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Abstract

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSOR
  8. REFERENCES

Objective

To evaluate whether low knee confidence at baseline is associated with poor baseline-to-3-year physical function outcome in the Osteoarthritis Initiative.

Methods

Knee confidence was assessed using an item from the Knee Injury and Osteoarthritis Outcome Score instrument. Physical function was assessed using self-report measures (Western Ontario and McMaster Universities Osteoarthritis Index [WOMAC] function score and Short Form 12 physical component scale) and performance-based measures (20-meter walk and chair stand test). 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 level and poor baseline-to-3-year function outcome, adjusting for potential confounders.

Results

The sample included 3,975 men and women with or at high risk of developing osteoarthritis of the knee, of whom 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., the adjusted odds ratios [95% confidence intervals] for the WOMAC function score for worsening confidence categories were 1.26 [1.07–1.49], 1.43 [1.16–1.77], and 2.05 [1.49–2.82], P for trend <0.0001). 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, body mass index, and joint space narrowing.

Conclusion

These findings indicate that worse knee confidence at baseline is independently associated with greater risk of poor function outcome by self-report measures, with evidence of a graded response; the relationship with performance measures is not significant in fully adjusted models.

Osteoarthritis (OA) of 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 US 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 in 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 decline in function include older age, female sex, higher 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 in the Knee Injury and OA Outcome Score (KOOS) instrument (12) via a question regarding how much the individual is troubled by lack of confidence in his or 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 that are thought to be 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 that includes individuals with or at high risk of developing 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 high risk of developing knee OA.

PATIENTS AND METHODS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSOR
  8. REFERENCES

Patient sample.

The OAI is a prospective, observational cohort study of incident and progressive knee OA in men and women ages 45–79 years who have or are at increased risk of developing symptomatic, radiographic knee OA. Participants were enrolled at one of the following 4 sites: Baltimore, Maryland; Columbus, Ohio; Pittsburgh, Pennsylvania; or Pawtucket, Rhode Island (see http://www.oai.ucsf.edu/datarelease/About.asp). All racial/ethnic groups were eligible to enroll, and the goal was to recruit 23% of the cohort from racial/ethnic minority groups.

To be eligible for the progression subcohort of the OAI, participants were required to have symptomatic, radiographic knee OA, defined as the presence of both of the following in at least 1 native knee at baseline: pain, aching, or stiffness in or around the knee on most days for at least 1 month during the past 12 months; and a definite tibiofemoral osteophyte (osteophyte grade of ≥1, according to the OA Research Society International [OARSI] atlas [13]). Participants 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 of 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; being overweight, which was defined according to sex- and age-specific cut points 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 of work; and 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, which were applied to the entire OAI cohort, included 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 magnetic resonance imaging examination of the knee because of contraindications (including a pacemaker, artificial valve, aneurysm clip or shunt, stent, implanted device, or ocular metallic fragment) or inability to fit in the scanner or in the knee coil (including men >285 lbs and women >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 >50% of the time in ambulation; comorbid conditions that might interfere with the ability to participate in a 4-year study; and current participation in a double-blind randomized trial.

Assessment of knee confidence.

Knee confidence was assessed using the question from the KOOS instrument, “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 patients 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 Mechanical Factors in Arthritis of the Knee [MAK] cohort at Northwestern University) on 2 occasions, 1 week apart, asking all KOOS quality of life subscale questions (including the knee confidence question). For intersession reliability, we analyzed responses on the 2 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 the Western Ontario and McMaster Universities OA Index (WOMAC) physical function scale, the physical component scale of the Short Form 12 (SF-12), the 20-meter walk rate, and the chair stand rate. 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 (1 of the 2 worst groups) or moving into a worse function group.

The WOMAC is a disease-specific self-report instrument with 17 questions in the physical function scale. It has been extensively validated and is 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 (scores of >20.19), second quintile (scores of >10.00 and ≤20.19), third quintile (scores of >3.40 and ≤10.00), fourth quintile (scores of >0 and ≤3.40), and fifth quintile (score of 0). The WOMAC outcome grid is shown in Table 1, with X representing a poor baseline-to-3-year WOMAC function outcome.

Table 1. Definition of poor WOMAC function outcome*
WOMAC score quintile at baselineWOMAC score quintile at 3-year followup
Quintile 1 (worst)Quintile 2Quintile 3Quintile 4Quintile 5 (best)
  • *

    Quintile groups were defined by the cutoff values of the baseline Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) function score quintiles. The baseline-to-3-year outcome was “poor” (indicated by X) when a participant moved into a worse function group or remained within the same low-functioning group (the 2 worst function groups).

 Quintile 1 (worst) (> 20.19)X    
 Quintile 2 (>10.00 and ≤20.19)XX   
 Quintile 3 (>3.40 and ≤10.00)XX   
 Quintile 4 (>0 and ≤3.40)XXX  
 Quintile 5 (best) (0)XXXX 

The SF-12 health survey is a generic self-report health-related quality of life instrument that uses 12 items from the Medical Outcomes Study Short Form 36 (SF-36). The SF-12 covers 8 domains, with 2 questions in 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 (scores of ≤41.20), second quintile (scores of >41.20 and ≤48.84), third quintile (scores of >48.84 and ≤53.20), fourth quintile (scores of >53.20 and ≤56.43), and fifth quintile (scores of >56.43).

Chair stand test performance, i.e., the time required to rise from a chair and sit down 5 times (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 stands/minute), second quintile (>21.60 and ≤26.40 stands/minute), third quintile (>26.40 and ≤30.60 stands/minute), fourth quintile (>30.60 and ≤36.60 stands/minute), and fifth quintile (>36.60 stands/minute). 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 meters/minute), second quintile (>68.65 and ≤76.09 meters/minute), third quintile (>76.09 and ≤82.48 meters/minute), fourth quintile (>82.48 and ≤89.55 meters/minute), and fifth quintile (>89.55 meters/minute).

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). The number of times a participant had fallen in the past 12 months was recorded. Physical activity was assessed using the Physical Activity Scale for the Elderly (PASE) (24). Alcohol consumption was determined by the response to the question, “How many drinks did you have in a typical week within the last 12 months?” Heavy alcohol intake was defined as ≥8 drinks per week. The 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. Patients were considered to have had a knee injury if they responded yes to “ever injured so badly, that it was difficult to walk for at least one week.” Patients were considered to have had knee surgery if they responded 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 the 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 (Metitur) at a knee angle of 60° from full extension (25, 26). Details of this protocol may be found at http://www.oai.ucsf.edu/datarelease/OperationsManuals.asp.

Statistical analysis.

Our analyses used the OAI public data release (V0.2.2 and V5.2.1). All analyses were conducted at the level of the individual. Baseline characteristics are summarized using percentages for categorical variables and the mean ± SD for continuous variables for the overall sample and for groups stratified by the 4 baseline knee confidence level categories. Descriptive statistics for outcomes are presented as the percentages of participants 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 in 1 of the 2 lowest functioning groups.

The relationship between baseline knee confidence level and poor baseline-to-3-year function outcome was examined using multiple logistic regression, primarily in the cohort as a whole and secondarily in 2 subgroups: participants with knee OA (defined by a Kellgren/Lawrence radiographic grade (27) of ≥2 in at least 1 knee) and participants without knee OA. Unadjusted and adjusted odds ratios (ORs) were calculated from a sequence of forward stepwise models based on prespecified blocks of covariates. The first model included knee confidence level (unadjusted OR). The second model included knee confidence level and demographic factors (age [continuous], sex, and race). The third model included the covariates of the first and second models and health-related factors (depression, comorbidity, BMI, physical activity, knee pain severity, and extensor strength as continuous variables, and number of falls, heavy alcohol intake, hip pain, ankle pain, foot pain, knee injury, knee surgery, and knee OA disease severity as yes/no variables). The associated 95% confidence intervals (95% CIs) that exclude 1 indicate statistically significant associations with the outcome, based on the predetermined nominal 5% significance level for testing. A graded relationship across the 4 baseline confidence level categories was evaluated by a trend test. Analyses were performed using SAS software version 9.2.

RESULTS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSOR
  8. REFERENCES

Among the 4,796 participants in the full OAI cohort at baseline, 526 did not return for, or did not participate in, any of the function assessments at the 36-month visit. Of the remaining 4,270 participants, 289 were excluded due to missing covariate data. In addition, 6 participants were excluded due to missing values in all of the 4 function outcomes. Some of the remaining 3,975 participants were missing baseline or followup function data, resulting in the following analysis samples for each of the 4 function outcomes: 3,935 participants for WOMAC function score (40 missing); 3,767 for SF-12 physical component score (208 missing); 3,597 for chair stand rate (378 missing); and 3,605 for 20-meter walk rate (370 missing).

Overall, 3,975 participants were included in at least 1 of the baseline-to-3-year function outcome analyses; their baseline characteristics and functional status are shown in Table 2. The 3,975 participants who were included in at least 1 function outcome analysis and the participants who were not included in any function outcome analysis (821 of 4,796 participants [17.1%]) did not differ significantly in sex, comorbidity, falls, BMI, physical activity, heavy alcohol intake, or knee injury or surgery, but the participants who were not included in any function outcome analysis were slightly younger (mean ± SD age 59.6 ± 9.2 years), were more frequently African-American (31.3%), had a higher average WOMAC knee pain score (mean ± SD 4.4 ± 4.3), more frequently had depression (14.3%), ankle pain (13.8%), and foot pain (14.4%), had a lower average extensor strength (mean ± SD 297.3 ± 129.3), and less frequently had hip pain (52.0%) or joint space narrowing (51.2% with no narrowing). In terms of function, participants who were not included in the analyses had slightly worse scores on average (mean ± SD WOMAC score 13.5 ± 14.3, SF-12 physical component score 46.5 ± 10.8, chair stand rate 27.4 ± 11.9 stands/minute, and 20-meter walk rate 78.2 ± 15.2 meters/minute).

Table 2. Baseline characteristics of the participants*
 Overall (n = 3,975)Not troubled by lack of confidence (n = 1,826)Mildly troubled (n = 1,231)Moderately troubled (n = 652)Severely or extremely troubled (n = 266)
  • *

    Except where indicated otherwise, values are the mean ± SD. CES-D = 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; SF-12 = Short Form 12.

  • Worse score of the 2 knees.

  • Highest (worst) joint space narrowing grade among the medial and lateral tibiofemoral compartments of both knees.

% of participants10045.931.016.46.7
Age, years61.5 ± 9.262.4 ± 9.161.1 ± 9.160.4 ± 9.159.5 ± 8.9
Sex, % female58.359.057.757.458.7
Race, %     
 White82.085.882.276.568.1
 African American15.512.214.821.228.2
 Other2.52.13.02.33.8
CES-D score ≥16, %9.45.29.814.722.6
Comorbidity score0.4 ± 0.80.4 ± 0.80.3 ± 0.80.5 ± 0.90.6 ± 1.0
No. of falls in past year0.5 ± 0.90.5 ± 0.80.6 ± 0.90.6 ± 0.90.8 ±1.0
BMI, kg/m228.6 ± 4.827.9 ± 4.528.3 ± 4.729.9 ± 4.931.4 ± 5.2
PASE score161.6 ± 81.2162.2 ± 79.3160.3 ± 80.0166.5 ± 85.8151.9 ± 86.9
Heavy alcohol intake, %13.813.515.711.811.7
Hip pain, %56.151.956.962.166.5
Ankle pain, %10.57.010.715.820.7
Foot pain, %11.08.211.513.721.1
Knee injury, %43.633.648.953.562.8
Knee surgery, %22.815.626.230.139.1
WOMAC knee pain score3.3 ± 3.61.7 ± 2.43.4 ± 3.15.7 ± 3.78.3 ± 4.2
Joint space grade, %     
 045.151.144.534.731.6
 142.742.342.045.342.9
 212.26.613.520.125.6
Extensor strength, N313.5 ± 122.7324.9 ± 120.1309.2 ± 122.3303.1 ± 126.1280.8 ± 125.8
WOMAC function score10.2 ± 11.54.2 ± 6.710.8 ± 9.618.8 ± 11.527.8 ± 13.5
SF-12 physical component score49.3 ± 8.852.4 ± 6.949.2 ± 8.044.8 ± 9.439.2 ± 9.9
Chair stand rate, no./minute28.9 ± 10.730.7 ± 10.329.1 ± 10.425.5 ± 10.524.3 ± 12.2
20-meter walk rate, meters/minute79.4 ± 12.581.2 ± 11.979.8 ± 12.376.3 ± 12.773.5 ± 13.7

Of the 3,975 participants, 1,826 (45.9%) were not troubled by lack of knee confidence, 1,231 (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 participants with poor baseline-to-3-year function outcomes were 37.4% (1,473 of 3,935) for the WOMAC function measure, 52.5% (1,978 of 3,767) for the SF-12 physical component measure, 45.4% (1,634 of 3,597) for the chair stand rate measure, and 46.7% (1,682 of 3,605) for the 20-meter walk rate measure. The percentage of participants within each baseline knee confidence category with poor baseline-to-3-year function outcome is shown for each of the outcome measures in Figure 1. With increasingly worse knee confidence at baseline, the percentage of participants with poor baseline-to-3-year function outcome progressively increased. This pattern was the case for each of the 4 function measures analyzed (Figure 1).

thumbnail image

Figure 1. Proportion of participants with poor baseline-to-3-year function outcome by baseline confidence level. Bars represent the percentage of participants in the indicated confidence category with a poor outcome. WOMAC = Western Ontario and McMaster Universities Osteoarthritis Index; SF-12 = Short Form 12.

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For both of the self-report measures (WOMAC function score and SF-12 physical component score), increasingly worse knee confidence (versus the reference group of persons not troubled by lack of confidence in their knees) was associated with a greater risk of poor function outcome (unadjusted P for trend < 0.0001) (Table 3). Even being mildly troubled by a lack of confidence was significantly associated with a higher risk of poor function outcome by these measures. Although there was some reduction in the ORs, 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 (Table 3). As shown in Table 3, the results were very similar in the subgroup of participants with knee OA. In participants who were at high risk of knee OA but did not have knee OA, the results were significant for the SF-12 physical component but not for the WOMAC function score.

Table 3. Association of baseline knee confidence with poor baseline-to-3-year self-report function outcome*
 Poor WOMAC function outcomePoor SF-12 physical component outcome
Mildly troubled by lack of confidenceModerately troubled by lack of confidenceSeverely troubled by lack of confidenceP for trendMildly troubled by lack of confidenceModerately troubled by lack of confidenceSeverely troubled by lack of confidenceP for trend
  • *

    Values are the odds ratio (95% confidence interval) for the relationship between category of knee confidence at baseline and baseline-to-3-year function outcome assessed using the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) function outcome and the Short Form 12 (SF-12) physical component outcome. For all models, the reference group was participants not troubled by lack of confidence. For the WOMAC function outcome, the full cohort consisted of 3,935 participants (1,729 without knee OA and 2,206 with knee OA). For the SF-12 physical component outcome, the full cohort consisted of 3,767 participants (1,662 without knee OA and 2,105 with knee OA).

  • Adjusted for age, sex, race, depressive symptoms, comorbidity, falls, body mass index, physical activity, alcohol use, hip pain, ankle pain, foot pain, knee injury, knee surgery, knee pain severity, knee OA disease severity, and extensor strength.

Full cohort        
 Unadjusted1.53 (1.32–1.79)2.29 (1.91–2.76)4.24 (3.23–5.56)<0.00011.56 (1.34–1.81)2.00 (1.66–2.41)2.73 (2.05–3.64)<0.0001
 Adjusted for age, sex, and race1.53 (1.31–1.79)2.26 (1.87–2.72)4.11 (3.12–5.40)<0.00011.61 (1.39–1.88)2.10 (1.73–2.54)2.89 (2.16–3.87)<0.0001
 Fully adjusted1.26 (1.07–1.49)1.43 (1.16–1.77)2.05 (1.49–2.82)<0.00011.43 (1.22–1.68)1.52 (1.22–1.89)1.69 (1.21–2.35)<0.0001
Participants without knee OA (fully adjusted)1.14 (0.89–1.47)1.23 (0.87–1.76)1.15 (0.64–2.07)0.25381.56 (1.23–1.98)1.53 (1.08–2.17)1.18 (0.65–2.14)0.0072
Participants with knee OA (fully adjusted)1.33 (1.07–1.66)1.54 (1.18–2.02)2.59 (1.75–3.83)<0.00011.32 (1.06–1.65)1.48 (1.12–1.96)1.83 (1.21–2.77)0.0006

For the chair stand rate measure, a relationship was detected between the chair stand rate and being moderately or severely troubled by lack of confidence, but these relationships were not significant in the fully adjusted models (Table 4). For the 20-meter walk rate, a 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 the chair stand rate in participants with knee OA and for the 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*
 Poor chair stand test outcomePoor 20-meter walk rate outcome
Mildly troubled by lack of confidenceModerately troubled by lack of confidenceSeverely troubled by lack of confidenceP for trendMildly troubled by lack of confidenceModerately troubled by lack of confidenceSeverely troubled by lack of confidenceP for trend
  • *

    Values are the odds ratio (95% confidence interval) for the relationship between category of knee confidence at baseline and baseline-to-3-year function outcome assessed using performance on the chair stand test and 20-meter walk rate. For all models, the reference group was participants not troubled by lack of confidence. For the chair stand test, the full cohort consisted of 3,597 participants (1,581 without knee osteoarthritis [OA] and 2,016 with knee OA). For the 20-meter walk, the full cohort consisted of 3,605 participants (1,581 without knee OA and 2,024 with knee OA).

  • Adjusted for age, sex, race, depressive symptoms, comorbidity, falls, body mass index, physical activity, alcohol use, hip pain, ankle pain, foot pain, knee injury, knee surgery, knee pain severity, knee OA disease severity, and extensor strength.

Full cohort        
 Unadjusted1.00 (0.86–1.17)1.56 (1.29–1.89)2.07 (1.57–2.74)<0.00011.24 (1.07–1.45)1.33 (1.10–1.60)1.90 (1.44–2.51)<0.0001
 Adjusted for age, sex, and race1.05 (0.90–1.23)1.65 (1.36–2.00)2.20 (1.65–2.92)<0.00011.37 (1.17–1.61)1.46 (1.20–1.78)2.15 (1.61–2.88)<0.0001
 Fully adjusted0.90 (0.76–1.07)1.20 (0.96–1.49)1.33 (0.95–1.85)0.09211.23 (1.04–1.46)1.09 (0.87–1.37)1.37 (0.97–1.92)0.0721
Participants without knee OA (fully adjusted)0.87 (0.68–1.12)1.12 (0.77–1.61)1.06 (0.57–1.98)0.88111.47 (1.15–1.90)1.12 (0.77–1.63)1.58 (1.04–1.07)0.0607
Participants with knee OA (fully adjusted)0.93 (0.74–1.17)1.24 (0.93–1.65)1.44 (0.90–2.15)0.05901.06 (0.84–1.34)1.04 (0.78–1.39)1.25 (0.83–1.87)0.4045

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 SF-12 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, female sex, African American race, 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 the 4 outcome measures. Overall, depression score, comorbidity, BMI, and joint space narrowing were associated with a poor function outcome for each of the 4 function measures studied (Table 5).

Table 5. Factors associated with poor baseline-to-3-year function outcome in each final multiple logistic regression model*
Baseline characteristicWOMACSF-12Chair stand test20-meter walk rate
  • *

    Values are the odds ratio (95% confidence interval) for each variable, adjusting for all other variables in the table, associated with poor baseline-to-3-year outcome for each of the 4 outcome measures. WOMAC = Western Ontario and McMaster Universities Osteoarthritis Index; SF-12 = Short Form 12; BMI = body mass index.

  • P < 0.05.

  • Measured with the Center for Epidemiologic Studies Depression Scale.

Knee confidence (reference: not troubled)    
 Mildly troubled1.26 (1.07–1.49)1.43 (1.22–1.68)0.90 (0.76–1.07)1.23 (1.04–1.46)
 Moderately troubled1.43 (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 troubled2.05 (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)
Female (vs. male)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. white)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 (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)

DISCUSSION

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSOR
  8. REFERENCES

In summary, 46% of the participants were not troubled by lack of knee confidence, 31% were mildly troubled, 16% were moderately troubled, and 7% were severely or extremely troubled. Depending on the outcome measure used, 37–53% of the participants had a poor 3-year outcome. With increasingly worse knee confidence at baseline, the percentages of participants 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 4 measures were depressive symptoms, comorbidity, BMI, and joint space narrowing.

To our knowledge, this is the first study of knee confidence and its relationship to physical function outcomes in persons who either have or are at risk of 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 address this question, given the large size of the OAI cohort, the longitudinal design, and the inclusion of persons at risk of knee OA as well as persons with knee OA at 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 of the self-report measures in the full cohort and in the subgroup of participants with knee OA. In participants without knee OA, a relationship was demonstrated between knee confidence and outcomes using the generic instrument (SF-12) but not using the OA-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 than with performance-based measures. The lack of a relationship between knee confidence and performance-based outcomes in the fully adjusted models 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 the 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 internal resources and take the course of action needed to meet given situational demands (28). 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 results of the present study are also consistent with our previous findings regarding self-efficacy in the MAK 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 (29–33). Since these other parameters were not assessed in the OAI, we were not able to examine the relationship between confidence and these variables; such an evaluation would be an important focus of a future study to further inform the development of strategies for prevention and intervention.

Depressive symptoms, comorbidity, BMI, and joint space narrowing were each independently associated with every measure of poor outcome we evaluated. These results are consistent with the results of previous cross-sectional (29, 34–37) and longitudinal (7, 8) studies demonstrating a relationship between depressive symptoms and worse function in persons with arthritis. Our findings for comorbidity are consistent with those of previous longitudinal studies (3, 11), as are our findings for BMI (7, 10, 38). Recently, White et al found evidence in the Multicenter OA 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 (39).

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 noted (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 (38) have used the outcome approach of the present study 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 (40), 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 (40).

These results have implications for future studies undertaken to develop strategies for the prevention of a decline in function in persons with knee OA. Knee confidence could be addressed using the principles of Bandura's social cognitive theory (41, 42), including the following: 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 the risk of poor outcome and benefits associated with its prevention, recognize and address impediments to improving confidence, be tailored to the individual's self-management capabilities, and use telephone counseling and/or linkage to supportive social networks (41, 42). 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 present study that should be noted. Given the composition of the OAI sample, these results are potentially generalizable only to individuals with or at high risk of developing 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, did 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 (43). However, PASE has been validated as a reasonable tool with which to discriminate relative activity in older adults with knee pain and physical disability (44).

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 high risk of 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.

AUTHOR CONTRIBUTIONS

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSOR
  8. REFERENCES

All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be published. Dr. Sharma had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study conception and design. Colbert, Song, Dunlop, Sharma.

Acquisition of data. Colbert, Song, Dunlop, Chmiel, Hayes, Cahue, Moisio, Chang, Sharma.

Analysis and interpretation of data. Colbert, Song, Dunlop, Chmiel, Hayes, Cahue, Moisio, Chang, Sharma.

ROLE OF THE STUDY SPONSOR

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSOR
  8. REFERENCES

None of the funding partners played a role in the design or conduct of the study, collection, management, analysis, or interpretation of the data, or preparation, review, or approval of the manuscript.

REFERENCES

  1. Top of page
  2. Abstract
  3. PATIENTS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. AUTHOR CONTRIBUTIONS
  7. ROLE OF THE STUDY SPONSOR
  8. REFERENCES