The distinctive bone structure and complex movement(s) of the sacroiliac joint are held together and powered by an extensive network of ligaments. The muscles surrounding the sacroiliac joint do not specifically move the joint, but the health of these muscles can influence the stability and motion of the joint.
The joint receives its blood supply from major and smaller arteries and has a wide network of nerves that are pain-sensitive.
The supporting and stabilizing ligaments of the sacroiliac joint connect the joint in several ways. While several ligaments connect the joint from the front and back, others are present between the joint surfaces, holding them together.
Interosseous sacroiliac ligament
The interosseous ligament is one of the strongest of all ligaments in the body.1 It bears significant forces while stabilizing the torso and supporting lower body movement(s).
The interosseous ligament:
- Connects the outer surface of the sacrum (triangular part of the lower spine) to the inner surface ilium (hip bone)
- Receives the greatest stresses of the ligaments associated with the sacroiliac joint.2
- Forms the major connection between the sacrum and the ilium.
- Prevents forward and downward movement of the sacrum.1
- Protects the joint by preventing excessive backward movement.2
The interosseous ligament has several layers. Since the back portion of the sacroiliac joint is not covered by a capsule (as in the front), this ligament helps prevent adverse movements of the joint toward the back.1
Superior intracapsular ligament (Illi’s ligament)
This ligament is a small band of fibrous tissue that may not always be present. The Illi’s ligament is thought to be an extension of the interosseous ligament and has little or no mechanical value.2
Anterior sacroiliac ligament
This ligament, sometimes called the ventral sacroiliac ligament, covers the front of the sacroiliac joint, which includes the articular (joint) capsule that encloses the joint in this area. The fibers of this capsule blend with the joint’s capsule in front and do not provide much support.2
Posterior sacroiliac ligament
The posterior SI ligament runs along the back of the sacroiliac joint and provides considerable stability.2 The ligament connects the back of the hip bones (posterior-superior iliac spine and iliac crest) to the sacrum.
There are two components of the posterior SI ligament:2
- Long posterior sacroiliac ligament
- Short posterior sacroiliac ligament
The long posterior sacroiliac ligament undergoes tension during the transmission of forces from the legs to the upper body and vice versa.1
Accessory sacroiliac ligaments
The following three accessory ligaments help enhance the stability of the sacroiliac joint1:
- Sacrotuberous ligament
- Sacrospinous ligament
- Iliolumbar ligament
The sacrotuberous and sacrospinous ligaments create the greater sciatic foramen and the lesser sciatic foramen.1,2 The largest nerve in the body, the sciatic nerve, passes through the greater sciatic foramen formed by these ligaments. Trauma to these ligaments, and the consequent inflammation, can lead to sciatic nerve pain, which runs down through the leg along the course of the nerve. (Read: What You Need to Know About Sciatica)
SI Joint Muscles
The muscles around the sacroiliac joint do not specifically power its movements; most of the joint’s movements are facilitated by tension on its ligaments.2 The surrounding muscle groups typically play a role in maintaining the stability and function of this important joint.
Anatomically, the SI joint is surrounded by over 40 muscles.2 The main muscle groups that affect the SI joint are the:
- Back muscles, such as the erector spinae, quadratus lumborum, and multifidus lumborum.2
- Hip muscles, such as the iliopsoas.2
- Core muscles, such as the rectus abdominis.2
- Buttock muscles, such as the gluteus maximus and piriformis.2
- Thigh muscles, such as the biceps femoris from the hamstring group.3
When these muscles become tight due to inadequate activity (such as from a sedentary lifestyle), they become shorter, and in turn, cause tension around the sacroiliac joint, making it stiff. Adequately stretching and activating these muscles allows the joint to be flexible and function without pain.3
Nerve Supply of the Sacroiliac Joint
The sacroiliac joint is well innervated, causing considerable pain and symptoms to arise from this region during trauma or inflammation. The pattern of nerve supply can vary among individuals. This variability may also be seen on the right and left sides of the same individual. It is believed that this inconsistency may be responsible for the diverse pattern of referred pain from the sacroiliac joint.1,2
- The sacroiliac joint receives its nerve supply from the major anterior branches (ventral rami) of the L4 and L5 spinal nerves, superior gluteal nerve, and other major branches (dorsal rami) of the spinal nerves L5 to S2.1
- The nerve supply of the sacroiliac joint includes sensory innervation, which means that the nerves conduct pain signals from the joint’s capsule, ligaments, and cartilage.2
- The joint’s nerves also possess special receptors called mechanoreceptors. These mechanoreceptors help relay information related to movement and joint position. This information helps to keep the body upright and balanced.2
The sacroiliac joint typically contains more pain-sensitive receptors than mechanoreceptors. The most pain-sensitive and highly innervated part of the sacroiliac joint is the cartilage.2
Blood Supply of the Sacroiliac Joint
A rich network of arteries supplies the sacroiliac joint through large and small branches that course along the front and back of the joint.
- The back portion of the joint is supplied by the median sacral artery and lateral sacral artery. Both these arteries arise from the internal iliac artery, commonly found at the spine levels L5-S2. These arteries join with the superficial branch of the superior gluteal artery.1,2
- The front portion of the joint is supplied by the iliolumbar artery, which originates either from the internal iliac or common iliac artery.1,2
The venous drainage of the joint flows into the internal iliac vein.
The vast majority of sacroiliac joint problems affect the joint’s ligaments and/or cartilage. Muscles surrounding the SI joint, such as the back, core, buttocks, and thigh muscles, may also influence the joint if they are tight and not well-conditioned. The joint’s natural development and mechanical loading pattern change with age to accommodate the growing physiological stresses and impacts on the pelvic and hip regions.