Coccyx

The coccyx (plural: coccyges) is the series of rudimentary vertebrae forming the caudal termination of the vertebral column and is positioned inferior to the apex of the sacrum. The coccyx is one leg of the tripod formed in conjunction with the ischial tuberosities for support in a seated position. Additionally, it serves as the insertion site for the muscles of the pelvic floor and those that contribute to voluntary bowel control and supports the position of the anus.

Terminology

For the purposes of numbering the vertebral segments, and stipulated by the Terminologia Anatomica (TA), "Co" is used as the abbreviation for each coccygeal level, e.g. Co1, Co2, etc. Clearly "C" is already used for the cervical vertebrae.

Gross anatomy

The coccyx is formed from four rudimentary vertebrae and does not contain a spinal canal, pedicles, laminae or spinous processes. The first segment is the largest, and the subsequent are smaller in size. Structure of the coccygeal vertebral junctions is variable and age-related, ranging from fully developed to rudimentary intervertebral discs with varying degrees of cystic or fibrotic change, to fusion of the vertebrae in the later decades.

The coccyx consists of an anterior and posterior surface, two lateral surfaces, an apex and a base.

• anterior surface: concave, marked with three transverse grooves representing the fusions of the four separate vertebrae

• posterior surface

  • convex, similarly marked with three transverse grooves

  • there is a vertical row of tubercles on either side, which are rudimentary articular processes of the coccygeal vertebrae

  • the superior pair are the largest and are called the coccygeal cornua. They articulate with the sacral cornua

  • the coccygeal and sacral cornua combine to form the foramen for the transmission of the posterior division of the fifth sacral nerve

• lateral surface

  • thin with several eminences that represent rudimentary transverse processes of the coccygeal vertebrae

  • the most superior eminences join the lateral edges of the sacrum, forming the foramen for the transmission of the anterior division of the fifth sacral nerve

  • it is the largest eminence and the inferior eminences subsequently decrease in size

  • anteriorly to posteriorly, the lateral border serves as attachment for the coccygeus, sacrospinous ligament, sacrotuberous ligament, and fibers of the gluteus maximus

• base: proximal oval surface for articulation with the sacrum

• apex: distal rounded prominence

Articulations

• sacrococcygeal symphysis

  • fibrocartilaginous joint that connects the apex of the sacrum to the coccyx

  • movement is passive minor flexion and extension

  • typically fuses with age

Attachments

Ligamentous

Five ligaments support the sacrococcygeal symphysis:

• anterior sacrococcygeal ligament: continuation of the anterior longitudinal ligament - connects to the anterior aspect of the 1^st and sometimes 2^nd vertebral bodies

• deep posterior sacrococcygeal ligament: connects from the 5^th sacral body to the dorsal surface of the coccyx

• superficial posterior sacrococcygeal ligament: begins on the medial sacral crest and inserts on the dorsal surface of the coccyx

• lateral sacrococcygeal ligament: joins the transverse process of the first coccygeal vertebra to the inferolateral angle of the sacrum. This completes the foramen for the fifth sacral nerve anteriorly. Posteriorly the foramen is closed by the sacral and coccygeal cornua connected by the intercornual ligament

• interarticular (intercornual) ligaments: connect the cornua of the sacrum to the cornua of the coccyx

• anococcygeal raphe: ligament that helps support the position of the anus

Musculotendinous

• gluteus maximus muscle: attaches to the lateral coccyx

• levator ani muscle: attaches to the anterior border and the apex of the coccyx

• sphincter ani externus: attaches to the apex of the coccyx

Relations

• upper surface: pelvic floor, ganglion impar

• lower surface: buttocks

Variant anatomy

• coccyx hypoplasia

• coccyx aplasia

• incomplete fusion

• bifid coccyx

Variation in position

One method of classification on that was proposed by Postacchini and Massobrio and subsequently modified by Nathan which classifies into 6 types ⁸

• type I: present in over half of people; coccyx has a gentle ventral curvature as a continuation of the natural curvature of the sacrum and a caudally pointing apex

• type II: (8-32%): more prominent ventral curvature with coccyx apex pointing anteriorly

• type III: (4-16%): acute anterior angulation of the coccyx but no subluxation

• type IV: (1-9%): focal anterior angulation with anterior subluxation

• type V: (1-11%): posteriorly angulated coccyx

• type VI: (1-6%): scoliotic deformity or lateral deviation of coccyx

Radiographic features

Plain radiograph

AP axial and lateral views are used to visualize fractures.

Development

The coccyx arises from a caudal eminence present from weeks 4-8 of gestation. This caudal eminence regresses by birth, leaving the four precursor vertebrae. Each coccygeal segment is ossified from one primary center, with the cornua of the first segment ossifying from seperate centers ¹. The first segment appears between ages one to four years, the second between ages five to ten years, the third between ten and fifteen years, and the fourth between fourteen and twenty years. Segments do not unite until after age twenty-five or thirty. The coccyx only fuses with the sacrum late in life, and this is more common in females than in males.

History and etymology

The word coccyx is derived from the Greek word for “cuckoo” because of its similarity to a cuckoo's beak when viewed from the side.

Related pathology

• coccygeal fracture

• coccydynia

• sacrococcygeal teratoma