Background Infrapatellar fat pad (IPFP), an intracapsular but extrasynovial structure, is situated in the knee under the patella, between the patellar tendon, femoral condyle and tibial plateau, and is structurally similar to subcutaneous adipose tissue. The exact function of IPFP in knee OA is still unknown.
Objectives To determine the cross-sectional and longitudinal associations between abnormal infrapatellar fat pad quality and knee osteoarthritic changes in older adults.
Methods A total of 971 subjects (mean 62.4 years, 50.1% female) selected randomly from local community were studied. Radiographic knee osteophyte and joint space narrowing (JSN) were assessed using the OARSI atlas. T1- or T2-weighted fat suppressed magnetic resonance imaging (MRI) was utilized to assess infrapatellar fat pad quality (signal intensity change and soft tissue thickening), cartilage volume, cartilage defects, and bone marrow lesions (BMLs). Knee pain was assessed by self-administered Western Ontario McMaster Osteoarthritis Index (WOMAC) questionnaire.
Results In cross-sectional analyses, both signal intensity change and soft tissue thickening were significantly and positively associated with knee osteophytes, cartilage defects, bone marrow lesions, and knee pain and stiffness, while negatively associated with patellar cartilage volume after adjustment for potential confounders. Soft tissue thickening was also significantly and positively associated with knee joint space narrowing and dysfunction. Longitudinally, signal intensity change was significantly and positively associated with increases in cartilage defects [OR: 1.46, 95% CI: 1.13-1.90 (medial tibiofemoral); OR: 1.45, 95% CI: 1.12-1.87 (lateral tibiofemoral)], bone marrow lesions [OR: 1.44, 95% CI: 1.05-1.99 (medial tibiofemoral); OR: 1.77, 95% CI: 1.31-2.40 (lateral tibiofemoral)], and keen pain when going up/down stairs (OR: 1.27, 95%CI: 1.01-1.59), whereas negatively associated with per annum change in lateral tibial cartilage volume (β: -15.4 mm3, 95% CI: -29.2, -1.7). Consistently, soft tissue thickening was significantly and positively associated with increases in cartilage defects [OR: 1.56, 95% CI: 1.18-2.06 (medial tibiofemoral); OR: 1.50, 95% CI: 1.14-1.98 (lateral tibiofemoral)], bone marrow lesions [OR: 1.82, 95% CI: 1.29-2.58 (medial tibiofemoral); OR: 1.65, 95% CI: 1.19-2.27 (lateral tibiofemoral)], keen pain when walking on flat surface (OR: 1.57, 95% CI: 1.14-2.16), going up/down stairs (OR: 1.56, 95% CI: 1.19-2.04), standing (OR: 1.48, 95% CI: 1.08-2.03), and knee dysfunction (OR: 1.27, 95% CI: 1.00-1.61), but negatively associated with per annum change in lateral tibial cartilage volume (β: -16.8 mm3, 95% CI: -31.8, -1.8).
Conclusions Signal intensity change and soft tissue thickening within infrapatellar fat pad were associated with clinical and structural abnormalities of knee joint cross-sectionally and over 2.6 years in older adults. These suggest that abnormal infrapatellar fat pad quality may have a detrimental role in the development or progression of knee osteoarthritis.
Acknowledgements We especially thank the participants who made this study possible, and we gratefully acknowledge the role of the staff and volunteers in collecting the data, particularly research nurses Boon C and Boon P. Warren R assessed MR images, and Dr Srikanth V and Dr Cooley H assessed radiographs.This study was supported by the National Health and Medical Research Council of Australia; Arthritis Foundation of Australia; Tasmanian Community Fund; Masonic Centenary Medical Research Foundation; Royal Hobart Hospital Research Foundation; and University of Tasmania Institutional Research Grants Scheme.
G Jones is supported by a National Health and Medical Research Council Practitioner Fellowship. C Ding is supported by an Australian Research Council Future Fellowship.
Disclosure of Interest None declared
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