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 [22
]. We also lacked preoperative data; while a baseline comparison is lacking, patients with TKA typically improve substantially compared to their preoperative condition.
Pain plays a role in the ability to use the quadriceps femoris muscles [17
]; weakness of the quadriceps has been implicated in the development of knee osteoarthrosis [2
]. Strength affects function [20
]; reduced physical capacity may be both a cause and a consequence of physical impairment and functional limitations [32
]. Patients with TKA were stable on tests of functional performance and self-report outcome measures during this investigation; however, lower scores on functional tests compared to controls indicate their level of function was not as good as individuals without osteoarthrosis. Aging affects strength of the knee extensors [6
] and played a role in declining strength [9
] in these individuals who were tested over the course of several years. The decline in strength in the nonoperated limb was clearly steeper than that of the control subjects suggesting more than aging was at play. Patients with a history of knee osteoarthrosis that led to a TKA risk more rapid declines in strength and increases in pain in their nonoperated knee compared with healthy counterparts.
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 [28
]. 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 [6
]. Although self-report questionnaires can be highly responsive in the early phases after TKA [23
], as function improves, tests of function become the best measures of performance [23
]. 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
]. 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
], 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 [3
], 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 [5
], 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 [3
]. Controls walked at least 100 m further than patients after TKA at both intervals, a difference considered clinically meaningful [35
]. Patients with osteoarthritis may have decreased cardiovascular status compared to controls [25
], and while cardiovascular fitness may improve after TKA [26
], 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.