Abstract
Background: The restoration of posterior femoral translation has
been shown to be an important factor in enhancing knee flexion after total
knee arthroplasty. The purpose of this study was to compare the ranges of
motion associated with standard and high-flexion posterior stabilized total
knee prostheses in patients managed with simultaneous bilateral total knee
arthroplasty.
Methods: Fifty patients (mean age, sixty-eight years) received a
standard fixed-bearing knee prosthesis in one knee and a high-flexion
fixed-bearing knee prosthesis in the contralateral knee. Two patients were
men, and forty-eight were women. At a mean of 2.1 years postoperatively, the
patients were assessed clinically and radiographically with use of the
knee-rating systems of the Knee Society and The Hospital for Special
Surgery.
Results: The mean postoperative Hospital for Special Surgery knee
score was 90 points for the knees treated with the standard fixed-bearing
prosthesis and 89.4 points for those treated with the high-flexion prosthesis.
At the time of the final follow-up, the knees with the standard prosthesis had
a mean range of motion of 135.8° (range, 105° to 150°) and those
with a high-flexion prosthesis had a mean range of motion of 138.6°
(range, 105° to 150°) (p = 0.41). No knee had aseptic loosening,
revision, or osteolysis.
Conclusions: After a minimum duration of follow-up of two years, we
found no significant differences between the groups with regard to range of
motion or clinical and radiographic parameters, except for posterior femoral
condylar offset.
Level of Evidence: Therapeutic Level II. See Instructions
to Authors for a complete description of levels of evidence.
The range of motion after total knee arthroplasty is an important
component of a patient's overall functional
outcome1. However,
patients rarely flex the knees beyond 120° following total knee
arthroplasty2-5.
Although the mechanisms that hinder more flexion are unclear, the ability to
restore posterior femoral translation has been shown to be an important factor
in enhancing knee flexion after total knee
arthroplasty4-7.
A reduction in posterior femoral translation has been found to cause
impingement of the posterior edge of the tibial component on the femoral
shaft, thus preventing a high degree of flexion of the
knee5,6.
The NexGen LPS-Flex total knee system (Zimmer, Warsaw, Indiana) was
introduced to enhance knee flexion after total knee arthroplasty. Compared
with the NexGen LPS prosthesis, the NexGen LPS-Flex system includes an
extension of the posterior condyle of the femoral component by 2 mm, a
modification of the cam and tibial spine, and a reduction of patellar
impingement. The purpose of the extended posterior condyle of the femoral
component is to extend the surface of the femoral component posteriorly to
increase the articular contact area at high flexion angles and thereby
increase posterior femoral translation and the range of flexion. The shape of
the femoral cam was modified to improve stability of the femoral component on
the articular surface and to reduce the bending moment applied to the
articular surface of the spine. The femoral cam design increases the
subluxation resistance and increases the contact surface between the cam and
the tibial spine beyond that of the standard design at flexion angles of
>130°. To decrease stresses on the quadriceps mechanism and to reduce
the potential for patellar impingement during high degrees of flexion,
material was removed from the anterior face of the polyethylene tibial
bearing.
We performed a prospective, randomized study to compare the ranges of
motion of the NexGen LPS and NexGen LPS-Flex total knee replacements in
patients who were managed with simultaneous bilateral total knee
arthroplasty.
Between July and September 2002, the senior author (Y.-H.K.)
performed fifty consecutive primary bilateral total knee arthroplasties in
fifty patients (100 knees). All fifty patients were enrolled in the present
study. The bilateral total knee arthroplasties were performed during the same
anesthesia session, with one side treated immediately after the other. No
patient was lost to follow-up. The study was approved by our institutional
review board, and all patients provided informed consent.
Randomization of the use of a NexGen LPS or a NexGen LPS-Flex prosthesis
was determined from a sequential pool on the basis of a table of random
numbers. Each of the fifty patients received a NexGen LPS total knee component
on one side and a NexGen LPS-Flex total knee component on the contralateral
side. The order of insertion of the two prostheses was assigned alternately to
each side. Two patients were men, and forty-eight were women. The mean age at
the time of the index operation was sixty-eight years (range, fifty-three to
eighty-one years). The diagnosis was osteoarthritis for forty-nine patients
and rheumatoid arthritis for one. No patient had had a previous knee
operation.
All procedures were performed through a midline skin incision measuring 9
to 12 cm in length, with a subvastus approach into the joint. The anterior and
posterior cruciate ligaments were excised in all patients in both groups.
Ligamentous balancing was done, and an attempt was made to resect 10 mm of
tibial bone distally from what was considered to be the intact articular
surface in order to achieve a surface that was perpendicular to the shaft of
the tibia in the coronal plane with a 7° posterior slope in the sagittal
plane. The distal and posterior femoral condylar resection was done with an
attempt to remove a length of bone that was equal to the thickness of the
femoral component to be inserted. The valgus angle of distal femoral
resection, made with use of an anterior referencing system, was the same in
the two groups. The amount of bone resected from the posterior femoral condyle
was 2 mm greater in the knees to be treated with the NexGen LPS-Flex
prosthesis than it was in the knees to be treated with the standard NexGen LPS
prosthesis. The patellar thickness was measured before the resection, and an
attempt was made to remove a segment of bone that was equal to or slightly
thicker than the component to be inserted. After pulsed lavage, all implants
were inserted with cement, which was pressurized.
A splint was applied with the knee in 15° of flexion, and it was worn
for the first twenty-four hours after the operation. The knee was placed in a
continuous-passive-motion machine after the splint was removed. All patients
began walking with crutches or a walker and started active and passive
range-of-motion exercise on the second day after the operation. The patients
used crutches or a walker, with full weight-bearing, for six weeks and then
used a cane for six weeks.
Clinical and radiographic evaluations were done at three months after the
operation, at one year, and then yearly thereafter. The mean duration of
follow-up was 2.1 years (range, 2.0 to 2.2 years). Each knee was rated
preoperatively and postoperatively according to the systems of the Knee
Society8 and The
Hospital for Special
Surgery9. In
addition, each patient completed the Short Form-36 (SF-36)
questionnaire10,
which is self-administered. The SF-36 consists of a visual analog scale for
the assessment of the severity, location, and frequency of pain as well as a
series of questions regarding the achievement of functional benchmarks (the
ability to climb stairs, to walk a certain distance, and to participate in
specific sports), the overall sense of well-being, and the level of
satisfaction with the operative result.
The active range of motion was determined with use of a standard (60-cm)
clinical goniometer before the operation and at the time of the review. The
patients were told to bend their knees as much as they could while lying in a
supine position. The range of motion was measured for all patients, on two
occasions, by one of the authors (Y.-H.K.) as well as by another author
(K.-S.S.) who was blinded to the type of implanted prosthesis. The range of
motion was considered to be the arc of motion instead of the flexion
angle.
Anteroposterior radiographs with the patient both standing and lying
supine, lateral radiographs, and skyline patellar radiographs were made
preoperatively and postoperatively and were assessed for the alignment of the
limb (tibiofemoral angle), the position of the components, and the presence
and location of radiolucent lines at the bone-cement interface according to
the recommendation of the Knee
Society8.
Posterior femoral condylar offset was evaluated on preoperative and
postoperative lateral radiographs by measuring the maximum thickness of the
posterior condyle projected posteriorly to the tangent of the posterior cortex
of the femoral shaft (Fig. 1).
The preoperative and postoperative measurements were then compared, after
correction for magnification, with use of the diameter of the femoral shaft 10
cm proximal to the femoral articular surface as a reference measurement.
The level of the joint line was determined on anteroposterior radiographs,
made with the patient lying supine, before and after surgery. This was done by
measuring the distance between the tip of the fibular head and the distal
margin of the lateral part of the femoral condyle preoperatively and the
distance between the tip of the fibular head and the distal margin of the
lateral femoral component postoperatively. Skyline patellar radiographs were
examined for patellar tilt, subluxation, or dislocation.
Statistical comparison of the clinical and radiographic results associated
with the two groups was done with analysis of variance, chi-square analysis,
Pearson regression analysis, the independent unpaired Student t test, and the
two-tailed Student t test.
Twenty patients were required to determine whether there was a significant
difference (power = 0.8 and p < 0.05) in the knee scores between the NexGen
LPS and NexGen LPS-Flex groups. Thirty-six patients were required to determine
whether there was a significant difference (power = 0.8 and p < 0.05)
between the two groups with regard to the radiographic parameters, including
the tibiofemoral angle, position of the components, presence of radiolucent
lines, posterior femoral condylar offset, and level of the joint line.
Forty-two patients were required to determine whether there was a significant
difference between the two groups (power = 0.8 and p < 0.05) with regard to
the range of motion of the knees.
Clinical Results
Knee Score
The preoperative and postoperative knee and pain scores are
summarized in Table I. The Knee
Society and The Hospital for Special Surgery knee scores did not differ
significantly between the two groups either preoperatively (p = 0.8583 and p =
0.9246, respectively) or postoperatively (p = 0.3356 and p = 0.7108,
respectively). In the NexGen LPS group, the mean postoperative knee score was
92.5 points (range, 82 to 100 points) according to the system of the Knee
Society and 90 points (range, 75 to 100 points) according to the system of The
Hospital for Special Surgery. In the NexGen LPS-Flex group, the mean
postoperative knee score was 91.6 points (range, 70 to 100 points) according
to the system of the Knee Society and 89.4 points (range, 70 to 100 points)
according to the system of The Hospital for Special Surgery.
Pain
The postoperative pain scores, according to both knee-scoring systems, did
not differ significantly between the groups (p = 0.2241 and p = 0.3825). Of
the fifty knees treated with the NexGen LPS implant, thirty-eight (76%) were
not painful at the time of the latest follow-up, twelve (24%) were mildly
painful, and none were moderately or severely painful. Of the fifty knees
treated with the NexGen LPS-Flex prosthesis, thirty-six (72%) were not
painful, thirteen (26%) were mildly painful, one (2%) was moderately painful,
and none were severely painful.
Range of Motion (Table
II)
Preoperatively, the mean knee flexion contracture was 6° (range, 0°
to 50°) in the NexGen LPS group and 5° (range, 0° to 50°) in
the NexGen LPS-Flex group. At three months, no knee had a measurable flexion
contracture. The mean range of flexion preoperatively, at three months
postoperatively, at one year, and at two years did not differ significantly
between the two groups (p = 0.41 at two years) (Figs.
2-A and
2-B).
Satisfaction
Thirty-eight patients (76%) were fully satisfied with the outcome of the
operation with the NexGen LPS prosthesis, and twelve patients (24%) were
satisfied. Thirty-six patients (72%) were fully satisfied with the result of
the operation with the NexGen LPS-Flex prosthesis, thirteen patients (26%)
were satisfied, and one patient (2%) was dissatisfied because of constant
moderate pain.
Radiographic Results
There were no significant differences between the groups with regard to the
position of the femoral and tibial components in the coronal and sagittal
planes, the alignment of the knee, the patellar angle (the angle between a
line along the patellar cut surface and a line joining the most proximal
margins of the femoral condyles of the component on the skyline radiograph),
the amount of the tibial surface area covered by the implants (tibial
capping), or the mean level of the joint line (all p > 0.05). The alignment
of the knee was a mean of 5.4° of valgus in the NexGen LPS group and
7° of valgus in the NexGen LPS-Flex group. There were no radiolucent lines
in either group. The preoperative posterior femoral condylar offset was
approximately the same (26.6 and 26.5 mm) in the two groups. However, the
postoperative posterior femoral condylar offset was significantly greater (p =
0.0012) in the NexGen LPS-Flex group (27.3 mm) than in the NexGen LPS group
(25.3 mm). On the lateral radiographs of the knees in full flexion, the NexGen
LPS-Flex prosthesis appeared to provide a greater contact area between the
femoral and tibial components (Figs.
2-A and
2-B). No knee had loosening of
the femoral, tibial, or patellar component, and no knee had subluxation or
dislocation of the tibiofemoral joint or a patellar dislocation.
It has been claimed that the anterior-posterior dimensions of the
tibial articular surface on the medial and lateral sides of an intact knee are
greater than the comparable dimensions of the tibial polyethylene of a NexGen
LPS total knee prosthesis. As a consequence, posterior femoral translation
results in contact with the posterior portion of the tibial articular surface
of the NexGen LPS system. This might result in edge-loading of the posterior
tibiofemoral joint and inhibit further posterior femoral translation. In the
NexGen LPS-Flex prosthesis, the posterior femoral condyles are elongated by 2
mm to provide a greater contact area between the femoral and tibial components
during high degrees of flexion. Therefore, the tibial component is placed more
posteriorly on the plateau, which may enhance posterior femoral translation
and the range of flexion. Also, the cam-spine mechanism of the NexGen LPS-Flex
prosthesis is designed to facilitate posterior femoral translation, thus
improving knee
flexion11.
In our study, the average range of postoperative motion was approximately
the same in the two groups. A high degree of flexion was achieved with both
types of prosthesis, which may have clouded the possible advantage of the
NexGen LPS-Flex knee. Several factors may have played an important role in the
achievement of this high degree of flexion, including the preponderance of
women, the low body mass index of the patients, the use of the subvastus
approach, the relatively good preoperative range of motion, and the effective
restoration of the joint
line12,13.
Postoperatively, the posterior femoral condylar offset was significantly
better (p = 0.0012) in the NexGen LPS-Flex group than it was in the NexGen LPS
group. This finding, however, did not appear to be clinically relevant because
it was not associated with a better range of motion.
Although the NexGen LPS-Flex group did not have a better range of motion
than the NexGen LPS group, lateral radiographs of the knees in full flexion
demonstrated that the NexGen LPS-Flex prosthesis provided a greater contact
area between the femoral and tibial components. Increased contact area can
reduce the peak stresses in the polyethylene tibial bearing and reduce wear of
the polyethylene. Consequently, the long-term risk of osteolysis may be
reduced.
Bellemans et al. showed that, in patients treated with cruciate-retaining
total knee arthroplasty, posterior femoral condylar offset correlated with the
range of flexion of the
knee2. They claimed
that restoration of posterior femoral condylar offset is important because it
allows a greater degree of flexion before impingement occurs. In the current
series, posterior femoral condylar offset was well restored compared with the
preoperative value in the NexGen LPS-Flex group but not in the NexGen LPS
group, in which the postoperative offset was decreased by a mean of 1.2 mm
compared with the preoperative value. In the NexGen LPS-Flex group, an
additional 2 mm of the posterior femoral condyle was resected, but this
additional resection was compensated for by a 2-mm extension of the posterior
condyle of the femoral component. Therefore, theoretically, the posterior
femoral condylar offset should have been the same in the two groups. The
reason why the NexGen LPS group had a 2.0-mm decrease in the offset compared
with the NexGen LPS-Flex group is not known. Although we attempted to remove
more posterior femoral condylar bone (2 mm) and the posterior femoral condyle
was elongated by 2 mm in the NexGen LPS-Flex group, the posterior femoral
condyle appeared to be resected more in the NexGen LPS group than in the
NexGen LPS-Flex group, possibly because of a technical error. Despite the fact
that the posterior femoral condylar offset was well restored in the NexGen
LPS-Flex group, the range of flexion was approximately the same as that in the
NexGen LPS group. This finding suggests that restoration of the posterior
condylar offset in a knee with a posterior stabilized total knee prosthesis
may not be as crucial for allowing a greater degree of flexion before
impingement as it is in a knee with a posterior cruciate-retaining total knee
prosthesis.
The present study demonstrated gratifying results in association with both
devices, with no differences between them in terms of clinical and
radiographic findings, except for posterior femoral condylar offset. We
believe that several factors were responsible for our superior results: better
cementing technique and design of the component, small and light patients, and
a relatively short duration of follow-up.
The NexGen LPS-Flex prosthesis did not demonstrate its theoretical
advantage of providing a better range of motion of the knee. Other factors
such as a good preoperative range of motion, flexion-space balancing,
posterior tibiofemoral articular contact stability, limb characteristics (long
and slender versus short and thick), and the patients' motivation may have
affected the clinical results and the range of motion.
The authors did not receive grants or outside funding in support of their
research or preparation of this manuscript. They did not receive payments or
other benefits or a commitment or agreement to provide such benefits from a
commercial entity. No commercial entity paid or directed, or agreed to pay or
direct, any benefits to any research fund, foundation, educational
institution, or other charitable or nonprofit organization with which the
authors are affiliated or associated.
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