Sacroiliac Joint Anatomy

The sacroiliac (SI) joints connect the sacrum at the base of the spine to the hip bone. They provide a great deal of strength to modulate powerful forces between the upper and lower body.^, These joints also protect the lowest motion segment of the spine, L5-S1, through their limited motion and the very strong ligaments that interconnect with the L5-S1 segment.

An extensive architecture of tough and resilient ligaments helps support the sacroiliac joints, permitting stable, yet flexible support to the upper body.

The Role of the Sacroiliac Joints: Stabilization, Shock Absorption, and Mobility

The sacroiliac joints provide a range of essential functions, which:

• Control and distribute the forces from the upper body into the legs.
• Act as a shock absorber for the spine and control the transmission of forces from the lower body into the spine, such as gravitational forces and forces transmitted upward during standing or walking.^,
• Allow walking, spinal and thigh movements, and changes in posture or position, such as changing from lying to standing and standing to sitting.
• Support the weight of the upper body.
• Facilitate pregnancy and childbirth in women by widening and becoming more mobile.

The nervous system connections to this joint transmit pain signals arising from within the joint and its surrounding ligaments. The nerves also provide a sense of position and balance.

Sacroiliac Joint Articulation, Location, and Shape

The articulation between the outer side of the spine’s sacrum and the inner side of the butterfly-shaped hip bone (ilium) forms the sacroiliac joint.

• The SI joints are located on either side of the sacral spine and are situated deep in the pelvis.
• Each SI joint is secured and well protected by strong ligaments.
• The joint surface extends from the S1 spinal segment to the middle of the S3 spinal segment. This location may vary slightly, beginning above the S1 spinal segment (near L5-S1 or lumbosacral joint) and ending toward the top of the S3 spinal segment.

When viewed from the front, the joints are aligned in a parallel fashion on either side of the lower spine.

Directly pressing the skin over the sacroiliac joint in the rear pelvic area may elicit pain if the joint is inflamed (sacroiliitis) or not functioning as usual (sacroiliac joint dysfunction).

Articular Surface of the Sacroiliac Joint: Characteristics and Function

The unique sacroiliac articulation between the sacrum and the hip provides a transition between the spine and the pelvis.

While the SI joint has the features of a typical moveable joint, the movements within the joint are very limited.

The sacroiliac joint has a C-shaped contour

A fully developed adult sacroiliac joint is formed by two C-shaped (or inverted L-shaped) layers of bone with several irregular ridges and depressions.

• The outer (convex) part of the C corresponds to the hip’s ilium bone. This surface is covered with thick hyaline cartilage.
• The inner (concave) part of the C corresponds to the fused sacral bones S1-S3. This surface is lined with thin fibrocartilage.
• In the sacroiliac joint, both types of cartilage have rough and coarse texture due to natural adaptation to stress.
• Both of these opposing joint surfaces articulate closely with each other.
• The joint surface can be divided into 3 regions that correspond with the sacral vertebrae. The region near S1 is the largest and the joint surface near S3 is the smallest.

The angle and texture of the connecting surfaces change mildly with age to overcome and resist forces and compression in this region. The ridges and depressions along the joint’s surfaces develop and deepen with age.

Joint Capsule and Synovial Fluid of the Sacroiliac Joint

The outer part of the SI joint is covered by a joint capsule made up of thick and tough membranous tissue. The inner portion of this capsule has a delicate lining membrane called the synovial membrane. This membrane secretes a fluid called the synovial fluid, which helps lubricate the joint.

The presence of this joint capsule and inner membrane classifies the SI joint into synovial and diarthrodial, which means moveable joint. The fibrous joint capsule is located along the front part of the sacroiliac joint and there is no capsule along the back of the joint. For this reason, some researchers classify the sacroiliac joint as an atypical synovial joint or a modified synarthrodial joint.

The outer portion of this capsule is thick and tough.

Sacroiliac Joint Movement

The primary movements within the sacroiliac joint are front-and-back (anterior-posterior). Movements also occur along different planes, such as upward and downward. Movements can be further grouped into:

• Nutation, which refers to the forward and downward (anterior-inferior) movement of the sacroiliac joint. The tailbone (coccyx) moves back in relation to the hip bone during this motion. The stability of the sacroiliac joint is increased by the nutation movement.
• Counternutation, which refers to the backward and upward (posterior-superior) movement of the joint. The tailbone moves forward in relation to the hip bone during this motion.

These movements are typically limited to 2 mm to 4 mm and 2° to 5° due to the bony architecture and the surrounding ligaments of the joint.^,,

A full range of motion of the sacroiliac joint is expressed when the extremes of hip motion are reached (such as while rotating the hip outward until the farthest point).

Factors that Influence SI Joint Movements

Several components, including age, sex, and the mechanical loads that are placed on the sacroiliac joint affect its movements. For example:

• The need for more mobility may be most necessary in the sacroiliac joints of younger individuals, women (around the time of pregnancy and childbirth), and athletes.
• The need for more stability may be most necessary in older persons, individuals who are overweight, and those who frequently carry heavy weights. This increased need for stability may be accommodated by the joint’s strong ligaments and the age-associated ridges and depressions along the joint surfaces, which may promote better interlocking.

Adaptive changes in this joint also occur during pregnancy, when the joints become more mobile to compensate for the growing uterus and pelvic expansion.

Age-Related Changes in the SI Joint

The bones of the sacroiliac joint undergo changes as part of the natural aging process. These changes help the sacroiliac joint accommodate more forces. For example:

• Stability in the joint is increased. As a person approaches the end of their teenage years and begins early adulthood, the joint’s stability is primarily maintained by the ligaments. In adulthood, bony interlockings on the joint surface become enhanced to improve stability.
• Mobility in the joint is decreased. As age progresses, the movement of the sacroiliac joint reduces and is usually very minimal around the eighth decade of life.'

As the joint ages, the space between the joining bones narrows, making injection treatments, such as the sacroiliac joint injection, more challenging.

If the joint moves more or less than usual due to an underlying problem and causes symptoms, it may indicate a medical condition called sacroiliac joint dysfunction.

See Sacroiliac Joint Dysfunction Symptoms and Causes

Osteophytes (bone spurs) and ankylosis may develop in the joints around the fourth decade of life to increase stability.

Variations of the Sacroiliac Joint: Implications for Pain and Discomfort

Anatomical variations of the sacroiliac joint exist in 3% to 50% of the general population. These variants may include changes to the cartilage or ligament tissues or be associated with structural changes in the adjacent bone, and typically include^,:

• Accessory sacroiliac joint
• Iliosacral complex
• Bipartite iliac bony plate
• Crescent-like bony plate

The accessory sacroiliac joint is generally prone to degenerative changes and the most common symptomatic variant.

Due to its constant role in weight-bearing and unique anatomy, the sacroiliac joint(s) can become a source of pain. Symptoms can develop within the joint if the cartilage or bone gets damaged and/or when the ligaments that support the joint are strained. The vast network of nerves around the joint is highly sensitive and can transmit painful signals originating in this region.

See Accurate Diagnosis of Sacroiliac Joint Dysfunction