Linear and Curvilinear Relationship between Knee Range of Motion and Physical Functioning in People with Knee Osteoarthritis: A Cross-Sectional Study

BACKGROUND: Knee range of motion (KROM) is associated with the ability to perform daily activities in people with knee OA. However, this association is weak, possibly through the use of linear analyses. Curvilinear associations appear much more relevant, as these allow the determination of relevant clinical thresholds in KROM. The goal of this study is to assess the curvilinear association between KROM and daily activities (self-reported and observed) in people with knee osteoarthritis (OA). METHODS: Demographic, functional and KROM (flexion and extension) data were collected from a convenience sample of people with knee OA awaiting total knee arthroplasty. Self-reported functioning was measured by use of the Knee Osteoarthritis Outcomes Scale and observed functioning with the timed up and go and six-minute walk test. The presence of curvilinear relationships between KROM and measures of functioning were tested by generalized additive modeling, piecewise regression modeling and receiver operated curves. RESULTS: Data from 110 participants (mean age ± standard deviation: 65 ± 9 and female: 54%) with knee OA were evaluated. Statistical modeling did not reveal linear nor curvilinear associations between KROM and self-reported or observed measures of functioning; except for statistical significant associations between reduced knee flexion and major difficulties standing (p<=0.01). However, further modeling did not provide convincing evidence for relevant clinical associations and thresholds. CONCLUSIONS: No clinically relevant relationship between KROM and self-reported or observed measures of physical functioning could be established, indicating that the limitations in range of motion in the affected knee OA alone do not contribute to poorer functional performance.