Abstract
Background: Alternative bearings have been explored in an attempt to
improve the longevity of total hip prostheses. A Food and Drug Administration
(FDA)-approved clinical study of a nonmodular acetabular component consisting
of a porous metal shell, compression-molded polyethylene, and a ceramic liner
inlay was discontinued following reports of early failures.
Methods: Between October 1999 and January 2003, 429 patients were
enrolled in a prospective study to evaluate a cementless ceramic-on-ceramic
total hip arthroplasty design (Hedrocel ceramic bearing cup; Implex,
Allendale, New Jersey). Two hundred and eighty-two patients (315 hips) were
treated with the experimental acetabular implant and 147 patients (157 hips)
were treated with an acetabular implant that consisted of the same porous
shell but an allpolyethylene liner. Clinical data including a Harris hip score
and responses to the Short Form-12 (SF-12) health survey were collected
preoperatively and at twelve and twenty-four months postoperatively. Serial
radiographs were made preoperatively; at six weeks, three months, six months,
and twelve months postoperatively; and annually thereafter. Retrieval analysis
was performed on all failed explanted components. Failure was defined as
fracture or displacement of the ceramic liner out of the acetabular component.
In addition, biomechanical testing was performed on unimplanted acetabular
components and mechanically altered cups in an effort to recreate the
mechanisms of failure. Finite element analysis was used to estimate stress and
strain within the ceramic liner under extreme physiologic loading
conditions.
Results: The ceramic liner failed in fourteen of the 315
experimental acetabular components; all of the failures were at the
ceramic-polyethylene interface. Patients with a body weight of >91 kg had a
4.76 times greater odds of the ceramic liner failing than those who weighed
=91 kg. Retrieval analysis demonstrated stripe and rim wear with evidence
of adhesive wear, indicating a potentially high-friction interaction at the
articulation. Finite element analysis demonstrated that the forces on the
ceramic liner in cups subjected to extreme loading conditions were
insufficient to cause fracture. Biomechanical testing was unable to reproduce
an initial ceramic liner displacement in vitro; however, when the ceramic
liner was forcibly displaced prior to biomechanical testing, complete
displacement and eventual fracture of the ceramic liner resulted.
Conclusions: We hypothesized that the combination of a high patient
body weight, an extensive range of motion, and subluxation of the femoral head
led to high friction at the articulation between the femoral head and the rim
of the liner, which initiated displacement of the ceramic liner. Subsequent
normal gait led to further displacement of the liner in all of the fourteen
failed components and eventually to ceramic fracture in twelve of the fourteen
components.
Level of Evidence: Prognostic Level I. See Instructions
to Authors for a complete description of levels of evidence.
Complications of polyethylene wear debris-induced osteolysis led to
the development of ceramic-on-ceramic total hip replacements and other
alternative bearing surfaces such as highly cross-linked polyethylene and
metal-on-metal
bearings1-4.
However, concerns have been raised about the increased stiffness associated
with all-ceramic and metal-backed ceramic acetabular components, which
potentially leads to increased rates of migration and
loosening2,3.
The design goal of so-called "sandwich" cup designs, with
polyethylene interposed between the ceramic bearing surface and the outer
metal shell, is to reduce the rigidity of the ceramic-on-ceramic coupling and
prevent impingement between the rim of the ceramic liner and the neck of the
femoral stem5. One
such design (Hedrocel ceramic bearing cup; Implex, Allendale, New Jersey)
consists of a porous tantalum shell, compression-molded polyethylene, and a
ceramic bearing insert (Fig.
1).
In 2003, patient enrollment into a prospective United States Food and Drug
Administration (FDA)-approved clinical study of the Hedrocel ceramic bearing
cup was halted following reports of three failed ceramic acetabular liners. As
of the time of writing, eleven additional failed ceramic liners have been
reported, for a total of fourteen. The resulting investigation, failure
analysis, and review of the literature led us to hypothesize that a
combination of high patient body weight and deep flexion activities caused
high frictional interaction and torque, which in turn led to dislodgment and
subsequent fracture of the ceramic acetabular liner.
Beginning in October 1999, a prospective, randomized, controlled
FDA-regulated investigational device exemption clinical trial was initiated in
the United States to assess the clinical performance of the Hedrocel ceramic
bearing cup. The study protocol was approved by the investigational review
boards at twenty-two institutions, and all subjects gave their informed
consent to participate in the study. The nonmodular cup comprised a porous
tantalum shell, direct-compression-molded ultra-high molecular weight
polyethylene, and an alumina ceramic bearing insert (Biolox Forte; CeramTec,
Stuttgart, Germany). The ceramic bearing was mechanically secured to the
polyethylene by an interference fit (Fig.
1). The study was designed to compare this cup with a nonmodular
design consisting of the same porous tantalum shell and an articulating
surface of direct-compression-molded polyethylene. All cups had a 28-mm
internal diameter, and the range of outer diameters was 48 to 64 mm in 2-mm
increments. All subjects received a 28-mm alumina ceramic head, which had 0,
+3, and +6-mm neck-length options (Biolox Forte). The modular femoral stems
used in the study included press-fit hydroxyapatite-coated titanium-alloy and
porous-coated cobalt-chromium-alloy implants (both ProxiLock designs; Implex)
and cemented cobalt-chromium-alloy implants (Cobrex; Implex).
The ratio of the random group assignment was two ceramic-on-ceramic
arthroplasties for every one ceramic-on-polyethylene arthroplasty. There were
fewer control patients because we had thought that another population of
control subjects would be available from an identically structured and
randomized FDA-regulated study of metal-on-metal implants that had used the
same control implants and our intention had been to pool the controls.
However, with the termination of this study, the two studies were divorced so
we did not have access to the second group of controls for the present
analysis. No attempt was made to blind the thirteen principal investigator
surgeons or the eleven coinvestigator surgeons with regard to the bearing
surface assignment.
Clinical outcome measures included radiographic analysis, Harris hip
scores6, and
assessments with the Short Form-12 (SF-12) Health
Survey7.
Radiographic analysis was performed by a radiologist who had not been involved
in the patient's care and included anteroposterior pelvic, anteroposterior
hip, and frog-leg lateral views. Harris hip scores and SF-12 data were
collected preoperatively and at twelve and twenty-four months postoperatively.
Hip range-of-motion values were derived from the motion subscale of the Harris
hip score. Radiographs were made at the preoperative visit; at six weeks and
three, six, twelve, and twenty-four months following the surgery; and annually
thereafter. The primary criteria for including a patient in the study were an
age between eighteen and seventy-five years, a body-mass index of <40, a
preoperative Harris hip score of <70 points, and clinical indications for
total hip replacement. Subjects with bilateral hip disease were randomized
once and received the same cup in both hips.
Biomechanical Data and Failure Analysis
Failed acetabular components retrieved at the time of revision were
analyzed both macroscopically and microscopically. Ceramic liners that were
retrieved intact (not fractured) and the corresponding femoral heads were
analyzed with profilometry and a coordinate measuring system (Mitutoyo FJ805;
Mitutoyo America, Aurora, Illinois), and the wear areas were documented with
scanning electron microscopy.
The mechanical integrity of the interference fit between the ceramic liner
and the polyethylene was measured in unimplanted cups with use of a lever-out
test, with torque calculated from the measured force and apparatus dimensions.
The lever-out test was performed by tangentially loading the ceramic liner
within a water bath, with testing performed at both room and body temperatures
(20° and 37°C). The load was increased, and maximum load was recorded
when the ceramic liner escaped capture. The maximum torque at failure was
calculated from the load applied and the distance from the center of rotation
of the ceramic liner (14 mm).
In an attempt to experimentally replicate the in vivo failures, unimplanted
cups were tested with use of hip-wear simulators under a variety of adverse
conditions, including creation of neck impingement, introduction of third-body
debris, and testing of the effects of deliberately roughened heads, partially
dislodged ceramic liners, and microseparation conditions. The hip simulators
included an MTS 3 Station 4-DOF (four-degrees-of-freedom) Hip Wear Simulator
(MTS Systems, Eden Prairie, Minnesota) with the components in the anatomical
position at room temperature and an AMTI-Boston 12 Station Hip Simulator
(AMTI, Watertown, Massachusetts) with the components in the inverted position
at 37°C. All testing was performed with a modified Paul curve in bovine
serum.
Finite element analysis of the ceramic bearing acetabular cup was performed
for extreme loading conditions (a near maximum range of motion of 60° with
a 5-kN [1124-lb] load) by EndoLab Mechanical Engineering (Rosenheim, Germany).
The purpose of the finite element analysis was to determine the location and
magnitude of the maximum stress within the materials that make up the
prosthesis (porous tantalum, polyethylene, and the alumina ceramic liner). The
femoral neck had a 120° maximum arc of motion before it impinged on the
acetabular component. Thus, the end point of motion used in the finite element
model was 60° as measured between the axis of the femoral neck and the
polar axis of the acetabular component. As a result of the nonaxisymmetric
load condition, a three-dimensional model was constructed with use of the
program MSC.MARC 2001 (MSC. Software, Ann Arbor, Michigan). Cups with 48 and
54-mm outer diameters were modeled. The finite element analysis model assumed
that the acetabular cup was loaded with a 28-mm alumina ceramic femoral
head.
Statistical Analysis
Summary clinical data and group comparisons were made for the
ceramic-on-ceramic and ceramic-on-polyethylene arms of the study as well as
within the ceramic-on-ceramic group to compare data between the failed and
non-failed cases. For the latter data analysis, patient weight, body-mass
index, gender, age, size of the acetabular cup, abduction of the acetabular
component, type of femoral stem, range of motion at one year, and Harris hip
score at one year were compared by using Mann-Whitney rank sum or Fisher exact
testing followed by logistic regression in univariate analysis, with failure
as the outcome of interest. Variables with p values of =0.25 in the
univariate analysis were entered into a multivariate logistic model. Alpha was
set at 0.05 for all assessments. Statistical calculations were performed with
MINITAB 14.11 (Minitab, State College, Pennsylvania).
Clinical
Three hundred and fifteen ceramic-on-ceramic total hip prostheses
were implanted in 282 patients, and 157 ceramic-on-polyethylene total hip
prostheses were implanted in 147 patients. Demographics were comparable
between the ceramic-on-ceramic and ceramic-on-polyethylene groups. In both
groups, 77% of the hips had a diagnosis of osteoarthritis, 17% had aseptic
necrosis, 3% had rheumatoid arthritis, 2% had a fracture, and 0.3% had another
diagnosis. Fifty-five percent of the patients were male, and 45% were female.
The mean age (and standard deviation) was 54 ± 12 years (range,
twenty-three to seventy-six years), and the mean body-mass index was 29
± 4.6 kg/m2 (range, 18.4 to 39.9 kg/m2). No
differences in the above factors were seen between the ceramic-on-ceramic and
ceramic-on-polyethylene groups.
At the time of analysis, 98.8% of patients had been followed for two years
or more. The mean preoperative and one-year postoperative Harris hip scores
were 45 points (range, 14 to 91 points) and 92 points (range, 36 to 100
points), respectively, in the ceramic-on-ceramic group and 43 points (range,
10 to 78 points) and 93 points (range, 51 to 100 points) in the control group;
these scores did not differ significantly between the groups. The SF-12 Mental
and Physical Component Summary scores were comparable between the
ceramic-on-ceramic and ceramic-on-polyethylene groups at all time-points, and
the radiographic results were also comparable. No progressive radiolucency,
cup migration, or evidence of osteolysis was seen in either patient group at
any follow-up time. Excluding hip revision necessitated by failure of the
ceramic liner, only two ceramic acetabular components were revised: one
because of recurrent dislocation and the other because of groin pain of
uncertain etiology. In the control group, one cup was revised at nineteen
months because of recurrent dislocation, one was revised because of persistent
groin pain, and one was revised during surgery for a periprosthetic femoral
fracture.
As of April 15, 2005, fourteen of the 315 implanted ceramic bearings were
reported to have failed. The time to failure averaged twenty-five months and
ranged from eight to forty-two months. The median weight and body-mass index
of the patients with a failed ceramic-on-ceramic prosthesis were 102.5 kg and
30.7 kg/m2, respectively (Table
I), with all but three failures occurring in patients weighing
>91 kg. In contrast, the median weight and body-mass index of the patients
with no report of failure were 83.4 kg (p = 0.004) and 27.9 kg/m2
(p = 0.09), respectively. Twelve of the fourteen failures occurred in men. The
results of univariate logistic regression analysis of subject characteristics
for the fourteen failed and 301 non-failed ceramic-on-ceramic implants are
shown in Table II. No
significant association was found between failure and age, range of motion,
Harris hip score at one year, acetabular cup size, stem size, stem type, or
cup abduction angle. Male gender (p = 0.03), a weight of >91 kg (p =
0.006), and an increased body-mass index (p = 0.045) increased the odds of the
ceramic liner failing.
On the basis of the univariate regression analysis, gender, a weight of
>91 kg, and acetabular cup size were assessed in a multivariate logistic
regression model. Body-mass index was excluded from the model as it is
calculated from patient weight and height and is therefore not an independent
variable. The results of the multivariate logistic regression analysis are
shown in Table III. After we
controlled for gender and acetabular cup size, we found that patients with a
body weight of >91 kg had a 4.76 times greater odds (95% confidence
interval, 1.14 to 19.92) of having a ceramic liner failure than those who
weighed =91 kg (p = 0.03). The difference based on gender seen in the
univariate analysis was explained when weight data were stratified by gender
for the entire ceramic-on-ceramic group. The mean weight for men was 94
± 15 kg, whereas it was 74 ± 15 kg for women (p < 0.0005).
The increased odds of failure seen in men were due to the confounding of the
increased mean weight of men compared with that of women in the study
group.
Anecdotal clinical information reported by the surgeons who performed the
revision procedures in five of the fourteen hips indicated that these patients
had experienced a noise or sensation during a deep-hip-flexion activity at the
approximate time of ceramic failure. One patient involuntarily experienced
deep flexion during a work-related accident. Another patient had frequent
unusual sensations about the replaced hip when he tied his shoe. One patient
with a failed bearing stated that she gardened often and typically in a
squatting position. The fourth patient reported the hip "going"
while in a deep-squat position, and the fifth such failure was in a body
builder who often engaged in deep-hip-flexion activities. The
"noise" or "sensation" was described as a
"squeak" emanating from the hip. This finding was not analyzed
statistically.
Retrieval Data
At the time of revision, twelve of the fourteen acetabular bearing surfaces
were found to have fractured. In the remaining two hips, the acetabular
bearing surface had disengaged from the polyethylene portion of the acetabular
component but remained intact. In both of these unfractured implants, the
ceramic liner was apparently entrapped by the femoral neck, as illustrated in
Figures 2 and
3. Visual analysis of the
acetabular polyethylene at the time of revision indicated that little or no
impingement had occurred between the femoral neck and the edge of the
polyethylene layer, the first point of contact in impingement, prior to
displacement of the ceramic liner. Metal transfer from the neck of the femoral
stem was observed on the rims of the two ceramic liners that were retrieved
intact (Fig. 3) and on the rims
of all of the fractured ceramic liners
(Fig. 4). The ceramic heads
corresponding to the two liners that were retrieved intact showed evidence of
stripe wear. This could not be definitively identified in the remaining heads
because they were damaged by the fractured acetabular liners. Scanning
electron microscopy identified abrasion of the ceramic surface with grain
pull-out within the striped wear area of the ceramic heads
(Fig. 5). Metrological analysis
of the two retrieved intact liners and their corresponding heads indicated
that the components had deformation of the surfaces outside of the original
manufacturing tolerances due to in vivo wear.
Biomechanical Data
The strength of the ceramic liner-and-polyethylene assembly of five
unimplanted cups of each size from the manufacturer's inventory (48 to 64 mm)
was measured with lever-out testing at both body and room temperatures. The
strength of the assembly (defined here as the resistance to torsional
dislodgment of the ceramic liner) at body temperature averaged 33.4 ±
3.8 Nm (295 ± 34 in-lb), with a range of 24.9 to 41.2 Nm. The lever-out
torque at body temperature was approximately 40% less than that at room
temperature, which is consistent with the decrease in elastic modulus and
yield strength of polyethylene for this change in temperature. The results of
the lever-out testing also showed no apparent loss of integrity of the capture
mechanism caused by shelf aging within the package. (The shelf ages of the
tested cups ranged from two to four years.)
In the hip wear/failure simulation testing, femoral neck impingement did
not result in displacement of the ceramic liner. With current hip-simulator
technology, we could not reproduce microseparation of the articulation in
combination with load and motion of the hip joint. Complete displacement of
the ceramic liner occurred only after partial displacement (experimentally
caused prior to testing). Finally, fracture of the ceramic liner occurred
subsequent to complete displacement of the ceramic liner from the remainder of
the acetabular component. Hip wear simulation testing, with use of standard
and modified gait loads as well as abrasives added to the lubricant for
acetabular cups that had not been altered prior to testing, could not
replicate the failure mechanism observed in vivo.
Finite element calculation of maximum principal tensile stress within the
ceramic liner indicated a value of 100 MPa (14,500 psi) for a 5-kN (1124-load)
load at a nearly maximum range of motion (60°). In contrast, the
manufacturer of the alumina ceramic liner (Biolox Forte) has reported the
four-point strength to be 580 MPa (84,000 psi), far more than the maximal load
predicted by our finite element
analysis8.
With the exception of the failed ceramic liners, the excellent
short-term clinical results associated with the ceramic-on-ceramic and
ceramic-on-polyethylene articulations in our study are consistent with the
results in other clinical studies of ceramic articulations in primary total
hip
replacement9-11.
The demographic and clinical similarity between the subjects with a
ceramic-on-ceramic articulation and those with a ceramic-on-polyethylene
articulation in our study suggests that selection bias did not play a role in
the observed failure rate. Analysis of the clinical data indicated that the
odds of ceramic liner failure were 4.76 times greater in patients who weighed
>91 kg than in those who weighed =91 kg after we controlled for gender
and the size of the acetabular cup. The Harris hip scores indicated that the
patients with and without failure of the liner were quite active with an
essentially unrestricted range of hip motion. Anecdotal information about the
activities of five of the patients with a failed bearing indicated a
relationship between symptoms and deep hip flexion.
Retrieval analysis of the two intact liner-and-head pairs demonstrated
striped head wear and rim wear. The wear damage observed macroscopically and
microscopically showed evidence of abrasion and grain pull-out, indicating
both abrasive and adhesive wear
mechanisms12. This
observation is in agreement with the findings of other reports.
Alumina-alumina ceramics have been reported to wear in vivo as a result of
rim/edge loading at the extremes of motion and/or by means of
microseparation4,12-14.
Walter et al. reported a 52% prevalence of rim and/or striped head wear of
ceramic-ceramic hip
bearings12.
Retrieved ceramic bearings in other studies have demonstrated stripe and rim
wear, and scanning electron microscopy has shown that wear was caused by grain
pull-out (adhesive wear) and abrasion (by the pulled-out
grains)12,15.
By carefully documenting the locations of the stripe and rim wear relative to
the osseous anatomy at the time of retrieval, Walter et al. concluded that
this mechanism was due to head-rim engagement with subluxation in a deep
hipflexion position.
The relevance of this finding is that adhesive wear occurs when two
opposing solid surfaces come into physical contact (e.g., when there is
complete breakdown of fluid boundary lubrication), thereby leading to high
friction/traction
forces15. This
causes high, localized friction at the contact interface. Prosthetic hip
bearings are known to demonstrate mixed-film boundary lubrication under
biological conditions (i.e., an absence of lubrication during stop-start,
direction reversals, and low-relative-velocity
conditions)16-18.
Such mixed-film lubrication conditions increase the probability of adhesive
wear, which becomes more likely for bearing surfaces damaged by wear because
of disruption of the fluid boundary layer. The coefficient of friction for
nonlubricated alumina-alumina contact has been reported to range from 0.5 to
1.0, whereas the value for lubricated contact ranges from 0.002 to 0.05,
depending on the
lubricant19-21.
Our laboratory simulations conclusively showed that neck impingement alone
cannot cause dislodgment of this ceramic liner. In addition, hip simulation of
normal gait cycles under a variety of adverse conditions did not generate
cyclic or singular torsional forces sufficient to dislodge the ceramic liner.
However, with the liner partially displaced, normal gait cycles resulted in
complete dislodgment and eventually fracture.
We suggest that high torsion across the articulation interface that leads
to ceramic dislodgment must originate with, and depends on, one or more of the
following: friction between the head and liner, subluxation and edge loading
of the head and liner, and the patient's weight. The relevance of patient
weight to the failure mechanism is that frictional torque at the articulation
interface is directly proportional to patient weight; the higher the patient's
weight, the larger the frictional force and associated torque transmitted
about the center of rotation of the hip. Given the force multiplier of five
times body weight in deep flexion, most of the patients in whom the ceramic
liner failed could exceed a load of 4460 N at the hip
articulation8.
Coupled with a high coefficient of friction of 0.5 to 1.0, there is a
resultant 2230 to 4460-N force on the ceramic liner acting tangentially 14 mm
from the center of the 28-mm-diameter head. This results in a torque of 31.2
to 62.4 Nm, which exceeds the strength of the assembly of the ceramic liner
within the polyethylene portion of the ceramic cup (33.4 ± 3.8 Nm at
body temperature).
Finite element analysis of the cup at the near end point of the range of
motion and with high load (5 kN) indicated a maximum stress within the ceramic
(100 MPa) that was substantially less than that required to cause fracture of
the ceramic (flexural strength = 580
MPa)8. Analysis of
the retrieved fractured and non-fractured inserts indicated that overload or
fatigue fracture of the ceramic liner alone did not cause failure but rather
that displacement of the liner from the polyethylene socket was a prerequisite
to failure.
In their case report, Akagi et al. noted head and liner wear of the ABS
nonmodular ceramic
cup1. They
speculated that torque generated by damage to the articulation caused by
microseparation during normal gait led to the gradual propagation of a defect
in the polyethylene layer, causing a loss of support of the ceramic liner and
eventual failure. Hasegawa et al. reported two ceramic liner fractures and one
liner dissociation in a study of thirty-five nonmodular
ceramic-polyethylenemetal acetabular
components22. They
suggested that the fractures were likely due to edge loading with the liner in
situ and the dissociation was due to the mechanism described by Akagi et al.
In contrast, we agree with Walter et
al.12 and believe
that the displacement of the ceramic liner occurs during subluxation and
reengagement of the head and liner during deep flexion.
In summary, we believe that the failures of the ceramic liner of the
nonmodular, so-called sandwich-design ceramic-on-ceramic cup were caused by
high torque transmitted from the femoral head to the ceramic liner, causing
dislodgment of the ceramic liner from the polyethylene socket. We postulated
that the origin of high torque is frictional interference between a
wear-damaged ceramic head and the rim of the ceramic liner. Striped wear
damage of the head occurs as a result of edge loading and rim wear, due to
subluxation and microseparation. The high frictional torque transmitted across
the articulation interface probably occurs during deep flexion or other high
load/extreme range-of-motion activities. Subsequent to dislodgment of the
ceramic liner and with continued articulation of the hip joint, the ceramic
liner completely displaces and eventually fractures in most cases.
A potential solution to the ceramic liner displacement is the addition of a
geometric irregularity such as a central peg or a change in the geometry of
the outer surface of the ceramic liner from a hemisphere to an angled surface
between the ceramic-polyethylene interface. This would increase the torque
required to displace the liner and reduce this mode of failure.
This investigation revealed a previously unknown phenomenon that is
inherent in ceramic-ceramic hip articulations, which is high friction and
torque associated with the tribological process that causes striped head wear
and rim liner wear. ?
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