Thesacroiliac joint, often referred to as the “SI joint” is the space located between the sacrum bone and the back of the hip bone. You might think of the SI joint as the place where the spine connects to the pelvis at and around the level of your hips.
You have two SI joints—one on either side of thesacrum.
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Bones of the SI Joint
Like mostjoints of the body, the sacroiliac is comprised of two bones, namely, the sacrum and the ilium. In front, it is classified as diarthrodial joint (which means it is a freely movable type of joint) comprised of the sacrum and the two hip bones in the back. These two hip bones (ilium, as mentioned above) are technically called the innominates of the pelvis.
The back of the SI joint is not as moveable as the front.
The sacrum is a triangularly shaped bone located just below thelumbar spine. The last lumbar vertebra (and the weight of the entire spinal column for that matter) sits on the top surface of the sacrum (which is an area known as the sacral base).
On either side, the ilium provides a kind of buttressing support to the sacrum.
In the back of the sacroiliac joint are the following ligaments:
Both the sacrotuberous and the sacrospinous ligaments limitflexion movementof the sacrum.Sacral flexion is also called nutation and is defined below.
In front, the anterior ligament is really just a thickened extension of the capsule surrounding the SI joint. The anterior ligament is small, especially in comparison to theligaments in the back.
Characteristics
The SI joint is interesting because it is classified as one type of joint in front and another in back.
In front, it is a synovial joint (discussed above as diarthrodial). This type of joint typically has a lot of movement, although not in the case if an SI. (Your shoulder and hips are synovial joints—think of the big movements they can make.)
The surfaces of the sacrum and the ilium—where the meeting between the two occurs—are covered with cartilage (the sacrum is lined with a different type of cartilage than the ilium), as well as a number of bony contours that resemble projections and valleys. (These are located in front only.)
As far as muscles go, portions of the gluteus maximus and the piriformis muscles cross the SI joint.
Function
The SI joint bears the weight of the spine. It is responsible for transmitting this weight into the area between the hip and foot inclusively, which is known as the lower extremity. It also transmits twisting (rotational) forces from the lower extremity that go all the way up into the spine.
For example, during walking, just as you finish taking a step forward, your back leg is extended behind you and you are pushing off the big toe. (This is called the “push off” phase of gait.) At this point, there is minimal space between the two bones of the SI joint. This is called joint congruency or the “close pack” position of the joint. The close pack position of the SI joint helps transmit forces arising from the big toe as they transfer up the lower extremity, and pass through the sacrum and into the spine.
The SI joint also transmits the weight of your spine onto the sitting bones (technically called the ischial tuberosities), during the act of sitting.
As mentioned above, even though the front part of the SI joint is classified as a synovial joint, it has only a small degree of movement available to it.This is unusual for a synovial joint. One reason for the limited movement is the joint’s large weight support and transfer responsibility, which requires stability and a close connection between the sacrum and the ilium.
The motions of the sacroiliac joint include:
Again the movements at the SI joint are small; also, when the sacrum moves, the hip bones may move along with it.
Relief for Mild to Severe SI Joint Dysfunction
4 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Kiapour A, Joukar A, Elgafy H, Erbulut DU, Agarwal AK, Goel VK.Biomechanics of the sacroiliac joint: anatomy, function, biomechanics, sexual dimorphism, and causes of pain.Int J Spine Surg. 2020;14(Suppl 1):3-13. doi:10.14444/6077Tsoi C, Griffith JF, Lee RKL, Wong PCH, Tam LS.Imaging of sacroiliitis: current status, limitations and pitfalls.Quant Imaging Med Surg. 2019;9(2):318-335. doi:10.21037/qims.2018.11.10Toyohara R, Ohashi T.A literature review of biomechanical studies on physiological and pathological sacroiliac joints: articular surface structure, joint motion, dysfunction and treatments.Clin Biomech (Bristol, Avon). 2024;114:106233. doi:10.1016/j.clinbiomech.2024.106233Poilliot A, Hammer N, Toranelli M, Doyle T, Gay-Dujak MH, Müller-Gerbl M.Influence of size and shape of the auricular surfaces on subchondral bone density distribution in the sacroiliac joint.J Anat. 2023;243(3):475-485. doi:10.1111/joa.13857
4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Kiapour A, Joukar A, Elgafy H, Erbulut DU, Agarwal AK, Goel VK.Biomechanics of the sacroiliac joint: anatomy, function, biomechanics, sexual dimorphism, and causes of pain.Int J Spine Surg. 2020;14(Suppl 1):3-13. doi:10.14444/6077Tsoi C, Griffith JF, Lee RKL, Wong PCH, Tam LS.Imaging of sacroiliitis: current status, limitations and pitfalls.Quant Imaging Med Surg. 2019;9(2):318-335. doi:10.21037/qims.2018.11.10Toyohara R, Ohashi T.A literature review of biomechanical studies on physiological and pathological sacroiliac joints: articular surface structure, joint motion, dysfunction and treatments.Clin Biomech (Bristol, Avon). 2024;114:106233. doi:10.1016/j.clinbiomech.2024.106233Poilliot A, Hammer N, Toranelli M, Doyle T, Gay-Dujak MH, Müller-Gerbl M.Influence of size and shape of the auricular surfaces on subchondral bone density distribution in the sacroiliac joint.J Anat. 2023;243(3):475-485. doi:10.1111/joa.13857
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
Kiapour A, Joukar A, Elgafy H, Erbulut DU, Agarwal AK, Goel VK.Biomechanics of the sacroiliac joint: anatomy, function, biomechanics, sexual dimorphism, and causes of pain.Int J Spine Surg. 2020;14(Suppl 1):3-13. doi:10.14444/6077Tsoi C, Griffith JF, Lee RKL, Wong PCH, Tam LS.Imaging of sacroiliitis: current status, limitations and pitfalls.Quant Imaging Med Surg. 2019;9(2):318-335. doi:10.21037/qims.2018.11.10Toyohara R, Ohashi T.A literature review of biomechanical studies on physiological and pathological sacroiliac joints: articular surface structure, joint motion, dysfunction and treatments.Clin Biomech (Bristol, Avon). 2024;114:106233. doi:10.1016/j.clinbiomech.2024.106233Poilliot A, Hammer N, Toranelli M, Doyle T, Gay-Dujak MH, Müller-Gerbl M.Influence of size and shape of the auricular surfaces on subchondral bone density distribution in the sacroiliac joint.J Anat. 2023;243(3):475-485. doi:10.1111/joa.13857
Kiapour A, Joukar A, Elgafy H, Erbulut DU, Agarwal AK, Goel VK.Biomechanics of the sacroiliac joint: anatomy, function, biomechanics, sexual dimorphism, and causes of pain.Int J Spine Surg. 2020;14(Suppl 1):3-13. doi:10.14444/6077
Tsoi C, Griffith JF, Lee RKL, Wong PCH, Tam LS.Imaging of sacroiliitis: current status, limitations and pitfalls.Quant Imaging Med Surg. 2019;9(2):318-335. doi:10.21037/qims.2018.11.10
Toyohara R, Ohashi T.A literature review of biomechanical studies on physiological and pathological sacroiliac joints: articular surface structure, joint motion, dysfunction and treatments.Clin Biomech (Bristol, Avon). 2024;114:106233. doi:10.1016/j.clinbiomech.2024.106233
Poilliot A, Hammer N, Toranelli M, Doyle T, Gay-Dujak MH, Müller-Gerbl M.Influence of size and shape of the auricular surfaces on subchondral bone density distribution in the sacroiliac joint.J Anat. 2023;243(3):475-485. doi:10.1111/joa.13857
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