The sacrococcygeal joint is an articulation between the apex of the sacrum and the base of the coccyx (tailbone). This joint is a symphysis, having the two bones lined by hyaline cartilage and connected by an interposed fibrous disc. In some individuals, the joint might be synovial.
Being a symphysis, the sacrococcygeal joint is only slightly mobile. The movements within the joint are entirely passive and are limited to one degree of freedom; flexion-extension. The function of the sacrococcygeal mobility is to increase the anteroposterior diameter of the pelvis during labor and defecation.
|Type||Secondary cartilaginous joint (symphysis)|
|Articular surfaces||Apex of sacrum, the base of coccyx|
|Ligaments||Anterior sacrococcygeal, superior posterior sacrococcygeal, deep posterior sacrococcygeal, lateral sacrococcygeal, intercornual ligaments|
|Innervation||Spinal nerves S4-Co|
|Blood supply||Inferior lateral sacral, median sacral arteries|
|Movements||Passive flexion and extension|
This article will discuss the anatomy and function of the sacrococcygeal joint.
The articular surfaces that comprise the sacrococcygeal joint are the apex of sacrum and the base of coccyx. Both of them are lined by a layer of hyaline cartilage. The surfaces are united by an ellipsoid fibrocartilaginous articular disc. The joint may become partially or completely obliterated later in life.
Ligaments and joint capsule
The sacrococcygeal joint doesn’t have an articular capsule. It is surrounded and stabilized by 5 ligaments;
- The anterior sacrococcygeal ligament, which extends from the anterior surface of the sacrum to the anterior surface of the apex of coccyx.
- The superficial posterior sacrococcygeal ligament, that arises from the margin of the sacral hiatus and attaches to the dorsal surface of the coccyx.
- The deep posterior sacrococcygeal ligament, which spans from the dorsal surface of the fifth sacral segment to the dorsal surface of coccyx.
- The lateral sacrococcygeal ligament, that attaches to the inferolateral angle of the sacrum and the transverse processes of the coccyx.
- The intercornual ligament, a band that connects the cornua of the sacrum and coccyx.
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The sacrococcygeal joint is innervated from the sacral spinal nerves S4-S5 and the coccygeal spinal nerve Co.
The blood supply to the sacrococcygeal joint comes from the inferior lateral sacral and median sacral arteries. The former is a branch of the internal iliac artery, while the latter arises from the terminal bifurcation of the abdominal aorta.
Since the sacrococcygeal joint is a symphysis, it is only slightly mobile. The movements within it are entirely passive, limited to a small degree of passive flexion and extension. There are no muscles acting directly upon the joint. Instead, these movements happen when the intra-abdominal pressure is increased, which is during labor and defecation.
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