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Information

Anatomy of Skull and Spine

  1. Upper dorsal part = syndesmosis
    = bone surfaces united by interosseous sacral ligaments
    Location: superior to ½ of joint
    • irregular syndesmotic margins
  2. Lower ventral part = synovial joint
    = anatomic characteristics of cartilaginous articulation (hyaline cartilage firmly attached to adjacent bone by fibrous tissue with inner capsule of synovial cells)
    Location: inferior to ½ of joint
    • smooth parallel joint margins
    • normal joint space width of 2.49 ± 0.66 mm in people <40 years of age
      1. 3–5 mm thick cartilage on sacral side
      2. 1 mm thick cartilage on iliac side
    • loss of joint space width in people >40 years of age to 1.47 ± 0.21 mm ± asymmetry of joint width
    • focal joint space narrowing + nonuniform ill-defined subchondral iliac sclerosis frequent >30 years of age
  3. Ligamentous stabilizers of SI joint
    1. Interosseous ligament
    2. Ventral + dorsal sacroiliac ligaments
    3. Sacrospinous ligament
    4. Sacrotuberous ligament
    5. Iliolumbar ligament

Positioning: oblique view + modified Ferguson view = AP projection with 23° cephalad angulation

Anatomic Variants of SI Joint

  1. Accessory sacroiliac joint (most common)
    Site: posterosuperior portion
  2. Iliosacral complex
    = iliac projection inserted into a complementary sacral recess
    Site: transition between ligamentous + synovial portion
  3. Bipartite iliac bone plate
    Site: posteroinferior portion