![Transmission electron microscopy (TEM) investigations of the interface of a DLC coating on a CoCr substrate. Both the TEM image (a), high resolution TEM image (b) reveal the presence of a metal carbide, which could lead to delamination when exposed to the environment of the body [139]. Reprinted by permission from Acta Materialia, Elsevier.](https://www.researchgate.net/publication/360285479/figure/fig2/AS:1159302446350342@1653410650911/Transmission-electron-microscopy-TEM-investigations-of-the-interface-of-a-DLC-coating_Q320.jpg)
![A comparison of the LC 2 values [36,45,51-54,134] (a) and specific wear rates from ball/pin-on-disc evaluations [46,49,51-54,129,131,134-134,173,179,183] (b) found for coatings currently being researched. The reported values are divided into groups based on the main constituents of the coating.](https://www.researchgate.net/publication/360285479/figure/fig3/AS:1159302450561033@1653410651117/A-comparison-of-the-LC-2-values-36-45-51-54-134-a-and-specific-wear-rates-from_Q320.jpg)
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Current status and future potential of wear-resistant coatings and
articulating surfaces for hip and knee implants
Charlotte Skj€
oldebrand
a
, Joanne L. Tipper
b
, Peter Hatto
c
, Michael Bryant
d
, Richard M. Hall
d
,
Cecilia Persson
a
,
*
a
Uppsala University, Department of Materials Science and Engineering, Uppsala, Sweden
b
University of Technology Sydney, School of Biomedical Engineering, Sydney, Australia
c
Ionbond UK Ltd, Consett, United Kingdom
d
University of Leeds, Department of Mechanical Engineering, Leeds, United Kingdom
ARTICLE INFO
Keywords:
Joint implants
Coatings
Surface layers
Ceramics
Biomaterials
Wear resistance
ABSTRACT
Hip and knee joint replacements are common and largely successful procedures that utilise implants to restore
mobility and relieve pain for patients suffering from e.g. osteoarthritis. However, metallic ions and particles
released from both the bearing surfaces and non-articulating interfaces, as in modular components, can cause
hypersensitivity and local tissue necrosis, while particles originating from a polymer component have been
associated with aseptic loosening and osteolysis. Implant coatings have the potential to improve properties
compared to both bulk metal and ceramic alternatives. Ceramic coatings have the potential to increase scratch
resistance, enhance wettability and reduce wear of the articulating surfaces compared to the metallic substrate,
whilst maintaining overall toughness of the implant ensuring a lower risk of catastrophic failure of the device
compared to use of a bulk ceramic. Coatings can also act as barriers to inhibit ion release from the underlying
material caused by corrosion. This review aims to provide a comprehensive overview of wear-resistant coatings
for joint replacements –both those that are in current clinical use as well as those under investigation for future
use. While the majority of coatings belong predominantly in the latter group, a few coated implants have been
successfully marketed and are available for clinical use in specific applications. Commercially available coatings
for implants include titanium nitride (TiN), titanium niobium nitride (TiNbN), oxidized zirconium (OxZr) and
zirconium nitride (ZrN) based coatings, whereas current research is focused not only on these, but also on
diamond-like-carbon (DLC), silicon nitride (SiN), chromium nitride (CrN) and tantalum-based coatings (TaN and
TaO). The coating materials referred to above that are still at the research stage have been shown to be non-
cytotoxic and to reduce wear in a laboratory setting. However, the adhesion of implant coatings remains a
main area of concern, as poor adhesion can cause delamination and excessive wear. In clinical applications zir-
conium implant surfaces treated to achieve a zirconium oxide film and TiNbN coated implants have however been
proven comparable to traditional cobalt chromium implants with regards to revision numbers. In addition, the
chromium ion levels measured in the plasma of patients were lower and allergy symptoms were relieved.
Therefore, coated implants could be considered an alternative to uncoated metal implants, in particular for pa-
tients with metal hypersensitivity. There have also been unsuccessful introductions to the market, such as DLC
coated implants, and therefore this review also attempts to summarize the lessons learnt.
1. Introduction
A damaged or diseased joint can cause severe pain and limited
mobility, which impairs the quality of life of the afflicted individual. To
treat this condition, it might be necessary to replace the joint with an
implant in a surgical procedure. Two of these types of implants, namely
hip and knee implants, will be the focus of this review. The number of
primary, or first time, total hip surgeries performed every year has
steadily increased and is predicted to continue to rise [1] as the ageing
population increases, and indications are that younger patients can also
benefit from these procedures. While hip joint implants have a generally
high survival rate of approximately 95% or more at 10 years for all
* Corresponding author.
E-mail address: cecilia.persson@angstrom.uu.se (C. Persson).
Contents lists available at ScienceDirect
Materials Today Bio
journal homepage: www.journals.elsevier.com/materials-today-bio
https://doi.org/10.1016/j.mtbio.2022.100270
Received 7 January 2022; Received in revised form 9 April 2022; Accepted 24 April 2022
Available online 30 April 2022
2590-0064/©2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Materials Today Bio 15 (2022) 100270
currently used material combinations, metal-on-polyethylene (MoP)
being the most common, [2–5]. However, the increasingly active, ageing
and younger populations require longer lasting implants and a focus of
current research is on prolonging the lifespan of implants to spare pa-
tients from further pain and revision surgeries. Similarly, the revision
rates for knee implants are reported to range from 4.3% [5] to 5.3% [4]at
10 years.
The primary cause of revision surgery for metal or ceramic hip
replacement components paired with polyethylene (PE) is aseptic loos-
ening of the implant (28–51% [2–10]). In these, one of the main causes
for this is believed to be the presence of wear debris from the articulating
surfaces, mainly polymeric particles. However, it should be noted that PE
wear has decreased dramatically since the introduction of highly cross-
linked polyethylene (XLPE) 20 years ago [11]. Necrosis, pseudo tumours
and pain have also been found to be the cause of revision surgeries
particularly in alternative metal on metal bearing systems. These com-
plications are believed to be caused by metallic particulate and soluble
(ionic) debris originating from the articulating and non-articulating
surfaces of the implant [12–14]. While the particulate debris may orig-
inate from any surface of the implant, it is primarily formed at the
articulating interface.
The most common materials currently implanted in this context are
cobalt chromium alloy (CoCr) and highly crosslinked polyethylene
(XLPE) in MoP implants [3,4,9,10], together with a zirconia/alumina
ceramic combined with PE (CoP) [4,10]. In these cases, the dominant
particulate debris is that of the polymer, which can range between 10 nm
and 1 mm in size. Particles in the size range 0.1–1.0
μ
m are believed to
play an important role in the activation of macrophages [15], which
might initiate a cascade of reactions that eventually cause wear-induced
osteolysis (loss of bone), possibly resulting in implant loosening [15–19].
Metal-on-metal (MoM) hip re-surfacing implants were considered an
alternative for young patients, however, the implants experienced high
short-term failure rates and an increased rate of revision surgeries [20].
Due to these catastrophic outcomes MoM hip replacements are now
rarely used [2,4,5,8]. Debris from MoM implants is generally in the
nanoscale range (<100 nm) and there is evidence to suggest that metal
particles in this size range and ions can cause a variety of biological ef-
fects such as hypersensitivity, pseudo-tumours [12,14,21–25] and
aseptic lymphocytic vasculitis-associated lesions [13]. In addition high
concentrations of cobalt and chromium ions are believed to cause e.g.
acute visual and auditory impairment, peripheral neuropathy and car-
diomyopathy [26]. It should be noted that the latter are extreme cases,
and the incidence of adverse reactions to metal debris has been found to
be less than 1.2% for MoM hip implants [27]. The revision per 1000
prosthesis years caused by adverse reactions to particulate debris for
MoM was found to be 9.90, which can be compared to 0.19 for MoP [5].
Several different strategies have been explored for reducing wear and
the negative biological effects of metal ions and PE wear debris. Among
the most noteworthy of these are the improvement of polymer wear
properties [11,28] and the development of e.g. textured surfaces [29].
However, this review will focus on the use of ceramic coatings or surface
modifications, such as nitriding, to reduce wear and ion release. These
aim to combine the advantages of a wear resistant ceramic with the
ductility and toughness of a metal. The ceramic coating can also act as a
barrier to, and decrease, ion release from the underlying metal [30].
Previous reviews [31,32] have focused on tantalum, graphite-like
carbon (GLC) [33,34], diamond-like carbon (DLC) [35–38], titanium
nitride based coatings [39–43] and chromium nitride (CrN) [44,45].
However, a number of novel experimental coatings have been reported
(e.g. silicon nitride (SiN) [46–53], multilayer structured coatings [42,44,
54]) as well as commercially available coatings (oxidized zirconium
(OxZr) [55] and titanium nitride (TiN) [56,57]), which are utilised in
interfaces of joint prostheses, and currently, there is no comprehensive
review available. This review aims to provide readers with a compre-
hensive overview of coatings that are (i) commercially available and (ii)
currently being researched, and to summarize the current status and
future potential of such coatings for hip and knee joint implants. We
focus on coatings and treatments for articulating surfaces, and as such,
other types of coatings, e.g. those for improved bone ingrowth and fix-
ation lie beyond the scope of this review.
2. Methodology
The material in this review is based on published research articles,
patents and product information in order to cover all phases from early
stage research to products in clinical use. The research articles were
found using the database SCOPUS with the primary search terms “wear”
AND “coatings OR film”AND “joint implant”for coatings and the search
terms “wear”AND “surface modification”AND “joint implant”for sur-
face modifications. An additional search with the terms “biocompati-
bility”AND “coatings OR film”AND “joint implant”was conducted to
include studies regarding the biocompatibility and finally, through ref-
erences, an additional 6 papers were identified. The patents were found
by using the same search terms on Espacenet, and the products were
found using Google (with search terms “joint replacement”“products”
“coating”). Research papers, patents and products concerning dental
implants, osteoinductive/conductive coatings, protein and other bio-
logical coatings as well as bulk materials were excluded and the joint
types were limited to hip and knee. This left a total number of 89 research
papers (27 of them being case reports or revision studies) and 4 patents.
Information such as wear rates and surface roughness was compiled in a
table that may be found in the supplementary information. Therefore,
some of the references are not included in the main paper but can be
found in the supplementary information. Fig. 1 shows a flow diagram of
the procedures followed in selecting the reports and associated data for
inclusion in this review.
3. Substrate material and deposition methods
A coating used for joint implants should be hard, wear resistant,
corrosion resistant, biocompatible, not release particulates and any par-
ticulates that are generated should be biocompatible. In addition, the
coating should have a low surface roughness and a good adhesion to the
substrate material. Some of these properties are specified in standards
such as ISO 7206-2 while others, such as wear resistance, have no
specified target value. The important properties and what to consider are
specified in Table 1. It should be noted that the table distinguishes be-
tween wear resistance and corrosion resistance, however theses are
processes that occur synergistically. Typically, these properties are
however reported separately.
These properties are covered in more detail in the supplementary
information section.
Ceramic coatings are typically deposited onto a metal implant using a
wide range of techniques [74–76]. Possible substrate materials include
the metals commonly used as bulk materials for joint implants i.e. CoCr
and stainless steel but also titanium. Titanium is not used in articulating
surfaces due to its poor wear properties [77–79], but this becomes
possible with a coating or surface treatment that improves such proper-
ties [75]. The use of titanium as a substrate material could even be ad-
vantageous in terms of coating adhesion [80]. It is however important to
keep in mind that the coating should be compared with the material it
aims to substitute, e.g. in the case of articulating surfaces typically CoCr.
The choice of deposition method or surface treatment will have a
strong influence on the above-mentioned parameters. The most common
deposition techniques used for coatings for articulating surfaces can be
divided into either physical vapor deposition (PVD) methods [81]or
chemical vapor deposition (CVD) methods [82], the former group is more
commonly used than the latter in the studies found in this review. These
deposition techniques, as well as their advantages and challenges, have
been covered in detail in previously published articles [82–86]. PVD
techniques include, amongst others, vacuum evaporation, magnetron
sputtering (MS), reactive magnetron sputtering (rMS), pulsed laser
C. Skj€
oldebrand et al. Materials Today Bio 15 (2022) 100270
2
deposition (PLD) and high-power impulse magnetron sputtering
(HiPIMS). They utilise metallic sources that are either evaporated, typi-
cally using an electron beam, or sputtered with the help of a plasma to
produce a flux of metal atoms/ions which is deposited onto the surface to
be coated in the presence of one or more reactive gases to form a layer of
ceramic material. Coatings deposited using PVD techniques typically
have a low surface roughness, are hard and wear resistant. There are
however limitations due to the technique being line-of-sight but these are
usually mitigated by manipulation of the substrate during deposition.
The second group of methods, CVD, entail exposing the substrate to a
mixture of gases that react at high temperature (typically >600 C) to
form a ceramic compound, though it is possible to use a plasma to
enhance the reactivity of the gas precursors (plasma-enhanced CVD
(PECVD)) to reduce the deposition temperature. Using CVD techniques it
is possible to grow uniform, well adhering coatings on complicated
substrates. However, there are limitations related to e.g. heat resistance
of the substrate. Another approach is to treat the surface by e.g. heat or
laser in the presence of gases such as nitrogen [87]. This often yields an
increased hardness and wear resistance, however the surface roughness
is often increased and the techniques are limited to materials that can
form suitable ceramic surface layers such as zirconium oxide and tita-
nium nitride.
Another family of surface deposition techniques is thermal spraying
in which heated or melted material is sprayed onto a surface [88]. The
coating material is either in the form of powder or a filament that is
typically melted by flame or plasma [75].
A harder surface can also be achieved by surface treatment processes
such as plasma electrolytic oxidation where metals are oxidized through
a process similar to anodization but with higher potentials [89].
A technique with future potential is additive manufacturing where it
is theoretically possible to manufacture implants with a surface layer
with different properties compared to the underlying material. However,
the manufactured materials currently require extensive post processing
to achieve smooth surfaces suitable for wear-resistant applications [90].
The most important properties relevant to coatings for joint implants,
as well as the methods used in their evaluation, are discussed in the
supplementary information section. It should be noted however that
somewhat different requirements exist depending on joint application,
e.g. hip joints may experience more complex movement patterns, as well
as more severe edge loading [91] than knee joints [92], and different
standards are available to evaluate their performance.
4. Commercial coatings
There are currently three types of commercially available coated or
surface treated hip and knee joint implants for clinical applications,
namely:
Titanium nitride and titanium niobium nitride (TiN and TiNbN) by
various companies including:
Implantcast [56].
Cellumed [93].
OHST medical technology [94].
Link orthopaedics [95].
Corin [96,97].
Zirconium nitride (ZrN):
Aesculap (B Braun) [98].
Oxidized zirconium (OxZr)
Smith and Nephew [99].
TiN coated implants are available with either a Ti6Al4V [100]ora
CoCr [101] substrate and the coated surface is usually paired with PE in
the articulating surfaces. These implants are aimed for young, active
patients, in particular patients with metal sensitivity [102] and studies
have shown them to be stable over time [103].
As TiN, ZrN and OxZr are commercially available it is possible to
study patient outcomes (Table 2). Since their introduction, long-term
follow up studies have found that OxZr implants perform well, with
low revision rates [55], comparable to uncoated CoCr implants [104,
105](Table 2). The implant has performed better, with lower rates of
corrosion and fretting wear, in comparison to CoCr, as seen in retrieved
implants [106]. Taking all these results together Oxinium implants show
great promise as a viable option for joint implants, not just for patients
suffering from or at risk of developing metal hypersensitivity but also for
otherwise healthy patients.
However, there have been cases with both TiN and OxZr in which
retrieved implants were reported to exhibit surface damage, including an
exposed substrate [100–103,108]. While the femoral heads in these
studies have been paired with a PE liner the oxidized surface has shown
damage and metallic transfer likely caused by contact with the metallic
shell following dislocations. The authors concluded that Oxinium
femoral heads should not be used in patients at risk of joint instability.
Fig. 1. Aflow chart showing how the data was collected and treated. The supplementary information contains additional references such as ISO and ASTM standards,
studies on background such as biological reactions to debris, and information regarding deposition techniques.
C. Skj€
oldebrand et al. Materials Today Bio 15 (2022) 100270
3
5. Potential candidate wear-resistant coatings
Several coatings are being investigated for their potential to reduce
wear in joint implants. The following section will examine the published
research, categorized into six groups according to coating composition:
diamond-like carbon, silicon nitride, chromium nitride, zirconium based,
titanium based and tantalum based. A summary of the properties can be
found in Table 3 (hardness, Young's modulus and adhesion) and Table 1
in supplementary information (surface roughness and wear properties).
The properties vary, with hardness ranging from 8 to 44 GPa and Young's
modulus from 100 to 466 GPa. The tribological properties such as wear
rates were obtained using different set-ups, with differences in contact
pressure and counter surface, leading to large inherent variation between
samples, making results difficult to compare.
5.1. Diamond-like carbon and nano-crystalline diamond
The term diamond-like carbon (DLC) covers a range of hard carbon-
based materials with a wide range of properties such as hardness and
wear resistance. The variety of structures and consequently properties for
carbon is explained by its ability to exist in three hybridizations (sp
1
,sp
2
and sp
3
). DLC coatings have a significant fraction of sp
3
bonds [137].
However, it is important to keep in mind that DLC coatings do not consist
solely of amorphous carbon (a-C) but also hydrogenated amorphous
carbon (a-C:H) alloys and the structure of the coating will largely depend
on the deposition method. For example sputtered coatings typically can
extend from sp
2
to sp
3
and plasma-enhanced CVD produced coatings
have a higher fraction of hydrogenated carbons [137].
DLC coatings have been considered for joint implants because of the
promise of a chemically inert, hard, wear resistant surface and significant
research has been conducted on DLC-coated implants industrially; e.g. in
2001 the company Implant Design AG put forward a DLC-coated knee
implant but had to withdraw it the same year after it was banned by the
Swiss Federal Office of Public Health (SFOPH). The implants failed due to
excessive wear caused by partial delamination of the coating, which led
to early revisions [128,138]. Since then research has focused on under-
standing the mechanism behind the failures as well as improving the
coating adhesion. Falub et al. observed an interlayer measuring
approximately 5 nm in thickness that occurred gradually between the
CoCr implant and DLC coating. This interlayer consisted mainly of car-
bides with an overall stoichiometry close to Me
2
C and delamination is
believed to be caused by the instability of Co carbides in this interlayer
(Fig. 1)[139]. The mechanism is believed to be stress-corrosion cracking,
i.e. delayed failure due to environmentally induced crack propagation
[140].
Attempts to improve the adhesion by using interlayers was made by
Thorwarth et al. [37] in a study whereby tantalum (Ta) was used as an
interlayer for a DLC coating. The results were promising, with little
noticeable wear for DLC coatings with Ta interlayers in the case of low
concentrations of oxygen impurities. However, it was hypothesised that
oxygen contamination would lead to an increased occurrence of the
β-phase in the Ta interlayer, which could lead to mechanical failure due
to its brittleness. Wang et al. on the other hand proposed the introduction
of a fullerene-like structure and incorporating fluoride (F-FLC) to obtain
long-term stability in in vitro environments [132]. The results revealed
coatings with lower coefficient of friction and wear compared with DLC
as well as promising in vitro results with e.g. cell adhesion of rabbit bone
marrow mesenchymal stem cells. In addition Cr has been used to in an
attempt to improve adhesion and these coatings exhibited improved
corrosion resistance compared with CoCr [141].
The closely related nanocrystalline diamond coatings (NCD) consist
of nano-sized diamond crystals in the range of 3–15 nm with a large
fraction of amorphous carbon at the grain boundaries [142]. These
coatings have similar wear properties to DLC coatings and have been
shown to reduce the wear compared to uncoated Ti6Al4V components
[143]. However, as previously mentioned CoCr is the common choice for
articulating surfaces due to its superior wear properties, and a compari-
son to this material is lacking. Cell studies using primary human bone
marrow cells have shown cell attachment, spreading and proliferation on
the coatings, i.e. non-cytotoxicity. The available biocompatibility studies
however seem to focus on osteoblasts, which may be more suitable for
coatings aimed at bone ingrowth [144–146].
After investigating and addressing the risk of delamination in DLC
coatings the use of interlayers could make them a viable option for joint
implants as observed from the low wear rates (Table 1 in supplementary
information section) [38,129,147].
5.2. Silicon nitride
Bulk silicon nitride, Si
3
N
4
, and related materials have been used in
applications such as combustion engines due to high temperature and
Table 1
The important properties of coatings for joint implants, their evaluation methods
and their target profile.
Property Target profile Typical method for evaluation
and standards
Hardness A high hardness will help
mitigate wear. However, the
stiffness should also be
considered as the ratio of
hardness to Young's modulus
gives a measure of the elastic
limit in the contact and hence
provides an indicator of wear
performance.
Nano- or microindentation.
Values are given in Pa or Vickers
hardness number (HV).
ASTM E2546-07 [58]
ISO 14577-4 [59]
Wear
resistance
A low wear rate is desirable,
but actual values will depend
on the specific tribological
situation and are therefore not
specified here. Attention
should be paid to the generated
wear debris, the size, shape and
volume will likely influence the
immune response in the final
application.
Tribological set-up ranging for
pin-on-disc to joint simulators.
The resulting wear is measured as
specific wear rate (mm
3
/Nm) or
mass loss per million cycles (mg/
Mc).
EN 1071-12 [60]
EN 1071-13 [61]
ASTM F732 [62]
ISO 20808 [63]
Corrosion
resistance
A coating should protect the
underlying metal from
corrosion as well as have a low
rate of degradation. However,
it is also important to consider
the character of the particles
and ions that inevitably are
released.
Measuring open circuit potential
(V) and corrosion current (
μ
A).
ASTM G5 [64]
ISO 16429 [65]
ISO 16773 [66]
Toxicity Ultimately the coating, and
more importantly the ions and
wear debris, should not elicit
an adverse immune response.
The toxicity will depend on the
volume of debris or ions, i.e. a
dose dependency, and the
volume will depend on the
wear properties of the coating.
In vitro studies using cell lines.
Results are given as cell viability.
ISO 10993 [67]
Surface
roughness
A smooth surface is necessary
to reduce PE wear. An R
a
value
of 20 nm has been specified
for ceramics in ISO 7206-2.
Optical or stylus methods for
surface characterization. The
most common parameter to report
is the average surface roughness,
Ra, (nm).
ISO 4287 [68,69]
ISO 4288 [70]
ISO 25178-604 [71]
Adhesion A coating that adheres well to
the substrate is of utmost
importance as delamination of
the coating could cause
excessive wear through the
release of abrasive debris. It is
important to consider factors
such as time and corrosive
environments when evaluating
the adhesion.
Most common methods are
scratch tests, from which critical
loads are obtained (N), or
Rockwell indentations that are
categorized according to a
standard.
ISO 26443 [72]
ISO 20502 [73]
C. Skj€
oldebrand et al. Materials Today Bio 15 (2022) 100270
4
Table 2
Revised implants and follow-up studies of coated implants. In the case of revision-retrieved samples one must be aware of the fact that they are failures and may depart
from the general performance of the cohort.
Coating Implant Product Number
of coated
implants
Average time of
implantation/
follow-up [months]
Revison
rate
Reason for revision/
retrieval
Key findings Reference
TiNbN Hip –1 53 n.a. Aseptic loosening No signs of metallosis. Coating
failure due to insufficient
adhesion, corrosion, and
involvement of third bodies.
Łapaj et al.,
2016 [112]
TiN Knee Implantcast ACS
and Corin Uniglide
5 16 Retrieval
study
(100%)
Aseptic loosening: 4 TiN coatings of knee
replacements undergo wear and
degradation related to presence
of third bodies and microscopic
defects on their surface.
Łapaj et al.,
2020 [113]Periprosthetic
inflammation: 1
TiNbN Knee –59 36 0 n.a. Chromium concentrations in
patient plasma increased from
0.25 to 0.75
μ
g/l in the coated
TKA group compared with of
0.25–1.30
μ
g/l in the standard
TKA group.
Postler et al.,
2018 [114]
TiN Knee ACS®MB system,
Implantcast
25 30.7 Infection: 11
Septic loosening: 4
Ligament instability: 2
Pain: 2
Aseptic loosening: 1
Dislocation: 3
Fracture: 1
Recurrent effusion: 1
TiN provides low wear rates and
little surface damage
Fabry et al.,
2017 [115]Retrieval
study
(100%)
TiN Knee B-P™knee system 1031 46 2.2% Malpositioning of tibial
component: 6
Traumatic event: 5
Pain (Isolated
patellofemoral,
instability and
arthrofibrosis): 11
Infection: 2
TiN coated total knee
replacements perform up to par
with conventional implants, but
does not solve the problem with
residual pain.
Breugem
et al., 2017
[116]
TiN Knee ACS®Basic,
Implantcast
51 62 5.8% Aseptic loosening: 2
Layer spinout: 1
No difference between coated
and conventional implants.
van Hove
et al., 2015
[117]
TiN Knee B-P™knee system 61 33 n.a. n.a. TiN coated implants showed a
high degree of satisfaction and
less intraoperative bone mass
removal compared to NexGen-
LPS implants.
Moon et al.,
2012 [118]
TiN Hip B-P™Integrated
Hip system,
Endotec
1 12 n.a. Unrelated causes Well-functioning implant, close
future monitoring needed
Harman
et al., 1997
[111]
OxZr Hip Oxinium, Smith &
Nephew
3 7 Retrieval
study
(100%)
Dislocation The Zr substrate may deform in
the case of dislocation because
of its low hardness
Kop et al.,
2007 [110]
OxZr Hip Oxinium, Smith &
Nephew
1 0.5 Retrieval
study
(100%)
Dislocation Damage to the ZrO
2
coating and
exposed Zr substrate after
dislocation
Evangelista
et al., 2007
[107]
OxZr Hip Oxinium, Smith &
Nephew
56 30 n.a. Not revised 2D wear analysis of radiographs
show reduced wear of oxinium
femoral heads compared to CoCr
Garvin et al.,
2009 [55]
OxZr Knee Genesis II, Smith &
Nephew
98 74.4 0% Not revised Survivorship of 98.7% at 7 years Innocenti
et al., 2010
[119]
OxZr Knee Oxinium, Smith &
Nephew
11 18.5 Retrieval
study
(100%)
Stiffness: 7
Infection: 1
Instability: 1
Mal-positioning: 2
Loosening: 1
Lower damage of both the OxZr
femoral component and PE tibial
component for Oxinium
compared to CoCr.
Heyse et al.,
2011 [120]
OxZr Hip Oxinium, Smith &
Nephew
1 48 h Retrieval
study
(100%)
Correction of leg length
discrepancy
Extensive PE wear, loss of the
ZrOx layer and Ti transfer from
the acetabular shell.
McCalden
et al., 2011
[108]
OxZr Hip Oxinium, Smith &
Nephew
60 24 n.a. Not revised Further follow-up needed to be
able to discern differences
between CoCr and Oxinium
Kadar et al.,
2011 [105]
OxZr Knee Oxinium, Smith &
Nephew
16 16.4 Retrieval
study
(100%)
Stiffness, infection,
instability and
dislocation
Wear comparable to
conventional MoP implants
Heyse et al.,
2011 [121]
OxZr Knee Oxinium, Smith &
Nephew
109 70.8 n.a. Not revised OxZr is an attractive option for
patients with metal sensitivity
Hofer et al.,
2014 [122]
(continued on next page)
C. Skj€
oldebrand et al. Materials Today Bio 15 (2022) 100270
5
wear resistance [148], and components made of Si
3
N
4
have been used in
several applications subjected to wear, e.g. bearings [149–152]. It is also
used in biomedical applications such as spinal implants [153].
The main advantage of silicon nitride, SiN
x
, as a coating for joint
implants has been shown to be, not only its ability to reduce wear, but
also to minimize the adverse immune response to released ions and
particles. SiN
x
dissolves in aqueous solutions into only biocompatible
elements [154], which could mean that the generated wear particles
would dissolve in the body without triggering the immune system
response that would eventually lead to bone resorption. It should be
noted however that ammonia is formed during the dissolution of SiN
x
,
which may result in an elevated pH. However, this has been found to be
beneficial in terms of antibacterial properties [155,156]. Further, there
needs to be a compromise between the dissolution of the wear particles
and the requirement of a coating that provides sufficient performance for
the intended period of use i.e. significantly greater than 20–25 years.
SiN
x
coatings have been manufactured both through PVD and CVD
methods with a wide range of hardness (12–24 GPa) [47,49,50,53] and
Young's moduli (173–293 GPa) [47,49,50]. Specific wear rate measure-
ments through pin-on-disc setups have found reduced wear compared to
CoCr [49,53]. The generated wear debris was investigated in a study
where a ball-on-disc set-up was used to produce the same, which was
found to lie in the range of 0.01–0.05
μ
m[157]. It was noticed that the
debris agglomerated and that these agglomerates were in the range of
0.15–1.96
μ
m, while the individual particles measured between 0.01 and
0.05
μ
m in size. It was also found that the pH increased from 7.45 to
around 8 after 20 days, likely due to the formation of ammonia [158,
159], as mentioned above.
Biocompatibility studies have mainly been focused on simulated wear
particles, often commercially available alternatives. The particles, both
Table 2 (continued )
Coating Implant Product Number
of coated
implants
Average time of
implantation/
follow-up [months]
Revison
rate
Reason for revision/
retrieval
Key findings Reference
and patients in risk of high rates
of wear (due to young age or
high activity levels).
OxZr Knee Oxinium, Smith &
Nephew
98 135.6 2.3% Loosening. Survival rate of OxZr of 97.8% at
10 years.
Innocenti
et al., 2014
[123]
OxZr Knee Oxinium, Smith &
Nephew
71 62 n.a. No revision for
loosening.
OxZr comparable to the
standard knee prosthesis but
further follow up needed.
Park et al.,
2014 [124]
OxZr Hip Oxinium, Smith &
Nephew
60 60 n.a. Not revised. Radiostereometric analysis was
used to determine OxZr was
comparable to, but not better
than, CoCr.
Jonsson
et al., 2015
[104]
OxZr Hip Oxinium, Smith &
Nephew
11 8.64* Retrieval
study
(100%)
Aseptic loosening: 14*
PE wear/osteolysis: 17*
Fracture: 1*
Instability: 5*
Infection: 10*
Malposition: 4*
Multiple reasons: 1*
No difference between OxZr and
CoCr femoral heads with regards
to fretting and corrosion,
however bulk ceramic
performed better than both OxZr
and CoCr.
Tan et al.,
2016 [106]
*Both bulk ceramic
and OxZr
*Both bulk ceramic and
OxZr
OxZr Hip Oxinium, Smith &
Nephew
3 57.3 Retrieval
study
(100%)
Pain, hip squeak and
limited movement.
Misuse of Oxinium heads
(pairing Oxinium femoral heads
with alumina liners) caused
damage to the coated surface
and high wear rates.
Ozden et el.
2017 [109]
OxZr Knee Oxinium, Smith &
Nephew
5969 144 7.7% Infection, loosening or
lysis, patellofemoral
pain, pain and
instability the most
common reasons for
revision.
The cumulative revision risk was
higher for Oxinium than CoCr
(7.7% and 4.8% respectively).
Loosening/lysis was the reason
for revision in 1.1% of cases.
Vertullo
et al., 2017
[125]
OxZr Knee Oxinium, Smith &
Nephew
10,477 156 0.46% Infection (the only
reason investigated)
Overall same risk of infection for
OxZr as CoCr.
Vertullo
et al., 2018
[126]
ZrN Knee Aesculap 1 18 Not
revised
n.a. The wound healed without
complications and the patients
eczema as well as the knee pain
had disappeared at the last
follow up of 18 months.
Thomsen
et al., 2011
[127]
ZrN, TiN
and
TiNbN
Knee Implantcast,
AlphaNorm (now
aquired by Corin),
Mathys, Link and
Aesculap
28 TiN(CoCrMo): 42
TiNbN(CoCrMo):
40.8
ZrN(CoCrMo): 7.8
TiN(Ti6Al4V): 114
TiNbN(Ti6Al4V): 12
Infection: 12
Aseptic loosening: 10
Instability/Luxation: 3
Arthrofibrosis: 1
Periprosthetic fracture:
1
Movement deficit: 1
Herbster
et al., 2020
[80]
DLC Hip Adamante®,
Biomecanique
101 110.4 25.8% Aseptic loosening: 41
Ossification: 1
Pain: 2
Infection: 1
Implant failure: 1
54% survival for DLC/PE
implants at 8.5 years compared
to 88.2% for Al
2
O
3
/PE implants.
Delamination of the coating
caused aggravated wear of the
PE liner.
Taeger et al.,
2003 [128]
C. Skj€
oldebrand et al. Materials Today Bio 15 (2022) 100270
6
micron-scale and nanoscale, did not give rise to any significant release of
proinflammatory cytokines in a study with primary human peripheral
blood mononuclear cells [160]. Bulk Si
3
N
4
has also been found to show
no cytotoxic effects [161,162] but instead antiviral properties [163].
The aforementioned dissolution behaviour of the coatings was
investigated in simulated body fluid (25 vol% foetal bovine serum
Table 3
Hardness and Young's modulus, as obtained with nanoindentation, and adhesion test values for the reviewed coatings.
Coating (substrate) Deposition technique H [GPa] E [GPa] Adhesion from scratch Test
[N]
a
Reference
DLC (CoCr) Unbalanced MS 13 100 Guo et al., 2015 [129]
DLC (CoCr) PECVD 24 Thorwarth et al., 2010 [130]
DLC (cemented carbide) Enhanced cathodic arc MS 16.7 166 Wang et al., 2015 [131]
F-FLC (Si) PECVD 16.41 132.65 Wang et al., 2020 [132]
SiN
x
(CoCr) HiPIMS 12–26 173–293 Skj€
oldebrand et al., 2017
[47]
SiN
x
and SiN
x
C
y
(CoCr and Si) HiPIMS 18 200 Pettersson et al., 2013 [49]
SiN
x
(CoCr and Si) RF MS 18–24 0–7 Olofsson et al., 2012 [53]
SiNO and F:SiCN (CoCr) Unbalanced MS 15 236 Shi et al., 2012 [50]
SiN
x
(CoCr) HiPIMS 14–88 Filho et al., 2019 [52]
SiN
x
, SiCN, SiCrN and SiNbN
(CoCr)
HiPIMS 13–25 148–286 Filho et al., 2019 [51]
SiN
x
(CoCr) HiPIMS Filho et al., 2020 [46]
TiCN (Ti6Al4V) Cathodic arc deposition 8–10 S
aenz de Viteri et al., 2015
[41]
TiN (CoCr) MS 21–23 45–70 Gallegos-Cantú et al., 2015
[42]
Multilayered TiN/CrN (CoCr) MS 8.0–13.5 50–70 Gallegos-Cantú et al., 2015
[42]
TiN (cemented carbide) Enhanced cathodic arc MS 23.6 397 Wang et al., 2015 [131]
TiAlN (cemented carbide) Enhanced cathodic arc MS 27.3 466 Wang et al., 2015 [131]
Multilayered TiAlN (Ti6Al4V) Closed field unbalanced magnetron sputter ion
plating
18.8–44.1 302.6–516.5 17-7-47.7 Yi et al., 2016 [54]
TiN (Ti or Ti6Al4V) Laser nitriding 997-1099
HV
Chan et al., 2017 [43]
Nitrated TNZT Laser nitriding 14 171 (Er) Chan et al., 2016 [133]
TiC (steel) PECVD 829-1500
HV
40–70 Vitu et al., 2008 [134]
CrN/NbN (CoCr) HiPIMS 34 447 50–100 Hovsepian et al., 2016
CrN (cemented carbide) Enhanced cathodic arc MS 17.9 422 Wang et al., 2015 [131]
CrN (CoCr) Plasma nitriding 12–19 Liu et al., 2013 [135]
CrCN (CoCr) Plasma carbonitriding 16–18 Liu et al., 2013 [135]
CrN and Cr2N (CoCr) Plasma nitriding 660-900 HV Wang et al., 2010 [136]
CrN/NbN (Stainless steel 304) MS 28 390 0.02 Huang et al., 2017 [45]
CrAlTiN (Stainless steel 304) MS 33 450 30.4 Huang et al., 2017 [45]
Multilayer TaC and Ta
2
C (CoCr) Thermal
treatment in molten salts
24–37 250–316 LC3: 11-48 Balagna et al., 2012 [36]
TaN(CoCr) RF sputtering 15–28 255–319 Corona-Gomez 2021
a
L
C2
according to ISO 20502 unless otherwise indicated.
Fig. 2. Transmission electron microscopy (TEM) investigations of the interface of a DLC coating on a CoCr substrate. Both the TEM image (a), high res-
olution TEM image (b) reveal the presence of a metal carbide, which could lead to delamination when exposed to the environment of the body [139].
Reprinted by permission from Acta Materialia, Elsevier.
C. Skj€
oldebrand et al. Materials Today Bio 15 (2022) 100270
7
diluted in phosphate buffer saline solution) for up to 60 days (Fig. 2). The
coatings successfully reduced the metal ion release by two orders of
magnitude, as compared to uncoated CoCr references. The dissolution
rates of the coatings were lower or comparable to the CoCr (0.2–1.2 nm/
day for SiN
x
coatings compared to 0.7–1.2 nm/day for CoCr) [48]. It
should be noted however, that the dissolution rate will depend on factors
such as composition and density of the coating - an increased nitrogen
content has e.g. found to yield lower dissolution rates [48]. Recent
studies have also found that alloying the SiN coating with Fe and C or Cr
and Nb could give reduced dissolution rates [164].
5.3. Chromium nitride
Similar to other ceramic coatings CrN coatings have been shown to
increase hardness and reduce wear compared to CoCr [44,165], as well
as reducing the release of metal ions. The added advantage of CrN
coatings is the possibility to achieve a CrN surface layer through e.g.
plasma nitriding. Because of its potential for strong adhesion CrN is also
sometimes used as an interlayer between the substrate and a top coating
[46,52].
In a study comparing the performance of CrN with TiN, TiAlN and
DLC, CrN was found to exhibit superior corrosion and wear resistance
[131]. CrN coatings have been subject to additional investigation
whereby coated CoCr femoral heads were paired with PE cups and tested
in a hip simulator for 5 million cycles. The CrN coatings were shown to
produce similar amounts of PE wear under standard conditions compared
to adverse conditions (9.5 mm
3
/mc and 12 mm
3
/mc for standard and
adverse conditions –for the latter alumina particles were introduced to
simulate third body abrasive wear), while the uncoated CoCr head
showed a large increase in wear for the adverse conditions (9.2 mm
3
/mc
and 469 mm
3
/mc under standard and adverse conditions, respectively)
[166]. Another study comparing TiN, CrN, CrCN and DLC coatings in a
hip simulator showed a 36-fold reduction in wear rate for self-mating CrN
and CrCN coatings compared with uncoated CoCrMo MoM implants. In
addition, the ion release was dramatically reduced [165].
Another method of creating a CrN rich surface is through incorpo-
ration of nitrogen into the surface of CoCr using reactive plasma. This
was reported by Wang et al. [136], who exposed CoCr substrates to a
plasma of NH
3
at a constant pressure of 500 Pa for 9 h. The formation of a
CrN and Cr
2
N layer was confirmed by X-ray diffraction (XRD), and
evaluation showed that the nitrided surface was harder and had lower
wear rates compared to the untreated surface when in contact with a
cemented carbide (WC/Co) ball during ball-on-disc wear tests. By using
different plasma gases it is possible to obtain both nitrided and e.g.
carbonitrided surface layers [135]. This was investigated by Liu et al.
[135], who found both plasma nitriding and plasma carbonitriding of
CoCr increased the hardness and wear resistance as compared to un-
treated CoCr. In addition, corrosion resistance was improved for both
treatments, and the carbonitrided surface showed a better corrosion
resistance compared to the nitrided surface.
Reported wear rate values of CrN based coatings have been consis-
tently low (110
6
to 8.810
7
mm
3
/Nm) [44,167] and adhesion tests
have given high critical load values (L
C2
) - up to 50 N for CrN/NbN
deposited on CoCr substrates [44,45]. Overall the low wear rates and
contingency of good adhesion make CrN-based coatings potentially
suitable for joint implants.
5.4. Zirconia and yttria-stabilized zirconia
Zirconia and yttria-stabilized zirconia have been investigated as po-
tential candidate coatings for orthopaedic implants because of their
ability to reduce wear. The number of studies available on these mate-
rials for this application are still limited but the results are comparable to
other investigated coatings [168,169]. These coatings were deposited
onto a substrate such as titanium and should not be confused with the
surface treated Oxinium implants.
The wear properties of yttria-stabilized zirconium dioxide (YSZ)-
coated titanium balls paired with UHMWPE have been evaluated in a
ball-on-disc set-up using different lubricants. The coatings were found to
decrease the wear of the PE under dry conditions and when lubricated
with a NaCl solution. However, when lubricated with a serum solution
i.e. under conditions more closely emulating those of a natural joint, the
results were similar to those obtained for the uncoated Ti spheres [169]
(3–1310
4
mm
3
/Nm for dry conditions and 3–810
4
mm
3
/Nm for
serum solution as a lubricant; the root mean square surface roughness of
the coatings ranged from 28 1nmto605 nm). The coatings did not
exhibit any cytotoxicity when tested with mesenchymal stem cells and
pre-osteoblast cell lines [168], indicating that they are biocompatible
and could be useful as coatings for orthopaedic implants.
One way of achieving a surface oxide layer is through thermal
oxidation, this was conducted by Luo et al. who oxidized a ZrNb alloy. An
increased treatment temperature (of 700 C compared to 500 C)
improved both the hardness and wear resistance [170].
The hydrolytic long-term stability of zirconia-based material remains
a concern however [171,172], and would have to be thoroughly inves-
tigated before being an option for biomedical implants.
5.5. Alloyed and structured titanium nitride and carbide
While titanium nitride has successfully made it to the market
(Implantcast, Cellumed, Link medical technology and Link orthopaedics)
there is still ongoing research aimed to improving these coatings. One
such strategy is to alloy the TiN coating with one or more elements and
another is using a multi-layer structure. These strategies have the po-
tential to further reduce the wear and/or improve adhesion.
Although alloying TiN with elements such as aluminium has been
found to increase the hardness and Young's modulus, and decrease the
wear rate compared to Ti6Al4V, however it should again be noted that
CoCr is more commonly used in articulating surfaces and should be the
material used for comparison [54]. Other investigated alloying elements
include niobium and carbon, which have been shown to be non-cytotoxic
[39]. Titanium carbide has also been shown to reduce wear compared to
uncoated Ti6Al4V when tested in a reciprocating pin-on-disc set-up
[173].
Another strategy for the improvement of wear properties is to deposit
multilayers. The mechanical and tribological properties of such a coating
(TiN/CrN) has been investigated and compared to a TiN monolayer. The
multilayer structures reduced the friction coefficient, however, the wear
rate was not reported [42].
Another option is to create a ceramic surface layer, by e.g. heat-
accelerated diffusion. There are several techniques available for alloy-
ing the surface of a material by exposure to elevated temperatures in a
controlled environment. Another example is the powder immersion re-
action assisted coating (PIRAC). Yet another process whereby nitrogen is
incorporated into the surface is laser nitriding, where the substrate is
irradiated with a laser in a chamber with nitrogen gas. The laser-
illuminated area of the surface will melt and create a plasma above it.
Subsequently the nitrogen, which is now ionized, will be absorbed by the
melted surface. All methods result in increased hardness as well as
increased wear resistance compared with Ti6Al4V. The PIRAC method
and laser melting did however yield an increased surface roughness
compared to an untreated reference [43,133,174–176].
While TiN based coatings offer a possibility to improve the wear
properties of Ti6Al4V, they are in many of the published current studies
compared only to uncoated Ti6Al4V, which makes it challenging to
assess the efficacy of these proposed coatings.
5.6. Tantalum carbide, tantalum oxide and tantalum nitride
Porous tantalum is used in orthopaedic applications such as cranio-
plasty plates and hip implant fixation due to its osseintegrative properties
[177]. Proven to be non-cytotoxic, tantalum-rich coatings, such as TaN
C. Skj€
oldebrand et al. Materials Today Bio 15 (2022) 100270
8
and TaO, have been proposed for application on the articulating surfaces
of joint implants due to their corrosion resistance, promising results in
vitro [177] and favourable mechanical properties.
Tantalum carbide coatings deposited on CoCr by thermal treatment in
molten salts have been investigated with regards to their adhesion, me-
chanical and tribological properties [36,178]. The coatings were found to
have a TaC or Ta
2
C-TaC structure depending on the carbon content,
manufacturing process of the CoCr substrate and temperature during
coating growth. In addition, the coatings were deposited in a multilayer
structure with layers comprising different carbon contents. The thickness
of the coatings varied depending on structure and ranged from 300 to
1000 nm. The structure of the coating proved important for adhesion,
where a multilayer structure (TaC-Ta
2
C-Ta) yielded higher critical loads
during scratch tests (delamination at 30 N compared with delamination
at 14 N for a single layer TaC) [36]. When evaluating hardness and
Young's modulus by nanoindentation (with a maximum load of 10 mN),
the hardness was significantly increased (27 GPa for a multi-layer
structure and 23 GPa for a single layer coating compared to 12 GPa for
the uncoated substrate) [178]. The wear volume obtained in a
pin-on-disc set-up with 25 vol% bovine serum diluted in distilled water as
lubricant was similar for the different multi-layer structures. The wear
was reduced compared to the uncoated substrate, however there were no
discernible differences between coatings in terms of wear performance.
Noteworthy is that during wear testing, third body abrasion was the most
prominent wear mechanism due to pull-out of carbides that acted as third
body abrasive particles.
Investigation of the corrosion and wear resistance of tantalum oxide
(TaO
2
) deposited onto Ti6Al4V has shown improved corrosion properties
with an increased corrosion potential, reduced anodic current and
reduced Ti ion release (I
corr
of 6.77010
8
A/cm
2
for TaO compared to an
I
corr
of 2.56010
7
A/cm
2
for Ti6Al4V). The wear volume of the coated
samples was reduced compared to Ti6Al4V (2.22 mm
3
/Nm compared to
7.78 mm
3
/Nm for uncoated Ti6Al4V) [179]. Again, a limitation of the
study was the comparison only to Ti6Al4V. Another study where TaN was
deposited on CoCr by RF sputtering the coatings were shown to have
comparable or lower wear rates of a PE counter surface to uncoated CoCr
[180].
In summary, tantalum carbide and oxide coatings have been shown to
be biocompatible, but their wear performance in the application requires
further investigations.
5.7. Alumina based coatings
In addition to the previously discussed coatings alumina based coat-
ings have been proposed as an option. These coatings include monolithic
micron alumina (IDA), micron alumina yttria-stabilized zirconia (YSZ)
composite coating (IDAZ), and nanostructured alumina titania/YSZ
(IDZAT) deposited on Ti-6Al-4V alloy and have been shown to have
better wear and corrosion compared to Ti6Al4V [181].
6. Discussion
This review has provided a comprehensive overview of both
commercially available coated implants, as well as coatings currently
being researched for potential application in joint implants. The coated
implants currently available in the market are ZrO
2
coated Zr (Smith and
Nephew), TiN coated CoCr or Ti6Al4V (Implantcast), Link orthopaedic),
TiNbN coated CoCr (OHST medical technology) and TiN coated Ti6Al4V
(Endotec). Reported follow up studies as well as case studies of retrieved
implants have revealed revision rates comparable to traditional CoCr
implants [104,105,124–126]. The potential coating materials currently
being investigated include SiN
x
, CrN, TaO, TaC, DLC, TiN, TiCN, ZrO
2
and ZrN. Looking at total knee replacements the use of coatings is more
widespread and the retrieval data reveals the coated implants to be
comparable to CoCr but there is not enough evidence to support a lower
amount of osteolysis-caused failures [80,113,116–125].
When comparing the specific wear rates of coatings currently being
researched (Fig. 3b) it is evident that all coating types (silicon nitride
based, titanium based, chromium nitride based, tantalum based and DLC)
have the potential for low wear rates. However, it is difficult to distin-
guish between them since i) different set-ups have been used to assess the
wear performance, and ii) different counter surfaces have been used in
the tests, i.e. hard-on-hard (e.g. alumina) or hard-on-soft (a polymer).
The available standards cover procedures for both types of material
combinations, including suitable testing conditions. However, even when
the testing is performed in accordance with the standards it might not
reflect the full range of loading scenarios the implant will be exposed to.
To better predict clinical outcome the current standardized approaches
require further development. Important considerations for such an
approach are of course the choice of material combinations, i.e. hard-on-
hard versus hard-on-soft as well as the effect of activities other than
steady state gait. Common activities such as standing up from a chair and
climbing stairs put a higher demand on the implant and will affect the
implant performance.
The previous introduction to and subsequent removal from the mar-
ket of DLC coated implants illustrates the need for more rigorous testing
before clinical use. DLC coated implants were removed from the market
after the revelation that exposure to synovial fluid over time led to a
delamination of the coatings due to the instability of metal carbides in the
interlayer, which in turn caused early revisions. Scratch and Rockwell
tests typically evaluate the adhesion of as-deposited coatings, but typi-
cally do not consider corrosion or changes to the coating over time. To
better predict the adhesion of the coating, the scratch or Rockwell tests
can e.g. be performed on coatings exposed to liquid at different soaking
times [52]. In this review, DLC coatings were included as materials that
are not currently in the market but show potential for implant applica-
tions. Having been introduced to the market and suffered from early
implant revisions, DLC coatings are often not considered an option.
However, the mechanisms causing the delamination have since been
investigated and are now better understood. By using interlayers, DLC
coatings could be stable long-term, reduce wear and potentially increase
implant longevity.
In summary, several of the investigated coatings show potential
because of their ability to reduce wear and ion release. Different coatings
carry different advantages, e.g. CrN on CoCr and TiN on Ti6Al4V can give
enhanced adhesion. One material that has the potential to provide
additional biological advantages is silicon nitride, which has demon-
strated antibacterial and antiviral properties.
Revision and follow up studies of commercially available coatings
have found revision rates to be comparable to, but not better than,
conventional CoCr implants. This could be due to low revision rates or
not long enough follow-up studies (Table 2)[55,80,96,104–128]. The
signs of wear and damage of the PE countersurface has been found to be
lower for oxidized zirconium knee implants compared with CoCr [120],
which is promising as it speaks to the potential for reduced generation of
PE debris. Since this debris is believed to be a major cause of revision its
reduction could lead to a longer implant lifetime. Assuming the coating
adheres well to the substrate it is possible to have a biocompatible surface
that generates low amounts of wear debris and ion release, which would
be ideal for articulating surfaces.
Whilst much of the coating work has focussed on the application of
coatings to bearing surfaces to reduce wear and corrosion, there is now
also a growing interest in translating such technologies to other in-
terfaces, such as hip modular-tapers, where fretting-corrosion processes
may dominate. The ability to develop novel multi-material systems also
paves the way for a new generation of multi-functional coating tech-
nologies to combat the aforementioned issues as well as the emerging
grand challenges within the area of orthopaedics (e.g. infection and
treatment of metastatic cancers).
C. Skj€
oldebrand et al. Materials Today Bio 15 (2022) 100270
9
7. Conclusions
In conclusion,
Coated implants are available in the marked. These implants include
TiN, TiNbN, ZrN coatings and surface treated Zr resulting in an
oxidized surface layer.
Several candidate coating materials such as carbon-based, silicon
nitride, chromium nitride, Ti-based, Zr-based, Ta-based and alumina-
based are being researched.
Coated implants exhibit comparable survival rates to uncoated im-
plants, however the basis for assessment is limited due to generally
low revision rates and short follow-up times.
Coatings could be relevant for other surfaces such as modular
interfaces.
Data availability
The data in this review consists of information found in published
research papers.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
Funding from the European Union's Seventh Framework Program
(FP7/2007–2013), grant agreement GA-310477 (Life-Long Joints) and
the European Union's Horizon 2020 research and innovation programme
under the Marie Skłodowska-Curie grant agreement No 812765 (NU-
SPINE).
Susan Peacock is gratefully acknowledged for her helpful comments
and invaluable discussions.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.mtbio.2022.100270.
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