A common clinical presentation in patients with low back pain is pain around the region of the posterior superior iliac spine (PSIS) and buttocks. Amongst other conditions various local entrapment neuropathies in the soft tissue can account for these symptoms. Some of these entrapment neuropathies have been well documented and others due to observations on dissection could be a potential site of a double crush syndrome.
The proposed sites of entrapment of the L5 and sacral rami giving PSIS and buttock pain are (from above down):
- External transforaminal ligaments of the lumbosacral spine.
- Psoas Major.
- Osteofibrous tunnel (formed by the thoracolumbar fascia and iliac crest).
- Lumbosacral ligament.
- L5 to sacral multifidus muscle.
- Long dorsal sacroiliac ligament.
- Between the thoracolumbar composite and sacrotuberous ligament.
- Potentially the sacroiliac joint.
Firstly the individual unique peculiarities of the anatomy of the muscles, ligaments, fascia and nerves are covered. Then these peculiarities in relation to the anatomy of various nerves in the Lumbar and Sacral rami are discussed regarding osteopathic practise.
Essentially most of the anatomical courses and relation of various structures are illustrated in the diagrams. The real value of the text is to illustrate how these structures are adhered to each other to get a better appreciation of their potential implications.
For instance when palpating the sacrum it’s easy to forget you’re palpating soft tissue as it’s so dense and tightly attached to the bone. When dissecting the multifidus over the sacrum, carefully moving one layer at a time, the sacral rami are so tightly adhered to the muscle bits of the nerve come off in the substance of the mutifidus (Cox & Fortin 2014). This level of detail which is important when reviewing the possible causes of an entrapment neuropathy of these sacral rami in causing sacroiliac joint pain can’t be illustrated in a diagram.
- Psoas Major fibers run from the transverse processes and interverterbral discs of the lumbar spine (except the L5-S1 disc) to the lesser trochanter.
- Lumbosacral plexus lies in the substance of the Psoas Major between the transverse process and vertebral body.
- Lumbosacral plexus exits along the medial edge of the Psoas Major distally.
- Superior Cluneal nerve passes through the Psoas Major.
The Psoas Major stabilises the lumbar lordosis by pulling L1-2 and L2-3 into extension, L3-4 and L4-5 downwards into compression and L5-S1 into flexion (Penning 2000). This creates a shear force at L5-S1.
Benglis et al (2009) found the lumbosacral plexus to lie in the substance of the Psoas Major between the transverse process and vertebral body exiting it distally along the medial edge of this muscle.
Tubbs et al (2010) found the Superior Cluneal nerve to pass through the Psoas Major and paraspinal muscles running posterior to the Quadratus Lumborum.
- Iliocostalis and spinalis sections of the erector spinae fuse in the lower lumbar spine to form the transversospinalis.
- The aponeurosis of the erector spinae muscle receives strong attachments from the multifidus.
The general rule of thumb with the erector spinae (as well as its investing fascia) is the lower down it goes the more densely fused it becomes.
Willard et al (2012) describes the two most lateral muscles of the erector spinae group often fused in the lower lumbar and sacral levels to be termed the ‘sacrospinalis muscle’.
These paraspinal muscles are completely covered by the dense erector spinae aponeurosis. Willard et al (2012) describes this aponeurotic band as extending laterally to approximately the inferior border of L3 while medially it extends further into the thoracic region. Williams & Warwick (1980) describes the aponeurosis as attached medially to the sacral crest and the spinous process from L5 to T11 (and the supraspinous ligament), medial part of the iliac crest and the lateral sacral crest where it blends with the dorsal sacroiliac ligament, sacrotuberous ligament and with some fibers running continuous with the Gluteus Maximus. Creze et al (2018) found the erector spinae to be formed of regular longitudinal orientated connective tissue fibers.
Being named the erector spinae aponeurosis would suggest when the erector spinae contracts the aponeurosis contracts. However the multifidus strongly attaches to the inner surface of this aponeurosis giving it the potential to exert a direct line of pull (Willard et al 2012). Creze et al (2018) found the superficial fibers of the multifidus to be attached to the erector spinae aponeurosis at the midline in the lumbar spine and at the sacral levels. It would therefore be more apt to name this aponeurosis ‘the erector spinae-multifidus aponeurosis’.
- 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 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 at the lumbar (close to the midline) and sacral levels (Creze et al 2018) 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).
Johnson and Zang (2002) found the multifidus, longissimus thoracis and thoracolumbar fascia as contributers to the supraspinous and interspinous ligaments.
Only the relevant anatomy of the fascia is discussed here in relation to entrapment neuropathies.
Paraspinal Reticular Sheath (PRS) and Thoracolumbar Composite (TLC)
- PRS (or vertebral aponeurosis) is the deep layer of the posterior thoracolumbar fascia enclosing the paraspinal muscles.
- The TLC is the lower part of the PRS in the pelvis.
- The TLC is a fusion of the thoracolumbar fascia, aponeurosis of the erector spinae and mutlifidus.
- The TLC is the “soft tissue” you’re palpating when palpating the sacrum and PSIS.
The thoracolumbar fascia is typically described as consisting of three layers in the lumbar region.
- Anterior layer: attaches to the lumbar transverse processes. Covers the quadratus lumborum.
- Middle layer: attaches to the tips of the lumbar transverse processes. Gives rise to the transverse abdominis and internal oblique aponeurosis.
- Posterior layer: attaches to the spinous processes and median sacral crest. Covers the deep back muscles. The posterior layer ascends from the pelvis and sacrotuberous ligament, covering the erector spinae, ascending to the inferior boarder of the serratus posterior inferior. Here it splits into the superficial and deep lamina encircling the aponeurosis of the serratus posterior inferior (Schuenke et al 2012). The superficial lamina provides attachments for the latissimus dorsi and serratus posterior inferior. The superficial part of the thoracolumbar fascia is the part that gives rise to the latissimus dorsi and serratus posterior inferior. The PRS is the deep part of the posterior lumbar fascia that encircles the paraspinal muscles up the spine. The vertebral aponeurosis is the posterior wall of the PRS in the thoracic region starting from where the PRS splits deep to the serratus posterior inferior (Willard et al 2012).
Superficial layer of the posterior thoracolumbar fascia
The origin of the superficial lamina is the gluteus maximus fascia. It attaches to the spinous processes and the median sacral crest. It fuses with the aponeurosis of the latissimus dorsi and serratus posterior inferior. Fibers of the superficial lamina can, in cases, ascend to be continuous with the aponeurosis of the trapezius muscle (Loukas et al 2008).
Deep layer of the posterior thoracolumbar fascia (aka the PRS) and vertebral aponeurosis
The PRS is attached medially to the spinous processes of the lumbar and thoracic vertebrae and the median sacral crest; laterally to the angles of the ribs; anteriorly it runs continuous with the aponeurosis of the transverse abdominis and internal oblique (via middle layer of the thoracolumbar fascia and lateral raphe), blends with the iliolumbar ligaments and ventral sacroiliac joint capsule; superiorly it run deep to the serratus posterior superior to cover the splenius capitis and blend with the deep fascia of the neck.
Bogduk & Macintosh (1984) found the PRS to form a series of accessory posterior ligaments that runs from the L2 to L5 spinous processes to the ilium. These accessory ligaments resist flexion. This all enables the muscles to tense the PRS and stabilize the Lumbar spine and pelvis.
The vertebral aponeurosis is the posterior wall of the PRS in the thoracic region (Willard et al 2012). Therefore anatomically it's the same structure. It is not an aponeurosis in the true sense of the term as it has no muscular component (Loukas et al 2008).
Being the posterior wall of the PRS Loukas et al (2008) identified the vertebral aponeurosis deep to the latissimus dorsi and blending with the superficial lamina of the thoracolumbar fascia at the serratus posterior inferior. The authors also identified the thoracic attachments of the PRS/vertebral aponeurosis as the thoracic spinous processes and rib angles.
Being part of the the PRS Loukas et al (2008) found the vertebral aponeurosis to blend cephalad deep to the serratus posterior superior and splenius capitis with the deep cervical fascia; caudad to run over the erector spinae to blend with the sacrotuberous ligament.
Loukas et al (2008) hypothesized the PRS/vertebral aponeurosis, due to its association with the serratus posterior inferior, may also play a role in proprioception as this muscle has been implicated in this function.
Below L5 the PRS changes its name to the ‘thoracolumbar composite’ (TLC). Anatomically it is the same structure although it becomes far more dense and tightly adhered to its surrounding structures.
Willard et al (2012) gives an anatomical description of this TLC. He describes it as being a fusion at or below the level of L5 of:
- the thoracolumbar fascia.
- aponeurosis of the erector spinae and multifidus muscles.
This fusion forms a thick composite. This composite attaches to the PSIS spreading caudolaterally to join the gluteus maximus and terminating by covering the sacrotuberous ligament. Distally the TLC receives attachments from the biceps femoris, semimembranosus and semitendinosus muscles.
Vleeming et al (2012) states about the TLC “while palpating the upper part of the sacrum lateral of the spinous processes, this composite of structures can give the impression of feeling hard bone. This could mistakenly suggest that it is the sacrum itself that can be directly felt, instead of the tight fascial and tendinous composite enclosing the multifidus and sacrospinalis muscles”.
- Blends with the PRS.
- The PRS and iliolumbar ligaments blends with the ventral sacroiliac joint capsule.
The iliolumbar ligament runs from L5 (sometimes L4) to the sacrum and ilium. Because of variations in its anatomical attachments to the L5 transverse process sometimes the lumbosacral ligament is termed an iliolumbar ligament. The iliolumbar ligament blends with the ventral sacroiliac ligaments and anterior portions of the PRS.
These ligaments are subject to fatty degeneration after the first decade of life and potentially ossification.
- Extends anteriorly from L5 vertebral body (or transverse process) to the sacrum.
- Hypertrophies in response to instability.
- 'Squashes' the L5 nerve root between the ligament anteriorly and sacrum posteriorly (Lumbosacral tunnel syndrome).
- The sympathetic ramus communicans to the L5 nerve root pierces and can become tethered in the lumbosacral ligament.
- The perineurium of the L5 nerve root has adhesions to the periosteum of the sacrum potentially causing a double crush syndrome with Lumbosacral tunnel syndrome.
- Tarsal tunnel syndrome can be associated with Lumbosacral tunnel syndrome and can act as a double crush syndrome.
Variations in the anatomy of the 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). 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.
Long Dorsal Sacroiliac Ligament
- Blends with the TLC, erector spinae (& Multifidus) aponeurosis and sacrotuberous ligament.
- Sacral rami can either run through tunnels in the LDSL, over the LDSL or under the LDSL.
The long dorsal sacroiliac ligament (LDSL) runs from the sacrum to the PSIS and the inner lip of the dorsal part of the iliac crest. This 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 LDSL, being connected to (Vleeming et al 2012):
- The deep lamina of the posterior lumbar fascia.
- Aponeurosis of the erector spinae muscle and multifidus muscle.
- Blending distally into the sacrotuberous ligament.
Traditionally lip service has been paid to these attachments of the LDSL but on dissection it must be remembered that these attachments are through the tough dense connective tissue sheaths of the TLC (Willard et al 2012).
- Acts as a caudal attachment for the TLC.
- This attachment forms tunnels the nerves to the sacral rami run through.
Anatomically the sacrotuberous ligament (STL) runs from the sacrum to the ischial tuberoisity. For the purposes here it is merely a terminal anchorage for the TLC so functionally it is a continuation of this structure.
The attachment of the TLC to the sacrotuberous ligament creates another tunnel that nerves from the sacral rami run through (Willard et al 1998).
External transforaminal ligaments lumbosacral spine
Several external transforminal ligaments exist in the lumbosacral spine but of particular importance is the corpotransvere ligament (Amonoo-Kuofi et al 1988). This ligament passes obliquely downwards, forwards and medially from the inferior aspect of the accessory process of the fifth lumbar vertebra to the lateral surface of the intervertebral disc and the adjacent parts of the bodies of the L5 and S1.
It has been proposed that changes in this ligament could be responsible for compression of the L5 nerve root (Kuofi et al 1988 & Maric et al 2015). This was confirmed by Qian et al (2011) who proposed entrapment of the nerve roots by transforaminal ligaments secondary to loss of intervertebral disc height whereby the ligament lowers down or pathological changes to the ligament itself.
Superficially, the ligament is related to another flat band - the lumbosacral hood (Kuofi et al 2015). Together these ligaments separate and provide openings for the sympathetic ramus, the ventral ramus and blood vessels related to the intervertebral foramen.
Anatomy of dorsal lumbar rami
- Innervates the L5-S1 facet joint and Multifidus.
- Can give pain along the dorsal segment of the iliac crest.
- Joins the S1 dorsal ramus ?contributes to S1 dorsal ramus pain.
The dorsal rami of the lumbar spinal nerve divides into medial and lateral branches. The L5 dorsal rami divides into medial and intermediate branches. Due to the absence of an attachment of the iliocostalis to L5 there is no lateral branch as in the other levels (Bogduk et al 1982).
The intermediate branch of the L5 dorsal rami innervates the Longissimus Thoracics as it attaches to the medial aspect of the dorsal segment of the iliac crest. The L5 nerve root can course through the lumbosacral tunnel (between the sacrum posteriorly and lumbosacral ligament anteriorly. The L4 nerve root passes in front of the lumbosacral ligament. The L4 and L5 nerve root unite below the level of the Lumbosacral ligament to form the lumbosacral trunk. Ebraheim et al (1997) found the L5 nerve root and lumbosacral trunk coursed across the sacroiliac joint and was relatively fixed to the sacral ala with fibrous connective tissue. It then runs down to and communicates with the S1 dorsal ramus.
The medial branch of the L5 dorsal rami innervates the L5-S1 facet joint and the multifidus.
Anatomy of the dorsal sacral rami
- Middle cluneal nerve comprises of afferent nerves of the dorsal rami of the S1-3 foramina supplying the skin of the posteromedial area of the buttock.
- Responsible for gluteal and sacroiliac joint pain.
- Covered and adhered to the multifidus and TLC.
- Nerves run through the LDSL and between the STL and TLC.
S1-4 not S5 arises from the dorsal sacral foramina. The S1-3 dorsal rami, that form the middle cluneal nerve, are covered by the multifidus and a dense fascial composite (TLC). From here they divide into medial and lateral branches.
The medial branches end in the multifidus and the dense fascia (TLC). The lateral branches join with each other along with branches from the L5 and S4 rami.
The nerves run from the PSIS to the coccyx either through (McGrath & Zhang 2008), beneath or over (Konno et al 2017) the long dorsal sacroiliac ligament (along with minute blood vessels potentially creating ischaemic zones Willard et al 1998) and Gluteus Maximus (Williams & Warwick, 1980). They then run through a tunnel created by the sacrotuberous ligament internally and an outer sheath of TLC (Willard et al 1998).
The levels at which the lateral branches of the dorsal sacral rami typically penetrate the LPSL are:
(McGrath & Zhang 2005)
Clinically entrapment of these nerves can cause sacroiliac joint pain. Interestingly when Murakami et al (2007) compared the effects of blocking injections into the sacroiliac joint and around the LPSL in patients with sacroiliac joint pain 100% got relief by blocking the LPSL and only 9 out of the 25 patients got relief from the intraarticular injection.
Anatomy of the superior cluneal nerve & osteofibrous tunnel
The superior cluneal nerve is a cutaneous branch of the T12 to L5 nerve roots. The nerve penetrates the psoas major and paraspinal muscles. It traverses through the superficial layer of thoracolumbar fascia before crossing the posterior iliac crest or through the gluteal fascia. After crossing the iliac crest it supplies the skin overlying the upper half of the gluteus muscles (Konno et al 2017).
When the medial branch of the superior cluneal nerve penetrates the gluteal fascia, the nerve passes through a space surrounded by the iliac crest and the fascia attached to the iliac crest, the so called “osteofibrous tunnel.” Predominantly it’s the L4 and L5 lateral branches that run through the osteofibrous tunnel (Konno et al 2017).
Lu et al (1998) found the medial branch of the superior cluneal nerve is confined within the osteofibrous tunnel but the intermediate and lateral branches of the superior cluneal nerve either pierces the thoracolumbar fascia or passes through an orifice or fissure in the thoracolumbar fascia.
The mean distance from the PSIS to the osteofibrous tunnel was 40.8mm (Konno et al 2017).
Sacroiliac joint and potential neuropathies
- Anteriorly and posteriorly the sacroiliac joint is closely related to lumbosacral trunk, obturator nerve and sacral rami.
- The sacroiliac joint allows a "leakage" of fluid anteriorly and posteriorly through defects in the joint capusle.
- Could this "leakage" of fluid include inflammation causing a neuropathy through irritation of the nerves or tightening of connective tissue? Could this account for the variety of sacroiliac joint referred pain patterns?
The ventral sacroiliac joint capusle 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 ventral 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 discontinous 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 dorsal 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.
Osteopathic practice encompasses many different treatment techniques the efficiency of which is not covered here. All of these treatment techniques aim to normalise soft tissue and articular dysfunction.
With a better appreciation of not only the anatomical location of the different soft tissue and neurological structures but an appreciation for their dense consistency and attachments a better appreciation can be obtained as to their relative importance in different low back and pelvic pain presentations.
Several authors for instance have noticed pain and tension in the LDSL and have attributed this to mechanics of the pelvis (Vleeming et al 1996) and entrapment of dorsal sacral rami in this ligament has been associated with producing pain around the posterior superior iliac spine (McGrath et al 2009).
Broadening this concept further the dense adherence of the various muscular, fascial and ligamentous structures having either a direct or indirect effect on neural tension can be responsible for many different pain presentations.
The dual innervation of the facet joint and the multifidus could explain why in facet joint pain there can be pain on segmental palpation associated with mutlfidus spasm (Stephen et al 2007) or central sensitization (Crosby et al 2014) and atrophy of the multifidus in cases of chronic back pain (Woodham et al 2014).
The influence of abnormalities in multifidus tone with its attachments to the sacral rami, LDSL, erector spinae aponeurosis and PRS/TLC has the potential to cause an array of different pain presentations be it via an entrapment neuropathy or other causes of MSK pain.
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