Patient Selection Criteria
Modular component exchange was considered for treatment of recurrent
dislocation only if the patients met specific preoperative and intraoperative
requirements. The preoperative criteria included (1) use of modular
components in the primary arthroplasty, (2) two or more dislocations
despite nonoperative management, (3) radiographic signs of stability
of the components, and (4) an otherwise satisfactory result of the hip
arthroplasty. The final decision to limit the procedure to modular
component exchange was based on intraoperative trial reductions
and range-of-motion assessment. Contraindications included a component
position that contributed to instability, inadequate soft-tissue
tension, abductor insufficiency, and, most importantly, inadequate
intraoperative stability.
Patient Selection
From our institution’s total hip arthroplasty database,
we identified 2935 primary arthroplasties performed with uncemented,
porous-coated femoral and acetabular components between 1983 and
1998. Of 118 first revisions, thirty-four were performed to correct
instability. Fourteen of the thirty-four patients met the previously
described criteria and were treated with unconstrained modular component
exchange. The average age of these four men and ten women at the
time of the primary operation was 59.3 years (range, twenty-six
to seventy-nine years). The underlying diagnosis prior to the arthroplasty
was osteoarthritis in twelve hips and osteonecrosis in two. The
average number of dislocations prior to revision was 2.9 (two, three,
or four), and the average time to the first dislocation was 1.3
years (range, two months to 6.3 years).
Primary Procedures
A posterior approach had been used for all fourteen primary operations.
The primary femoral stems included ten Anatomic Medullary Locking
stems (DePuy, a Johnson and Johnson Company, Warsaw, Indiana), three
Prodigy stems (DePuy), and one S-ROM stem (Joint Medical Products,
Stamford, Connecticut). The heads included seven 28-mm balls, six
32-mm balls, and one 26-mm ball. There were seven Duraloc (DePuy),
six Arthropor (Joint Medical Products), and one Tri-Lock+ (DePuy)
acetabular components. Eight of the primary hip replacements had
a polyethylene liner with a 10° elevated lip, and six had a liner
with no augmentation (see Appendix).
Acetabular orientation was evaluated on preoperative anteroposterior
radiographs. The mean cup abduction angle was 44.6° (range, 33°
to 63°). The amount of version was not quantified because of the
uncertainty associated with an assessment based on a single anteroposterior
radiograph.
Modular Exchange Procedure
The modular exchange included any combination of the following:
use of a longer neck length, use of a larger ball size, conversion
to a polyethylene liner with an elevated rim, reorientation of an
existing polyethylene liner with an elevated rim, or use of a thicker
polyethylene liner. The operation could also involve the removal
of any bone or soft tissue causing impingement.
For the procedure, the patient was positioned on the operating table
with the affected lower limb prepared and draped free to allow evaluation
of unencumbered range of motion during trial reductions. We then
exposed the hip through the previous incision and assessed the direction
or directions of instability. During the range-of-motion testing
process, we sought to discover all factors that contributed to instability,
including soft-tissue laxity and impingement of components, bone,
or soft tissues.
After removal of the polyethylene liner, we confirmed the stability
of fixation of both components. Sources of bone or soft-tissue impingement
that contributed to hip instability were removed. We then performed
a series of trial reductions, with use of various offset and lipped
unconstrained trial polyethylene liners and trial modular heads
in an effort to improve stability. With each trial reduction, the
range of motion until dislocation was assessed.
We limited the procedure to a modular component exchange only
if the hip was stable in maximum flexion, in full extension with
external rotation, and in at least 45° of internal rotation with
the hip in 90° of flexion and maximum adduction. If such stability
was not achieved, we proceeded with another revision technique.
We thought that the contribution of component position to instability
was more reliably addressed during these trial reductions than by
preoperative radiographic assessment; therefore, intraoperative
stability was given clear priority as the factor in the decision
of whether to proceed with another option. We did not attempt to
repair the capsule after the revision because the posterior aspect
of the capsule had been excised in each of the hips during the primary arthroplasty.
The revision operations were performed at a mean of 2.6 years
(range, six months to ten years) after the primary total hip arthroplasty.
During the intraoperative examination, one hip was found to dislocate
only anteriorly, and one hip could be dislocated both anteriorly
and posteriorly because of excessive soft tissue laxity. The other
twelve hips could be dislocated only posteriorly. Osseous impingement
was encountered in ten hips, and it was treated with excision of portions
of the acetabular rim or the anterior aspect of the greater trochanter,
or both.
In the revision operations, one 26-mm ball and five 28-mm balls
were replaced with a 32-mm ball. In two hips, a 28-mm ball was converted
to a 28-mm ball with a longer neck. In five hips, a 32-mm ball was
converted to a 32-mm ball with a longer neck (Figs. 1-A and 1-B).
In one hip with a 32-mm ball, the head diameter and neck length
were not changed. In this hip, the 10° lipped liner was exchanged
for a 20° lipped liner. In four hips, the neck length was increased
with the use of a so-called skirted-neck segment.
Among all hips, the mean neck length was increased by 6.1 mm
(range, 0 to 13 mm), but the total lengthening along the axis of
the femoral neck was 8 mm (range, 0 to 16.1 mm) when the lateralization
achieved with the polyethylene exchanges was taken into account.
Because the neck angle of each femoral prosthesis was 135°, the
total lengthening procedure effectively increased both the offset
and the limb length by an average of 5.7 mm (range, 0 to 11.4 mm).
In five hips, a 10° lipped liner was exchanged for one with a 20°
lip oriented in the posterior direction. In three hips, a nonlipped
polyethylene liner was converted to a 10° lipped liner. A nonlipped
liner was exchanged for a liner that was 4 mm thicker (lateralized)
in three hips, in one of which the 4-mm lateralized liner also had
a 10° elevated lip. In two hips, a 10° lipped liner was exchanged
for a new 10° lipped liner with the same geometry. One primary 10°
lipped liner was converted to a 10° lipped liner that was 3 mm thicker.
Postoperatively, all fourteen patients were instructed to limit hip
flexion to 70°, to limit abduction to 20°, and to avoid internal
rotation and adduction. The patient with anterior instability also
was advised to avoid external rotation of the hip with full extension.
Poor voluntary compliance with range-of-motion restrictions was
anticipated from three patients. One of these patients was placed
in a spica cast for one month, commencing one week after the revision
operation, and the other two patients were instructed to wear an abduction
brace for six weeks.
Outcome Measurement
The outcome of this technique was evaluated by (1) recording the
number of dislocations after the revision operation; (2) documenting
Harris hip scores prior to the primary total hip arthroplasty, immediately
prior to the revision operation, and at the time of the most recent
follow-up; and (3) asking the patients whether they were satisfied
with the result of the hip operation. Statistical analysis was performed
with use of SPSS software (version 8.0; SPSS, Chicago, Illinois).
The Wilcoxon signed-rank test was used to compare Harris hip scores,
and the Fisher exact test was employed to compare the results associated
with categorical variables that had only two possible values. A
p value of £0.05 was considered significant.
Intraoperative stability was improved in all fourteen hips. All thirteen
hips that could be dislocated posteriorly before the component exchange
could no longer be dislocated by simply flexing the hip maximally
after the exchange; adduction and internal rotation of >45°
was required to dislocate the hip. The hip that had had anterior
instability before the exchange required an extreme amount of external
rotation (70°) in full extension with abduction to produce dislocation
after the exchange.
Thirteen of the fourteen patients were evaluated at a mean of 5.8
years (range, 2.8 to 11.8 years) after the revision operation. One
patient was lost to follow-up 0.9 years after the revision; at that
time, the patient had not sustained a dislocation. Ten of the thirteen
patients had no dislocation subsequent to the revision operation.
The hips of three patients dislocated despite exchange of the modular
components. One, a twenty-nine-year-old woman, had a total of three
postrevision dislocations. She was treated nonoperatively with bracing
and a hip-spica cast, and the hip has not dislocated in the past
five years. The second patient, who had undergone the revision at
the age of seventy-five years, had one subsequent dislocation, 5.3
years later. He was treated at another institution with open reduction
and a spica cast without revision of components. By the time of
writing, the hip had not subsequently dislocated. The hip of a third patient,
a woman with Ehlers-Danlos syndrome, dislocated 1.1 years after
the revision operation. The dislocation was reduced at another institution,
and no more dislocations occurred.
The mean Harris hip score prior to the primary total hip arthroplasty
was 54 points (range, 44 to 66 points). Before the revision operation,
the Harris hip score averaged 85 points (range, 71 to 99 points).
At a mean of 5.8 years after the revision operation, the Harris
hip score averaged 90 points (range, 62 to 100 points). Although
the overall Harris hip scores prior to revision and at the most
recent follow-up examination were not significantly different (p = 0.13),
the ability of patients to put on socks and to tie shoes was significantly
improved (p = 0.01). There was no significant change in
the other components of the Harris hip score (0.10 < p £ 1.0).
In the group of thirteen patients who had a minimum two-year follow-up,
one of the four hip replacements with a so-called skirted neck dislocated
after the revision operation, whereas two of the nine that did not
have a skirted neck dislocated. With the number of patients in the
present study, this difference was not significant (p = 1.0).
At the most recent follow-up evaluation, all twelve patients who
were questioned indicated that they were satisfied with the result
of the revision hip operation. Information regarding satisfaction
was not available for one patient who had had a hip dislocation
after the revision operation and had been treated with open reduction
and a spica cast. Despite the fact that the neck length of all fourteen
hips was increased as a result of the revision operation, no patient
reported problems with limb-length inequality.
The results of revision operations for hip instability have been disappointing,
with reported success rates ranging from 61% to 83%6-9,19,20. In the present series, three
of thirteen patients had at least one hip dislocation after the
revision operation. Two of these patients were treated nonoperatively,
and the other patient had open reduction and application of a spica
cast. Two of these three patients had only one dislocation. Thus,
we believe that recurrent dislocation was eliminated as a problem
in twelve of the thirteen hips that underwent modular component exchange.
Although the success rate in the present series seems comparable
with that reported for other operative techniques, we should be
cautious in making direct comparisons. Prior studies included various
treatments with different durations of follow-up, patient demographics,
and patient selection criteria. Our series was selective, so it
is essential to reemphasize that this option should be considered
only in the few cases that meet the described preoperative and intraoperative
criteria. In our experience, inadequate component position or fixation, excessive
soft-tissue laxity, and abductor insufficiency were the most frequent
contraindications noted.
The use of a so-called skirted femoral ball to lengthen the neck
segment in some of our patients could be criticized because potentially
the range of motion could have been limited by impingement of the
neck on the acetabulum. We were primarily concerned with intraoperative
stability, and if use of a longer neck resulted in improved and
satisfactory stability, we chose this option over that of a shorter
neck that did not meet our criteria for intraoperative stability.
This choice does not seem to have been detrimental to the outcome,
at least in this limited number of patients.
We did not set strict preoperative radiographic criteria for acceptable
component position for several reasons. The only measurement that
can be easily performed on an anteroposterior radiograph is that
of acetabular abduction. Estimation of acetabular and femoral component
version is difficult on plain radiographs. Also, such measurements
cannot quantify component-to-component relationships or patient
anatomy, both of which are of primary importance. We have found
intraoperative assessment more reliable than radiographic analysis
for detecting component malposition that contributes to instability.
In addition to component malposition, it is critical to seek
all other potential factors that contribute to the unstable hip
after arthroplasty. Evidence of soft-tissue or osseous impingement must
be actively sought, and excision of offending bone or capsular structures
is an essential element of the operative technique. Among the fourteen
patients in this series, ten required removal of impinging bone
or capsule to obtain adequate stability.
There are obvious advantages to modular component exchange. The
operation is technically easier and potentially is associated with
much less morbidity compared with the removal of a well-fixed femoral
stem or acetabular cup. Furthermore, because fixation of the stem
and cup is not an issue with simple component exchange, the patient
may be mobilized early with weight-bearing as tolerated. Additional
studies should be done to determine the success rate of modular component
exchange for the treatment of recurrent dislocation when other surgical
approaches and modes of fixation have been used.
We believe that exchanging only the modular head and polyethylene
liner in carefully selected patients is an acceptable option for
treatment in selected instances of recurrent instability of the
hip. The procedure is not technically demanding and obviates the
need to revise a well-fixed component.