To report the prevalence and relationship of self-reported knee instability to physical function in a sample of subjects with knee osteoarthritis (OA), and to discuss the implications of these observations for rehabilitation.
To report the prevalence and relationship of self-reported knee instability to physical function in a sample of subjects with knee osteoarthritis (OA), and to discuss the implications of these observations for rehabilitation.
Subjects were 105 individuals (80 females) with knee OA who rated their knee instability severity on a 6-point numeric scale in response to the query “To what degree does giving way, buckling, or shifting of the knee affect your level of daily activity?” A principal component analysis was used to combine The Western Ontario and McMaster Universities pain, stiffness, and physical function subscale scores, and the Timed Get Up and Go Test score into a principal component score for physical function (PCPF). Other variables that could affect the PCPF such as age, sex, years with knee OA, radiographic severity of knee OA, knee pain, knee motion, and quadriceps strength were also recorded. The prevalence of self-reported knee instability was determined by calculating the proportion of subjects who reported each severity level of knee instability. Hierarchical regression analysis was performed to determine if the level of self-reported knee instability could predict the PCPF, even after accounting for the effects of the other variables.
Sixty-three percent of the subjects reported knee instability during activities of daily living, and 44% reported that instability affects their ability to function. The severity of self-reported knee instability was associated with the PCPF (eta2 = 0.40, P < 0.001), and after controlling for all other independent variables, significantly increased the prediction of the PCPF (r2 = 0.56, r2 change = 0.05; P < 0.001).
The results indicate that a substantial proportion of individuals with knee OA report episodes of knee instability during activities of daily living, and instability affects physical function beyond that which can be explained by contributions from other impairments such as knee pain, range of motion, and quadriceps strength. Knee instability is a problem that should be specifically addressed in rehabilitation programs and may require interventions beyond those that address pain, joint motion, and muscular strength, to maximize the effectiveness of rehabilitation for individuals with knee OA.
Knee osteoarthritis (OA) is a prevalent condition that contributes significantly to functional limitations and disability in the elderly population (1). Physical impairments such as knee pain, loss of knee motion, and decreased quadriceps strength have been associated with knee OA and are believed to contribute to physical disability and progression of the disease (2–7). Recently, as part of a larger study conducted by our research group, we observed that a substantial number of subjects reported the sensation of knee instability (buckling, shifting, or giving way of the knee) during activities of daily living. We hypothesized that perhaps reports of knee instability may be related to measures of physical function, beyond that which could be accounted for by physical impairments such as pain, knee range of motion, and quadriceps strength. We report the prevalence and the relationship of self-reported knee instability to physical function in our sample of subjects with knee OA, and discuss the implications of these observations for rehabilitation.
Subjects were 105 individuals (80 females, 25 males) who had been diagnosed by their physician with osteoarthritis of the knee (tibiofemoral and/or patellofemoral involvement). Subjects were included in the study if they were ≥ 45 years of age, had knee radiographs performed within the last 12 months, met the 1986 American College of Rheumatology (ACR) clinical and radiographic criteria for knee osteoarthritis (8), and had grade II or greater Kellgren/Lawrence radiographic changes (9). The 1986 ACR criteria for diagnosis of knee osteoarthritis includes knee pain with osteophytes and at least one of the following: age ≥ 50 years, morning stiffness lasting less than 30 minutes, or crepitus with active motion of the knee, such as when squatting while weightbearing (8). Subjects were excluded from the study if they had limitations in knee motion that prevented them from comfortably positioning their knee for the quadriceps torque test, had undergone total knee arthroplasty, exhibited uncontrolled hypertension, had a history of cardiovascular disease, or had other pre-existing conditions that would place them at risk for injury to the extensor mechanism during quadriceps torque testing (e.g., a history of patellar tendon autograft anterior cruciate ligament reconstruction, a history of patellar or quadriceps tendon rupture, a history of patellar fracture, a history of prolonged corticosteroid injection of the patellar or quadriceps tendons). Subject characteristics are provided in Table 1.
|Mostly sedentary||37 (35.2)|
|Somewhat sedentary with substantial walking required||18 (17.1)|
|Moderately active, walking, some lifting and carrying||42 (40.0)|
|Demanding physical activity, heavy lifting and carrying||8 (7.6)|
|Radiographic knee OA†|
|Grade II||27 (26)|
|Grade III||35 (33)|
|Grade IV||43 (41)|
|Medial compartment||95 (90)|
|Lateral compartment||44 (42)|
|Patellofemoral compartment||51 (49)|
|Bilateral knee OA||68 (64.8)|
|Years with knee OA‡|
|Prior knee injury or surgery||34 (32.4)|
|Age in years, mean (range)||62.0 ± 9.6 (46–84)|
|Body mass index (kg/m2), mean (range)||33 ± 6 (18–53)|
Subjects were recruited from physician's offices of the University of Pittsburgh Medical Center (UPMC) Arthritis Network, the UPMC Arthritis Network Registry, and from physical therapy clinics of the Centers for Rehabilitation Services of the UPMC Health System. All subjects signed an informed consent form approved by the University of Pittsburgh Institutional Review Board prior to participation in the study.
Data for each subject were collected during one testing session that lasted approximately 1.5 hours. During the session, subjects first completed a demographic and health history questionnaire, a self-report measure of function, and rated the severity of knee pain and knee instability. Following completion of the questionnaires, physical performance measurements of function were administered.
Subjects rated the severity of knee instability on a 6-point numeric scale in response to the query “To what degree does giving way, buckling, or shifting of the knee affect your level of daily activity?” Definitions for the 6 levels of instability are provided in Table 2. This self-report rating of knee instability was adapted from the Knee Outcome Survey-Activities of Daily Living Scale (10). We have estimated the test-retest reliability of our self-report rating of knee instability on 50 subjects with a variety of knee pathologies, including knee OA, using an intraclass correlation coefficient (ICC formula 2,1) and have determined it demonstrates adequate test-retest reliability (ICC 0.72).
|Knee instability ratings||Frequency||%||Cumulative %|
|0 = The symptom prevents me from all daily activity||1||1||1|
|1 = The symptom affects my activity severely||8||8||9|
|2 = The symptom affects my activity moderately||13||12||21|
|3 = The symptom affects my activity slightly||24||23||44|
|4 = I have the symptom but it does not affect my activity||20||19||63|
|5 = I do not have giving way, buckling, or shifting of the knee||39||37||100|
The Get Up and Go test was used as a physical performance measure of function. The test was performed as described by Hurley et al (11). To perform the test, subjects were seated on a standard height chair with armrests. On the command “go” subjects stood up and walked along a level, unobstructed corridor as fast as possible. A stopwatch was used to measure the length of time it took for the subject to stand and walk 15 meters. We found good intra-tester and inter-tester reliability (ICC 0.95, 098, respectively) for this test at our facility. A longer time to complete the Get Up and Go test represents greater functional limitations.
The Western Ontario and McMaster Universities (WOMAC) Osteoarthritis Index was used as a self-report measure of function. The WOMAC is a disease specific measure of pain, stiffness, and physical function for individuals with knee OA, which includes 5 items related to pain, 2 related to stiffness, and 17 related to physical function. Each item is scored on a five-point Likert scale. Reliability and validity of the WOMAC has been established (12) with higher scores representing greater limitations in function.
To gain a more comprehensive understanding of the relationship between self-reported knee instability and physical function, we examined this relationship after accounting for other factors that could relate to physical function. These factors included age, sex, number of years with a diagnosis of knee OA, radiographic severity of knee OA, pain, knee motion, and quadriceps strength.
The severity of radiographic knee OA was rated by an experienced rheumatologist, using the method described by Kellgren and Lawrence (9). Subjects rated their current amount of knee pain, the greatest amount of knee pain in the past 24 hours, and the least amount of knee pain in the past 24 hours, using a 0 to 10 numeric pain scale, (0 = no pain and 10 = worst pain imaginable.) The score for knee pain was recorded as the sum of the numeric ratings for all 3 conditions. Knee motion was recorded as the total combined motions of knee flexion and extension, measured using standard goniometry.
Quadriceps strength was determined by measuring maximum voluntary isometric quadriceps torque output. Subjects were seated on an isokinetic dynamometer (Biodex System 3 Pr, Shirley, NY) with the dynamometer force-sensing arm secured to the ankle. The knee was positioned in 60 degrees of flexion, with the lateral femoral epicondyle aligned with the dynamometer's axis of rotation. A thigh strap, waist strap, and 2 chest straps were then secured to stabilize the subject in the dynamometer chair. Following a practice period in which subjects practiced producing isometric contractions at 50%, 75%, and 100% maximum effort, the subjects were asked to exert as much force as possible while extending the knee against the fixed force-sensing arm of the dynamometer. A torque target line was displayed on the computer monitor to provide subjects with visual feedback in an effort to maximize their ability to produce maximum torque output during the test. The torque target was placed at a torque level slightly greater than the peak torque produced during the practice maximum voluntary isometric contraction. If subjects exceeded this torque target during a given trial, the target was reset at a higher level for the next trial. The test was repeated for up to 4 trials. It has been demonstrated that individuals should achieve maximum muscle activation during a maximum voluntary isometric contraction within 4 trials (13). Maximum voluntary isometric quadriceps torque output was expressed as a ratio of peak isometric quadriceps torque output produced during the 4 trials, divided by the subject's body mass index.
To determine the incidence of self-reported knee instability, we tabulated the frequency of each response to the question “To what degree does giving way, buckling, or shifting of the knee affect your level of daily activity?” (see Table 2). Prior to examining the relationship between self-reported knee instability and physical function, we used a principal component analysis to combine the WOMAC stiffness, pain, and physical function scores as well as the Get Up and Go Test score into a single principal component score for physical function.
Several authors suggested that because there is only a modest relationship between performance-based and self-reported measures of function, these measures may be measuring different aspects of the construct of physical function (14–17). We decided to combine the self-report and physical performance measures of function to provide a more comprehensive measure of function. The Get Up and Go test score and the 3 components of the WOMAC (stiffness, pain, and physical function) scores were entered into the principal component analysis. The eigenvalue > 1.0 rule was used to determine the number of principal components to extract (18). In this study, only 1 principal component had an eigenvalue > 1, implying that there was a single dimension underlying the 4 original variables and it was thus appropriate to combine the scores into a single principal component. The decision to combine the 4 measures into a single score was strengthened by the fact that the measures were well correlated with the principal component of physical function (WOMAC pain 0.92, WOMAC stiffness 0.89, WOMAC physical function 0.94, Get Up and Go test 0.62).
The principal component analysis allowed us to combine the 4 original variables into a single principal component score of a physical function (PCPF) for each subject. This was accomplished by transforming each subject's score on each of the 4 measures of physical function to a Z score. The Z score for each variable was then multiplied by the principal component score coefficient for that variable, and the resulting products were summed to create a principal component score for each subject. The principal component scores were normalized scores that had a mean of 0 and a standard deviation of 1. An example of these calculations is provided in Table 3.
|Variable||Component score coefficient||Z score||Component score coefficient × Z score|
|WOMAC physical function||0.345||−1.67944||−0.5794|
|Get Up and Go Time||0.211||−0.96809||−0.2043|
Pearson's correlation coefficients were calculated to determine the relationship between the PCPF and age, number of years with a diagnosis of knee OA, pain ratings, knee motion, and quadriceps strength. Point biserial correlations were calculated to determine the association between the PCPF and sex. The Pearson's and biserial correlation coefficients were converted to coefficients of determination (r2) to determine the strength of association between aforementioned variables and PCPF. Eta2 was calculated to determine the strength of association between the severity of radiographic knee OA with the PCPF as well as between self-reported knee instability and the PCPF. The level of significance was set at 0.05.
Hierarchical regression analysis was performed to determine if the level of self-reported knee instability can predict physical function, even after accounting for the effects of age, sex, number of years with a diagnosis of knee OA, radiographic severity of knee OA, knee pain, knee motion, and quadriceps strength. The PCPF was used as the dependent variable to represent physical function. Variables were entered into the regression analysis in order of age, sex, number of years with diagnosis of knee OA, radiographic severity of knee OA, knee pain, knee motion, and quadriceps strength, and self-reported knee instability entered last. The self-reported rating of knee instability was entered last so that its effect on physical function could be determined after controlling for all other variables.
The number and percentage of subjects reporting each new level of knee instability are shown in Table 2. The data indicate that 63% (66) of 105 subjects reported giving way, slippage, or buckling of their knees during activities of daily living and 44% (46) reported that instability affects their ability to function. Table 4 provides Pearson's correlation coefficients, coefficients of determination, and Eta2 values for the relationships between the PCPF and the independent variables. Age and severity of radiographic knee OA were not significantly correlated with the PCPF. All other independent variables were significantly correlated with the PCPF. The results of the regression analysis are summarized in Table 5. The analysis indicates that after controlling for all other independent variables, the self-reported rating of knee instability significantly increased the prediction of the PCPF.
|Number of years with knee OA||0.26||0.07||0.01|
|Radiographic severity of knee OA||–||–||0.01||0.77|
|Knee pain||0.58||0.34||< 0.001|
|Knee motion||−0.46||0.21||< 0.001|
|Quadriceps strength||−0.36||0.13||< 0.001|
|Self-reported knee instability||–||–||0.40||< 0.001|
|Variable||r2||r2 Change||df||Partial F||P|
|Severity of radiographic knee OA||0.09||0.000||1,100||0.00||0.99|
|Numeric pain rating||0.40||0.31||1,99||50.62||< 0.001|
|Knee motion||0.49||0.09||1,98||17.80||< 0.001|
|Knee instability rating||0.56||0.05||1,96||10.13||< 0.001|
Our results indicate that a substantial number of individuals with knee OA report episodes of knee instability during activities of daily living and that many of these individuals consider knee instability to be a limiting factor in their ability to perform functional tasks. The regression analysis indicated that reports of giving way, buckling, or slippage of the knee affects physical function beyond that which can be explained by contributions from other impairments that can influence physical function such as knee pain, range of motion, and quadriceps strength. This finding may imply that knee instability is a problem that should be specifically addressed in rehabilitation programs, and may require interventions beyond those that address pain, joint motion, and muscular strength, to maximize the effectiveness of rehabilitation for individuals with knee OA.
Knee instability experienced by individuals with knee OA is most likely a multi-factorial problem that may be the result of factors such as increased capsuloligamentous laxity, structural damage to the knee, and altered lower extremity muscular strength and neuromuscular control. Investigators have reported increased passive knee laxity in individuals with knee OA (19, 20). Passive varus/valgus laxity appears to be more prevalent in knee OA than anterior-posterior laxity (20). The laxity has been described as “pseudo-laxity” because although capsuloligamentous structures remain intact, it is believed that the laxity results from reduced tension in the joint capsule and ligaments, secondary to progressive degenerative changes in the joint and increased joint space narrowing (20). In short, the passive restraints slacken as the disease process progresses. Recently, Sharma et al reported that greater amounts of passive varus/valgus laxity were associated with greater amounts of bony attrition and joint space narrowing of the knee, providing some support for the notion of pseudo-laxity (20).
Although passive knee laxity may be associated with the progression of degenerative changes in the joint, the degree to which passive knee laxity is associated with reports of knee instability and limitations in physical function has not been clearly determined for individuals with knee OA. Studies including patients with ligament deficient knees have reported little to no relationship between passive knee laxity and physical function (21–25). Sharma et al reported that as passive varus/valgus laxity increased, the relationship between lower extremity muscle strength and physical function decreased in individuals with knee OA (26). Sharma et al also reported a small, but significant relationship between passive knee laxity and the WOMAC physical function subscale (r 0.20; r2 0.04); however, there was no relationship between passive knee laxity and the chair-stand test, which is a performance-based measure of physical function (r −0.15) (26). Although our data indicated that reports of knee instability correlate with limitations in physical function, we did not measure knee laxity and are unable to determine at this time to what degree the laxity measures account for reports of knee instability and limitations in physical function. Further study is needed to clarify the relationship between passive knee laxity and physical function.
It is important to consider that passive knee laxity and functional knee instability are not synonymous terms. Passive laxity is a clinical sign that indicates either lack of tension in capsuloligamentous structures of a joint, or the degree of “joint looseness” on passive motion testing of the joint (27). Functional knee instability is a symptom that refers to the sensation of buckling, slippage, or giving way of the knee during functional activities (27). Passive laxity can contribute to functional knee instability and deficits in physical function, but there are other factors, such as pain, muscle weakness, and alterations in joint proprioception and muscle activity patterns (28–34) that can also contribute to joint instability experienced by individuals with knee OA.
Patients with knee OA who experience episodes of knee instability may be a subgroup of patients who require interventions that specifically target knee instability to maximize the effectiveness of rehabilitation. Sharma et al suggested that because increased knee laxity reduces the relationship between muscle strength and function, strengthening alone may not be enough to overcome functional deficits when a patient demonstrates increased knee laxity (26). Treatment interventions, which may include orthotics or interventions that promote neuromuscular control may be needed to supplement exercise therapy programs for individuals with knee OA who experience episodes of instability (26).
Recently, we have developed a training program that consists of agility and perturbation training techniques for individuals with knee OA, which are used in conjunction with range of motion, flexibility, and strengthening exercises. The program was modified from a similar training program that was found to be effective in returning individuals with anterior cruciate ligament deficient knees to high-level activities without experiencing episodes of knee instability (35). A detailed description of this modified program is provided in a recently published case report (36). The idea of including agility and perturbation training techniques is to expose the individual to potentially destabilizing loads in a controlled manner during training so that their neuromuscular system may learn to adapt to conditions that would induce knee instability during activities of daily living.
It has been demonstrated that with practice of a given movement task, muscle activity patterns shift from generalized co-contraction of agonist/antagonist pairs, to well timed, selected bursts of activity between agonist/antagonist pairs, as the learner achieves a higher level of skilled performance (37, 38). The key to the transition from less coordinated to more agile, coordinated muscular responses is the repeated exposure of the individual to the desired movement experience (37–39). The implication for rehabilitation in individuals with knee OA is that the individual must have exposure to movement experiences that challenge knee stability during training in a controlled manner, so that when the need arises in daily activities, the neuromuscular system has been prepared to react rapidly and efficiently to maintain knee stability.
We believe our observations indicate that further attention needs to be directed at reports of knee instability in individuals with knee OA. Further study is needed to examine the degree to which factors such as passive knee laxity, muscle strength, lower extremity neuromuscular control, and progression of disease interact to cause the knee instability experienced by individuals with knee OA. Clinical trials are also needed to determine if the addition of agility and perturbation training programs, such as the one we have described, or orthotic interventions designed to promote knee stability, can improve the effectiveness of rehabilitation for individuals with knee OA whose functional activity levels are limited by episodes of knee instability.
We would like to thank the University of Pittsburgh Medical Center Arthritis Network Registry for their assistance with subject recruitment for this study.