Sacroiliac (SI) Joint: Anatomy, Function, and Clinical Significance

What is the name of the joint between the sacrum and the pelvis?

The joint between the sacrum and the pelvis is known as the Sacroiliac (SI) Joint. There are actually two such joints, one on each side of the sacrum, connecting it to the ilium, which is the largest part of the pelvic bone.

Understanding the Sacroiliac (SI) Joint

The Sacroiliac (SI) joint is a critical, yet often misunderstood, articulation in the human body. Located at the base of the spine, where the sacrum (a triangular bone formed by the fusion of five vertebrae) meets the two iliac bones (the large, flaring bones that form the upper part of the pelvis), these joints play a pivotal role in transferring forces between the upper body, spine, and lower extremities. While classified as a synovial joint, its unique structure and extensive ligamentous support limit its mobility, prioritizing stability over movement.

Anatomy and Structure of the SI Joint

The intricate design of the SI joint is crucial for its function:

• Bones Involved:
  • Sacrum: A triangular bone at the base of the spine, wedged between the two ilia. It forms the posterior wall of the pelvis.
  • Ilium: The largest and uppermost part of the innominate bone (pelvis). Each ilium articulates with one side of the sacrum.
• Joint Surfaces: The articulating surfaces of the sacrum and ilium are known as the auricular surfaces due to their ear-like shape. These surfaces are covered with different types of cartilage: the sacral side is typically covered with hyaline cartilage, while the iliac side is covered with fibrocartilage. This difference contributes to the joint's limited motion.
• Ligamentous Support: The SI joint is one of the strongest joints in the body, primarily due to a robust network of ligaments that provide significant stability and limit excessive movement. Key ligaments include:
  • Anterior Sacroiliac Ligaments: Thinner and less robust, located on the front of the joint.
  • Posterior Sacroiliac Ligaments: Stronger and more numerous, located on the back of the joint, providing significant stability.
  • Interosseous Sacroiliac Ligaments: Extremely strong and deep ligaments connecting the tuberosities of the sacrum and ilium, forming the primary bond between the bones.
  • Sacrotuberous Ligament: Connects the sacrum to the ischial tuberosity (sit bone).
  • Sacrospinous Ligament: Connects the sacrum to the ischial spine. These ligaments collectively act to resist shear forces and control the subtle movements of the joint.

Function and Biomechanics of the SI Joint

Despite its limited range of motion, the SI joint performs several vital biomechanical functions:

• Load Transfer: Its primary role is to transmit the forces of the upper body and trunk down through the pelvis to the lower limbs, and vice-versa during ground reaction forces. It acts as a crucial link in the kinetic chain.
• Shock Absorption: The SI joints, along with the pubic symphysis, contribute to dispersing impact forces, protecting the spine and brain from excessive jarring during activities like walking, running, and jumping.
• Subtle Movement: While not designed for large movements, the SI joint undergoes small, complex motions, including:
  • Nutation: The sacral base moves anteriorly and inferiorly, while the apex moves posteriorly and superiorly. This motion occurs during trunk flexion and hip extension.
  • Counternutation: The opposite of nutation, where the sacral base moves posteriorly and superiorly, and the apex moves anteriorly and inferiorly. This occurs during trunk extension and hip flexion. These subtle movements are essential for efficient gait mechanics and maintaining postural stability.
• Stability: The intricate design of the joint surfaces (form closure) and the extensive ligamentous network (force closure) work together to provide inherent stability, which is paramount given its role in load bearing.

Clinical Significance and Common Issues

Understanding the SI joint is crucial for fitness professionals, athletes, and anyone dealing with lower back or hip pain:

• SI Joint Dysfunction (SIJD): This is a common cause of lower back, buttock, or even leg pain. It typically arises from either:
  • Hypomobility: Too little movement, often due to stiffness or fixation, leading to localized pain and restricted motion.
  • Hypermobility: Too much movement, often due to ligamentous laxity (common in pregnancy or after trauma), leading to instability and pain as the joint struggles to bear load.
• Referred Pain: Pain originating from the SI joint can be referred to various areas, including the groin, hip, thigh, and even the foot, often mimicking sciatica.
• Impact on Exercise: Imbalances or dysfunction in the SI joint can significantly impact exercise performance, posture, and increase the risk of injury. For instance, a dysfunctional SI joint can affect squat depth, deadlift mechanics, and running efficiency.

Maintaining SI Joint Health

Promoting SI joint health is integral to overall spinal and pelvic stability. Strategies include:

• Core Stability: Strengthening the deep core muscles (transversus abdominis, multifidus, pelvic floor) helps provide an internal "corset" that supports the lumbar spine and SI joints.
• Balanced Strength and Flexibility: Addressing muscular imbalances around the hips and pelvis is crucial. This includes strengthening glutes, hamstrings, and hip abductors, while ensuring appropriate flexibility in hip flexors, quadriceps, and adductors.
• Proper Lifting Mechanics: Utilizing correct biomechanics during resistance training, especially for movements like squats, deadlifts, and lunges, minimizes undue stress on the SI joints.
• Unilateral Training: Incorporating single-leg exercises can help identify and correct asymmetries that might contribute to SI joint dysfunction.
• Awareness and Listening to Your Body: Paying attention to pain signals and avoiding activities that aggravate the SI joint is important.
• Professional Consultation: If persistent pain or dysfunction is suspected, consulting with a physical therapist, chiropractor, or sports medicine physician is recommended. They can provide an accurate diagnosis and develop a targeted rehabilitation plan.

By appreciating the unique anatomy and critical function of the Sacroiliac (SI) joint, individuals can take proactive steps to maintain its health, optimize movement, and prevent pain.