Abstract
Background: A dark metallic-appearing smear, resembling a lead
pencil mark, may be seen on a ceramic femoral head component at revision total
hip arthroplasty. The purpose of this study was to investigate the hypothesis
that such a mark on a retrieved ceramic femoral head is associated with
increased surface roughness of the head and increased polyethylene liner wear
in total hip replacement.
Methods: Fifteen ceramic prosthetic femoral heads retrieved from
fifteen patients at revision arthroplasty were examined in this study.
Thirteen heads had been in vivo for an average of 10.8 years (range, 7.8 to
14.2 years). The remaining two heads had been in vivo for less than one month.
The surface roughness characteristics of the explanted ceramic heads, the
linear wear of the polyethylene liner, and the patient activity levels after
the primary replacement and before the revision were determined.
Results: Four of the thirteen ceramic heads that had been in vivo
for =7.8 years had severe smears (>6% of the surface area), and the
remaining nine heads had slight smears (<6% of the surface area). The two
heads that had been in vivo less than one month had severe smears. The mean Ra
and Rpm, the values for surface roughness, were 44.95 nm and 571.15 nm,
respectively, in the hips with slightly smeared regions and 180.77 nm and
1245.88 nm, respectively, in the hips with severely smeared regions (p =
0.002). The mean linear liner wear rate was 0.10 mm/yr in the hips with
slightly smeared heads and 0.19 mm/yr in the hips with severely smeared heads
(p = 0.002). The activity score for all patients was 5 or 6 points on a
6-point scale.
Conclusions: The results of this study confirm the hypothesis that a
visual dark metallic-appearing smear on a ceramic femoral head correlates with
increased surface roughness of the head and increased polyethylene wear. These
findings imply that contact of a ceramic femoral head with a metallic
material, such as may occur with femoral head reduction or dislocation of a
total hip replacement, is best avoided to prevent this metallic smear
phenomenon.
Wear-related complications have been a major cause of revision following
total hip arthroplasty. The surface roughness of the prosthetic femoral head
plays an important role in generating polyethylene wear debris after total hip
arthroplasty1,2.
It has been reported that cobalt-chromium heads have rougher surfaces than
ceramic heads3 and
produce more wear than ceramic heads in studies with use of a hip
simulator4-7;
however, studies on the wear of polyethylene liners in vivo have had
conflicting
results8-17.
At revision surgery, a dark metallic-appearing smear resembling a lead
pencil mark may be seen on a ceramic femoral head. The purpose of this study
was to investigate the hypothesis that ceramic femoral heads exhibiting a
metallic-appearing smear, compared with those that do not, have a rougher
surface and are associated with greater in vivo polyethylene wear.
The study was approved by our institutional review board, and all patients
provided informed consent. We examined fifteen ceramic femoral heads retrieved
at the time of revision from fifteen patients who had a mean age at the time
of the operation of 49.9 years (range, thirty-one to sixty-one years), a mean
weight of 65.8 kg (range, 48 to 78 kg), and a mean height of 164.8 cm (range,
154 to 176 cm). The mean duration of implantation was 10.8 years (range, 7.8
to 14.2 years) for thirteen heads and less than one month for two heads. One
of these two heads was retrieved at a primary total hip arthroplasty because
the ceramic head had been scratched very much by the rim of the metallic
acetabular component during reduction of the femoral head. The other one was
retrieved at open reduction of a dislocated total hip replacement. All heads
articulated with a polyethylene liner made of ram-extruded 415 GUR
polyethylene. The polyethylene had been irradiated in a vacuum and was
packaged in a vacuum state. All hips had a 28-mm femoral head.
Analysis of Retrieved Femoral Heads
The surface characteristics of the fifteen explanted ceramic heads were
evaluated with use of three different methods: visual assessment,
interferometry (Wyko RST 500 interferometer; Wyko, Tucson, Arizona), and
environmental scanning electron microscopy. On the basis of visual assessment,
the femoral head was considered to be nonsmeared if no smeared region was
visible on the femoral head. The femoral head was considered to be slightly
smeared if the smeared region was <6% of the total head surface, and it was
considered to be severely smeared if the smeared region was >6% of the
total head surface.
The interferometry measurements were undertaken at two different
magnifications with use of 20× and 40× lenses. The areas of
analysis were 125 by 125 µm and 64 by 64 µm, respectively. At each
magnification, six measurements were made of the smeared and nonsmeared
regions. The results are presented in terms of Ra and Rpm. The parameter Ra is
defined as the mathematical average of all deviations (peaks and valleys) from
the mean line of the surface profile. The parameter Rpm (the mean leveling
depth) is defined as the distance between the mean line and a line parallel to
it, which passes through the highest point. The intrinsic errors of the
measurement of Ra and Rpm were 0.0219 nm and 0.157 nm, respectively. The
measurement resolution of the interferometer is <1 Å in phase shift
interferometry mode and <1 nm in vertical shift interferometry mode.
Therefore, the phase shift interferometry mode is more accurate for highly
polished surfaces. The vertical shift interferometry mode is appropriate for
surfaces that are rough, or highly contoured, or for sharp peaked surfaces
typically >0.6 µm in
height17.
The roughness values in the nonsmeared region of each head were used to
estimate the roughness values of each head before implantation. In addition,
two ceramic heads that had not been implanted were analyzed to confirm that
the nonsmeared regions of the retrieved specimens reflected the surface
roughness of heads before implantation and thus were appropriate to use as
controls.
Further analysis was completed with use of a Camscan 4 environmental
scanning electron microscope (University of Leeds, Leeds, United Kingdom) by
two scientists who had no knowledge of the clinical and experimental results.
Secondary and backscattered images were reviewed at various magnifications
(particularly at 250×) to attempt to identify the composition and
origins of the materials adherent to the head surfaces as well as to assess
the pits and scratches on the head surfaces.
A stereoscopic zoom microscope (Nikon, Melville, New York) was used to
examine grossly the polyethylene liners corresponding to each femoral head for
evidence of embedded particle debris. The evidence for particle embedding was
examined qualitatively by a research associate who had no knowledge of the
clinical and experimental results.
Radiographic Analysis
We measured the linear wear of polyethylene radiographically by determining
the migration of the center of the femoral head relative to the center of the
cup, according to the computer-aided technique of Kim et
al.13. The 95%
confidence interval was considered a measure of reproducibility. Intraobserver
error was ±0.047 mm.
We compared the radiographic measurements with the direct measurements of
all fifteen cups to validate this wear-measurement technique. The linear wear
was measured directly from the retrieved polyethylene liners with use of a
three-dimensional coordinate measuring device (BHN 305; Mitutoyo, Tokyo,
Japan). Validation testing revealed that the measuring device tended to
underestimate the true amount of penetration by a mean of 0.08 mm. Therefore,
the radiographic measurements of penetration were thought to be
reproducible.
Activity Levels of Patients
The activity level of the patients between the primary and the revision
total hip arthroplasty was assessed with the activity score of Tegner and
Lysholm18. The
activity grading scale, in which work and sports activities were graded
numerically, was constructed as a complement to the functional score. The
patients were given a score according to the activities in which they engaged
in daily life. The score ranged from 0 points for a hip-related disability to
10 points for participation in competitive sports at a national level.
Statistics
The Student t test was used to determine possible correlations between the
rate of penetration and several specific variables: patient age, gender, and
activity level; duration of implantation; and type of head, stem, and cup.
Linear regression analysis was used to reveal any relationship between the
surface roughness values and the patient age, gender, weight, and activity
level; duration of implantation; and type of head, stem, and cup. The Wilcoxon
rank-sum test was used to determine any statistical difference in surface
roughness between heads articulating with liners with or without embedded
debris. A p value of <0.05 was considered significant.
Analysis of Retrieved Femoral Heads
Thirteen heads were alumina ceramic, and two heads were zirconia ceramic.
Patient demographic information and component data are summarized in the
Appendix. On visual evaluation, severe smears were observed on six components
and slight smears were seen on nine components
(Fig. 1). Four of the thirteen
heads that had been in vivo for more than 7.8 years had severe smears, and the
remaining nine heads had slight smears. Both of the heads that had been in
vivo less than one month had severe smears. The area of smearing on the heads
ranged from <1% to 10% of the total surface of the head. The results of the
measurements by interferometer on the smeared and nonsmeared regions on all
heads are summarized in Table
I, and the results of the measurements by interferometer on all
heads are graphically illustrated in the Appendix.
The roughness values for the nonsmeared regions of the ceramic heads
compared well with the roughness values for two heads that had never been
implanted. The mean Ra and Rpm values for the ceramic heads that had not been
implanted were the same as those for the nonsmeared regions of the fifteen
implanted ceramic heads (Ra, 7.81 nm; Rpm, 76.49 nm). The mean Ra and Rpm
values for the slightly smeared regions (nine heads) were 44.95 nm and 571.15
nm, respectively. The mean Ra and Rpm values for the severely smeared regions
(six heads) were 180.77 nm and 1245.88 nm, respectively. The differences in Ra
and Rpm values between the slightly smeared and severely smeared regions were
significant (p = 0.002), irrespective of magnification.
The mean surface roughness values for the slightly smeared regions in the
nine ceramic heads without third-body debris embedded in the corresponding
polyethylene liner (Ra = 44.95 nm; Rpm = 571.15 nm) were lower than those for
the severely smeared regions in six ceramic heads associated with embedded
debris (Ra = 180.77 nm; Rpm = 1245.88 nm). The differences in roughness values
between the two groups were significant (p = 0.02 for both). However, no
correlation was found, on the basis of the numbers, between roughness
parameters of the ceramic heads (slightly or severely smeared) and the
following parameters: age, gender, weight, and activity level of the patient;
duration of implantation; and type of head, stem, and cup (p > 0.05
according to linear regression analysis).
Environmental scanning electron microscopic evaluation of the ceramic heads
revealed small pits and scratches on the surfaces of ceramic heads in six of
nine slightly smeared heads and in all of six severely smeared heads. The
difference was not found to be significant on the basis of the numbers (p =
0.078) (Fig. 2).
Environmental scanning electron microscopic evaluation of the smeared
regions demonstrated a higher atomic number than the ceramic substrate in the
backscattered image (Fig. 3).
This is indicative of metal deposits on the surface of the ceramic head.
However, confirmation of this was difficult because the film of deposited
material was very thin (on the order of 3 µm). Therefore, complete chemical
identification of the material with energy-dispersive x-ray spectrometry was
not possible. Some regions also had particles with a lower atomic number than
that of the ceramic substrate, which perhaps was attributable to organic
contamination of the head surface.
Radiographic Analysis
Validation testing of the measuring technique revealed a good correlation
between the radiographic and direct measurements of polyethylene wear
(R2 = 0.95), and it demonstrated that the radiographic measurement
underestimated the direct measurement by a mean of 0.08 mm (see Appendix). The
mean linear wear rate was 0.10 mm/yr (range, 0.09 to 0.12 mm/yr) in the hips
with slightly smeared heads and 0.19 mm/yr (range, 0.16 to 0.24 mm/yr) in the
hips with severely smeared heads. This difference was significant (p =
0.002).
The mean linear wear rate of the polyethylene was 0.10 mm/yr (range, 0.09
to 0.12 mm/yr) in the hips with a low mean Ra and mean Rpm (44.95 nm and
571.15 nm, respectively) and 0.19 mm/yr (range, 0.16 to 0.24 mm/yr) in the
hips with a high mean Ra and mean Rpm (180.77 nm and 1245.88 nm,
respectively). This difference was significant (p = 0.002).
The mean polyethylene linear wear rate was 0.20 mm/yr (range, 0.16 to 0.24
mm/yr) for the cups associated with third-body embedded debris and 0.09 mm/yr
(range, 0.08 to 0.12 mm/yr) for the cups without third-body embedded debris.
This difference was significant (p = 0.03). The mean polyethylene linear wear
rate was 0.21 mm/yr (range, 0.16 to 0.24 mm/yr) for femoral heads with surface
pits and scratches and 0.11 mm/yr (range, 0.09 to 0.12 mm/yr) for the heads
without surface pits and scratches. This difference was significant (p =
0.002).
No correlation was found, on the basis of the numbers available, between
the wear rate of the polyethylene and the following parameters: patient age,
gender, and activity level; duration of implantation; and type of head, stem,
and cup (p > 0.05) (see Appendix).
Activity Score
Many patients were quite active despite the usual cautions to avoid
activities involving impact after total hip replacement. All patients had an
activity score of 5 or 6 points before revision, indicating participation in
strenuous farm work (a score of 5 points) or participation in tennis (a score
of 6 points).
The surface characteristics of the femoral head play an important role in
generating polyethylene wear debris after total hip
arthroplasty1,2.
It has been reported that the adherence of third-body particles to the femoral
head increases its surface roughness, and the resultant abrasive wear process
increases the wear rate of the polyethylene liner in the total hip
arthroplasty11. In
the current series, metallic-appearing smears on the ceramic head were found
to be transferred metallic debris on the basis of the findings of
environmental scanning electron microscopic and energy-dispersive x-ray
spectrometric examinations. This debris increased the surface roughness and
consequently increased the wear of the polyethylene liner through either an
abrasive or a third-body wear mechanism. An increase in surface roughness and
wear as a result of transferred metal debris may be one explanation for the
sporadic cases of excessive wear of alumina-on-alumina bearings and the poor
results with alumina-on-polyethylene
bearings14-17,19-25.
We confirmed the hypothesis in our study that metallic transfer onto the
ceramic femoral head increases surface roughness and consequently increases
the wear rate of the polyethylene liner.
The transfer of metal debris to the ceramic head occurs even with
relatively minor scratching. Black discoloration of a ceramic head can occur
simply by lightly scratching the head on a metal surface. Luchetti et
al.26 reported that
metal was transferred to a zirconia head when the head was scratched on the
metal shell during a closed reduction of a dislocated total hip prosthesis.
Therefore, caution is required to avoid contact of the ceramic head with
metallic materials during the operation. Because of the hardness of ceramic,
one can expect more metal to be transferred to ceramic than to other materials
used for a prosthetic
head27.
This study also found that the surface roughness of the ceramic head and an
increased polyethylene wear rate were associated with the presence of embedded
debris in the articulating polyethylene liner. Therefore, third-body
contamination should be avoided as much as possible.
Our study has several limitations. The surface characteristics and wear
pattern of retrieved femoral heads at revision hip arthroplasty may not
represent the surface characteristics and wear pattern of well-functioning
total hip replacements. The number of heads examined was small, limiting our
conclusions. In addition, the examination was not performed in a population of
patients who all had identical components; femoral heads from different
manufacturers may have different initial roughness. However, we attempted to
normalize the effect of the manufacturing technique on the initial surface
roughness by using a nonsmeared area of each head to estimate the
preimplantation roughness of that head.
The results of this study demonstrate that the area of a visual smear
resembling a lead pencil mark (metallic transfer) on the ceramic femoral head
increases surface roughness and that a severe smear (>6% of surface area)
correlates with increased polyethylene wear. These findings imply that contact
of a ceramic femoral head with metallic material, such as may occur with
femoral head reduction or dislocation of a total hip replacement, is best
avoided to prevent this metallic smear phenomenon.
Tables presenting demographic information and component data, a comparison
of the direct and radiographic measurements of wear, and the relationship
between wear and demographic data and figures showing the roughness of each
femoral head are available with the electronic versions of this article, on
our web site at
(go to
the article citation and click on "Supplementary Material") and on
our quarterly CD-ROM (call our subscription department, at 781-449-9780, to
order the CD-ROM). ?
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