Entrapment Neuropathies Causing Low Back & Pelvic Pain

Written By Darren Winter

Introduction

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).

  • Iliolumbar membrane.

  • 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.

Muscular anatomy

Psoas major

  • 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.

The L1-5 spinal nerves roots emerge in front of the lumbar transverse processes and enter into the part of the psoas major which is in between the muscle fibres originating from (i) the vertebral bodies and intervertebral discs and (ii) the muscle fibres originating from the transverse processes. 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.

Erector Spinae

  • Iliocostalis and longissimus sections of the erector spinae fuse in the lower lumbar spine to form the sacrospinalis.

  • The aponeurosis of the erector spinae muscle receives strong attachments from the multifidus.

The erector spinae is made from lateral to medial the iliocostalis, longissimus and spinalis. There is a fibrous separation between the longissimus muscle and more medial multifidus with both these muscles occupy the gutter between the spinous processes and the transverse processes. The spinalis dorsi is sometimes not included in the erector spinae muscle group as it is aponeurotic in the lumbar region (Sami 2015).

An intermuscular cleft between the iliocostalis and longissimus is often described as being absent in the lower lumbar and sacral levels so these muscles fuse forming the ‘sacrospinalis muscle’ (Willard et al 2012). However Bogduk (1980) refuted this claiming the longissimus and iliocostalis were separated in the lumbar region by (i) the intermuscular aponeurosis (ii) erector spinae aponeurosis (Bogduk 1980).

The intermuscular aponeurosis is attached just anterior to the PSIS extending 2-3 cm lateral along the iliac crest. It fans out towards the L1-4 transverse processes but is separated from them by a fat-filled space. This fat-filled space attaches on to the transverse processes (separating the attachments of the iliocostalis and longissimus) and extends caudally attaching on to the ilium anterior to the lumbar intermuscular aponeurosis. The lateral branches of the upper lumbar dorsal rami course through this fat filled space dividing into their branches. The posterior edge of the intermuscular aponeurosis is attached to the anterior edge of the erector spinae aponeurosis forming one continuous structure. As well as being continuous posteriorly with the more superficial vertebral aponeurosis both the intermuscular aponeurosis and vertebral aponeurosis gives origin to the lumbar erector spinae. Separating the medial (longissimus thoracis) and lateral (iliocostalis lumborum) erector spinae muscle groups the medial surface of the intermuscular aponeurosis gives rise to the longissimus thoracis and the lateral side of the intermuscular aponeurosis gives rise to just a few fibers of the iliocostalis lumborum (Bogduk 1980).

The dense erector spinae aponeurosis is formed of regular longitudinal orientated connective tissue fibers (Creze et al 2018) that with the intermuscular aponeurosis divides the erector spinae into medial and lateral divisions. Covering the paraspinal muscles lying underneath the thoracolumbar fascia the erector spinae aponeurosis shares the same location on the sacrum, ilium and spinous processes as the thoracolumbar fascia (Creze et al 2019) contributing to the thoracolumbar composite (Willard & Carreiro 2010) (refer ‘fascia anatomy; posterior layer of the thoracolumbar fascia). It attaches to the medial part of the iliac crest, sacral crest (S3, Creze et al 2019), dorsal sacroiliac ligament and sacrotuberous ligament and gluteus maximus. It attaches to the spinous processes from L5 (Williams & Warwick 1980) to T5 (Creze et al 2019) (and the supraspinous ligament). At L5 the most medial tendons of the erector spinae aponeurosis crosses the mid-line (with the superficial lamina of the posterior thoracolumbar fascia) to attach to the contralateral side of the posterior edge of the L5-S1 spinous process, however, when the supraspinous ligament is present at this level it attaches to the lateral boarder of the ipsilateral spinous process (Heylings 1976). Its anterior edge is continuous with the intermuscular aponeurosis forming one continuous structure (refer above, Bogduk 1980). The erector spinae aponeurosis extends laterally to approximately the inferior border of L3 (Willard et al 2012) then tapers as it extends further up to T5 (Creze et al 2019).

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’.

The erector spinae aponeurosis functions in (i) providing a functional link between the upper and lower extremities. This function is attributed the erector spinae aponeurosis rather than the 'thin' sacral thoracolumbar fascia (McGrath and Nicholson 2009); (ii) along with the thoracolumbar fascia stores energy from tendons in order to provide a recoil mechanism responsible for stabilisation and extension moments of the spine and the pelvis e.g. when sitting to standing (Creze et al 2019).

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.

Piriformis

Piriformis attaches from S2-4 and the fascial origin at the anterior sacroiliac joint to the greater trochanter and hip joint capsule.

  • Piriformis is in contact with S1-3 nerve roots and S1 nerve.

  • Piriformis can cause an entrapment neuropathy of the superior gluteal nerve causing low back pain at it innervates the sacroiliac joint. It is also associated with sciatic symptoms.

The piriformis muscle arises from S2-4. There is also a fascial origin arising from the capsule of the sacroiliac joint whose fibres pass inferiorly rather than laterally. This brings the piriformis in contact with the anterior ligament of the sacroiliac joint and the S1-3 nerve roots (Han et al 2017). The first sacral nerve is located just behind the layer of parietal fascia covering the piriformis (Florian-Rodriguez et al 2017).

Spanning the pelvis the piriformis attaches to the summit and medial aspect of the greater trochanter as well as having variable attachments to the fibrous capsule of the hip joint (Roche et al 2013). 

The piriformis forms a canal with the gemellus superior through which the sciatic nerve passes (Lewis et al 2016). It is enclosed by a common sheath and is supported by the piriformis just behind where the sacrospinous ligament attaches to the ischial spine (Hernando et al 2015).

Simonova (1979) claimed the suprapiriform canal should be considered a fascial osseous canal. It is formed by the upper margin of the greater sciatic notch being covered with a thin fascia, fascia of the gluteal and piriformis muscles and the parietal layer of the pelvic fascia. The proper fascial vaginae(*) of the upper gluteal vessels and nerves are adhered to the fascial walls of the canal.

*: this presumably relates to the fascia of the blood vessels as vaginae can be defined as a sheath-like structure, especially a sheath formed around a stem.

Containing the superior gluteal nerve Diop et al (2002) found hypertrophy of the piriformis can narrow this tight fascial suprapirifom canal and cause superior gluteal nerve entrapment.

With the relations of the S1-3 nerve roots (Han et al 2017) and S1 nerve (Florian-Rodriguez et al 2017) to the piriformis could there be a potential for an entrapment neuropathies of these nerves in causing low back pain?

Fascia anatomy

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.

Thoracoulumbar fascia: posterior, middle and anterior layers

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) and lumbar intermuscular aponeurosis (Bogduk 1980), 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. The divisions of the thoracolumbar fascia are the posterior (superficial and deep lamina), middle and deep layer:

(1) Posterior layer.

The posterior layer is made up of a superficial and deep lamina It bears most of the stress of forces transferred between the spine, pelvis, and legs.

  • Superficial lamina of the posterior layer of the thoracolumbar fascia

The superficial lamina is a thick fibrous membrane that runs posterior to the erector spinae (Przybycień et al 2023). It divides into three layers: (i) superficial: continuation of the thin epimysial fascia that connects the latissimus dorsi and gluteus maximus (ii) middle: aponeurosis of the latissimus dorsi; (iii) deep: fibers separating the superficial from the deep lamina of the posterior layer of thoracolumbar fascia, or, in the upper lumbar levels, the aponeurosis of the serratus posterior inferior (Willard et al 2012). Here it will just be considered as one.

The serratus posterior inferior and latissimus dorsi muscles originate from the superficial layer which attaches thoracic and lumbar spinous processes, median sacral crest and coccyx, as well as the iliac crest (Przybycień et al 2023). It fuses with the erector spinae aponeurosis, giving rise to the latissimus dorsi, serratus posterior inferior, parts of the external oblique and trapezius (Schuenke et al 2018).

It originates at the PSIS where it fuses with the deep lamina of the posterior layer of the thoracolumbar fascia and the gluteus maximus and its fascia, the median sacral crest, it then descends as a combined aponeurotic structure (‘refer TLC’). Ascending the lumbar region the fused superficial and deep lamina of the posterior thoracolumbar fascia attach to the L1-4 spinous processes helping form the supraspinous ligament, as there is no supraspinous ligament at L5 the collagen fibres of the superficial lamina cross the midline to interlace with the contralateral side (Sami 2015) enabling walking with opposing oscillations of the arms (Adamietz et al 2021). Ascending the lumbar spine the fused superficial and deep lamina split to encircle the aponeurosis of the serratus posterior inferior (Schuenke et al 2012). Laterally, the serratus posterior inferior aponeurosis fibers blend with the superficial lamina (latissimus dorsi fibers) above and the deep lamina of the posterior layer of thoracolumbar fascia below. Past the lumbar spinous processes the superficial lamina attaches to the thoracic spinous processes up to C7 forming the supraspinous ligament, and then to the ligamentum nuchae eventually attaching to the occiput (Adamietz et a 2021).

The superficial lamina connects parts of the external oblique to the trapezius through their fascia, to pass under and fuse with the trapezius and rhomboids (as these muscles share the same origin to the latissimus dorsi) to eventually join the fascia nuchae.

  • Deep lamina of the posterior layer of the thoracolumbar fascia: PRS and vertebral aponeurosis

Deep lamina of the posterior layer fuses over the iliac crest with the aponeurosis of the gluteus medius; attaches to the PSIS where it fuses with the superficial lamina of the posterior layer of the thoracolumbar fascia and the gluteus maximus and its fascia; long dorsal sacroiliac ligament; sacrum. Fusing with the adjacent epimysium at L5 and S1 spinous process (Adamietz et al 2021), and the erector spinae in the depression between the median sacral crest and PSIS (Vleeming 2016), it then descends through the pelvis attaching to the median sacral crest as a combined aponeurotic structure (‘refer thoracolumbar composite (TLC)’).

As it ascends the lumbar region the fused superficial and deep lamina of the posterior thoracolumbar fascia split to encircle the aponeurosis of the serratus posterior inferior (Schuenke et al 2012). Laterally, the serratus posterior inferior aponeurosis fibers blend with the superficial lamina (latissimus dorsi fibers) above and the deep lamina of the posterior layer of thoracolumbar fascia below. It attaches medially to S1 (Creze et al 2019) fusing with adjacent epimysium (Adamietz et al 2021), the lumbosacral interspinous ligament, spinous processes of L4-T6 (T7, Creze et al 2019). It’s T6 attachment coincides with a marked transition in the midline connective tissue organisation and the presence of the decussating fibers of trapezius. This composite of fibers from the deep lamina of the posterior layer of the thoracolumbar fascia, dense connective tissue and decussating fibers of trapezius attaches as a single layer of connective tissue directly to the lateral aspect of the T6–T9 spinous processes (Johnson & Zhang 2002).

Bogduk & Macintosh (1984) found the deep lamina of posterior thoracolumbar fascia to form a series of accessory posterior ligaments that runs from the L2 to L5 spinous processes to the ilium that resist flexion and enables the muscles to tense the deep lamina and stabilise the lumbar spine and pelvis. It blends with the iliolumbar ligaments and anterior sacroiliac joint capsule. The iliolumbar membrane is a connective tissue membrane originating from fibres of the thoracolumbar fascia at the lateral boarder of the erector spinae and resembles a fibrous septum. From here it fans out laterally dividing into fascial septa that traverse the subcutaneous tissue to attach on to the superficial fascia. Along its course it fuses with (i) the gluteus medius fascia at the iliac crest; (ii) along its lateral boarder with the deep fascia over the lateral boarder of the latissimus dorsi and external oblique (Loukas et al 2008).

The deep lamina of the posterior thoracolumbar fascia fuses with the aponeurosis of the serratus posterior inferior at the upper lumbar level (Patel 2014) to form the posterior retinaculum sheath (PRS) that encircles the paraspinal muscles. The PRS is externally reinforced by (i) laminae from the transverse abdominis and internal oblique: as the transverse abdominis and internal oblique approach the lateral boarder of the PRS they bifuricate into two distinct lamina (Schuenke et al 2012); (ii) transverse abdominis: the transverse abdominis attaches on to the PRS between T12-L4; (iii) middle layer of the thoracoulumbar fascia runs along the anterior boarder of the PRS to attach on to the transverse processes and iliolumbar ligaments; (iv) quadratus lumborum: the lateral boarder of the quadratus lumborum is wider than the paraspinal muscles inferiorly but superior to L2 it is narrower. Therefore, below L2, the lateral margin of the paraspinal muscles is reinforced by the quadratus lumborum and its fascia (Schuenke et al 2012). Internally the PRS is reinforced by epitenon of the erector spinae aponeurosis as it blends with the PRS (Willard & Carreiro 2010).

In the lumbar spine the PRS encircles the paraspinal muscles originating postero-medially: spinous process; laterally: rib angles; anteriorly: continuous with the middle layer of the thoracolumbar fascia and lateral raphe; antero-medially: L2-4 transverse processes (due to its middle layer of the thoracolumbar fascia attachments); superiorly: rib 12.

In the thoracic spine the PRS encircles the paraspinal muscles being composed of anteriorly the posterior aspects of the ribs and the associated fascia to which the paraspinal muscles attach (Willard et al 2012). In the thoracic spine the posterior wall of the PRS (deep lamina of posterior thoracolumbar fascia) is called the vertebral aponeurosis. The vertebral aponeurosis is attached: medially: spinous processes; laterally: rib angles; superiorly: deep to the serratus posterior superior and splenius capitis to fuse with the deep cervical fascia. Although it is called the vertebral aponeurosis it has no muscular component, having, due to its association with the serratus posterior inferior, a potential proprioceptive function (Loukas et al 2008).

The function of the paraspinals and erector spinae aponeurosis in the PRS is not to pull on their tendons to create movement but to increase stiffness in the PRS and store energy in order to provide a recoil mechanism responsible for stabilisation and extension moments of the spine and the pelvis e.g. when sitting to standing (Creze et al 2019), 30–40% of the muscle force is not transferred to the tendons, but is distributed directly to the fascia system (Adamietz et al 2021). Consequently, as the PRS restricts the radial expansion of the erector spinae muscles, this restriction can increase the tension created during the contraction of the erector spinae by up to 30% (Sami 2015).

At the PSIS, the PRS, along with its fused gluteus maximus attachments, may function in resisting the action of the transverse abdominis and oblique muscles in compressing the sacroiliac joint by approximating the ASIS and distracting the PSIS (Willard & Carreiro 2010).

  • Thoracolumbar composite (TLC)

Below L5 the PRS (fusion of the superficial and deep lamina of the posterior thoracolumbar fascia) 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:

(i). The thoracolumbar fascia (fusion of the superficial and deep lamina of the posterior thoracolumbar fascia).

(ii). Aponeurosis of the erector spinae and multifidus.

McGrath and Nicholson (2009) disputed the fusion of the thoracolumbar fascia to the erector spinae aponeurosis. They found this layer of thoracolumbar fascia forms a thin fascial structure that also does not attach to the LDSL. The TLC attaches to the iliac crest (with the aponeurosis of the gluteus medius) PSIS, lateral sacral tubercles (Siccardi & Vale 2020), posterior sacroiliac ligament (Todorov et al 2018), spreading caudolaterally to join the gluteus maximus and terminating by covering the sacrotuberous ligament to attach onto the ischial tuberosity. 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”.

(2) Middle layer.

The middle layer is aponeurotic providing stability. It is made up of three layers (Willard et al 2012):

  • Posterior extension of the transverse abdominis aponeurosis (passing posterior to the quadratus lumborum) and the internal oblique aponeurosis. In the lumbar region, on the lateral border of the erector spinae muscle, the thoracolumbar fascia is formed by a common tendon of the transversus abdominis muscle which reaches the lumbar transverse processes (Przybycień et al 2023).

  • Deep lamina of the posterior layer of the thoracolumbar fascia (PRS).

  • Posterior epimysium of the quadratus lumborum.

It attaches to the tips of the lumbar transverse processes, iliolumbar ligaments, iliac crest and is in an anatomic relationship with the proximal course of the obturator nerve (Siccandi & Valle 2020). Passing between the paraspinal muscles and the quadratus lumborum muscle it gives rise to the transverse abdominis and internal oblique aponeurosis. Gilchrist et al (2003) also found the iliocostalis lumborum attaches on to the middle layer of the thoracolumbar fascia (< L1-2 level, Sami 2015). It firmly attaches to rib 12, ascending to the cervical region where it surrounds the anterior spinal muscles (Siccandi & Valle 2020).

In between the attachment sites on the transverse processes the middle layer of thoracolumbar fascia forms an arch. The dorsal ramus passes through this arch to gain entrance into, and traverse in a lateral direction, the middle layer of the thoracolumbar fascia, posterior to the quadratus lumborum muscle, before entering the erector spinae muscles. The anterior layer of the thoracolumbar fascia normally fills this arch surrounding the ventral rami as they pass anterior to the quadratus lumborum before entering the psoas (Willard & Carreiro 2010).

(3) Anterior layer (transversalis fascia).

This thin layer is the anterior extension of the transverse abdominis fascia that runs between the twelfth rib and the iliac crest anterior to the erector spinae and posterior to the quadratus lumborum and psoas major. It attaches to the rectus muscles, psoas fascia and then the transverse processes and iliolumbar ligaments due to its associated with the middle layer. The anterior layer of the thoracolumbar fascia normally fills the arch formed by the middle layer of the thoracolumbar fascia attachments to adjacent transverse processes surrounding the ventral rami as they pass between the quadratus lumborum and psoas (Willard & Carreiro 2010).

Ligamentous anatomy

Iliolumbar ligaments

  • Blends with the PRS.

  • The PRS and iliolumbar ligaments blends with the anterior sacroiliac joint capsule.

The iliolumbar ligament runs from L5 (sometimes L4), the intertransversarii ligament to the sacrum, ilium and sacroiliac ligaments. 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 anterior sacroiliac ligaments and anterior portions of the PRS.

These ligaments receive attachments from the iliacus, quadratus lumborum, iliocostalis lumborum and their fascia.

Lumbosacral ligaments

  • 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).

Sacrotuberous Ligament

  • 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

  • Transforaminal ligaments traverse the intervertebral foramen subdividing it into different compartments that contain specific anatomical structures.

Several external transforminal ligaments exist in the lumbosacral spine but of particular importance is the corpotransvere ligaments (Amonoo-Kuofi et al 1988). The superior corporotransverse ligament attaches from the posterolateral corner of the vertebral body to the accessory process of the transverse process of the same vertebra; the inferior corporotransverse ligament connects the same posterolateral corner of a vertebral body to the transverse process below.

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. However, some authors ascribe these ligaments as having more of a protective role.

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.

L4 & L5 nerve root

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. 

Anatomy of dorsal lumbar rami

  • Dorsal lumbar rami divides into medial and lateral branches within the intertransverse ligaments (an extension of the middle lamina of the TLF).

  • Medial branch of the dorsal rami innervates the facet joints and multifidus.

  • Lateral branch of L5 dorsal ramus joins the S1 dorsal ramus ?contributes to S1 dorsal ramus pain.

Dorsal rami

Spinal cord --> dorsal root (sensory) & ventral root (motor) --> spinal nerve (mixed sensory & motor) --> ventral rami (motor > sensory) & dorsal rami (sensory > motor). Dorsal rami --> medial branch of dorsal rami & lateral branch of dorsal rami.

The dorsal rami enters the back through a foramen bounded by the superior border of the transverse process, the anterior aspect of the superior articular facet joint and the intertransverse ligament (Zhou et al 2012). It runs posteriorly on the medial aspect of the intertransversarri muscles (Shuang et al 2015).

The dorsal ramus then divides into the into medial and lateral branches at the junction of the facet joint and the proximal superior border of the transverse process (Zhou et al 2012). Bogduk and Long (1979) and Masini et al (2005) found the L1-L4 dorsal rami to divide into the medial and lateral branches within the intertransversarii ligament (an extension of the middle laminae of the TLF). The L5 dorsal rami gives rise to the medial and intermediate branches as it runs in the groove formed by the S1 superior articular process and the sacral ala. The L5 dorsal rami lacks a lateral branch as there is no attachment of the iliocostalis lumborum to L5 as it is replaced by the iliolumbar ligament.

Dorsal rami: medial branch

The medial branch of the dorsal rami passes through a groove formed between the root of the transverse process and root of the superior articular process. At this point Bogduk et al (1982) found the medial branch bound to the periosteum by connective tissue extending between the superior articular process and transverse process.

The medial branch of the dorsal rami then passes through a fibroosseous canal formed by the junction of the transverse process and the lateral aspect of the superior articular process. The roof of the canal is formed by the mammilloaccessory ligament. 

The medial branch of the dorsal rami then penetrates the deep fascia near the median line to enter the subcutaneous tissue.

The medial branch of the dorsal rami innervates:

  • Facet joints: innervates the two to three adjacent facet joints e.g. the L4 facet joint is innervated by the L3 and L4 medial branches. To innervate the facet joint the proximal nerve runs between the intertransversarii and the most lateral fibres of multifidus; the distal nerve runs deep to the multifidus (Bogduk et al 1982).

  • Multifidus: Shuang et al (2015) found this innervation to be highly specific. They found each medial branch ran on the deep aspect of the multifidus and was solely innervated by this one branch without any communicating branches. This finding was disputed by Wu et al (1997) who found the multifidus to be polysegmentally innervated.

  • Interspinous ligament and muscle: the nerve weaves medially between the fascicles of the multifidus to reach the interspinous space (Bogduk et al 1982).

  • Supraspinous ligament.

Dorsal rami: lateral branch

The lateral branch of the lumbar dorsal rami innervates the tissues lateral to the facet joint line i.e. erector spinae (iliocostalis and longissimus muscles) and cutaneous innervation of the back and pelvis.

The lateral branch of the L5 dorsal ramus descends and merges into the S1 dorsal ramus (Zhou et al 2012). Bogduk et al (1982) described the L5 dorsal rami as lacking a lateral branch dividing into medial and intermediate branches. This was due to the absence of an attachment of the iliocostalis to L5 that was replaced by the iliolumbar ligament. However, both these authors also described the intermediate branch (Bogduk et al 1984) and lateral branch (Zhou et al 2012) of the L5 dorsal ramus as innervating the Longissimus Thoracics as it attaches to the medial aspect of the dorsal segment of the iliac crest.

The lateral branch of dorsal rami lies in an osseous groove on the superior transverse process. The upper lumbar dorsal rami then pass through a fat filled space that separates the intermuscular aponeurosis (refer ‘muscular anatomy; erector spinae’) from the L1-4 transverse processes (Bogduk 1980). It then sends branches to the iliocostalis and longissimus muscles. After passing the iliocostalis muscle, the main lateral branch descends approximately two vertebral segments before it pierces the dorsal layer of the thoracolumbar fascia to descend to supply the skin in the posterolateral buttock.

Anatomy of the superior cluneal nerve, osteofibrous tunnel & iliolumbar membrane

The superior cluneal nerve (SCN). This arises from T12-L4 lateral branch of dorsal rami. Innervates skin of upper 2/3 of buttocks (passes over PSIS). There is no cutaneous branches of L5 lateral branch.

Iwanaga et al (2019) found the SCN to be at L1: 75% of the dorsal rami; L2: 90% of the dorsal rami; L3: 95% of the dorsal rami contributes to the SCN; L4: 45% of the dorsal rami contributes to the SCN; L5: 10% of the dorsal rami contributes to the SCN. SCN terminates in:

(i) Medial branches (L4>L5>L3). Passs through the osteofibrous tunnel (<L4 & L5) to anastomose with the upper branches of the MCN (S1 & S2) (Kuniya et al 2014) accounting for pseudosciatic symptoms (Konno et al 2017).

(ii) Intermediate branches. Either pierces the thoracolumbar fascia or passes through an orifice or fissure in the thoracolumbar fascia (Lu et al 1998).

(iii) Lateral branches. Either pierces the thoracolumbar fascia or passes through an orifice or fissure in the thoracolumbar fascia (Lu et al 1998).

The nerve penetrates the psoas major and paraspinal muscles. After the dorsal rami divides into the medial and lateral branches within the intertransverse ligament (Bogduk and Long 1979 & Masini et al 2005) the lateral branch lies in an osseous groove on the superior transverse process. It then sends branches that course within the iliocostalis and longissimus. After piercing the iliocostalis, the main lateral branch descends approximately two vertebral segments before it passes through the superficial layer of the posterior thoracolumbar fascia lateral to the erector spinae mass just superior to the iliac crest (Przybycień et al 2023). They then either pass through the osteofibrous tunnel (40.8mm lateral to the PSIS, Konno et al 2017) (medial branch), or the thoracolumbar fascia (medial and lateral branches). Netter (1997, pg 237) illustrates the SCNs passing through the latissimus dorsi where it attaches to the ilium just lateral to the paraspinal mass to descend into the pelvis and pierce the gluteal fascia to supply the skin in the upper 2/3 of the posterolateral buttock.

At the lateral boarder of the erector spinae the SCN’s traverse the iliolumbar membrane. The iliolumbar membrane originates from fibres of the thoracolumbar fascia at the lateral boarder of the erector spinae and resembles a fibrous septum. From here it fans out laterally dividing into fascial septa that traverse the subcutaneous tissue to attach on to the superficial fascia. Along its course it fuses with (i) the gluteus medius fascia at the iliac crest; (ii) along its lateral boarder with the deep fascia over the lateral boarder of the latissimus dorsi and external oblique (Loukas et al 2008). Bogduk (1982) found a similar ‘bridge of connective tissue’ extending lateral to the origin of the iliocostalis lumborum that held the L3 lateral branch of the dorsal rami down to the iliac crest. A similar ‘rigid fascial edge’ has been described as causing entrapment of the superior cluneal nerve (Yuruk et al 2022). Obesity has been postulated as a potential cause of hypertrophy of the iliolumbar membrane accounting for superior cluneal nerve pain (Loukas et al 2008).

Dorsal rami: intermediate branch

The intermediate branch of the dorsal rami is commonly viewed as the muscular branch of the lateral dorsal rami.

The L3 and L4 dorsal rami (and sometimes L1 and L2) give off intermediate branches which supply the lumbar fibers of the longissimus thoracis (Zhou et al 2012).

Anatomy of the dorsal sacral rami

  • Middle cluneal nerve comprises of afferent nerves of the dorsal rami of the S1&2 > - S4 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 S1-4 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). Kikuta et al (2020) found they pierced the gluteus maximus by two different pathways and supplied the gluteal skin or the gluteus maximus muscle. 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): S1: 4%; S2: 96%; S3: 100%; S4: 59%.

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.

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 capsule.

  • 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 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 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 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:

  • 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 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.

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Darren Winter https://www.ducodigital.com
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