The Low Contact Stress mobile-bearing knee prosthesis (DePuy, Warsaw,
Indiana) was introduced to address the so-called kinematic conflict in
fixed-bearing knees by allowing a highly conforming articular surface to
coexist with free
rotation1-9.
The rotating-platform Low Contact Stress mobile-bearing prosthesis was
designed to have unlimited rotation but a limited range of anteroposterior and
mediolateral translation, which occurs primarily as a result of the femoral
component sliding on the surface of the polyethylene
insert10
(Figs. 1-A and 1-B). A
second-generation rotating-platform Low Contact Stress prosthesis
(Anterior-Posterior Glide; DePuy) was developed to approximate the kinematics
of the natural knee more closely by incorporating unlimited anterior-posterior
translation and rotation. The anterior-posterior-glide prosthesis has a
conforming rotating polyethylene tibial insert that can translate along a bar
on the tibial component in the sagittal plane. This modification of the distal
articulation allows for anteroposterior translation and rotary movement
between the flat distal polyethylene surface and the highly polished proximal
tibial tray (Figs. 2-A and
2-B).
The anterior-posterior-glide and rotating-platform Low Contact Stress
mobile-bearing total knee replacements have been studied
independently4,6,7,9-14.
A comparison of the results in the same patients eliminates the variability
that is introduced by differences in gender, age, weight, comorbidities, bone
quality, and activity level and allows for a more meaningful comparison of the
impact of fixation on the outcome of total knee arthroplasty. However,
variability in terms of the preoperative severity of the arthritis cannot be
eliminated because the severity of the disease on both sides is rarely
identical.
We performed a prospective, randomized study to compare the clinical and
radiographic results associated with the anterior-posterior-glide and
rotating-platform Low Contact Stress total knee replacements in patients who
were managed with simultaneous bilateral total knee arthroplasty.
Between May 1996 and June 1998, the senior author (Y.-H.K.) performed 196
consecutive primary bilateral total knee arthroplasties in 196 patients (392
knees). All 196 patients were enrolled in the present study. The bilateral
total knee arthroplasties were performed during the same anesthetic session,
with one side treated immediately after the other. Six patients were lost to
follow-up, leaving 190 patients (380 knees) available for inclusion in the
study. The study was approved by our institutional review board, and all
patients provided informed consent.
Randomization between the use of an anterior-posterior-glide Low Contact
Stress mobile-bearing prosthesis or a rotating-platform Low Contact Stress
mobile-bearing prosthesis was determined from a sequential pool on the basis
of a table of randomized numbers. Each of the 190 patients received an
anterior-posterior-glide Low Contact Stress mobile-bearing total knee
component on one side and a rotating-platform Low Contact Stress
mobile-bearing total knee component on the contralateral side. The order of
insertion of the anterior-posterior-glide and rotating-platform mobile-bearing
prostheses was assigned alternately to each side. Eleven patients (twenty-two
knees) were men, and 179 patients (358 knees) were women. The mean age of the
patients at the time of the index operation was sixty-four years (range,
forty-seven to seventy-six years). The diagnosis was osteoarthritis for 162
patients and osteonecrosis of the medial femoral condyle for twenty-eight
patients. One hundred and thirty-three patients (70%) had had no previous knee
operation, and fifty-seven patients (30%) had had arthroscopic
débridement of one or both knees.
All procedures were performed through a midline skin incision measuring 10
to 12 cm in length, with a subvastus approach into the joint. The anterior
cruciate ligament was excised in all patients. The posterior cruciate ligament
was relatively well preserved in all knees as none of the patients had
inflammatory arthritis. In all knees that were treated with the
anterior-posterior-glide prosthesis, the posterior cruciate ligament was
preserved. In all knees that were treated with the rotating-platform
prosthesis, the posterior cruciate ligament was excised.
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 being made to remove a length of bone that was equal to the thickness
of the femoral component to be inserted. 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. All
implants were inserted with cement after pulsed lavage, drying, and
pressurization of the cement.
A splint was applied with the knee in 15° of flexion and was worn for
the first twenty-four hours after the operation. The knee then was placed in a
continuous passive-motion machine. All patients began walking with crutches or
a walker and started active and passive range-of-motion exercises 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 six weeks, three months,
six months, and one year after the operation, and then yearly thereafter. The
mean duration of follow-up was 6.4 years (range, five to seven years). Each
knee was rated preoperatively and postoperatively according to the systems of
the Knee Society15
and the Hospital for Special
Surgery16. In
addition, each patient completed a self-administered questionnaire that
consisted of (1) a visual analog scale for the assessment of the severity,
location, and frequency of pain and (2) 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.
Anteroposterior radiographs with the patient standing and supine, lateral
radiographs, and skyline patellar radiographs were made preoperatively and
postoperatively and were assessed for alignment of the limb, 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
Society15.
The levels of the joint lines were determined on anteroposterior
radiographs made before and after surgery with the patient supine 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 between the tip of the
fibular head and the distal margin of the lateral femoral component
postoperatively. The skyline patellar radiographs were examined for patellar
tilt, subluxation, or dislocation. Osteolysis around the three components was
recorded. No intraobserver or interobserver analysis of the radiographic
findings was done.
Statistical comparison of the clinical and radiographic results associated
with the two groups was done with analysis of variance, chi-square analysis,
and the two-tailed Student t test.
Survivorship analysis was performed to determine the cumulative rate of
survival of the implant during the period of the
study17,18.
The end point for this analysis was revision surgery (or a recommendation for
revision surgery) for any reason.
Clinical Results
Knee Score
The preoperative and postoperative knee scores, pain scores, walking
distance, range of motion, use of walking support, and ability to negotiate
stairs in both groups are summarized in
Table I. The preoperative Knee
Society and Hospital for Special Surgery knee scores for the two groups were
not significantly different (p = 0.5242). In the anteriorposterior-glide
group, the mean postoperative knee score was 90.2 points (range, 45 to 100
points) according to the system of the Knee Society and 89.4 points (range, 69
to 100 points) according to the system of the Hospital for Special Surgery. In
the rotating-platform group, the mean postoperative knee score was 89.2 points
(range, 52 to 100 points) according to the system of the Knee Society and 88.6
points (range, 45 to 100 points) according to the system of the Hospital for
Special Surgery. The postoperative knee scores for the two groups were not
significantly different, with the numbers available (p = 0.7242). Three knees
in each group had a poor result.
Pain
The postoperative pain scores according to both knee-scoring systems were
not significantly different between the groups, with the numbers available (p
= 0.4652). At the time of the latest follow-up of the 190 knees in the
anterior-posterior-glide group, 132 (69%) were not painful, fifty-eight (31%)
were mildly painful, and none were moderately or severely painful according to
the system of the Knee Society, whereas 155 (82%) were not painful,
thirty-five (18%) were mildly painful, and none were moderately or severely
painful according to the system of the Hospital for Special Surgery. In
comparison, of the 190 knees in the rotating-platform group, 134 (71%) were
not painful, fifty-six (29%) were mildly painful, and none were moderately or
severely painful according to the system of the Knee Society, whereas 149
(78%) were not painful, forty (21%) were mildly painful, none were moderately
painful, and one (0.5%) was severely painful according to the system of the
Hospital for Special Surgery.
Ability to Walk
At the time of the latest follow-up, 148 patients (78%) were able to walk
an unlimited distance without pain or with only mild discomfort, ten patients
were able to walk six to ten blocks, fourteen patients were able to walk one
to five blocks, and eighteen patients were able to walk less than one block.
One hundred and seventy-five patients (92%) did not require support for
walking, eleven patients required one cane, and four patients required two
crutches. One hundred and thirteen patients (59%) were able to negotiate
stairs without support, and seventy-seven patients used support.
Range of Motion
Preoperatively, the mean flexion contracture was 7° (range, 0° to
53°) in the anterior-posterior-glide group and 6.8° (range, 0° to
35°) in the rotating-platform group. The mean flexion was 132° (range,
95° to 150°) in the anterior-posterior-glide group and 131.6°
(range, 85° to 150°) in the rotating-platform group. At the time of
the latest follow-up, no knee had a flexion contracture. Postoperatively, the
mean flexion was 128.3° (range, 70° to 150°) in the
anterior-posterior-glide group and 128.0° (range, 90° to 150°) in
the rotating-platform group; this difference was not significant (p = 0.8578),
with the numbers available. Although all knees obtained at least 120° of
passive flexion with the use of a continuous passive-motion machine during the
hospitalization, the active range of flexion was reduced to 70° or 90°
in several patients at the time of the final follow-up.
Satisfaction
With regard to the knees in the anterior-posterior-glide group, 114
patients (60%) were fully satisfied with the outcome of the operation,
sixty-six patients (35%) were satisfied, and ten patients (5%) were
dissatisfied. With regard to the knees in the rotating-platform group, 106
patients (56%) were fully satisfied, seventy-three patients (38%) were
satisfied, and eleven patients (6%) were dissatisfied. Of the twenty-one
patients in both groups who were dissatisfied, ten patients had constant mild
pain and stiffness, four had constant moderate pain, and the remaining seven
had an inadequate range of motion.
Radiographic Results
The radiographic results are summarized in
Table II. With the numbers
available, there were no significant differences between the groups with
regard to the position of the femoral and tibial components in coronal and
sagittal planes, the alignment of the knee, the patellar angle, or the tibial
surface area covered by the implants (p > 0.05).
With the numbers available, the mean level of the joint line was not
significantly different between the two groups preoperatively (p = 0.465) or
postoperatively (p = 0.758). In both groups, while the mean range of motion
(and standard deviation) was 115° ± 18.06° in the knees in
which the post-operative joint line changed by >1 cm compared with the
preoperative joint line, it was 125° ± 10.98° in the knees in
which the postoperative joint line changed by <1 cm compared with the
preoperative joint line. This difference was significant (p = 0.003).
One hundred and sixty knees (84%) in the anterior-posterior-glide group and
173 knees (91%) in the rotating-platform group had no evidence of radiolucent
lines around any component (Figs. 3-A,
3-B, 3-C, 3-D).
Therefore, the prevalence of radiolucent lines around one component was 16%
(thirty knees) in the anterior-posterior-glide group and 9% (seventeen knees)
in the rotating-platform group. This difference was not significant (p =
0.098), with the numbers available. No knee had loosening of the femoral,
tibial, or patellar component. No knee had a patellar dislocation.
Kaplan-Meier
analysis17 with
revision as the end point revealed a seven-year survival rate of 100% (95%
confidence interval, 0.94 to 1.0) in both groups.
Complications
One knee (0.5%) in the rotating platform group was associated with
persistent severe pain. One knee with an anterior-posterior-glide prosthesis
was associated with permanent tibial and deep peroneal nerve palsies, perhaps
due to a compartment syndrome. One knee with a rotating-platform prosthesis
had a permanent deep peroneal nerve palsy resulting from an unknown cause.
The rotating-platform and anterior-posterior-glide Low Contact Stress
mobile-bearing prostheses have been used successfully for many
years4,6,8,9,11-14,19.
The present study demonstrated gratifying results in association with both
devices, with no differences between the two prostheses in terms of clinical
and radiographic findings.
It has been demonstrated that a greater range of motion can be obtained
with a properly inserted posterior cruciate-retaining anterior-posterior-glide
Low Contact Stress prosthesis compared with a posterior cruciate-sacrificing
rotating-platform Low Contact Stress
prosthesis14. In
the current study, the average range of motion was approximately the same in
both groups. In a previous study, we evaluated factors affecting range of
motion after fixed-bearing and mobile-bearing total knee arthroplasties and
concluded that preoperative range of motion and restoration of the joint line
were the most important
factors7. The
findings of the current study confirmed this view.
Oakeshott et al. reported synovitis and recurrent effusion in eighteen
(60%) of thirty patients who had had a total knee arthroplasty with use of an
anterior-posterior-glide Low Contact Stress prosthesis. The authors reported
that these problems were due to impingement of anterior soft tissues and that
they resolved after subsequent exchange of the tibial
polyethylene14. In
the current series, no patient with an anterior-posterior-glide Low Contact
Stress prosthesis had synovitis or a recurrent effusion.
Waslewski et al. reported on thirteen patients (sixteen knees) who had
early incapacitating instability secondary to posterior cruciate ligament
deficiency following posterior cruciate-retaining total knee
arthroplasty20.
Morberg et al. reported that six of sixteen patients who had been managed with
the anterior-posterior-glide Low Contact Stress prosthesis had adverse events
related to excessive translation in the sagittal plane secondary to failure of
the posterior cruciate ligament after
surgery21. In the
current study, no knee that had been treated with the anterior-posterior-glide
prosthesis had early or delayed clinical failure of the posterior cruciate
ligament. Careful assessment and protection of the posterior cruciate ligament
during the operation is important. Leaving a bone island to protect the
insertion of the posterior cruciate ligament on the posterior tibial plateau
may minimize iatrogenic damage to the insertion site, but care must be taken
to avoid impingement of the polyethylene insert against the bone island during
anterior-posterior translation.
Buechel and
Pappas4 and
Goodfellow and
O'Connor22
postulated that the mobile-bearing knee replacement would minimize
bone-prosthesis stress at the tibial surface. In the current study, the
prevalence of radiolucent lines in fewer than two zones around the tibial
component was 16% in the anterior-posterior-glide group and 9% in the
rotating-platform group. No knee had radiolucent lines in more than three
zones around the tibial, patellar, or femoral component. Therefore, it appears
that the mobile bearing minimizes bone-cement stress at the interfaces of all
components.
Mobile-bearing total knee arthroplasty was introduced to increase articular
conformity and to minimize contact stresses, thereby reducing linear wear and
subsurface fatigue failure. The current study demonstrated no measurable wear
of the tibial or patellar polyethylene bearing in either group. There also was
no evidence of periprosthetic osteolysis in either group, although early
detection of osteolysis after a total knee arthroplasty can be difficult on
plain radiographs. Our finding of no osteolysis in either group suggests that
solid fixation of these prostheses with good cementing technique limits the
so-called effective joint
space23. It is also
possible that the rate of osteolysis was low because the duration of follow-up
was not long enough.
The results associated with the anterior-posterior-glide and the
rotating-platform Low Contact Stress mobile-bearing total knee replacements
were favorable after a minimum duration of follow-up of five years. The
results for the two groups were comparable with regard to the total knee
score, pain and functional scores, range of motion, polyethylene wear, aseptic
loosening, and periprosthetic osteolysis.