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Inferior rectus muscle

The inferior rectus muscle is one of the six extraocular muscles responsible for eye movement. It is located in the orbit, originating from the common tendinous ring (annulus of Zinn) at the orbital apex and inserting onto the inferior surface of the eyeball. This muscle plays a key role in downward gaze and other complex eye movements, contributing to ocular alignment and coordinated binocular vision.

Synonyms

  • Inferior rectus

  • Musculus rectus inferior (Latin)

  • IR muscle (abbreviation in clinical practice)

Function

  • Primary action: Depression of the eyeball (moves the eye downward)

  • Secondary actions: Extorsion (rotates the top of the eye away from the nose) and adduction (moves the eye toward the midline)

  • Innervation: Oculomotor nerve (cranial nerve III), inferior division

  • Blood supply: Branches of the ophthalmic artery

MRI Appearance

  • T1-weighted imaging:

    • The inferior rectus muscle appears as a well-defined, iso- to mildly hypointense structure relative to surrounding orbital fat.

    • Homogeneous muscle signal intensity under normal conditions.

  • T2-weighted imaging:

    • The muscle is iso- to mildly hypointense compared to orbital fat.

    • Inflammatory or pathological changes (e.g., myositis) may cause increased T2 signal (hyperintensity) due to edema.

  • STIR (Short Tau Inversion Recovery):

    • Normal muscle appears slightly hyperintense (mildly bright) compared to T2 and T1 due to fat suppression.

    • Pathological processes (e.g., edema, inflammation) show marked hyperintensity on STIR images, helping to detect subtle changes not seen on conventional T1 or T2 sequences.

CT Appearance

  • On CT, the inferior rectus muscle is visualized as a soft tissue density band inferior to the globe.

  • Well-defined borders, uniform size and density under normal conditions.

MRI images

Inferior rectus muscle mri image