Sacroiliac Joint (SIJ) Pain: Understanding Its Diverse Causes

What Causes Sacroiliac Joint (SIJ) Pain?

Sacroiliac joint (SIJ) pain arises from dysfunction or inflammation within one or both of the joints connecting the sacrum to the ilium, often resulting from mechanical stress, inflammatory conditions, trauma, or altered biomechanics.

Understanding the Sacroiliac Joint

The sacroiliac joints are two robust, L-shaped joints located on either side of the spine, connecting the sacrum (the triangular bone at the base of the spine) to the ilium (the largest part of the pelvic bone). Unlike highly mobile joints such as the knee or shoulder, the SIJs are designed for stability rather than extensive movement. Their primary roles include:

• Load Transfer: They act as crucial shock absorbers, transferring forces between the upper body and the lower limbs during movements like walking, running, and lifting.
• Stability: A complex network of strong ligaments and muscles surrounds and supports the SIJs, providing stability to the pelvis and spine.
• Limited Motion: While their motion is minimal (only a few degrees of rotation and millimeters of translation), this small degree of movement is vital for normal biomechanics and shock absorption.

When the normal function or structural integrity of one or both SIJs is compromised, it can lead to pain, typically felt in the lower back, buttocks, groin, or even radiating down the leg.

Primary Causes of SIJ Pain

SIJ pain is multifactorial, stemming from a variety of mechanical, inflammatory, traumatic, and degenerative origins. Understanding these causes is crucial for accurate diagnosis and effective management.

• Mechanical Stress or Dysfunction:

  • Hypermobility (Too Much Movement): When the ligaments supporting the SIJ become loose or stretched, the joint can move excessively. This instability can lead to irritation and pain as the joint surfaces rub or shift abnormally. Common causes include repetitive asymmetrical loading, prolonged sitting or standing, or conditions that weaken ligaments.
  • Hypomobility (Too Little Movement): Conversely, if the joint becomes stiff or "locked," it can also cause pain. This lack of normal movement can lead to increased stress on the joint itself and surrounding structures, as it can no longer adequately absorb forces. This often results from muscle imbalances, poor posture, or degenerative changes.

• Inflammatory Conditions (Spondyloarthropathies):

  • Certain systemic inflammatory diseases, primarily spondyloarthropathies, can directly affect the SIJs. Conditions like ankylosing spondylitis, psoriatic arthritis, reactive arthritis, and inflammatory bowel disease-associated arthritis can cause inflammation (sacroiliitis) and eventual fusion of the SI joints. This inflammation is a common cause of chronic SIJ pain, particularly in younger individuals.

• Trauma:

  • Direct trauma to the pelvic region can injure the SIJ, its ligaments, or surrounding structures. This can occur from:
    • Falls: Landing directly on the buttocks or hip.
    • Motor Vehicle Accidents: Especially side-impact collisions.
    • Repetitive Microtrauma: Activities involving repeated twisting, bending, or asymmetrical loading (e.g., certain sports, occupational movements).

• Pregnancy and Postpartum:

  • Pregnancy is a common period for SIJ pain due to several physiological changes:
    • Hormonal Changes: The hormone relaxin increases during pregnancy, loosening ligaments throughout the body, including those supporting the SIJs. This increased laxity can lead to hypermobility and instability.
    • Weight Gain: The added body weight increases stress on the pelvic girdle.
    • Altered Biomechanics: The shifting center of gravity and changes in posture (e.g., increased lumbar lordosis) can alter forces transmitted through the SIJs.
    • Childbirth: The trauma of delivery can also directly impact the SIJs.

• Leg Length Discrepancy and Gait Abnormalities:

  • A significant difference in leg length can lead to uneven loading and compensatory movements in the pelvis and spine. This asymmetrical stress can chronically irritate one or both SIJs.
  • Abnormal gait patterns, whether due to a leg length discrepancy, foot problems, or neurological conditions, can also disrupt the normal biomechanics of the SIJ, leading to pain.

• Muscle Imbalances and Weakness:

  • The stability of the SIJ relies heavily on the synergistic action of surrounding muscles. Imbalances or weakness in key muscle groups can compromise this stability:
    • Weak Core Muscles: The transverse abdominis and multifidus are crucial for lumbar and pelvic stability.
    • Weak Gluteal Muscles: Gluteus medius and maximus contribute significantly to pelvic stability and hip extension/abduction.
    • Tight Hip Flexors or Adductors: Can alter pelvic tilt and mechanics, increasing stress on the SIJ.
    • Asymmetrical Muscle Tension: Can pull the pelvis out of alignment, leading to uneven SIJ loading.

• Post-Surgical SIJ Pain:

  • Individuals who have undergone lumbar spinal fusion surgery may develop SIJ pain. Fusing segments of the lumbar spine can alter the biomechanics of the adjacent SIJ, increasing the stress and load it must bear. This is sometimes referred to as "adjacent segment disease" affecting the SIJ.

• Degenerative Changes:

  • While less common than in other joints, osteoarthritis can affect the SIJs, particularly in older individuals. Wear and tear on the joint cartilage over time can lead to pain, stiffness, and reduced function.

• Infection:

  • Though rare, the SIJ can become infected (septic sacroiliitis) through bacterial spread from other parts of the body or direct inoculation. This is a serious condition requiring immediate medical attention.

When to Seek Professional Help

If you experience persistent lower back, buttock, or leg pain that you suspect originates from the SIJ, it is important to consult a healthcare professional. A thorough evaluation, including physical examination and potentially imaging or diagnostic injections, can help determine the exact cause of your pain and guide appropriate treatment. Early intervention can often prevent chronic issues and improve outcomes.