Coccyx

1. Overview

The coccyx, commonly known as the tailbone, is the terminal portion of the vertebral column. It is a small, triangular bony structure composed of 3 to 5 fused vertebrae, usually four. The coccyx is considered a vestigial structure, the remnant of a tail in humans, but it still plays important roles in posture, muscle attachment, and weight-bearing when sitting. Despite its small size, injury or dysfunction of the coccyx can lead to significant pain and discomfort.

2. Location

The coccyx is located at the inferior end of the vertebral column:

• Superiorly: Articulates with the sacrum at the sacrococcygeal joint.

• Inferiorly: Ends in an apex that may be slightly curved or pointed.

• Posteriorly: Lies just beneath the skin at the base of the spine.

In seated posture, it is positioned anterior to the gluteal cleft and can be palpated externally.

3. Structure

The coccyx is typically made up of four small fused vertebrae (Co1–Co4), though variation exists:

• Co1: The largest and most prominent segment, often retains rudimentary transverse processes and may articulate with the sacrum.

• Co2–Co4: Progressively smaller; lack true pedicles, laminae, and spinous processes.

• Sacrococcygeal joint: A fibrocartilaginous joint (sometimes synovial) between the sacrum and coccyx; may become fused with age.

• Ligamentous support: Includes the anterior and posterior sacrococcygeal ligaments and the intercornual ligament connecting the sacral and coccygeal cornua.

4. Function

The coccyx serves several biomechanical and support functions:

• Supports weight in sitting: Helps bear body weight when seated, especially when leaning backward.

• Muscle and ligament attachment: Serves as an anchor for several pelvic floor muscles and ligaments.

• Shock absorption: Acts as a slight buffer or flex point during falls or sudden seating pressure.

5. Physiological role(s)

Despite being vestigial, the coccyx plays important physiological roles:

• Pelvic floor support: Provides attachment for muscles such as the levator ani, coccygeus, and external anal sphincter, which support pelvic organs and control continence.

• Postural balance: Contributes to distribution of pressure and posture during sitting and dynamic movement.

• Parturition (childbirth): May move slightly backward to increase the pelvic outlet during vaginal delivery.

6. Clinical Significance

The coccyx can be a source of clinical concern, especially in cases of trauma or dysfunction:

• Coccydynia (tailbone pain):

  • Pain in the coccygeal region, often from falls, prolonged sitting, childbirth, or idiopathic causes.

  • May involve inflammation, dislocation, or spasm of surrounding muscles (e.g., coccygeus, levator ani).

• Fracture or dislocation:

  • Results from direct trauma (e.g., falling onto a hard surface); can be very painful and slow to heal.

• Hyper- or hypomobility:

  • Excessive or restricted movement at the sacrococcygeal joint may cause chronic pain or functional limitations.

• Surgical consideration:

  • In rare, refractory cases of coccydynia, coccygectomy (surgical removal of the coccyx) may be considered.

• Referred pain:

  • Pelvic floor dysfunction or nerve entrapment in the region may mimic coccygeal pain and require differential diagnosis.