BACKGROUND:The pathogenesis of knee intraarticular adhesion is yet unknown. Excessive proliferation of fibroblasts is considered to cause knee intraarticular adhesion. OBJECTIVE:To study the preventive effects of methotrexate on knee intraarticular adhesion through fibroblast apoptosis induced by endoplasmic reticulum stress. METHODS:The viability of the cultured fibroblasts treated with methotrexate(10-5-10-9mol/L)or PBSwas determined after 24 hours. Fibroblast apoptosis was detected by Hoechst33342 staining. Endoplasmic reticulum stress-and apoptosis-related proteins, including cleaved-PARP, CHOP, Bax and Bcl-2, were determined by western blotassay. Eighteen healthy male New Zealand white rabbits were used to establish the knee intraarticular adhesion models, and equaly randomized into three groups, and received topical application of 2 or 1 g/L methotrexate, or normal saline (control). The preventive effects of methotrexate on knee intraarticular adhesion and CHOP expression in scar tissue were observed. RESULTS AND CONCLUSION:Methotrexate inhibited the proliferation and viability of fibroblasts in a dose-dependent manner. The number of apoptotic fibroblasts was significantly increased compared with control group. Protein expression of cleaved-PARP, CHOP, and bax was increased, while protein expression of bcl-2 was decreased with time. The animal experiment showed that preventive effects of 2 g/L methotrexate on knee intraarticular adhesion were superior to 1 g/L methotrexate treatment. CHOP expression in the scar tissue in the methotrexate groups was higher than the control group and that was higher in high-dose methotrexate group. Our results suggest that methotrexate prevents knee intraarticular adhesionviaendoplasmic reticulum stress-induced fibroblast apoptosis.