Hemispheric Titanium Porous Coated Acetabular Component Without Screw Fixation
Lawrence D. Dorr, MD*, Zhinian Wan, MD*, and
Jondy Cohen, MD*
One hundred fifteen hips in 108 patients with primary
total hip arthroplasty using the anatomic porous replacement
hemispheric acetabular component implanted without adjunctive
screw fixation had a mean postoperative follow-up time of 6
years (range 5-7.4 years). Clinical evaluation was performed
using the Harris hip score and patient self assessment using a
modified Short Form-36 questionnaire. Radiographs were
measured for radiolucent lines, polyethylene wear, osteolysis,
migration, and fractures. No acetabular metal shell had been
revised for loosening or was radiographically loose with or
without migration (more than 3 mm) at final follow up.
Reoperation was done in nine (8%) hips because of polyethylene
insert wear or disassembly. No fracture of the acetabular bone
occurred at the time of surgery or was observed on radiograph.
Fixation of the metal shell was stable, with progressive
radiolucent lines observed at final follow up in 2% of the
hips. Osteolysis was recorded in one patient with two
acetabular components. The fixation of noncemented hemispheric
porous coated acetabular components is more related to the
technique of acetabular bone preparation and press fit
implantation than to whether additional screws or peg fixation
are used. Fixation of this acetabular component without screws
at an average of 6 years after surgery is reproducible and
predictable in primary hip arthroplasty. The design of modular
polyethylene inserts has been improved and should reduce the
wear rate of reoperations of the polyethylene insert.
Revision and reoperation of the acetabulum is the most
common reason for repeated surgery in total hip arthroplasty. 7,12, 23, 43, 44 Radiographic evidence of loosening
or migration has been reported in as many as 11% to 50% of
acetabular components in cemented total hip arthroplasty at 10
to 21 years after surgery. 7,21,35, 43 With noncemented
acetabular components, fixation failure has occurred less
frequently, but reoperation for wear and disassembly of the
polyethylene insert has been common. 11, 13, 34, 36, 43
Additional concerns with noncemented modular
acetabular components include the increased wear measured of
the polyethylene and the incidence of expansile osteolytic
defects in the first 5 to 10 years. 20, 25, 34, 40, 43,
46 The use of screws as adjunctive fixation for the
metal acetabular component has been implicated as causing
impingement on the polyethylene inert, fretting wear,
corrosion between the screws and the shell, screw fracture,
and the vascular injury. 21-23, 41 Acetabular
components with adjunctive peg fixation have been reported to
have an increased incidence of osteolysis that ranges from 9%
to 205 in 5 to 10 years. 19, 25, 46
In 1988 the anatomic porous replacement hemispheric porous
coated acetabular component (APR, Sulzer Medica, Austin,
THERAPY) first was implanted without adjunctive screw
fixation. Morsher et al 31, 32 and Gordon Hill (as
reported by Schmalzried et al) 36 were using a
similar technique with different components. Morscher et
al31, 32 subsequently published their experience
that fixation with bone screws made no difference as compared
with that without the use of screws. Schmalzried et
al36 reported that none of these implants were
loose or revised at 5 years after surgery. In addition,
laboratory studies subsequently suggested that components with
press fit fixation were stable. Won et al42 and
Kwong et al26 observed in the laboratory that if
the acetabular component was inherently stable by press fit
techniques supplemental screw fixation did not significantly
enhance the stability of the implant. The current study
assesses the results seen with a press fit porous coated
hemisphere with a minimum of 5 years or postoperative
follow-up.
MATERIALS AND METHODS
From a computer database of 207 primary total hip
arthroplasties performed from June 1988 to November 1990, 132
primary total hip arthroplasties with press fit acetabular
components were performed by the senior author (LDD). All hips
during this period that were reconstructed by this technique
were reviewed. An additional 75 hips were implanted with the
same component using additional screw fixation. The first
acetabular components implanted without screws were done so
selectively to be able to observe the early results of this
technique and confirm that early migration would not occur.
Hips were selected by intraoperative confidence that the
components was secure without screw fixation. From June 1988
to December 1988, 30% of noncemented cups were press fit; in
1989, 68% and in 1990, 93%. All of these hips were included in
this study. Seventeen hips were excluded from statistical
analysis because 10 patients with 11 treated hips died before
5 years of follow-up, and five patients with six treated hips
were lost to follow-up. These 15 patients (17 hips) had no
radiographic loosening or revision of acetabular components,
and had satisfactory clinical results before death or at last
follow-up. One hundred fifteen hips in 108 patients with a
minimum 5-year follow-up were included in this study.
The APR hemispheric porous coated acetabular component was
used in all hips. The acetabular component is a hemisphere and
is manufactured from titanium +6% aluminum, +4% vanadium alloy
with a pure titanium porous coating named cancellous
structured titanium. The porous coating is heat sintered to
the alloy with a pore size of 490 um and 55 volume % ingrowth
available. The metal shell has 3-mm thick walls. Thus, a 49-mm
shell with a 28-mm femoral head would have a 7.5-mm thick
polyethylene insert. The modular polyethylene inserts used in
these hips were circumferentially incongruent with the metal
shell (a gap) by 0.5 to 0.75 mm and had rotatory motion of 0.5
mm (fig 1) The femoral head was cobalt chrome.
The femoral components used were the APR 1 stem in 54
hips, with 32 noncemented and 22 cemented; the APR II in 21
hips, with 17 noncemented and four cemented; and the APR II
stem with a sleeve that was noncemented in 40 hips. In these
hips the choice of the stem and fixation was based on the age
of the patient , the quality of bone of the femur, and the
stage of development of the femoral stem prosthesis. The
design of the acetabular component did not change.
A posterior surgical approach was used in all hips. 8 The goal of the acetabular preparation technique
was to optimize contact between the metal shell and the bone
and to provide a press fit between metal and bone. The
acetabulum was reamed progressively with hemispheric reamers
to obtain circumferential bone contact with bleeding
cancellous or subchondral bone. The initial reamer size used
was 4 mm smaller than the size expected to be needed from
templating the radiograph. Reaming was done with a to and fro
motion, rather than a rotatory motion, from a transverse to a
horizontal inclination of the reamer. The final reamer was
inclined firmly under the superior acetabular rim to ream this
bone to a hemisphere from its normal sloped anatomy. The final
reamer size was determined by complete rim contact of the
reamer, even if this meant thinning of the medial bone. A
trial shell the same size as the final reamer was inserted. If
this trial shell showed loosening in either the superoinferior
or anteroposterior (AP) plane, the next size trial shell was
inserted. If this second trial shell fit satisfactorily, the
implant for that trial shell was selected. If the second trial
shell could not be seated fully, the rim of the acetabulum was
reamed by that size reamer. The reamer was not seated fully.
This technique resulted in a flaring of the acetabulum because
it had been reamed deeply to a size 2 mm smaller than the rim.
The second trial shell now would have a secure fit. The actual
porous coated acetabular component was 1 mm larger than the
final trial shell used. For example, if a 52-mm trial shell
was used, a 53-mm acetabular component was implanted. This
technique meant that in many patients the acetabulum was
underreamed by 1 to 3 mm. The position of the acetabular
component was preferred to be 40 mm abduction (theta angle)
and 20° anteversion.
Clinical follow-up information was obtained by interview
and physical examination by one of the authors (LDD) or the
clinical fellows for each patient. Harris hip scores 17 were determined preoperatively, 6 and 12 months
after the operation, and annually thereafter. For this study,
the preoperative, 2-year, and final follow-up scores were
compared. Patient self-assessment was available at final
follow-up for 78 hips. The self assessment used was a modified
Short Form-36 using 23 questions (Form HBK21, Orthographics,
Salt Lake City, UT.) The questions from this form that were
evaluated and compared with the physician?s score were (1) the
patient?s grade of operation, (2) pain, and (3) limp.
Radiographic examination included an AP radiograph of the
pelvis centered over the pubic symphysis, which included the
proximal femur so that the femoral stem could be visualized,
and a 17-inch Lowenstein lateral of the hip. These radiographs
were obtained preoperatively; 3, 6, and 12 months after
surgery; and annually thereafter. No jig was used for these
radiographs, but two radiographic technicians obtained all of
the postoperative radiographs, and emphasis was placed on
patient position and radiographic exposure. T he magnification
of radiographs was corrected by the k known diameter of the
metal femoral head. Radiographic measurements were performed
on the preoperative, 3-month, 2-yeare, and final follow up
postoperative films. The preoperative radiographs were
measured for Type A, B, and C femoral bone10 to be
able to evaluate the influence of osteoporotic bone on
fixation. The diagnosis of the etiology of the arthritis also
was classified from the preoperative film. From postoperative
radiographs the angle of inclination of the acetabular
component was measured with the method first described by
Yoder et al 45 and modified by Callaghan et
al.6 The coverage of the acetabular component by
host bone on the AP radiograph was measured as a percent of
the acetabular circumference. Migration of the acetabular
component was defined as linear in the direction of medial,
superior (or both), or rotational, which is a change in the
angle of inclination of the component. The method modified by
Callaghan et al16 was used for determining a fixed
reference on the pelvis. A horizontal line connected the
inferior points of the bony teardrops, and vertical lines were
drawn through the medial edge of the teardrops; the position
of the metal shell was measured from these reference
points.
The acetabulum was divided into three zones as described by
DeLee and Charnley9 for assessment of the
radiolucent lines in the AP and lateral radiographs. The
contact of the surface of the acetabular component with
acetabular bone was measured by zones. Regions in which the
surface of the acetabular component was not in contact with
bone appeared as radiolucent lines on the immediate
postoperative radiographs and were classified as gaps to
distinguish them from radiolucent lines that appeared on
subsequent radiographs in zones where no gaps initially had
existed. The radiographic measurements were made by one
observer (ZW), who performed the measurements without clinical
knowledge of the patient. The width of the radiolucent line in
each zone was measured visually using a micrometer.
Radiolucent lines were recorded as increased if in serial
radiographs the number of radiolucent lines increased;
decreased if fewer radiolucent lines were observed; and stable
if there was no change in the occurrence of radiolucent lines.
If there was a complete radiolucent line around the acetabular
component and the width of the radiolucent line was greater
than or equal to 2 mm with or without acetabular migration,
the component was classified as loose. Migration of more than
3 mm was considered loosening.
Acetabular fracture was identified by visual assessment
during surgery and by radiographic assessment immediately
after operation.
Polyethylene wear was measured by modification40 of the technique of Livermore et al.28 Femoral head
displacement that was measured on the 3-month radiograph was
considered creep of the polyethylene,2,4,11 and
subsequent displacement was considered wear. Three head sizes
were used (26 mm, 28 mm, and 32 mm). The head size was
selected randomly by surgeon choice with no firm criteria for
selection.
Fixation of the stem was measured by zonal analysis or
radiolucent lines as described by Gruen et al.16 Cemented fixation was grade by the ABCD grading scale of
Barrack et al.3 Noncemented fixation was graded by
a modification of the system of Engh et al.14 The
modification is used for smooth diaphyseal femoral stems with
metaphyseal bone ingrowth fixation because the radiographic
pattern of remodeling differs. Almost no proximal osteopenia
occurs, and the radiographic classification of stable fibrous
fixation is not applicable.11 Type IA fixation has
radiolucency only in Zone 4, Type IB has three to five
radiolucent zones around the smooth stem, Type II has
radiolucent zones in all smooth zones; and Type III has
divergent radiolucency or migration of the
stem.30
All data were analyzed statistically using SPSS software
(SPSS Inc. Chicago, IL). The Wilcoxon test was used to compare
the Harris hip score and the pain score, limp score, and
rating of clinical results between the physician grade and
patient self assessment. The chi squared test was used to
analyze radiolucent lines between hips with Types A and B
compared with bone Type C.
RESULTS
Forty-two patients with 45 treated hips were woman, and 66
patients with 70 treated hips were men. The average follow up
was 6 years (range, 5-7.4 years). T he average age was 60
years (range, 24-86 years). Fifteen patients were younger than
45 years of age, 36 were older than 60 years of age. The
average weight was 177 pounds (range, 106-325 pounds).
Twenty-five patients weighed less than 150 pounds, 61 weighed
150 to 200 pounds, and 29 weighed more than 200 pounds. The
reason for the operation was osteoarthritis in 97 hips,
osteonecrosis in eight hips, rheumatoid arthritis in four
hips, developmental dysplasia in four hips, and traumatic
arthritis in two hips.
The preoperative mean Harris hip score was 48.1, the
postoperative score at 2 years was 94.3, and a final follow-up
was 91.2. Before surgery, the mean pain score was 17.9. At 2
years and final follow-up, the scores were 42.5 and 43.2
respectively. The preoperative mean limp score was 5, the limp
score at 2 years after surgery was 10.3, and at final
follow-up was 9.7. According to the Harris hip score, 67.8% of
hips were rated as excellent. 20.1% of hips as good, 8.6% as
fair, and 2.6% as poor. Thirteen hips were rated as fair and
poor with the reason in five patients being spine disease;
four had stem Type II or III fixation; two had a weak gluteal
medius with rapid onset of buttock pain; one had rheumatoid
arthritis; and one had cancer. Patient self assessment
evaluation at final follow showed 71.8% of patients rated
their hip as excellent, 15.8% as very good, 7.7% as good, 2.6%
as fair, and 1.3% as poor. The average pain score was 38.9 and
the average limp score was 9.6. Comparison of the physician?s
clinical evaluation with he patient?s self assessment gave no
statistical difference for limp score (p = 0.38) and rating of
clinical results (p>0.05). The pain score was statistically
different (p=0.006), with the patient listing more pain than
did the physician. Patient evaluation showed that 20 hips had
more pain than physicians reported. Six patients had back
pain; two had psychiatric difficulties; two had rheumatoid
arthritis; one had fracture o the femur; one had Reiter?s
syndrome; and in nine patients, no reason was identified for
the difference.
All hips had at least 90% coverage of the acetabular
component on radiograph, with 100% coverage in 84 hips, 95% in
28 hips, and 90% in three hips. The coverage of acetabular
components was not related to radiolucent lines. No acetabular
fractures were identified, either during surgery or on
radiographs. The average acetabular angle of abduction
inclination (theta angle) was 46.4° (range, 35° -60° ).
Anteversion could not be measured accurately on the
postoperative radiographs because of the metal acetabular
shell.
The average polyethylene insert thickness for the 115 hips
was 10.5 mm+ 1.9 mm. For 49 hips with 26-mm femoral
heads, the average polyethylene thickness was 10.7 mm+ 1.8 mm; for 28-mm heads, the polyethylene thickness was 10.7
mm+ 1.6 mm; and for 22 hips with 32-mm heads,
polyethylene insert thickness was 9.8 mm+ 2.3 mm. The
average linear polyethylene wear for 115 hips was 0.168 mm per
year. In 104 hips that did not require operation, polyethylene
wear was 0.128 mm per year, compared with 0.336 mm per year in
11 hips with reoperation. Between the hips with and without
reoperation there was no statistical difference for patient
age, gender, weight, and diagnosis (all p > 0.05). One of
the 11 hips had polyethylene thickness less than 8 mm. In this
hip, a 49 mm metal shell was used with a 3-mm thick
polyethylene insert, and the polyethylene insert disassociated
(Table 1).
Overall, measurement of fixation showed that all
postoperative periods no acetabular component had migration
more than 3 mm (from the 3-month postoperative film).
Radiolucent gaps appeared in 39 hips on the initial
postoperative radiographs. Twenty hips had a radiolucent gap
in two zones. Zone 2 most frequently had a postoperative
radiolucent gap, and these gaps were the ones that most often
persisted at final follow-up. All 20 hips with two zones had a
radiolucent line in Zone 2, with an accompanying Zone 1 or
Zone 3 line (Table 2). None of these gaps were greater than 1
mm. The incidence or radiolucent gaps was higher on AP
radiographs than on lateral radiographs. As was the incidence
of final radiolucent lines. In 26 of 39 hips, these
radiolucent gaps were not present on final radiographs, and in
13, the radiolucent gap on the initial radiograph was measured
as a constant radiolucent line on final radiographs (Fig 2).
Three hips that did not have a radiolucent gap on the
postoperative radiograph did have a radiolucent line on the
final follow-up radiograph. Thus, 16 hips had radiolucent
lines on final radiographs, with three of these being
progressive (Table 2) All three of these radiolucent lines
appeared in the first two years after surgery. On the
radiographs obtained 2 years after surgery, as compared with
those obtained immediately after surgery, no change in the
incidence of radiolucent lines was present in 85 (73.9%) hips.
At 2 years after surgery, radiolucent lines decreased in 27
(23.5%) hips and appeared in three (4.3%) hips.
At the final postoperative follow-up, 99 hips had
radiolucent lines around the acetabular component. Eleven hips
had a radiolucent lines in one zone and five hips in two
zones. All five hips with the radiolucent line in two zones
had a radiolucent line in Zone 2 with a second line in either
Zone 1 o 3 (Fig 2). In a comparison of the occurrence of
radiolucent lines measured at 2 years after surgery with that
at final follow-up, there was no change in 109 (94.9%) hips,
and radiolucent lines had decreased in six hips (5.2%). No new
radiolucent lines occurred after 2 years of postoperative
follow-up.
Twenty-seven hips had bone Type A, 73 had bone Type B, and
15 had Bone Type C. On radiographs obtained immediately after
surgery, 30% of hips with Type A bone had radiolucent gaps,
compared with 33% of hips with Type B bone, and 46% of hips
with Type C bone. There was no statistical difference among
the bones types (p = 0.51). At final follow-up, 7.4% of hips
with Type A bone had a radiolucent line, compared with 17.8%
of hips with Type B bone, and 6.7% of hips with Type C bone (p
=0.28).
Focal osteolysis40 was present in one patient
with bilateral total hip arthroplasty. By 4 years after
surgery, the linear wear in this patient was 3.5 mm in the
left hip and 3 mm in the right hip. The metal shell size was
49 mm, femoral head size was 26 mm, and polyethylene insert
thickness was 8.5 mm, and polyethylene insert thickness was
8.5 mm. Osteolysis was present in the femur and acetabulum of
the left hip and the acetabulum of the right hip. Reoperation
was performed for exchange of the polyethylene and femoral
head. Four years after reoperation the patient has a Harris
hip score of 93 in the left hip and 100 in the right hip.
Of 115 hips, 25 had cementless stems, and 90 had
noncemented stems. In 25 cemented stems after surgery, cement
technique was Grade A in 17 hips, Grade B in two, and Grade C2
in six. At final follow-up, there were no loose cemented
stems. At final follow-up in noncemented stems, 21 hips had
fixation Type 1A; 63 hips had Type IB; four hips had Type II;
and two hips had Type III.
Eleven hips had reoperations or revisions (see Table 1). In
all 11 hips the polyethylene insert and the femoral head were
changed. In each of these 11 hips the acetabular component was
examined for fixation and no acetabular shell was found to be
loose, and none was revised. The polyethylene insert and
femoral head were replaced in seven hips because of severe
polyethylene wear and the presence of osteolysis in the femur.
Disassembly of the polyethylene occurred in two hips, and in
one hip the polyethylene insert was worn severely and
fractured. One hip was surgically treated again for chronic
dislocation, and a new hooded polyethylene insert and a longer
modular femoral head were placed. This patient had no
additional dislocation and at final follow-up had a Harris hip
score of 100. One hip was revised for a loose noncemented
femoral stem. In all reoperations, the replacement
polyethylene insert and femoral head size permitted the use of
at least 8 mm of polyethylene, and the insert had better
congruency than did the initial insert used (Fig1).
DISCUSSION
In these hips the acetabular components implanted without
adjunctive screw fixation functioned well. None of the metal
shells required revision for loosening, none were loose, and
only 2.6% had progressive radiolucent lines at final
follow-up. The mechanical failure rate (revised & Loose0
for fixation of these sockets was 0%. The clinical results in
this study were influenced more by the function of the femoral
stem and the acetabular polyethylene insert than by the
fixation of the acetabular component. The mechanical failure
rate for all of the arthroplasties (loose or revised femoral
and acetabular components) was 2.6%. Reoperation for worn or
loose polyethylene inserts was 7.8%. Clearly, the fit of the
polyethylene insert is the weakness of this noncemented
modular acetabular component. Wear and disassembly also have
been reported with other noncemented acetabular
components.20,25,34,43 Changes have been made by
most manufacturers to correct this weakness (Fig1).
Fixation of the metal shells was stable and predictive of
durability of the acetabular component. As measured by
revisions, at a similar postoperative time, fixation with this
hemispheric porous coated socket, press fit without adjunctive
screws or pegs, was not better than that with cemented
fixation or fixation with the use of screws or pegs. At a
similar follow-up time of 5 to 7 years, Hozack et
all22 and Schulte et al37 had a 1%
revision of the Charnley cup. With the Harris design II
(Howmedica, Rutherford, NJ) all polyethylene component, Harris
et al18 had 2% revision at 6 years. Zicat et
al46 reported 1% revision at 7 years using the
anatomic medullary locking (AML, DePuy, Warsaw, IN) acetabular
components with pegs. Schmalzried and Harris34 had
no revisions at 5 years with the Harris-Galante prosthesis
1(HGPI, Zimmer, Warsaw IN) with adjunctive screw fixation.
However, using the criteria of radiographic loosening or
progressive radiolucent lines, fixation was better with this
noncemented implant and technique. Radiographic loosening
rates are related to the technique of measurement used and the
person who does the measurement.5 In this study,
there were no loose components, there was an overall 2.6%
incidence of progressive radiolucent lines, and no case had
progressive radiolucent lines after the second postoperative
year. Cemented sockets have shown a high rate of progression
of loosening and radiolucent lines between 5 and 10 years
after surgery. Stauffer,38 using the Charnley cup,
had 6.5% loose at 5 years of follow-up and 11.3% loose at 10
years of follow-up. Harris et al18 and Mulroy and
Harris33 reported 2% loosening at 6 years of
follow-up, with an increase of 42% at 11 years of follow-up.
Hozack et al22 had 25% with either loose or
migrated cups at 10 years using the Charnley cup. Ziegler and
Lachiewicz47 reported on the trial all polyethylene
acetabulum (Johnson and Johnson, Braintree, MA) and had 15%
loose at 5 years of follow-up; at 10 years, 30% were loose.
These results show some differences associated with the design
of the polyethylene acetabular component used. Femoral head
size also has been implicated in the data of Kelley and
Johnston24 with the Iowa all polyethylene cup
(Zimmer, Warsaw, IN). At 6 years after surgery, Kelley and
Johnston24 had 21.4% loose cups when using a 28-mm
femoral head and 4.3% loose when using a 22-mm femoral
head.
The importance of surgical technique to fixation is shown
by the difference in results with noncemented components. As
observed in this study, the absence of progression of
radiolucent lines after the second postoperative year to the
seventh postoperative year has been reported by two other
studies. Zicat et al46 using the AML porous coated
component with pegs and Latimer and Lachiewicz27 using the HGPI acetabulum with an adjunctive screw fixation
had no progressive radiolucent lines. Although Latimer and
Lachiewicz27 used screw fixation and Zicat et all
used pegs, all used the technique of underreaming and required
a stable fit of the acetabular component, just as was done in
the hips in this study. This intraoperative stability of
fixation before adjunctive pegs or screws seems to be
important for the absence of progressive radiolucent lines.
Schmalzried and Harris,34 using a line to line fit
of the acetabular component and primary screw fixation,
reported 27% with progressive radiolucent lines. The PCA
(porous coated anatomic Howmedica, Rutherford, NJ) acetabular
component had adjunctive peg fixation but was fitted with line
to line reaming. Heeking et al19 reported 9% with
migration and progressive radiolucent lines at 5 to 7 years
after surgery.
Wear in these acetabular components was not less than in
the sockets with adjunctive fixation. The most common reason
for reoperation in these hips was wear of the acetabular
inset. The current authors? overall linear wear rate was 0.168
mm per year. With 28-mm femoral heads, the wear rate was 0.14
mm per year, which was the same as that reported by Woolson
and Murphy43 using the HGP 1 acetabulum. The linear
wear in these sockets is lighter than that of 0.08 mm per year
reported for the all polyethylene Charnley socket used by
Schulte et al.37 The highest wear in this study
occurred with 26-mm heads and was 0.29 mm per year. This
accelerated wear with 26-mm heads was attributed to rapid
creep of the polyethylene. 11,29,39 Because of the
gap caused by the creep, the polyethylene insert became out of
round with the femoral head and permitted accelerated linear
wear.29
Clearly, the weakness of noncemented modular acetabular
components is the wear that occurs and particularly the
possibility of severe accelerated wear in some hips. In the
hips in this study, , there often was 0.5 mm eccentricity of
the femoral head on the 3-month radiograph. This eccentricity
was caused by creep that occurred because there was a
0.5-0.75-mm gap between the polyethylene insert and the metal
shell.11 Loading of the femoral head after surgery
closed the gap in the axial direction of load and created an
immediate eccentricity in the polyethylene. This out of round
articulation allowed sliding of the femoral head, and this
sliding promotes accelerated wear.11
In this study the incidence of identified acetabular
osteolysis is 2%, which is comparable with the 1.2% reported
by Schmalzried and Harris 34 to have occurred with
screw fixation. Latimer and Lachiewicz27 using
screw fixation observed no osteolysis and Schmalzried et
al36 observed no osteolysis without screw fixation.
There is no evidence that the use of screws increases the
incidence of osteolysis. An increased incidence of osteolysis
is reported in sockets with pegs. Using the porous coated
anatomic socket, Kim and Kim25 reported 9%
osteolysis at 5 years, and Astion et al1 reported
20% for the same period. Engh et al15 reported 18%
with the AML cup at an average of 9 years follow-up. Both of
these hip systems have femoral components that had
circumferential coating and a low incidence of femoral
osteolysis. The APR and HGP femoral components did not have
circumferential coating, and this resulted in a greater
incidence of femoral osteolysis. Perhaps this was the reason
the APR and HGP acetabular components had lesser acetabular
osteolysis. Debris will seek the path of least
resistance.35 This path of least resistance differs
for these hip systems, with the femoral component being the
pathway for the APR and the HGP systems, whereas the
acetabulum is the pathway for the PCA and AML hip systems.
The assessment of clinical outcome of these hips provide a
significant finding regarding grading of pain. Patients graded
pain greater than did physicians, although the patients graded
their overall result the same as the physician (and even
somewhat better). With patient self assessment, 3.9% of
patients rated their results as far and poor, whereas by the
Harris hip score, physicians rated 11.2% as fair and poor. The
average Harris pain score at final follow-up by the patient
was 38.9, compared with 42.2 by the physician. Patients did
not discriminate hip pain from leg pain because of the wording
of the question on the self assessment form. Future outcome
instruments need to help the patient better distinguish these
pains for a more accurate measure of hip pain.
The conclusions of this study regarding fixation emphasize
the importance of the technique of preparation of the
acetabular bone to provide an intrinsically stable press fit
of the acetabular component. The technique of an underreamed
press fit acetabular component has provided the most stable
fixation interface. The use of adjunctive screw fixation is
not necessary but can be used safely at the discretion of the
surgeon. The advantages of not using adjunctive screw fixation
are the time saved at the initial surgery for insertion of the
screws and, more importantly, the time saved for removal of
the screws at any future necessary revision surgery. The use
of additional screw fixation is most needed when performing
total hip replacement in hips with osteoporotic (Type C)
bone.
References
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