We report the case of a 28-year-old man presenting with a medial orbital wall trapdoor fracture with a missing medial rectus muscle. We believe this to be the first case report of an adult medial orbital wall trapdoor fracture. Trapdoor fractures most commonly occur in the pediatric population, and those involving the medial orbital wall generally occur in areas with less developed ethmoid air cells. Since the present case followed neither pattern, a different injury mechanism was considered. The ethmoid air cells in this case were well developed, which may have played an important role in the pathogenesis of this adult medial orbital wall trapdoor fracture. Based on our findings, we propose a possible mechanism for a medial orbital wall trapdoor fracture in an adult. The cellular frames enable the medial bone to shift just minimally, regardless of the high orbital pressure during a blow. The excess volume of the orbital content escapes into the cells through narrow cracks; therefore, after a blow, it cannot move back completely into the orbit. Consequently, it pushes the shifted bone towards the orbit, becoming trapped in a manner similar to that of a check-valve mechanism.