Abstract
Background:
In an effort to decrease the rate of aseptic loosening, certain cemented
femoral components were designed to have a roughened or textured
surface with a methylmethacrylate precoating. Reports differ as
to whether this step has increased or decreased the rate of failure.
This study was designed to evaluate this issue.
Methods:
Five hundred and fourteen hips treated with a cemented Harris Precoat
stem (Zimmer, Warsaw, Indiana) were evaluated clinically and radiographically
and compared with 254 hips treated with an uncoated Harris Design-2
stem (Howmedica, East Rutherford, New Jersey). Prostheses that had
been removed at revision were examined. The cementing and surgical
techniques were identical and the population demographics were similar
for these two groups.
Results:
The mean durations of follow-up were 8.4 and 13.5 years for the
Precoat and uncoated Design-2 stems, respectively. At those times,
at least forty-nine (9.5%) of the 514 Precoat components and at
least ten (3.9%) of the 254 uncoated Design-2 stems had failed (p
= 0.006). Five Precoat stems fractured, and no uncoated Design-2
stems fractured. Component failure was associated with use in young,
active, heavy men with a diagnosis of avascular necrosis and generally
with the use of smaller components. The cementing technique was
satisfactory in the majority of the patients, and there were no
qualitative differences in cementing technique between the hips
that failed and those that did not. The mechanisms of failure of
the Precoat prostheses included bone-cement loosening, focal osteolysis,
stem fracture, and prosthesis-cement debonding. Fractures of smaller
components occurred as a result of fatigue failure and were associated
with good distal fixation but proximal stem loosening.
Conclusions:
The rate of failure of roughened, precoated, cemented femoral components
was considerably higher and occurred earlier than that of femoral
components that were neither textured nor precoated with methylmethacrylate.
Younger patients with avascular necrosis had a higher risk of failure;
however, this factor alone did not completely explain the differences
in outcome between these two components. The causes of aseptic loosening
are multifactorial and may be related to component design and size
as well as to precoating and surface finish.
Aseptic loosening is the most common cause of stem failure after
total hip arthroplasty performed with cement. It has been suggested
that this is related in part to debonding of the femoral component
from the cement
1-8
. Thus, it would seem that methods to improve this bond should decrease
the incidence of loosening, and, with this goal, prostheses have
been textured proximally, roughened, or precoated with methylmethacrylate.
Laboratory studies and finite element analyses have confirmed that
both precoating and roughening enhance the bonding of cement to
the prosthesis and decrease the stresses at the prosthesis-cement
interface
1,2,6,9-12
. Some clinical studies have demonstrated excellent seven to ten-year
rates of survival of precoated prostheses
2,4,13-21
, whereas others have shown higher rates of loosening
1,3,12,22-28
.
In 1987, we switched from the use of nontextured uncoated femoral
stems (Harris Design-2; Howmedica, East Rutherford, New Jersey)
to the use of precoated stems (Harris Precoat; Zimmer, Warsaw, Indiana),
some of which were also textured proximally. Our initial follow-up
suggested that this change in prostheses increased the rate of femoral
failure. Accordingly, we sought to evaluate these findings more
completely.
We retrospectively reviewed the results of 768 primary total hip
arthroplasties performed by one surgeon (M.E.S.) between 1984 and
1994. Two hundred and fifty-four uncoated Harris Design-2 components
were inserted between 1984 and 1987. The mean interval between the
surgery and the follow-up was 13.5 years (range, twelve to fifteen
years). Five hundred and fourteen Harris Precoat components were
inserted between 1987 and 1994. The mean interval from insertion
to follow-up was 8.4 years (range, five to twelve years). The two series
were consecutive, and only rarely were other components used during
these time-periods.
A posterolateral surgical approach was used in all patients. Second-generation
cementing techniques (retrograde introduction of cement) with Simplex
cement (Howmedica, London, England) were used. We did not use vacuum-mixing, centrifugation,
or stem centralizers.
Patients were seen at six weeks; at three, six, and twelve months;
and yearly thereafter. Radiographs were made at three, six, and
twelve months and every two years thereafter. Patients were evaluated
clinically with Harris hip scores both preoperatively and postoperatively.
We defined the result as a clinical failure when the hip underwent
or was scheduled to undergo a surgical revision. Our definition
of radiographic failure was a progressive radiolucent line of ≥2
mm in thickness completely surrounding the cement column, migration
of the prosthesis, or fracture of the prosthesis. Prostheses that
were removed were examined grossly to determine the mode of failure.
The cementing technique was evaluated radiographically, shortly
after the initial operation as well as at the time of failure, with
use of the grading system described by Mulroy et al.
29
.
Prostheses
The Harris Design-2 stem is a cemented, nonmodular, collared
component made of forged chromium-cobalt (Vitallium alloy; Howmedica)
(
Fig. 1
). It is neither textured nor precoated, and it has anterior and posterior
grooves. It has a surface roughness of 33 Ra (microinch). The size
array included small, medium, and large components that were 110
mm long and an extra-large component that was 165 mm long. Different
head sizes were available, although 32-mm diameter heads were used
exclusively in this series. Three neck lengths were available: short
(28 mm), medium (35 mm), and long (40 mm).
The Harris Precoat stem was designed as a successor to the Harris
Design-2 stem by the same surgeon who designed the Harris Design-2
stem. It is a modular component with a Morse-type-taper neck and
a medial collar that is smaller than the collar of the Harris Design-2
stem. It is made of forged high-strength cobalt-chromium-molybdenum
(Zimaloy). Its geometry is similar but not identical to that of
the Harris Design-2 stem. It is precoated proximally with methylmethacrylate
at the factory. The earlier components, referred to as "Precoat,"
were not textured and had a surface roughness of 80 Ra (microinch).
Some had anterior and posterior grooves, and some did not. Later
components, referred to as "Precoat-Plus," were modified to include
proximal texturing under the methacrylate coating, had a surface
roughness of 87 Ra (microinch), and had no grooves
10
(
Fig. 1
). The size array included CDH stems, 100, 150, and 200 mm long;
small stems, 120 mm long; medium and large stems, 140 mm long; and
extra-large stems, 170 mm long. All of the heads used in this series
were 28 mm in diameter. Short, medium, and long necks were available
in lengths ranging from 29 to 50 mm, depending on the size of the
stem. Both Precoat and Precoat-Plus components were used in this
series, although we could not determine the exact number of each type
implanted.
Femoral Components
Eighty-five (17%) of the 514 Precoat components and sixty-two
(24%) of the 254 Design-2 components were lost to follow-up, leaving
429 Precoat and 192 Design-2 prostheses available for evaluation.
The mean time since the arthroplasties with the Precoat stems was
8.4 years (range, five to twelve years), and the mean time since
the arthroplasties with the Design-2 stems was 13.5 years (range,
twelve to fifteen years).
At least forty-nine (9.5%) of the 514 Precoat stems failed at
a mean of 5.7 years (range, two to eleven years). Forty-three were
revised, and six were radiographically loose but had not yet been
revised. Five stems fractured. At least ten (3.9%) of the 254 Design-2
stems failed at a mean of 9.9 years (range, eight to thirteen years).
Eight were revised, and two were loose but had not yet been revised.
No stem fractured. The difference in the failure rate between the
Precoat and Design-2 stems was significant (p = 0.006).
The rates of failure of both components were greater in patients
with avascular necrosis than they were in patients with degenerative
arthritis, and the rate of failure of the Precoat components was
greater than that of the Design-2 components in patients with either
diagnosis (
Table I
).
Acetabular Components
In the combined series, seventeen (2.2%) of the 768 acetabular
components failed. Thirteen of the failed cups were cemented M�ller
all-polyethylene components, all used with Design-2 stems. In addition,
one Optifix cup (Smith and Nephew Orthopaedics, Memphis, Tennessee)
and three Ti-Bac II cups (Zimmer), used with Precoat stems, failed.
None of the Harris-Galante-1 porous acetabular components, all used
with Precoat stems, failed.
Demographic Factors
The age and gender of the patients with Precoat and Design-2 components
were similar. The mean age of the patients with a Precoat component
was fifty-five years, and the female-to-male ratio was 1.5 to 1.
The mean age of the patients with a Design-2 component was fifty-nine
years, and the female-to-male ratio was 1.4 to 1. With a mean age
of 54.3 years (range, thirty-one to seventy-one years), the patients
with a failed Design-2 component were slightly younger than the
entire group of patients treated with that stem. The mean age of
the patients with a failed Precoat component was 42.3 years (range,
twenty-one to seventy-eight years), which was significantly younger
than the mean age of the Precoat group as a whole (p = 0.0261).
Failures were more common in men than in women. One-half (five)
of the ten failed Design-2 stems and 53% (twenty-six) of the forty-nine
failed Precoat stems were in men, although only 42% (sixty-five)
of the 156 patients treated with a Design-2 stem were men and only
40% (125) of the 313 treated with a Precoat stem were men.
The majority of patients in the series had a diagnosis of either avascular
necrosis or osteoarthrosis. The others had a diagnosis of developmental
dysplasia of the hip (thirty-one hips) or had miscellaneous diagnoses
(eighty-seven hips). Patients with a Precoat stem had a higher rate
of avascular necrosis and a somewhat lower rate of degenerative
joint disease than did those with a Design-2 stem (see Appendix).
Characteristics of Failed Precoat Components
Thirty-three (67%) of the forty-nine failed Precoat stems were the
smallest components (
Table II
). Failure was not associated with a greater neck length or with
either the earlier or the later component design (Precoat or Precoat-Plus).
Cementing Technique
The cementing technique for thirty-eight failed components was
retrospectively evaluated on radiographs made after the initial
surgical procedure and was compared with that of thirty-three surviving
components chosen at random. The cementing technique was graded
as A or B in the majority of hips, and no difference in cementing
technique was seen between those that eventually failed and those
that did not, although a somewhat greater number of failed hips
had a C2 cementing grade (see Appendix).
Radiographs made just prior to revision of forty-one failed prostheses
were reviewed, and the failure was graded as one of four types (see
Appendix). The majority showed extensive bone-cement radiolucency,
often with subsidence of the prosthesis and fracture of the cement.
Only rarely was there evidence of debonding alone, although it was
occasionally seen in conjunction with bone-cement demarcation. When
present, the debonding invariably originated in Gruen
30
zone 1. In only four instances was there evidence of focal osteolysis
occurring before gross loosening or debonding.
A somewhat different picture was seen on gross examination of
thirty-four retrieved stems. Twenty had partial or complete debonding
of the cement, although this was not obvious from the radiographs
alone. Nine prostheses had little or no debonding and had been extracted
with most or all of the cement mantle attached (
Fig. 2
). The prostheses with partial debonding had separation of the cement
proximally while the distal cement remained firmly attached; loosening
had occurred at the distal part of the bone-cement interface.
Five stems fractured. The findings were characteristic, with the
distal portion of the stem firmly fixed within the cement column
and the removal performed with some difficulty. The fracture line
was seen passing through one of the laser etchings on the anterior
aspect of the stem, usually at or below the junction of the middle
and distal one-thirds. The proximal portion of the prosthesis was
grossly loose and was completely devoid of cement. In addition,
the remaining proximal cement column was usually loosened from the
bone and fragmented. Varying amounts of osteolysis were present.
The fracture pattern was consistent with fatigue failure (
Figs. 3-A
and
3-B
). Fractures occurred in both textured and non-textured components.
All five patients with a fractured prosthesis were young, heavy,
and active. All fractures were in the left hip, and four of the
five were of a small prosthesis. Three of the five were patients
with avascular necrosis (see Appendix).
Initially, some clinical studies of precoated femoral components
demonstrated excellent results
2,4,13-21
and others showed a higher incidence of loosening
3,22-28,31,32
. Gardiner and Hozack, in 1994, reported aseptic loosening at the
bone-cement interface of seventeen cemented precoated or roughened
implants at a mean of thirty-seven months
24
. They speculated that improvement of the cement-prosthesis bond
caused stresses to be transferred to the bone-cement interface,
promoting early failure at this site. Studies that compared the
polished and matte-finish versions of the Exeter prosthesis revealed
that roughening of the surface increased the incidence of failure
25,27,28,31,32
. When loosening occurred, rougher surfaces generated more cement
debris than smooth surfaces did, a situation that could lead to
bone lysis and stem loosening
4,10,33
. Thus, a number of investigators have concluded that an enhanced
cement-prosthesis bond may actually be deleterious rather than helpful
with regard to the long-term survival of the stem
1,2-4,10,12,22-28,33
.
In 1996, Woolson and Haber reported a 5% rate (six of 121) of
loosening of the Harris Precoat femoral component at six years
34
. Ten additional hips had femoral osteolysis, and two components
fractured. In 1997, Woolson et al. reported that ten of 654 Precoat
or Precoat-Plus stems fractured
35
. Six were in patients who weighed more than 80 kg. All fractures
occurred at the site of laser etching on the stem surface, and nine
of the ten were in the left hip, where the etching was on the anterior
or tensile aspect of the stem. Scanning electron microscopy indicated
that the metal had undergone localized melting and weakening from
the high temperature of the laser beam.
Dowd et al., in 1998, reported on a series of 154 total hip arthroplasties
in which a precoated femoral component was used
3
. Twenty-three (15%) of these components failed, with an average
time to revision of 3.9 years. The most common mechanism of failure
was loosening at the bone-cement interface with debonding of the
stem as a later finding. The authors concluded that the failure
was related to a thin cement mantle, which they attributed to use
of an undersized rasp, lack of centralization, and vacuum-mixed
cement.
The Centralign precoated hip prosthesis (Zimmer) was designed
to improve on certain aspects of the earlier Precoat and Precoat-Plus
prostheses. The surface roughness of these components was similar,
but the Centralign had precoating applied both proximally and distally
over areas of macrotexturing and had proximal and distal centralizers.
Recently, Sylvain et al.
7
reported their results with eighty-four Centralign components.
Although radiographs indicated excellent cementing technique, the
rate of failure was 12% at an average of thirty-one months after
insertion.
Although several authors have identified poor cementing technique,
a thin cement mantle, and the use of vacuum mixing as possible causes
of failure
3,9,11,18,29,36-39
, the literature does not uniformly support these conclusions. Good-to-excellent
results with the Precoat prosthesis have been reported by authors
who routinely used vacuum mixing
2
as well as by those who used neither vacuum mixing nor centrifugation
13
. In addition, a high percentage of good and excellent clinical results
with a low failure rate has been reported in series in which many
hips were found to have a thin, and supposedly poor, cement mantle
2,13,15,21,31,34
.
In the present study, we found little difference in cementing technique
between the Precoat hips that failed and those that did not. Although
a grade of C2 was more common in the hips that failed, the cementing
technique was graded A or B in the majority of hips in both groups.
We also found a low incidence of failure of older Harris Design-2
prostheses that had been fixed with a similar second-generation
cementing technique, with no stem centralization or porosity reduction
40,41
. Of the ten hips that failed, four had grade-A cementing technique;
four, B; and two, C2.
The demographic characteristics of the patient population in the
present study differed considerably from those in most others
2,17,36,38
. Since our medical center has an active osteonecrosis program,
a large number of patients required hip replacement because of osteonecrosis.
These patients were generally younger and more active than patients
with degenerative joint disease. As in previous studies
28,36
, we found a higher failure rate in the patients with osteonecrosis.
The percentage of patients with osteonecrosis was higher in the
group treated with the Precoat stem than it was in the group treated
with the Design-2 stem, and this accounted in part for the higher
overall rate of failure of the Precoat components. However, the
failure rate of the Precoat prostheses was also considerably higher
than that of the Design-2 components in patients with degenerative
arthritis (
Table I
).
There are probably multiple factors responsible for prosthetic failure
in patients with osteonecrosis, including concomitant disease, use
of alcohol and corticosteroids, metabolic alterations in bone, younger
age, and greater activity. These patients often have thicker femoral
cortices and smaller medullary canals than do patients with degenerative
arthritis, and therefore smaller stems with thinner cement mantles
are often required.
Some authors have indicated that failure of a Precoat prosthesis
begins with cement debonding
35
, whereas others have found loosening at the bone-cement interface
to be the primary cause of failure
3
. In our series, four separate failure modes were noted: loosening
at the bone-cement interface, cement-stem debonding, early focal
osteolysis, and stem fracture. On radiographic examination, the
majority of the failures appeared to have been due to loosening
at the bone-cement interface; however, on gross examination most
components also showed evidence of cement-stem debonding.
Thirty-three (67%) of the forty-nine prostheses that failed were
the smallest sizes (small or CDH design). Of the twenty-one prostheses
whose model could be ascertained, five were Precoat and sixteen
were Precoat-Plus.
Patients who sustained a fracture of the component were young,
heavy, and quite active. Four of the fractured prostheses were small
and one was medium, and all were in the left hip. The mode of fracture
was similar to that described by others
35
. The fracture line passed through a laser etching on the anterior
aspect of the distal portion of the stem and appeared to be due
to fatigue failure. The manufacturer has since modified the technique
for marking prostheses. All fractures occurred in prostheses that
remained well fixed distally but had loosened proximally.
Although the present study focused primarily on the rate, mode,
and causes of failure of the Harris Precoat stem, certain comparisons
were made with the Harris Design-2 prosthesis, which was used routinely
at our institution before the Precoat stem was employed. All of
the surgical procedures, with either the Harris Design-2 or the
Harris Precoat component, were performed by one surgeon. The surgical
approach, cementing technique, and postoperative care were similar
for the two groups. Second-generation cementing was used without
centralizers or porosity reduction. The age and gender of the patients
were similar between the two groups, although there was a higher
prevalence of avascular necrosis in the group treated with the Precoat
stem. However, the rate of failure of the Precoat prosthesis was
higher than that of the Design-2 stem both in patients with avascular
necrosis and in those with degenerative arthritis.
The rate of failure of a modern precoated femoral component was
found to be higher than one would have anticipated and stands in
marked contrast to the failure rate of an older prosthesis with
nearly twice the duration of follow-up. The causes of failure are
multifactorial, but it would appear that an important element in
the present series was the design and surface treatment of the femoral
component, including the increased surface roughness and precoating
with methylmethacrylate.
Tables presenting diagnostic categories for all hips and those that
failed, detailed radiographic findings, and the demographic characteristics
of the patients with a fracture of the Precoat stem are available
with the electronic versions of this article, on our web site at
www.ejbjs.org (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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