Sacroiliac joint

Function

Where the spine meets the pelvis

The sacroiliac joint is where the sacrum meets the ilium, so it links the spine to the pelvis¹. The two sacroiliac joints are the largest joints of the axial skeleton (the bones of the head and trunk)². The first three segments of the sacrum typically form the joint, and its bony shape varies widely between people⁵.

In adults the joint surface is shaped like an ear, with a short upper limb and a longer lower limb⁵. Cartilage (a firm, rubbery tissue, softer than bone) covers both surfaces⁶. The lower portion of the joint is synovial, with a fluid-filled space between the bones that lets them move freely⁵. The top of its upper limb is more fibrous⁵.

Engraving of the right half of the pelvis and the lower spine seen from the front. The anterior sacroiliac ligament crosses the joint between the sacrum and the ilium as a sheet of thin bands, with the iliolumbar and lumbosacral ligaments above it. Lower down, the sacrospinous and sacrotuberous ligaments run from the side of the sacrum to the ischium, and the pubic bones meet at the interpubic fibrocartilage. — Sacroiliac joint and pelvic ligaments from the frontHenry Gray, Anatomy of the Human Body (1918), Fig. 319 (after Quain). Public domain, via Wikimedia Commons. (source, Public domain)

How it carries the trunk

A 1918 anatomy text gives the pelvic girdle’s most important mechanical job as passing the weight of the trunk and arms to the legs⁶. Body weight falls on the sacrum at the lumbosacral joint, the joint of the lower back and the sacrum⁶. The back arch of the pelvis then carries it⁶.

The sacrum is wider at the top than the bottom and wider in front than behind⁵. It wedges between the two ilia like a keystone⁵. That shape is adapted to resist shearing, the sliding of one surface across another, under vertical compression⁵. Body weight drives this wedge down and tends to push the hip bones apart⁶. The sacroiliac and iliolumbar ligaments, and the ligaments of the pubic symphysis at the front of the pelvis, resist that spreading⁶.

Rough surfaces and squeezing forces

An examination of 200 preparations found the bony joint surfaces smooth until puberty, with ridges and grooves appearing later⁵. Sectioned human joints show cartilage-covered ridges and depressions that match across the two surfaces, more pronounced in men than in women⁷. In a laboratory study, surfaces with both a coarse texture and such ridges had high friction⁸. A review of the joint’s anatomy states that these interlocking ridges contribute to the joint having the highest friction of any freely movable joint⁵.

The same review describes two ways a joint can hold steady⁵:

Form closure is a theoretical joint whose surfaces fit so closely that it is stable with no extra force⁵. A sacrum that fitted so perfectly would make movement practically impossible⁵.
Force closure needs a sideways squeezing force plus friction to withstand vertical load⁵. That force comes from tension in ligaments, fasciae and muscles, and from the push of the ground⁵.

The ligaments that bind it

The joint’s core ligaments (tough bands that hold bones together) lie at the front, at the back and between the bones⁵. They are numerous and strong⁵. The sacrotuberous, sacrospinous and iliolumbar ligaments are strong accessory ligaments⁵.

Ligament	What the sources report
Anterior sacroiliac	Numerous thin bands that join the front of the sacrum’s side to the edge of the iliac joint surface⁶
Interosseous	The strongest of the joint’s ligaments; it fills the space behind and above the synovial part⁵
Posterior sacroiliac	Lies in a deep hollow between the two bones at the back; a 1918 text calls it their chief bond⁶
Long dorsal	The most superficial of the joint’s ligaments at the back⁵
Sacrotuberous	A systematic review concluded it may limit forward tilting of the sacrum but appears to add little to pelvic stability⁹
Iliolumbar	In 12 embalmed cadaver specimens, cutting both significantly increased the joint’s forward and backward mobility¹⁰

Engraving of the right half of the pelvis and the sacrum seen from behind. The short and long posterior sacroiliac ligaments run from the back of the sacrum to the ilium, covering the joint, with the iliolumbar ligament above. Below them the sacrotuberous ligament fans down from the sacrum to the ischial tuberosity, and the sacrospinous ligament runs to the ischial spine. — Sacroiliac joint and pelvic ligaments from behindHenry Gray, Anatomy of the Human Body (1918), Fig. 320 (after Quain). Public domain, via Wikimedia Commons. (source, Public domain)

How far it moves

The ilium and sacrum rotate against each other, mainly about a side-to-side axis⁵. Forward nodding is called nutation and backward nodding counternutation⁵. Nutation increases in load-bearing positions such as standing and sitting⁵. It tightens most of the sacroiliac ligaments and raises compression across the joint⁵.

One literature review put the range at about 3 degrees in flexion and extension, 1.5 in axial rotation and 0.8 in side bending². Two studies measured the movement in patients with an X-ray method called radiostereometric analysis^11,12.

Who was measured	Movement found
25 patients with sacroiliac joint disorders	Rotation averaging 2.5 degrees and sliding averaging 0.7 mm between positions¹¹
11 patients with long-lasting, severe pelvic girdle pain	0.5 degrees of total rotation in each joint during a single-leg stance, and no sliding¹²

In the 25 patients, movement did not differ between painful and painless joints¹¹. In radiostereometric studies of patients with pelvic girdle pain, average mobility was about 40% lower in men than in women⁵. A review of the joint’s anatomy states that during pregnancy the joint’s fibrous structures loosen under the hormone relaxin, which increases mobility^5,6.

What the muscles add

The erector spinae and multifidus, muscles that run along the spine, help pull the sacrum into nutation⁵. The gluteus maximus (the large muscle of the buttock) pulls the sacrum sideways into the ilium⁵. In six healthy women, joint stiffness rose significantly when single muscles contracted without moving the joint¹³. The rise was clearest for the erector spinae, the gluteus maximus and the biceps femoris, one of the hamstrings¹³.

The load figures that follow come from a computer model, not from measurements in people¹⁴. In that static 3-D model of standing, trunk load was balanced mainly by vertical shear force at the sacroiliac joints¹⁴. When the model then forced that shear down by another 20%, on top of an earlier reduction, joint compression rose by 400%¹⁴. It did so by switching on the transversus abdominis, the deepest flat abdominal muscle, and the pelvic floor muscles¹⁴.

Breakdown

Wear is common on scans of joints with no known problem

In 373 pelvic CT scans of patients with no past low back or pelvic girdle pain, 65.1% showed sacroiliac joint degeneration³. In the same study this degeneration (gradual wear and breakdown) became steadily more common with age, reaching 91% of subjects in their eighties³. In CT scans of 818 patients without sacroiliac joint disease, 45.7% had sclerosis (hardening of bone) and 46.8% had osteophytes¹⁵.

A review of normal imaging findings states that such changes can appear as early as the twenties and probably result from strain¹⁶. Among 2,845 skeletal pelves, bone bridged across the joint in 12.27% of males and 1.83% of females¹⁷. A review of anatomical studies concluded that fusion inside the joint seldom occurs⁵.

What MRI shows in healthy and active people

Sacroiliitis is inflammation of the sacroiliac joint¹⁸. It is the earliest sign of the disease spondyloarthritis, and its most common one¹⁸. MRI, a scan that shows soft tissue in detail, has also found changes at the joint in healthy and active people^16,19.

Who was scanned	Share with the finding
47 healthy volunteers with no back pain history	23.4% had scans that met a standard MRI definition of sacroiliitis¹⁹
24 frequent runners	12.5% had scans that met the same definition¹⁹
20 healthy hobby runners	35% had bone marrow oedema on MRI meeting a published set of criteria, ASAS 2009¹⁶
22 ice hockey players	41% had bone marrow oedema meeting those criteria¹⁶

In the study of volunteers and runners, no healthy volunteer, runner or patient with chronic back pain had deep bone marrow oedema¹⁹. It was found in 42 of 47 patients with axial spondyloarthritis¹⁹.

Injury, and who reports pain

Among 54 patients whose sacroiliac pain was confirmed by injection, 44% reported a traumatic event beforehand²⁰. Another 21% reported a cumulative injury, and 35% reported a spontaneous onset²⁰.

Figures have been published for three groups, which the sacroiliac joint pain page covers in more detail^21,22,23.

After spinal surgery. A meta-analysis (a study pooling earlier studies) of 12 studies of spinal surgery patients put the incidence of new sacroiliac joint pain at 9.40%²¹.
Athletes. A systematic review of 13 studies of athletes found that reported prevalence (the share who have it) of sacroiliac joint pain or dysfunction varied widely²². Its authors concluded that an accurate prevalence cannot be reported²².
Pregnancy. A European guideline group put the share of pregnant women with pelvic girdle pain at any one time at about 20%²³.

Recovery

The joint reshapes under load

A review of sacroiliac anatomy describes the roughening of the joint surfaces as an adaptation for stability⁵. The gain in body weight, especially after puberty, possibly drives it⁵.

After childbirth

In an MRI study of 35 women, 77% showed sacroiliac bone marrow oedema within 10 days of giving birth⁴. After 6 months, 46% still showed it⁴. In the same study, 60% met an MRI definition of sacroiliitis immediately after birth, 15% at 6 months and 12% at 12 months⁴.

A second study followed 19 women for 5 years after an uncomplicated vaginal delivery²⁴. Six of the 19 had bone marrow oedema on MRI at 5 years²⁴. No significant development of structural lesions (lasting changes to the bone or joint) was seen after a single delivery²⁴.

After six weeks of hard training

One study scanned 22 military recruits without back pain before and after 6 weeks of intense, standardised physical training²⁵. Before training, 40.9% already had bone marrow oedema at the sacroiliac joints, and 50.0% had it afterwards²⁵. The change was not statistically significant²⁵. Structural lesions were present in 6 recruits at both time points²⁵.

Connections

The joint links the sacrum, at the base of the spine, to the ilium of the pelvis¹.

Several muscles and sheets of tissue meet its ligaments^6,26:

The whole back surface of the sacrotuberous ligament gives origin to the gluteus maximus⁶.
A 1918 anatomy text describes that ligament’s lower border as continuous with the tendon of the biceps femoris, one of the hamstrings⁶. A systematic review found the extent of these muscle links unclear⁹.
A dissection study found the long dorsal ligament closely related to the erector spinae²⁶. It was also closely related to the thoracolumbar fascia, a sheet of tough tissue over the back²⁶.

A narrative review describes the front of the joint as supplied by branches of the lumbosacral plexus, a nerve network, and the superior gluteal nerve¹⁸. At the back, it describes side branches of the sacral nerves, S1 to S4, as the supply¹⁸. One study took ligaments from 5 cadavers (bodies donated for study) with no history of sacroiliac pain²⁷. The anterior and interosseous ligaments held nerve fibres carrying two pain-related markers²⁷.