Sacroiliac Joint
Introduction
The sacroiliac joint (SIJ) 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 SIJ can give a better understanding of the mechanical injuries affecting the joint as well as the different manipulative and diagnostic procedures used in SIJ 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.
Entheses causing sacroiliac joint (PSIS) pain.
Entrapment neuropathies in the pelvis causing sacroiliac joint pain.
Sacroiliitis.
Diagnostic testing for the sacroiliac joint.
Review of the structures: capsular, ligamentous, myofascial and neurological
The SIJ is located between the sacral and iliac articular surfaces that interlock with each other. 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 (i) a syndemosis (fibrous) part and (ii) a synovial part.
Syndemosis (fibrous) part: this lies in the superior two-thirds of the SIJ parallel to S1-2 and is composed of the interosseous sacroiliac ligaments (Zou et al 2015).
Synovial part: this lies in the inferior one-third parallel to S3–S4 levels with a true synovial-lined joint space located within the capsule (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 with a true joint space located within the capsule (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 articular surfaces are covered by two different types of cartilage: the sacral surface is covered by hyaline cartilage (3mm) and the iliac surface, above the greater sciatic notch, by fibrocartilage (0.5mm) (Huec et al 2020). The hyaline cartilage on the sacrum is thicker anteriorly than posteriorly in adults. This cartilage has been proposed to have a mechanical effect on restricting SIJ movement (Hammer et al 2013).
The joint transmits pain and proprioceptive signals from its innervation from the (Huec et al 2019):
Superior gluteal nerve.
Obturator nerve.
Lateral branch of the dorsal rami of L5, S1 and S2 nerve roots.
Ventral rami of L4, L5 and sacral nerve roots.
As well as the L5-S1 branches of the lateral dorsal rami the S2 and S3 lateral branches of the dorsal rami, and the S1-2 and S2-3 communicating branches are directed towards the interosseous ligaments. The interosseous ligaments cover the SIJ, and, at this level the joint has no capsule which creates a gap that leads directly into the SIJ cavity (Steinke et al 2022). The superior two-thirds of the SIJ (S1-2) is syndemotic (fibrous) being composed of the interosseous sacroiliac ligaments, whilst the lower third (S3-4) has a true synovial-lined joint space located within the capsule (Zou et al 2015). Therefore, it is possible, but not confirmed, that small S2 and S3 branches from the lateral branches of the dorsal sacral rami may continue past the interosseous ligaments to gain direct access to the sacroiliac joint. This is not the case for the L5-S1 branches or the S1-2 and S2-3 communicating branches that don’t reach the joint. With the ambiguity over this it is assumed that the posterior SIJ has no innervation (Steinke et al 2022).
The anterior sacroiliac joint capsule, however, does receive innervation from the S2 ventral rami and possibly the lateral branches of the L5 dorsal rami (Steinke et al 2022). The ventral sacral rami (containing the pelvic parasympathetic nerve fibers) are ensheathed posteriorly-medially by the anterior surface of the piriformis, anteriorly by the parietal pelvic fascia (including the fascia of the piriformis) (Williams and Warwick 1980) and complex of internal iliac vessels (Shafarenko et al 2022) and laterally by the obturator internus (Gaertner 2006). Also, when present, accessory fibers of the piriformis cross the anterior sacral foramen (Sen & Rajesh 2011) especially at S2 (Larionov et al 2022).
Articular capsule
Both the anterior and posterior sacroiliac joint capsules attach close to both articular margins of the joint.
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 referring to the iliacus. More recently Standring (2017) claimed the iliacus originated from the anterior sacroiliac ligament but not the joint capsule. Anterior rotation of the innominate can cause a separation of the ilial and sacral origins of the iliacus accounting for pain in this muscle (Dontigny 2000).
Sacrospinous ligament (Stout 2010).
Otsuru et al (2017) found a ureteric entrapment inside the SIJ.
Posterior sacroiliac joint capsule
The interosseous ligaments cover the SIJ, and, at this level the joint has no capsule making it discontinuous (Fortin et al 1999) which creates a gap that leads directly into the SIJ cavity (Steinke et al 2022). The superior two-thirds of the SIJ (S1-2) is syndemotic (fibrous) being composed of the interosseous sacroiliac ligaments, whilst the lower two thirds (S3-4) is s a true synovial-lined joint space located within the capsule (Zou et al 2015).
Ligaments
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).
Lumbosacral ligaments.
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). This tunnel, although not a constant finding (Hanson & Sorenson 2000), is formed by the ala of the sacrum posteriorly and the lumbosacral ligament anteriorly (lumbosacral tunnel).
The fifth lumbar nerve root passes through the L5-S1 intervertebral foramen and then the lumbosacral tunnel. A branch of the fourth lumbar nerve root passes in front of the lumbosacral ligament to join the fifth below the ligament to form the lumbosacral 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.
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 PIIS 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 SIJ capsule.
McCory and Bell (1999) found there is a fascial origin for the piriformis arising from the capsule of the SIJ. As these fibres pass inferiorly rather than laterally it brings the piriformis in contact with the anterior SIJ ligament and S1-3 nerve roots. 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).
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. As the horizontal axis of SIJ movement goes through these ligaments they are the strongest ligaments, are subject to the highest strain during movement, but in relation to the other ligaments don't proportionately contribute to restricting movement (Kiapour et al 2020).
Short posterior sacroiliac ligament: extension of the interosseous sacroiliac ligament passing from S1 and S2 to the ilium.
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 (LPSL): 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 the more superficial posterior sacroiliac ligament lies the dorsal rami of the sacral spinal nerves. The dorsal sacral rami as they leave the sacral foramen pass through an aponeurotic arch that originates from the medial aspect of each posterior sacral foramen and then continues as an aponeurotic tunnel that carries the lateral branches of the dorsal sacral rami as they wend their path from the dorsal sacral foramina (McGrath 2014), either along the dorsum of the sacrum, or variably through spaces between the laminae of the posterior sacroiliac ligament (Dreyfuss et al 2008). This aponeurotic tunnel terminates by blending laterally with the erectors spinae aponeurosis at the LPSL (McGrath et al 2010) providing a protective and continuous layer of loose connective and adipose tissue surrounded by a dense fibrous connective tissue that allows the nerves terminate in the gluteal region. In contrast, the medial branches of dorsal sacral rami pierce through this fibrous tunnel (McGarth et al 2010) before its LPSL attachment to blend with, and innervate, the multifidus (Cox & Fortin 2014).
The LPSL can either be penetrated by the lateral branches of the dorsal 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 LPSL 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.
Lip service shouldn’t be paid to these attachments of the LPSL as on dissection these attachments have been found to be through the tough dense sheaths of the thoracolumbar fascia (Willard et al 2012). Distinguishing between the LPSL and throacolumbar fascia as they both rise together from the lateral sacral crest and deeper ligaments. At the region of the PIIS the LPSL, sacrotuberous ligament (and gluteal aponeurosis) and thoracolumbar fascia merge (Steinke et al 2022).
Sacrotuberous ligament.
Runs from the PSIS, posterior sacroiliac ligaments (with which it is partly blended), lateral sacral crest and upper coccyx to the ischial tuberosity and ramus.
Its soft tissue attachments are:
Blends with the fascial sheet of the internal pudendal vessels and pudendal nerve.
Piriformis (Solonen 1957).
Lowest fibers of the gluteus maximus. Due to their intimate connection the different lamellas of the sacrotuberous ligament maybe the same thing as the different laminaes of the gluteal aponeurosis (Steinke et al 2022).
Blends partially with the sacrospinous ligament.
Biceps femoris and semimembranosis (Solonen 1957).
Thoracolumbar fascia: having traversed the LPSL the lateral branches of the dorsal sacral rami (S2-3) descend to a tunnel formed anteriorly by the sacrotuberous ligament and posteriorly by the outer thoracolumbar fascia (that extends up to the gluteus medius) and then pierces the fascial roof of this tunnel and the gluteal aponeurosis (Willard et al 1998).
Deep pelvic fascia (Poilliot et al 2019).
Obturator fascia.
Nerves that pierce this ligament are:
Nerves from the coccygeal plexus.
Perforating cutaneous nerve.
Sacrospinous ligament
Lies anterior to the sacrotuberous ligament. Extends from the lateral margins of the sacrum and coccyx to the ischial spine.
The sacrospinous ligament attaches to the sacrotuberous ligament (Standring 2017), anterior sacroiliac joint capsule (Stout 2010) and coccygeus (the anterior surface of this ligament is the coccygeus muscle and 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 (superior fibers); axial rotation (transverse fibers); upwards displacement of the coccyx during nutation (inferior fibers) (Huec et al 2019).
Interosseous sacroiliac ligament: sacral nutation and axial rotation. The posterior interosseous ligament takes the load in standing rather than transferring the weight directly to the SIJ (Dontigny 2000).
Long and short posterior sacroiliac ligament: sacral counternutation; axial rotation (transverse portions); sacral nutation (lower portion); internal rotation of the innominate separating the PSIS’s and caudal movement of the PSIS (Dontigny 2000).
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, despite its interlocking grooves and ridges the joint doesn’t have much stability against the vertical shear loads (Kiapour et al 2020) caused by gravity and muscles that act in either a longitudinal direction, for example psoas and rectus abdominis and, also, the piriformis* (Pel et al 2008). These vertical shear loads cause a caudal shift of the sacrum, with an accompanying nutation, in relation to the innominates, and a cephalic shift of the innominates, with an accompanying posterior rotation, in relation to the sacrum. These movements (i) wedges the sacrum between the innominates due to the sacral base being wider superiorly than inferiorly, and also wider anteriorly than posteriorly; (ii) loads the sacrotuberous ligament; this all serves to reduce vertical shear allowing the self-bracing mechanism of the SIJ to compresses the joint which in turn reduces vertical shear even further. This SIJ compression, by internally rotating the innominate around a vertical axis, approximates the grooves and ridges of the sacroiliac joint providing stability, protecting the ligamentous system and supporting the transfer of load from the trunk to the legs and vice versa; it involves activation of the transversely oriented muscles, primarily the transversus abdominis and the pelvic floor muscles (levator ani and coccygeus muscles) (Kiapour et al 2020) but also the piriformis*, gluteus maximus and external and internal obliques (Pel et al 2008).
When all these forces between muscle, ligament, and joint surfaces are balanced out, and pass through one point, there is ‘force equilibrium’. In relation to innominate and sacral movement with SIJ vertical shear force, equilibrium is achieved by activation of the external and internal obliques, iliacus, psoas, rectus abdominis, rectus femoris, tensor fasciae lata and a loading of the sacrotuberous ligament. As equilibrium is achieved and vertical shear is reduced this allows SIJ compression to occur. Force equilibrium from SIJ compression is mainly achieved by activation of the hip flexors (adductor longus, iliacus, pectineus, sartorius and rectus femoris), hip extensors (gluteus medius and minimus and piriformis*) with an unloading of the sacrotuberous ligament and loading of the sacrospinous ligaments; in the absence of transversely oriented muscles at the posterior side of the SIJ, to counterbalance activation of the transverse abdominis during SIJ compression, the iliolumbar and posterior sacroiliac ligament are placed under loads (Pel et al 2008).
*: note in this model the piriformis has a dual action in creating a vertical shear at the SIJ and resisting this vertical shearing through SIJ compression.
SIJ 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 is less than 1.2° (Kiapour et al 2020).
Movements of the sacrum on the innominate bone
Goode et al (2008) found movement at the SIJ in six planes. Three were pivoting around three axes and three were translating along these same three axes. The axes are:
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.
Vertical axis (sacral Y-axis): this permits sacral rotation in the horizontal plane (i.e. rotation) and translation superiorly along this axis.
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:
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.
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.
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.
Hammer et al (2019) found on standing the sacrum moved inferiorly to the innominate, or vice versa, the innominate moved superiorly (0.31mm), this downwards and forwards movements of the sacrum on weight bearing into nutation creates a self locking mechanism to stabilise the pelvis. Forst et al (2006) associated lumbosacral extension with nutation and flexion with counternutation. While lying, hip flexion (posteriorly rotating the innominate) creates sacral nutation, and hip extension (anteriorly rotating the innominate) creates sacral counternutation, counternutation normally takes place in unloaded situations such as lying prone (Buchanan et al 2022).
Movements of the innominate bone around the sacrum
Innominate ranges of motion has been documented in:
Rotation around a transverse axis.
Rotation around a vertical axis.
Rotation around a sagittal axis.
Vertical shear along a vertical axis.
Rotation of the innominate around a transverse axis: anterior and posterior rotation
Anterior rotation of the innominate is produced by:
Hip extension: hip extension produces 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 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 parallels Barakatt et al (1996) findings who found doing the opposite, i.e. elevating the leg, (as opposed to letting it hang down), posteriorly rotates the innominate.
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 that separates the ilial and sacral 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, a presentation that can be accompanied by an anteriorly rotated innominate ‘switching off’ the gluteus medius (Dontigny 2000).
Biomechanically the horizontal axis of the SIJ 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 to create an oblique axis. This vertical shear of the PIIS from an anteriorly rotated innominate creates 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 femur 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. Although this finding was contradicted by Kibsgard et al (2017) who found during an ASLR there was greater movement in posterior rotation of the innominate on the contralateral side this could be from the effect of hip flexor contraction on the ipsilateral side trying to anteriorly rotate the innominate.
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 posteriorly rotates the ipsilateral innominate (and anteriorly rotate the contralateral innominate) (Bussey et al 2009).
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 SIJ) 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).
Rotation of the innominate around a sagittal axis
Hammer et al (2019) found on standing there was a rotation of the innominate in the vertical plane with the iliac crest moving inwards to the cranial aspect of the sacrum.
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). Hammer et al (2019) found on standing the innominate moved cranially relative to the sacrum, or, vice versa, the sacrum to moved inferiorly (0.31mm). 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 SIJ 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 SIJ can occur secondary to a SIJ injury. However altered neuromuscular control to the muscles that support the SIJ from other causes can cause SIJ injury by directly force closing the joint (Barker et al 2014).
Mechanical loading
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 SIJ the sacrum rotates to the left and nutates.
At the left SIJ the sacrum rotates to the left and counternutates.
The right short and LPSL relaxes.
The left short and LPSL 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 has a greater caudad shift.
It’s not just in a HAbER position that altered axes of motion are produced for the SIJ. 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 SIJ 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 and gluteus maximus.
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 SIJ 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 SIJ pain try to enhance force closure of the SIJ by faulty motor control of the muscles that stabilise the joint (O’Sullivan et al 2002). Therefore tightness in the myofascial structures associated with SIJ stabilisation (and compression) may be indicative of a SIJ dysfunction or intern directly cause it (Barker et al 2014). The myofascial structures that induce force closure of the SIJ include (Wingerden et al 2004):
Gluteus maximus. The lower fibers of the gluteus maximus, with the piriformis, prevents further posterior movement of the distal part of the sacrum during nutation (Dontigny 2000), while the superior fibers of the gluteus maximus provide a compressive load across the SIJ (Barker et al 2014). The ilial attachments of the piriformis and gluteus maximus also prevent anterior rotation of the innominate (Dontigny 2017). A separation of the ilial and sacral fibers of the gluteus maximus during anterior innominate rotation can cause a separation in the gluteus maximus muscle fibers that extend from just caudad to the PSIS to the greater trochanter, this along with ‘switching off’ the gluteus medius can produce greater trochanteric and iliotibial band pain (Dontigny 2000).
Piriformis. The piriformis has a dual action in creating a vertical shear at the SIJ (caudal movement of the sacrum with nutation, and a cephalic movement of the innominate with posterior rotation) and resisting this vertical shearing through SIJ compression (internal rotation of the innominate around a vertical axis) (Pel et al 2008). This weight bearing function of the piriformis not only ‘locks’ the sacroiliac joint but tilts the pelvis down laterally (Söztanacı et al 2021) possibly due to its function as a hip abductor transferring bodyweight to the ipsilateral side on weight bearing.
Consequently, Dontigny (2017) found an anterior innominate rotation causes the innominate to rotate cephalad and laterally at the PIIS placing a vertical shear and separation movement between S3 and the PIIS, that in turn, separates the ilial and sacral 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.
Biceps femoris (via the sacrotuberous ligament).
Latissimus dorsi via the posterior layer of the thoracolumbar fascia.
Transverse abdominis, external and internal obliques and thoracolumbar fascia. The abdominal muscles brace the anteriorly rotated innominate during, for instance, trunk flexion (Dontigny 2000) by approximating the ASIS and distracting the PSIS (Willard & Carreiro 2010) which serves to compress the SIJ.
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 SIJ force closure. The latissimus dorsi has the small effect (Wingerden et al 2004).
Entheses causing sacroiliac joint (PSIS) pain
The thoracolumbar fascia is typically described as consisting of three layers in the lumbar region. All three layers fuse laterally at the lateral raphe (rib12 —> iliac crest giving attachment to transverse abdominis > internal oblique, with additional attachments from the latissimus dorsi and maybe the external oblique, Schuenke et al 2012) to redistribute muscular tensions to all layers of the thoracolumbar fascia, and, caudally to the PSIS and sacrotuberous ligament (Tabesh et al 2021). This is why entheses of not only the erector spinae (Todorov et al 2018) but also the posterior layer of the thoracolumbar fascia (Tabesh et al 2021) gives pain along its attachment from the PSIS spanning superior and laterally along the posteromedial portion of the iliac crest.
Entrapment neuropathies in the pelvis causing sacroiliac joint pain
The medial branch of the dorsal sacral rami penetrates and can be entrapped in the:
Multifidus.
The lateral branches of the dorsal sacral and L5 rami refers pain mimicking SIJ pain. The dorsal sacral rami penetrates and can be trapped in the:
Interosseous sacroiliac ligament
Erector spinae and gluteus maximus.
LPSL.
Between the sacrotuberous ligament and thoracolumbar fascia.
Gluteus maximus.
The ventral branches of the dorsal sacral rami can be entrapped by the:
Piriformis and its fascia.
All these soft tissues cause force closure of the SIJ. Their anatomy has been discussed in 'review of the structures: capsular, ligamentous, myofascial and neurological' . Their function in stabilising the SIJ 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 lateral branches of the dorsal sacral rami in reference to entrapment neuropathies in these myofascial and ligamentous structures. This has been shown to mimic SIJ pain in the LPSL (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 SIJ and around the LPSL in patients with SIJ pain. 100% got relief by blocking the LPSL and only 9 out of the 25 patients got relief from the intraarticular injection. Could this relief be due to the anatomy of the LPSL in trapping the lateral branches of the dorsal sacral rami causing SIJ pain?
Interosseous sacroiliac ligament
The lateral branch of each dorsal sacral rami has a communicating branch to the lower dorsal lateral sacral rami. The caudal communicating branch of the L5-S1 lateral branch of the dorsal rami passes downwards from the L5 root at the intervertebral foramen, adherent to the S1 segment, to attach on to the lateral branch of the S1 dorsal rami. These L5-S1 branches innervate the interosseous sacroiliac ligaments. There may also be some shared innervation to the interosseous ligaments from the lateral branches of the S2 dorsal rami. Therefore, injury to the axial interosseous sacroiliac ligament, a relatively weak and small part of the interosseous ligament, may account for L5 and S1 referred sciatic pain (Steinke et al 2022).
Lateral branches of S2 and S3 dorsal rami, and the S1-2 and S2-3 communicating branches are also directed towards the interosseous ligaments. The interosseous ligaments cover the SIJ, and, at this level the joint has no capsule which creates a gap that leads directly into the SIJ (Steinke et al 2022). The superior two-thirds of the SIJ (S1-2) is syndemotic (fibrous) being composed of the interosseous sacroiliac ligaments, whilst the lower third (S3-4) is a true synovial-lined joint space located within the capsule (Zou et al 2015).
Therefore, it is possible, but not confirmed, that small S2 and S3 branches from the lateral branches of the dorsal sacral rami may continue past the interosseous ligaments to gain direct access to the sacroiliac joint. This is not the case for the S1-2 and S2-3- communicating branches. With the ambiguity over this it is assumed that the posterior SIJ has no innervation (Steinke et al 2022).
Multifidus
Multifidus attaches on to the PSIS and dorsal sacroiliac ligament.
Medial branches of the sacral dorsal rami attach to the multifidus.
The multifidus arises from the spinous process to run caudally to attach on the mammillary processes, facet joint capsules next to the mamillary processes (to support the facet joint), sacrum (as low as S4 foramen), soft tissues overlying the sacrum, PSIS, dorsal sacroiliac ligament and erector spinae aponeurosis. The longest fibers of the multifidus run from the spinous processes of L1 and L2 to the dorsal segment of the iliac crest. Both the multifidus and longissimus occupy the gutter between the spinous processes and the transverse processes but are separated by a fascial septum that creates an isolated muscle compartment for the multifidus (Willard & Carriero 2010). At the lumbosacral junction the separate muscle attachments of the multifidus and longissimus are replaced by a fascial sheet that extends laterally from the capsule and lateral surface of the superior articular process of S1; this fascial band could represent either a fusion of the lowermost fascicles of multifidus and longissimus or a superficial component of the iliolumbar ligament complex (Weatherley et al 2010).
With the interspinous ligament (Johnson & Zang 2002) the tendinous fascia from the multifidus covers, and attaches onto, the posterior aspect of the joint capsule and superior articular process (anterior part of the capsule receives attachments from the ligamentum flavum that is continuous again with the interspinous ligament) (Gorniak & Condrad 2015); is tightly adhered to the erector spinae aponeurosis at the lumbar (close to the midline) and sacral levels (Creze et al 2018); is adherent, at its the sacral attachment, to the medial branches of the sacral dorsal rami as the nerves pass through it (Cox & Fortin 2014).
Johnson and Zang (2002) found the multifidus, longissimus thoracis and thoracolumbar fascia contributes to the supraspinous-interspinous ligaments and in turn the interspinous-ligamentum flavum complex (Iwanga et al 2019) that forms the investing tendonous-fascial-ligamentous support of the facet joint capsule (Gorniak & Condrad 2015). The cervical facet joint capsules are enhanced by the tendinous fibers of the deep cervical muscles (Lowis et al 2018) e.g. multifidus and semispinalis cervicis.
Erector spinae and gluteus maximus
The S1-S4/5 dorsal sacral rami as they leave the sacral foramen pass through an aponeurotic arch. This arch originates from the medial aspect of each posterior sacral foramen, under the multifidus, and continues as an aponeurotic tunnel which is pierced by the medial branches of dorsal sacral rami (McGarth et al 2010) as they blend with, and innervate, the multifidus (Cox & Fortin 2014). However, unlike their medial branches, the lateral branches of the dorsal sacral rami course in these tunnels as they wend their path from the dorsal sacral foramina (McGrath 2014), either along the dorsum of the sacrum, or variably through spaces between laminae of the posterior sacroiliac ligament (Dreyfuss et al 2008) to terminate in the gluteal region via the LPSL, or having traversed the LPSL to descend over the sacrotuberous ligament and pierce the ligament’s overlying thoracolumbar fascia and gluteal aponeurosis (Willard et al 1998). This aponeurotic tunnel terminates by blending laterally with the erectors spinae aponeurosis at the LPSL (McGrath et al 2010). It provides a protective and continuous layer of loose connective and adipose tissue surrounded by a dense fibrous connective tissue. This means the lateral branches of the dorsal sacral rami can be relatively untouched by the myo-fascial-ligamentous structures they pass through but reactive changes to the erector spinae, multifidus, gluteus maximus may narrow these gaps and give rise to SIJ pain (Steinke et al 2022).
Long Posterior Sacroiliac Ligament
Under the posterior sacroiliac ligament the lateral branches of dorsal sacral rami and their communicating branches, as they course in their dense fibrous tunnels that extend from the medial aspect of each sacral foramen to the the erectors spinae aponeurosis at the LPSL (McGrath et al 2010), anastomose to form plexi (Steinke et al 2022). These plexi give rise to amongst other nerves the middle cluneal nerves (MCN) (Steine et al 2022).
After coursing through the aponeurotic tunnels that terminate at the LPSL (McGrath et al 2010) the MCN either pass through (McGrath & Zhang 2005), or goes beneath or over (Konno et al 2017) the LPSL (along with minute blood vessels potentially creating ischaemic zones Willard et al 1998). The MCN pass through the LPSL between (i) the PSIS, whereby the S1 MCN runs closest to the PSIS, possibly accounting for PSIS pain in this area, and (ii) the PIIS, where the LPSL, sacrotuberous ligament (and gluteal aponeurosis) and thoracolumbar fascia in this area merge (Steine et al 2022).
The levels at which the lateral branches of the dorsal sacral rami typically penetrate the LPSL are (McGrath & Zhang 2005):
S1: 4%.
S2: 96%.
S3: 100%.
S4: 59%.
Sacrotuberous Ligament
Having traversed the LPSL the S2 and S3 lateral branches of the dorsal sacral rami (Steinke et al 2022) then descend to a tunnel formed by the underlying sacrotuberous ligament and overlying sheath of the thoracolumbar fascia (Willard et al 1998) that attaches cephalically to the gluteus medius (Steinke et al 2022). Having pierced the roof of this tunnel formed by the overlying thoracolumbar fascia (Willard et al 1998) it is not clear if branches from the S2 and S3 lateral dorsal sacral rami reach the overlying gluteus maximus’s anterior surface. However, the different lamellas of the sacrotuberous ligament and gluteal aponeurosis could be the same anatomical structure that form gaps which the nerve can run through (Steinke et al 2022).
Gluteus Maximus
The lateral branches of the dorsal 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).
Piriformis
Unlike the posterior SIJ the anterior sacroiliac joint capsule, however, does receive innervation from the S2 ventral rami and possibly the lateral branches of the L5 dorsal rami (Steinke et al 2022). The piriformis comes into contact with the ventral sacral rami as they are ensheathed posteriorly by the anterior surface of the piriformis, anteriorly by the parietal pelvic fascia (including the fascia of the piriformis) (Williams and Warwick 1980) and complex of internal iliac vessels (Shafarenko et al 2022) and laterally by the obutrator internus (Gaertner 2006). Also, when present, accessory fibers of the piriformis cross the anterior sacral foramen (Sen & Rajesh 2011) especially at S2 (Larionov et al 2022). Not only does the S2 ventral rami innervate the anterior sacroiliac joint capsule (Steinke et al 2022) but it also provides somatic and autonomic fibers to the pelvis and the leg (sciatic nerve, pudendal nerve, inferior and superior gluteal nerves, femoral nerve and posterior femoral cutaneous nerve) (Larionov et al 2022).
Sacroiliitis
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 SIJ 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 SIJ and neural structures. These were:
Posterior extravastation into the dorsal sacral foramen.
Superior recess extravastation at the sacral alar level to the L5 epiradicular sheath.
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 SIJ 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 SIJ has received mixed reviews due to the complexities of the joint and its soft tissues.
The SIJ tests associated with stretching the different sacroiliac ligaments are (Kim et al 2014):
Anterior sacroiliac ligament: thigh thrust test and Gaenslens test.
Interosseous sacroiliac ligament: compression test, distraction and sacral apex pressure test, thigh thrust test, Patrick FAbER and Gaenslens test.
LPSL: Gaenslens test (minimal).
Short posterior sacroiliac ligament: thigh thrust test, Patrick FAbER and Gaenslens test.
Sacrospinous ligament: Gaenslens tests (minimal).
Sacrotuberous ligament: Gaenslens test (minimal).
van de Wurrf et al (2000) reviewed clinical testing for the SIJ 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 LPSLt. 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 lateral branches of the dorsal sacral rami to the LPSL.
Active straight leg raising test (ASLR) has also been shown to be a more reliable test for the diagnosis of SIJ 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 SIJ during this test but the altered neuromuscular control in the muscles trying to force close the SIJ (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 SIJ (O’Sullivan et al 2002). This is why pain in the SIJ 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 SIJ.
Innominate movement patterns in healthy and SIJ 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:
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.
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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