Structure and Connections
The coccyx is formed by the fusion of rudimentary coccygeal vertebrae. Different sources indicate the fusion of 2–4 vertebrae (Co₁–Co₄) or 4–5 vertebrae.
- The first coccygeal vertebra (Co₁) has rudimentary articular processes — coccygeal cornua (cornua coccygea), which articulate with the sacral cornua.
- The sacrococcygeal symphysis contains an intervening fibrocartilaginous disc.
Topography and Ligamentous-Muscular Apparatus
The coccyx serves as an attachment site for ligaments and pelvic floor muscles.
- The levator ani muscle (m. levator ani) is the deepest layer of the pelvic floor muscles. It comprises the pubococcygeus muscle (m. pubococcygeus), iliococcygeus muscle (m. iliococcygeus), and ischiococcygeus muscle (m. ischiococcygeus).
Innervation
In the region of the coccyx lies the coccygeal nerve (n. coccygeus, Co₁). Like other spinal nerves, it gives rise to 4 groups of branches:
- Ventral branches (rr. ventrales)
- Dorsal branches (rr. dorsales)
- Meningeal branches (rr. meningei)
- Communicating branches (rr. communicantes)
The ventral branches of the coccygeal nerve, together with the sacral nerves S₄–S₅, form the coccygeal plexus (plexus coccygeus). It innervates the skin of the coccygeal region and the anococcygeal zone.
What is the obstetrical significance of the coccygeal vertebrae?
The coccyx has clinical significance in obstetrics:
- Pelvic boundaries: The sacrococcygeal symphysis forms the posterior boundary of the plane of the narrow pelvic cavity; the anteroposterior diameter here is 11.5 cm. The apex of the coccyx serves as the posterior boundary of the pelvic outlet plane.
- Dimension expansion during childbirth: The anteroposterior diameter of the pelvic outlet plane from the inferior margin of the pubic symphysis to the apex of the coccyx is normally 9.5 cm. As the fetal head passes through, the coccyx deviates, allowing this diameter to increase by 1–2 cm, reaching 11.5 cm.
- Biomechanism of labor: In anterior asynclitism / brow presentation during the third stage of labor, the fetal occiput rests against the coccyx, forming a second pivot point, after which the facial part is born from beneath the pubic arch.
- Differential diagnosis: In frank breech presentation, the fetal coccyx and ischial tuberosities can be mistaken for facial bones, and the anus for the mouth.
Class and exam questions
How many vertebrae form the coccyx?
The coccyx consists of 2–5 fused rudimentary vertebrae. Osteology notes the fusion of 2–4 vertebrae (Co₁–Co₄), whereas obstetrical anatomy notes 4–5 vertebrae.
Which nerves form the coccygeal plexus?
The coccygeal plexus (plexus coccygeus) is formed by the ventral branches of the coccygeal nerve (Co₁) along with the sacral nerves S₄ and S₅. It innervates the skin of the coccygeal region and the anococcygeal zone.
How does the coccyx assist in childbirth?
During the passage of the fetal head, the coccyx deviates, and the anteroposterior diameter of the pelvic outlet increases from 9.5 cm to 11.5 cm.
What else to study on this topic
- Sacrum (os sacrum)
- Pelvic floor and perineal muscles
- Coccygeal plexus (plexus coccygeus)
- Pelvic planes and dimensions
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