Incarceration of the inferior oblique muscle branch of the oculomotor nerve in two cases of orbital floor trapdoor fracture

Incarceration of the inferior oblique muscle (IO) branch of the oculomotor nerve may occur in cases of orbital floor trapdoor fracture. Two orbital floor trapdoor fracture cases, with lesions located just outside of the inferior rectus muscle but without its incarceration, were examined pre- and postoperatively for visual acuity, intraocular details, the nine diagnostic ocular positions of gaze, binocular single vision field with the Hess chart, and by computed tomography (CT). One case was also examined by magnetic resonance imaging (MRI; T1-weighted images). A forced duction test was conducted intraoperatively. Each case presented good visual acuity and neither globe showed any injury. Motility disturbance of the IO was shown in each case by binocular single vision field testing and the Hess chart. The possibility of the incarceration of the IO branch of the oculomotor nerve, which runs from the incarcerated lesion to the superior belly of the IO, in an orbital floor trapdoor fracture was shown on CT and MRI. Intraoperative forced duction testing revealed a restriction due to the incarceration of the connective tissue septa. As inferred from the CT and MRI analyses conducted in this study, IO palsy may be one of the causes of ocular motility disturbance of the IO in an orbital floor trapdoor fracture, in addition to the ocular motility disturbance due to the connective tissue septa.