The sacroiliac joint is responsible for many different pain presentations. Clinical testing and the effects of joint manipulation has received mixed reviews.
A full understanding of the anatomy and mechanics of the sacroiliac joint can give a better understanding of the mechanical injuries affecting the joint as well as the different manipulative and diagnostic procedures used in sacroiliac joint pain management.
The article is separated into:
Review of the structures: capsular, ligamentous, myofascial and neurological.
Movements of the sacroiliac joint: movement in health and injury.
Mechanisms of sacroiliac joint dysfunction.
Entrapment neuropathies in the pelvis causing sacroiliac joint pain.
Diagnostic testing for the sacroiliac joint.
Review of the structures: capsular, ligamentous, myofascial and neurological
The sacroiliac joint is located between the sacral and iliac articular surfaces. Zou et al (2015) measured the width of the joint space at 2–3-mm which declines with age.
The joint surface is flat in childhood but then becomes irregular in adulthood. The curves and irregularities are reciprocal e.g. a convexity on the iliac surface is met with a concavity on the sacral surface.
The joint consists of (1) a syndemosis (fibrous) part and (2) a synovial part.
(1) Syndemosis (fibrous) part: this lies in the superior two-thirds of the sacroiliac joint parallel to S1-2 and is composed of the interosseous sacroiliac ligaments (Zou et al 2015).
(2) Synovial part: this lies in the inferior one-third parallel to S3–S4 levels (Zou et al 2015).
Brun-Rosso et al (2016) found that under vertical loads (e.g. standing) the sacrum rotates around a horizontal axis (nutation) with some accompanied translation movement. This horizontal axis goes through the interosseous ligaments.
This ligament exerts the greatest strain in restricting movement (Eichenseer et al 2011). If the movement in the superior two thirds of the joint has to be relatively restricted so the interosseous ligament can provide this stable horizontal axis then the movement will be greater farther away from the axis in the inferior one third of the joint.
This would explain the presence of the synovial part of the joint in the lower third (Zou et al 2015). It would also explain how stiffness in the sacrotuberous and sacrospinous ligament increases the range of motion of the sacrum by pulling on this distal mobile area (Hammer et al 2013).
The sacrum is covered by hyaline cartilage which is thicker anteriorly than posteriorly in adults. The hyaline cartilage on the ilium is thinner. This cartilage has been proposed to have a mechanical effect on restricting sacroiliac joint movement (Hammer et al 2013).
Both the anterior and posterior sacroiliac joint capsules attach close to both articular margins of the joint.
(1) Anterior sacroiliac joint capsule.
The anterior sacroiliac joint capsule is relatively thin. It relates closely to the nerve fibers of the lumbosacral trunk (L4 and L5 nerve roots) and the nerve bundles of the obturator nerve (Vleeming et al 2012).
Soft tissues that blend with the anterior sacroiliac joint capsule include:
Piriformis (Solonen, 1957 & McCory and Bell 1999).
Iliacus: Solonen (1957) claimed that the Iliopsoas muscle partly originates from the anterior sacroiliac joint capsule although the author was probably refering to the Iliacus. More recently Standring (2017) claimed the Iliacus originated from the anterior sacroiliac ligament but not the joint capsule.
The sacrospinous ligament (Stout 2010).
Otsuru et al (2017) found a ureteric entrapment inside the sacroiliac joint.
(2) Posterior sacroiliac joint capsule.
The posterior sacroiliac joint capsule is discontinuous (Fortin et al 1999) and is closely related to the nerves exiting the sacral foramen.
(1) Iliolumbar ligaments.
The iliolumbar ligaments run from L5 to the sacrum and ilium. The iliolumbar ligaments blends with the thoracolumbar fascia and anterior sacroiliac joint capsule.
Mens et al (1999) illustrated the functional relationship of the shared anatomy between the iliolumbar ligament, ilium, and anterior joint capsule. “As the right innominate rotates anteriorly and produces a caudad shift of the pubis the right illiolumbar ligament pulls L4-5 into left rotation and right lateral flexion”.
Snijders et al (2008) found contrary to this describing the iliolumbar ligament as being stretched in counternutation of the pelvis and flexion of the lumbar spine (and Miyasaka et al 2000).
(2) Lumbosacral ligament
The anatomical attachments of the lumbosacral ligament to L5 and the sacrum can vary.
L5 attachments can be:
- Antero-inferior aspect of the L5 transverse process (costal process) (Hanson & Sorensen 2000).
- L5 vertebral body and transverse process of L5 (Protas et al 2017).
- L5 vertebral body (Protas et al 2017).
- L5 pedicle (Hanson & Sorenson 2000).
Sacrum attachments can be:
- Ala of the sacrum (Hanson & Sorenson 2000).
- Sacral promontory (Hanson & Sorenson 2000).
The lumbosacral ligament can, in some cases, be attached by a thin fascia to the iliolumbar ligament, ventral sacroiliac ligament and/or L5 nerve root (Hanson & Sorenson 2000).
Hanson & Soreen (2000) determined from the position of the ligament its primary mechanical function is to restrict contralateral lateral flexion and probably also extension.
Lumbosacral tunnel syndrome
The lumbosacral ligament forms, with its attachments to L5 and the sacrum, an osteofibrotic tunnel as an extension of the intervertebral foramen (Nathan et al 1982). Although this tunnel is not a constant finding (Hanson & Sorenson 2000).
The 5th lumbar nerve root passes through the L5-S1 intervertebral foramen and through this tunnel formed by the ala of the sacrum posteriorly and the lumbosacral ligament anteriorly (lumbosacral tunnel).
A branch of the 4th lumbar nerve root passes in front of the lumbosacral ligament to join the 5th below the ligament to form the lumbo-sacral trunk.
The sympathetic ramus communicans to the L5 root always penetrates the lumbosacral ligament at its superior border and reaches the nerve inside the lumbosacral tunnel. Protas et al (2017) found the piercing of the rami communicants through the lumbosacral ligament forms a tethering point between the L5 ventral ramus and adjacent sympathetic trunk.
Protas et al (2017) defined lumbosacral tunnel syndrome (LSTS) as a narrowing of the lumbosacral tunnel leading to compression of the L5 nerve root against the ala of the sacrum, causing radiculopathy.
To complicate the mechanical predisposition of the L5 nerve root to injury Kelihues et al (2001) found the perineurium of the L5 nerve root to have adhesions to the periosteum of the sacrum. This made the nerve root manually undetachable. These adhesions were found to be located at the level between the ilium attachments of the iliolumbar ligament and the sacral attachment of the lumbosacral ligament.
Clinically LSTS can be associated with Tarsal Tunnel Syndrome (Protas et al 2017) potentially forming a double crush syndrome.
Symptoms of LSTS are L5 radiculopathy with normal strength and no signs of muscle atrophy.
(3) Anterior sacroiliac joint ligament.
The anterior sacroiliac ligament is a thickening of the anterior-inferior joint capsule. It is particularly well developed near the arcuate line and the posterior inferior iliac spine where it connects S3 to the lateral side of the preauricular sulcus. It is thin elsewhere.
Solonen (1957) claimed the anterior sacroiliac ligaments were direct continuations of fibers from the Piriformis and iliopsoas (the author is probably referring to Iliacus). He claimed that part of the origin of these muscles originate, in part, from inside the capsule of the sacroiliac joint.
McCory and Bell (1999) found there is a fascial origin for the Piriformis arising from the capsule of the sacroiliac joint. As these fibres pass inferiorly rather than laterally it brings the Piriformis in contact with the anterior ligament of the sacroiliac joint and the roots of the 1st to 3rd sacral nerves. The Iliacus has more recently been documented as originating partly from the anterior sacroiliac ligament (Standring 2017) but not the anterior sacroiliac joint capsule as claimed by Solonen (1957).
(4) Interosseus sacroiliac ligament (including the short posterior sacroiliac ligament).
The interosseous sacroiliac ligament is a major bond between the ilium and sacrum forming the syndemosis part of the superior two thirds of the joint. Kiapour et al (2020) found these to be the strongest ligaments as although they have the highest strains under different spine motions they don't proportionately contribute to restricting movement.
Short posterior sacroiliac ligament: extension of the interosseous sacroiliac ligament passing from S1 and S2 to the ilium.
(5) Posterior sacroiliac ligament (including the long posterior sacroiliac ligament).
Lies superficial to the interosseous ligament and blends with the sacrotuberous ligament. Connects the intermediate and lateral sacral crests to the PSIS and the posterior end of the internal lip of the iliac crest.
Long posterior sacroiliac ligament: extension of the posterior sacroiliac ligament. Fibers extend from S3 and S4 to the PSIS and the posterior end of the internal lip of the iliac crest.
Between the deeper interosseous ligament and more superficial posterior sacroiliac ligament lies the dorsal rami of the sacral spinal nerves.
The long posterior sacroiliac ligament can either be penetrated by the sacral rami (McGrath & Zhang 2008) or have the nerves run underneath or over it (Konno et al 2017). There is a wide-ranging variation among fibers of the long posterior sacroiliac ligament being connected to (Vleeming et al 2012):
The deep lamina of the posterior lumbar fascia.
Aponeurosis of the erector spinae muscle and multifidus muscle.
Gluteus Maximus (Barker et al 2014).
Blending distally into the sacrotuberous ligament.
Traditionally lip service has been paid to these attachments of the long posterior sacroiliac ligament but on dissection it must be remembered that these attachments are through the tough dense sheaths of the thoracolumbar fascia (Willard et al 2012).
(6) Sacrotuberous ligament.
Runs from the PSIS, posterior sacroiliac ligaments (with which it is partly blended), lateral sacral crest to the upper coccyx to the ischial tuberosity and ramus.
Its soft tissue attachments are:
(a) Blends with the fascial sheet of the internal pudendal vessels and pudendal nerve.
(b) Piriformis (Solonen 1957).
(c) Lowest fibers of the Gluteus Maximus.
(d) Blends partially with the sacrospinous ligament.
(e) Biceps Femoris and semimembranosis (Solonen 1957).
(f) Thoracolumbar fascia: between the outer thoracolumbar fascia and inner sacrotuberous ligament a tunnel is formed that nerves from the sacral rami run through (Willard et al 1998).
(g) Deep pelvic fascia (Poilliot et al 2019).
(h) Obturator fascia.
Nerves that pierce this ligament are:
(a) Nerves from the coccygeal plexus
(b) Perforating cutaneous nerve
(6) Sacrospinous ligament.
Extends from the ischial spine to the lateral margins of the sacrum and coccyx anterior to the sacrotuberous ligament.
The sacrospinous ligament attaches to:
(a) Sacrotuberous ligament (Standring 2017).
(b) Anterior sacroiliac joint capsule (Stout 2010).
(c) Coccygeus muscle: the anterior surface of this ligament is the coccygeus muscle.
(d) Obturator fascia.
Usually ligaments restrict movement. Hammer et al (2013) found this to be the case with the sacrotuberous and sacrospinous ligaments reducing movement at the acetabulum and the pubic symphysis. However at the sacrum they found increased stiffness in the sacrospinous and sacrotuberous ligaments increased sacral movement. They attributed this to their distal attachments and the relative fixation of the upper part of the sacrum. In other words the stiffness in these ligaments pull on the more flexible lower part of the sacrum.
Biomechanics of the sacroiliac ligaments
Different trunk positions have been shown to place maximum strains on the sacroiliac ligaments (Kiapour et al 2020):
Anterior sacroiliac ligament: sacral nutation by the superior fibers and by transverse portions axial rotation.
Interosseous sacroiliac ligament: sacral nutation and axial rotation.
Long and short posterior sacroiliac ligament: sacral counter nutation and by transverse portions axial rotation. The lower portion of the ligament resists sacral nutation.
Sacrospinous Ligament: sacral nutation.
Sacrotuberous Ligament: sacral nutation.
Movements of the sacroiliac joint: movement in health and injury
The relatively flat shape of the SIJ along with its ligaments transfers large bending moments and compression loads; however, the joint does not have as much stability against shear loads despite its interlocking grooves abd ridges. The transversus abdominis and the pelvic floor muscles (levator ani and coccygeus muscles) play a major role in SIJ stability as they increase the compression load across the SIJ to resist shear loads (Kiapour et al 2020).
Sacroiliac joint movements includes (1) movements of the sacrum on the innominate bone and (2) movements of the innominate bone on the sacrum. The main motions of the sacrum are lateral rotation and nutation, which are less than 1.2° (Kiapour et al 2020).
(1) Movements of the sacrum on the innominate bone.
Goode et al (2008) found movement at the sacroiliac joint in six planes. Three were pivoting around three axes and three were translating along these same three axes. The axes are:
(a) Transverse axis (sacral X-axis): courses mediolateral through the left and right PSIS. This permits sacral rotation in a sagittal plane (i.e. nutation and counternutation) and translation transversely along this axis.
(b) Vertical axis (sacral Y-axis): this permits sacral rotation in the horizontal plane (i.e. rotation) and translation superiorly along this axis.
(c) Sagittal axis (sacral Z-axis): courses anterior-posterior midway between the anterior superior iliac spines (ASIS). This permits sacral rotation in the coronal plane (i.e. sidebending) and translation anteriorly along this axis.
Ranges of motion:
(a) Transverse axis (sacral X-axis): rotation around this axis (nutation-counter nutation) ranged between −1.1 and 2.2 degrees (3 degrees, Kiapour et al 2020). Translation along the axis ranged between −0.3 and 8.0mm.
(b) Vertical axis (sacral Y-axis): rotation around this axis (rotation) ranged between −0.8 and 4.0 degrees (1.5 degs, Kiapour et al 2020). Translation along the axis ranged between −0.2 and 7.0mm.
(c) Sagittal axis (sacral Z-axis) rotation around this axis (sidebending) ranged between −0.5 and 8.0 degrees (0.8 degs Kiapour et al 2020). Translation along the axis ranged between −0.3 and 6.0 mm.
Forst et al (2006) associated lumbosacral extension and flexion with movement of the sacrum around a transverse axis in nutation and counternutation respectively.
(2) Movements of the innominate bone around the sacrum.
Innominate ranges of motion has been documented in:
(a) Rotation around a transverse axis.
(b) Rotation around a vertical axis.
(c) Vertical shear along a vertical axis.
(a) Rotation of the innominate around a transverse axis: anterior and posterior rotation.
Anterior rotation of the innominate is produced by:
Hip extension: extension of the hip produces an anterior rotation (Barakatt et al 1996) and moves the innominate away from the sacrum (Stout 2010). Mens et al (1999) contradicted this saying that during hip flexion the innominate rotates anteriorly. This closely allies Kibsgard et al (2017) (refer to posterior innominate rotation).
Short leg: Coopersten and Lew (2010) found the innominate bone rotates anteriorly on the side of the short leg.
Inferior vertical shear of the innominate: When standing on a step with one leg the other leg hanging down undergoes anterior rotation of the innominate bone (Mens et al 1999). This relates to Barakatt et al (1996) who found doing the opposite i.e. elevating the leg (as opposed to letting it hang down) posteriorly rotates the innominate bone.
Dontigny (2017) found anterior innominate rotation causes the innominate to rotate cephalad and laterally at the PIIS. This places a vertical shear and separation movement between S3 and the PIIS.
This cephalic and lateral movement places a vertical shear on and separates the ilial origins of both the gluteus maximus and the piriformis at the superior margin of the greater sciatic notch. This results in buttock pain, piriformis syndrome and sciatica. Tension can also be transmitted through the tensor fascia lata into the lateral knee.
Biomechanically the horizontal axis of the sacroiliac joint passes through the interosseous ligament at S2. Therefore a vertical shear at the PIIS can also result in a vertical shear on the ipsilateral side of this horizontal axis.
Diagnostically the vertical shear placed upon the PIIS from an anterior innominate rotation results in a tender point between the PIIS and S3.
- Posterior rotation of the innominate is produced by:
Standing: the forces coming up from the ground along the long axis of the leg runs up through the hip joint and posteriorly rotates the innominate (Hungerford et al 2004).
Hip flexion: Stout (2010) found on hip flexion the innominate rotates posteriorly, translates caudad down the vertical axis and compresses the sacrum. This finding however was contradicted by Kibsgard et al (2017) who found during an active SLR (unilateral hip flexion) there was greater movement in posterior rotation of the innominate on the contralateral side that wasn’t undergoing hip flexion.
Superior vertical shear of the innominate: Barakatt et al (1996) found elevating the innominate by either placing a platform under the foot or an ischial tuberosity posteriorly rotates the ipsilateral innominate.
Long leg: Barakatt et al (1996) findings of an elevated leg posteriorly rotating the innominate was confirmed by Cooperstren and Lew (2009) who performed a literature review on leg length discrepancy and innominate rotation. They found the literature supported a posteriorly rotated innominate on the side of the longer leg.
Prone hip abduction and external rotation (HABER): this has been shown to posteriorly rotate the ipsilateral innominate (and anteriorly rotate the contralateral innominate) (Bussey et al 2009).
(b) Rotation of the innominate around a vertical axis.
Kibsgard (2017) noted when performing an ASLR the innominate bone of the rested leg (the one not being tested) had an inward tilt of 0.3. degs and a posterior rotation of 0.8°.
The compression and decompression of the sacrum noted by Stout (2010) in posterior and anterior rotation respectively presumably related to rotation around a vertical axis. Whether she defined compression and decompression as an inward tilt (which would cause force closure of the sacroiliac joint) or outward tilt around the sacrum was not specified.
Bussey and Milosavljevic (2013) investigated movement of the innominate bone around a vertical axis using hip abduction and external rotation (HABER). They found in healthy controls the rotation of the innominate was either reciprocal (e.g. with the right hip in HABER each innominate bone will rotate in opposing directions) or unilateral (e.g. if the right hip is in HABER both innominate bones would rotate to the right and if the left hip is in HABER both innominate bones would rotate to the left).
(c) Vertical shear of the innominate along a vertical axis.
Greenman (1986) identified movement of the innominate bone in a cephalad or caudad direction along a vertical axis creating a superior innominate shear (upslip innominate) or inferior innominate shear (downslip innominate). Mens et al (1999) found an inferior vertical shear of the innominate (downslip innominate) anteriorly rotates the innominate bone.
Mechanisms of sacroiliac joint dysfunction
Mechanisms for sacroiliac joint dysfunction are:
Mechanical loading: mechanical loading produces an eccentric load that can ‘contort’ the pelvis causing articular and soft tissue strains.
Hormonal changes and an enlarged uterus.
- Altered neuromuscular control: altered neuromuscular control to the muscles that support the sacroiliac joint can occur secondary to a sacroiliac joint injury. However altered neuromuscular control to the muscles that support the sacroiliac joint from other causes can cause sacroiliac joint injury by directly force closing the joint (Barker et al 2014).
The far reaching effects of pelvic mechanics on the soft tissues has been summarised by Adhia et al (2016). They found when you induce an oblique axis by placing the subject in prone hip abuction and external rotation on the right leg (HABER) the following:
The right innominate rotates posteriorly and externally rotates.
The left innominate rotates anteriorly and internally rotates.
At the right sacroiliac joint the sacrum rotates to the left and nutates.
At the left sacroiliac joint the sacrum rotates to the left and counternutates.
The right short and long posterior sacroiliac ligaments relaxes.
The left short and long posterior sacroiliac ligaments tenses.
The right anterior sacroiliac, interosseous, sacrotuberous, sacrospinous and iliolumbar ligaments tense.
The left anterior sacroiliac, interosseous, sacrotuberous, sacrospinous and iliolumbar ligaments relaxes.
Adhia et al (2016) refers to a posterior rotation of the innominate causing tension in the ipsilateral iliolumbar ligament which is the opposite to Mens et al (1999) who found anterior rotation of the innominate caused tension in the iliolumbar ligament. However in the model used by Mens et al (1999) this anterior rotation was secondary to a leg hanging off a step where the innominate bone shifts caudad.
It’s not just in a HABER position that altered axes of motion are produced for the sacroiliac joint. Fortin and Flaco (1997) highlighted how a vertical shear of the innominate from injuries such as landing in mid-stance or ‘missing a step’ can cause complications. The result of this ipsilateral upward shear is that the sacrum no longer rotates around a straight horizontal axis but eccentrically pivots around an oblique axis.
For example if you missed a step with the right leg the right innominate will shear superiorly along a vertical axis and be higher. If the right innominate is higher the right side of the transverse axis will be higher. If the right side of the transverse axis is higher when the sacrum nutates instead of the sacral base ‘nodding’ directly forwards it will ‘nod’ or nutate around an oblique axis to the right.
This cephalic movement of the PIIS with resultant change in horizontal axis of the sacroiliac joint was also noted by Dontigny (2017). The vertical movement of the PIIS in this case however was due to an anteriorly rotated innominate. Due to the Piriformis attachment to the PIIS this author attributed this lesion pattern to placing adverse strain on the Piriformis.
It’s not just the sacrum that moves incorrectly when subject to injury. Hungerford et al (2004) found that posterior rotation of the innominate occurred during weight bearing in healthy subjects. This movement pattern optimises stability of the pelvic girdle during weight bearing. In symptomatic individuals the innominate bone anteriorly rotated during weight bearing. This faulty pattern may indicate abnormal articular function due to altered axis of movement through the pelvis.
Hormonal changes and an enlarged uterus
Hormonal changes and an enlarged uterus causes an increased lumbar hyperlordosis, pelvic anteversion, and widening of the pubic symphysis. The sacroiliac joints oppose this rotation causing an increase of mechanical tension of the pelvic ligaments, which eventually results in lower back pain (Sipko et al 2010).
Altered neuromuscular control
Subjects with sacroiliac joint pain try to enhance force closure of the sacroiliac joint by faulty motor control of the muscles that stabilise the joint (O’Sullivan et al 2002). Therefore tightness in the myofascial structures associated with sacroiliac joint stabilisation (and compression) may be indicative of a sacroiliac joint dysfunction or intern directly cause it (Barker et al 2014). The myofascial structures that induce force closure of the sacroiliac joint include (Wingerden et al 2004):
Gluteus Maxims (Barker et al 2014).
Posterior layer of the thoracolumbar fascia.
Transverse abdominals and external and internal obliques.
Aponeurosis of the erector spinae (including the Multifidus).
Pelvic floor (O’Sullivan et al 2002).
Diaphragm (O’Sullivan 2002).
The erector spinae aponeuriosis (with the Multifidus), the biceps femoris, gluteus maximus, oblique and transverse abdominis muscles were shown to have the greatest effect on sacroiliac joint force closure. The latissimus dorsi has the small effect (Wingerden et al 2004).
Entrapment neuropathies in the pelvis causing sacroiliac joint pain
The dorsal sacral rami refers pain mimicking sacroiliac joint pain. The dorsal sacral rami penetrates and can be trapped in the:
(2) Long Posterior Sacroiliac Ligament.
(3) Between the Sacrotuberous Ligament and Thoracolumbar fascia.
(4) Gluteus Maximus.
All these soft tissues cause force closure of the sacroiliac joint. Their anatomy has been discussed in 'review of the structures: capsular, ligamentous, myofascial and neurological' . Their function in stabilising the sacroiliac joint and contributing to its injury is discussed in 'mechanisms of sacroiliac joint dysfunction (altered neuromuscular control)'.
What is discussed here is the anatomy of the dorsal sacral rami in reference to entrapment neuropathies in these myofascial and ligamentous structures. This has been shown to mimic sacroiliac joint pain in the long posterior sacroiliac ligament (McGrath and Zhang 2005, Murakami et al 2007) and be at the very least a potential source of double crush in the other tissues.
Murakami et al (2007) compared the effects of blocking injections into the sacroiliac joint and around the Long Posterior Sacroiliac Ligament in patients with sacroiliac joint pain. 100% got relief by blocking the Long Posterior Sacroiliac Ligament and only 9 out of the 25 patients got relief from the intraarticular injection. Could this relief be due to the anatomy of the Long Posterior Sacroiliac Ligament in trapping the dorsal sacral rami causing sacroiliac joint pain?
The Multifidus arises from the spinous process of L5 to as low as the fourth sacral foramen, PSIS and dorsal sacroiliac ligament. The longest fibers of the Multifidus run from the spinous processes of L1 and L2 to the dorsal segment of the iliac crest.
Along with being tightly adhered to the erector spinae aponeurosis the sacral attachment of the multifidus is also tightly adhered to the medial branches of the sacral dorsal rami. To illustrate how tight this adherence is when the multifidus was removed piecemeal many of these nerves were removed along side with it (Cox & Fortin 2014).
Long Posterior Sacroiliac Ligament
The dorsal sacral rami pass either through (McGrath & Zhang 2005), beneath or over (Konno et al 2017) the long posterior sacroiliac ligament (along with minute blood vessels potentially creating ischaemic zones Willard et al 1998).
The levels at which the lateral branches of the dorsal sacral rami typically penetrate the Long Posterior Sacroiliac joint Ligament are:
(McGrath & Zhang 2005)
The sacral rami run posterior to the sacrotuberous ligament (Standring 2017) through a tunnel created by the sacrotuberous ligament internally and an outer sheath of the thoracolumbar fascia (Willard et al 1998).
The sacral rami run from the sacrotuberous ligament forming loops under the Gluteus Maximus. These nerves then pierce the Gluteus Maximus along a line running from the PSIS to the apex of the coccyx (Standring 2017).
Sacroiliitis has been associated with inflammatory changes (Almodovar et al 2014, Fortin et al 1999) as well as changes to the bone (Panwar et al 2017) and joint space (Zou et al 2015).
The diffuse local and lower extremity pain referred from sacroiliac joint problems has been associated with (1) the joint's innervation and (2) its close association with neighbouring nerve trunks.
The anterior sacroiliac joint capsule relates closely to the nerve fibers of the lumbosacral trunk (L4 and L5 nerve roots) and the nerve bundles of the obturator nerve (Vleeming et al 2012). The anterior sacroiliac joint capsule being relatively thin allows substances in the joint space to leak out and potentially irritate the lumbosacral trunk (Vleeming et al 2012).
The dorsal sacroiliac joint capsule is discontinuous (Fortin et al 1999) and is closely related to the nerves exiting the sacral foramen. This discontinuous capsule allows once again for extravastation of joint fluid to potentially irritate the neighbouring nerves.
Fortin et al (1999) found three pathways between the sacroiliac joint and neural structures. These were:
(1) Posterior extravastation into the dorsal sacral foramen.
(2) Superior recess extravastation at the sacral alar level to the L5 epiradicular sheath.
(3) Ventral extravastation to the lumbosacral plexus.
Due to the discontinuous nature of the posterior sacroiliac joint capsule the most common pattern of extravastation was posteriorly. Whilst it is not known if this ‘inflammatory leakage’ is a pathological mechanism for neuropathies its potential effects not only directly on the nerves but indirectly by its effects on the connective tissue could be a potential mechanism for neuropathies.
Four zones of the sacroiliac joint have been identified accounting for different symptoms when stimulated (Kurosawa et al 2015). These zones are:
Zone 1 (upper section): pain was referred around the PSIS.
Zone 2 (middle section): pain was referred to the middle buttock.
Zone 3: (lower section): pain was referred to the lower buttock.
Zone 0 (cranial portion of the ilium outside the SIJ): pain was referred mainly to the upper buttock along the iliac crest.
All subjects complained of groin pain, which was slightly relieved by lidocaine injection into zones 1 and 0.
Diagnostic testing for the sacroiliac joint
Diagnostic testing for the sacroiliac joint has received mixed reviews due to the complexities of the joint and its soft tissues.
The sacroiliac joint tests associated with stretching the different sacroiliac ligaments are (Kim et al 2014):
(a) Anterior sacroiliac ligament: thigh thrust test and Gaenslens test.
(b) Interosseous sacroiliac ligament: compression test, distraction and sacral apex pressure test, thigh thrust test, Patrick FAbER and Gaenslens test.
(c) Long posterior sacroiliac ligament: Gaenslens test (minimal).
(d) Short posterior sacroiliac ligament: thigh thrust test, Patrick FAbER and Gaenslens test.
(e) Sacrospinous ligament: Gaenslens tests (minimal).
(f) Sacrotuberous ligament: Gaenslens test (minimal).
van de Wurrf et al (2000) reviewed clinical testing for the sacroiliac joint and found the two most reliable tests were Gaenslens and the thigh thrust test.
This was later confirmed by Arnbak et al (2017) who advocated these two tests but also a pain provocation sign when palpating the long posterior sacroiliac ligament. However these authors only found these tests accurate in men.
Werner et al (2013) found a 100% accuracy when performing a PSIS distraction test. This was performed as a pain provocation test where a punctual force was applied to the PSIS in a medial to lateral direction. The rationale for the high degree of sensitivity was the relationship of the sacral rami to the long posterior sacroiliac ligament.
Active straight leg raising test (ASLR) has also been shown to be a more reliable test for the diagnosis of sacroiliac joint pain. However in this test there has been shown to be a greater posterior rotation of the innominate on the side not being tested (Kibsgard et al 2017). As a whole, the research papers tend to point not so much towards the shearing movement of the sacroiliac joint during this test but the altered neuromuscular control in the muscles trying to force close the sacroiliac joint (Shadmehr et al 2012). Changes in neuromuscular control when performing an ASLR can even effect the control of respiratory muscles in order to try and force close the sacroiliac joint (O’Sullivan et al 2002). This is why pain in the sacroiliac joint is reduced during this test with bilateral compression to the ASIS in a medial direction to replicate the action of these muscles in force closing the sacroiliac joint.
Innominate movement patterns in healthy and sacroiliac joint pain subjects were examined in prone hip abduction and external rotation (HABER). Innominate movement around a horizontal and vertical axis were analysed and they found:
(a) Healthy controls: posterior rotation of the ipsilateral innominate (Adhia et al 2016) with rotation around a vertical axis either of both innominate bones towards the side being tested or in opposing directions (Bussey & Milosavljevic 2013).
Example: if the left hip was placed in HABER the left innominate would rotate posteriorly with either or both innominate bones rotating to the left (not to the right) or the left innominate would rotate to the left (not to the right) and the right innominate to the right.
(b) Sacroiliac joint positive subjects: posterior rotation of the innominate decreases as rotation around a vertical axis increases (Adhia et al 2016). This rotation around a vertical axis is always unilateral (i.e. both innominate bones always rotate to the same side) and this unilateral pattern was always to the same side regardless of what hip was placed in HABER.
Example: if the left hip was placed in HABER the left innominate would posteriorly rotate but with a reduced movement. Rotation about a vertical axis would always include both innominate bones to one side only regardless of what leg was tested.
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