Tailbone
Os Coccygis
location_on Inferior tip of the vertebral column, below the sacrum
The coccyx is a small triangular bone formed by the fusion of three to five rudimentary vertebrae, representing the vestigial tail. It articulates superiorly with the sacrum and provides attachment for muscles of the pelvic floor, the gluteus maximus, and the anococcygeal ligament. Despite its small size, it plays an important weight-bearing role in the seated position.
Key Anatomical Features
- Coccygeal cornua are the vestigial superior articular processes of the first coccygeal vertebra
- Transverse processes of the first segment may articulate with the sacrum
- Pelvic surface provides attachment for the levator ani and coccygeus muscles
- Tip is the inferior terminus of the entire vertebral column
- Three to five segments, with the first often remaining separate
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Coccygeus (ischiococcygeus) | Lateral coccyx | Supports the pelvic floor and flexes the coccyx |
| Levator ani | Coccyx via the anococcygeal ligament | Supports pelvic viscera and constricts anorectal junction |
| Gluteus maximus | Posterior coccyx and sacrotuberous ligament | Extends and laterally rotates the hip |
| External anal sphincter | Tip of coccyx via anococcygeal ligament | Maintains closure of the anal canal |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Sacrococcygeal joint | Cartilaginous symphysis (often fuses with age) | Sacrum |
| Intercoccygeal joints | Cartilaginous (often fused) | Adjacent coccygeal segments |
Common Pathologies
Coccydynia (coccygodynia)
Pain in and around the coccyx, typically worsened by sitting. May result from a fall onto the buttocks, childbirth, or be idiopathic. More common in women (5:1 ratio).
Coccyx fracture
Fracture or dislocation of the coccyx, usually from a fall onto the buttocks. Diagnosis is clinical; imaging is often unnecessary as management is conservative.
Pilonidal cyst
An abscess or sinus tract in the natal cleft near the coccyx, thought to arise from ingrown hair. Common in young hirsute males.
Clinical Relevance
Coccydynia is diagnosed clinically by eliciting pain on direct palpation of the coccyx. Dynamic seated and standing lateral radiographs can show excessive coccygeal mobility or subluxation. Most cases resolve with conservative treatment (donut cushion, NSAIDs, pelvic floor therapy). Coccygectomy is reserved for severe refractory cases and has a success rate of about 80-90%.
Development and Ossification
The coccygeal segments ossify from one center each, appearing between birth and puberty in a craniocaudal sequence. The first coccygeal segment fuses with the sacrum between ages 25 and 30. Inter-coccygeal fusion proceeds superiorly to inferiorly, often remaining unfused throughout life.
Did You Know?
- The word coccyx comes from the Greek kokkyx meaning cuckoo bird, because its shape was thought to resemble a cuckoo's beak
- The coccyx is the vestige of a tail that our primate ancestors possessed
- Despite being considered vestigial, the coccyx serves as an important anchor for pelvic floor muscles and bears weight when sitting
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