Abstract
Background: There is disagreement about whether so-called minimally
invasive approaches result in faster recovery following total knee
arthroplasty. It is also unknown whether patients are exposed to excess risk
during the surgeon's learning curve. We hypothesized that a minimally invasive
quadriceps-sparing approach to total knee arthroplasty would allow earlier
clinical recovery but would require longer operative times and compromise
component alignment during the learning period compared with a traditional
medial parapatellar approach.
Methods: The first 100 minimally invasive total knee arthroplasties
done by a single high-volume arthroplasty surgeon were compared with his
previous fifty procedures performed through a medial parapatellar approach,
with respect to operative times, implant alignment, and clinical outcomes.
Radiographic end points and operative times for the minimally invasive group
were evaluated against increasing surgical experience, in order to
characterize the learning curve.
Results: Overall, the minimally invasive approach took significantly
longer to perform, on the average, than a medial parapatellar approach (86.3
and 78.9 minutes, respectively; p = 0.01); this was the result of especially
long operative times in the first twenty-five patients in the minimally
invasive group (mean, 102.5 minutes). After the first twenty-five minimally
invasive operations, no significant difference in the operative times was
detected between the groups. The first twenty-five minimally invasive
procedures had significantly less patellar resection accuracy (p < 0.001)
and significantly more patellar tilt than the last twenty-five (p = 0.006).
Other end points for implant alignment, including the frequency of
radiographic outliers, were not different between the minimally invasive and
traditional groups. The patients who had the minimally invasive approach
demonstrated significantly better clinical outcomes with respect to the length
of hospital stay (p < 0.0001), need for inpatient rehabilitation after
discharge (p < 0.001), narcotic usage at two and six weeks postoperatively
(p = 0.001 and p = 0.01, respectively), and the need for assistive devices to
walk at two weeks postoperatively (p = 0.025).
Conclusions: A quadriceps-sparing minimally invasive approach seems
to facilitate recovery, but a substantial learning curve (fifty procedures in
the hands of a high-volume arthroplasty surgeon) may be required. If this
experience is typical, the learning curve may be unacceptably long for a
low-volume arthroplasty surgeon.
Level of Evidence: Therapeutic Level III. See
Instructions to Authors for a complete description of levels of evidence.
The efficacy of total knee arthroplasty is not in
question1, but
traditional approaches have resulted in demanding and lengthy recuperative
periods, which may contribute to patient dissatisfaction or knee stiffness, or
both2-4.
For these reasons, minimally invasive surgical approaches have been
developed.
Advocates of minimally invasive total knee arthroplasty have cited faster
recovery times, shorter hospital stays, and improved short-term functional
outcomes as the principal reasons for adopting these new approaches, and their
originators have claimed that no compromises occur with respect to component
positioning or to the soft tissues about the
knee5-10.
Critics have raised questions about implant malalignment and the length of the
learning
curve11,12.
To date, there is little evidence in the literature as to the extent of the
learning curve for minimally invasive total knee
arthroplasty12 or
the efficacy of the procedure in the hands of surgeons who were not involved
in either the development of the procedure or the instrumentation.
The present study compared one surgeon's previous fifty traditional total
knee arthroplasties performed with a medial parapatellar approach and his
first 100 minimally invasive total knee arthroplasties performed with a
so-called quadriceps-sparing approach as described by its originators. This
study tested three hypotheses. First, during the learning curve, the operative
time for minimally invasive total knee arthroplasty is longer than the
operative time for traditional total knee arthroplasty. Second, during the
learning curve, implant alignment is compromised in patients having minimally
invasive total knee arthroplasty compared with those having a traditional
total knee arthroplasty. Third, the minimally invasive approach results in a
shortened recuperative period, as judged by the length of the hospital stay,
need for inpatient rehabilitation, consumption of narcotic analgesics, and use
of assistive devices for walking.
Patients
The operative logs of the senior author (S.S.L.) were used to identify the
previous fifty consecutive primary total knee arthroplasties he had done using
a traditional medial parapatellar approach and the first 100 primary total
knee arthroplasties he had performed with a minimally invasive
(quadriceps-sparing) approach. There was no overlap of the two groups, but in
the initial part of the learning curve the senior author selected patients for
the minimally invasive approach who were considered height-weight proportional
and who had knees with mild-to-moderate deformities. Specifically with respect
to obesity, of the first twenty-five patients in the minimally invasive
cohort, fifteen had a body-mass index of <30, seven had a body mass index
from 30 to 34, two had a body mass index between 35 and 40, and one had a body
mass index of >40; among those whose body mass index was >30, the
minimally invasive approach was performed because most of the excess weight
was carried centrally and the limb girth was not considered to be excessive.
The patients in the minimally invasive cohort had the operation in the period
from April 2004 to June 2005, and those in the traditional total knee
arthroplasty cohort had the operation in the period from June 2003 to April
2004. The first quartile (Cases 1 through 25) in the minimally invasive group
represented 38% of the sixty-five primary total knee arthroplasties performed
during this period by the senior author, the second quartile (Cases 26 through
50) represented 89% of the twenty-eight primary total knee arthroplasties
performed during that time, the third quartile (Cases 51 through 75)
represented 86% of the twenty-nine total knee arthroplasties performed, and
the final quartile (Cases 76 through 100) represented all of the total knee
arthroplasties performed during that period. Each patient in the minimally
invasive group was informed that this was a new procedure. In the first
twenty-five minimally invasive procedures, the patient was told exactly which
number he or she was in the learning curve (in the second group of
twenty-five, patients were told that they were among the "first
fifty" in whom the new approach was used). Informed consent for the
operation was obtained from all patients. Baseline demographic and clinical
data are presented in Table
I.
This study was approved by the institutional review board of the sponsoring
institution, and after review by the institutional review board a waiver of
informed consent for the medical records and radiographic review was granted
by the human subjects review board.
The senior author, who is a fellowship-trained high-volume arthroplasty
surgeon, performed all of the total knee arthroplasties in this series with
the assistance of a physician assistant and mid-level orthopaedic residents as
part of a university-based teaching service.
Data Abstraction and Radiographic Methods
Electronic medical records were available for all patients; the senior
author was not involved in data collection. Radiographs were reviewed by a
blinded independent investigator who was not involved in the surgery or the
aftercare of the patients. The parameters recorded were the tibial component
coronal and sagittal alignment, patellar tilt, patellar subluxation, and
patellar resection asymmetry. All patellar measurements were made from
radiographs made at six weeks postoperatively. Additional details on the
radiographic techniques are given in the Appendix.
Surgical Approaches and Aftercare
Each patient in both the minimally invasive and traditional total knee
arthroplasty groups had implantation of the same cemented, modular condylar
prosthesis (NexGen; Zimmer, Warsaw, Indiana). In the traditional group,
thirty-five cruciate-retaining implants (NexGen CR) and fifteen
cruciate-substituting implants (NexGen LPS) were used, with the milder
deformities treated with cruciate-retaining implants. In the minimally
invasive group, all patients received cruciate-substituting implants as this
was the implant that the originator of this technique recommended for this
approach, and it was the implant used at the surgical course on the technique.
Also, as recommended at the course for surgeons early in their experience with
the technique, the NexGen LPS-Flex implant was used early on (in the first
thirty-seven patients); subsequently, the standard NexGen LPS implant was
used. All of the patellas in both groups were resurfaced with a cemented,
three-pegged, all-polyethylene patellar component.
The traditional total knee arthroplasties were done by means of a medial
parapatellar approach. For the patients who underwent minimally invasive total
knee arthroplasty, the procedure was performed according to the technique
described by Tria and
Coon10. Details of
both surgical approaches are presented in the Appendix. The principal
differences between the minimally invasive technique and the traditional total
knee arthroplasty approach include the following: there is no eversion of the
patella in the minimally invasive operation (instead, lateral patellar
subluxation during tibial and femoral preparation is used); freehand resection
of the patella is done with use of the same landmarks as in the traditional
total knee arthroplasty group, but the cut is made with the patella tipped
90°; the tibiofemoral joint is not dislocated during bone preparation (the
tibial cut is made in situ with a cutting block positioned anteromedially); no
knee is flexed to >90° until trial components are in place and
range-of-motion testing is performed; and a shorter anteromedially placed
incision, rather than a longer directly anterior incision, is used.
Anesthesia and aftercare were similar in both groups, including early
aggressive range-of-motion protocols, weight-bearing as tolerated,
patient-controlled analgesia, preoperative and postoperative antibiotic
therapy, and thromboprophylaxis. The anesthesiology department had final say
over anesthetic choices for all patients in both groups; their approach was to
use regional anesthesia in all patients whenever possible during the study
period. In the traditional total knee arthroplasty group, an indwelling
epidural catheter typically was used for anesthesia, while in the minimally
invasive group, single-shot spinal anesthesia with use of bupivacaine and 100
mcg of morphine sulfate (Duramorph; Baxter Healthcare, Deerfield, Illinois)
typically was used. The only indication for general anesthesia in either group
was a failed block or a patient who refused regional anesthesia. Indwelling
epidural catheters and patient-controlled epidural analgesia pumps were
maintained for forty-eight hours in the traditional total knee arthroplasty
group when they provided good analgesia; the epidural catheters were
discontinued earlier if analgesia was inadequate, and patient-controlled
anesthesia pumps were used if the epidural was discontinued on the first
postoperative day. This management decision was made in all instances by the
acute pain anesthesia team who managed the patients separately from the
surgical service. No femoral or other peripheral nerve blocks were used. The
same physical therapy team managed all patients with the use of aggressive
range-of-motion and early weight-bearing protocols. No special emphasis was
placed on shortening the length of stay for the patients in the minimally
invasive group in this state university hospital, and no adjustments to
physical therapy protocols were made for the minimally invasive group. The
criteria for hospital discharge to home and those for admission to the
adjacent inpatient rehabilitation unit were identical in the traditional and
minimally invasive groups, and these decisions were made by the physical
therapy and rehabilitation teams, not the operating surgeon.
Preoperative Training in Minimally Invasive Surgery
Before starting the minimally invasive series, the operating surgeon took a
standard industry-sponsored course (Zimmer Institute course on the MIS
[Minimally Invasive Solutions] Quad-Sparing Total Knee Procedure; Zimmer,
Warsaw, Indiana), which consisted of didactic instruction, familiarization
with the instruments with use of Sawbones models (Pacific Research
Laboratories, Vashon, Washington), and three cadaveric operations (one
observing the procedure, one assisting the procedure, and one performing the
procedure). The particular course that the senior surgeon attended was taught
by one of the originators of the procedure and the designer of the minimally
invasive surgery instrument set. Following that, and before starting the
minimally invasive series, four additional minimally invasive total knee
arthroplasty procedures were performed on cadaver specimens in the senior
author's laboratory. Lastly, a trip was made to observe one of the originators
of the procedure perform an additional six minimally invasive procedures at
his hospital.
Statistical Analysis
After the initial data verification, independent t tests were used to
determine differences between mean performance measures. When comparing
quartiles in the minimally invasive total knee arthroplasty series, a one-way
analysis of variance was performed and was followed by post hoc comparisons
with use of the Scheffe procedure as appropriate. Pearson chi-square tests
were used to determine the significance of binary outcomes. All analyses were
conducted with use of SPSS 13.0 for Windows (SPSS, Chicago, Illinois). The
level of significance was set at p < 0.05.
Demographics, Baseline Data, and Follow-Up
Baseline demographic and clinical data are presented in
Table I. The majority of
patients in both groups had osteoarthritis (ninety-five of 100 patients in the
minimally invasive group and forty-five of fifty patients in the traditional
group; p = 0.25), with the remainder having various forms of inflammatory
arthritis. In the traditional total knee arthroplasty group, fifteen patients
had neutral alignment preoperatively, five had a valgus alignment, and thirty
had a varus alignment. Preoperative limb alignment in the minimally invasive
total knee arthroplasty group was varus in fifty-eight patients, neutral in
twenty-seven patients, and valgus in fifteen patients. The same medial-sided
minimally invasive approach, described below, was used in all patients
regardless of alignment. No lateral releases were performed in either group;
the "rule of no thumb" was used to assess patellar
tracking13.
Five patients in the traditional total knee arthroplasty group who were
from out of state were followed locally by other surgeons and were excluded
from the study, resulting in a control group of forty-five traditional total
knee arthroplasties. No patient in the minimally invasive group was lost to
follow-up. To date, there have been no clinical failures, no radiographic
evidence of prosthesis loosening, no infections, and no revisions performed in
either group.
One procedure early in the series began as a minimally invasive total knee
arthroplasty, but it was converted to a traditional total knee arthroplasty
intraoperatively because of difficulty with exposure. The recovery of the
patient was uncomplicated and the implant alignment was excellent, but his
results were excluded from this report.
Operative Time
The mean operative time in the overall minimally invasive group was longer
than that for the traditional total knee arthroplasties (86.3 and 78.9
minutes, respectively; p = 0.01). However, operative time demonstrated a
significant learning curve effect, and the overall difference in operative
time was largely due to particularly lengthy operative times in the first
quartile (Cases 1 through 25) of the minimally invasive group. The mean
operative time (and standard deviation) was 102.5 ± 15.9 minutes for
the first quartile of minimally invasive patients, 84.6 ± 9.2 minutes
for the second quartile (Cases 26 through 50), 82.0 ± 8.3 minutes for
the third quartile (Cases 51 through 75), and 76.0 ± 6.7 minutes for
the fourth quartile (Cases 76 through 100). The decreasing time over each
quartile was significant (p < 0.001). Comparing quartiles within the
minimally invasive group with those in the traditional total knee arthroplasty
group, we found that the times for the minimally invasive group were
significantly longer only in the first quartile (p < 0.001), with no
significant difference in the remaining quartiles (p = 0.16, 0.43, and 0.48,
respectively).
Radiographic Data
A significant difference was detected between the minimally invasive group
and the control group with respect to anteroposterior tibial alignment
(89.8° and 91.1°, respectively; p < 0.001), with better results in
the minimally invasive group; however, this difference is unlikely to be
clinically important. Lateral tibial alignment was excellent in both groups
and was not significantly different from the desired norms. Tibial alignment
in both the coronal and sagittal planes in the minimally invasive total knee
arthroplasty group was consistently good over all four quartiles of the
learning curve, with no significant differences among the quartiles. No
significant difference between the minimally invasive and the traditional
group was detected in terms of the frequency of radiographic tibial component
alignment outliers (<87° or >93° on the anteroposterior
radiograph); there were no tibial component alignment outliers in the
minimally invasive group and one (5° of varus alignment) in the
traditional group (p = 0.88).
Patellar tilt averaged 3.3° in the minimally invasive total knee
arthroplasty group and 3.0° in the traditional group (p = 0.65), but the
minimally invasive group showed significant improvement over the learning
curve, with the first twenty-five patients having a mean tilt of 4.6° and
the last twenty-five patients having a mean tilt of 1.6° (p = 0.006). With
use of a strict standard for the definition of radiographic outliers for
patellar tilt (tilt of >5°), the number of outliers observed in the
minimally invasive group (seventeen of 100 patellae) and the traditional group
(seven of forty-five knees) was not significantly different, with the numbers
available (p = 0.92); sixteen of the seventeen outliers in the minimally
invasive group were in the first fifty procedures performed with that
technique. The mean amount of patellar subluxation in both groups (average,
1.8 mm in both) was negligible and was not significantly different, with the
numbers studied, between the minimally invasive group and the traditional
group; also, no significant difference was detected over the course of the
learning curve in the minimally invasive group with respect to patellar
subluxation (p = 0.78). There was no significant difference on the basis of
these numbers in terms of the frequency of outliers (subluxation of >2 mm)
between the minimally invasive and traditional total knee arthroplasty groups;
with use of this strict standard, fourteen of 100 patellae in the minimally
invasive group were outliers compared with five of forty-five patellae in the
traditional group (p = 0.76). Only two patellae in the minimally invasive
group had =5 mm of subluxation (5 mm in one and 6 mm in the other).
Patellar resection asymmetry averaged 2.5 mm for the minimally invasive
total knee arthroplasty group and 2.2 mm for the traditional total knee
arthroplasty group (p = 0.35). In the minimally invasive group, there was a
significant improvement in resection accuracy over the course of the learning
curve; the first twenty-five patients had a mean of 2.7 mm of asymmetry, while
the last twenty-five patients had a mean of 1.4 mm (p < 0.001).
Radiographic outliers of patellar resection asymmetry, with use of a strict
standard of >3 mm of difference from the medial to the lateral edge of the
patellar component, were not significantly more frequent, with the numbers
available, in the minimally invasive total knee arthroplasty group than in the
traditional total knee arthroplasty group (p = 0.30); there were only two
outliers in the last fifty minimally invasive total knee replacements (both
measuring 4 mm of asymmetry).
Clinical Outcomes
Clinical outcomes favored the minimally invasive group fairly consistently
over the traditional group (Table
II), including a shorter mean length of hospital stay (p <
0.0001), a higher percentage of patients discharged to home rather than to
inpatient rehabilitation (p < 0.001), a lower percentage of patients who
used narcotic pain medications at two and six weeks after surgery (p = 0.001
and p = 0.01, respectively), and less frequent use of assistive walking
devices at two weeks after surgery (p = 0.025). The minimally invasive and
traditional total knee arthroplasty groups at six weeks were not significantly
different, with the numbers available, with respect to the range of motion
either in flexion (118.6° compared with 117.8°; p = 0.65) or extension
(1.7° compared with 1.3°; p = 0.24) and with respect to the use of
assistive devices for walking (92% of those in the minimally invasive group
compared with 84% in the traditional group were able to walk independently; p
= 0.2).
Complications
In the minimally invasive group, one patient required a manipulation under
anesthesia. No manipulations were performed in the traditional group, and no
other patient in either group underwent reoperation for any reason. The
trigger for manipulation was flexion of <90° at the two-week follow-up
evaluation. The patient who underwent manipulation achieved 0° to 100°
of rotation at six weeks following the index procedure and 0° to 110°
at one year. In the traditional total knee arthroplasty group, four patients
had distal deep-vein thromboses suspected on the basis of symptoms and
confirmed by venous Doppler and duplex ultrasound. There were no deep-vein
thromboses diagnosed in the minimally invasive group. There were no wound
complications in either group.
There is no standardized minimally invasive approach for total knee
arthroplasty7-10.
Similarly, there is no agreement as to whether an approach that allegedly
spares the quadriceps but goes a centimeter or two (or more) into the
medial-sided soft tissues should be described as a "quadriceps
sparing," a "mini midvastus," or a "mini
subvastus"
approach14. In the
present study, minimally invasive total knee arthroplasty was defined as an
approach that ranged from purely transcapsular to one in which a very small
(<2-cm) extension was made into the midvastus or subvastus plane, depending
on the local anatomy, as described by the original developers of the
approach10. It has
been theorized that the handling of the patella (in minimally invasive
procedures, it was not everted), the treatment of the tibiofemoral joint
itself (in minimally invasive procedures, the tibia was cut in situ and no
anterior dislocation of the joint was made during preparation of the femur or
the tibia), and the management of the soft tissues (in minimally invasive
procedures, the so-called mobile window approach was used to minimize
retractor tension) all may result in less trauma to the joint than occurs with
traditional approaches.
The nascent literature on minimally invasive total knee arthroplasty is far
from universally
supportive11,15.
Most series that have been published in support of these surgical approaches
are the work of implant or instrument designers or they are first reports by
originators of particular techniques or aftercare protocols, or
both5-10.
We sought to determine whether one of these new approaches is generalizable in
the hands of a surgeon who was not involved in the development of the
instruments or the approach, and to define the shape of the learning curve in
the hands of a surgeon who had not developed it.
This study demonstrated that it took just over twenty-five procedures
before minimally invasive operative times were comparable with those achieved
with use of the traditional approach and about fifty procedures until patellar
alignment was optimized with the minimally invasive approach. No replacements
in this series required revision, but it is possible that some patients with
less accurate patellar resections may develop problems over time. Although
patellar alignment in all quartiles of the minimally invasive group was within
the range of that reported in studies with use of traditional
approaches16, there
were clear improvements over time in the minimally invasive group. This
suggests that, at least potentially, there was some additional risk to being a
patient in the minimally invasive group early in the learning curve. Clinical
outcome data in this series tended to support the claims of proponents of
minimally invasive total knee arthroplasty suggesting accelerated
recovery12, but
they also echoed the concerns of one skeptical report that clinical results
tend to converge with those after use of a traditional approach once outside
the early postoperative
period11. The
clinical differences observed in the present report must be tempered by the
fact that this study, while controlled, was retrospective; as such, the
patients and caregivers were not blinded to the treatments administered. This
introduces the issue of both patient and provider expectations into the
assessment of outcomes, and there are subjective elements present in all
clinical research of this design.
A previous
report11 noted that
the minimally invasive approach was more likely to result in varus tibial
alignment than was the traditional approach with which it was compared. By
contrast, we found that tibial alignment was, in fact, significantly better
with the minimally invasive approach, although the mean difference was so
small as to likely be clinically unimportant. On the other hand, we identified
patellar malalignment issues, which appeared to be the result of
underresection of the lateral facet, early in the minimally invasive learning
curve. Surgeons adopting the minimally invasive approaches should be
conscientious about verifying that the entire patella is clearly visible prior
to resection in order to confirm that the saw comes out beneath the true
chondral (or osteophytic) rim, and that the patella is symmetric following
resection.
The present study has important methodological limitations, and conclusions
drawn from it should be interpreted in light of them. This study must be
considered a best-case scenario, as it drew from the practice of a high-volume
arthroplasty subspecialist and the initial twenty-five patients in the
minimally invasive group were selected on the basis of approximate
height-weight proportionality, which was determined by body mass index and
limb girth, and the absence of severe deformity. Because of such issues, this
study, like most retrospective studies, is at risk of inflating apparently
beneficial treatment effects of the new approach compared with what might be
found in a well-designed prospective
trial17-19.
On the other hand, the traditional cohort in this study was assembled before
the start of the minimally invasive series so that the control group of
traditional total knee arthroplasties represents an "all comers"
population, and it does not include the procedures that were done after the
start of the minimally invasive series; this should have prevented the
traditional group from experiencing an additional bias toward patients with
more difficult cases. There were subtle differences between the traditional
and minimally invasive total knee arthroplasty groups with regard to the
anesthetic technique; the anesthesiology department used regional anesthesia
whenever possible in both groups but opted for forty-eight-hour indwelling
epidural catheters in the traditional group and single-shot bupivacaine spinal
anesthesia in the minimally invasive group. One might theorize that, if
anything, the longer duration of regional anesthesia achieved with indwelling
catheters for two days might have conferred a slight rehabilitative benefit on
the traditional total knee arthroplasty group.
It is worth noting that the learning curve in this series was facilitated
by a hands-on course, additional cadaver dissections, and an off-site visit to
observe the technique in the hands of a developer. Taken together, these steps
represent substantial investments of effort and time that may not be typical
of surgeons who adopt new procedures. Despite these steps, the minimally
invasive total knee arthroplasty learning curve in this series was
approximately fifty procedures.
The strengths of the study include its size, the inclusion of a control
group, the use of independent evaluators, and the presentation of data or
alignment and operative time as a function of the case number (representing
increasing surgical experience). The fact that the last twenty-five operations
in the minimally invasive group were consecutive (representing all primary
total knee arthroplasties performed during the period in question), and that
seventy-five of the last eighty-two operations performed during the
time-period in question were done with use of the minimally invasive
technique, both speak to the issue of adaptability of the minimally invasive
technique to challenging total knee arthroplasties, once a surgeon is outside
the early part of the learning curve. Other points supporting the internal
validity of this study are the fact that the same conventional condylar total
knee arthroplasty implant design, physical therapy protocols, and criteria for
inpatient rehabilitation admission were employed in both study groups.
Recently, ethical issues have been raised about the adoption of minimally
invasive total knee
arthroplasty15. In
anticipation of learning-curve issues, the first twenty-five patients in the
minimally invasive portion of this series were told exactly which number in
the surgeon's operative experience they were, the second twenty-five were told
that they were among the "first fifty" patients, and all 100 were
told that there was little short-term and no long-term documentation of
outcomes with use of this approach.
On the basis of these results, we are cautiously optimistic about minimally
invasive total knee arthroplasty. It may have a role in the hands of
high-volume arthroplasty surgeons who are willing to invest the time to learn
and practice the procedure before performing it in patients. On the other
hand, if our learning curve represents a typical experience with this
procedure, it would appear not to be a worthwhile technique for the low-volume
(five to ten procedures per year) arthroplasty surgeon to attempt, as the
learning curve would apparently stretch out over a period of years.
Details of the radiographic measurements and surgical technique are
available with the electronic versions of this article, on our web site at
(go to
the article citation and click on "Supplementary Material") and on
our quarterly CD-ROM (call our subscription department, at 781-449-9780, to
order the CD-ROM). ?
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