov. Objavljeni rezultati dolgoro~nih preiskav ka`ejo odli~ne klini~ne rezultate najmanj 15 let po
operacijah TEP. Uporaba novih, izbolj{anih kirur{kih metod in novih, izbolj{anih vsadkov iz naprednih biomaterialov, nakazuje
bolj{e klini~ne rezultate. Kljub odliènim rezultatom omenjenih operacij, pribli`no 10 % implantatov prezgodaj odpove.
Asepti~no omajanje in periprosteti~na oku`ba sta najpogostej{a vzroka za odpoved implantanta. Ortopedska klinika Ljubljana
opravi od 80 do 100 revizijskih operacij endoprotez kolena in kolka letno. Najpogostej{i vzroki za revizijske operacije so
asepti~no omajanje in oku`ba vsadkov. Za vse zdravljene bolnike je natan~no upo{tevan klini~ni protokol zdravljenja, vklju~no
z rentgensko dokumentacijo. Odstranjene endoproteze se po{ljejo na bakteriolo{ko analizo, potem pa se ohranijo za nadaljnje
preiskave. Analize povr{in in mikrostrukture endoprotez kolka in kolena so bile izvedene v sodelovanju z In{titutom za kovinske
materiale in tehnologije (IMT) Ljubljana z naprednimi integriranimi analitskimi tehnikami elektronske spektroskopije. [tudirali
smo dve novi in dve uporabljeni endoprotezi. Povr{insko kemijo in mikrostrukturo novih in uporabljenih titanovih zlitin in
CoCrMo zlitin, ki se uporabljajo za endoproteze kolka in kolena, smo dolo~ili z uporabo SEM (morfologija), EBSD (fazna
analiza) ter AES in XPS (povr{inska analiza). Slike SEM, SE in BE prikazujejo mikrostrukturo, medtem ko EBSD analizo faz
materialov. Med proizvodnjo endoprotez kolka in kolena so ti materiali izpostavljeni termomehanskim obremenitvam in
fizikalno-kemijskim postopkom, ki so odlo~ilni za kakovost CoCrMo zlitin. Rezultati AES in XPS so pokazali, da so tanki
oksidni filmi na: a) Ti6Al4V zlitini predvsem TiO2 z majhno koli~ino Al2O3 , b) Ti6Al7Nb zlitini je predvsem TiO2 z majhno
koli~ino Al2O3 in Nb2O5 in c) na zlitini CoCrMo je predvsem Cr2O3 z majhnimi koli~inami Co in Mo oksidov.
Klju~ne besede: biomateriali, kolenske in kolen~ne endoproteze, mikrostruktura, faze, povr{inska analiza, SEM, EDS, EBSD,
AES in XPS
Materiali in tehnologije / Materials and technology 52 (2018) 1, 89–98 89
UDK 620.1:67.017:615.461:615.477.2 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(1)89(2018)
1 INTRODUCTION
The endoprosthetics of hip and knee joints (TJR) is
nowadays the established method for treating advanced
degenerative changes in these joints and one of the most
successful and often performed orthopaedic procedures.
Annually, more than a million hip endoprostheses and
nearly two million knee endoprostheses are implanted
worldwide.1 Indications in patients with hip or knee pain
are varied, and the insertion of a TJR, as a rule, elimi-
nates the pain in these patients, improves the mobility
and the quality of life.
The most common biomaterials used to make hip and
knee endoprostheses are titanium alloys, cobalt-chro-
mium-molybdenum alloys, trabecular tantalum, ceramics
and polymers. In spite of the good physical properties,
problems associated with wear and corrosion occur in
the use of these materials, which in the long run leads to
the instability of the embedded implants. Recently, tribo-
logical investigations of orthopaedic implants (problems
of lubrication, friction and wear among individual parts
of embedded endoprostheses) and related research on the
influence of individual wear and corrosion products on
the human tissue have been established, which will
enable us in the future to further clarify the causes of
premature failure of orthopaedic implants.
According to the latest literature related to the
problem of the premature failure of orthopaedic im-
planted hip and knee endoprostheses, there are two main
causes that are responsible for this problem. These are
aseptic loosening and infections.1–3 Aseptic loosening of
the endoprosthesis is the loss of endoprosthesis in its
bone bed. The most common result is the wear of endo-
prostheses and consequent osteolysis. Other etiologies
represent poor initial implant stability, poor implantation
(inadequate geometry and surface treatment of the im-
plant) and consequently unsuccessful osseointegration of
the implant. Infection of the hip or knee endoprosthesis
is a serious complication; the most common cause is
infection by staphylococcus aureus. Such infections
require a long-term antibiotic treatment, often the re-
moval or replacement of the endoprosthesis, a prolonged
hospital stay, and longer and more demanding rehabili-
tation, all of which greatly increases the cost of the
treatment.
The latest studies show a potential link between the
"aseptic-non-bacterial" implantation of the implant and
latent implant infection. They found that the diagnosis
was wrong, and the latent "low-grade infection" was
crucial for the premature loosening of the implant. There
are studies which show that for the implants a free sur-
face competition is in progress, and if the host cells first
occupy the surface of the implant, a strong integration of
the tissue and the formation of a barrier occurs to prevent
the adhesion and colonization of microbes and the
formation of a biofilm. 4 A strong osseointegration and
the prevention of infection are both criteria necessary for
the success and long-term use of the implant, which
should be taken into account in the design of the
implants.2
Recent research and possible improvements to ortho-
paedic implants are being carried out, in particular, in the
direction of improving the surface treatment of implants,
which would allow for faster and better osteointegration
of the implant, while also preventing the possible surface
colonization of bacteria and the formation of a bacterial
biofilm on the implant.2
In recent years, new surgical methods have also been
applied, and implants from novel, better materials have
been improved. The postoperative rehabilitation of these
patients is also faster.
This paper highlights the basic novelties in this field
of orthopaedic surgery and preliminary results of surface
chemistry and microstructure analysis of retrieved hip
and knee endoprostheses.
D. DOLINAR et al.: BIOMATERIALS IN ENDOPROSTHETICS
90 Materiali in tehnologije / Materials and technology 52 (2018) 1, 89–98
Figure 1: a) cemented total hip arthroplasty (THA), b) uncemented THA. Currently, THA usually uses an uncemented titanium (TiAl6V4,
TiAl6Nb7) or cemented cobalt-chromium alloy (CoCr28Mo6) femoral stem (fixed into place for uncemented or cemented with PMMA) and a
modular cobalt-chrome alloy or ceramic head. The head articulates on a ceramic or ultra-high molecular weight polyethylene (UHMWPE)
acetabular cup fitted into a titanium or cobalt-chromium cup liner that is press fitted or screwed into place or a complete UHMWPE cemented
2 NOVELTIES AND IMPROVEMENTS IN HIP
ENDOPROSTHETICS
From the starting idea, at the beginning of the last
century, several decades have passed since the artificial
hip joint was developed. In the late 50s, Sir John Charn-
ley introduced the use of bone cement to strengthen the
components of hip endoprosthesis into the bone,
developed the shape, and laid out the biomechanical
principles of hip endoprosthetics.
The modern low-friction hip arthroplasty era began in
the early 1970s with the hard-on-soft bearing concept
where polyethylene was used in the cup, stainless steel in
the femoral head and stem and poly-methyl-methacrylate
(PMMA) as bone cement.5,6 The total hip arthroplasty
(THA) developed by Charnley has been successful ever
since, with 77 % to 81 % survivorship after a 25-year
follow-up reported in the first-generation results of his
low-friction arthroplasty with the revision of any
component as the endpoint. Long-term survival in more
than 90 % of patients over 60 years of age has been
associated with Charnley’s THA.7,8
By implanting artificial hip joints, some perioperative,
short-term and long-term complications are also en-
countered. Among the most common perioperative
complications are hematoma, seroma, pulmonary embo-
lism, and paresis of the femoral or sciatic nerve. The
most common short-term complications are infection and
dislocation, and in the long-term, it is particularly
important to mention the aseptic loosening of the
artificial hip joint, which requires the replacement of one
or both parts of the endoprosthesis. Decades have passed
before scientists have at least partially managed to
resolve the cause of the aseptic loosening of endopro-
stheses. The isolation and identification of micron and
submicron particles from the tissue at the prostheses,
their biological activity, the formation of macrophages,
their activation, the release of cytokines, etc., were
discoveries that led to the finding that the process of
loosening is a very complex one. Today, it is known that
most of the particles found in the tissue in the vicinity of
the hip endoprosthesis are polyethylene particles, hence
the "disease of polyethylene particles". Polyethylene
acetabular cups are the main cause of the aseptic loosen-
ing of an artificial hip joint.1
Due to the known possible complications in the
replacement of hip joints with artificial ones, new mini-
mally invasive operative methods are being introduced,
as well as ever-better materials (both in cemented and
non-cemented hip endoprostheses). These improvements
allow the faster recovery of patients to live a normal life,
as well as prolongation of the expected lifespan of hip
endoprostheses (Figure 1). Today, we estimate that the
normal life expectancy of modern hip endoprostheses is
in the range from 20 years to 25 years.
2.1 Novel materials in modern endoprosthetics of the
hip joint
Due to the polyethylene particles disease, which is
the main cause of aseptic loosening of hip endopro-
stheses, recently we have used newer polyethylene
acetabular cups or polyethylene acetabular inserts from
the mechanically resistant, highly cross-linked polyethy-
lene-HXLPE. Cross-linking can be accomplished using
gamma radiation. A subsequent annealing stage is per-
formed in order to reduce the free radicals that are
produced by the radiation (first-generation HXLPE). Test
data from contemporary hip simulators have shown an
80 % to 90 % reduction in wear with highly cross-linked
polyethylenes.1 One promising approach to improving
oxidation resistance is the addition of antioxidants such
as 
-tocopherol (or vitamin E) to the material, thereby
preventing oxidation of the polymer, while allowing for
cross-linking or sterilizing irradiation in the absence of a
post-irradiation thermal stabilization (second-generation
HXLPE). Vitamin E (VE) is an effective biological anti-
oxidant, helping to prevent the oxidative degradation of
cell-membrane phospholipids. When added to
UHMWPE, VE performs a similar role, helping to pre-
vent oxidation of polyethylene chains.9
Modern methods of cementing have increased the
quality of cement endoprostheses (vacuum mixing of
cement, pulse cleansing of the femoral channel, retro-
grade cement introduction into the femoral channel, low
viscosity of cement, use of cement with antibiotics).10
In modern orthopaedics, cement-free hip endopro-
stheses made of titanium are increasingly being used. As
a biocompatible material, titanium has been known for
its excellent osteointegrative capacity. The use of tita-
nium-based materials reduces the stress-shielding effect
and debris generation. This is because its modulus of
elasticity is closer to cortical bone than that of Co-Cr-Mo
alloys. Ti-based implants can be inserted without bone
cement, which is typically used with Co-Cr-Mo femoral
stems to improve the stress distribution to the bone. The
drawback of titanium alloys relates to their softness com-
pared with Co-Cr-Mo alloys and also to their relatively
poor wear and frictional properties. For this reason, tita-
nium alloys are rarely used when hardness or wear resis-
tance is considered to be the optimal property.
A non-cemented femoral component is an excellent
choice for young patients as well as for patients with
good bone density (especially for those with a thick
femoral cortex and a smaller diameter of the femoral
canal). However, it is sometimes difficult to obtain good
initial stability with a cement-free implant in patients
with a broad femoral channel and porous bone. There-
fore, the cement implant is an appropriate choice in
elderly, physically less active patients with a poor bone
structure (thin cortical bone and broad femoral canal).1
Cement-free implants are usually coated with a porous
material to increase the area for bone ingrowth. More-
over, the rough surface allows for greater initial implant
D. DOLINAR et al.: BIOMATERIALS IN ENDOPROSTHETICS
Materiali in tehnologije / Materials and technology 52 (2018) 1, 89–98 91
stability, which is crucial for the long-term success of the
endoprostheses. The porous coating can be limited to the
proximal part of the femoral component, or it may take
place over the entire length of the implant. This varies
according to the philosophy of designing the component
itself. Proximal coated implants are fixed in a metaphysis
or metadiaphysis. Widely coated implants are fixed dis-
tally in the femoral diaphysis. Some non-cemented pros-
theses are also coated with hydroxyapatite, which pro-
motes bone formation and allows for an even faster
ingrowth of the endoprosthesis.
We use either self-contained cement-less acetabular
cups, and recently, especially press-fit acetabular cups,
which are pressed into a ready-made acetabular bed or
"highly cross-linked" polyethylene acetabular inserts,
especially in younger patients, and more and more often
ceramic acetabular inserts. Until recently, we have
avoided ceramic hip endoprostheses (ceramic head-
ceramic acetabular insert) due to the brittleness of the
ceramics, and the complications because of this.
For novel, more robust and mechanically resistant
ceramics (Biolox delta ceramics – an alumina matrix
composite made up of 82 % alumina, 17 % zirconia,
0.6 % strontium oxide and 0.3 % chromium oxide) these
problems no longer exist, providing both endoprosthesis
components are correctly mounted.10,11 Due to the
mechanical strength of ceramics, the wear of these endo-
prostheses is minimal.
In recent years tantalum implants have been used in
adult hip and knee surgery. Tantalum is an elemental
metal with great characteristics for biocompatibility,
corrosion resistance, and a porous geometry. Tantalum,
marked as Trabecular Metal, has a uniform and conti-
nuous structure that allows for greater strength, lower
stiffness, higher volumetric porosity, and a higher coeffi-
cient of friction compared to other porous metals. The
porous structure has higher porosity than other materials
used in orthopaedic implants like sintered beads (30–35 %)
and fibre metal (40–50 %)8, providing a great increase in
elasticity and making porous tantalum mechanical pro-
perties very close to those of the subchondral bone. The
great number of pores and their dimensions positively
affect the stability of the implant, increasing the friction
coefficient, up to even three times higher than sintered
bead materials.12 In the case of marked bone loss, trabe-
cular implants have been used to supplement the fixation
in total hip and knee arthroplasty.13
In selected revision cases with extensive bone defi-
ciencies of the femur or tibia, tumour cases or difficult
cases after the treatment of periprosthetic joint infection,
silver-coated mega implants are used. The infection rate
of modular mega-endoprostheses is stated in the litera-
ture as 4–36 %. Various in-vitro studies have shown that
silver coatings effectively inhibit or even prevent the
formation of biofilms on the metal surfaces of different
bacteria. Several clinical studies have confirmed the
positive effects of silver coatings in preventing infections
in endoprostheses.14
For all hip implants, the femoral head must match the
acetabular insert. The endoprosthesis can be metallic or
ceramic. The metal heads are made of cobalt-chrome
alloys. Different sizes from 22 mm to 38 mm are avail-
able (sometimes even more). The size of the head
influences the motion, stability and tendency to wear.
With the enlargement of the head, the range of motion
increases and the tendency for laxation is lowered, but at
the same time the wear of the material and the formation
of the wear particles increases. With improved materials
and, consequently, smaller wear of the contact surfaces
(ceramic head in combination with a ceramic or HXLPE
acetabular insert), more and more surgeons select for the
use of a larger femoral head (32 mm and 36 mm).
From the biotribological point of view the increased
head size coupled with decreased clearance has been
proved to improve lubrication, as happens for both
metal-to-metal (MoM) large head and ceramics-to-cera-
mics (CoC) implants, which can operate under a
fluid-film lubrication regime (favourable regarding the
wear of the articulating surfaces). The best lubrication
behaviour is estimated for CoC implants: their high
surface finishing (i.e., very low roughness) balances the
low film thickness, guaranteeing a fluid-film regime. It is
worth noting that the clearance must be dimensioned
properly, avoiding both large values, which would lead to
the boundary regime and low values, which might cause
edge contact and thus lubricant starvation (unfavourable
conditions regarding wear). Conversely, larger head sizes
increase the frictional torque, consequently the stresses
are larger on tapers and bone-implant interfaces (un-
favourable regarding implant survival) and in
metal-to-polymer (MoP) or (ceramics-to-polymer) CoP
bearings more polyethylene wear is due to a longer
sliding distance per step.15
Several years ago metallic contact surfaces were rein-
troduced in the endoprosthetics of the hip joint, using
modern metal implants. Such component contacts have
proved to exhibit reduced wear compared to the standard
contact surface where the metal head slides on polyethy-
lene. The most popular was primarily arterial arthro-
plasty, which is the biomechanical best approximation to
the normal hip condition. Subsequent studies have shown
that the metal-to-metal contact surface did not yield
clinical advantages, but caused an even greater number
of revisions.16 For these patients strongly elevated levels
of cobalt and chromium ions in the serum, erythrocytes,
and urine were registered. This is associated with the
potential for an increased incidence of certain cancers.
An even greater problem is the local release of metal par-
ticles and the vigorous response of the body. Pseudo-
tumours are formed, and loosenings of endoprostheses
are more frequent. The use of prostheses with a metal-
contact surface is therefore being abandoned.
D. DOLINAR et al.: BIOMATERIALS IN ENDOPROSTHETICS
92 Materiali in tehnologije / Materials and technology 52 (2018) 1, 89–98
2.2 Minimally invasive surgical methods for replacing
the hip joint
In recent years, when replacing a hip joint with an
artificial one, we often decide for a minimally invasive
approach that reduces blood loss during surgery and
enables faster and less painful rehabilitation of the
patient. Until recently, the operations were carried out in
such a way that the individual circumferential muscles
were separated by means of cutting or releasing the
increments of the individual muscles to the bone. With
the newer approach, the muscles are only released,
which is significantly more favourable, since the sur-
rounding muscles are not damaged. Also, the skin cuts
are shorter; therefore, a specially adapted instrument is
used, which enables normal and precise replacement of
the hip joint. A patient can step on the foot and load it to
pain the first day after the surgery and after the fifth to
the seventh day after the surgery, the patient is released
from the hospital and can return to normal life much
more quickly than before.
2.3 Novelties in the knee joint endoprosthetics
The modern era of the replacement of defective knee
joint starts in 1974, when Insall designed the so-called
total condylar prosthesis with a recess to contact with a
knee pad. To replace the tibial part of the joint, a poly-
ethylene insert attached to the metal plate was used.
Various improvements followed, and basically such pros-
theses are still used today (Figure 2).1
The goal of inserting knee endoprosthesis is to
achieve a painless, stable and easily-moving knee joint,
thus improving the quality of life for patients with
worn-out knee joints.
The knee is not a simple hinged joint, but has a
complex mobility around and along all three axes, which
are even changing in the course of movement. This
means that the basic motion, bending-out, does not take
place around the fixed axes, but around the constantly
changing rotation centres. In addition, the movement is
combined with rotations and slips between the femoral
and tibial condyles. Because of this, biomechanically, a
knee endoprosthesis is much more complicated than a
hip. The ideal knee endoprosthesis should provide nor-
mal movements, to a normal extent in all three axes.
Patients should be aware that the complexity of the func-
tioning of a normal knee joint, despite modern techni-
ques and implants, cannot be fully achieved.
Cemented endoprostheses provide very good long-
term clinical results and therefore constitute a "gold
standard". Long-term results of non-cemented knee
endoprostheses are significantly worse, according to the
data of internationally published national registries,
mainly due to the more frequent loosening of the tibial
part of endoprosthesis. Recent advanced models of non-
cemented knee endoprostheses (modern porous coatings,
the use of hydroxyapatite and, above all, the use of trabe-
cular metals), show encouraging, medium-term, clinical
results that are even better than cemented-type knee
endoprostheses.17
In unilateral wear of the knee joint unicondylar knee
endoprostheses are used. In severe knee joint deformities
with markedly ligamental instability or major bone
defects, a hinge-type knee prosthesis is used with cemen-
ted or non-cemented femoral and tibial intramedullary
extensions, which improve the stability of knee endo-
prostheses.
Recently, we have learned that the lifespan of knee
endoprostheses, considering the correct biomechanical
incorporation and according to the results from the
literature, is up to 15 years after installation (in 95 % of
cases).1 An experienced surgeon will allow for the longer
mechanical stability of these endoprostheses by aligning
the mechanical axis of the lower limb with the centre of
the implant, by aligning the transverse axis of the knee
with the base plane and by providing the ligament’s
D. DOLINAR et al.: BIOMATERIALS IN ENDOPROSTHETICS
Materiali in tehnologije / Materials and technology 52 (2018) 1, 89–98 93
Figure 2: Total knee endoprosthesis is nowadays the most common implant used to replace a defective knee joint. It consists of a metallic
femoral part (cobalt-chromium) with a recess forming a contact surface with a knee pad and a polyethylene tibial part with a metal carrier
(cobalt-chrome, titanium) and an intramedullary extension that anchors the part into the bone. The femoral and tibial components are usually
attached to the bone with bone cement (polymethyl methacrylate)1
stability and the balance of the knee joint. In recent
years, the computerized installation of knee endopros-
theses has been progressively implemented. The surgeon
is computer-assisted in a more precise biomechanical
installation, especially for minimally invasive operative
methods, where the knee joint’s visibility is worse and
therefore the possibility of improper insertion of the knee
endoprosthesis becomes greater. Computer-assisted
methods are still improving and are not routinely used in
clinical practice, mainly due to the longer surgical
operating times.
In modern endoprosthetics of the knee joint, we also
use novel technologies and materials that are the same as
for endoprosthetics of the hip joint (modern metallic
femoral part, "highly cross-linked" polyethylene tibial
part, improved techniques of positioning and positioning
of knee endoprosthesis components).18 The use of cera-
mics is also becoming important. In clinical use, metal
femoral components with a ceramic coating (e.g., Oxi-
nium – oxidized zirconium is a metallic alloy with a
ceramic surface that provides wear resistance without
brittleness) are introduced. With advanced models of
primary knee endoprostheses, we approach the ideal
implant model, which must ensure a normal or almost
normal range of motion in all three axes (i.e., a free-
gliding endoprosthesis), and the extent of movement
should be limited by soft tissues, in particular the liga-
ments.
3 PRELIMINARY RESULTS OF SURFACE
CHEMISTRY AND MICROSTRUCTURE
ANALYSES OF RETRIEVED HIP AND KNEE
ENDOPROSTHESES
Recent investigations of metallic biomaterials in-
volved corrosion, bio-tribocorrosion, taper corrosion,
wear, metalosis-cobaltosis–Co metal ions in patients’
blood and urine MoM. In addition there are the object
studies of the influence of acetabular diameter on hip
osseointegration, multifunctional coatings of hip endo-
prostheses and the prevention of biofilm formation on
the implant surfaces. Later studies showed the compe-
tition for a free implant surface of host cells and bacteria
(staphylococcus aureus). If the host cells first occupy the
surface of the implant, a strong integration of the tissue
and the formation of a barrier occurs, which prevents the
adhesion and colonization of bacteria and the formation
of biofilm.2
Our key contribution is a detailed surface characteri-
zation, required for a better understanding and explo-
itation of the surface properties. Microstructure is a
neglected factor in implant design and for that reason a
detailed microstructure characterization is required to
determine the role of prematurely failed implants that
determine the biological responses such as the compo-
sition and structure of the surface oxide film, the surface
contamination and the surface topography.19
3.1 Materials and methods
Implants, retrieved and new, listed in Table 1, were
provided by the Orthopeadic Clinic of the University
Medical centre Ljubljana. We studied CoCrMo alloys,
Ti6Al4V and Ti6Al7Nb alloys.
The samples were cut from new and retrieved endo-
prostheses using a water jet. The samples were prepared
by standard metallographic procedures, using Struers
devices. All the samples for the surface-chemistry anal-
yses had highly polished surfaces.
We have used XRF, ICP-OES for the chemical anal-
ysis, SEM for the visualization, SEM/EDS for the che-
mical analysis, SEM/EBSD for the phase analysis and
the grain orientation, AES for the surface analysis.
4 RESULTS
4.1 Chemical analysis
X-ray fluorescence (XRF) and inductively coupled
plasma optical emission spectrophotometry (ICP-OES)
analyses of the implant biomaterials showed that the
chemical compositions of the alloys are within the
requirements of ASTM F 1295 (Ti6Al7Nb alloy); ASTM
F136 (Ti6Al4V alloy), and ASTM F75 (CoCrMo alloy).
4.2 SEM and EBSD analyses of Ti6Al4V, Ti6Al7Nb
and CoCrMo alloys
The microstructures of the samples were investigated
using a JEOL JSM 6500F field-emission scanning elec-
tron microscope (SEM) equipped with a HKL Nordlys II
electron-backscatter diffraction (EBSD) camera using
D. DOLINAR et al.: BIOMATERIALS IN ENDOPROSTHETICS
94 Materiali in tehnologije / Materials and technology 52 (2018) 1, 89–98
Table 1: The investigated components of retrieved and new implants
KNEE /HIP endoprostheses KNEE EP1 KNEE EP2 KNEE EP3 HIP P1
PRODUCER Depuy Depuy Not known Smith & Nephew
Served in human new 3 month 22 years new
Femoral component CoCrMo CoCrMo CoCrMo -
Tibial component Ti6Al4V Ti6Al4V Ti6Al4V -
Polyethylene liner UHMWPE UHMWPE UHMWPE -
HIP-stem - - - Ti6Al7Nb
HIP acetabular - - - Ceramics Biolox delta
HIP acetabular polymer cup - - - UHMWPE
HIP acetabular metal cup - - - cp Ti/Al
Channel 5 software. Individual diffraction patterns were
obtained together with mapping of the area of the
interest. The instrument was operated at 15 kV and
approximately 2-nA current for the EBSD analysis, with
a tilting angle of 70°. The instrument has both secon-
dary-electron (SE) and backscattered-electron (BE)
imaging modes for morphological analyses of the sam-
ples. For the SE or BE imaging, the instrument was
operated with an acceleration of 15 kV at a current of
approximately 0.5 nA. The vacuum was maintained
below 1·10–6 mbar.
Figure 3, backscattered-electron (BE) images, shows
the cast microstructure of the CoCrMo alloy with Cr-rich
carbides, Ti6Al4V and Ti6Al7Nb alloys a two-phase
dendrite structure in a matrix of Ti6Al4V and Ti6Al7Nb,
respectively, studied by SEM. The microstructures of the
new and retrieved components are very similar, so only
representative images of each composition are shown.
EBSD analyses showed that the microstructure of the
CoCrMo alloys, the face-centred cubic (fcc) and the
hexagonal close-packed (hcp) crystalline structures
co-exist. Typically, the (fcc) phase is predominant at
room temperature, but the (fcc) to (hcp) transformation
could be isothermally or strain induced. The CoCrMo
alloys show different hard phases after casting and
further heat treatments, such as the M23C6 type (M = Cr,
Mo,Co), M7C3, and intermetallic phases of Mo-, Co- and
Si-like (-type) phases, which have the same size as the
eutectic carbides. Figure 4 shows in a) band-contrast
D. DOLINAR et al.: BIOMATERIALS IN ENDOPROSTHETICS
Materiali in tehnologije / Materials and technology 52 (2018) 1, 89–98 95
Figure 4: a) band-contrast image, b) IPF Z direction, phase-map image: green-Co (hcp), red Cr23C6 (fcc), yellow-Co (fcc), c) colour legend for
IPF Z colouring
Figure 3: Backscattered-electron (BE) images of clinical implant microstructures, a) CoCrMo alloy (retrieved after 22 years), b) Ti6Al4V alloy
(retrieved after 22 years), (c) Ti6Al7Nb alloy (new component)
Figure 5: Ti6Al4V EBSD mapping of phases: a) band-contrast image,
b) IPF Z direction
image, b) IPF Z direction, phase-map image: green-Co
(hcp), red Cr23C6 (fcc), yellow-Co (fcc), c) color legend
for IPF Z coloring.
EBSD analyses showed that microstructures of the
Ti6Al4V alloy contain small grains, which are mainly
alpha hexagonal close-packed (hcp) Ti-type, and some
small grains of beta Ti-(bcc) body-centred cubic struc-
ture (Figure 2). The microstructure of the Ti6Al7Nb
alloy similarly contains small grains, the majority alpha
(hcp)Ti with a small amount of Ti beta grains (bcc)
structure, as shown in the EBSD analyses. Figure 5
shows Ti6Al4V EBSD mapping of phases a) band-con-
trast image, b) IPF Z direction. Figure 6 shows
Ti6Al7Nb EBSD mapping of phases a) band-contrast
image, b) IPF Z direction.
4.3 AES analysis
The Auger electron spectroscopy (FE-AES) instru-
ment used in this study was a Thermo Scientific Micro-
lab 310-F spectrometer, equipped with a thermally
assisted Schottky field-emission electron gun (FEG) that
provides a stable electron beam in the accelerating volt-
age range 0.5–25 kV, and a sphericalsector electron kine-
tic energy analyser. The spectra were usually acquired
with a constant retarding ratio (CRR) of 4, which pro-
vides an energy resolution that is 0.5% of the pass
energy. The parameters used in the AES analysis
included a 10-keV primary electron beam at a current of
1 nA, an angle of incidence of 0°, and an Auger emission
angle of 30°. The AES depth profiling was performed by
argon-ion sputtering at 3 keV and scanning the ion beam
over a 2 mm × 2 mm area. The sputtering rate was about
0.7 nm/min.
D. DOLINAR et al.: BIOMATERIALS IN ENDOPROSTHETICS
96 Materiali in tehnologije / Materials and technology 52 (2018) 1, 89–98
Figure 7: AES depth profiles of thin oxide films on: a) Ti6Al4V, b)
Ti6Al7Nb, c) CoCrMo alloy
Figure 6: Ti6Al7Nb EBSD mapping of phases: a) band-contrast
image, b) IPF Z direction
The thin oxide films on the surfaces of the CoCrMo
and Ti6Al4V, Ti6Al7Nb alloys were analysed by AES.
The thicknesses of the thin oxide films on the Ti6Al4V
and Ti6Al7Nb (primarily of TiO2) were estimated using
AES depth profiling. Ti, O and C Auger peaks were de-
tected in the AES analysis. Figure 7a shows the AES
depth profile through the thin oxide film on the Ti6Al4V
alloy. The estimated oxide thickness was about 7 nm,
consisting primarily of TiO2. The amount of Al2O3 is
below the detection limit (<0.1 % of mass fractions), and
no V was detected on the surface. Figure 7b shows the
AES depth profile of the thin oxide film on the
Ti6Al7Nb with an estimated thickness of 5 nm, primarily
of TiO2 with the small amounts of Al2O3 and Nb oxides
being below the detection limit. Figure 7c shows the
AES depth profile of a thin oxide film on the CoCrMo
alloy. The estimated thickness of this thin oxide film,
primarily a mixture of Cr2O3 and Co oxides, was found
to be about 2 nm.
4.4 Conclusions of preliminary research
The present study established the surface microstruc-
tures of biomaterials for hip and knee endoprostheses, Ti
alloys and CoCrMo alloys. Stable and adherent thin
oxide films protect the Ti alloys from intergranular and
crevice corrosion attack and provide excellent biocom-
patibility and osteointegration of Ti alloys. The micro-
structures and mechanical properties of the Ti6Al4V
alloys are very dependent on their thermomechanical
processing treatments, especially homogenization heat
treatments. The AES results showed that thin oxide films
on: a) Ti6Al4V are primarily a mixture of TiO2 with a
small amount of Al2O3, while the V is depleted the
estimated thickness is 6 nm, b) Ti6Al7Nb is primarily a
mixture of TiO2 with a small amount of Al2O3 and Nb2O5
of estimated thickness 5 nm and c) CoCrMo is a mixture
of Co and Cr oxides, primarily of Cr2O3 with small
amounts of Co3O4 and MoOx, thickness was estimated to
be 2 nm. We need to keep an optimal microstructure
regarding corrosion and mechanical properties, which
can be controlled through processing parameters and be
standardized in the near future. The physical and chemi-
cal processes on the surface will be studied in detail to
prevent the adhesion of bacteria and formation of biofilm
to determine the implants’ longevity.
5 CONCLUSIONS
The implantation of an artificial hip or knee is one of
the most successful surgical procedure in orthopaedic
surgery. With further research, including the analysis of
the surface of biocompatible materials, monitoring
novelties and carefully and selectively integrating them
into everyday clinical practice, we want to improve the
performance of these interventions.
Acknowledgement
This research was financially supported by the Ter-
tiary project of the University Medical Center Ljubljana
UKCLJ20160146 and by Slovenian Research Agency
ARRS-0206-0132. Authors acknowledge A/Prof. Dr.
Igor Beli~ for valuable discussions.
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