Free fatty acids are responsible for the hidden blood loss in total hip and knee arthroplasty.

Hidden blood loss (HBL), commonly seen post total knee or hip arthroplasty, causes postoperative anemia even after reinfusion or blood transfusion based on the visible blood loss volume. The mechanism of HBL remains unclear although more than one theory had tried to explain it. Free fatty acids, metabolites of fatty emboli that are generated during TKA, THA and other surgery manipulating the medullary canal of femur, had been demonstrated to stimulate the neutrophils in producing reactive oxygen species such (ROS) as hydrogen peroxide and chlorous peroxide. Erythrocytes injury was also shown in parasitic infection, chronic renal disease and paroxysmal nocturnal hemoglobinuria in a mechanism of oxidation of membrane polyunsaturated fatty acids and cytosolic hemoglobin by ROS. Based on these results we hypothesize that free fatty acids generated from fatty emboli in blood circulation are responsible for the hidden blood loss through peroxidating injury of membrane molecules of RBC and hemoglobin. Antioxidants administered intra- or post-operatively are predicted to play a protective role in erythrocytes oxidation and potentially reduce the volume of hidden blood loss after arthroplasty.