Association of physical activity with loss of knee joint space width over two years: a compositional data analysis in the Osteoarthritis Initiative.

OBJECTIVE: There is continued debate as to how engaging in physical activity (PA), including moderate-to-vigorous PA (MVPA), light PA (LPA), and sedentary time (SED), affects one's risk for knee osteoarthritis (OA). Traditional regression methods do not account for the codependence of these categories of PA, whereby when one category increases, the others must decrease. Thus, we used compositional data analysis (CoDA) to examine time spent in each category of PA, or PA composition, and its association with loss of knee joint space width (JSW), a common indicator of knee OA progression. METHODS: We performed a secondary analysis of data from a subset of participants in the Osteoarthritis Initiative. These participants had minute-by-minute activity data collected over 7 days at baseline; we then categorized each minute as MVPA, LPA, or SED. Our exposure, PA composition, represented min/day spent in each category. Our outcome, medial JSW loss, was the difference in medial tibiofemoral JSW from baseline to 2 years later. We employed CoDA, using an isometric log-ratio transformation, to examine the association of PA composition with medial JSW loss over 2 years, adjusting for potential confounders. RESULTS: We included 969 participants (age: 64.5 years, 56% female, body mass index [BMI]: 28.8 kg/m2). Mean PA composition was: MVPA 9.1 min/day, LPA 278 min/day, SED 690 min/day. Per adjusted regression models, higher MVPA was not associated with greater medial JSW loss (ß = -0.0005, P = 0.97), nor was LPA (ß = 0.06, P = 0.27) or SED (ß = -0.06, P = 0.21). CONCLUSION: Using CoDA, PA composition was not associated with medial JSW loss over 2 years.

Range-of-motion affects cartilage fluid load support: functional implications for prolonged inactivity.

OBJECTIVE: Joint movements sustain cartilage fluid load support (FLS) through a combination of contact migration and periodic bath exposure. Although there have been suggestions that small involuntary movements may disrupt load-induced exudation during prolonged inactivity, theoretical studies have shown otherwise. This work used well-controlled explant measurements to experimentally test an existing hypothesis that the range-of-motion must exceed the contact length to sustain non-zero FLS. METHOD: Smooth glass spheres (1.2-3.2 mm radius) were slid at 1.5 mm/s (Péclet number >100) against bovine osteochondral explants under varying normal loads (0.05-0.1 N) and migration lengths (0.05-7 mm) using a custom instrument. In situ deformation measurements were used to quantify FLS. RESULTS: Non-zero FLS was maintained at migration lengths as small as 0.05 mm or <10% the typical contact diameter. FLS peaked when track lengths exceeded 10 times the contact diameter. For migration lengths below this threshold, FLS decreased with increased contact stress. CONCLUSIONS: Migration lengths far smaller than the contact diameter can sustain non-zero FLS, which, from a clinical perspective, indicates that fidgeting and drifting can mitigate exudation and loss of FLS during prolonged sitting and standing. Nonetheless, FLS decreased monotonically with decreased migration length when migration lengths were less than 10 times the contact diameter. The results demonstrate: (1) potential biomechanical benefits from small movement (e.g., drifting and fidgeting); (2) the quantitative limits of those benefits; (3) and how loads, movement patterns, and mobility likely impact long term FLS.

The modes and competing rates of cartilage fluid loss and recovery.

Cartilage loses, recovers, and maintains its thickness, hydration, and biomechanical functions based on competing rates of fluid loss and recovery under varying joint-use conditions. While the mechanics and implications of load-induced fluid loss have been studied extensively, those of fluid recovery have not. This study isolates, quantifies, and compares rates of cartilage recovery from three known modes: (1) passive swelling - fluid recovery within a static unloaded contact area; (2) free swelling - unrestricted fluid recovery by an exposed surface; (3) tribological rehydration - fluid recovery within a loaded contact area during sliding. Following static loading of adult bovine articular cartilage to between 100 and 500 µm of compression, passive swelling, free swelling, and tribological rehydration exhibited average rates of 0.11 ± 0.04, 0.71 ± 0.15, and 0.63 ± 0.22 µm/s, respectively, over the first 100 s of recovery; for comparison, the mean exudation rate just prior to sliding was 0.06 ± 0.04 µm/s. For this range of compressions, we detected no significant difference between free swelling and tribological rehydration rates. However, free swelling and tribological rehydration rates, those associated with joint articulation, were ∼7-fold faster than passive swelling rates. While previous studies show how joint articulation prevents fluid loss indefinitely, this study shows that joint articulation reverses fluid loss following static loading at >10-fold the preceding exudation rate. These competitive recovery rates suggest that joint space and function may be best maintained throughout an otherwise sedentary day using brief but regular physical activity. STATEMENT OF SIGNIFICANCE: Cartilage loses, recovers, and maintains its thickness, hydration, and biomechanical functions based on competing rates of fluid loss and recovery under varying joint-use conditions. While load-induced fluid loss is extremely well studied, this is the first to define the competing modes of fluid recovery and to quantify their rates. The results show that the fluid recovery modes associated with joint articulation are 10-fold faster than exudation during static loading and passive swelling during static unloading. The results suggest that joint space and function are best maintained throughout an otherwise sedentary day using brief but regular physical activities.