Hemarthrosis promotes TNF-α production. (A) TNF-α was measured by ELISA in serum isolated from F8^−/− mice (5.5 ± 0.2 pg/mL; n = 7) 2 days after needle-puncture injury or in intra-articular hemorrhagic material isolated from the right knee of the same F8^−/− mice (83 ± 34 pg/mL; n = 6). As control, TNF-α was also measured in heparin-treated blood (0.6 ± 0.6 pg/mL; n = 6) and in clotted blood (1.2 ± 1.2 pg/mL; n = 6) from F8^−/− mice. (B) Measurement of TNF-α mRNA expression by qPCR following needle-puncture injury of F8^−/− mice showed significantly increased expression at 3, 7, 14, and 30 days after the injury (13-, 5-, 6.4-, and 3.6-fold increase, respectively, compared with control; n = 3 independent experiments for each group and time point). For each experiment and time point, 2 or 3 joint tissues were pooled. (C) Joint exudates from patients following ACL injury showed a correlation between the hemoglobin concentration (an indicator of lysed blood in the joint fluid) and the levels of TNF-α. *P < .05 vs control.