The sacroiliac joint is a synovial joint formed between the ilium and the sacrum. The left and right sacroiliac joints, together with the pubic symphysis and the sacrococcygeal joint, compose the articulations of the pelvic girdle. The sacroiliac joints connect the hip bones posterolaterally, while the pubic symphysis connects them anteriorly.
The main function of the joint is to bear the weight of the axial skeleton and transfer it to the hip bones. The weight can then be distributed to the two femurs in the standing position, or the ischial bones in the seated position.
This article will discuss the anatomy and function of the sacroiliac joint.
|Type||Synovial plane joint; nonaxial|
|Articular surfaces||Auricular surface of ilium, auricular surface of sacrum|
|Ligaments||Anterior sacroiliac ligament, posterior sacroiliac ligament (short, long and interosseous sacroiliac ligament); sacrotuberous ligament, sacrospinous ligament|
|Innervation||S1-S2 spinal nerves, superior gluteal nerve, obturator nerve,
|Blood supply||Iliolumbar, superior gluteal, lateral sacral arteries|
|Movements||Slight gliding and rotatory movements|
- Articular surfaces
- Joint capsule
- Blood supply
- Muscles acting on the sacroiliac joint
The sacroiliac joint is a synovial-type articulation between the auricular surfaces of the sacrum and ilium. Even though it is classified as a synovial plane joint, the auricular surfaces of the sacroiliac joint show marked irregularities in forms of depressions and elevations. However, these irregularities are reciprocal and congruent in which the central sacral concavity fits the iliac convexity, therefore interlocking the two auricular surfaces and providing stability. The auricular surface of the ilium is covered with some type of fibrocartilage, while the corresponding auricular surface of the sacrum is covered with hyaline cartilage.
Some authors describe the sacroiliac joint as being in part synovial on its anterior side, and in part a syndesmosis on its posterior side, formed between the tuberosities of the sacrum and ilium. A syndesmosis is a type of fibrous joint in which the bones are joined by fibrous connective tissue that forms an interosseous membrane.
The sacroiliac joint is completely encircled by a fibrous capsule that attaches on the articular margins of the sacrum and ilium. The internal surface of the capsule over the non-articular surfaces of the joint is lined by synovial membrane.
Due to its position and weight-bearing nature, the sacroiliac joint has to be stabilized with several strong ligaments. The main ligaments of the joint are anterior and posterior sacroiliac ligaments, while additional stability is provided with accessory ligaments: sacrotuberous ligament and sacrospinous ligament.
Anterior sacroiliac ligament
The anterior sacroiliac ligament is composed of many thin strands that form a strong, broad and flat band. It lies on the pelvic surface of the joint, forming the anteroinferior component of the joint capsule. The anterior sacroiliac ligament extends from the ala of the ilium, just anterior to the auricular surface, to the pelvic surface of the sacrum.
The part of the pelvic surface that lies between the auricular surface and the upper rim of the greater sciatic notch often shows a rough preauricular sulcus that provides an attachment site (origin) for the lower fibers of the anterior sacroiliac ligament that then insert onto the third sacral segment. This part of the ligament is particularly well developed, whereas the rest of the ligament is rather thin.
Posterior sacroiliac ligament
The posterior sacroiliac ligament is a compound ligament composed of three distinct bands. It lies posterior and superior to the joint, filling the space between the corresponding tuberosities of the ilium and the sacrum. The posterior sacroiliac ligament is much thicker and stronger than its anterior counterpart. The bands that compose this ligament are as follows:
- Interosseous sacroiliac ligament: a short but very strong band that spans the gap between the sacrum and ilium on the posterior side of the joint by attaching on their tuberosities, just behind the auricular surfaces. Occasionally, one or two accessory joint cavities can be identified within the ligament.
- Short posterior sacroiliac ligament: a short ligament found superficial to the interosseous sacroiliac ligament. It fills the upper part of the gap between the ilium and sacrum. The short posterior sacroiliac ligament originates on the first and second transverse tubercles of the sacrum and extends horizontally to attach on the tuberosity of the ilium.
- Long posterior sacroiliac ligament: a long band that is the most superficial of the posterior sacroiliac ligaments. It extends from the posterior superior iliac spine to the third and fourth transverse tubercles of the sacrum.
The accessory ligaments aid the main ligaments and contribute to the stability of the sacroiliac joint. The main function of these ligaments is to prevent forward tilting of the sacral promontory.
- Sacrotuberous ligament: a flat, triangular ligament that has several superior attachments; one band of the ligament arises on the gap between the posterior superior and posterior inferior iliac spines, where it is partially blended with the posterior sacroiliac ligaments. A second band arises on the lateral side of the sacrum below the auricular surface, and the third on the lateral side of the upper part of the coccyx. The three bands converge and extend inferiorly, forming a triangular shape before inserting on the lower margin of the ischial ramus. Right before the insertion occurs, the bands diverge again, thereby prolonging the inferior attachment point. This prolonged attachment is called the falciform process and is situated just below the pudendal canal, where it blends with the fascial sheath of the internal pudendal vessels and pudendal nerve. The posterior surface of the ligament is blended with the gluteus maximus, whereas the inferior fibers of the ligament continue into the tendon of biceps femoris.
- Sacrospinous ligament: a thin, triangular band found deep to the sacrotuberous ligament. It originates from the border of the lower sacral and upper coccygeal segments, anterior to the sacrotuberous ligament. The ligament is broad in its origin, but narrows as it extends laterally to insert onto the ischial spine. The sacrospinous ligament is thought to be a fibrous part of the coccygeus muscle, as it is largely blended with it.
Besides their main function, the sacrotuberous and sacrospinous ligaments also transform the greater and lesser sciatic notches into the greater and lesser sciatic foramina, respectively.
- Greater sciatic foramen: an opening bounded by the posterior border of the iliac bone anterosuperiorly, the sacrotuberous ligament posteromedially, and the sacrospinous ligament and ischial spine inferiorly. The contents of this foramen are the piriformis muscle, gluteal vessels and nerves, internal pudendal vessels, pudendal nerve, sciatic and posterior femoral cutaneous nerves and the nerves to obturator internus and quadratus femoris.
- Lesser sciatic foramen: an opening bounded by the ischial body anteriorly, spine of the ischium and sacrospinous ligament superiorly, and the sacrotuberous ligament posteriorly. Its contents include the tendon of obturator internus, the nerve to obturator internus, and the internal pudendal vessels and pudendal nerve.
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The sacroiliac joint receives innervation primarily from branches of the anterior and posterior rami S1-S2 spinal nerves and the superior gluteal nerve. Additionally, the joint receives contributions from the obturator nerve and lumbosacral trunk.
The sacroiliac joint receives arterial blood supply from branches of the iliolumbar, superior gluteal and lateral sacral arteries. The venous blood is drained from the corresponding veins into the internal iliac vein.
Due to its arrangement and strong ligaments, the sacroiliac joint allows little to no movement. Like other plane-type synovial joints, the sacroiliac ligament only allows for some amount of gliding and rotatory movements. The rotatory movements of the sacroiliac joint are described as nutation and counternutation.
- Nutation: occurs as the superior segment of the sacrum tilts downwards and anteriorly, while the inferior portion of the sacrum and the coccyx tilt posteriorly in the opposite direction. This movement is also referred to as sacral flexion, and occurs only in conjunction with trunk flexion or hip extension.
- Counternutation: occurs as the superior segment of the sacrum tilts upwards and posteriorly, while the inferior portion of the sacrum and the coccyx tilt anteriorly in the opposite direction. This movement is also referred to as sacral extension and occurs only in conjunction with trunk extension or hip flexion.
The main function of the sacroiliac joint lies in the fact that it comprises one of the pillars of the pelvic mechanism for transmitting the weight of the head, trunk and upper limbs to the lower limbs. The lack of mobility in the sacroiliac joint favors strength and stability to maximize weight bearing capabilities. These characteristics are especially important in order to maintain stability in the erect position during standing or walking. Interestingly, during childbirth, this changes due to the hormone relaxin, which loosens the joint and its ligaments to allow for more mobility. This puts the sacroiliac joints in counternutation, which is important to increase the pelvic inlet and outlet diameters to allow for the passage of the fetus.
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Muscles acting on the sacroiliac joint
No muscles have a direct action on the sacroiliac joint. Instead, there are muscles that attach onto the joint and provide to its stabilization. Some of these muscles include the internal abdominal oblique, external abdominal oblique, erector spinae, hamstring muscles, piriformis, the superior fibers of gluteus maximus, psoas minor, coccygeus, levator ani and others.