Topics

Topic

design image
Common bony crus

The common bony crus (crus commune) is the shared osseous channel formed by the posterior limb of the superior semicircular canal and the upper limb of the posterior semicircular canal. It is part of the bony labyrinth of the inner ear and serves as a unifying passage before these limbs open into the vestibule.

This structure stabilizes the architecture of the semicircular canals and plays a crucial role in transmitting endolymph movement for angular acceleration sensing. It forms a key anatomical landmark in temporal bone imaging, vestibular surgery, and evaluation of dehiscence or congenital anomalies.

Synonyms

  • Crus commune

  • Common crus of semicircular canals

  • Bony common limb

Location and Structure

  • Located in the posterior superior part of the bony labyrinth.

  • Formed by the fusion of:

    • Posterior limb of the superior (anterior) semicircular canal

    • Superior limb of the posterior semicircular canal

  • Opens directly into the vestibule, just posterior to the utricle.

  • Bony walls are thin, compact, and smoothly contoured, surrounding the endolymphatic duct within the membranous labyrinth.

Relations

  • Superiorly: Tegmen mastoideum (thin superior temporal bone plate)

  • Inferiorly: Posterior semicircular canal loop

  • Anteriorly: Superior semicircular canal arch

  • Medially: Vestibule of the bony labyrinth

  • Laterally: Lateral semicircular canal (separate, not part of the crus)

  • Posteriorly: Proximity to posterior cranial fossa dura

Function

  • Provides a shared structural pathway for endolymph flow entering the vestibule.

  • Maintains precise orientation of semicircular canals for acceleration detection.

  • Serves as a fixed geometric landmark for vestibular system symmetry.

  • Plays a role in maintaining fidelity of angular movement sensing in orthogonal planes.

Clinical Significance

  • Dehiscence: Rare defects may cause vestibular symptoms or third-window phenomena.

  • Congenital malformations: Dysplastic semicircular canals often involve abnormal development of the common crus.

  • Labyrinthitis and otic capsule infection: May extend to the crus region.

  • Surgical relevance: Key in superior canal dehiscence repair, posterior fossa approaches, and cochleovestibular interventions.

  • Imaging importance: Assessed in trauma, congenital anomalies, chronic otitis, and vestibular disease.

MRI Appearance

T1-weighted (Pre-Contrast)

  • Bony walls: Low signal intensity (black), representing dense otic capsule bone.

  • Endolymph/fluid lumen: Low-to-intermediate signal (fluid is darker on T1).

  • Adjacent structures:

    • Vestibule: fluid-filled, low signal

    • Surrounding temporal bone: uniformly hypointense

T1 Post-Contrast (Fat-Sat)

  • Normal: No enhancement of the common bony crus (bone and endolymph do not enhance).

T2-weighted / High-Resolution 3D T2 (SPACE / FIESTA / CISS)

  • Normal appearance:

    • Endolymph/perilymph fluid: bright hyperintense signal

    • Bony walls: low-signal black margin

    • Common crus: appears as a smooth, tubular bright channel joining two semicircular canals to

CT Appearance

Non-Contrast CT (Temporal Bone High-Resolution)

  • Best modality for visualizing the common bony crus.

  • Bony walls: High-attenuation dense cortical bone forming a smooth, narrow canal.

  • Canal lumen: Low-attenuation channel corresponding to fluid space.

MRI images

Common bony crus mri axial image 1