The coccyx, also known as the tailbone, is a triangular-shaped bone formed by three to five (usually four) fused rudimentary coccygeal vertebrae. It forms the terminal part of the spine. The coccyx is located inferior to the sacrum and articulates with it at the sacrococcygeal joint.

The coccyx participates in the formation of the posterior wall of the pelvis, and it serves as an attachment site for various muscles and ligaments, such as the gluteus maximus muscle and anterior sacrococcygeal ligament. Besides the mentioned functions, the coccyx plays a significant role in weight bearing when a person is seated.

The coccyx is characterized by its shape and small size. It lacks vertebral arches and does not participate in the formation of the vertebral canal. The coccyx of an adult usually presents as a single bone formed by four fused vertebrae. However, the number of coccygeal vertebrae varies among people.

Landmarks

The coccyx is a triangular-shaped bone with a base, apex and four surfaces - anterior, posterior and two lateral. The coccygeal base is directed upward, while the apex is pointed downward. The base of the coccyx has one articular facet for articulation with the sacrum.

The anterior surface is concave and presents several transverse lines or ridges that mark the fusion sites of the rudimentary coccygeal vertebrae. The posterior surface is convex and has the same transverse lines. Two rudimentary superior articular processes project upward from the posterior surface of the first coccygeal vertebra. They are called the coccygeal cornua (sing. cornu; horns).

The lateral surfaces of the first coccygeal vertebra present with two eminences called the transverse processes that extend from the body of the vertebra in the lateral direction.

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Coccyx