The Sacrum and Coccyx Are Distinct Anatomical Structures
No, the sacrum and coccyx are not the same—they are two separate bones that articulate with each other at the sacrococcygeal joint. 1
Anatomical Distinctions
The Sacrum
- The sacrum is a triangular bone formed by the fusion of five sacral vertebrae (S1-S5) that sits at the base of the spine 2
- It develops from 58 to 60 separate ossification centers that fuse in an organized pattern from the fetal period through age 30 3
- The sacrum contains four pairs of sacral foramina (anterior and posterior) in its most common anatomical configuration 2
- Its upper surface articulates with the L5 vertebra at the lumbosacral joint, while its lower portion connects to the coccyx 2
The Coccyx (Tailbone)
- The coccyx is a separate bone composed of 4-6 small vertebrae (typically 4) that represents the vestigial tail 4
- It develops from eight coccygeal ossification centers distinct from those of the sacrum 3
- The first coccygeal vertebra is the most developed, with subsequent segments becoming progressively smaller 4
- The coccyx is located below the sacrum and above the anus 5
The Sacrococcygeal Joint
- These two bones connect at the sacrococcygeal symphysis, an amphiarthrodial joint (a slightly movable cartilaginous joint) 2
- While the sacrum and coccyx can fuse together in some individuals (creating what appears as a single bone on imaging), this represents a variation rather than normal anatomy 2, 6
- Fusion between the sacrum and coccyx may occur unilaterally or bilaterally and is not uncommon, but when present, it can contribute to coccydynia (tailbone pain) by compressing coccygeal nerves 6
Clinical Relevance
Location Matters for Clinical Assessment
- When examining sacral dimples in infants, distinguishing between the sacrum and coccyx is critical 1, 7
- Dimples located on the flat part of the sacrum (above the gluteal cleft) are pathologic lumbosacral dermal sinus tracts requiring neurosurgical referral 7
- Dimples at or near the coccyx (within the gluteal cleft) are benign coccygeal dimples requiring no intervention 7
Pain Localization
- Coccydynia specifically refers to pain in the coccyx region (below the sacrum, above the anus), not sacral pain 5
- The distinction is important for targeted treatment approaches including injections and physical therapy 5
Common Pitfall to Avoid
Do not assume the sacrum and coccyx are a single continuous bone—while they may appear fused on imaging in some individuals, they develop from entirely separate ossification centers and represent distinct anatomical entities with different clinical implications 3, 4