Sciatica: it’s not just about the Piriformis
Introduction
Sciatica is a common condition in the population. In the absence of active pathological findings in the lumbar spine deep gluteal space problems can cause a dynamic or non-dynamic compression of the sciatic nerve. Whilst piriformis syndrome is typically associated with this and can be commonly described simply as a “tight piriformis” the range of problems is more diverse and the pathology of deep gluteal problems as a whole more complex.
Anatomy of the sciatic nerve
The sciatic nerve is made up of the tibial and common peroneal nerve.
Nerve root
Tibial nerve: ventral divisions of the ventral rami of L4-S3 (Butz et al 2015).
Common peroneal nerve: dorsal divisions of ventral rami of L4–S2 (Butz et al 2015).
Throughout their lumbopelvic course the sciatic nerve and its roots are lightly tethered by innumerable filmy fibrous strands to the adjoining fascia, discs and spinal/sacral periosteum (<foramina). At the foramina these adhesions extend from the lateral extension of the posterior common [?longitidinal] spinal ligament.
The ventral sacral rami (carrying the pelvic parasympathetic neurones) are enclosed between the anterior surface of the piriformis posteriorly, the parietal pelvic fascia (including the fascia of the piriformis) (Williams and Warwick 1980) and complex of internal iliac vessels (Shafarenko et al 2022) anteriorly and the obturator internus laterally (Gaertner 2006).
Nerve trunk
The sciatic nerve is formed on the anteromedial surface of the piriformis (Goddard & Reid 1965). The piriformis then 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 (Fernandez et al 2015). In normal anatomy the sciatic nerve passes under the piriformis, but in a small portion of cases one of the divisions of the sciatic nerve passes through or over the muscle.
Carro et al (2016) found six possible anatomical relationships between the sciatic nerve and the piriformis muscle:
Sciatic nerve passes below the piriformis.
A divided nerve passes through and below the piriformis.
A divided nerve passes above and below the piriformis.
An undivided nerve passes through the piriformis.
A divided nerve passes through and above the piriformis.
A smaller accessory piriformis with its own separate tendon and sciatic nerve passes through the piriformis.
As the sciatic nerve continues to descend connective tissue attaches it to the obturator-gemelli complex (obturator internus muscle and superior and inferior gemellus muscles above and below it) (Balius et al 2017). Stecco (2014) simply stated the sciatic nerve was ensheathed usually down to the popliteal fossa but sometimes the proximal thigh by the piriformis fascia and Nutter (1936) described the fascial tube enclosing the sciatic nerve as a prolongation of the parietal pelvic fascia that gets drawn laterally through the great sciatic notch by the the piriformis and sciatic nerve.
The sciatic nerve then runs under the gluteus maximus beside the ischium (Ripani et al 2006). Martin et al (2011) found the sciatic nerve ran between 1.2 to 2mm from the most lateral part of the ischial tuberosity. Fascial bands run from the anterior aspect of the gluteus maximus that expand to the proximal attachment of the hamstring muscles at the ischial tuberosity. Laterally this fascial expansion splits to form a canal that surrounds the sciatic nerve and the posterior femoral cutaneous nerve (Perez-Bellmunt et al 2015). Farfan et al (2020) found the sciatic nerve covered by transverse connective tissue from the biceps femoris.
Therefore, from the ventral sacral rami to the nerve trunk exiting the gluteal region the sciatic nerve is enclosed in a fascial/connective tissue sheath. From the top down this sheath is associated with the piriformis, obturator-gemelli complex and gluteus maximus/proximal hamstring attachment and biceps femoris. Stecco (2014) simply stated the sciatic nerve was ensheathed usually down to the popliteal fossa but sometimes the proximal thigh by the piriformis fascia.
Innervation
Tibial nerve: (motor) hamstring muscle, posterior compartments of the calf, muscles of the sole of the foot (Lewis et al 2016).
Common peroneal nerve: (motor) short head of biceps femoris, and the muscles of the lateral and anterior compartments of the calf and the dorsum of the foot (Lewis et al 2016).
It was believed that after the branching of these two nerves occurs there was no further communication between them. A recent paper found that 75% of the sciatic nerves examined showed some variation of connection between these two nerves. This concludes the division of the sensory and motor innervation of these two nerves may not be as clearly divided as once believed (Lewis et al 2016).
Anatomy of the pudendal nerve
Nerve root: ventral divisions of ventral roots of the S2-S4.
Nerve trunk: It extends from the pelvis under the inferior border of the piriformis, crossing the ischial spine posteriorly and reenters the pelvis through the lesser sciatic foramen. It then reaches the pudendal canal on the medial surface of the ischial tuberosity, where it divides into its terminal branches: the perineal nerve, the dorsal nerve of the penis or clitoris, and the inferior rectal nerve.
In some cases the pudendal nerve (or pudendal plexus, or ventral branch of S2) can give off the perforating cutaneous nerve of the sacrotuberous ligament nerve <between the sacrospinous ligament and sacrotuberous ligament and ischial spine. The nerve then perforates the sacrotuberous ligament —> posterior surface of the sacrotuberous ligament —> ischioanal fossa —> innervate inferomedial area of the buttocks (and possibly the medial area of the perineal skin) (Shafarenko et al 2022).
Innervation: sensory: skin of the pelvic muscles and the genitals except the area around of the mons pubis and the anterior part of the scrotum or labia. Motor (somatic and parasympathetic): pelvic muscles and the external genitalia.
Anatomy of the piriformis
The piriformis arises from S2-4 and the PIIS. However, the attachment of the piriformis can also be to the second anterior sacral foramina's medial border or completely covering and filling up of the S2 foramen (Larionov et al 2022). 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).
Spanning the pelvis the piriformis attaches to the summit and medial aspect of the greater trochanter where its tendon fuses with the tendons of the superior gemellus, obturator internus and inferior gemellus to form the conjoint tendon (Goidescu et al 2021). The conjoint tendon attaches to the greater trochanter, proximal part of the anterior intertrochanteric line, joint capsule, posterior margin of gluteus medius and obturator externus tendon (Solomon et al 2010). The prirformis also has variable attachments to the fibrous capsule of the hip joint (Roche et al 2013). The piriformis, along with the obturator internus and coccygeus, comprises the pelvic wall.
The piriformis forms a canal with the superior gemellus 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 (Fernandez et al 2015).
Haladaj et al (2015) found four variations of the piriformis:
‘Text book’ piriformis morphology where the muscle is pear-shaped.
The piriformis is divided into two parts with the common peroneal nerve running between them.
Fusion of the piriformis with the gluteus medius.
Fusion of the piriformis with, as well as the gluteus medius, the superior gemellus, and in rare cases, the obturator internus.
The piriformis is innervated by branches from the sacral plexus, specifically arising from S1–S2 nerve roots (Butz et al 2015). 40% innervation comes from the S1 sacral plexus nerve root (S1), 20% from L4/L5, 20% from the S2 sacral plexus nerve root and 20% from a double innervation from the S1 and S2 sacral plexus nerve roots (Larionov et al 2022).
Piriformis fascia (parietal pelvic fascia)
The fascia of the piriformis is a thin fibrous layer which adheres firmly with the muscle. It’s boundaries are (Stecco 2014):
Medially: fuses with the periosteum at the front of the sacrum around the margins of the anterior sacral foramen, where it ensheathes the sacral nerves, and sides of the greater sciatic foramen. The fascia anterior to the piriformis communicates with the presacral space and thus with the piriformis fascia from the opposite side. Posteriorly the piriformis fascia doesn’t cross the midline attaching to the lateral boarder of the sacrum (Lanzier & Hilal 1984).
Laterally: covers gluteus minimus and merges with the periosteum of the ilium.
Inferiorly: ensheathes the sciatic nerve usually to the popliteal region but can terminate in the proximal thigh.
The piriformis fascia is thickened at its superior and inferior end filling the gaps between its related structures (Nutter 1936): (i) at the upper boarder of the piriformis where it is contiguous with the lower edges of the gluteus medius and minimus. (ii) At the lower boarder of the piriformis, in the space between the piriformis and superior gemellus, immediately lateral to the sciatic nerve. The thickening of the fascia at these levels can create definitive bands that can act as a potential source of nerve entrapment involving the superior gluteal nerve superiorly, or, the sciatic, posterior femoral cutaneous or inferior gluteal nerve inferiorly.
The piriformis fascia (along with the obturator internus fascia) blends with the arcus tendineus fasciae pelvis (refer ‘anatomy of the obturator internus’; ‘obturator fascia’) (Roch et al 2021). At its sacral attachment the ventral sacral rami (carrying the pelvic parasympathetic neurones) are enclosed between the anterior surface of the piriformis posteriorly, the parietal pelvic fascia (including the fascia of the piriformis) (Williams and Warwick 1980) and complex of internal iliac vessels (Shafarenko et al 2022) anteriorly and the obturator internus laterally (Gaertner 2006). The pelvic preganglionic parasympathetic neurones pass from the S2-4 ventral sacral rami, after they emerge from the sacral foramina, to pass through the parietal pelvic fascia to form usually three pelvic splanchnic nerves. These pelvic splanchnic nerves then enter the inferior hypogastric plexus to synapse with postganglionic fibers that terminate at their target organ (Goidescu et al 2022).
Accessory piriformis
Accessory fibers can originate from the anterior inferior-lateral portion of the first sacral segment and cover the second anterior sacral foramen. With this arrangement the S2 rootlets emerging from the foramen must pass through this portion of the piriformis (Chapman & Bakkum 2012).
Accessory fibers can cross anterior to the sacral foramen and the sacral nerve (Sen & Rajesh 2011) especially at S2 (Larionov et al 2022).
Accessory fibers can originate from the sacrotuberous ligament and the fascia overlying the gluteus medius to merge with the main tendinous part of the piriformis. The main trunk of the sciatic nerve was found deep to the accessory slip (Ravindranath et al 2008).
An accessory piriformis was found inferior to the main piriformis with its own separate tendon attached to the greater trochanter. The sciatic nerve is sandwiched between the main piriformis superiorly and this smaller accessory piriformis inferiorly (Carro et al 2016).
Tanyeli et al (2006) found not just a double piriformis in the same buttock but also a double superior and inferior gemelli and quadratus femoris.
Inferiorly located accessory fibers of the piriformis have not just been associated with sciatica but also coccygodynia (Butz et al 2015).
Function of the piriformis
The piriformis functions in the abduction and external rotation of the hip during its flexion (Pierce et al 2017), to, along with the other hip rotators and ischiofemoral ligament, compress the hip joint along a single axis (Baba et al 2022).
Roche et al (2013) found in 90 degs hip flexion the piriformis lies directly behind the hip joint. This might reasonably be considered to contribute to the stability of the joint by restricting posterior translation of the femoral head (Carro et al 2016).
Also at 90 degs hip flexion Keskula & Tamburello (1992) found the rotary component of the piriformis decreased. They found it had a significant abductor component and functions as an internal rotator of the hip. Consequently Carro et al (2016) found hip flexion, adduction and internal rotation stretches the piriformis muscle and cause narrowing of the space between the inferior border of the piriformis, superior gemellus and sacrotuberous ligament, however, Baba et al (2022) described the piriformis as only restricting internal rotation of the hip at 0° flexion.
The piriformis has a dual action in creating a vertical shear at the sacroiliac joint (caudal movement of the sacrum with nutation, and a cephalic movement of the innominate with posterior rotation) and resisting this vertical shearing through sacroiliac joint 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), whilst along with the other hip external rotators and ischiofemoral ligament, compresses the hip joint (Baba et al 2022); this reflects 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.
Anatomy of the obturator internus
The obturator internus muscle originates (i) rami surrounding the obturator foramen and (ii) quadrilateral plate —> lesser sciatic foramen —> obturator internus (and gemelli) + piriformis = conjoint tendon —> greater trochanter, proximal part of the anterior intertrochanteric line, joint capsule, posterior margin of gluteus medius and obturator externus tendon (Solomon et al 2010). The obturator internus, along with the coccygeus and piriformis, comprises the pelvic wall.
Obturator fascia (parietal pelvic fascia)
The obturator fascia is located on the lateral side of the pelvic cavity covering the obturator internus, it is firmly attached to this muscle above the arcus tendinous levator ani (Chin et al 2021). It envelopes the obturator internus as well as the gemelli separating these muscles with intermuscular septa. It is a continuation of the iliac fascia that follows the obturator internus, gradually separating from the iliac fascia as the obturator internus leaves the pelvic cavity through the lesser sciatic foramen. It covers the neighbouring superior gemelli superiorly, and inferior gemelli, and from there the quadratus femoris and sacrotubeorus ligament inferiorly (Stecco 2014).
The obturator fascia attachments are (Chin et al 2021):
Superiorly: inferior border of the superior ramus of pubic bone.
Inferiorly: ischial spine and arcus tendinous fasciae pelvis.
Anteriorly: fused with the fascia of the puborectalis muscle (anterior part of the levator ani).
Posteriorly: anterior boarder of the greater sciatic notch.
Muscle fibers from the levator ani, that are continuous with the coccygeus superiorly and the external anal sphincter inferiorly, attach to the obturator fascia. The levator ani moves in conjunction with the obturator internus through the levator ani’s attachments to the obturator fascia (Muro et al 2023):
The anterior part of the levator ani, at its pubic bone attachment, has tendinous attachments to the obturator fascia.
The posterior part of the levator ani originates from the obturator fascia (arcus tendinous levator ani). The fascia of the levator ani also fuses with the obturator fascia above the arcus tendinous levator ani (Chin et al 2021).
The levator ani originates from the inferior region of the obturator fascia, in a position entirely different from the conventional ‘arcus tendinous levator ani’, between the obturator canal and ischioanal (or rectal) fossa.
Septa-like fibers originating from the gluteus maximus pass inferiorly/medially to attach it on to the obturator internus muscle (and its fascia by the inferior ramus of the ischium) and the urogenital diaphragm. This fascia then extends from the obturator internus/urogenital diaphragm to the levator ani fascia. This myofascial complex distributes load between the gluteus maximus and urogenital diaphragm, which, if disturbed can exert a lateral traction force on the sphincter muscles embedded in the urogenital diaphragm causing urinary incontinence. Additionally, the fascial connections of gluteus maximus and obturator internus, around the inferior ramus of the ischium, form a neurovascular sheath for the inferior rectal nerve and vessels as they enter the pudendal canal and descend to innervate the external anal sphincter, which in turn, connects this sheath to the sphincter (Siess et al 2023).
The obturator fascia contains the arcus tendinous levator ani, the thickening of the parietal fascia that attaches onto the ischial spine and pubic bone. Traditionally the arcus tendinous levator ani been described as giving rise to the levator ani, but this has been disputed by Chin et al (2021) who described it, not as a site of origin for the levator ani, but a line of fusion for the levator ani fascia and obturator internus fascia. Could the arcus tendinous levator ani, along with the arcus tendineus fasciae pelvis*, to which, along with the obturator internus and piriformis (Roch et al 2021), it is attached, transmit forces between the hip (obturator internus) and pelvis (levator ani)? (Amorim et al 2017). These authors described the pubococcygeus and iliococcygeus as extending from the arcus tendinous levator ani to the inferior boarder of superior ramus of pubic bone. The would make the origin of all three parts of the levator ani (pubococcygeus, iliococcygeus and puborectalis) as being continuous with the pubic symphysis and inferior border of superior ramus of the pubis which would provide stronger support for the pelvic organs than if they were just attached to the arcus tendinous levator ani (Chin et al 2021).
Due to the attachments to the levator ani to the pubic symphysis Ramin et al (2015) provided a myofascial link from the abdominal wall to the obturator fascia: internal oblique and transverse aponeurosis, that blend into each other at the pubic symphysis, forming the urogenital diaphragm —> central tendon of perineum —> levator ani. From the levator ani (i) superficial layer, above the levator ani, formed by the superior band of the pelvic diaphragm, bends posteriorly, through the arcus tendinous fasciae pelvis* —> obturator internus aponeurosis; (ii) through the levator ani —> deep portion of the anococcygeal ligament —> presacral fascia —> iliac fascia of the iliopsoas; (iii) deep layer, below the levator ani, formed from the the lower band of the pelvic diaphragm, that connects with the aponeurosis of the internal oblique.
Superior to the pudendal canal, the superior and inferior fascia of the pelvic diaphragm joins the obturator fascia (Muro et al 2023).
*: arcus tendineus fasciae pelvis: thickening of the endopelvic fascia that extends from the pubic symphysis to the ischial spine.
These myofascial attachments between the pelvic floor and obturator internus fascia are the proposed mechanism by which hip rehabilitation contributes to the strengthening of the pelvic floor muscles to improve stress incontinence (Muro et al 2023).
Hip joint compression: external rotators & ischiofemoral ligament
The ischiofemoral ligament is the primary contributor to hip stability by restricting internal rotation of the hip at < 30° flexion as it pulls the femoral head into the acetabulum (Baba et al 2022). This stabilising function is augmented by the external rotators. Whilst their individual cross sectional areas has minimal impact on hip joint compression when looked upon as a collective exceeds that of the gluteus minimus (Parvaresh et al 2019).
The superior gemellus, obturator internus, inferior gemellus, and obturator externus are essentially fused. With the addition of quadratus femoris and piriformis, the collective “short external rotators” become a substantial force-producing unit. Considering them as a unit, with a large physiological cross sectional area and short fiber length, enables to have a stabilising function (Parvaresh et al 2019) < the quadratus femoris (Takao et al 2018). This stabilising effect occurs by their accumulated external function combining with their rotational antagonists (gluteus minimus, pectineus, and adductors), to provide a simultaneous internal and external rotational contraction, that creates a medial compressive force that facilitates dynamic stabilisation of the hip joint (Parvaresh et al 2019).
Greater sciatic foramen
The boundaries of the greater sciatic foramen are:
Anterolaterally: greater sciatic notch of the ilium.
Posteromedially: sacrotuberous ligament.
Inferiorly: sacrospinous ligament and the ischial spine.
Superiorly: anterior sacroiliac ligament.
The piriformis, runs through the greater sciatic foramen and occupies most of its volume. Above the piriformis the greater sciatic foramen is termed the suprapiriform canal and inferior the to the piriformis it is termed the infrapiriform canal. As the sciatic nerve passes over (or lateral) to the ischial spine (Hanna et al 2024) it can become compressed in the infrapiriform canal between the inferior margin of the piriformis and the superior margin of the ischial spine (Goidescu et al 2022).
The pudendal nerve (ventral divisions of the S2-S4 spinal nerves) can be compressed during contraction of the piriformis and is prone to entrapment as it leaves the greater sciatic foramen as it is closely associated with the rigid superior border of the sacrospinous ligament, passes between the piriformis and the sacrospinous ligament and then between the ligament and the upper border of the sciatic spine (Goidescu et al 2022).
The most common site of entrapment of the pudendal nerve occurs between the sacrospinous ligament and sacrotuberous ligament and ischial spine, which coincidentally is the most common site for the perforating cutaneous branch of the sacrotuberous ligament to arise from the pudendal nerve (Shafarenko et al 2022). But entrapment can also occur through the pudendal (Alcocks) canal (obturator internus fascia and the sacrotuberous ligament), directly by the obturator internus muscle medial to the ischium (Martin et al 2017), between the anterior-inferior part of the piriformis and superior border of the sacrospinous ligament or between the obturator internus, the superior gemellus and the inferior border of the ischial spine (Goidescu et al 2022). A block can be performed with the patient sidelying with hips flexed at 90° with inserting the needle lateral to ischial tuberosity and advancing it in a caudal to cranial direction in relation to the sacrotuberous ligament, to target the medial/superior surface of the obturator internus (Almeida 2022).
Suprapiriform canal
Simonova (1979) found the suprapiriform canal to be 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.
Infrapiriform canal
The infrapiriform canal contains the inferior gluteal nerve, pudendal nerve, sciatic nerve, posterior femoral cutaneous nerve, nerve to obturator internus and nerve to quadratus femoris.
As well as the suprapiriform canal hypertrophy of the piriformis has been associated with compression of the infrapiriform canal (Grgić 2013). Also Carro et al (2016) found hip flexion, adduction and internal rotation stretches the piriformis muscle causing a narrowing of the space between the inferior border of the piriformis, superior gemellus and sacrotuberous ligament.
Deep gluteal space (Carro et al 2016)
Anatomy and clinical relevance of the deep gluteal space
Martin et al (2015) defined the boarders of the deep gluteal (posterior peritrochanteric) space as:
Posteriorly: gluteus maximus.
Anteriorly: posterior acetabular column (posterior acetabulum and ischium), hip joint capsule and proximal femur.
Laterally: femur (lateral lip of linea aspera and gluteal tuberosity).
Medially: sacrotuberous ligament and falciform fascia.
Superiorly: inferior margin of the greater sciatic notch.
Inferiorly: proximal origin of the hamstrings at the ischial tuberosity.
Clinically the deep gluteal space is relevant as fibrous bands and hypertrophic thickening from surrounding structures can cause restricted movement as well as direct compression (dynamic and non-dynamic). Considering that a neuropraxia can be produced at 6% strain, and that a complete block can occur at 12% strain, the nerve kinematics in the deep gluteal space is a crucial aspect of the entrapment’s pathophysiology.
Movement of the sciatic nerve in the deep gluteal space has been measured (Martin et al 2015):
Hip flexion: the sciatic nerve has 28 mm of excursion during hip flexion. This motion is affected by anatomic variances between the sciatic nerve and piriformis.
Hip flexion, abduction and external rotation: the sciatic nerve glides across the posterior border of the greater trochanter. Also in this position the semimembranosus origin and the posterior edge of the greater trochanter can come into contact.
Knee flexion: the nerve moves posterolateral.
Knee extension: the nerve moves deep into the tunnel in the deep gluteal space.
Passive hip internal rotation: turns the sciatic nerve from a straight structure to a curved structure due to connective tissue attachments of the sciatic nerve to the gemellus-obturator system (Balius et al 2017).
Passive hip external rotation: causes the sciatic nerve to relax due to connective tissue attachments of the sciatic nerve to the gemellus-obturator system (Balius 2017).
Pathologies causing deep gluteal space problems
Carro et al (2016) identified the pathologies associated with deep gluteal space as:
Fibrous and fibrovascular bands.
Piriformis syndrome.
Gemelli-obturatus internus syndrome.
Quadratus femoris and ischiofemoral impingement syndrome.
Hamstring conditions (ischial tunnel syndrome).
Sacroiliac Joint Dysfunction.
Fibrous and fibrovascular bands
Fibrotic bands (with or without blood vessels) entrap the sciatic nerve. Nutter (1936) found these bands can be associated with the piriformis fascia as it condenses to fill any gaps superior and inferior to the muscle. The sciatic nerve should be able to stretch and glide to accommodate joint movement. Diminished or absent sciatic nerve mobility during hip and knee movements due to these bands can be a precipitating cause of sciatic neuropathy (ischaemic neuropathy).
There are three primary types of bands:
Fibrovascular bands: fibrous bands with blood vessels.
Fibrous bands: pure fibrous bands without blood vessels.
Vascular bands: bands exclusively formed by a blood vessel without any fibrous tissue.
Based on their location they can be classified as:
Proximal: affects the sciatic nerve in the vicinity of the greater sciatic notch. Martin et al (2011) found adhesions over the ischium anchoring the sciatic nerve laterally to the ischium.
Middle bands: located at the level of the piriformis and obturator internus-gemelli complex.
Distal: affects the ischial tunnel region between the quadratus femoris and proximal insertion of the hamstrings.
Based on their pathology the bands can compress the sciatic nerve in different ways:
Compressive or bridge-type bands (type 1): compresses the sciatic nerve in an anterior to posterior (type 1A) or posterior to anterior (type 1B) direction.
These fibrous bands usually extend from the posterior border of the greater trochanter and surrounding soft tissues (distal insertions are variable) to the gluteus maximus. These bands adhere onto the sciatic nerve and can extend as high as the greater sciatic notch.
Adhesive bands or horse-strap bands (type 2): these bind strongly to the sciatic nerve anchoring it in a single direction.
These type 2 bands can be:
Type 2a: attaches to the sciatic nerve laterally from the greater trochanter. This can include adhesions from the trochanteric bursa.
Type 2b: attaches to the sciatic nerve medially from the sacrotuberous ligament. The sciatic nerve runs between 1.2 to 2mm from the most lateral part of the ischial tuberosity (Martin et al 2011)
Bands anchored to the sciatic nerve with undefined distribution (type 3): These kinds of bands with an erratic distribution are characterized by anchoring the nerve in multiple directions.
Piriformis syndrome
The potential sources of pathology related to the piriformis muscle include:
Peh and Reinus (1995) found an enlarged piriformis bursa causing compression of the sciatic nerve. Their case showed an extravasation of hip joint contents into the piriformis bursa instead of the more common iliopsoas bursa.
Entrapment of the superior gluteal nerve in the suprapiriform canal (Diop et al 2002).
Hypertrophy of the piriformis causing an increase in thickness, area and volume of the muscle (Huang et al 2018).
Dynamic sciatic nerve entrapment by the piriformis.
Entrapment sites for the sciatic nerve from the piriformis include:
Sacral origins of the piriformis: nerve root compression. The ventral sacral rami (carrying the pelvic parasympathetic neurones) are enclosed between the anterior surface of the piriformis posteriorly, the parietal pelvic fascia (including the fascia of the piriformis) (Williams and Warwick 1980) and complex of internal iliac vessels (Shafarenko et al 2022) anteriorly and the obturator internus laterally (Gaertner 2006). When the piriformis attaches to the medial boarder of second anterior sacral foramina or complete covers and fills up this foramina. The S2 root of the sacral plexus sends 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) innervating the obturator internus, the piriformis and the gemellus superior (Larionov et al 2022).
Infrapiriform space: as the nerve trunk passes over (or lateral) to the ischial spine (Hanna et al 2024) it can become compressed between the inferior border of the piriformis and the upper border of the ischial spine (Goidescu et al 2022).
Other structures apart from the sciatic nerve have been documented as being compressed/irritated within the infrapiriform foramen. These include various neural and vascular structures (Grgić 2013):
Inferior gluteal nerve: atrophy of gluteal muscles.
Posterior femoral cutaneous nerve: pain, paraesthesia and sensory disturbances in the posterior thigh.
Pudendal nerve: pudendal neuralgia, dyspareunia, sexual dysfunction, changes in bowel and/or bladder habits.
Inferior gluteal artery: ischaemic buttock pain.
Inferior pudendal artery: ischaemic pain in the area of external sex organs, perineum and rectum, sexual dysfunction, urination and defecation problems.
Inferior gluteal vein: venous stasis in gluteal area.
Inferior pudendal vein: venous stasis in external sex organs and rectum.
Grgić (2013) is the only author to comment on vascular symptoms associated with piriformis syndrome.
Piriformis syndrome is not only bought on and causes sciatic symptoms by mechanical effects but also physiological. Akçali (2009) presented a patient with CRPS in the foot secondary to piriformis syndrome. Alonso-Blanco et al (2011) attributed central sensitisation to the generation of myofascial trigger points whilst Md Abu Bakar Siddiq et al (2014) gave a case report of a patient with piriformis syndrome associated with the central sensitisation along side fibromyalgia.
Gemelli-obturator internus syndrome
Obturator internus/gemelli complex pathology is rare. As the sciatic nerve passes under the belly of the piriformis and over the superior gemelli/obturator internus, a scissor-like effect between the two muscles can be the source of sciatic nerve entrapment.
Balius et al (2017) found connective tissue attaching the sciatic nerve to the gemmeli-obturator system. This caused a tethering of the sciatic nerve at two points during internal rotation of the hip.
Additional anatomical considerations is potential fusion of the piriformis tendon to the obturator internus tendon and the falciform process that is an expansion of the sacrotuberous ligament that fuses with the fascia of the obturator internus (Fernandez et al 2015).
Quadratus femoris and ischiofemoral pathology
Ischiofemoral impingement syndrome (IFI) is characterised by a narrowing of the space between the ischial tuberosity and the femur. Impingement between these structures is rare, considering that the distance between the lesser trochanter and ischium is approximately 20 mm (Lee et al 2013)
Narrowing of the ischiofemoral space leads to muscular, tendon and neural changes. Lee (2013) attributed the symptoms mainly to the narrowing of the ischiofemoral space leading to oedema, fatty infiltration or tearing of the quadratus femoris muscle and other hip muscles, especially the hamstring muscle. This results in:
Irritation of sciatic nerve caused by the proximity of the oedematous quadratus femoris.
Snapping hip sensation. Wilson and Keene (2016) theorised that it is the psoas tendon trapping and then abruptly rolling over the lateral edge of the swollen quadratus femoris muscle that produces the ‘snap’.
Groin pain: an enlarged quadratus femoris can press anteriorly creating a bursa-like formation surrounding the lesser trochanter.
Pain lateral to the ischium and/or in the centre of the buttock (Carro et al 2016).
Hamstring conditions (ischial tunnel syndrome)
The sciatic nerve runs between 1.2 to 2mm from the most lateral aspect of the ischial tuberosity (Martin et al 2011). This close proximity can render the sciatic nerve to be affected by a wide spectrum of hamstring origin enthesopathies.
Farfan et al (2020) found transverse connective tissue bands extending from the biceps femoris over the sciatic nerve. Could this be a site of potential entrapment?
Fascial bands run from the anterior aspect of the gluteus maximus and expand to the proximal attachment of the hamstring muscles. Laterally this fascial expansion splits to form a canal that surrounds the sciatic nerve and the posterior femoral cutaneous nerve (Perez-Bellmunt et al 2015). Ripani et al (2006) found sciatic symptoms arising in the hamstring region where the nerve runs under the gluteus maximus beside the ischium.
Sacroiliac joint dysfunction
Whilst the piriformis had no direct contact with a sacroiliac joint the L5 and S1 nerves are in contact with the sacroiliac joint’s anterior projection. Therefore, any inflammatory response of the sacroiliac joint may elicit neurological effects on the L5–S1 anterior spinal branch. The S1 ventral branches innervating the piriformis are located at the area of sacroiliac joint projection explaining piriformis pain, posterolateral gluteal pain, and any unusual pain in the buttock area without sciatic nerve compression (Larionov et al 2022).
Diagnosis of the piriformis and stretching of the piriformis/obturator internus
Diagnosis
Han et al (2017) highlighted the diagnostic procedure for piriformis syndrome. This included:
Palpation to test for tenderness over the origin (sacroiliac joint) or insertion of the short external rotators behind the trochanter.
Provocation tests for pain and weakness including:
Pace test: resisted abduction and external rotation of the thigh in a sitting position.
Freiberg's test: pain on forced passive internal rotation of the extended thigh.
Lasègue's sign/SLR: buttock and leg pain during passive straight leg raising.
Han et al (2017) described the piriformis as firm and hard to palpation from the greater sciatic notch to the posterior aspect of the greater trochanter. In physical examination they described tenderness of the piriformis the most consistent finding in 83% of cases. This was confirmed by Huang et al (2018) who found tenderness in the piriformis a constant diagnostic finding but tenderness in the suprapiriform canal more variable attributing it to either piriformis syndrome or other causes of sciatica. Martin et al (2015) suggested palpation of the muscle as a diagnostic test with the muscle under stretch.
Stretching of the piriformis quadratus coxa & quadratus femoris
Stretching of the piriformis
Lewis et al (2016) described Gulledge et al (2016) research on the effectiveness of stretching the piriformis. They described flexion/adduction/internal rotation stretches designed to decrease the pain from piriformis syndrome. The stretches decrease compression of the sciatic nerve by relaxing the piriformis muscle by increasing the resting length. They used CT imaging to measure the impact of the stretches on the piriformis. They found that stretches lasting 20–30 seconds for a total of 7–14 stretches over 5 min increased the length of the piriformis by 30–40%. Increasing flexion from 90 degs to 115-120 degs elongates the piriformis by 15%.
Wright and Drysdale (2008) found similar results with post-isometric relaxation and reciprocal inhibition in increasing hip internal rotation which they hypothesised as being due to increase piriformis length.
Stretching of the quadratus coxa (piriformis, obturator internus and superior gemellus)
Whilst the piriformis restricts hip internal rotation only at 0° of hip flexion (Baba et al 2022) maximum lengthening of the obturator internus (and inferior gemellus) was found to be at 45 degs hip internal rotation from a neutral position (McGovern et al 2017), although other authors have described the obturator internus (and obturator externus) restricting hip internal rotation at 0° and 60° hip flexion (Baba et al 2022).
However, although known as the 'external rotators' stretching of the quadratus coxa occurs during hip flexion and adduction. In the same study McGovern et al (2017) found the piriformis and superior gemellus best stretched in 90 degs hip flexion and 30 degs adduction. Varrbakken et al (2014) found the piriformis and obturator internus lengthened by 35mm and 30mm respectively in 105 degs hip flexion and 10 degs adduction. This emphasis on hip flexion and adduction when stretching the obturator internus was confirmed by Hodges et al (2014) who identified the obturator internus as primarily a hip extensor, (then external rotator) and then abductor.
Stretching of the quadratus femoris
Vaarbakken et al (2015) found the maximal length of the quadratus femoris to be delivered in flexion with adduction/abduction and internal rotation.
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