Osteology : Coccyx

 Coccyx (Tailbone) The coccyx is the small, triangular bone at base of the vertebral column, formed by the fusion of 3 to 5 coccygeal vertebrae. It acts as an attachment site for muscles, ligaments, and tendons, playing a role in pelvic support and posture.

1. General Features of the Coccyx

Feature	Description
Shape	Small, triangular, and  curved bone
Location	Lower end of the vertebral column, below the sacrum vertebra
Formation	It is formed by the fusion of 3 to 5 coccygeal vertebrae (typically 4)
Curvature	Slightly curves anteriorly (more in males, less in females)
Base	The superior, broader part that articulates with the sacrum vertebra
Apex	The inferior, pointed end that does not articulated with any bone
Cornua (Coccygeal Horns)	Two small projections at the base, connecting with the sacral cornua
Transverse Processes	Small lateral extensions present in the first coccygeal vertebra
Articulation	It articulates with the sacrum at the sacrococcygeal joint
Function	It supports body weight during sitting, attachment site for pelvic muscles and ligaments

 

2. Differences Between Coccygeal Vertebrae

• Co1 (First Coccygeal Vertebra):
• Largest and most developed.
• Has transverse processes and cornua (horn-like projections).
• Articulates with the sacrum.
• Co2 to Co4 (or Co5):
• Become progressively smaller and simpler.
• Lack transverse processes.
• Eventually fuse into a single bony mass.

3. Ligaments and Muscles Attached to the Coccyx

Ligaments:

• Anterior sacrococcygeal ligament – Connects sacrum to coccyx (like the anterior longitudinal ligament of the spine).
• Posterior sacrococcygeal ligament – Similar to the posterior longitudinal ligament.
• Lateral sacrococcygeal ligaments – Stabilize the sacrococcygeal joint.
• Intercoccygeal ligaments – Connect coccygeal vertebrae before they fuse.

Muscles:

• Levator ani (pubococcygeus & iliococcygeus) – it supports pelvic organs.
• Coccygeus muscle – it helps with defecation and pelvic floor stability.
• Gluteus maximus – Partly attaches to the coccyx for hip movement.
• Sphincter ani externus – Controls anal opening.

4. Clinical Importance

 Fracture/Dislocation is common in falls or during childbirth, causing coccydynia (tailbone pain).
 Coccygodynia – Chronic pain due to injury, prolonged sitting, or muscle strain.
 Childbirth Adaptation – The female coccyx is more flexible and moves backward during delivery.
 Vestigial Structure – Considered a remnant of a tail from evolutionary history.