Biomechanics and Kinesiology of the Sacroiliac Joint

Functions of the Sacroiliac Joint

• Acts as a link between the axial skeleton (spine) and lower extremities
• Transmits forces from upper extremity to lower while standing
• Transmits forces from upper extremities and axial skeleton to ischial tuberosities while sitting
• Provides attachment for large muscles of trunk and lower extremities
• Absorbs ground reaction forces during standing and sitting

Anatomy Overview

• Pelvic Complex includes:
  • Two innominates (right and left)
  • Sacrum between L4 and L5
  • Sacroiliac joint, pubic symphysis (cartilaginous joint)
  • Articulation between L5 and S1
  • Acetabular femoral joint (hip)
• Sacroiliac joint requires a complex ligamentous network for stabilization

Sacrum Anatomy

• Triangular bone made of five fused bones (S1-S5)
• Superior base and inferior apex
• Anterior surface: smooth and concave
• Posterior surface: rough for ligament and muscle attachments
• Auricular surfaces articulate with innominates

Lumbar Facet Joints Orientation

• L1 to L5 in sagittal plane, transitioning to frontal plane as moving down
• Important for sacroiliac joint stability

Lumbosacral Junction

• Transition zone, frequent area of pain and dysfunction
• Composed of intervertebral discs and facet joints

Ligaments and Stabilization

• Iliolumbar Ligament: stabilizes L5 and S1
• Anterior and Posterior Longitudinal Ligaments: restrain movement and provide stability
• Anterior shear forces can lead to spondylolisthesis if unchecked

Lumbosacral Angle

• Calculated between L5 and S1
• Typical angle: 40 degrees; increases can lead to increased shear forces
• Frontal plane articulation prevents anterior translation

Sacroiliac Joint Ligaments

• Three primary stabilizing ligaments:
  • Anterior Sacroiliac Ligament
  • Posterior Sacroiliac Ligament (Long Dorsal Ligament)
  • Interosseous Sacroiliac Ligament
• Three accessory stabilizing ligaments:
  • Iliolumbar Ligament
  • Sacrotuberous Ligament
  • Sacrospinous Ligament

Sacral Motions

• Nutation: base of sacrum tilts anteriorly, apex moves posteriorly
• Counternutation: base tilts posteriorly, apex moves anteriorly
• Motions are associated with innominate motion

Sacroiliac Joint Stability

• Form closure: due to joint shape
• Force closure: due to muscle and ligament tension
• Nutation is the most stable position of the sacroiliac joint

Muscles Involved in Stability

• Erector Spinae and Multifidus
• Rectus Abdominus
• Biceps Femoris and Gluteus Maximus

These notes cover the key points discussed in the biomechanics and kinesiology of the sacroiliac joint, focusing on its functions, anatomy, stabilization mechanisms, and significance in maintaining posture and movement. The sacroiliac joint plays a crucial role in force transmission and stability within the pelvic complex.