Coccyx anatomy

Osteology and joints of the coccyx

The coccyx (os coccygis) is formed from a small number of rudimentary bone elements—from three to five coccygeal vertebrae (vertebrae coccygeae). Depending on individual anatomical features of the body, these vertebrae may exist as separate bone fragments or be completely fused into a single bone.

The coccyx articulates with the overlying spinal column through the sacrum (os sacrum). This junction forms the sacrococcygeal symphysis (symphysis sacrococcygea). In its tissue structure, this joint is fibrocartilaginous, which provides strength and a certain cushioning in this area.

Ligamentous apparatus of the pelvic region

The coccyx and sacrum are integrated into the complex biomechanical system of the pelvis thanks to a robust ligamentous apparatus. The area of the symphysis itself is reinforced by the sacrococcygeal ligaments (ligamenta sacrococcygea). The anterior longitudinal ligament of the spine (ligamentum longitudinale anterius) also runs along the anterior surface of the vertebral bodies.

For a comprehensive understanding of pelvic anatomy, it is necessary to consider other vital ligaments fixing the pelvic ring:

• The pubic symphysis (symphysis pubica), closing the pelvis anteriorly.
• The inguinal ligaments (ligamentum inguinale).
• The anterior sacroiliac ligaments (ligamenta sacroiliaca anteriora), stabilizing the joint of the same name.
• The iliolumbar ligaments (ligamenta iliolumbalia).
• The sacrospinous (ligamentum sacrospinale) and sacrotuberous (ligamentum sacrotuberale) ligaments, stretched between the sacrum and the structures of the ischium.

X-ray anatomy of the pelvis in anteroposterior projection

When studying a plain radiograph of the pelvis, the coccyx is visualized as a continuation of the sacrum in the lower part of the image. For accurate topographical orientation, one must be able to identify all key bony structures.

Main radiographic landmarks:

1. Structures of the ilium: iliac crest (crista iliaca), anterior superior and anterior inferior iliac spines (spina iliaca anterior superior and spina iliaca anterior inferior), as well as the posterior inferior iliac spine (spina iliaca posterior inferior).
2. Structures of the pubis: superior ramus (ramus superior ossis pubis) and inferior ramus (ramus inferior ossis pubis).
3. Structures of the ischium: ischial tuberosity (tuber ischiadicum).
4. Joints and foramina: sacroiliac joint (articulatio sacroiliaca), acetabular roof (acetabulum), and obturator foramen (foramen obturatum).

Features of the development of the vertebral column

The development of the coccyx follows the general laws of axial skeleton formation, yet it has distinct characteristics. At birth, the length of the entire vertebral column is approximately 40% of the child's total body length. During the first two years of life, rapid growth occurs, and the length of the spine doubles.

The most important characteristic of this process is growth heterochrony—the uneven enlargement of various segments. In the first year of life, the maximum growth rate is recorded in the lumbar region (pars lumbalis). At the same time, the coccygeal region (pars coccygea) demonstrates the minimal growth rate among all segments of the vertebral column.

Comparative osteogenesis of the skeleton

Processes of ossification (osteogenesis) in the spine occur in parallel with the development of other structures, particularly the skull. Between the ages of 1 and 3, cartilage tissue is actively replaced by bone tissue in the skeleton.

Growth vectors of various parts of the skeleton shift depending on age. While the coccyx grows extremely slowly in the first year of life, the facial skull, for example, begins to grow at a sharply accelerated rate in the 2nd–3rd year of life (which is physiologically driven by the eruption of deciduous teeth and increased load on the masticatory muscles). In the pubic and postpubertal periods (13–20 years), bone thickening occurs with the simultaneous development of their air-containing spaces (pneumatization), which compensatorily reduces the overall skeletal mass.

Class and exam questions

What else to study on this topic

• X-ray anatomy of the pelvic ring and joints in anteroposterior projection
• Ligamentous apparatus of the pelvis: topography and biomechanics
• Growth heterochrony of vertebral column segments in children
• Anatomical and morphological features of the newborn skull
• Chronology of growth and ossification of skull bones up to 20 years

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.