Sacroiliac Joint

The sacrum is the large, wedge-shaped bone formed from five fused vertebrae at the base of the spine. It sits between the two hip bones and, with the coccyx below it, completes the back wall of the pelvis. On each side of the sacrum lies a sacroiliac joint, where the sacrum articulates with the ilium, the broad upper part of the hip bone. Because there is one on the left and one on the right, the sacrum is effectively slung between the two iliac bones, and the whole arrangement forms a closed bony loop known as the pelvic ring.

The joint surface itself is described as auricular, meaning ear-shaped, because its outline resembles the curve of an ear, typically forming a broad L or C shape. These matching surfaces on the sacrum and the ilium face each other and are held in intimate contact. The joint lies deep within the pelvis, low in the back just beside the dimples that can often be seen above the buttocks, and it is far more solidly anchored than joints such as the hip or the shoulder that are built for a wide range of motion.

The sacroiliac joint is anatomically distinctive because it does not fit neatly into a single category. Much of it is a true synovial joint, or diarthrosis: it has a genuine joint cavity containing a small amount of lubricating synovial fluid, its surfaces are lined with cartilage, and it is enclosed by a fibrous capsule. Yet the rear and upper portions of the junction are not a gliding cavity at all but a dense mass of ligament directly binding bone to bone, which behaves more like a fibrous joint. For this reason the sacroiliac joint is often classed as a modified or part-synovial joint, and functionally it is frequently likened to an amphiarthrosis, a joint built for stability with only slight movement, much like the pubic symphysis at the front of the pelvis.

The cartilage arrangement is itself unusual. The sacral side of the joint is covered by hyaline cartilage, the same smooth material found on most joint surfaces, while the iliac side is covered by the tougher fibrocartilage. This pairing of two different cartilage types across a single joint is rare in the body and is thought to suit a joint whose job is to bear and transmit large loads rather than to move freely.

The shape of the sacroiliac surfaces is one of the keys to its stability. In early life the surfaces are relatively flat and smooth, but from adolescence and through adulthood they progressively develop matching ridges and grooves. A raised crest tends to form along the iliac surface and a corresponding furrow along the sacral surface, so that the two bones come to interlock like complementary pieces, roughening and deepening with age.

This bony interlocking is the essence of what Vleeming and colleagues termed form closure. Form closure describes a state in which the close fit of the articular surfaces alone resists shearing, so that, in theory, no additional external force would be needed to keep the joint stable when it is loaded. A perfectly form-closed joint, however, would be so tightly locked that almost no movement or shock absorption would be possible. The sacroiliac joint therefore does not rely on shape alone. Its ridged surfaces greatly increase the friction and resistance to sliding, and this bony contribution to stability tends to increase over a lifetime as the surfaces become more coarsely textured, but the shape provides only part of the answer.

The second principle, force closure, describes how additional compression is applied across the joint to hold it firm. Because a joint held only by its shape would be either unstable or completely immobile, the body actively presses the two surfaces together. This compression is generated by the tension in the surrounding ligaments and by the pull of muscles and fascia that cross the joint. The greater the compression squeezing the surfaces together, the greater the friction between them, and the better they resist the tendency to shear apart under load.

Vleeming and colleagues identified particular muscles whose lines of pull are well placed to compress the sacroiliac joint. These include the gluteus maximus of the buttock, the latissimus dorsi of the back, the erector spinae running alongside the spine, and the biceps femoris of the hamstrings, which connects into the sacrotuberous ligament. A key arrangement is the way the gluteus maximus on one side links, through the sheet of connective tissue called the thoracolumbar fascia, to the latissimus dorsi on the opposite side. This diagonal muscular partnership, often called the posterior oblique sling, tensions the fascia across the back of the pelvis and draws the sacroiliac joint surfaces together, especially during walking, when the arms and legs swing in opposition. Force closure is thus an active, adjustable form of stability, tuned moment by moment by the nervous system according to the load being handled.

The sacroiliac joint is bound by some of the strongest ligaments in the body, and it is these, more than any muscle, that hold the sacrum to the ilia. The most substantial is the interosseous sacroiliac ligament, a thick, deep mass of fibres filling the space behind and above the joint cavity; it is regarded as the strongest ligament of the region and does much of the work of binding the two bones. Behind it lie the posterior sacroiliac ligaments, and in front the thinner anterior sacroiliac ligament, which is really a thickening of the joint capsule.

Beyond the joint itself, a set of accessory ligaments spans between the sacrum and other parts of the pelvis and adds greatly to stability. The sacrotuberous ligament runs from the sacrum down to the ischial tuberosity, the bony point one sits on, while the sacrospinous ligament runs from the sacrum to the spine of the ischium. These accessory ligaments resist the forward tipping of the sacrum and are important participants in the force-closure system, particularly the sacrotuberous ligament, into which the hamstring and gluteal tissues connect. The overall ligamentous arrangement is so robust that in a healthy adult the sacroiliac joint scarcely moves under everyday loads.

Although the sacroiliac joint is built for stability, it is not completely rigid, and its slight movement matters. Careful measurement studies, most famously the roentgen stereophotogrammetric work of Sturesson and colleagues, have shown that the movement available is very small: on the order of a couple of degrees of rotation and around a millimetre of translation, mostly occurring about a side-to-side, or transverse, axis. Reviews of the literature broadly agree that flexion and extension amount to only a few degrees, with even smaller amounts of twist and side-tilt.

This small movement is nonetheless functionally important. It allows the sacrum to make fine adjustments as the trunk and legs move, contributes to the pelvis acting as a shock absorber, and permits the subtle shape changes of the pelvic ring needed during activities such as walking and, in women, childbirth. The joint is therefore best understood not as a hinge that moves through a range, but as a stable structure that yields just enough to distribute forces smoothly and to prevent the transmission of jarring shocks directly up the spine.

The principal movement of the sacroiliac joint is described in terms of nutation and counternutation, words that refer to the tilting, or nodding, of the sacrum between the iliac bones. In nutation, the top of the sacrum tips forwards and downwards while the lower end and the coccyx move backwards, relative to the ilia. In counternutation, the reverse happens: the top of the sacrum tips backwards and upwards while the coccyx swings forwards.

These movements are closely bound up with stability. Nutation is the more stable, close-packed position of the joint. As the sacrum nutates, it winds up and tightens several of the key ligaments, notably the interosseous, the short posterior sacroiliac and the sacrotuberous ligaments, which increases the compression across the joint and locks it more firmly. For this reason nutation is regarded as the joint preparing itself for load: it tends to occur naturally when the body takes up weight, as in standing, sitting down or lifting. Counternutation, by contrast, slackens these ligaments and is the less stable, more loosely packed position, associated with lying down and unloading. During walking, the sacrum has been observed to nutate on the side taking weight during the stance phase and to counternutate as that leg swings through, an elegant rhythm that keeps the loaded side braced.

A helpful way to picture the sacrum's role is to compare it with the keystone of a Roman arch. In an arch, the central wedge-shaped stone at the top is pushed down by the weight it carries, and because it is wedged between the stones on either side, that downward force jams it ever more tightly into place rather than pushing it out. The sacrum sits in a comparable way between the two iliac bones. It is broader at the top than the bottom and is wedged between the ilia, so that the downward load coming from the spine tends to drive it more snugly into the pelvic ring, tightening rather than loosening the sacroiliac joints. This is why the joint is often said to become more stable, not less, the more it is loaded.

It should be noted that this classic keystone image, while widely taught, is not universally accepted. Some authors argue that because the sacrum is actually wider at the front than the back at the level of the joint, downward load is partly resisted by the sacrum being suspended from the ilia by the strong posterior ligaments, more like a load hung on straps than a wedge jammed into an arch. Both descriptions capture the same underlying truth: the pelvic ring is designed so that weight-bearing increases, rather than reduces, the grip on the sacrum, whether by bony wedging, ligamentous suspension, or a combination of the two.

Bringing form closure and force closure together produces what Vleeming and colleagues called self-bracing, sometimes described as a self-locking mechanism. The idea is that the interlocking shape of the surfaces, form closure, and the active compression from ligaments, muscles and fascia, force closure, reinforce one another. When the body loads the pelvis, the sacrum nutates, the ligaments tighten, the muscles of the slings engage, and the compression across the joint rises. Higher compression combined with the ridged surfaces produces greater friction, and greater friction means the joint resists shearing more effectively just when the demand upon it is greatest. In effect, the joint braces itself in proportion to the load it is being asked to carry.

This self-adjusting quality is why a healthy sacroiliac joint can remain stable across such a wide span of activity, from quiet standing to heavy lifting to running, without either seizing up or giving way. The system depends on all its parts working in concert: the shape of the bones, the integrity of the ligaments, the coordinated timing of the muscles, and the tension in the fascia. It is a balance between enough stability to transfer load safely and enough give to absorb shock and allow the pelvis to adapt.

The sacroiliac joint sits at a crucial crossroads in the skeleton. Above it, the entire weight of the head, arms and trunk bears down through the spine onto the sacrum. Below and to each side, the pelvis connects through the hip joints to the legs. The sacroiliac joints are the twin gateways through which this load must pass in both directions.

Travelling downwards, the weight carried by the spine arrives at the top of the sacrum and is split to left and right, passing across the two sacroiliac joints into the iliac bones, then down through the hip sockets into the thigh bones and on to the ground. Travelling upwards, the ground-reaction forces generated each time a foot strikes the ground during walking or running are transmitted from the legs into the pelvis and must cross the sacroiliac joints again to reach the sacrum and the spine above. The joint therefore handles a two-way traffic of forces and, by yielding slightly and by the self-bracing mechanism, helps to smooth and dampen these forces rather than letting them pass as sharp jolts. In this sense the sacroiliac joints act as load-distributing and shock-absorbing hubs linking the axial skeleton to the lower limbs.

Pregnancy alters the mechanics of the sacroiliac joint in ways that illustrate how finely the balance between stability and mobility is tuned. Hormonal changes, in particular the hormone relaxin together with the effects of oestrogen and progesterone, increase the laxity of the pelvic ligaments during pregnancy. This softening allows the pelvic joints, including the sacroiliac joints and the pubic symphysis at the front, to loosen and become slightly more mobile, which helps the pelvis to widen and adapt in preparation for childbirth.

Greater mobility, however, comes at the cost of some of the joint's usual stiffness, so the force-closure system of muscles and fascia takes on a larger share of the work of keeping the joint stable. Added to this are the mechanical demands of a growing uterus: increasing weight, a shift in posture and centre of gravity, and altered muscle balance. Studies have noted that the pattern of laxity matters as much as its degree, with uneven, asymmetrical loosening between the two sides being of particular interest. These pregnancy-related changes are a normal physiological adaptation, and the reasons they can sometimes be associated with discomfort are beyond the scope of this educational description.

The sacroiliac joint is a masterpiece of structural compromise. It is built overwhelmingly for stability rather than movement, yet it retains just a few degrees of nutation and counternutation that allow it to distribute forces and absorb shock. Its stability rests on two collaborating principles: form closure, from the interlocking ridged shape of the bony surfaces, and force closure, from the active compression supplied by the powerful ligaments and by the muscles and fascia of the slings. The sacrum sits like a keystone within the pelvic ring, so that load tends to tighten rather than loosen the joint, and the whole system self-braces in proportion to the demand placed upon it. Positioned at the junction between the spine and the legs, the sacroiliac joints carry weight downwards into the pelvis and pass ground-reaction forces back upwards, adapting even to the special demands of pregnancy. Understanding this mechanical design is the foundation for understanding the joint in health.

The sacroiliac joint is the paired junction on either side of the pelvis where the triangular sacrum, the base of the spine, meets the ilium, the large wing-shaped bone of the pelvis. It is one of the least mobile and most heavily reinforced joints in the human body. Its central task is not movement but the safe transfer of load: it channels the weight of the head, arms and trunk down from the spine into the pelvis and legs, and it passes the reaction forces from the ground back up again during standing, walking and running. To do this it relies on two complementary principles described by the anatomist Andry Vleeming and colleagues. The first is form closure, the way the ridged, interlocking shape of the bones themselves resists movement. The second is force closure, the way muscles, ligaments and fascia actively compress the joint surfaces together so that they grip and do not shear. Together these produce a self-bracing, self-locking mechanism. The sacrum sits within the pelvic ring rather like the keystone of an arch, and the joint permits only a few degrees of a small tilting movement known as nutation and counternutation. This page explains, in strictly educational terms, how the sacroiliac joint is built and how it works mechanically. It is not medical advice.

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This information is for general patient education and is not a substitute for an individual consultation. Always seek personalised advice from a qualified spinal specialist.