Abstract
Our objectives were to evaluate whether low knee confidence at baseline is associated with poor baseline-to-3-year physical function outcome in the Osteoarthritis Initiative (OAI).
Knee confidence was assessed using an item from the KOOS instrument. Physical function was assessed using self-report (WOMAC function, SF-12 physical component scale) and performance-based (20 m walk, chair stand test) measures. Poor function outcome was defined as moving into a worse function group or remaining in the 2 worst function groups between baseline and 3 years. Logistic regression was used to evaluate the relationship between baseline knee confidence and poor baseline-to-3-year function outcome, adjusting for potential confounders.
The sample included 3975 men and women with or at higher risk to develop knee OA. 37-53% had poor baseline-to-3-year function outcome. For both self-report measures, increasingly worse knee confidence was associated with a greater risk of poor function outcome and trend tests supported a graded response [e.g., for WOMAC, adjusted OR (95% CI) for worsening confidence categories: 1.26 (1.07, 1.49), 1.43 (1.16, 1.77), 2.05 (1.49, 2.82), p for trend <.0001]. Similar associations between confidence and performance-based function outcome were observed but statistical significance did not persist in adjusted analyses. Factors independently associated with poor function outcome for all 4 outcome measures were depressive symptoms, comorbidity, BMI, and joint space narrowing.
Worse knee confidence at baseline was independently associated with greater risk of poor function outcome by self-report measures, with evidence of a graded response; the relationship was not significant for the performance measures in the fully adjusted models.
INTRODUCTION
Osteoarthritis (OA) at the knee is a highly prevalent condition and a leading cause of chronic disability (1). Given the expected growth and life expectancy of the older segment of the U.S. population, the societal impact of disability from knee OA is likely to increase.
A reduced ability to meet personal, social, or occupational demands is often first manifested as a limitation in physical function. Identification of factors associated with decline in physical function will aid the development of strategies to prevent disability onset and progression. In longitudinal studies of persons with knee OA, factors shown to be associated with greater function decline include greater age, female gender, greater body mass index (BMI), knee pain, comorbid medical conditions, depressive symptoms, varus-valgus laxity, malalignment, and proprioceptive inaccuracy; greater physical activity, aerobic exercise, strength, self-efficacy, and social support each have been associated with a reduced risk of decline (2-11).
Confidence in the knees is a variable assessed within the KOOS (Knee Injury and Osteoarthritis Score) (12) instrument using a question concerning how much the individual is troubled by lack of confidence in his/her knee(s). Given the central role of the knee in all weight-bearing activity, an individual’s confidence in the knees may be a proximal factor influencing activity decisions (what and how much) and self-efficacy, factors thought critical to physical functioning. The relationship between confidence in the knees and physical function has not previously been examined. The Osteoarthritis Initiative (OAI) is a large ongoing cohort study including persons with or at higher risk to develop symptomatic, radiographic knee OA, providing an optimal setting to evaluate the role of knee confidence as it relates to change in physical functioning over time.
We tested the hypothesis that low knee confidence at baseline is associated with poor baseline-to-3-year physical function outcome, using both self-report and performance-based function measures, in persons with or at higher risk to develop knee OA.
METHODS
Sample
The OAI is a prospective, observational cohort study of incident and progressive knee OA in men and women, ages 45-79 years, all with or at increased risk to develop symptomatic, radiographic knee OA, who were enrolled at one of four sites: Baltimore, Maryland; Columbus, Ohio; Pittsburgh, Pennsylvania; and Pawtucket, Rhode Island (see http://www.oai.ucsf.edu/datarelease/About.asp). All racial/ethnic groups were eligible to enroll, and the recruitment goal was 23% of the cohort from racial/ethnic minority groups. To be eligible for the progression subcohort of the OAI, persons were required to have symptomatic, radiographic knee OA, defined as the presence of both of the following in at least one native knee at baseline: pain, aching, or stiffness in or around the knee on most days for at least one month during the past 12 months; and a definite tibiofemoral osteophyte [osteophyte grade ≥ 1, using the OARSI atlas (13)]. Persons were eligible for the incidence subcohort of the OAI if they did not have symptomatic, radiographic knee OA in either knee at baseline, but had characteristics that placed them at increased risk for developing it during the study. Age-specific criteria for determining increased risk were identified from within the following set of established risk factors: knee symptoms in a native knee in the past 12 months; overweight, defined using gender and age-specific cutpoints for weight; knee injury causing difficulty walking for at least a week; history of any knee surgery; family history of a total knee replacement for OA in a biological parent or sibling; Heberden’s nodes; repetitive knee bending at work or outside work; age 70-79 years. (See http://www.oai.ucsf.edu/datarelease/About.asp and Appendix B at that site for greater detail regarding the rationale and approach taken to derive the criteria.)
Exclusion criteria, applied to the entire OAI cohort, were: rheumatoid arthritis or inflammatory arthritis; severe joint space narrowing in both knees on the baseline knee radiograph, or unilateral total knee replacement and severe joint space narrowing in the other knee; bilateral total knee replacement or plans to have bilateral knee replacement in the next 3 years; inability to undergo a 3.0T MRI exam of the knee because of contraindications (including pacemaker, artificial valve, aneurysm clip or shunt, stent, implanted device, ocular metallic fragment) or inability to fit in the scanner or in the knee coil (including men over 285 lbs and women over 250 lbs); positive pregnancy test; inability to provide a blood sample for any reason; use of ambulatory aides other than a single straight cane for more than 50% of the time in ambulation; comorbid conditions that might interfere with the ability to participate in a 4-year study; current participation in a double-blind randomized trial.
Assessment of Knee Confidence
Knee confidence was assessed using the KOOS question: “How much are you troubled with lack of confidence in your knees?” Possible responses include not at all, mildly, moderately, severely, and extremely. The KOOS is a valid, reliable, and responsive self-administered instrument, developed with an overall goal of evaluating short-term and long-term symptoms and function in persons with knee injury and OA (12). The knee confidence question lies within the quality of life subscale. In our previous pilot testing of this question, participants had difficulty distinguishing “severely” from “extremely”; therefore, we evaluated these responses as a single category. To assess reliability, we interviewed 26 individuals with knee OA (belonging to the MAK, Mechanical Factors in Arthritis of the Knee, cohort at Northwestern University) on two occasions, one week apart, asking all KOOS quality of life subscale questions (including the knee confidence question). For inter-session reliability, we analyzed responses on the two occasions using weighted kappa coefficients with quadratic weights (14). The coefficient specifically for the knee confidence item was 0.84, suggesting excellent agreement per Landis and Koch (15).
Assessment of Baseline-to-3-Year Physical Function Outcome
Physical function was assessed at baseline and at 3 years using: 1) the WOMAC physical function scale; 2) the physical component scale of the SF-12; 3) the rate of 20 meter walk performance; and 4) the rate of chair-stand performance. To characterize the baseline to 3-year function experience of each participant, quintile grids were used, with poor outcome defined as remaining within the same low functioning group (the two worst groups) or moving into a worse function group.
The WOMAC is a disease-specific self-report instrument with 17 questions comprising the physical function scale. It is extensively validated and widely recommended and used in studies of individuals with knee OA (16,17). A higher score indicates worse function. Participants were categorized by WOMAC function score quintile derived from the OAI cohort at baseline, ranging from worst to best function, as follows: first quintile (>20.19); second quintile (>10.00 and ≤20.19); third quintile (>3.40 and ≤10.00); fourth quintile (>0 and ≤3.40); and fifth quintile (0). The WOMAC outcome grid is shown in Table 1, with shaded squares representing a poor baseline-to-3-year WOMAC function outcome.
TABLE 1. Definition of Poor WOMAC Function Outcome.
The table illustrates how poor outcome was defined in terms of the WOMAC function measure. Quintile groups were defined by the cut-off values of the baseline WOMAC function score quintiles. The baseline-to-3-year outcome was “poor” when a participant moved into a worse function group or remained within the same low functioning group (the two worst function groups).
| WOMAC Quintile Group at 3 year Follow-Up | |||||
|---|---|---|---|---|---|
| WOMAC Quintile (Q) at Baseline | Q1 (worst) | Q2 | Q3 | Q4 | Q5 (best) |
| Q1 (worst) (>20.19) | |||||
| Q2 (>10.00, ≤20.19) | |||||
| Q3 (>3.40, ≤10.00) | |||||
| Q4 (>0, ≤ 3.40) | |||||
| Q5 (best) (0) | |||||
The 12-item Short Form Health Survey (SF-12) is a generic self-report health-related quality of life instrument that uses 12 items from the Medical Outcomes Study 36-Item Short Form (SF-36). The SF-12 covers 8 domains, with 2 questions comprising the physical component scale. A higher score indicates better function. Criterion validity and reliability have been demonstrated for each of the individual scales (18-20). Participants were categorized by baseline SF-12 physical component score quintile, ranging from worst to best function, as follows: first quintile (≤41.20); second quintile (>41.20 and ≤48.84); third quintile (>48.84 and ≤53.20); fourth quintile (>53.20 and ≤56.43); and fifth quintile (>56.43).
Chair stand test performance, i.e. time required for five repetitions of rising from a chair and sitting down (21), was evaluated as a rate (number of stands per minute calculated from the time required to complete 5 stands). Participants were categorized by baseline chair stand rate quintile, ranging from worst to best function, as follows: first quintile (≤21.60); second quintile (>21.60 and ≤26.40); third quintile (>26.40 and ≤30.60); fourth quintile (>30.60 and ≤36.60); and fifth quintile (>36.60). The 20-meter walk was evaluated as a rate (meters per minute). Participants were categorized by baseline walk rate quintile, ranging from worst to best function, as follows: first quintile (≤68.65); second quintile (>68.65 and ≤76.09); third quintile (>76.09 and ≤82.48); fourth quintile (>82.48 and ≤89.55); and fifth quintile (>89.55).
Assessment of Covariates
Depression was assessed using the Center for Epidemiologic Studies Depression Scale (22). Medical comorbidity was assessed using a questionnaire version of the Charlson Index (23). Falls in the past year were scored according to how many times a participant had fallen in the past 12 months. Physical activity was assessed using the Physical Activity Scale for the Elderly (PASE) (24). Alcohol consumption was defined by response to: “How many drinks did you have in a typical week within the last 12 months?” Heavy alcohol intake was defined as eight or more drinks per week. Presence of hip, ankle, and foot pain was defined as pain, aching, or stiffness on most days of the month during the last 12 months on the right or left side. Knee injury was defined by the report of yes to “ever injured so badly, that it was difficult to walk for at least one week”. Knee surgery was identified by a reply of yes to “ever had any kind of surgery to either knee”. Pain was assessed using the WOMAC pain scale, adapted by the OAI to score pain separately for each knee; data from the worse knee was used in analysis. To assess OA disease severity within each tibiofemoral compartment, joint space was graded in the medial and lateral compartments separately using an adaptation of the OARSI atlas approach (13) in which 0 = none (OARSI grade 0), 1 = narrowed (OARSI grade 1 or 2), and 2 = severely narrowed (OARSI grade 3). Bilateral isometric knee extensor strength was measured using the Good Strength isometric strength chair at a knee angle of 60° from full extension. (Metitur, Jyvaskyla, Finland) (25,26). Details of this protocol may be found at http://www.oai.ucsf.edu/datarelease/OperationsManuals.asp.
Statistical Analyses
Our analyses utilized the OAI public data release (V0.2.2 and V5.2.1). All analyses were at the level of the person. Baseline characteristics are summarized using percentages for categorical variables and means ± standard deviations (SDs) for continuous variables, overall and stratified by the four baseline confidence level categories. Descriptive statistics for outcomes are presented as percentages of persons with poor baseline-to-3-year function outcome stratified by baseline knee confidence level, for each of the physical function measures. As described above, poor baseline-to-3-year function outcome was defined as moving into a worse function group or remaining within the two lowest functioning groups.
The relationship between baseline knee confidence level and baseline-to-3-year poor function outcome was examined using multiple logistic regression, primarily in the cohort as a whole and secondarily in 2 subgroups, persons with knee OA (defined by the presence of K/L radiographic grade ≥ 2 in at least one knee) and persons without knee OA. Unadjusted and adjusted odds ratios (ORs) were calculated from a sequence of forward stepwise (based on prespecified blocks of covariates) models, that entered: first, knee confidence (unadjusted OR); second, also included demographic factors [age (continuous), gender, and race]; and third, also included, health-related factors (as continuous variables, depression, comorbidity, BMI, physical activity, knee pain severity, and extensor strength, and, as yes/no variables, falls, heavy alcohol intake, hip pain, ankle pain, foot pain, knee injury, knee surgery, radiographic joint space narrowing). The associated 95% confidence intervals (CIs) that exclude 1 indicate statistically significant associations with the outcome, based on the pre-determined nominal 5% significance level for testing. A graded relationship across the four baseline confidence level categories was evaluated by a trend test. Analyses were performed using SAS software version 9.2 (SAS Institute Inc., Cary, NC).
RESULTS
Among the 4796 participants in the full OAI cohort at baseline, 526 persons did not return for or did not participate in any of the function assessments at the 36 month visit. Of the remaining 4270 participants, 289 were excluded due to missing covariate data. In addition, 6 persons were excluded due to missing values in all of the four function outcomes. Some of the remaining 3975 participants were missing baseline or follow-up function measure data, resulting in the following analysis samples for each of the four function outcomes: 3935 participants for WOMAC function (40 missing); 3767 for SF12 physical component (208 missing); 3597 for chair stand rate (376 missing); and 3605 for 20-meter walk rate (370 missing).
Overall, 3975 participants were included in at least one of the baseline-to-3-year function outcome analyses; their baseline characteristics and functional status are shown in Table 2. Compared to these 3975 participants, persons not contributing to any function analysis (821 of 4796, 17.1%) did not differ significantly in gender, comorbidity, falls, BMI, physical activity, heavy alcohol intake, knee injury or surgery, but were slightly younger (59.6 ± 9.2 years), were more frequently African-American (31.3%), had greater average WOMAC knee pain (4.4 ± 4.3), more depression (14.3%), ankle pain (13.8%), and foot pain (14.4%), and lower average extensor strength (297.3 ± 129.3), and less hip pain (52.0%) and joint space narrowing (51.2% with no narrowing). In terms of function, persons not included had slightly worse scores on average (WOMAC 13.5 ± 14.3, SF12 physical component score 46.5 ± 10.8, chair stand rate 27.4 ± 11.9, and 20-meter walk rate 78.2 ± 15.2).
TABLE 2.
Baseline Characteristics of Overall Analysis Sample (n = 3975 OAI participants)
| Baseline Characteristics | Overall (n = 3975 ) | Not troubled by lack of confidence (n = 1826, 45.9%) | Mildly troubled (n = 1231, 31.0%) | Moderately troubled (n = 652, 16.4%) | Severely or extremely troubled (n = 266, 6.7%) | |
|---|---|---|---|---|---|---|
| Age, years, mean ± SD | 61.5 ± 9.2 | 62.4 ± 9.1 | 61.1 ± 9.1 | 60.4 ± 9.1 | 59.5 ± 8.9 | |
| Gender, % Women | 58.3% | 59.0% | 57.7% | 57.4% | 58.7% | |
| Race, %: | Caucasian | 82.0% | 85.8% | 82.2% | 76.5% | 68.1% |
| African American | 15.5% | 12.2% | 14.8% | 21.2% | 28.2% | |
| Other | 2.5% | 2.1% | 3.0% | 2.3% | 3.8% | |
| Depressive symptom (CESD) score ≥ 16, % | 9.4% | 5.2% | 9.8% | 14.7% | 22.6% | |
| Comorbidity score, mean ± SD | 0.4 ± 0.8 | 0.4 ± 0.8 | 0.3 ± 0.8 | 0.5 ± 0.9 | 0.6 ± 1.0 | |
| Falls in past year, mean ± SD | 0.5 ± 0.9 | 0.5 ± 0.8 | 0.6 ± 0.9 | 0.6 ± 0.9 | 0.8 ±1.0 | |
| BMI (kg/m2), mean ± SD | 28.6 ± 4.8 | 27.9 ± 4.5 | 28.3 ± 4.7 | 29.9 ± 4.9 | 31.4 ± 5.2 | |
| Physical activity (PASE) score, mean ± SD | 161.6 ± 81.2 | 162.2 ± 79.3 | 160.3 ± 80.0 | 166.5 ± 85.8 | 151.9 ± 86.9 | |
| Heavy alcohol intake, % | 13.8% | 13.5% | 15.7% | 11.8% | 11.7% | |
| Hip pain, % | 56.1% | 51.9% | 56.9% | 62.1% | 66.5% | |
| Ankle pain, % | 10.5% | 7.0% | 10.7% | 15.8% | 20.7% | |
| Foot pain,% | 11.0% | 8.2% | 11.5% | 13.7% | 21.1% | |
| Knee injury, % | 43.6% | 33.6% | 48.9% | 53.5% | 62.8% | |
| Knee surgery, % | 22.8% | 15.6% | 26.2% | 30.1% | 39.1% | |
| WOMAC knee pain score*, mean ± SD | 3.3 ± 3.6 | 1.7 ± 2.4 | 3.4 ± 3.1 | 5.7 ± 3.7 | 8.3 ± 4.2 | |
| Joint space grade**, %: | 0 | 45.1% | 51.1% | 44.5% | 34.7% | 31.6% |
| 1 | 42.7% | 42.3% | 42.0% | 45.3% | 42.9% | |
| 2 | 12.2% | 6.6% | 13.5% | 20.1% | 25.6% | |
| Extensor strength (N)*, mean ± SD | 313.5 ± 122.7 | 324.9 ± 120.1 | 309.2 ± 122.3 | 303.1 ±126.1 | 280.8 ± 125.8 | |
| WOMAC function score*, mean ± SD | 10.2 ± 11.5 | 4.2 ± 6.7 | 10.8 ± 9.6 | 18.8 ± 11.5 | 27.8 ± 13.5 | |
| SF12 physical component score, mean ± SD | 49.3 ± 8.8 | 52.4 ± 6.9 | 49.2 ± 8.0 | 44.8 ± 9.4 | 39.2 ± 9.9 | |
| Chair stand rate (number/minute), mean ± SD | 28.9 ± 10.7 | 30.7 ± 10.3 | 29.1 ± 10.4 | 25.5 ± 10.5 | 24.3 ± 12.2 | |
| 20 meter walk rate (m/minute), mean ± SD | 79.4 ± 12.5 | 81.2 ± 11.9 | 79.8 ± 12.3 | 76.3 ± 12.7 | 73.5 ± 13.7 | |
Abbreviations: SD, standard deviation; CESD, Center for Epidemiologic Studies Depression Scale; BMI, body mass index; PASE, Physical Activity Scale for the Elderly; WOMAC, Western Ontario and McMaster Universities Osteoarthritis Index; SF12 Short form-12 health status questionnaire
Worse of the two knees
Highest (worst) joint space narrowing grade among the medial and lateral tibiofemoral compartments of both knees
Of the 3975 participants, 1826 (45.9%) were not troubled by lack of knee confidence, 1231 (31.0%) were mildly troubled, 652 (16.4%) were moderately troubled and 266 (6.7%) were severely or extremely troubled. Baseline characteristics stratified by baseline knee confidence category are shown in Table 2.
The percentages of persons with poor baseline-to-3-year function outcomes were: 37.4% (1473/3935) for the WOMAC function measure; 52.5% (1978/3767) for the SF-12 physical component measure; 45.4% (1634/3597) for the chair stand rate measure; and 46.7% (1682/3605) for the 20-meter walk rate measure. The percentages of persons within each baseline knee confidence category with poor baseline-to-3-year function outcome are shown for each of the outcome measures in Figure 1. With increasingly worse knee confidence at baseline, the percentages of participants with poor baseline-to-3-year function outcome progressively increased. This pattern was the case for each of the four function measures analyzed (see Figure 1).
FIGURE 1. Proportion of Persons with Poor Baseline-to-3-Year Function Outcome by Baseline Confidence Level.
Each bar represents the % of persons with poor outcome among persons who belong to the given confidence category.
For both self-report measures (WOMAC function and SF-12 physical component), increasingly worse knee confidence (vs. the reference group of persons not troubled by lack of confidence in their knees) was associated with greater risk of poor function outcome (unadjusted p for trend < 0.0001; Table 3). Even being mildly troubled by lack of confidence was significantly associated with a higher risk of poor function outcome by these measures. Although there was some reduction of the OR, a significant relationship between each category of worsening knee confidence and function outcome persisted in the fully adjusted models, with the trend tests supporting a graded response (see Table 3). As shown in Table 3, results were very similar in the subgroup of persons with knee OA. In persons at higher risk but without knee OA, results were significant for SF-12 physical component but not for WOMAC function (see Table 3).
TABLE 3. Association of Baseline Knee Confidence with Poor Baseline-to-3-Year Self-Report Function Outcome.
The table shows the odds ratios (OR) and 95% confidence intervals (CI) for the relationship between category of knee confidence at baseline and baseline-to-three-year function outcome assessed using WOMAC Function Outcome and SF12 Physical Component Outcome. For all models, the reference group was persons “not troubled by lack of confidence”. Results are shown first for the full cohort and next for each subgroup, persons without knee OA and persons with knee OA.
| POOR WOMAC FUNCTION OUTCOME Full cohort (N=3935) | POOR SF12 PHYSICAL COMPONENT OUTCOME Full cohort (N=3767) | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Covariate groups* | Mildly troubled by lack of confidence OR (95% CI) | Moderately troubled OR (95% CI) | Severely troubled OR (95% CI) | p for trend | Mildly troubled by lack of confidence OR (95% CI) | Moderately troubled OR (95% CI) | Severely troubled OR (95% CI) | p for trend | |
| None | 1.53 (1.32, 1.79) | 2.29 (1.91, 2.76) | 4.24 (3.23, 5.56) | <.0001 | 1.56 (1.34, 1.81) | 2.00 (1.66, 2.41) | 2.73 (2.05, 3.64) | <.0001 | |
| Adjusted for age, gender, race | 1.53 (1.31, 1.79) | 2.26 (1.87, 2.72) | 4.11 (3.12, 5.40) | <.0001 | 1.61(1.39, 1.88) | 2.10 (1.73, 2.54) | 2.89 (2.16, 3.87) | <.0001 | |
| Fully adjusted* | 1.26 (1.07, 1.49) | 1.43 (1.16, 1.77) | 2.05 (1.49, 2.82) | <.0001 | 1.43 (1.22, 1.68) | 1.52 (1.22, 1.89) | 1.69 (1.21, 2.35) | <.0001 | |
| Persons without knee OA (N=1729) | Persons without knee OA (N= 1662) | ||||||||
| Fully adjusted* | 1.14 (0.89, 1.47) | 1.23 (0.87, 1.76) | 1.15 (0.64, 2.07) | .2538 | 1.56 (1.23, 1.98) | 1.53 (1.08, 2.17) | 1.18 (0.65, 2.14) | .0072 | |
| Persons with knee OA (N=2206) | Persons with knee OA (N=2105) | ||||||||
| Fully adjusted* | 1.33 (1.07, 1.66) | 1.54 (1.18, 2.02) | 2.59 (1.75, 3.83) | <.0001 | 1.32 (1.06, 1.65) | 1.48 (1.12, 1.96) | 1.83 (1.21, 2.77) | .0006 | |
Fully adjusted model was adjusted for age, gender, race, depressive symptoms, comorbidity, falls, BMI, physical activity, alcohol use, hip pain, ankle pain, foot pain, knee injury, knee surgery, knee pain severity, knee OA disease severity, and extensor strength.
For the chair stand rate outcome, a relationship was detected between being moderately troubled by lack of confidence or being severely troubled, but these relationships were not significant in the fully adjusted models (Table 4). For the 20-meter walk rate outcome, the relationship was detected for each level of worse confidence, but again, these were not significant in the fully adjusted models. Results were similar in the subgroups, but approached significance for chair stand rate in persons with OA and for 20-meter walk rate in persons without knee OA (Table 4).
TABLE 4. Association of Baseline Knee Confidence with Poor Baseline-to-3-Year Performance-Based Function Outcome.
The table shows the odds ratios (OR) and 95% confidence intervals (CI) for the relationship between category of knee confidence at baseline and baseline-to-three-year function outcome assessed using chair stand performance and 20-meter walk rate. For all models, the reference group was persons not troubled by lack of confidence. Results are shown first for the full cohort and next for each subgroup, persons without knee OA and persons with knee OA.
| POOR CHAIR STAND OUTCOME (N=3597) | POOR 20-METER WALK RATE OUTCOME (N=3605) | |||||||
|---|---|---|---|---|---|---|---|---|
| Covariate groups* | Mildly troubled by lack of confidence OR (95% CI) | Moderately troubled OR (95% CI) | Severely troubled OR (95% CI) | p for trend | Mildly troubled by lack of confidence OR (95% CI) | Moderately troubled OR (95% CI) | Severely troubled OR (95% CI) | p for trend |
| None | 1.00 (0.86, 1.17) | 1.56 (1.29, 1.89) | 2.07 (1.57, 2.74) | <.0001 | 1.24 (1.07, 1.45) | 1.33 (1.10, 1.60) | 1.90 (1.44, 2.51) | <.0001 |
| Adjusted for age, gender, race | 1.05 (0.90, 1.23) | 1.65 (1.36, 2.00) | 2.20 (1.65, 2.92) | <.0001 | 1.37 (1.17, 1.61) | 1.46 (1.20, 1.78) | 2.15 (1.61, 2.88) | <.0001 |
| Fully adjusted* | 0.90 (0.76, 1.07) | 1.20 (0.96, 1.49) | 1.33 (0.95, 1.85) | .0921 | 1.23 (1.04, 1.46) | 1.09 (0.87, 1.37) | 1.37 (0.97, 1.92) | 0.0721 |
| Persons without knee OA (N=1581) | Persons without knee OA (N=1581) | |||||||
| Fully adjusted* | 0.87 (0.68, 1.12) | 1.12 (0.77, 1.61) | 1.06 (0.57, 1.98) | .8811 | 1.47 (1.15, 1.90) | 1.12 (0.77, 1.63) | 1.58 (1.04, 1.07) | 0.0607 |
| Persons with knee OA (N=2016) | Persons with knee OA (N=2024) | |||||||
| Fully adjusted* | 0.93 (0.74, 1.17) | 1.24 (0.93, 1.65) | 1.44 (0.90, 2.15) | .0590 | 1.06 (0.84, 1.34) | 1.04 (0.78, 1.39) | 1.25 (0.83, 1.87) | .4045 |
Fully adjusted model was adjusted for age, gender, race, depressive symptoms, comorbidity, falls, BMI, physical activity, alcohol use, hip pain, ankle pain, foot pain, knee injury, knee surgery, knee pain severity, knee OA disease severity, and extensor strength.
Table 5 shows the relationships between poor baseline-to-3-year function outcomes and all variables included in the final multiple logistic regression models. For the WOMAC function measure, being troubled by lack of knee confidence, depression, comorbidity, BMI, ankle pain, knee pain severity, and joint space narrowing were each independently associated with poor outcome. For the SF12 physical component score, being troubled by lack of knee confidence, age, depression, comorbidity, BMI, ankle pain, knee pain severity, and joint space narrowing were each independently associated with poor outcome. For the chair stand rate measure, age, depression, comorbidity, BMI, heavy alcohol intake and joint space narrowing were independently associated with poor outcome. For the 20-meter walk measure age, gender (female), being African American, depression, comorbidity, BMI, and joint space narrowing were each associated with a poor outcome. Extensor strength was associated with a modest reduction in the odds of poor outcome for 3 of 4 outcome measures. Overall, depression score, comorbidity, BMI, and joint space narrowing were associated with a poor function outcome for each of the four function measures studied (see Table 5).
TABLE 5. Factors Associated with Poor Baseline-to-3-year Function Outcome in each Final Multiple Logistic Regression Model.
The table shows the odds ratios (ORs) and 95% confidence intervals (CIs) for each variable, adjusting for all other variables in the table, associated with poor baseline-to-3-year outcome, for each of the four outcome measures. Statistically significant results are shown bold and italicized.
| Baseline Characteristics | WOMAC OR (95% CI) | SF12 OR (95% CI) | Chair Stand OR (95% CI) | 20 Meter Walk Rate OR (95% CI) | |
|---|---|---|---|---|---|
| Knee Confidence (reference: Not troubled): | |||||
| Mildly Troubled | 1.26 (1.07, 1.49) | 1.43 (1.22, 1.68) | 0.90 (0.76, 1.07) | 1.23 (1.04, 1.46) | |
| Moderately Troubled | 1.43 (1.16, 1.77) | 1.52 (1.22, 1.89) | 1.20 (0.96, 1.50) | 1.09 (0.87, 1.37) | |
| Severely or extremely Troubled | 2.05 (1.49, 2.82) | 1.69 (1.21, 2.35) | 1.33 (0.95, 1.85) | 1.37 (0.97, 1.92) | |
| Age (per year) | 0.99 (0.98, 1.00) | 1.02 (1.02, 1.03) | 1.03 (1.02, 1.04) | 1.05 (1.04, 1.06) | |
| Women (vs. men) | 1.00 (0.84, 1.20) | 0.94 (0.79, 1.11) | 0.88 (0.74, 1.05) | 1.24 (1.04, 1.49) | |
| African American (vs. Caucasian) | 0.97 (0.79, 1.18) | 0.94 (0.77, 1.16) | 1.19 (0.97, 1.47) | 1.33 (1.08, 1.64) | |
| Depressive symptoms (CESD) (per unit) | 1.28 (1.01, 1.63) | 1.75 (1.36, 2.25) | 1.36 (1.06, 1.75) | 1.63 (1.26, 2.11) | |
| Comorbidity score (per unit) | 1.11 (1.02, 1.21) | 1.15 (1.05, 1.25) | 1.17 (1.02, 1.22) | 1.22 (1.11,1.33) | |
| Falls in past year (per unit) | 0.98 (0.90, 1.06) | 1.04, (0.96, 1.12) | 0.99 (0.91, 1.07) | 0.97 (0.89, 1.05) | |
| BMI (per unit) | 1.05 (1.03, 1.06) | 1.05 (1.03,1.07) | 1.05 (1.04, 1.07) | 1.05 (1.04, 1.07) | |
| Physical activity (per 10 units) | 1.01 (1.00, 1.02) | 1.00 (0.99, 1.00) | 0.99 (0.98, 1.00) | 1.00 (0.99, 1.01) | |
| Heavy alcohol intake (yes vs. no) | 0.98 (0.80, 1.20) | 1.04 (0.85, 1.26) | 1.23 (1.01, 1.51) | 0.96 (0.78, 1.18) | |
| Hip pain (yes vs. no) | 1.15 (1.00, 1.33) | 1.05 (0.91, 1.21) | 1.10 (0.95, 1.27) | 1.08 (0.93, 1.25 | |
| Ankle pain (yes vs. no) | 1.48 (1.18, 1.86) | 1.32 (1.04, 1.67) | 1.09 (0.86, 1.39) | 1.17 (0.92, 1.49) | |
| Foot pain (yes vs. no) | 0.93 (0.74, 1.16) | 0.98 (0.78, 1.23) | 0.98 (0.77, 1.23) | 1.03 (0.82, 1.31) | |
| Knee injury(yes vs. no) | 1.04 (0.89, 1.20) | 0.90 (0.77, 1.04) | 0.97 (0.83, 1.13) | 1.02 (0.88, 1.20) | |
| Knee surgery (yes vs. no) | 1.11 (0.93, 1.33) | 1.14 (0.96, 1.37) | 1.01 (0.84, 1.22) | 0.93 (0.77,1.12) | |
| WOMAC knee pain score (per unit) | 1.04 (1.02, 1.07) | 1.03 (1.01, 1.06) | 1.02 (1.00, 1.04) | 1.01 (0.98, 1.03) | |
| Joint space grade 1 (vs. 0) | 1.32 (1.14, 1.54) | 1.03 (0.89, 1.19) | 1.30 (1.12, 1.52) | 0.90 (0.77, 1.05) | |
| Joint space grade 2 (vs. 0) | 2.00 (1.59, 2.52) | 1.33 (1.05, 1.69) | 1.41 (1.11, 1.80) | 1.52 (1.19, 1.95) | |
| Extensor strength (per 10 units) | 0.98 (0.98, 0.99) | 0.99 (0.99, 1.00) | 0.97 (0.97, 0.98) | 0.98 (0.98, 0.99) | |
Abbreviations: OR, odds ratio; CI, confidence interval; BMI, body mass index; WOMAC, Western Ontario and McMaster Universities Osteoarthritis Index.
DISCUSSION
In summary, 46% of participants were not troubled by lack of knee confidence, 31% were mildly troubled, 16% moderately troubled, and 7% severely or extremely troubled. Depending upon the outcome measure used, 37% to 53% of persons had a poor 3-year outcome. With increasingly worse knee confidence at baseline, the percentages of persons with poor outcome progressively increased, and this pattern was the case for each outcome measure. For both self-report measures, worse knee confidence was associated with a greater risk of poor function outcome; trend tests supported a graded response in adjusted analyses. Similar associations between confidence and performance-based measures of function were observed but statistical significance did not persist in adjusted analyses. Factors independently associated with poor function outcome for each of the four measures were depressive symptoms, comorbidity, BMI, and joint space narrowing.
To our knowledge, this is the first report on knee confidence and its relation to physical function outcomes in persons either with or at risk for developing knee OA. While confidence in a joint in general or in the knees in particular is a concept that is sometimes addressed by clinicians in practice, we were unable to find literature regarding its evaluation. The OAI provided an excellent opportunity to evaluate this question, given the large size of the OAI cohort, the longitudinal design, and the inclusion of persons at risk for knee OA as well as all stages of radiographic severity. Worse knee confidence at baseline was independently associated with an increased risk of poor function outcome as assessed by both self-report measures in the full cohort and in persons with knee OA. In persons without knee OA, a relationship was demonstrated with the generic instrument (SF-12) but not with the OA disease-specific instrument (WOMAC). Worse knee confidence was not significantly associated with poor performance outcomes assessed using the chair stand test and 20 meter walk rate in the fully adjusted models. Because confidence is self-reported, it is not surprising that knee confidence is more strongly associated with outcome measures that are also self-reported vs. the performance based measures. The lack of relationship in the fully adjusted models for the performance-based outcome measures may reflect that confidence is not important to physical task performance over time, when other important factors are considered. However, it remains possible that confidence is associated with performance of other physical tasks.
It is likely that confidence is closely related to self-efficacy, which has been defined by Bandura as the belief in one’s capacities to mobilize the internal resources and course of action needed to meet given situational demands (27). Rejeski et al. demonstrated that low self-efficacy is associated with poor function outcome in older adults with knee pain, after adjusting for strength and pain severity (5). The current results are also in keeping with our findings for self-efficacy in the MAK (Mechanical Factors in Arthritis of the Knee) Cohort (7). Confidence may lie proximal to self-efficacy on a causal pathway. It is also likely that confidence is related to other factors such as anxiety, pain-related fear, and subjective instability, which may be associated with function in persons with knee OA (28-32). As these other parameters were not assessed in the OAI, we were not able to look at the relationship between confidence and these variables; such an evaluation would be an important focus of a future study to further inform prevention/intervention strategy development.
Depressive symptoms, comorbidity, BMI, and joint space narrowing were each independently associated with every measure of poor outcome we evaluated. These results are in keeping with previous cross-sectional (28,33-36) and longitudinal (7,8) reports describing a relationship between depressive symptoms and worse function in persons with arthritis. Our findings for comorbidity are consistent with previous longitudinal studies (3,11), as are our findings for BMI (7,10,37). Recently White et al found evidence in the MOST study that persons with worsening OA (compared with those with stable radiographic OA) had more than a doubling of risk of incident severe functional limitation (38).
Measurement of function outcomes over time in studies of knee OA is necessary to better understand the impact of the disease. However, the best way to evaluate function outcomes in knee OA has not been established. As we previously described (7), a focus on change ignores those with persistently high or low function, effectively lumping them into the same group, and reducing the ability to detect the effects of factors responsible for an individual’s state of function. In a disease that is slow to evolve, such as knee OA, factors related to persistent low- or high-function states are particularly important. We (7) and others (37) have used the outcome approach of the current manuscript to address this issue. Our findings confirm that factors associated with self-reported function outcome differ from those associated with performance-based function outcome, and that studies should include both. As Jordan et al have noted (39), objective measures may not mirror activities considered important to individuals or the range of activities experienced during daily life. Self-report measures may better capture wider aspects of functioning and better define change in function over time at the individual level (39).
These results have implications for future studies to develop strategies to prevent function decline in persons with knee OA. Knee confidence could be addressed using principles of Bandura’s social cognitive theory (40,41), including: participation in physical therapy with reproduction of modalities learned within sessions (learning by vicarious reinforcement); anticipation of future events and planning for consequences of actions (forethought activity); short-term goal setting to make the connection between current actions and future outcomes (self-regulatory capabilities); continued self-evaluations to appraise goal attainment (self-reflecting capability); and social and intrinsic rewards to serve as meaningful motivators of performance as well as to help individuals persist through difficulties and setbacks (self-reinforcement). Specific strategies could incorporate education concerning risk of poor outcome and benefits associated with its prevention, recognize and address impediments to improving confidence, tailor to the individual’s self-management capabilities, and use telephone counseling and/or linkage to supportive social networks (40,41). Future studies should work to identify the sources of confidence; these parameters could then become the target of additional strategies.
There are limitations to the study that need to be recognized. Given the composition of the OAI sample, these results are potentially generalizable only to individuals with or at higher risk to develop knee OA. As noted above, persons not included had slightly worse scores on average; it is difficult to estimate what impact this may have had on our findings. Self-efficacy, fear, and anxiety were not measured in the OAI, precluding exploration of the relationship between these factors and knee confidence. Our analytic strategy, while useful in addressing possible confounding, does not allow inference regarding potential sequential relationships between variables (such as for confidence and physical activity). Physical activity may be overestimated when assessed using self-report (42). However, PASE has been validated as a reasonable tool to discriminate relative activity in older adults with knee pain and physical disability (43).
In conclusion, with self-report measures, worse knee confidence was associated with a greater risk of poor baseline-to-3-year function outcome in persons with or at higher risk for knee OA; trend tests supported a graded response in adjusted analyses. Similar associations between confidence and performance-based measures of function outcome were observed but statistical significance did not persist in adjusted analyses.
Acknowledgments
Support: The OAI is a public-private partnership comprised of five contracts (N01-AR-2-2258; N01-AR-2-2259; N01-AR-2-2260; N01-AR-2-2261; N01-AR-2-2262) funded by the NIH and conducted by the OAI Study Investigators. Private funding partners include Merck Research Laboratories, Novartis Pharmaceuticals Corporation, GlaxoSmithKline, and Pfizer, Inc. Private sector funding for the OAI is managed by the Foundation for the NIH. This manuscript was prepared using an OAI public use data set (V0.2.2 and V5.2.1) and does not necessarily reflect the opinions or views of the OAI investigators, the NIH, or the private funding partners.
Contributor Information
Dr Carmelita J. Colbert, Department of Medicine, Feinberg School of Medicine, Northwestern University, Chicago, IL.
Ms Jing Song, Institute for Healthcare Studies, Feinberg School of Medicine, Northwestern University, Chicago, IL.
Dr Dorothy Dunlop, Institute for Healthcare Studies, Feinberg School of Medicine, Northwestern University, Chicago, IL.
Dr Joan S. Chmiel, Department of Preventive Medicine, Feinberg School of Medicine, Northwestern University, Chicago, IL.
Dr Karen W. Hayes, Department of Physical Therapy and Human Movement Sciences, Feinberg School of Medicine, Northwestern University, Chicago, IL.
Ms September Cahue, Department of Medicine, Feinberg School of Medicine, Northwestern University, Chicago, IL.
Dr Kirsten C. Moisio, Department of Physical Therapy and Human Movement Sciences, Feinberg School of Medicine, Northwestern University, Chicago, IL.
Dr Alison H. Chang, Department of Physical Therapy and Human Movement Sciences, Feinberg School of Medicine, Northwestern University, Chicago, IL.
Dr Leena Sharma, Department of Medicine, Feinberg School of Medicine, Northwestern University, Chicago, IL.
References
- 1.Guccione AA, Felson DT, Anderson JJ, Anthony JM, Zhang Y, Wilson PW, et al. The effects of specific medical conditions on the functional limitations of elders in the Framingham Study. Am J Public Health. 1994;84:352–7. doi: 10.2105/ajph.84.3.351. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Davis M, Ettinger W, Neuhaus John, Mallon K. Knee osteoarthritis and physical functioning: evidence from the NHANES I Epidemiologic Followup Study. J Rheumatol. 1991;18:591–8. [PubMed] [Google Scholar]
- 3.Ettinger W, Davis M, Neuhaus J. Long-term physical functioning in persons with knee osteoarthritis from NHANES I: effects of comorbid medical conditions. J Clin Epidemiol. 1994;47:809–15. doi: 10.1016/0895-4356(94)90178-3. [DOI] [PubMed] [Google Scholar]
- 4.Sharma L, Song J, Felson D, Cahue S, Shamiyeh E, Dunlop D. The role of alignment in disease progression and functional decline in knee osteoarthritis. JAMA. 2001;286:188–95. doi: 10.1001/jama.286.2.188. [DOI] [PubMed] [Google Scholar]
- 5.Rejeski W, Miller M, Foy C, Messier S, Rapp S. Self-efficacy and the progression of functional limitations and self-reported disability in older adults with knee pain. J Gerontol B Psychol Sci Soc Sci. 2001;56:S261–5. doi: 10.1093/geronb/56.5.s261. [DOI] [PubMed] [Google Scholar]
- 6.Miller ME, Rejeski WJ, Messier SP, Loeser RF. Modifiers of change in physical functioning in older adults with knee pain: the Observational Arthritis Study in Seniors (OASIS) Arthritis Rheum (Arthritis Care and Research) 2001;45:331–9. doi: 10.1002/1529-0131(200108)45:4<331::AID-ART345>3.0.CO;2-6. [DOI] [PubMed] [Google Scholar]
- 7.Sharma L, Cahue S, Song J, Hayes K, Pai Y, Dunlop D. Physical functioning over three years in knee osteoarthritis. Arthritis Rheum. 2003;48:3359–70. doi: 10.1002/art.11420. [DOI] [PubMed] [Google Scholar]
- 8.Dunlop DD, Semanik P, Song J, Manheim L, Shih V, Chang R. Risk factors for functional decline in older adults with arthritis. Arthritis Rheum. 2005;52:1274–82. doi: 10.1002/art.20968. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.van Dijk GM, Dekker J, Veenhof C, van den Ende CHM. Course of functional status and pain in osteoarthritis of the hip or knee: a systematic review of the literature. Arthritis Rheum (Arthritis Care and Research) 2006;55:779–85. doi: 10.1002/art.22244. [DOI] [PubMed] [Google Scholar]
- 10.Dunlop DD, Song J, Semanik PA, Sharma L, Chang RW. Physical activity levels and functional performance in the Osteoarthritis Initiative: a graded relationship. Arthritis Rheum. 2011 Jan;63(1):127–36. doi: 10.1002/art.27760. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.Van Dijk GM, Veenhof C, Spreeuwenberg P, Coene N, Burger BJ, van Schaardenburg D, van den Ende CH, Lankhorst GJ, Dekker J. Prognosis of limitations in activities in osteoarthritis of the hip or knee: A 3-year cohort study. Arch Phys Med Rehabil. 2010;91:58–66. doi: 10.1016/j.apmr.2009.08.147. [DOI] [PubMed] [Google Scholar]
- 12.Roos EM, Roos HP, Lohmander L, Ekdahl C, Beynnon B. Knee Injury and Osteoarthritis Score (KOOS) – development of a self-administered outcome measure. J Orthop Sports Phys Ther. 1998;78:88–96. doi: 10.2519/jospt.1998.28.2.88. [DOI] [PubMed] [Google Scholar]
- 13.Altman RD, Hochberg M, Murphy WA, Jr, Wolfe F, Lequesne M. Atlas of individual radiographic features in osteoarthritis. Osteoarthritis Cartilage. 1995;3(Supplement A):3–70. [PubMed] [Google Scholar]
- 14.Cohen J. Weighted kappa: Nominal scale agreement with provision for scaled disagreement or partial credit. Psychological Bulletin. 1968;70:213–20. doi: 10.1037/h0026256. [DOI] [PubMed] [Google Scholar]
- 15.Landis RJ, Koch GG. The measurement of observer agreement for categorical data. Biometrics. 1977;33:159–74. [PubMed] [Google Scholar]
- 16.Bellamy N, Buchanan WW, Goldsmith CH, Campbell J, Stitt LW. Validation study of WOMAC: a health status instrument for measuring clinically important patient relevant outcomes following total hip and knee arthroplasty in osteoarthritis. J Ortho Rheum. 1988;1:95–108. [PubMed] [Google Scholar]
- 17.Bellamy N, Buchanan WW, Goldsmith CH, Campbell J, Stitt LW. Validation study of WOMAC: a health status instrument for measuring clinically important patient relevant outcomes to anti-rheumatic drug therapy in patients with osteoarthritis of the hip or knee. J Rheumatol. 1988;15:1833–40. [PubMed] [Google Scholar]
- 18.McHorney CA, Ware JE, Raczek AE. The MOS 36-item short-form health survey (SF-36): II. Psychometric and clinical tests of validity in measuring physical and mental health selection. Med Care. 1993;31:247–63. doi: 10.1097/00005650-199303000-00006. [DOI] [PubMed] [Google Scholar]
- 19.McHorney CA, Ware JE, Jr, Lu JF, Sherbourne CD. The MOS 36 item Short-Form Health Survey (SF-36). III. Tests of data quality, scaling assumptions, and reliability across diverse patient groups. Med Care. 1994;32:40–66. doi: 10.1097/00005650-199401000-00004. [DOI] [PubMed] [Google Scholar]
- 20.Gandhi S, Salmon J, Zhao S, Lambert B, Gore P, Conrad K. Psychometric Evaluation of the 12-Item Short-Form Health Survey (SF-12) in osteoarthritis and rheumatoid arthritis clinical trials. Clin Ther. 2001;23:1080–98. doi: 10.1016/s0149-2918(01)80093-x. [DOI] [PubMed] [Google Scholar]
- 21.Guralnik JM, Ferrucci L, Simonsick EM, Salive ME, Wallace RB. Lower-extremity function in persons over the age of 70 years as a predictor of subsequent disability. N Engl J Med. 1995;332:556–61. doi: 10.1056/NEJM199503023320902. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 22.Radloff L. The CES-D Scale: a self-report depression scale for research in the general population. Applied Psychological Measurement. 1977;1:385–401. [Google Scholar]
- 23.Katz JN, Chang LC, Sangha O, Fossel AH, Bates DW. Can comorbidity be measured by a questionnaire rather than medical record review? Med Care. 1996;34:73–84. doi: 10.1097/00005650-199601000-00006. [DOI] [PubMed] [Google Scholar]
- 24.Washburn RA, Smith KW, Jette AM, Janney CA. The Physical Activity Scale for the Elderly (PASE): development and evaluation. J Clin Epidemiol. 1993;46:153–62. doi: 10.1016/0895-4356(93)90053-4. [DOI] [PubMed] [Google Scholar]
- 25.Rantanen T, Era P, Heikkinen E. Maximal isometric strength and mobility among 75-year old men and women. Aging. 1994;23:132–7. doi: 10.1093/ageing/23.2.132. [DOI] [PubMed] [Google Scholar]
- 26.Rantanen T, Era P, Heikkinen E. Physical activity and the changes in maximal isometric strength in men and women from the age of 75 to 80 years. J Am Geriatr Soc. 1997;45:1439–45. doi: 10.1111/j.1532-5415.1997.tb03193.x. [DOI] [PubMed] [Google Scholar]
- 27.Bandura A. Self-efficacy mechanism in physiological activation and health-promoting behavior. In: Madden J, editor. Neurobiology of learning, emotion, and affect. Vol. 229 New York: Raven Press; 1991. [Google Scholar]
- 28.Scopaz KA, Piva SR, Wisniewski S, Fitzgerald K. Relationships of fear, anxiety, and depression with physical function in patients with knee osteoarthritis. Arch Phys Med Rehabil. 2009;90:1866–73. doi: 10.1016/j.apmr.2009.06.012. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 29.Somers TJ, Keefe FJ, Pells JJ, Dixon KE, Waters SJ, Riordan PA, Blumenthal JA, McKee DC, LaCaille L, Tucker JM, Schmitt D, Caldwell DS, Kraus VB, Sims EL, Shelby RA, Rice JR. Pain catastrophizing and pain-related fear in osteoarthritis patients: relationships to pain and disability. J Pain Symptom Manage. 2009;37:863–72. doi: 10.1016/j.jpainsymman.2008.05.009. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 30.Fitzgerald GK, Piva SR, Irrgang JJ. Reports of joint instability in knee osteoarthritis: its prevalence and relationship to physical function. Arthritis Rheum. 2004;51:941–6. doi: 10.1002/art.20825. [DOI] [PubMed] [Google Scholar]
- 31.Felson DT, Niu J, McClennan C, Sack B, Aliabadi P, Hunter DJ, et al. Knee buckling: prevalence, risk factors, and associated limitations in function. Ann Intern Med. 2007;147:534–40. doi: 10.7326/0003-4819-147-8-200710160-00005. [DOI] [PubMed] [Google Scholar]
- 32.Steultjens MPM, Dekker J, Bijlsma JWJ. Avoidance of activity and disability in patients with osteoarthritis of the knee: the mediating role of muscle strength. Arthritis Rheum. 2002;46:1784–8. doi: 10.1002/art.10383. [DOI] [PubMed] [Google Scholar]
- 33.van Baar ME, Dekker J, Lemmens JAM, Oostendorp RAB, Bijlsma JWJ. Pain and disability in patients with osteoarthritis of hip or knee: the relationship with articular, kinesiological, and psychological characteristics. J Rheumatol. 1998;25:125–33. [PubMed] [Google Scholar]
- 34.O’Reilly S, Jones A, Muir KR, Doherty M. Quadriceps weakness in knee osteoarthritis: the effect on pain and disability. Ann Rheum Dis. 1998;57:588–594. doi: 10.1136/ard.57.10.588. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 35.Salaffi F, Cavalieri F, Nolli M, Ferracciolli G. Analysis of disability in knee osteoarthritis. Relationship with age and psychological variables but not with radiographic score. J Rheumatol. 1991;18:1581–1586. [PubMed] [Google Scholar]
- 36.Summers MN, Haley WE, Reveille JD, Alarcon GS. Radiographic assessment and psychologic variables as predictors of pain and functional impairment in osteoarthritis of the knee or hip. Arthritis Rheum. 1988;31:204–209. doi: 10.1002/art.1780310208. [DOI] [PubMed] [Google Scholar]
- 37.Mallen CD, Peat G, Thomas E, Lacey R, Croft P. Predicting poor functional outcome in community-dwelling older adults with knee pain: prognostic value of generic indicators. Ann Rheum Dis. 2007;66:1456–61. doi: 10.1136/ard.2006.067975. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 38.White DK, Zhang Y, Niu J, Keysor JJ, Nevitt MC, Lewis CE, Torner JC, Neogi T. Do worsening knee radiographs mean greater chances of severe functional limitation? Arthritis Care Res (Hoboken) 2010;62:1433–9. doi: 10.1002/acr.20247. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 39.Jordan KP, Wilkie R, Muller S, Myers H, Nicholls E. Measurement of change in function and disability in osteoarthritis: current approaches and future challenges. Curr Opin Rheumatol. 2009;21:525–30. doi: 10.1097/BOR.0b013e32832e45fc. [DOI] [PubMed] [Google Scholar]
- 40.Bandura A. Health promotion by social cognitive means. Health Educ Behav. 2004;31:143–64. doi: 10.1177/1090198104263660. [DOI] [PubMed] [Google Scholar]
- 41.Bandura A. Social cognitive theory: an agentic. Annu Rev Psychol. 2001;52:1–26. doi: 10.1146/annurev.psych.52.1.1. [DOI] [PubMed] [Google Scholar]
- 42.Tucker JM, Welk GJ, Beyler NK. Physical activity in U.S.: adults compliance with the Physical Activity Guidelines for Americans. Am J Prev Med. 2011;40:454–61. doi: 10.1016/j.amepre.2010.12.016. [DOI] [PubMed] [Google Scholar]
- 43.Martin KA, Rejeski WJ, Miller ME, James MK, Ettinger WH, Jr, Messier SP. Validation of the PASE in older adults with knee pain and physical disability. Med Sci Sports Exerc. 1999;31:627–33. doi: 10.1097/00005768-199905000-00001. [DOI] [PubMed] [Google Scholar]