The sacroiliac (SI) joint is located deep within the pelvis, forming a connection between the spine and the lower body. It functions as a crucial link, allowing for the transfer of forces between the upper body and the legs. This article clarifies the bones forming the SI joint and explains its primary functions.
Understanding the Sacroiliac Joint Bones
The sacroiliac joint is formed by the articulation of two distinct bones: the sacrum and the ilium. Each human body possesses two such joints, positioned on either side of the sacrum. The sacrum is a large, triangular bone at the base of the vertebral column. It comprises five fused vertebrae and is thick, enabling it to support and transmit the body’s weight.
The ilium is the largest and uppermost bone of the hip, forming the flared, wing-shaped upper portion of the pelvic girdle. The articulation between the sacrum and the ilium occurs at specialized surfaces known as auricular surfaces. The auricular surface of the sacrum is typically covered with hyaline cartilage, while the corresponding auricular surface of the ilium is covered with fibrocartilage. These surfaces have irregular ridges and depressions that interlock, contributing to the joint’s stability.
Where the Sacroiliac Joint is Located and What it Does
The sacroiliac joints are located on either side of the sacrum, deep within the pelvis. They connect the sacrum (part of the axial skeleton) to the ilium (part of the appendicular skeleton). This positioning allows the SI joints to transfer forces between the trunk and the lower extremities.
A primary function of the sacroiliac joint is weight distribution. It supports the weight of the upper body and transfers this load to the lower limbs during activities such as standing, walking, or running. The joint’s strong network of ligaments allows it to withstand pressure and distribute weight evenly, which helps to reduce stress on other joints like the hips and knees.
Beyond weight bearing, the sacroiliac joint also plays a role in shock absorption. It reduces the impact of physical forces on the spine and other joints, particularly during high-impact movements. The joint’s strong ligaments and cartilage help to absorb these forces, thereby protecting the spine and lower body from excessive strain and preventing potential injuries. While the SI joint does permit a small amount of movement, typically ranging from 2 to 4 millimeters, its design prioritizes stability. This limited motion, combined with the interlocking nature of the bones and the extensive ligamentous support, ensures the pelvis remains stable during various movements and activities.