Sacrococcygeal joint

The sacrococcygeal joint (Latin: articulatio sacrococcygea) is a fibrocartilaginous articulation between the sacral apex and the base of the coccyx (tailbone). Between both bones is an intervertebral disc.

Classification of sacrococcygeal joint

The sacrococcygeal joint is a secondary cartilaginous joint (also called symphysis). This joint may be more mobile in some individuals and have synovial joint characteristics. Based on the provided movements (functionally), the sacrococcygeal joint is an amphiarthrosis. Therefore, it allows only slight motions. Nevertheless, the mobility of this joint ensures an increase in the pelvic diameters during labor.

Note: To read more in detail about the joint types and their function, please see our article on the classification of joints.

Articulating structures

The sacrococcygeal joint is formed between the following structures:

• Facet for the coccyx of the sacrum
• Superior and inferior surfaces of the intervertebral disc
• Articular facet of the coccyx

Ligaments

Several ligaments stabilize the sacrococcygeal joint, and they are as follows:

• Superficial posterior sacrococcygeal ligament
• Deep posterior sacrococcygeal ligament
• Anterior sacrococcygeal ligament
• Lateral sacrococcygeal ligaments
• Intercornual ligaments

The superficial posterior sacrococcygeal ligament originates from the posterior surface of the fifth sacral vertebra (S5), free margin of the sacral hiatus and the most inferior aspect of the median sacral crest and extends to the posterior surface of the coccyx.

The deep posterior sacrococcygeal ligament stretches from the posterior aspect of the fifth sacral (S5) vertebral body to the posterior surface of the coccyx. It is a continuation of the posterior longitudinal ligament.

The anterior sacrococcygeal ligament descends from the sacrum to the coccyx across the anterior surfaces of both vertebrae. This band is a continuation of the anterior longitudinal ligament.

The lateral sacrococcygeal ligaments are a pair of bands found on each side of the sacrum. They connect the transverse processes of the coccyx (Co1) with the inferior lateral angles of the sacrum. The intercornual ligaments connect the sacral and coccygeal cornua.

Both ligaments complete the foramina for the fifth sacral spinal nerves. The lateral sacrococcygeal ligaments form the anterior aspects of the foramina, while the intercornual ligaments complete the posterior portions of the foramina.

Neurovascular supply

The arterial blood to the sacrococcygeal junction is supplied by the median sacral artery and the inferior portion of the lateral sacral arteries. The median sacral artery arises from the abdominal aorta, while the lateral sacral arteries branch off the internal iliac artery.

The venous drainage of the sacrococcygeal joint occurs through the subchondral venous plexus, or venous blood is carried to the veins of the narrow spaces between two adjacent vertebrae.

The sacrococcygeal joint is innervated by the fourth and fifth sacral (S4, S5) and coccygeal (Co) spinal nerves.

Movements

As mentioned previously, the sacrococcygeal joint provides slight movements, which are all described as entirely passive. Only a small degree of passive flexion and extension of the coccyx is possible, and it happens due to an increase in the intra-abdominal pressure, for example, during defecation or labor.

References:

1. Kayalioglu, G., Paxinos, G., Watson, C. (2009). The Spinal Cord (1st Edition). Elsevier
2. Gray, H., & Carter, H. (2021). Gray’s Anatomy (Leatherbound Classics) (Leatherbound Classic Collection) by F.R.S. Henry Gray (2011) Leather Bound (2010th Edition). Barnes & Noble.