The SI joint is a diarthrodial synovial joint. It is surrounded by a fibrous capsule containing a joint space filled with synovial fluid between the articular surfaces. The articular surface is made up of 2, strong, C-shaped layers. It is distinguished from other synovial joints by the unusual articulation of 2 different types of cartilage. The sacral capsular surface is composed of hyaline cartilage while the iliac capsular surface is composed of fibrocartilage.
It is also unique in that its articular surfaces have multiple ridges and depressions. During the natural bone maturation process, the morphology and characteristics of the sacroiliac joint change. The joint surfaces are relatively flat in early life. However, with ambulation, the sacroiliac joint surfaces develop distinct angulations and lose their flat surface. They also develop an elevated ridge along the iliac surface and a depression along the sacral surface. This produces a functional interlocking between the 2 bones. This interlocking limits the movement at the SI joint. The increase in the sacroiliac joints’ stability makes dislocations very rare.
Biomechanically, the sacroiliac joint performs several functions. Primarily, its purpose is to attenuate distribution of force loads from the lower extremities. It functions both as a shock absorber for the spine above and conversion of torque from the lower extremities into the rest of the body. The primary plane of motion is anterior-posterior along a transverse axis. Nutation refers to anterior-inferior movement of the sacrum while the coccyx moves posteriorly relative to the ilium. Counternutation refers to a posterior-superior movement of the sacrum while the coccyx moves anterior relative to the ilium. In most individuals, this motion is limited to 2 to 4 mm of movement.
A strong ligamentous architecture stabilizes the SI joint. The interosseous sacroiliac ligament forms the major connection between the sacrum and the ilium. It is the strongest ligament in the body and prevents anterior and inferior movement of the sacrum. The posterior (dorsal) sacroiliac ligament connects the posterior-superior iliac spine (PSIS) and the iliac crest with the third and fourth segments of the sacrum. The ligament resists counternutation and can be palpated directly below the PSIS. The sacrotuberous ligament blends with the posterior sacroiliac ligament and resists nutation of the joint. It passes behind the sacrospinous ligament, which goes from the ischial spine to the lateral sacrum and also opposes nutation of the sacrum during weight bearing and gait. The presence of the sacrotuberous and sascrospinous ligaments creates the greater sciatic foramen and the lesser sciatic foramen. These ligaments help prevent flexion and rotation of the ilium past the sacrum. Of note, the pudendal nerve lies between these two ligaments and may become entrapped, leading to perineal pain. The anterior sacroiliac ligament is an anterior-inferior thickening of the fibrous capsule that is not as well defined as other SI ligaments. It is the thinnest of all ligamentous structures and most vulnerable to injury and thus a common source of pain.
The blood supply to the sacroiliac joint stems from 3 branches of the internal iliac artery: the superior gluteal artery, lateral sacral artery, and Iliolumbar artery.
The sacroiliac joint is well innervated. The sacroiliac joint receives its innervation from the ventral rami of L4 and L5, superior gluteal nerve and dorsal rami of L5-S2. The nerve supply to the SI joint varies between individuals and innervation may be almost exclusively derived from the sacral dorsal rami. This may account for the variable patterns of referred pain from the SI joint.
The incidence of lower back pain originating from the sacroiliac joint may be as high as 27%. Patients will often describe a specific trauma or inciting event, although this is not always true. Pain originating from the sacroiliac joint is likely more common than most providers realize due to difficult in localizing symptoms and referred pain patterns. Patients with pain originating from the sacroiliac joint may describe symptoms aggravated by getting out of a chair, prolonged standing or sitting, with climbing stairs or morning stiffness. Pain can transfer to other areas, including the buttocks, hip, groin, and leg. Numbness and tingling in the leg may also be present. Risk factors include leg length discrepancy, age, arthritis, history of spine surgery, pregnancy, and trauma.
Sacroiliac Joint Dysfunction
Sacroiliac joint dysfunction is a broad term that refers to the abnormal biomechanical function of the SI joint. Some causes of sacroiliac joint dysfunction include the following:
Increased mobility of the SI joint may be the result of injury to the supporting ligaments. This can be seen in high impact injuries such as a motor vehicle accident, fall, or repetitive trauma from weight lifting or sports.
Hypermobility can also be genetic, a term often labeled "multidirectional instability" for ligamentous laxity at multiple joints, not strictly the sacroiliac joint. Other causes of inherited laxity can include Marfan syndrome and Ehlers-Danlos syndrome.
Hormonal changes may also cause hypermobility. The ligaments of the sacroiliac joint slacken during pregnancy due to the hormone relaxin. This loosening, along with that of the related symphysis pubis, permits the pelvic rim to widen during the birthing process. The ligaments may also be stretched due to increased lumbar lordosis. Therefore, SI joint pain is common in pregnancy. However, this pain may persist after pregnancy if the ligaments do not return to physiologic tension. Risk factors include extended labor and delivery of large babies.
Structural abnormalities of the SI joint may also cause dysfunction. Individuals with altered gait pattern, spinal deformities or leg-length discrepancies can have reduced interlocking ability leading to laxity and pain. This equates to repetitive and uneven stress to the SI joint articular surfaces, causing laxity and pain.
Osteoarthritis of the SI joint is also common, and incidence increases with age. Other risk factors for osteoarthritis include previous trauma.
Inflammation of the sacroiliac joints is termed sacroiliitis. There are many causes of sacroiliitis, with some mentioned above. It may also be the presentation of arthritis from brucellosis. Sacroiliitis may also be related to other inflammatory conditions such as inflammatory bowel disease and seronegative spondyloarthropathies.
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