Structure and Articulations
The coccyx is formed by the fusion of rudimentary coccygeal vertebrae. Literature varies, describing the fusion of 2–4 vertebrae (Co₁–Co₄) or 4–5 vertebrae.
- The first coccygeal vertebra (Co₁) features rudimentary articular processes known as coccygeal horns (cornua coccygea), which articulate with the sacral horns.
- The sacrococcygeal symphysis contains an intervertebral disc-like fibrocartilaginous layer.
Topography and Musculoligamentous Apparatus
The coccyx serves as an attachment site for ligaments and pelvic floor muscles.
- Levator ani muscle (m. levator ani) forms 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
The coccygeal nerve (n. coccygeus, Co₁) is located in the region of the coccyx. Like other spinal nerves, it gives rise to four 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 S₄–S₅ sacral nerves, form the coccygeal plexus (plexus coccygeus). It innervates the skin of the coccygeal and anococcygeal regions.
What is the Obstetric Significance of the Coccygeal Vertebrae?
The coccyx has significant obstetric importance:
- Pelvic planes and boundaries: The sacrococcygeal symphysis marks the posterior boundary of the plane of least pelvic dimensions (narrow cavity of the lesser pelvis), where the anteroposterior diameter is 11.5 cm. The apex of the coccyx serves as the posterior boundary of the pelvic outlet.
- Expansion during labor: The anteroposterior diameter of the pelvic outlet from the inferior margin of the pubic symphysis to the coccygeal apex is normally 9.5 cm. As the fetal head passes through, the coccyx retropels (deflects posteriorly), allowing this diameter to increase by 1–2 cm, reaching 11.5 cm.
- Biomechanism of labor: In an occiput anterior presentation, during the third movement of labor, the fetal occiput presses against the coccyx, forming a secondary fulcrum (fixed point) around which the face is born from beneath the pubic arch.
- Differential diagnosis: In a frank breech presentation, the fetal coccyx and ischial tuberosities can be mistaken for facial bones, and the anus can be mistaken for the mouth.