Template:Joints

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Description

The sacroiliac joint (simply called the SI joint) is the joint connection between the spine and the pelvis. It is a large diathrodial joint made up of the sacrum and the two innominates of the pelvis. Each innominate is formed by the fusion of the three bones of the pelvis: the ilium, ischium, and pubic bone.

Sacroiliac joint

Ligaments & Joint Capsule

Joint Capsule

The sacroiliac joint capsule is made up of two strong layers.  It attaches to both articular margins of the joint and becomes thicker as it moves inferiorly.

Ligaments:

• Anterior Sacroiliac- an anteroinferior thickening of the fibrous capsule and is weak and thin when compared to the other ligaments of the joint.  It connects the third sacral ligament to the lateral side of the preauricular sulcus and is better developed closer to the arcuate line and the PSIS.  This ligament is injured most often and is a common source of pain because its thinness.
  
• Interosseus Sacroiliac- forms the major connection between the sacrum and the innominate and is a strong, short ligament deep to the posterior sacroiliac ligament.  It resists anterior and inferior movement of the sacrum.
  
• Posterior (Dorsal) Sacroiliac- connects the PSIS with the lateral crest of the third and fourth segments of the sacrum and is very stong and tough.   Nutation, which is anterior motion of the sacrum, slackens the ligament, and counternutation, which is posterior motion will make the ligament taut.  It can be palpated directly below the PSIS and can often be a source of pain.
  
• Sacrotuberous- consists of three large fibrous bands and is blended with the posterior (dorsal) sacroiliac ligament.  It stabilitzes against nutation of the sacrum and counteracts against posterior and superior migration of the sacrum during weight bearing.
  
• Sacrospinous- triangular shaped and thinner than the sacrotuberous ligament and goes from the ichial spine to the lateral parts of the sacrum and coccyx and then to the ischial spine laterally.  Along with the sacrotuberous ligament, it opposes forward tilting of the sacrum on the innominates during weight bearing

Muscles

There are 35 muscles that attach to the sacrum or innominates which mainly provide stability to the joint rather than producing movements.

Muscles that attach to the sacrum or innominates:

• Latissimus dorsi
• Erector spinae
• Semimembranosus
• Semitendonosus
• Biceps femoris
• Sartorius
• Inferior gamellus
• Multifidus
• Obturator internus
• Obturator externus
• Piriformis
• Tensor fascia lata
• External oblique
• Internal oblique
• Transversus abdominus
• Rectus abdominis
• Gluteus medius
• Gluteus maxiumus
• Gluteus minimus
• Quadratus femoris
• Superior gemellus
• Gracilis
• Iliacus
• Adductor magnus
• Rectus femoris
• Quadratus lumborum
• Pectineus
• Psoas minor
• Adductor brevis
• Adductor longus
• Levator ani
• Sphincter urethrae
• Superficial transverse perineal ischiocavernous
• Coccygeus
• Pyramidalis

Resources

Recent Related Research (from Pubmed)

References

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