Sacroiliac Joint

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The sacroiliac joint, formed at the junction of the bilateral iliac wings with the sacrum, is a common source of acute and chronic low back pain. It has a complex structure.

The sacrum sits at the base of the vertebral column where all longitudinal forces are ultimately transmitted. It is situated between the two iliac bones, making up the posterior wall of the pelvis, and this allows forces from the vertebral column to be transmitted sideways into the pelvis and lower limbs. Conversely, forces from the lower limbs can be transmitted up from the lower limbs into the pelvis, sacrum, and spine.

Bony and Cartilaginous Structures

The sacroiliac joint is around 1-2mm wide and is formed within S1, S2, and S3. The concave sacral surface joins with the convex ilium surface. The sacral surface is irregular, with ridges, prominences, troughs, an depressions. These are matched by reciprocal contours on the ilium giving an interlocking mechanism. This gives the joint the highest friction coefficient in the whole body. These contours give the sacroiliac joint a sinuous appearance in the frontal view. A notable contour is a major depression on the sacral surface, on the S2 segment. This receives a major prominence from the ilium known as Bonnaire's tubercle.

The articular surface of the sacrum is twisted. At S1, the dorsal edge extends more laterally than the ventral edge; the posterior width of the sacrum is greater than its anterior width. While at S3 the ventral edge extends more laterally than the dorsal edge; the anterior width is greater than its posterior width. The sacrum is therefore wedge shaped in transverse view but in opposite directions at opposite ends of the sacroiliac joint.

The joint space decreases with age, and becomes stiffer and less effective at shock absorption.

The sacroiliac joint has mostly been described as a diarthrodial synovial joint. However it has also been reported as being an amphiarthrosis, and this appearance is more common with age. The interosseous portion differentiates it from other diarthrodial joints.

Reports disagree as to the cartilage lining of the joint, with some authors concluding that the iliac side is lined by fibrocartilage and the sacral side lined by hyaline cartilage. Other authors state there is a mixture on both sides. The sacral side is 2-3 times thicker than the iliac side, and the sacral side is even thicker in women. The articular surfaces display degenerative changes such as erosions and cortical irregularities from as young as 20 years old, more prominent on the iliac side.

The anterior aspect of the joint is a synovial joint, while the posterior aspect is more of a syndesmosis with multiple myoligamentous attachments. Joint stability is provided by multiple ligaments and muscles. The gluteal muscles form some of the connections between the pelvis and hip, and can be painful in those with sacroiliac joint dysfunction.

Ligaments

Several ligaments help maintain stability and reduce the mechanical stress applied through the joint. The capsule is often indistinguishable from the surrounding ligaments. The sacroiliac joint consists of intrinsic (capsular) and extrinsic (accessory) ligaments. The intrinsic ligaments include the anterior sacroiliac ligament (ASL), the interosseous sacroiliac ligament (ISL), the posterior sacroiliac ligament (PSL), and the long posterior sacroiliac ligament (LPSL). The extrinsic ligaments include the iliolumbar ligament (ILL), the sacrotuberous ligament (STL), and the sacrospinous ligament (SSL).

Anterior sacroiliac ligament

This thin ligament is a thickening of the anterior and inferior aspects of the joint capsule. It passes from the anterior aspect of the sacrum, crosses the SIJ, and inserts on the anteromedial aspect of the ilium near the articular margin.

Posterior sacroiliac ligamentous complex

The fibres merge with other ligamentous structures. The LPSL attaches to the inferior lateral sacral crest. The PSL attaches to variable sites on the sacrum. The ISL is deep to the PSL. The LPSL is lateral to the PSL.

Interosseous sacroiliac ligament

This fills the syndesmotic joint space as the deepest layer posteriorly. It is only present in the cranial joint aspect. It attaches to the iliac tuberosity.

Posterior sacroiliac ligament

Found posterior to the sacroiliac joint. There are two groups, the cranial and caudal groups. The cranial group runs from the lateral edge of the posterior aspect of the sacral and attaches to the ridge on the ilium. Superiorly it merges with the ASL. The caudal group is the true PSL, and passes from the iliac tuberosity to the first sacral segment, lateral sacral tubercle, and the base of the articular process. Some fibres attach to the PSIS.

Long posterior sacroiliac ligament

Runs from the PSIS to the third or fourth lateral sacral tubercles. It is penetrated by S2-S4 dorsal sacral rami. The LPSL is under tension during SIJ counternutation (posterior rotation), and is slack during nutation. It is closely approximated with the erector spinae muscles, the posterior layer of the thoracolumbar fascia, and parts of the sacrotuberous ligament. It forms a functional link between legs, spine, and arms. The medial cluneal nerves can be associated with it. It is a potential pain generator.

Iliolumbar ligament

Runs from the transverse process of L5 to the posterior part of the iliac crest. It may arise from L4.

Sacrospinous and sacrotuberous ligaments

These two ligaments prevent sacral tilting. the SSL runs from the anterolateral aspects of the caudal 2 sacral segments, and the first coccygeal segment, along to the ischial spine. The STL runs from the PSIS to the iliac border to the caudal sacral and coccygeal edge, and also to the lateral sacral crest, and inserts on the ischial tuberosity. There are various muscular and fascial attachments as well (erector spinae, piriformis, gluteus maximus, biceps femoris, and thoracolumbar fascia). The SSL is the main ligamentous structure that restricted sacral nutation.

Innervation

The anterior aspect of the joint is innervated by L4-S2, sometimes L3, and the sacral plexus. The posterior aspect of the joint is supplied by a plexus that is formed by the lateral branches of the posterior rami of L5 to S4. The posterior innervation varies from person to person, side to side, and level to level.The superior gluteal nerve may contribute to both anterior and posterior innervation.

Biomechanics

The sacrum forms a joint with each ilium bone. The purpose of the sacroiliac joint has been a topic of debate. Some authors have dismissed the joint as having little or no movement, while others have stated that it has important primary movements that should be assessed like any other joint in the body. The truth lies somewhere in the middle. The sacroiliac joint does move , but the angles are very small (between 1-3° in all three axes), and there are no muscles that can allow the joint to move actively. Structurally and functionally it can be considered to be more like the intertarsal joints of the foot with the joints only able to be moved passively.

The sacroiliac joint is a stress-relieving joint. It is in a strategic site, at a location of maximal torsional stress. Without the joint, the fused pelvic ring would be susceptible to very large twisting forces around its transverse axis especially with walking. In elderly females, where there is a relatively ankylosed sacroiliac joint and reduced bone density, sacral insufficiency fractures can occur. Here, the torsional forces, which are usually buffered by the sacroiliac joint, are transferred to the sacrum, which then fractures. Tellingly, the fractures run vertically through the ala of the sacrum parallel to the sacroiliac joint. The sacroiliac joint.

The sacroiliac joint must be able to manage twisting forces imparted on it by the lower limbs, and have those forces be absorbed into the ligaments reducing the tendency for fracture. In the parasagittal plane the anti-torsion functions are achieved through a gliding planar surface. The sacroiliac joint must also be strong and stable, allowing force transmission from the spine to the lower limbs. This is achieved through a bony locking mechanism sparing the ligaments from static and longitudinal loads. It therefore retains both a locking mechanism for vertical loads and the ability to absorb twisting forces for twisting forces. The term form-closure refers to the vertical support of load, and force-closure refers to the horizontal support of load.

Injury can be caused by axial loading followed by abrupt axial rotation. Repetitive torsional forces can cause inflammation. Pathology can be present in any combination of the sacroiliac joint synovium, capsule, or ligaments, which may lead to hypo- or hypermobility.

Sacral nutation: anterior tilt of the superior sacrum. Limited primarily by the SSL

Sacral counternutation: posterior tilt of the superior sacrum. Limited primarily by the LPSL, and also by the thoracolumbar fascia and latissimus dorsi.

Gallery

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  Bony landmarks

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  Posterior ligaments

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  Posterior ligaments

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  Anterior ligaments

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• Review by Poilliot et al, free access