A satisfactory postoperative range of motion is an important component
of a successful result of total knee arthroplasty. Previously, we
conducted an analysis to compare the weight-bearing and non-weight-bearing
ranges of motion in subjects with a fixed-bearing posterior-cruciate-retaining
or posterior stabilized total knee replacement1. The purpose of
this follow-up study was to determine the difference between the weight-bearing
and non-weight-bearing ranges of motion in patients with either
a mobile-bearing posterior stabilized total knee replacement or
a fixed-bearing posterior stabilized total knee replacement. A secondary
goal of this study was to assess if the weight-bearing range of
motion was affected by the implant type—that is, a fixed-bearing
or a mobile-bearing posterior stabilized total knee replacement.
The range of motion of the knee in forty-six patients was evaluated
with use of video fluoroscopy. Two patient subgroups—those
with a fixed-bearing posterior stabilized total knee replacement
(Sigma PS; DePuy, a Johnson and Johnson company) (twenty-three patients)
and those with a mobile-bearing posterior stabilized total knee
replacement (LCS PS; DePuy, a Johnson and Johnson company) (twenty-three
patients)—were separately analyzed and were compared. All
patients evaluated in this study were randomly selected on the basis
of the following criteria: (1) no reported knee pain, functional limitation
related to the knee, or previous injury and (2) a clinical examination
demonstrating no measurable ligamentous instability.
Preoperative clinical assessment included determination of the
passive non-weight-bearing range of motion and the Hospital for
Special Surgery knee-rating scores. There was no significant difference
between the two subgroups with regard to the preoperative range
of motion, preoperative or postoperative Hospital for Special Surgery
knee-rating scores, patient age, or gender according to a two-sample
unequal variance Student t test, but the duration of follow-up was
significantly different between the two groups (p = 0.0462)
(Tables I and II). All patients were
examined with use of a High Frequency Pulsated Fluoroscopy Unit
(Radiographic and Data Solutions, Minneapolis, Minnesota). Each
patient performed three successive weight-bearing deep knee bends
to the maximum flexion attainable (Fig. 1). Subsequently, the passive non-weight-bearing
range of motion was determined by having an erect, standing patient raise
the foot from the ground to the point of maximum knee flexion. To
ensure maximum passive knee flexion, the subject was allowed to
grasp and pull the foot superiorly (Fig. 2).
To ensure accurate assessment, the maximum range of motion for
each subject was determined with use of a digitization technique
previously described for fluoroscopic studies. Points F1 through
F4 on the femur and T1 through T4 on the tibia were located and
digitized (Fig. 3).
Lines were then constructed on the fluoroscopic images from point
F1 to F2, F3 to F4, T1 to T2, and T3 to T4. After measurement of
the length of each line, the midpoints were located and denoted
as points F5, F6, T5, and T6. The longitudinal axes of the femur
and tibia were derived by constructing lines through points F5 and
F6 and through points T5 and T6. The angle between the longitudinal
axes was measured digitally to a tenth of a degree and denoted as
the maximum range of motion. Statistical comparisons of the patient
subgroups and the testing methods were performed with a two-tailed
distribution, two-sample unequal variance Student t test.
We assessed the effect of weight-bearing and prosthetic design
on knee range of motion in our previous study, which included subjects
with normal knees, those with a fixed-bearing posterior stabilized
total knee replacement, and those with a fixed-bearing posterior-cruciate-retaining
total knee replacement1. All three knee subgroups demonstrated a
significant decrease in the range of motion when it was measured during
weight-bearing as compared with when it was measured during non-weight-bearing
(p < 0.045 for the normal knees, p < 0.001 for
the posterior stabilized replacements, and p < 0.001 for
the posterior-cruciate-retaining replacements). This reduction in
motion was greatest in the posterior-cruciate-retaining subgroup
(20° reduction). The normal knee subgroup exhibited better flexion
than either of the knee-replacement subgroups, regardless of whether
it was measured during passive non-weight-bearing (p < 0.001)
or active weight-bearing (p < 0.001). The maximum mean
ranges of postoperative flexion in the posterior-cruciate-retaining (123°)
and posterior stabilized (127°) subgroups were similar under passive
non-weight-bearing conditions (p > 0.176). However, under
weight-bearing conditions, patients with a posterior stabilized
total knee replacement exhibited a significantly greater mean range
of motion than those with a posterior-cruciate-retaining replacement
(113° compared with 103°, p < 0.024). This finding occurred
despite the fact that the posterior-cruciate-retaining subgroup
demonstrated a greater mean knee flexion (118° compared with 108°)
and had a higher mean Hospital for Special Surgery score (65.2 compared
with 58.7 points) than the posterior stabilized subgroup preoperatively
and also had a younger mean age (53.7 compared with 65.5 years),
although these differences were not significant. No significant
difference in knee range of motion related to gender was observed
in any subgroup or under either testing condition.
Similar to the findings in our previous study, in the present study
subjects with either a fixed-bearing or a mobile-bearing posterior
stabilized total knee replacement had significantly less motion
under weight-bearing conditions than they had under non-weight-bearing
conditions. Also, subjects with a fixed-bearing posterior stabilized
total knee replacement had a significantly greater weight-bearing
range of motion than subjects with a mobile-bearing posterior stabilized
total knee replacement. Preoperatively, there was no significant
difference between the ranges of motion of the two groups under non-weight-bearing
conditions. It can be hypothesized that two factors may have contributed
to the greater weight-bearing range of motion in the subjects with
a fixed-bearing posterior stabilized total knee replacement. First,
those subjects had had the implant for significantly longer than
did the subjects with a mobile-bearing posterior stabilized total
knee replacement. Second, the mobile-bearing replacement chosen
for this study had a polyethylene insert with a high posterior lip, which
allows for sagittal conformity during the first 30o of knee flexion.
Also, the post is more posterior than is the post of the fixed-bearing
replacement, which allows for earlier cam-post engagement to occur.
Therefore, it is hypothesized that the cam-post mechanism engages
early and near 90o of knee flexion and the knee becomes very tight
as a result of a wedging effect between the cam-post mechanism and
the high posterior lip of the polyethylene. Under weight-bearing
conditions, with the muscle in high tension, a subject could have
a tight knee that would minimize the range of motion. Finally, the
results of this study have demonstrated that the effects of bearing
mobility did not lead to a larger range of motion in the subjects
who were assessed since subjects who had a fixed-bearing total knee
replacement had greater range of motion.
This study demonstrated that the range of motion during weight-bearing
is diminished compared with that during passive non-weight-bearing
in patients with either a fixed-bearing or a mobile-bearing posterior
stabilized total knee replacement. Under weight-bearing conditions,
subjects with a fixed-bearing posterior stabilized total knee replacement exhibited
significantly greater knee flexion than did subjects with a mobile-bearing
posterior stabilized total knee replacement.
Acknowledgment: Radiographic and Data Solutions, Minneapolis,
Minnesota