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SACROILIAC JOINT (SI joint) ANATOMY

 Structure and kinematics

Itis the joint between the sacrum and the ilium.

Type of joint; Bi-condylar, Synovial, non-axial, Plane joint

Shape of joint; C-shaped or L-shaped

Movement; small amount of movement (2-18◦)

Close-pack position; full nutation

Translational motion; 1-2 mm

Rotational motion; 0.2-2◦

Function;

  • Antagonist innominate tilt during gait
  • Transmit weight from the upper body through the vertebral column to the hip bones
  • Great stability and has very little mobility
  • Connects pelvis to spine
  • Absorbs vertical forces from spine and transmits them to pelvis and lower extremities.
  • It functions as shock absorber for the spine above and conversion of torque from the lower extremities into the rest of the body.

JOINT MOTION

Motion that occur at the SI joint are;

Nutation

  • Also referred as Sacral flexion.
  • It occurs when the base of the sacrum moves anteriorly and inferiorly, causing the inferior portion of the sacrum and coccyx to move posteriorly relative to the ilium.
  • The pelvic outlet becomes larger
  • AP diameter of pelvic brim is decreased
  • Nutation occurs with trunk flexion or hip extension
  • Nutation increases stability by increasing compression and frictional forces
  • Forces that create nutation torque include; *Gravity, *passive tension in stretched ligaments, * Muscle tension

Counter-nutation

  • Also referred as Sacral extension
  • The base of the sacrum moves posteriorly and superiorly, causing the tip of the coccyx to move anteriorly relative to the ilium
  • The pelvic inlet becomes larger
  • AP diameter of pelvic brim is increased
  • Counter-nutation occurs with trunk extension or hip flexion
  • This motion is limited to 2 to 4 mm of movement.

FACTORS THAT PROVIDE STABILITY TO SI JOINT

  • Stability is maintained by
  • Interlocking of articular surfaces
  • Thick and strong interosseous and posterior sacroiliac ligaments
  • Vertebro-pelvic ligaments
  • Partial synostosis of joint takes place which further reduces movements

MUSCLES THAT STABILIZE SI JOINT

  • Erector spinae
  • Lumbar multifidi
  • Abdominal muscles
  • External and internal oblique
  • Rectus abdominus
  • Transverse abominus
  • Hamstrings (biceps femoris)

NERVE SUPPLY

  • Superior gluteal nerve (Ventral rami)
  • lateral branches of dorsal rami of first and second sacral nerve

BLOOD SUPPLY

  • Posterior division of internal iliac artery
  • Lateral sacral artery
  • Superior gluteal artery

LIGAMENTS THAT STABILIZE SI JOINT

  • Sacrotuberous ligament
  • Sacrospinous ligament
  • Iliolumbar ligament
  • Long posterior sacroiliac ligament
  • Short posterior sacroiliac ligament
  • Interosseous sacroiliac ligament
  • Anterior sacroiliac ligament

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