Sacroiliac Joint

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. It's primary purpose is for stress-relief. 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.Bogduk, Nikolai. Clinical and radiological anatomy of the lumbar spine. Edinburgh: Elsevier/Churchill Livingstone, 2012
==Structure==
===Bones===
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. This bony locking prevents downward gliding of the sacrum under body weight.
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 sacrum is aligned obliquely between the ilia, with the anterior end leaning forward. With longitudinal load, the sacrum tends to tilt forwards and downwards, rotating around Bonnaire's tubercle. The wedge shape of the sacrum opposes this tendency. With forward sacral rotation, the wider posterior end of S1 moves inferiorly and tend to cause separation of the iliac. While, at the same time the wider anterior end of the S3 segment moves upwards and also tends to separate the ilia. Clamping of the sacrum between the ilia prevents these movements from occurring with the engaged corrugations preventing downward sliding. With this prevention of separation, the wedge shape of the sacrum does not allow forward rotation. The sacroiliac joint ligaments are vital to keeping the ilia locked up against the sacrum.
File:Pelvic Girdle.jpg|Bony landmarks
File:Innominate bony landmarks.jpg|Bony landmarks
===Cartilage===
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. Bogduk states that the sacral side is hyaline cartilage 1-3mm thick, and the iliac side macroscopically looks like fibrocartilage but is microscopically hyaline < 1mm thick but with a cell density higher than the sacral cartilage. The sacral cartilage is even thicker in women. The subchondral plate of the ilium is 50% thicker than that of the sacrum. One hypothesis for these differences is that the sacral cartilage is designed for transmission of load from the spine to the pelvis, while the iliac cartilage is designed to absorb the load. 
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. The anterior aspect of the joint is a synovial joint, while the posterior aspect is more of a syndesmosis with multiple myoligamentous attachments.
===Ligaments===
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. 
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).
Ligament 	Description 	Function
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. The ASL is a thickening of the anterior capsule. 	Prevents anterior diastasis of the SIJ
Interosseous sacroiliac ligament 	This is the most important ligament of the sacroiliac joint. It is composed of dense, thick short collagen fibres. It fills the syndesmotic joint space as the deepest layer posteriorly, connecting the ligamentous surface of the sacrum with that of the ilium. It is only present in the cranial joint aspect. It attaches to the iliac tuberosity. 	Binds the ilium strongly to the sacrum, thus securing the bony interlocking mechanism. Grays's anatomy recognises a superior superficial portion called the short posterior sacroiliac ligament the function of which is to prevent posterior diastasis of the SIJ.
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. It is superficial to the ISL, and medial to the LPSL.	
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. It is closely approximated with the erector spinae muscles, the posterior layer of the thoracolumbar fascia, and parts of the sacrotuberous ligament. The medial cluneal nerves can be associated with it. It is a potential pain generator. 	The LPSL is under tension during SIJ counternutation (posterior rotation), and is slack during nutation, and therefore it's function is to prevent backward rocking (counternutation) of the sacrum with respect to the ilium. It forms a functional link between legs, spine, and arms.
Iliolumbar ligament 	Runs from the transverse process of L5 to the posterior part of the iliac crest. It may arise from L4. 	Prevents horizontal translation of L5
Sacrospinous and sacrotuberous ligaments	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). 	These two ligaments prevents upward tilting of the lower end of the sacrum (nutation) by anchoring it to the ischium. The SSL is the main ligamentous structure that restricted sacral nutation.
The capsule is difficult to discern due to the multiple thick ligaments. The posterior capsule is rudimentary or absent, while the anterior capsule is a thickening of the anterior sacroiliac ligament. 
File:Posterior pelvis ligaments.jpg|Posterior ligaments
File:Posterior sacroiliac joint.png|Posterior ligaments
File:Anterior sacroiliac joint.png|Anterior ligaments
==Age Changes==
The sacroiliac joint undergoes changes with age. Young joints have a flat surface and are able to glide in all directions. A bony locking mechanism develops restricting movements to nodding movements along the longitudinal axis of the curved surface. The range of movements decreases further with age with the development of fibrous ankylosis and osteophytosis. 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 joint space decreases with age, and becomes stiffer and less effective at shock absorption.
Age 	Changes
Fetal Development	The future joint is apparent during the second month of gestation. It is a strip of mesenchyme between the cartilaginous future ilia and sacrum. Full cavitation is achieved by the seventh month. Synovial membrane development occurs by 37 weeks. In embryos the articular surface is smooth and flat.
First 10 years 	The joint enlarges but its surfaces remain flat. The anterior capsule thickens.
10-20 	Corrugation starts to develop. This starts with a depression along the articular surface of the sacrum, with a reciprocal ridge along the ilium.
Adulthood 	Prominences of the iliac ridge increases. The articular cartilage surfaces show superficial fibrillation and erosion, especially on the iliac side. Osteophytes appear. The joint margins, capsule, and synovium become thicker and more fibrous.
40-50s 	More prominent prominent changes of above. Cartilage becomes thinner, and becomes fibrillated and eroded. Debris and fibrous tissue connecting the bones fill the joint cavity. Sinuous corrugations along the joint appear by the 50s.
70s 	Large osteophytes with interdigitation. Plentiful intra-articular fibrous adhesions. Cartilage is thinner to <1mm on the sacrum, and <0.5mm on the ilium.
==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. Bogduk in 2012 does not make a definitive claim as to the innervation of the SIJ, rather outlining the various findings in different studies. 
==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, 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.
It is very difficult to realistically assess and study sacroiliac joint movement. This is because studies on cadavers do not necessarily correlate with living joints, and the complex structure of the joint makes assessment in vivo very difficult radiographically. Accurate assessment requires multiple three-dimensional implanted devices or rigidly fixed external devices in living individuals. Studies using these methods have found very small ranges of motion of rotation ranging from 0-2°. The same type of trunk movement can cause the sacrum to move 1° forward in some individuals and 1° backwards in just as many others. Therefore the true mean range of motion is 0°, and although sacroiliac joints move, their movements are irregular. More recent studies examining movement in a variety of postures and movements show a range of motion of only about 1°.
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.
;Terminology
==Recommended Articles==
*Review by Poilliot et al, free access 
==Videos==
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==References==
Category:Sacroiliac Joint
Category:Joints
Category:Joints of the Spine