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Inferior oblique muscle

The inferior oblique muscle is one of the six extraocular muscles responsible for controlling eye movements. Unlike the other extraocular muscles that originate from the annulus of Zinn (apex of the orbit), the inferior oblique originates from the orbital surface of the maxilla, specifically lateral to the lacrimal fossa, and inserts on the posterolateral aspect of the eyeball. This unique origin and insertion give the inferior oblique its distinctive role in ocular motility, especially in upward and outward (extorsion) movements of the eye.

Synonyms

  • Musculus obliquus inferior

  • Inferior oblique extraocular muscle

  • IO muscle

Function

  • Primary action: Extorsion (external rotation) of the eyeball

  • Secondary actions: Elevation and abduction of the eyeball

  • Acts antagonistically to the superior oblique muscle

  • Essential in coordinated eye movements, especially in upward gaze and when the eye is adducted

MRI Appearance

  • T1-weighted imaging:

    • The muscle appears isointense to other extraocular muscles and adjacent soft tissues

    • Well-defined, fusiform structure located inferior and lateral to the globe

  • T2-weighted imaging:

    • The muscle remains isointense to slightly hypointense compared to the orbital fat

    • Pathologies (e.g., inflammation, myositis) may cause increased T2 signal

  • STIR (Short Tau Inversion Recovery):

    • Normal muscle is isointense to slightly hypointense

    • Increased signal indicates edema, inflammation, or infiltration

    • Helpful for detecting subtle pathologies affecting the muscle

CT Appearance

  • On CT, the inferior oblique muscle appears as a well-defined, soft tissue-density structure

  • Located inferior and lateral to the globe, running obliquely from the maxilla to the lateral, posterior aspect of the eyeball

MRI images

Inferior oblique muscle mri image