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Topic

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Co (Coccyx)

The coccyx is the terminal segment of the vertebral column, forming the inferior-most part of the spine. It is a small triangular bone composed of fused coccygeal vertebrae and represents the vestigial tail of humans. The coccyx provides attachment for ligaments, muscles, and pelvic floor structures and plays an important role in pelvic stability and weight transmission in the seated position.

It is a key anatomical region in spine imaging, trauma assessment, pelvic floor evaluation, and musculoskeletal radiology.

Synonyms

  • Tailbone

  • Coccygeal spine

Location

  • Situated at the inferior end of the vertebral column

  • Inferior to the sacrum

  • Forms the posterior boundary of the pelvic outlet

  • Midline structure at the caudal aspect of the spine

  • Articulates superiorly with the sacral apex at the sacrococcygeal joint

Anatomical components

  • Coccygeal vertebrae:

    • Usually 3 to 5 small fused segments (most commonly 4)

  • Coccygeal cornua:

    • Small upward projections articulating with sacral cornua

  • Intercoccygeal joints:

    • Fibrocartilaginous or synovial joints (may be fused with age)

  • Ligamentous attachments:

    • Anterior sacrococcygeal ligament

    • Posterior sacrococcygeal ligament

    • Lateral sacrococcygeal ligaments

Relations

Anteriorly:

  • Rectum

  • Anococcygeal raphe

  • Pelvic floor muscles

Posteriorly:

  • Skin and subcutaneous tissues of the gluteal cleft

Superiorly:

  • Sacral apex and sacrococcygeal joint

Inferiorly:

  • Tip of the coccyx

  • Anococcygeal ligament insertion

Laterally:

  • Levator ani muscles

  • Coccygeus muscles

Developmental anatomy

  • Derived from caudal vertebral segments

  • Initially separate coccygeal vertebrae at birth

  • Progressive fusion occurs from adolescence to adulthood

  • Degree of fusion and curvature varies between individuals

X-ray appearance

Lateral and AP sacrococcygeal views:

  • Coccyx: Curved triangular bony structure inferior to the sacrum

  • Segments: Individual coccygeal vertebrae may be visible or fused

  • Sacrococcygeal joint: Seen as a thin joint space or fused interface

  • Alignment: Anterior or posterior angulation appreciable on lateral view

CT appearance

Pre-contrast CT:

  • Cortical bone: Hyperdense outer margins of coccygeal segments

  • Medullary cavity: Lower-density central marrow space

  • Segmental anatomy: Individual coccygeal segments and fusion planes clearly defined

  • Sacrococcygeal joint: Well visualized articulation with sacrum

  • Surrounding soft tissues: Rectum anteriorly and gluteal soft tissues posteriorly identified

MRI appearance

T1-weighted images:

  • Cortical bone: Low signal intensity

  • Marrow: Intermediate to high signal depending on fatty content

  • Intercoccygeal joints: Thin low-signal joint lines

  • Surrounding fat: High signal in presacral and subcutaneous regions

T2-weighted images:

  • Bone marrow: Intermediate signal

  • Intervertebral fibrocartilage: Intermediate signal

  • Presacral soft tissues: High signal fluid and fat interfaces visible

STIR:

  • Bone marrow: Suppressed fat signal with low background signal

  • Ligaments and joints: Well delineated low-signal structures

  • Soft tissues: High signal fluid-sensitive contrast around the coccyx

MRI image

coccyx MRI SAG anatomy Location and Imaging Appearance -img-00000-00000_00001

CT image

coccyx MRI SAG anatomy Location and Imaging Appearance -img-00000-00000

X Ray image

X Ray coccyx