High levels of fat and (n-6) fatty acids in cancellous bone in osteoarthritis

BACKGROUND: Osteoarthritis (OA) is strongly linked with obesity and patients with osteoporosis (OP) have a low body mass index. Anecdotal evidence, clinical and laboratory, suggests that OA bone contains more fat. However, conversion of osteoblasts to adipocytes is reported in OP and this would suggest that the more porous OP cancellous bone would have a high fat content. OBJECTIVES: To test the hypothesis that OA bone contains more fat than OP bone. METHODS: Cores of cancellous bone were obtained from femoral heads of patients undergoing surgery for either OA or OP. Lipids were extracted using chloroform-methanol, weighed and expressed as a fraction of core mass and volume. A fatty acid analysis was performed using gas chromatography. RESULTS: OA bone contained twice as much fat per unit volume of tissue as OP. Levels of n-6 fatty acids were elevated in OA, especially arachidonic acid (C20:4 n-6) which was almost double that found in OP. CONCLUSIONS: These data support the hypothesis that lipids may play a significant role in the pathogenesis of OA and may provide part of the key to understanding why OA and OP lie at opposite ends of the spectrum of bone masses.