The sacroiliac (SI) joint, located deep within the pelvis, connects the spine to the hip bones. Many questions arise about its movement capabilities. Understanding the SI joint’s function and movement is important for appreciating its role in overall body mechanics.
Understanding the Sacroiliac Joint
The sacroiliac joint forms where the triangular sacrum bone meets the two iliac bones of the pelvis. There are two SI joints, one on each side of the sacrum. These are diarthrodial joints, possessing a capsule and lubricating synovial fluid, similar to more mobile joints like the knee or shoulder.
Unlike highly mobile joints, the SI joint has unique characteristics that limit its movement. Its surfaces feature irregular ridges and depressions that interlock, providing inherent stability. A network of strong ligaments surrounds and reinforces the joint, further contributing to its stability. These ligaments include the anterior sacroiliac, posterior sacroiliac, interosseous, sacrotuberous, and sacrospinous ligaments.
The Truth About SI Joint Movement
The SI joint does move, but its motion is subtle. Rather than large, obvious movements, it exhibits minimal rotation and translation. Its rotation is limited to a few degrees, typically 1 to 4 degrees, primarily in movements known as nutation (forward and downward) and counternutation (backward and upward). The joint also undergoes slight translation or gliding motions, usually on the order of a few millimeters, averaging about 0.7 mm and rarely exceeding 2 to 4 mm.
This limited, adaptive movement is primarily designed for specific functions like shock absorption and load transfer. A common misconception is that the SI joint can “slip out” or “lock up.” However, due to its strong ligamentous support and unique interlocking surfaces, the SI joint is a stable structure, making true dislocation extremely rare.
Factors Influencing SI Joint Mobility
Several factors can influence SI joint mobility. Pregnancy is one factor, where hormonal changes, particularly an increase in the hormone relaxin, cause ligaments throughout the pelvis to become more lax. This increased laxity allows for greater SI joint mobility, a necessary adaptation for childbirth, but it can also contribute to discomfort and altered joint mechanics. The additional weight gain and shifts in posture during pregnancy further contribute to altered stress on the joint.
Traumatic injuries, such as direct impacts or repetitive stress, can also affect SI joint mobility. These events may result in hypermobility (excessive movement) or hypomobility (restricted movement). Both too much or too little movement can disrupt the joint’s normal function and lead to pain. Age also influences SI joint mobility, as the joint surfaces gradually become more irregular and movement tends to decrease over time, becoming less mobile in older individuals.
Underlying medical conditions can also impact SI joint mobility. Various forms of arthritis, including osteoarthritis, rheumatoid arthritis, psoriatic arthritis, and especially ankylosing spondylitis, can cause inflammation (sacroiliitis) and changes in the joint. Ankylosing spondylitis can lead to erosion and eventual fusion of the joint, severely restricting its movement. Other conditions like general ligamentous laxity, leg length discrepancies, or structural pelvic asymmetries may also contribute to altered SI joint mechanics.
The Role of SI Joint Stability
The SI joint’s limited movement is fundamental to its role in overall body function. One of its primary responsibilities is efficient load transfer. The SI joint transmits forces and weight from the upper body and spine to the lower limbs during activities like standing, walking, and running. This function helps distribute stress and maintain balance.
Beyond load transfer, the SI joint also performs a significant role in shock absorption. It helps dissipate forces during impact activities, protecting the spine and other joints from excessive stress. This spring-like action mitigates movement impact. The joint’s stability is further enhanced by its unique interlocking design and strong surrounding ligaments, which form a self-bracing mechanism contributing to pelvic girdle stability. Maintaining this optimal balance of limited movement and high stability is important for proper posture and efficient movement, as both excessive or insufficient motion can compromise these functions and lead to pain.