The aim of this study was to investigate the quantitative association between active/passive maximum mouth opening (AMMO/PMMO) and the severity of simulated temporomandibular joint (TMJ) bony ankylosis. Twenty-eight male sheep were divided randomly and equally into surgical and control groups. Surgical group animals underwent bilateral TMJ osteotomy during which left lateral pterygoid muscle function was blocked. Control animals did not undergo surgery. Body weight, AMMO/PMMO, and TMJ morphological features were evaluated preoperatively and at 12 and 24 weeks post-surgery. In the surgical group, only the right TMJ complexes with maintained lateral pterygoid muscle function developed TMJ bony ankylosis. The AMMO/PMMO and end-feel distance in the surgical group were significantly lower than those in the control group (P 0.001, both) at 12 and 24 weeks post-surgery. Moreover, AMMO (r = -0.940 and -0.952, P 0.001, both) and PMMO (r = -0.944 and -0.953, P 0.001, both) were negatively correlated with the area (mm