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.
Tibial nerve: The tibial nerve arises from ventral divisions of the ventral rami of L4-S3 (Butz et al 2015).
Common peroneal nerve: The common peroneal nerve arises from dorsal divisions of ventral rami of L4–S2 (Butz et al 2015).
The ventral sacral rami are enclosed between the Piriformis fascia and the Piriformis (Williams and Warwick 1980).
The Piriformis muscle forms a canal with the gemellus superior through which the sciatic nerve passes (Lewis et al 2016). It is enclsoed 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 muscle, 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:
(1) Sciatic nerve passes below the Piriformis muscle.
(2) A divided nerve passes through and below the Piriformis muscle.
(3) A divided nerve passes above and below the Piriformis muscle.
(4) An undivided nerve passes through the Piriformis.
(5) A divided nerve passes through and above the Piriformis muscle.
(6) 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 the superior and inferior gemellus muscles above and below it) (Balius et al 2017)
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.
As you can see 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.
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 Piriformis
The Piriformis muscle arises from S2-4 and the PIIS. 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 enclsoed 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:
(1) ‘Text book’ Piriformis morphology where the muscle is pear-shaped.
(2) The Piriformis is divided into two parts with the common peroneal nerve running between them.
(3) Fusion of the Piriformis with the Gluteus Medius.
(4) Fusion of the Piriformis muscle with (as well as the gluteus medius), the superior gemellus muscle and in rare cases with the obturator internus.
The fascia of the Piriformis is thin and fuses with the periosteum at the front of the sacrum around the margins of the anterior sacral foramen.
At its sacral attachment it ensheaths the nerves emerging from these foramina (Williams and Warwick 1980). Florian-Rodriguez et al (2017) found the first sacral nerve was located just behind the layer of parietal fascia covering the piriformis muscle.
The Piriformis is innervated by branches from the sacral plexus, specifically arising from S1–S2 nerve roots (Butz et al 2015).
(1) 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 muscle (Chapman & Bakkum 2012).
(2) Accessory fibers can cross anterior to the sacral foramen and the sacral nerve (Sen & Rajesh 2011).
(3) 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 muscle. The main trunk of the sciatic nerve was found deep to the accessory slip (Ravindranath et al 2008).
(4) An accessory Piriformis was found inferior to the main Piriformis with it’s 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 gemelli superior and inferior 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).
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.
The Piriformis may function in innominate rotation.Dontigny (2017) found an anterior innominate rotation causes the innominate to rotate cephalad and laterally at the PIIS. This places a vertical shear and separation movement between S3 and the PIIS.
This cephalic and lateral movement places a vertical shear on and separates the ilial origins of both the gluteus maximus and the piriformis at the superior margin of the greater sciatic notch resulting in buttock pain, piriformis syndrome and sciatica. Tension can also be transmitted through the tensor fascia lata into the lateral knee.
As this anterior innominate rotation stretches the Piriformis could tightening of this muscle stabilise the innominate in posterior rotation.
Greater sciatic foramen
The boundaries of the greater sciatic foramen are:
- Anterolaterally by the greater sciatic notch of the ilium.
- Posteromedially by the sacrotuberous ligament.
- Inferiorly by the sacrospinous ligament and the ischial spine.
- Superiorly by the 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.
The pudendal nerve is prone to entrapment as it leaves the greater sciatic foramen. The most common site of entrapment occurs between the Sacrospinous Ligament and Sacrotuberous Ligament and Ischial spine. But entrapment can also occur through the Pudendal (Alcocks) canal formed by the Obturator Internus fascia and the Sacrotuberous ligament or directly by the Obturator Internus muscle medial to the ischium (Martin et al 2017).
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.
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 the piriformis muscle.
- 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: turned the sciatic nerve from a straight structure to a curved strructure due to connective tissue attachments of the sciatic nerve to the gemellus-obturator system (Balius et al 2017).
- Passive hip external rotation: caused 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:
(1) Fibrous and fibrovascular bands.
(2) Piriformis syndrome.
(3) Gemelli-Obturatus internus syndrome.
(4) Quadratus Femoris and Ischiofemoral Impingement Syndrome.
(5) Hamstring conditions (Ischial Tunnel Syndrome).
1. Fibrous and fibrovascular bands
Fibrotic bands (with or without blood vessels) entrap the sciatic nerve. 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 (ischemic neuropathy).
There are three primary types of bands:
(a) Fibrovascular bands: fibrous bands with blood vessels.
(b) Fibrous bands: pure fibrous bands without blood vessels.
(c) Vascular bands: bands exclusively formed by a blood vessel without any fibrous tissue.
Based on their location they can be classified as:
(a) 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.
(b) Middle bands: located at the level of the Piriformis and obturator internus-gemelli complex.
(c) 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:
(a) 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.
(b) 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)
(c) 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.
2. 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 muscle causing an increase in thickness, area and volume of the muscle (Huang et al 2018).
- Dynamic sciatic nerve entrapment by the Piriformis muscle.
- 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.
3. 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 sciatice nerve at two points during internal rotation of the hip.
Additonal anatomical considerations is potential fusion of the Piriformis tendon to the Obturator Internus tendon and the falciform process that is an expansion of the scarotuberous ligament that fuses with the fascia of the obturator internus (Fernandez et al 2015).
4. 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) theorized 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).
5. 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 sciactic 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.
Diagnosis of the Piriformis and stretching of the Piriformis/Obturator Internus
Han et al (2017) highlighted the diagnostic procedure for Pirifomis 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 muscle 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 muscle 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 muscle. They found that stretches lasting 20–30 seconds for a total of 7–14 stretches over 5 min increased the length of the Piriformis muscle 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)
McGovern et al (2017) found maximum lengthening of the Obturator Internus (and Inferior Gemellus) with the hip 45 degs internally rotated from a neutral position.
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 Gemmulus 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.
Streching 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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