Located at the heart of the pelvic girdle, the sacroiliac joints connect the pelvic bones on either side to the spine. Key joints between the lower and upper body, they can be the seat of pain.

The sacroiliac joints, or SI joints, refer to the two joints connecting the ilium os in the pelvis to the sacrum of the spine. Located deep, at the bottom of the spine to the right and left of the sacrum, they are in a way the bridge connecting the spine to the bones of the legs.

It is a synovial-type joint: it has an articular capsule containing fluid. Its structure changes with age: the joint capsule is well developed in children, then thickens and becomes fibrosis over the years. Conversely, the cartilage covering the articular surfaces becomes thinner and almost disappears after 70 years.

Each joint is surrounded and reinforced by a complex network of intrinsic ligaments at the front, the ventral ligaments, and at the rear, the dorsal ligaments (superficial ligament, iliotranverse ligaments, ilio-transverse sacral ligament, or iliosacral, interosseous ligament) , and extrinsic. Finally, each SI joint is connected to powerful muscle groups including the hamstrings (posterior face of the thigh), the psoas (anterior face of the hip), the iliotibial band (lateral face of the thigh), the piriformis (buttock) and the rectus femoris (anterior aspect of the thigh).

Real central pivot, the sacroiliac joints distribute the weight of the body between the top and the bottom and play a role of support of the spine.

The SI joints can make complex nutation and counter-nutation movements, in particular depending on the movement of the coccyx, when bending forward or carrying a load, for example, but these movements remain of low amplitude. The two SI joints are interdependent on each other: movement on one side causes movement on the other. Their movement also depends on those of another key joint in the pelvis: the pubic symphysis.

Degeneration

A joint that is very stressed on a daily basis, the SI joint is a very common site of osteoarthritis.

Sacroiliac syndrome

Sacroiliac joint syndrome, or sacroiliac syndrome, refers to a painful mechanical phenomenon. It manifests as pain often on one side in the lower back, buttock, groin and even thigh, difficulty sitting. It is therefore often mistaken for a lumbar problem or sciatica.

Different factors can be at the origin of this syndrome:

• inequality of the lower limbs;
• hyperlodosis (excessive arching of the back);
• a fall on the buttocks;
• repetitive movements involving the lumbar region and pelvis;
• difficult childbirth;
• a lumbar sprain;
• excessive effort;
• prolonged work squatting on the buttocks.

Inflammatory disease

The SI joints are often the first to be affected in ankylosing spondyloarthritis, a chronic inflammatory rheumatic disease. This is manifested by pain in the buttocks called “rocking”, because sometimes affecting the right buttock, sometimes the left.

The SI joint is also a very frequent location for other inflammatory spondyloarthropathies, even rare infectious diseases grouped under the term seronegative spondylitis: ankylosing spondylitis, spondylitis associated with psoriasis, Reiter’s syndrome, certain inflammatory diseases of the digestive tract.

The sacroiliac syndrome can be managed by physiotherapy, chiropractic. 

The treatment of spondyloarthritis aims to stop the pain, the progression of the disease and prevent the onset of ankylosis. This support is multidisciplinary, with:

• analgesic and anti-inflammatory treatments to relieve symptoms:
• DMARDs to treat the disease;
• local treatments for painful joints;
• functional rehabilitation.

The clinical examination

It includes palpation and certain maneuvers and tests used to evaluate the function of the joint: tripod maneuver, spreading maneuver towards iliac wings, Gaensen maneuver, etc. The absence of neurological symptoms (numbness, loss of strength, modification of the tendon reflexes) makes it possible to differentiate the sacroiliac syndrome from lumbosaciatric disorders. The practitioner must also check the absence of systemic symptoms (fever, cough, fatigue, etc.) which may accompany rheumatic disease.

Medical imaging exams

Radiography of the pelvis and sacroiliacs is the first-line examination. 

MRI of the sacroiliacs allows it to early evaluate an infectious or inflammatory disease. It is particularly useful in the diagnosis of spondyloarthritis. The images will then show erosions.