The sacrococcygeal symphysis

Symphysis

A symphysis is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint.1.A symphysis is an amphiarthrosis, a slightly movable joint.2.A growing together of parts or structures...

(sacrococcygeal articulation, articulation of the sacrum and coccyx) is an amphiarthrodial joint, formed between the oval surface at the apex of the sacrum

Sacrum

In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...

, and the base of the coccyx

Coccyx

The coccyx , commonly referred to as the tailbone, is the final segment of the vertebral column. Comprising three to five separate or fused vertebrae below the sacrum, it is attached to the sacrum by a fibrocartilaginous joint, the sacrococcygeal symphysis, which permits limited movement between...

.

It is a slightly moveable joint which is frequently, partially or completely, obliterated in old age, homologous

Homology (biology)

Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 with the joints between the bodies of the vertebrae.

Disc

The sacrococcygeal disc or interosseus ligament is similar to the intervertebral disc

Intervertebral disc

Intervertebral discs lie between adjacent vertebrae in the spine. Each disc forms a cartilaginous joint to allow slight movement of the vertebrae, and acts as a ligament to hold the vertebrae together.-Structure:...

s but thinner, thicker in front and behind than at the sides, and with a firmer texture. The articular surfaces are elliptical with longer transversal axes. The surface on the sacrum is convex and that on the coccyx concave. Occasionally the coccyx is freely movable on the sacrum, most notably during pregnancy

Pregnancy

Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...

; in such cases a synovial membrane is present.

Ligaments

The joint is strengthened by a series of ligaments:

• The ventral or anterior sacrococcygeal ligament is an extension of the anterior longitudinal ligament
  Anterior longitudinal ligament
  The anterior longitudinal ligament is a ligament that runs down the anterior surface of the spine. It traverses all of the vertebral bodies and intervertebral discs....
  
   (ALL) that runs down along the spine on the anterior sides of the bodies of the vertebrae
  Body of vertebra
  The body is the largest part of a vertebra, and is more or less cylindrical in shape. For vertebrates other than humans, this structure is usually called a centrum....
  
  . It consists of a few irregular fibers that attach to the anterior sides of the sacrum and coccyx and blend with the periosteum
  Periosteum
  Periosteum is a membrane that lines the outer surface of all bones, except at the joints of long bones. Endosteum lines the inner surface of all bones....
  
  .
• The dorsal or posterior sacrococcygeal ligament has a deep and a superficial part:
  • The deep dorsal ligament is a flat band which corresponds to the posterior longitudinal ligament
    Posterior longitudinal ligament
    The posterior longitudinal ligament is situated within the vertebral canal, and extends along the posterior surfaces of the bodies of the vertebræ, from the body of the axis, where it is continuous with the membrana tectoria, to the sacrum....
    
     (PLL) that run down inside the vertebral canal on the posterior surfaces of the bodies of the vertebrae. From the posterior side of the fifth sacral body inside the sacral canal, the dorsal ligament stretches to the posterior side of the coccyx, to attach deep to the superficial dorsal ligament.
  • The superficial dorsal ligament corresponds to the ligamenta flava
    Ligamenta flava
    The ligamenta flava are ligaments which connect the laminae of adjacent vertebrae, all the way from the axis to the first segment of the sacrum...
    
     and closes the posterior aspect of the distal end of the vertebral canal. It stretches from median sacral crest
    Median sacral crest
    In the middle line of the dorsal surface of the sacrum, it displays a crest, the median sacral crest, surmounted by three or four tubercles, the rudimentary spinous processes of the upper three or four sacral vertebrae....
    
     and the free margin of the sacral hiatus
    Sacral hiatus
    The laminae of the fifth sacral vertebra, and sometimes those of the fourth, fail to meet behind, and thus a sacral hiatus occurs in the posterior wall of the sacral canal....
    
     to the dorsal surface of the coccyx.

• The lateral sacrococcygeal ligament
  Lateral sacrococcygeal ligament
  In the human body, the lateral sacrococcygeal ligaments is a pair of ligaments stretching from the lower lateral angles of the sacrum to the transverse processes of the first coccygeal vertebra....
  
  s run from the lower lateral angles of the sacrum to the transverse processes of the first coccygeal vertebra to complete the foramina for the last sacral nerve
  Sacral spinal nerve 5
  The sacral spinal nerve 5 is a spinal nerve of the sacral segment..It originates from the spinal column from below the 5th body of the sacrum S5 supplies the Coccygeus muscle...
  
  . Three lateral ligaments have been reported on either side.
• The interarticular or intercornual sacrococcygeal ligaments stretches from the cornu
  Cornu
  Cornu is a Latin word for horn.Cornu may also refer to:*Cornu , an ancient musical instrumentPeople:*Dominique Cornu, a Belgian road and track cyclist, born 1985....
  
   of the sacrum to the cornu of the coccyx.

Movements

Movements in the joint are restricted to flexion

Flexion

In anatomy, flexion is a position that is made possible by the joint angle decreasing. The skeletal and muscular systems work together to move the joint into a "flexed" position. For example the elbow is flexed when the hand is brought closer to the shoulder...

 and extension

Extension (kinesiology)

In kinesiology, extension is a movement of a joint that results in increased angle between two bones or body surfaces at a joint. Extension usually results in straightening of the bones or body surfaces involved. For example, extension is produced by extending the flexed elbow. Straightening of...

. These essentially passive movements occurs during defecation

Defecation

Defecation is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material from the digestive tract via the anus. Waves of muscular contraction known as peristalsis in the walls of the colon move fecal matter through the digestive tract towards the rectum...

 and labour

Childbirth

Childbirth is the culmination of a human pregnancy or gestation period with the birth of one or more newborn infants from a woman's uterus...

. When movements in the sacrum increase the anteroposterior diameter of the pelvic outlet

Pelvic outlet

The lower circumference of the lesser pelvis is very irregular; the space enclosed by it is named the inferior aperture or pelvic outlet.-Boundaries:It has the following boundaries:* anteriorly: the pubic arch* laterally: the ischial tuberosities...

, movements in the sacrococcygeal joint can further increase this diameter.

Palpation

The joint is palpable

Palpation

Palpation is used as part of a physical examination in which an object is felt to determine its size, shape, firmness, or location...

 deep within the natal cleft

Gluteal cleft

The intergluteal cleft, also known as the natal cleft, the vertical gluteal crease, and the gluteal cleft, is the groove between the buttocks that runs from just below the sacrum to the perineum, so named because it forms the visible border between the external rounded protrusions of the gluteus...

, and can be felt as a horizontal groove. With the palpating finger on the dorsal surface of the coccyx, a degree of rotation can be produced with an applied forward pressure.

See also

• Anococcygeal raphé
  Anococcygeal raphe
  The fibers of the Levator ani pass downward and backward to the middle line of the floor of the pelvis; the most posterior are inserted into the side of the last two segments of the coccyx; those placed more anteriorly unite with the muscle of the opposite side, in a median fibrous raphe, the...
  
  
• Coccydynia
  Coccydynia
  Coccydynia is a medical term meaning pain in the coccyx or tailbone area, usually brought on by sitting too abruptly.-Diagnosis:A number of different conditions can cause pain in the general area of the coccyx, but not all involve the coccyx and the muscles attached to it. The first task of...
  
   (coccyx pain, tailbone pain)
• Ganglion impar
  Ganglion impar
  The pelvic portion of each sympathetic trunk is situated in front of the sacrum, medial to the anterior sacral foramina. It consists of four or five small sacral ganglia, connected together by interganglionic cords, and continuous above with the abdominal portion...
  
  
• Rump (croup)