Total knee arthroplasty successfully reduces pain and provides a functional
range of motion for patients with severe knee
osteoarthritis1-3.
Despite these positive outcomes, walking and stair-climbing speeds have been
reported to be as much as 50% below those of age-matched controls at one year
after surgery4.
Quadriceps weakness has been reported at the time of long-term postoperative
assessment3-6
and has been correlated with disability in individuals with knee
osteoarthritis7-9.
Quadriceps weakness may be a factor that propagates continued functional
limitations after total knee arthroplasty.
Despite its potential impact on functional outcome, quadriceps strength is
not typically assessed in studies of the postoperative results of total knee
arthroplasty. Investigations of acute postoperative changes are particularly
rare, but the existing evidence suggests that patients lose approximately half
of their preoperative quadriceps strength in the first month after
surgery10,11.
Perhaps the most commonly held belief as to why patients are weak early after
surgery is that the pain associated with surgical trauma evokes failure of
voluntary muscle activation, also known as muscle inhibition. Failure of
voluntary muscle activation is a reduction in the maximal force output of a
muscle resulting from an inability to recruit all of the muscle's motor units
or to attain the maximal discharge rate from the motor units that are
recruited12. The
results of preliminary studies have confirmed that reduction in muscle
activation contributes substantially to early postoperative
weakness10,11,
but the contribution of a loss in muscle cross-sectional area to a loss in
strength is unknown.
Understanding how atrophy and the failure of voluntary muscle activation
contribute to quadriceps weakness following total knee arthroplasty is
important when directing postoperative care. The purpose of the present study
was to determine the role of failure of voluntary muscle activation and muscle
atrophy in the early loss of quadriceps strength after surgery. We
hypothesized that (1) voluntary activation, maximal cross-sectional area, and
strength of the involved quadriceps muscle decrease substantially after
surgery, (2) changes in voluntary activation and cross-sectional area account
for a majority of the loss of strength, (3) the change in muscle activation
accounts for more of the loss of quadriceps strength than does the change in
muscular cross-sectional area, and (4) a worsening of knee pain compared with
the preoperative level accounts for a considerable portion of the worsening of
voluntary activation after surgery.
Subjects
This prospective study included a total of twenty subjects (eight women and
twelve men) who were scheduled to undergo primary unilateral total knee
arthroplasty for the treatment of knee osteoarthritis. All subjects underwent
tricompartmental total knee arthroplasty with cement fixation through a medial
parapatellar surgical approach. All of the operations were performed by
experienced surgeons who extended the proximal incision into the quadriceps
tendon. Potential subjects were excluded from the study if they were
considered to be morbidly obese (that is, if they had a body-mass index
[calculated as the weight in kg divided by the height in meters squared] of
>40) or if they had been diagnosed with uncontrolled blood pressure,
diabetes mellitus, neoplasms, or neurological disorders (e.g., Parkinson
disease or stroke). Subjects who had substantial impairment in any of the
other lower-extremity joints were also excluded. The average age was 62
± 8 years (range, fifty-two to eighty-two years), and the average
body-mass index was 31 ± 5 kg/m2 (range, 22 to 40
kg/m2).
Postoperatively, all subjects underwent standardized inpatient and
home-therapy protocols before testing and were functioning clinically as
expected. The average maximal active knee flexion was 119° ±
13° (range, 95° to 141°) before surgery and 95° ±
14° (range, 75° to 121°) at the time of the follow-up test. The
study was approved by the Human Subjects Review Board at the University of
Delaware, and all subjects signed an informed consent form before
participation.
Measurement of Quadriceps Strength and Voluntary Activation
Knee extensor strength and voluntary activation were assessed in all
patients at an average of 10 ± 4 days (range, three to sixteen days)
before and 27 ± 2 days (range, twenty-three to thirty-two days) after
surgery. Measurement of maximal voluntary isometric contraction of the
quadriceps muscle was assessed with use of a burst-superimposition technique,
which was described in detail in a previous
publication11.
Subjects were seated in a dynamometer with the knee flexed to 75°. All
subjects were able to achieve 75° of knee flexion without additional
discomfort. Seat adjustments and transducer settings were recorded to allow
for an identical setup for subsequent postoperative testing.
Each subject performed two submaximal contractions (perceived to be 50% to
75% of maximal effort) and one maximal voluntary contraction lasting two to
three seconds each in order to warm-up the muscle and to gain familiarity with
the testing procedure. After three minutes of rest, the subject was instructed
to contract the quadriceps muscle maximally for approximately three seconds.
Approximately two seconds into the contraction, a stimulator delivered a
supramaximal burst of electrical stimulation through two electrodes that had
been placed on the motor points of the quadriceps.
If maximal voluntary force output was achieved and no augmentation of force
was observed in association with the stimulation (that is, there was optimal
muscle recruitment), then the testing session was concluded for that limb. If
force augmentation was present during the application of the electrical
stimulus, the test was repeated. Three minutes of rest were provided between
contractions in an effort to minimize fatigue. A maximum of three trials was
recorded. The trial with the highest volitional force achieved during the
three attempts was used for analysis.
The extent of voluntary activation of the quadriceps muscle was quantified
with use of the central activation ratio described by Kent-Braun and Le
Blanc12. The
central activation ratio is calculated by dividing the maximal volitional
force by the maximal force produced by the combination of volitional effort
and the superimposed burst (Fig.
1). A central activation ratio of 1.0 indicates complete
activation of the muscle, with no augmentation of the maximal volitional force
being observed during the electrical stimulation.
Measurement of Knee Pain
A numeric rating scale was used to quantify knee pain during
burst-superimposition testing. Subjects were asked to verbally rate the pain
in and around the knee during the burst-superimposition test on a scale from 0
to 10, with 0 representing no pain and 10 representing the worst pain
imaginable. The knee pain rating that was assigned during the attempt that
produced the greatest force was used for analysis.
Health-Status Questionnaires
Health-status questionnaires were completed by all subjects at the time of
the strength assessment and included the Medical Outcomes Survey Short Form 36
(SF-36)13 and the
Activities of Daily Living Scale of the Knee Outcome
Survey14. The
Activities of Daily Living Scale of the Knee Outcome Survey is a fourteen-item
scale designed to assess how knee symptoms and knee condition affect the
ability to perform daily functions. Scores are presented as a percentage of
the maximal score, with 100% representing full perceived knee function during
activities of daily living.
Magnetic Resonance Imaging
Each subject underwent magnetic resonance imaging of the quadriceps muscle
an average of 2 ± 2 days after both the preoperative and postoperative
strength assessments. Three-dimensional images were acquired with a spoiled
gradient-echo sequence (flip angle, 30°) with use of a body coil in a
1.5-T magnet (General Electric Medical Systems, Milwaukee, Wisconsin). Images
were acquired with an encoding matrix of 256 × 256 × 28, a field
of view of 24 cm, a pulse-repetition time of 31 ms, and an echo time of 10 ms.
Seven-millimeter slices were acquired along the entire length of the thigh
with use of chemically selective fat suppression to enhance the definition
between muscles. The cross-sectional area of each individual knee extensor
muscle was determined with use of a validated, custom-designed, interactive
computer program that allows for correction of partial volume-filling
effects15.
Nonmuscular regions, such as subcutaneous fat, were excluded from these
measurements. The sum total of each of the four muscles of the quadriceps
provided an anatomical maximal cross-sectional area for each slice. The slice
with the largest combined cross-sectional area was used for analysis. All
cross-sectional area measurements were performed by one person who had a high
intratester reliability for determining the maximum cross-sectional area, with
an intraclass correlation coefficient (ICC [2,1]) of 0.97 (95% confidence
interval, 0.94 to 0.99).
Data Management and Statistical Methods
All statistical analyses were performed with SPSS for Windows (Version
11.5.1; SPSS, Chicago, Illinois). The contribution of changes in voluntary
activation and atrophy to the change in quadriceps strength was analyzed with
use of multiple linear regression analysis. The influence of knee pain during
strength-testing on voluntary activation of the quadriceps was analyzed with
use of linear regression analysis. The level of alpha was set at 0.05 for all
regression analyses. Differences in the mean values between the preoperative
and postoperative conditions were compared with use of paired t tests, with a
Bonferroni correction for multiple corrections. An adjusted alpha level of
0.007 (determined by dividing the original alpha by the number of comparisons
[i.e., 0.05/7]) was used to determine significance for all statistical tests
performed to compare means.
The average score on the Activities of Daily Living Scale of the Knee
Outcome Survey was 50% ± 20% before surgery and 54% ± 17% one
month after surgery (p = 0.33). Preoperatively, the average physical component
and mental component summary scores of the SF-36 were 34 ± 11 and 58
± 8, respectively. The postoperative physical component summary score
(31 ± 9) was not significantly different from the preoperative score (p
= 0.42), whereas the postoperative mental component summary score (52 ±
11) approached a significant decrease compared with the preoperative score (p
= 0.03).
The quadriceps muscle of the involved limb was significantly weaker after
surgery than it had been before surgery; specifically, the average normalized
strength of the involved quadriceps muscle was decreased by 62% compared with
the preoperative value (p < 0.001) (Fig.
2). In addition, the average voluntary muscle activation of the
involved quadriceps was decreased by 17% compared with the preoperative value
(p = 0.002) and the maximal cross-sectional area of the involved quadriceps
was decreased by 10% compared with the preoperative value (p = 0.004).
Multiple regression analysis revealed that the percent change in voluntary
muscle activation and the percent change in maximal cross-sectional area
explained 85% of the relative change in quadriceps strength (r2 =
0.85, p < 0.001) (Fig. 3).
The relative contribution of the percent change in the central activation
ratio was nearly twice the relative contribution of the percent change in
maximal cross-sectional area in the regression equation that was used to
predict the loss of quadriceps strength after total knee arthroplasty.
The postoperative score for knee pain with muscle contraction was not
significantly different from the preoperative score (average, 3.6 ± 3.9
compared with 2.4 ± 3.0; p = 0.31). Knee pain with muscle contraction
explained a small but significant portion of the variance in voluntary
activation of the quadriceps at the time of the preoperative assessment
(r2 = 0.29, p = 0.015), but it did not have a significant effect at
the time of the postoperative assessment (r2 = 0.20, p = 0.05). The
change in knee pain during muscle contraction between the preoperative and
postoperative tests did not account for a significant amount of the change in
voluntary muscle activation (r2 = 0.12, p = 0.14)
(Fig. 4). Half of the subjects
reported no knee pain during the quadriceps strength test preoperatively, and
the same proportion reported no knee pain during the same test
postoperatively.
We found that patients who had undergone total knee arthroplasty
experienced a profound loss of quadriceps strength, marked failure of
voluntary muscle activation, and a decrease in quadriceps cross-sectional area
when evaluated one month after surgery. The loss of strength was largely
explained by a combination of failure of voluntary muscle activation and
atrophy. Failure of voluntary muscle activation explained much more of the
strength loss than atrophy did; however, the increased activation failure
after total knee arthroplasty was not explained by increased pain.
The loss of >62% of the preoperative quadriceps strength was dramatic
and closely matched the 60% loss of strength that we reported previously in a
similar study involving a different group of
patients11. Not
only was the change in strength after surgery pronounced, but the preoperative
quadriceps strength also appears to have been below normal. The preoperative
force production reported in the present study (18.1 N of force/body-mass
index) was 25% less than the force production reported for healthy older
adults who were tested previously in our laboratory (24.2 N of force/body-mass
index)16,17.
Failure of voluntary muscle activation is likely to have contributed to the
low preoperative quadriceps force production. The subjects in the present
study had an average central activation ratio of 0.867 at the time of
preoperative testing, whereas healthy older adults with no known knee
abnormalities have been reported to have an average central activation ratio
of
0.95516,17.
Two recent studies involving the use of electrical burst-superimposition
strength-testing showed that patients with less advanced knee osteoarthritis
(grade-2 or 3 according to the scale of Kellgren and
Lawrence18) did not
have such a low level of voluntary muscle activation (as indicated by central
activation ratios of
0.92819 and
0.9648). Individuals
who undergo total knee arthroplasty represent a population of patients who
clearly have substantial deficits in voluntary muscle activation.
Not only was there considerable failure of voluntary muscle activation
before surgery, but the degree to which it worsened was remarkable. In
contrast, it has been previously reported that patients who had undergone
anterior cruciate ligament reconstruction did not exhibit abnormal voluntary
activation of the quadriceps muscle eight weeks after
surgery20. Suter et
al. reported an unexpected lack of worsening of voluntary muscle activation at
six weeks in patients who had undergone arthroscopic surgery for the treatment
of anterior knee
pain21. A large
reduction in voluntary activation following total knee arthroplasty bodes
poorly for the recovery of strength as patients with large activation deficits
have been reported to have negligible improvement in strength even after
intensive
rehabilitation22.
Some improvement in voluntary muscle activation is expected during the
subsequent recovery period, a point that was not addressed in this
investigation. In fact, Berth et al., in a long-term follow-up study of
patients managed with total knee arthroplasty, demonstrated that voluntary
activation of the quadriceps improves over
time5. Specifically,
the level of voluntary activation of the quadriceps improved from 76%
preoperatively to 85% at the time of the thirty-three month follow-up. While
this improvement was substantial, the intervention of total knee arthroplasty
did not result in resolution of activation impairments as the level of
voluntary activation of the quadriceps remained much less than that in healthy
controls at both testing times.
A relatively small cross-sectional area of the quadriceps at the time of
the preoperative assessment also appears to have contributed to the overall
reduction in knee extensor strength. The preoperative maximal cross-sectional
area in the present study was much lower than the typical crosssectional areas
found in healthy older
adults23,24
and was slightly lower than the value found in individuals with less advanced
osteoarthritis25.
The change in maximal crosssectional area was smaller than expected as the
average knee extensor strength decreased to less than half of preoperative
strength. To our knowledge, the only other investigation that has assessed
acute changes in quadriceps cross-sectional area associated with total knee
arthroplasty also demonstrated only a small amount of atrophy (a 5% reduction)
compared with the preoperative
assessment10.
Unexpectedly, the change in knee pain did not account for a significant
amount of the large reduction in voluntary activation of the quadriceps
muscle. A similar moderate relationship between knee pain and muscle
activation has been reported in previous investigations of patients managed
with total knee
arthroplasty11,26.
Most of the activation failure does not appear to be due to knee pain during
muscle contraction in this patient population. Assuming that muscle activation
will improve as perioperative knee pain subsides, therefore, may not be
valid.
In the present study, patients who had been managed with total knee
arthroplasty had profound impairment in terms of quadriceps force-producing
ability one month after surgery. Both failure of voluntary muscle activation
and atrophy contributed to the strength loss; however, the major factor
appeared to be failure of voluntary activation. Since activation failure was
not strongly related to knee pain after surgery, pain control alone may be
insufficient to prevent loss of strength. It appears that efforts that are
taken specifically to address deficits in voluntary muscle activation in the
early postoperative period may improve the outcome in terms of quadriceps
strength. Exploring the use of exercise programs that encourage high-intensity
muscle contractions and interventions that facilitate activation (e.g.,
biofeedback and neuromuscular electrical stimulation) appears to be warranted
to counter the large deficit in quadriceps strength following total knee
arthroplasty. ?
Note: The authors thank Glenn Walter, PhD, and Supriya Shidore,
BPT, for their assistance in the analysis of the magnetic resonance
images.