Anatomy of Skull and Spine
- Upper dorsal part = syndesmosis
= bone surfaces united by interosseous sacral ligaments
Location: superior ⅔ to ½ of joint
- irregular syndesmotic margins
- 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 ⅓ to ½ of joint
- smooth parallel joint margins
- normal joint space width of 2.49 ± 0.66 mm in people <40 years of age
- 35 mm thick cartilage on sacral side
- 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
- Ligamentous stabilizers of SI joint
- Interosseous ligament
- Ventral + dorsal sacroiliac ligaments
- Sacrospinous ligament
- Sacrotuberous ligament
- Iliolumbar ligament
Positioning: oblique view + modified Ferguson view = AP projection with 23° cephalad angulation
Anatomic Variants of SI Joint
- Accessory sacroiliac joint (most common)
Site: posterosuperior portion - Iliosacral complex
= iliac projection inserted into a complementary sacral recess
Site: transition between ligamentous + synovial portion - Bipartite iliac bone plate
Site: posteroinferior portion