Modeling impairment: using the disablement process as a framework to evaluate determinants of hip and knee flexion

Elders often present to health care providers with multiple inter-related conditions that determine an individual's ability to function. The disablement process provides a generalized sociomedical framework for investigating the complex pathways from chronic disease to disability. At each stage of the main pathway, associations may exist among primary physical factors and modifying variables that ultimately have downstream effects on the progression toward disability. The purpose of the present analysis is to examine the inter-relationships between a cohesive set of variables primarily at the level of impairment that may affect hip and knee flexion range of motion (ROM). The San Antonio Longitudinal Study of Aging enrolled 833 community dwelling Mexican (MA) and European American (EA) elders aged 64-78 years between 1992 and 1996. Of these, 647 had complete data from both a home-based and performance-based battery of assessments for these analyses. Concerning impairments, hip ROM was measured using an inclinometer, and knee ROM using a goniometer. Pain location and intensity were assessed using the McGill Pain Questionnaire. Peripheral vascular disease was assessed using doppler brachial and ankle systolic blood pressures. Ankle and knee reflexes, and vibratory sensation were assessed by a standardized neurological examination. As to diseases, diabetes was assessed using a combination of blood glucose levels and self-report, and arthritis by self-report. Concerning modifying variables, height and weight were directly measured and used to calculate BMI. Activity level was assessed with the Minnesota Leisure Time Questionnaire. Analgesic use was assessed by direct observation of medications taken within the past two weeks. We used structural equation modeling to test associations between the variables that were specified a priori. These analyses demonstrate the central role of BMI as a determinant of hip and knee flexion ROM. For an increase in level of BMI, the coefficients [SEM] for changes in levels of hip and knee ROM were -0.38 [0.05] and -0.26 [0.05], respectively. A higher BMI resulted in lower hip and knee ROM. BMI was also directly associated with prevalent diabetes (0.10 [0.05]) and arthritis (0.17 [0.05]). However, after adjustment for BMI, diabetes and arthritis did not have direct independent associations with either hip or knee ROM. BMI was also indirectly associated with knee, but not hip, ROM through paths including lower-leg pain, pain intensity, and neurosensory impairments. Diabetes had an indirect association with hip, but not knee ROM, through a path including peripheral vascular disease. In conclusion, BMI is a primary direct determinant of hip and knee ROM. The paths by which diabetes and arthritis lead to physical disability may be mediated, in part, at the level of impairment by BMI's association with joint range of motion. Interventions designed to decrease the impact of diabetes and arthritis on disability should track changes in BMI and joint ROM to measure the paths that account for the intervention's success. The observed associations suggest that interventions targeted to decrease BMI itself may lead to improved function in part through improved joint ROM.