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- SUBJECTS AND METHODS
Physical disability is an important public health outcome for older adults. Roughly 40% of persons over the age of 65 years report limitations in their ability to perform activities of daily living. This high prevalence has been attributed, in large part, to the onset of various age-associated chronic diseases (1). Osteoarthritis (OA) is the most common rheumatic disease and the most common chronic disease reported by community-dwelling older adults. OA affects an estimated 20 million older adults in the United States (2, 3). The knee is the second most affected joint in OA and is the most common cause of chronic disability among older persons (4, 5). A review by Hurley (6) suggests that pain and quadriceps weakness are more important determinants of disability in knee OA than is radiographic evidence of knee OA. Given the clinical significance of this chronic disease, the objectives of the current study were to describe how chronic knee pain and radiographic knee OA affect the progression of disability as older adults age, and to identify the role that knee strength might play in this relationship.
Data from the first National Health and Nutrition Examination Survey indicate that 43% of persons with radiographic knee OA report pain, a statistic that is similar to prevalence rates from the Framingham cohort (7, 8). Interestingly, asymptomatic persons with radiographic evidence of knee OA report little or no disability (7, 9), whereas individuals with symptomatic knee OA report limitations in instrumental activities of daily living (e.g., walking, carrying and lifting objects, etc.) and various avocations (10, 11). It is also well established that quadriceps strength is compromised early in the development of knee OA (6, 12), and that loss of quadriceps strength is implicated in the progression of disability with the disease (13). Indirect evidence even suggests that deficits in sensorimotor function of the quadriceps muscle may be important to the pathogenesis of knee OA (6).
Other evidence has linked the prevalence or progression of knee OA to body weight, sex, race, and sociodemographic risk factors (4, 14, 15). Risk factors for disability among older adults include sociodemographic factors such as increased age, lower levels of education, being single, being nonwhite (16, 17), and having multiple comorbidities (18). Finally, although radiographic evidence of knee OA has been linked to disability, the real clinical malady with this disease is pain (19).
Despite the relevance of knee OA and physical disability to the public health of older adults, we are unaware of any prospective studies that describe the rate of decline in physical function as older adults with radiographic knee OA age. Moreover, there is limited information concerning factors that may modify the progression of disability with this disease. Using data from 30 months of followup in the Observational Arthritis Study in Seniors (OASIS), this paper will examine the rate of decline in both functional limitations (transfer and ambulatory measures) and self-reported disability among older adults with radiographic knee OA.
Decline in abilities over time that may be related to the aging of the participant can be thought of as the longitudinal effect of aging. The analyses reported in this paper focus on how the longitudinal effect of aging on functioning may be modified by several a priori selected baseline characteristics. First, we investigate how changes in longitudinal measures of transfer and ambulatory-based functional limitations and self-reported disability are related to radiographic knee OA, knee strength, and knee pain intensity at baseline. Second, we determine whether baseline pain and knee strength possibly mediate the relationship between radiographic knee OA and progression of functional limitations and self-reported disability.
- Top of page
- SUBJECTS AND METHODS
One objective of this investigation was to characterize the 30-month progression of both self-reported and performance-related functioning among older adults reporting knee pain with or without radiographic knee OA. Based on our evaluations of interactions in a multivariate model, there was no evidence that the rate of decline in self-reported disability was any different for individuals who had knee pain only as compared with those who had knee pain and radiographic evidence of knee OA. Such a finding supports the position that knee pain is the real clinical malady with this disease (19).
With respect to decline in functioning, several points deserve emphasis. First, on average, participants in this community sample experienced statistically significant increases in self-reported disability and time to complete a stair climb or car task over 30 months of followup. Although the presence of radiographic OA did not differentially affect the rate of decline for self-reported disability, in bivariate analyses it was significantly associated with faster progression in the time to complete the stair climb and car tasks. Other studies have found symptomatic knee OA to be an important determinant in the prevalence of physical disability in older adults (11, 31, 32). To our knowledge, this study is the first to illustrate that radiographic knee OA is associated with the rate of decline in both ambulatory and transfer-based functional limitations. As explained below, this association is most probably indirect and related to levels of knee strength and knee pain.
Second, we also found that knee strength and knee pain intensity may serve to mediate the effect of knee OA on decline in performance-based functioning. Based on the work of Baron and Kenny (33), evidence of mediation is obtained when a significant relationship between a predictor and an outcome is diminished when a third factor (that is related to the predictor) is entered into the model. For the stair climb time and car time tasks, the coefficient associated with the effect of knee OA is reduced by at least 50% when knee pain and knee strength are entered into each of these models. Although the relationship between the cross-sectional measurements of radiographic knee OA, knee strength, and knee pain collected at baseline do not represent a temporal, causal relationship, it is possible that increased knee pain and decreased knee strength follow the development of knee OA. However, other evidence suggests that decreased functioning of the quadriceps muscle may be related to the development of knee OA (6). The relationship between knee OA, knee pain, and knee strength results in confounding between these 2 factors and knee OA. Additionally, knee strength and knee pain intensity are likely interrelated, because pain has been shown to inhibit muscle contraction (34, 35). These models underscore the potential that may exist for combatting the physical disability that occurs in older adults with knee OA through strength training. Interventions targeted to improve the strength of the knee extensors may also be of benefit for older adults with knee pain (36).
Third, across the 30-month study period, a higher percentage of the study population experienced decline in stair climb performance (∼71%) and car time performance (∼85%) than in self-reported difficulty with activities of daily living (ADLs) (∼53%). One explanation for this pattern in the data is that ADLs do not depend exclusively on lower extremity function, nor do they typically require the demand inherent in a stair climb or car time task. To address this concern, in unreported analyses we refit all models after removing non–mobility-related tasks consisting of items related to managing money, using the telephone, eating, and dressing from the self-report disability measure. Conclusions from these analyses were consistent with those from analyses based on use of all items. Finally, the variability in rate of progression for both performance and self-reported ADL disability underscores the importance of considering potential modifiers of aging as it affects the progression of disability with this disease.
One limitation of our study is the large amount of missing performance outcomes at the 30-month visit (approximately 35%). For this reason, we used maximum likelihood repeated-measures analyses, permitting intermittent missing data, rather than analyzing change scores for individuals with complete data. These techniques help protect against biases due to missing data when the likelihood of an outcome being missing is dependent on previously observed data (29, 30). An alternative approach to handling the missing data would have been to use an imputation technique. However, simply imputing a very large time for those who did not complete the task would most certainly result in an overestimate of the 30-month change in either performance task, whereas imputing the last observed value carried forward would have resulted in a vast underestimate of this change. Our analysis technique probably results in an underestimate of the change that is intermediate between what would have resulted from imputation of a very large value and use of the last observed value. This limitation underscores the need to select performance tests that are related to disability, are sensitive to change in function, and are relatively easy for more disabled adults to complete but do not have “floor” effects.
The lack of radiographic assessment of the patellofemoral joint is another limitation of this study. In a study of community-dwelling subjects who were older than age 55, a little more than half of whom reported knee pain, isolated symptomatic patellofemoral joint OA was noted in 8% of the women and 2% of the men (37). In that study, somewhat higher scores for disability on the Stanford Health Assessment Questionnaire were found in the subjects with patellofemoral joint OA compared with those with medial compartment OA. Therefore, lack of assessment of the patellofemoral joint in the present study may have resulted in an underestimate of radiographic OA and the association between radiographic disease and functional loss.
Levels of pain and strength could be used to screen for older adults with knee pain who are at higher risk for experiencing decline in function over a 30-month period. The next step in our investigation will be to explore whether characteristics identified as associated with those persons who are predicted to experience the most decline may be precursors to changes in other variables that may mediate the observed change in functioning. For instance, higher levels of baseline knee pain may result in reduced levels of physical activity that lead to loss in muscular strength and subsequent decline in function. Through identification of possible mediating factors, interventions can be planned to help slow the decline in functioning among older adults.