* Corresponding author. Tel.: #[phone]; fax: #39-02-

2399-3360.

E-mail address: [e-mail] (R. Pietrabissa)

Biomaterials 21 (2000) 907}913

The in-vivo wear performance of prosthetic femoral heads

with titanium nitride coating

Manuela Teresa Raimondi, Riccardo Pietrabissa*

Laboratory of Biological Structure Mechanics, Dipartimento di Bioingegneria, Politecnico di Milano, Milano, Italy

Received 10 November 1998; accepted 21 October 1999

Abstract

This paper reports the study performed on four titanium nitride (TiN) coated prosthetic femoral heads collected at revision surgery

together with patient data. Surface topology has been examined using Scanning Electron Microscopy (SEM) and elemental analysis of
both coating and substrate have been evaluated using energy-dispersive X-ray spectrometry. Quantitative assessment of the surface
topography is achieved using contacting pro"lometry. The average R! roughness value is calculated at "ve di!erent locations for each

femoral head. The UHMWPE counterface worn volume has been measured directly on the acetabular components. TiN fretting and
coating breakthrough occurred in two of the four components examined. In the damaged coating areas the surface pro"le is
macroscopically saw-toothed with average tooth height 1.5

lm. The average R! value is 0.02 lm on the undamaged surfaces and

0.37

lm on the damaged ones. Failure of the coating adhesion resulted in the release of TiN fragments and of metallic particulate from

the substrate fretting corrosion and in the increase of the head surface roughness a!ecting counterface debris production. Our results
suggest that TiN-coated titanium alloy femoral heads are inadequate in the task of resisting third body wear mechanisms
in vivo.

( 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Hip arthroplasty; Femoral head; Coating; Titanium nitride; UHMWPE; Wear

1. Introduction

Wear particles from total joint replacements are

thought to play a critical role in implant failure by
generating periprosthetic bone lysis phenomena, as a re-
sult of a debris-induced macrophage in#ammatory re-
sponse [1}7]. Increased durability of a hip replacement
therefore appears to be related to the reduction of the
arti"cial joint wear [6,8]. Dramatic wear rates are re-
ported in literature for several di!erent joint couplings,
namely stainless-steel alloys-polytetra#uorethylene [9]
(PTFE) and later titanium alloys-ultra high molecular
weight polyethylene (UHMWPE) [10,11]. In titanium
alloy components extensive fretting corrosion is reported
in the presence of third-body hard particles (such as
acrylic cement and/or metal particles) entrapped between
the articulating bearing surfaces. Lower polymeric wear
rates are currently achieved by the clinical application of

highly polished Co}Cr}Mo alloy-UHMWPE and ce-
ramic-UHMWPE couplings [12,13].

Di!erent solutions aimed at improving the tribological

properties of the femoral head-acetabular cup system are
being developed. For the acetabular cup, metal [14,15]
or ceramic [16] bearing surfaces, improved UHMWPE
[17}19] and various carbon-"bre reinforced synthetic
materials [20] are under investigation. For the titanium
alloy femoral head, surface hardening and coating tech-
niques are being developed, such as nitrogen di!usion-
hardening [21], multi-layer titanium}niobium ceramic
coatings and titanium nitride (TiN) coatings [22,23].

The combination of a TiN-coating applied to a pre-

hardened titanium head is intended to combine in
a proper way the pro"table bulk material properties of
the Ti6Al4V alloy, extensively used in orthopaedic im-
plants for its biocompatibility and corrosion resistance,
with the superior fretting resistance of a hard nitride
layer [23]. The wear resistance of TiN-coated surfaces is
compared in friction and wear experimental studies with
those of standard materials [22,23]. Limited data are
available to evaluate the clinical performance of TiN-
coated femoral heads and concerns have been raised

0142-9612/00/$ - see front matter

( 2000 Elsevier Science Ltd. All rights reserved.

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Fig. 1. Location of the pro"lometer scans performed on the femoral
heads. Notations Sup, Inf, Ant and Post refer to the superior, inferior,
anterior and posterior areas, respectively. Two scans are performed on
each location, in directions parallel and perpendicular to the neck axis.
On the apex the scans are taken on the antero-posterior and medio-
lateral planes.

regarding the coating resistance to long-term wear re-
lated damage mechanisms [24].

In this paper we present an analysis of four TiN-coated

femoral heads retrieved at revision after a period of
in vivo articulation against UHMWPE liners. The heads
have been examined to assess the surface damage. The
worn volume is measured on the polymeric acetabular
components. A coating failure scenario is proposed and
discussed.

2. Materials and methods

Four hip prostheses have been harvested at revision

from four patients 18 to 96 months after hip arthroplasty.
All the femoral stems were built of Ti6Al4V alloy and
were implanted with modular TiN-coated femoral heads
articulating on UHMWPE counterfaces. Clinical and
radiographic data were available for all the patients, two
of which were total hip arthroplasty (THA) cases and two
were hemiarthroplasty cases. The THA hips were
cemented MuKller stems articulating on cemented all-poly
cups. The hemiarthroplasty hips were one cemented sel-

#ocking

stem and one cementless Moore stem, both

articulating on biarticular heads of the Bateman's type.
The four hips have been replaced with cemented THA
components. The patient sex, height, weight, activity level
and age at primary and at revision surgery, the primary
diagnosis, the reason for removal, the prosthesis type and
stability at revision have been recorded for each patient

908

M. Teresa Raimondi, R. Pietrabissa / Biomaterials 21 (2000) 907} 913

Fig. 2. SEM images of the transition area between the gold-coloured and the discoloured zones. Coating failure and isolated fragments on the exposed
substrate (a); a double edge (b); #aked fragment and the scratched titanium alloy substrate (c); detail of the porous anchorage layer binding the coating
to the substrate (d).

(Table 1). At revision all the implants were loose and were
collected without disassembling the prosthetic heads
from the femoral stems.

Surface topology has been examined using scanning

electron microscopy (SEM) and elemental analysis of
both coating and substrate have been evaluated using

energy-dispersive X-ray spectrometry. Both analyses
have been carried out using a Stereoscan S260 electron
microscope (Leica Cambridge Ltd., Cambridge, UK).

Quantitative assessment of the surface topography has

been achieved using a Form Talysurf series 2 S4C induc-
tive gauge contacting pro"lometer (Rank Taylor Hobson

M. Teresa Raimondi, R. Pietrabissa / Biomaterials 21 (2000) 907} 913

909

Fig. 3. X-ray spectra are consistent with TiN on the intact coating (a)
and on the intermediate layer (b), and consistent with the Ti6Al4V alloy
on the exposed substrate (c).

Ltd., Leicester, UK), equipped with a diamond tip, 2

lm

radius and 903 cone angle. Ten pro"les for each head
have been acquired in the areas represented in Fig. 1. The
adopted cut-o! is 0.25 mm, evaluation length is 1.25 mm
and sampling is 5000 points per pro"le. The form sub-
traction adopted is a least-squares circle. The roughness
pro"les have been obtained after data "ltering [25] and
the arithmetic mean roughness R! has been calculated for

each roughness pro"le. At the di!erent locations the
average R! value is calculated from the two values ob-

tained in the directions parallel and perpendicular to the
neck axis.

The

prosthetic

heads

articulated

in

vivo

on

UHMWPE counterfaces. The UHMWPE worn volume
has been measured directly on the polymeric compo-
nents. A cloud of points has been acquired on the worn
surface using a Werth Video Check IP 250/400 coordi-
nate measuring machine (Werth Messtechnik GmbH,
Giessen, G). The coordinate data have been input in
the commercial solid modelling code Catia (Dassault
Syste`mes, Paris, F) which gave the 3-D model of the
worn surface. The solid model of the worn material has
been obtained as the space between the worn and the
original cup surfaces. The worn volume has been cal-
culated too.

3. Results

Gross examination of the articulating surfaces shows

multidirectional slight scratching (3 mm or less in length)
on all the retrieved heads. Two components show a dull
grey discoloration on the gold-coloured TiN coating,
located on the anterior head region and involving about
10% of the articulating surface in one head (component
no. 1 in Table 1), located on a wide band surrounding the
complete head equator and involving about 70% of the
articulating surface in another head (component no. 3 in
Table 1).

SEM images of the transition surfaces between the

gold-coloured and the discoloured zones are shown in
Fig. 2. Isolated TiN coating fragments have been ob-
served, which appear bonded to the exposed substrate.
X-ray spectrometry (Fig. 3) produces spectra consistent
with the TiN on the intact coating and with the Ti6Al4V
alloy on the substrate, indicating extensive abrasion of
the TiN coating. At a greater magni"cation (Fig. 2c)
double-edge boundaries are observed on the TiN frag-
ments indicating #aking of the multi-layer coating. De-
tailed inspection of the #akes shows a porous anchorage
layer binding the TiN coating to the substrate. X-ray
spectrometry produces spectra consistent with a TiN on
the intermediate layer. The titanium alloy exposed sub-
strate shows severe scratching whereas slight scratching
is visible on the coating fragments and on the undamaged
coating areas.

910

M. Teresa Raimondi, R. Pietrabissa / Biomaterials 21 (2000) 907} 913

Fig. 4. Pro"les acquired on the undamaged (a) and on the damaged (b) coating areas (raw data are treated with least square circle form subtraction).

Fig. 4 shows two pro"lometer scans acquired on the

undamaged and on the damaged coating areas. Only
form subtraction is performed on the raw pro"le data, to
reproduce the actual shape and size of the asperities. In
the damaged coating area the pro"le is saw-toothed with
average tooth thickness about 1.5

lm. Fig. 5 shows the

average values of the R! parameter calculated on the

roughness pro"les, for each femoral head and at the vari-
ous head locations. The average R! value is 0.02 lm on the

undamaged surfaces and 0.37

lm on the damaged ones.

The calculated worn volumes for each UHMWPE cup

are shown in Table 1.

4. Discussion

Two of the four examined components (nos. 2 and 4)

show a macroscopically intact TiN coating 6 to 8 years
after implant. The presence of slight scratching, visible to
the naked eye, suggests that third body abrasion was
however taking place prior to implant failure. Harman

et al. [24] have reported about wear debris originating
from a TiN-coated femoral head as delaminated surface
asperities, after one year of in situ operating. This may be
the mechanism initiating abrasion of the TiN coating;
additional hard particles (such as acrylic cement par-
ticles) may take part in this abrasive process at a second
stage.

TiN fretting and coating breakthrough occurred in

two of the four examined components. In the presence of
hard particles entrapped between the articulating
counterfaces high local stresses can develop following
micro-contact of two hard materials under load (Hertz-
ian forces). In this occurrence, the peak stresses are
located below the contact surface (pellicular loads), and
failure of coating adhesion is likely to occur as a conse-
quence of the coating brittleness. Detachment of TiN
macroscopic fragments accelerates this failure mecha-
nism and extensive breakthrough of the coating is pos-
sible even at low loads.

Component no. 1 articulated in vivo with a biarticular

head on a patient with a low activity level and a coating

M. Teresa Raimondi, R. Pietrabissa / Biomaterials 21 (2000) 907} 913

911

Fig. 5. The average values of the R! parameter calculated, for each femoral head, at the superior, inferior, anterior, posterior and apex locations.

breakthrough initiation site was already present 1.5 years
after implant. As a matter of fact the entrapment of third
body particulate is favoured in the inner bearing of biar-
ticular joints [26]. This occurrence, associated to a rela-
tively high patient body weight, may have been the
critical failure factors for the coating of this component.
Component no. 3 articulated on a "xed socket on a high
demand heavy patient and the coating was almost com-
pletely disappeared from about 70% of the head surface
after 8 years of in situ operating. Extensive fretting cor-
rosion has been observed on the titanium alloy substrate
and this might have accelerated the periprosthetic bone
resorption and the implant failure.

The UHMWPE wear rates reported in Table 1 are

consistent with the wear data published by other authors
with reference to failed arthroplasties [13,27}29]. A com-
parison between the wear rates measured in the biarticu-
lar inner couplings and those measured in the THA
couplings is not possible, since the expected range of
relative motion is greater for the THA joints [30,31].
Various authors [32}35] have shown that a statistical
correlation is present between increased harder surface
roughness and UHMWPE worn volume. The correla-
tion is exponential according to several authors [36,37].
The shape and dimension of the asperities in the
damaged areas (Fig. 4) and the calculated R! values

(Fig. 5) suggest that the coating breakthrough can lead to
an increase of polymeric debris production rate. Further-
more, the coating fragments may favour third body wear
mechanisms. Failure of the coating adhesion results in
the following events, all proved [1}8] to adversely a!ect
the implant clinical outcome:

1. release of TiN fragments in the periprosthetic tissues;
2. release of metallic particulate from the titanium sub-

strate fretting corrosion;

3. increase of the head surface roughness a!ecting the

counterface debris production rate.

Our results suggest that TiN coated titanium alloy

femoral heads are inadequate in the task of resisting
in vivo third body wear mechanisms. Their use should
not be advocated.

Acknowledgements

The authors are grateful to Professor Giuseppe Silva of

Dipartimento di Meccanica for making available both
the SEM equipment and his competence and to Carlo
Santambrogio of Taylor Hobson S.p.A. for his help in the
pro"lometry studies.

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