There were no differences on self-report outcome measures (p > 0.070), the stair climbing test (p = 0.311), or the 6-minute walk test (p = 0.180) over time in patients with TKA (Table 1).
The predictive model for the stair climbing test was affected by strength and pain. One year after TKA, strength of both limbs accounted for 31% of the variance (R2 = 0.330, p < 0.001; Table 3) of the time to complete the stair climbing test. By 2 years, strength of both limbs and pain in the operated limb accounted for 33% of the variance in the time (R2 = 0.325, p = 0.003; Table 3). Three years after TKA, it was the nonoperated knee pain alone that accounted for 33% of the variance on the stair climbing test (R2 = 0.330, p < 0.001; Table 3). The predictive model for the 6 MW was affected by strength and knee pain. Strength of the operated quadriceps and operated knee pain accounted for 36% of the variance in the distance walked (R2 = 0.358, p = 0.032; Table 2) 1 year after TKA; by 2 years, strength of both operated quadriceps explained 25% of the variance (R2 = 0.248, p = 0.022; Table 2). Three years after TKA, strength of the operated quadriceps and nonoperated knee pain explained 44% of the variance of the distanced walked (R2 = 0.440, p = 0.005; Table 2) during the 6 MW.
In healthy control subjects, there were no changes in self-report outcome measures (p > 0.20), or 6 MW (p = 0.1) with time; however, the stair climbing test declined with time (p = 0.031) (Table 4). Quadriceps strength had an effect of time with the right limb weakening (p = 0.025); however, there were no side-to-side differences (p > 0.215) (Table 4).
Compared to patients after TKA, the healthy controls were stronger at both intervals (p < 0.001, Table 4), performed better on the 6 minute walk test (p = 0.007; Table 4), and self-reported better function (p < 0.1; Table 4) and less knee pain (p < 0.001; Fig. 2). Controls were faster on the stair climbing test when compared to patients 1 year after TKA (p < 001; Table 4), but not at 3 years after TKA (p = 0.088; Table 4).
The objective of this investigation was to assess changes over time in quadriceps strength, self-report outcome measures, and tests of functional performance in patients 1, 2, and 3 years after TKA, as well as in a cohort of controls without a history of osteoarthritis. We sought to determine the influence of quadriceps strength and self-report knee pain on the results of tests of functional performance in patients after unilateral TKA.
There are limitations to this investigation that warrant consideration when interpreting the results. The diminishing cohort over time in both groups was high, resulting in a dropout rate of nearly two thirds in the TKA group and a dropout rate of 50% in the control group. These subjects were tested longitudinally, and attrition of approximately 20% at each interval was expected, albeit our control subjects dropped out at a higher rate than expected. Our choice of using 4 out of 10 knee pain scores in the contralateral knee at the time of the index procedure was chosen because it represented the patient perception of that knee's pain regardless of the radiographic status. By using it as a screening question, it eliminated from participation individuals with more severe pain in both knees. The lack of radiographic data on the nonoperated knee limits conclusions that can be drawn regarding the status of that knee and how it fared over time; knees with more severe osteoarthritis may not fare as well as knees with little or no osteoarthritis at the time of the index procedure . We also lacked preoperative data; while a baseline comparison is lacking, patients with TKA typically improve substantially compared to their preoperative condition.
Self-report of function plateaued in the 3 years after TKA, and the Global Rating Scale peaked 1 year after TKA in this study, whereas the Knee Outcome Survey remained stable; previously, self-report of function was reported to occur 3 years after TKA . Self-report of function continued to be lower in patients with TKA than in control subjects without osteoarthrosis, similar to other reports in the literature . Although self-report questionnaires can be highly responsive in the early phases after TKA , as function improves, tests of function become the best measures of performance . The questionnaires do, however, provide insight into these individuals' perception of function; the Knee Outcome Survey remains stable, thus these individuals may not perceive the decline in strength.
Nonoperated knee pain was the primary contributor to performance on the stair climbing test and 6 MW in patients 3 years after TKA. The results of the stair climbing test and 6 MW did not change with time, yet there was a shift in the determinant of function in these tasks. The strength of both limbs was relatively stable between 1 and 2 years after TKA and both contributed to performance of the 6 MW and stair climbing test. Three years after TKA, however, the nonoperated limb was considerably weaker and more painful and only operated limb strength and nonoperated knee pain were the determinants of 6 MW and stair climbing test performance, suggesting a change to reliance on the operated knee for these subjects.
The cohort of controls without osteoarthritis weakened considerably over time, and slowed on their results of the stair climbing test. As stated, aging affects strength of the knee extensors [6, 10]. We believe the slower stair climbing score is a result of declining strength: stair ascent requires concentric muscle action against gravity repeatedly, while the descent requires eccentric control of the quadriceps. Quadriceps weakness would make this task more challenging; quadriceps function is also influenced by implant design [33, 34], thus altering muscle use in the lower extremity. The 6-minute walk score remained stable in both groups; walking requires many muscle groups; this test was originally designed as a measure of respiratory fitness , another factor that was not tested in this investigation.
When folks with TKA were compared to controls, patients with TKA were heavier, weaker, and had lower self-report outcome measures at both intervals. Patients with TKA were slower on the stair climbing test only at 1 year after TKA compared to the first test session of controls. Both groups weakened over time, yet the control slowed on this test at a greater rate than did patients after TKA. These results were puzzling, and may be due to the small sample size; however, aging results in greater demand on the hip extensors , and a shift in muscle use may play a role; hip extensors were not tested in this investigation. The 6-minute walk test was originally described as a measure of respiratory fitness . Controls walked at least 100 m further than patients after TKA at both intervals, a difference considered clinically meaningful . Patients with osteoarthritis may have decreased cardiovascular status compared to controls , and while cardiovascular fitness may improve after TKA , it was not tested in this investigation.
After unilateral TKA, the nonoperated knee worsened over the 2-year period of this study with increased pain and quadriceps weakness. Strength and pain stabilize in the index knee, and self-report outcome measures and functional performance plateau in the 1 to 3 years after TKA. Patients with TKA never reached the levels of function of a comparable group of healthy control subjects despite the fact that controls also declined in strength and function over the same time interval. The use of the nonoperated leg as the so-called healthy limb in patients after a unilateral TKA may underestimate disability; therefore, a comparison group should be the standard for assessments of functional abilities.
We thank Stuart Binder-Macleod, principle investigator of NIH T32-HD07490. We thank Leo Raisis, M.D., Alex Bodenstab, M.D., and William Newcomb, M.D., all of First State Orthopaedics for their patient referrals. We also thank Jennifer Stevens-Lapsley, PT, PhD, Ryan Mizner, PT, PhD, Stephanie Petterson, PT, PhD, and Yuri Yoshida, MS, for their assistance and support.
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