Anatomy of the Sacroiliac (SI) Joint

Introduction

• Presenter: Kevin Tockoff
• Platform: Catalyst University
• Focus: Stability of the sacroiliac joints (SI joints) and how they support body weight.

Overview of SI Joint Structure

• Previous Video Recap: Discussed ligaments and SI joint structure.
• Anatomical Challenge: Sacrum bears weight of the entire upper body, yet remains stable in SI joints.
• Weight Supported: Lumbar, thoracic, cervical vertebrae, head, torso, abdomen, neck, and arms.
• Stability versus Mobility: SI joints sacrifice mobility for stability; they are extremely stable with negligible mobility.

Mechanisms of Stability

Form Closure and Force Closure

• Form Closure: Vertical support via bone structure; ilium provides partial support underneath the sacrum.
• Force Closure: Horizontal support through internal mechanisms; ilia are pushed towards midline creating friction, preventing sacrum from falling through.
  • Muscles: Cross-bracing arrangement of muscles that force ilia together.

Self-Locking Mechanism

• Comparison to Legos: Articular surfaces of SI joint (sacrum and ilium) interlock like Legos.
  • Complementary Fit: Interlocking surfaces prevent sacrum from dropping through.

Potential Movements

Nutation and Counternutation

• Nutation: Anterior rotation of superior sacrum; posterior rotation of inferior sacrum and coccyx.

  • Restriction: Mainly by the sacrotuberous ligament and partly by the sacrospinous ligament.
  • Muscles Involved: Biceps femoris (hamstring muscle) aids in restriction.

• Counternutation: Posterior tilt of superior sacrum; anterior rotation of coccyx and inferior sacrum.

  • Restriction: Mainly by the long dorsal sacroiliac ligament.
  • Muscles Involved: Latissimus dorsi through thoracolumbar fascia.

Key Takeaways

• Stability Factors: Form and force closure, self-locking interlocking surfaces, and ligament restrictions.
• Mobility: Extremely limited (around 2 degrees), nearly undetectable.
• Importance of Stability: Prevents dislocations and supports effective load transfer.
• Risk of Dysfunction: Ligament disruption can lead to extra mobility and SI joint dysfunction.

Conclusion

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• Note: Understanding SI joint stability is crucial for comprehending body mechanics and joint function.