Outline View

SI Joint Separation

Ligaments have two basic functions:

  • hold a joint stable within normal range of motion
  • regulate the actions of muscles that attach to the bones that make up the joint

When the ligaments sprain, the bones separate slightly, proportionate to the degree of sprain.

The Syndesmosis
Within the weight-bearing portion of the SIJ is the interosseous ligament, which contains blood and lymph vessels, connective and fatty tissue [1]. This ligament is not a single continuous band. Rather, it is composed of many individual bands that go between the sacrum and ilium.

Ligament Separation from Joint Surfaces
When sprained, these interosseous ligament bands do not tear; instead, they pull away from their insertion into either the sacrum or ilium [1, 2]. As a result, the joint surfaces separate slightly.

As in any joint, fluid (edema) from the surrounding tissues, as well as from blood vessels within the joint, infiltrates the space between the sacrum and ilium and creates swelling, inflammation, and pain.

One benefit of the edema is that it provides a cushion that keeps pressure off of the vessels and nerves within the joint. But, the main benefit is stability, as explained below.

Ligament Stretching
The internal pressure caused by the edema pushes out on the joint surfaces, stretching the ligaments. Nerves within the ligaments respond by activating the ligamento-muscular reflex which, in turn, causes protective muscle contraction or inhibition (see Ligamento-Muscular Reflex). The internal pressure of the edema (pushing out) is countered by the external compression of the muscles (pushing in); this forms the body’s attempt to provide stability.

Stress Maintained by Edema
The Serola Theory suggests that the edema within the space keeps the joint surfaces apart, preventing the re-integration of the ligament into the bone and, thereby, maintains the lesion; this is a key point in understanding this complex syndrome.

Over time, due to a number of factors, the muscle contractions become chronic (see Chronicity Factors), directly affecting the structure and causing many compensatory problems throughout the musculoskeletal system (see Overcompensation Patterns – Structural Adaptations).

As long as the edema remains in the joint space, the ligaments cannot re-integrate into the surface. To heal the joint, the surfaces must be brought and held together so that the ligaments can re-integrate into the joint surfaces; this can only be done by one method (See Pelvic Blocking). Although the Serola SI Belt can help keeping the joint surfaces within the normal range of motion, allowing significant relief and stability while it is worn, no belt can heal the SI joint. Proper treatment involves wedges precisely placed to provide specific compression, with exact timing, or more structural damage can occur.

Conversely, movements that cause separation of the joint surfaces will allow increased edema within the SI joint, intensifying stress on the ligaments, and increasing muscular tightness and inhibition (see Rehab Principles, Mechanisms of Injury and Do’s and Don’ts).


  1. Sashin, D., A critical analysis of the anatomy and the pathologic changes of the sacro-iliac joints. The Journal of Bone and Joint Surgery, 1930. 12: p. 891.
  2. Grieve, G.P., The sacro-iliac joint. Physiotherapy, 1976. 62(12): p. 384-400.
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