Non-Discogenic Sciatic Nerve Entrapment: a Pictorial Review of Common Causes from a High-Volume Centre

Non-Discogenic Sciatic Nerve Entrapment: a Pictorial Review of Common Causes from a High-Volume Centre Poster No.: R-0071 Congress: 2016 ASM Type: Educational Exhibit Authors: A. Imrie, R. Pow, N. McGrigor, J. Fitzgerald ; QLD/AU, NSW/ AU Keywords: Musculoskeletal soft tissue, Pelvis, MR, Diagnostic procedure, Education, Athletic injuries DOI: 10.1594/ranzcr2016/R-0071 1 2 1 1 1 2 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply RANZCR's endorsement, sponsorship or recommendation of the third party, information, product or service. RANZCR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold RANZCR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies,.ppt slideshows,.doc documents and any other multimedia files are not available in the pdf version of presentations. Page 1 of 32

Learning objectives To improve understanding of the magnetic resonance imaging (MRI) features of common causes of non-discogenic sciatic nerve entrapment. We present this entrapment as the clinical entity of deep gluteal syndrome and highlight the relevant anatomy and pathological context of each cause. Background Posterior hip and buttock pain in a patient with a non-diagnostic lumbar spine MRI is a challenging radiological and clinical proposition. Piriformis syndrome has long been considered the primary cause of non-discogenic 'sciatica.'[1] The progression of MRI and introduction of MR neurography however has identified other causes. These aetiologies are encompassed by the term deep gluteal syndrome (DGS).[2,3] Deep Gluteal Syndrome DGS describes unilateral posterior hip, buttock or proximal thigh pain with or without radicular symptoms (pain, paraesthesia, dysaesthesia) caused by tethering or mechanical entrapment of the sciatic nerve within the sub-gluteal window (SGW). Entrapment can simultaneously involve the inferior gluteal or posterior femoral cutaneous nerves producing non-dermatomal symptoms. Motor dysfunction is rare.[2] Other features include intolerance of sitting for more than 20 minutes, nocturnal pain that improves during the day and gluteal and retro-trochanteric tenderness.[4,5] Sub-Gluteal Window The SGW contains fatty tissue between the deep and middle gluteal aponeurosis layers. These layers are rarely visible on MRI due to their close association with the deep fascia. The SGW is defined by anatomical boundaries: Boundary Anatomical Structure(s) superior greater sciatic notch anterior posterior acetabular column hip joint capsule posterior gluteus maximus Page 2 of 32

medial sacrotuberous ligament lateral gluteal tuberosity inferior ischial tuberosity Piriformis, superior gemellus, obturator internus, inferior gemellus and quadratus femoris (QFM) muscles lie within the SGW from superior to inferior (Fig. 1 on page 3). Piriformis divides the SGW into supra- and infra-piriform spaces. Its most important neuro-vascular structures are the sciatic nerve, posterior femoral cutaneous nerve, nerves to short and long heads of biceps femoris, inferior gluteal nerve and artery, and ascending circumflex femoral artery (Fig. 2 on page 4). Figures 3 and 4 demonstrate the relationships of these structures on T1-weighted images (Fig. 3 on page 5, Fig. 4 on page 7). Sciatic Nerve The sciatic nerve forms from the lumbosacral trunk (L4-5) and sacral plexus (S1-3). Anterior pre-axial L4-S3 branches form the anteromedial tibial division and post-axial L4-S2 branches form the posterolateral peroneal division. These divisions unite within a common sheath that passes under the greater sciatic notch and through the greater sciatic foramen to enter the SGW. It stretches and glides within the SGW to accommodate strain or compression associated with hip and knee joint movement. It has 28mm medial excursion during hip flexion.[6,7] Mechanical Entrapment and Tethering Any restriction to movement of the sciatic nerve within the SGW can cause DGS. The commonest causes from a high-volume musculoskeletal radiology clinic are: piriformis syndrome QFM strain ischiofemoral impingement (IFI) proximal hamstring tendinosis inferior gluteal venous varicosities Images for this section: Page 3 of 32

Fig. 1: Neuromuscular contents of the SGW (SP: sacral plexus, SN: sciatic nerve, STL: sacrotuberous ligament, P: piriformis, SG: superior gemellus, OI: obturator internus, IG: inferior gemellus). Page 4 of 32

Fig. 2: Neurovascular structures of the SGW (ITB: iliotibial band, F: femur, QFM: quadratus femoris, SN: sciatic nerve, GM: gluteus maximus, SM: semimembranosus tendon, B-ST: conjoint tendon of biceps femoris and semitendinous, IT: ischial tuberosity, 1: ascending circumflex femoral artery, 2: nerve to short head of biceps femoris, 3: vasa nervorum of sciatic nerve, 4: posterior femoral cutaneous nerve, 5: inferior gluteal artery and nerve, 6: nerve to long head of biceps femoris) Page 5 of 32

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Fig. 3: Normal anatomy of the right SGW on consecutive axial T1-weighted images - a) level of piriformis (sciatic nerve (arrow), P: piriformis, GM: gluteus maximus); b) level of obturator internus (sciatic nerve (arrow), OI: obturator internus, GM: gluteus maximus); c) level of the ischial tuberosity (sciatic nerve (arrow), QFM: quadratus femoris, GM: gluteus maximus, H: hamstring tendons, IT: ischial tuberosity). Page 7 of 32

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Fig. 4: Normal anatomy of the right SGW on consecutive coronal T1-weighted images - a) posterior (sciatic nerve (arrow), GM: gluteus maximus, SIJ: sacroiliac joint, SP: sacral plexus); b) anterior (sciatic nerve (arrow), P: piriformis, OI: obturator internus, H: hamstring tendons). Page 9 of 32

Imaging findings OR Procedure details Sciatic Nerve Injury Preferred imaging for DGS is 3T MR neurography. The sciatic nerve is most easily identified on axial T1-weighted images (Fig. 5 on page 13). It is oval-shaped with 40-60 uniform internal fascicles. It is isointense to muscle but has high signal peri-fasicular and peri-neural fat. Sciatic nerve injury is best identified on fat-suppressed T2-weighted and proton-density (PD) images on axial, axial-oblique or sagittal-oblique orientations. It is revealed as: endoneural and peri-fasicular hyper-intensity (Fig. 6 on page 15) loss of normal fasicular appearance neural thickening peri-neural oedema [2] Piriformis Syndrome Anatomy Piriformis is an external rotator and abductor of the hip joint that arises from the anterior surface of S2-4 vertebrae, the sacroiliac joint capsule and anterior sacrospinous ligament. It passes through the greater sciatic foramen and inserts laterally on the piriform fossa of the medial greater trochanter. There are six described anatomical relationships of the sciatic nerve to piriformis. The most common (Type A) sees an undivided sciatic nerve pass below piriformis into the infra-piriform space (Fig. 7 on page 15). This is considered a normal course in 83.1% of limbs.[9] The second commonest (Type B) is a divided sciatic nerve that passes through and below piriformis (Fig. 8 on page 16). A sub-set of this variation involves a smaller accessory piriformis with its own muscle belly and tendon (Fig. 9 on page 17). Pathology and Imaging Findings Piriformis syndrome is a controversial entity because it lacks validated diagnostic criteria. We consider it to represent dynamic sciatic nerve entrapment by piriformis. This is believed to establish a self-amplifying pain-spasm-inflammation-irritation cycle.[10] It has primary and secondary radiological variations. Primary relates to an anomalous piriformis attachment or accessory intra- or extra-pelvic muscular slip.[2] The most common attachment anomaly is an intra-foraminal origin that leads to impingement on S2-S3 nerve roots.[11] An intra-pelvic muscle slip arises from the anterior sacrum. It is best appreciated on axial PD-weighted images (Fig. 10 on page 18). An extrapelvic slip arises from the posterosuperior ilium and merges with the distal piriformis Page 10 of 32

musculotendinous junction. Secondary piriformis syndrome is asymmetric piriformis hypertrophy leading to anterior sciatic nerve displacement (Fig. 11 on page 19). Quadratus Femoris Strain Anatomy QFM originates from the anterior ischial tuberosity and inserts into the quadrate tubercle of the posteromedial proximal femur. It is bordered anteriorly by obturator externus and inferiorly by adductor magnus. The sciatic nerve passes intimately over its broad posterior surface (Fig. 12 on page 21). Pathology and Imaging Findings Tears most commonly involve its distal musculotendinous junction.[12] They are best seen on fat-suppressed PD or T2-weighted axial images as hyper-intense oedema with or without proximal muscle fibre retraction. Oedema from high-grade strains can encase the sciatic nerve and track to the lesser trochanter (Fig. 13 on page 21). Tears involving the femoral tendinous insertion are rare. Ischiofemoral Impingement Anatomy IFI is repetitive entrapment of the QFM muscle belly due to narrowing of ischiofemoral and quadratus femoris spaces. The ischiofemoral space is the narrowest distance between the lateral ischial tuberosity and medial lesser trochanter. In healthy controls it measures 23(±8)mm (Fig. 14 on page 23). The quadratus femoris space is between the superolateral hamstring tendon and lesser trochanter surfaces. It is 12(±4)mm in normal adults (Fig. 14 on page 23).[13] Pathology and Imaging Findings Congenital narrowing is most commonly caused by a larger femur cross-section at the level of the lesser trochanter or by a posteromedial femur position. Acquired causes include bony spurring of the ischial tuberosity, fractures of the lesser trochanter, osteoarthritis with superomedial femur migration and expansile bone lesions. IFI is best identified on fat-suppressed PD or T2-weighted axial images as hyper-intense oedema within the QFM muscle belly (Fig. 15 on page 23). This correlates with the site of maximal narrowing.[13,14] In severe cases an oedematous QFM can efface the sciatic Page 11 of 32

nerve. Oedema and bursa-like formations can also form around the sciatic nerve and lesser trochanter. Long-standing IFI is reflected by QFM atrophy and fatty infiltration and sub-cortical cysts within the ischium or lesser trochanter.[15] Proximal Hamstring Tendinosis Anatomy Semitendinosus and long head of biceps femoris arise as a common tendon from the inferomedial ischial tuberosity. Semitendinosus has a thin tendon-like appearance while long head of biceps femoris merges with its short head. The origin of semimembranosus is a crescent-shaped facet on the superolateral ischial tuberosity. Its proximal tendon has connections to long head of biceps femoris muscle and adductor magnus tendon. [16] The sciatic nerve passes 12(±2)mm lateral to the ischial tuberosity (Fig. 16 on page 25).[17] Pathology and Imaging Findings Tendinosis can involve one or both conjoint and semitendinosis tendons. It is characterised by hyper-intense intra-substance oedema best seen on fat-suppressed axial T2-weighted images. The investing paratenon is also usually thickened. Longitudinal partial thickness tears, ischiogluteal bursitis and reactive ischial tuberosity marrow oedema are also seen. Oedema and chronic adhesions can form around the sciatic nerve and limit its excursion (Fig. 17 on page 27). Inferior Gluteal Venous Varicosities Anatomy Inferior gluteal veins are venae comitantes of the inferior gluteal artery. They course 8(±4)mm medial and superficial to the sciatic nerve in the infra-piriform space.[18] As a neurovascular bundle with the inferior gluteal nerve they dive between superior gemellus and piriformis to enter the greater sciatic foramen. Pathology and Imaging Findings Varicosities in the SGW form as an asymmetrical cluster of vessels or single large vessel. They are appreciated on axial fat-suppressed PD and T2-weighted images by their hyperintensity. A venous plexus can encompass the sciatic nerve or it can be compressed by a single large varicosity.[19] Page 12 of 32

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Fig. 5: Normal appearance of the right sciatic nerve on consecutive axial T1-weighted images - a) level of the sciatic notch; b) level of the ischial tuberosity (sciatic nerve (arrow)). Fig. 6: Sciatic nerve injury within the left sciatic notch on fat-suppressed T2-weighted images - a) axial; b) coronal (sciatic nerve (arrow), normal right sciatic nerve for comparison (arrowhead)). It is characterised by T2-hyper-intensity and neural thickening. Page 15 of 32

Fig. 7: Normal (Type A) course of the sciatic nerve into the infra-piriform space - an undivided sciatic nerve passes below piriformis. Page 16 of 32

Fig. 8: Type B course of the sciatic nerve into the infra-piriform space - a divided sciatic nerve passes through and below piriformis. Page 17 of 32

Fig. 9: Type B sub-set - a smaller accessory piriformis passes between a divided sciatic nerve or below an undivided nerve. Page 18 of 32

Fig. 10: Right intra-pelvic piriformis muscle slip emerging from the anterior sacrum on axial PD imaging (intra-pelvic muscle slip (white arrowhead), GM: gluteus maximus, P: piriformis). This leads to dynamic sciatic nerve entrapment and establishes a pain-spasminflammation-irritation cycle. Page 19 of 32

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Fig. 11: Left piriformis hypertrophy on axial PD images - case 1; case 2 (sciatic nerve (arrow), OI: obturator internus, P: piriformis, GM: gluteus maximus). This leads to anterior sciatic nerve displacement and entrapment. Fig. 12: Normal appearance of the right quadratus femoris on axial T1-weighted imaging (sciatic nerve (arrow), QFM: quadratus femoris, GM: gluteus maximus, OE: obturator externus). Page 21 of 32

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Fig. 13: High-grade right quadratus femoris strains on axial fat-suppressed T2-weighted images - case 1; case 2 (sciatic nerve (arrow)). Case 1 involves 90% of its distal muscle fibres. Oedema tracks deep to the sciatic nerve and encases it at the level of the ischial tuberosity. Case 2 is a grade 2 strain and oedema outlines the sciatic nerve. Fig. 14: Normal right ischiofemoral and quadratus femoris spaces on axial T1-weighted imaging (ischiofemoral space (red line), quadratus femoris space (white line), sciatic nerve (arrow), GM: gluteus maximus, OE: obturator externus). Page 23 of 32

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Fig. 15: Right IFI with hyper-intense oedema through the quadratus femoris muscle belly on axial fat-suppressed T2-weighted images - case 1; case 2 (sciatic nerve (arrow), ischiofemoral space (red line), quadratus femoris space (white line), QFM: quadratus femoris). In case 1 oedema correlates with the site of maximal ischiofemoral and quadratus femoris space narrowing. Case 2 shows severe ischiofemoral and quadratus femoris space narrowing to 8mm and 5mm respectively. Page 25 of 32

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Fig. 16: Normal anatomy of the right proximal hamstring complex on axial T1weighted imaging (sciatic nerve (arrow), GM: gluteus maximus, BF: biceps femoris, ST: semitendinosus, SM: semimembranosus). Page 27 of 32

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Fig. 17: Severe right proximal hamstring origin tendinosis involving the conjoint and semimembranosus tendons on consecutive axial fat-suppressed T2-weighted images - a) superior; b) inferior (sciatic nerve (arrow), QFM: quadratus femoris, GM: gluteus maximus, AM: adductor magnus). Image a) shows hyper-intense oedema involving both tendons with moderate ischiogluteal bursitis. There is also evidence of IFI. Oedema extends laterally on image b) to partially encompass the sciatic nerve. Fig. 18: Prominent cluster of right inferior gluteal venous varicosities partially encompassing the sciatic nerve on axial fat-suppressed T2-weighted imaging (inferior gluteal venous varicosities (arrowhead), sciatic nerve (arrow)). The sciatic nerve is focally swollen at the level of the posterior ischial spine. Page 29 of 32

Conclusion DGS is an under-recognised cause of unilateral posterior hip and buttock pain. It is caused by mechanical entrapment of the sciatic nerve in the SGW that is best identified on fat-suppressed T2-weighted and PD images as endoneural hyper-intensity and neural thickening. The sciatic nerve can also lose its fasiculated appearance. The most common causes of DGS include primary and secondary piriformis syndrome, QFM strain, IFI, proximal hamstring tendinosis and inferior gluteal venous varicosities. Personal information Andrew Imrie MBBS (Hons) Redcliffe Hospital Anzac Avenue Redcliffe QLD 4020 Australia Email: Andrew.Imrie@health.qld.gov.au References 1. Halpin RJ, Ganju A. Piriformis syndrome: a real pain in the buttock? Neurosurgery. 2009;65(4):A197-A202. 2. Hernando MF, Cerezal L, Pérez-Carro L, Abascal F, Canga A. Deep gluteal syndrome: anatomy, imaging, and management of sciatic nerve entrapments in the subgluteal space. Skeletal Radiol. 2015;44(7):919-34. 3. Martin HD, Reddy M, Gómez-Hoyos J. Deep gluteal syndrome. J Hip Preserv Surg. 2015;2(2):99-107. Page 30 of 32

4. Martin HD, Shears SA, Johnson JC, Smathers AM, Palmer IJ. The endoscopic treatment of sciatic nerve entrapment/deep gluteal syndrome. Arthroscopy. 2011;27(2):172-81. 5. Martin HD, Kivlan BR, Palmer IJ, Martin RL. Diagnostic accuracy of clinical tests for sciatic nerve entrapment in the gluteal region. Knee Surg Sports Traumatol Arthrosc. 2014;22(4):882-8. 6. Güvençer M, Akyer P, Iyem C, Tetik S, Naderi S. Anatomic considerations and the relationship between the piriformis muscle and the sciatic nerve. Surg Radiol Anat. 2008;30(6):467-74. 7. Coppieters MW, Alshami AM, Babri AS, Souvlis T, Kippers V, Hodges PW. Strain and excursion of the sciatic, tibial, and plantar nerves during a modified straight leg raising test. J Orthop Res. 2006;24(9):1883-9. 8. Cassidy L, Walters A, Bubb K, Shoja MM, Tubbs RS, Loukas M. Piriformis syndrome: implications of anatomical variations, diagnostic techniques, and treatment options. Surg Radiol Anat. 2012;34(6):479-86. 9. Smoll NR. Variations of the piriformis and sciatic nerve with clinical consequence: a review. Clin Anat. 2010;23(1):8-17. 10. Benzon HT, Katz JA, Benzon HA, Iqbal MS. Piriformis syndrome: anatomic considerations, a new injection technique, and a review of the literature. Anesthesiology. 2003;98(6):1442-48. 11. Windisch G, Braun EM, Anderhuber F. Piriformis muscle: clinical anatomy and consideration of the piriformis syndrome. Surg Radiol Anat. 2007;29(1):37-45. 12. Kassarjian A, Tomas X, Cerezal L, Canga A, Llopis E. MRI of the quadratus femoris muscle: anatomic considerations and pathologic lesions. AJR Am J Roentgenol. 2011;197(1):170-4. 13. Torriani M, Souto SC, Thomas BJ, Ouellette H, Bredella MA. Ischiofemoral impingement syndrome: an entity with hip pain and abnormalities of the quadratus femoris muscle. AJR Am J Roentgenol. 2009;193(1):186-90. 14. Tosun O, Algin O, Yalcin N, Cay N, Ocakoglu G, Karaoglanoglu M. Ischiofemoral impingement: evaluation with new MRI parameters and assessment of their reliability. Skeletal Radiol. 2012;41(5):575-87. 15. Taneja AK, Bredella MA, Torriani M. Ischiofemoral impingement. Magn Reson Imaging Clin N Am. 2013;21(1):65-73. 16. Koulouris G, Connell D. Hamstring muscle complex: an imaging review. RadioGraphics. 2005;25(3):571-86. Page 31 of 32

17. Miller SL, Gill J, Webb GR. The proximal origin of the hamstrings and surrounding anatomy encountered during repair. A cadaveric study. J Bone Joint Surg Am. 2007;89(1):44-48. 18. Martin HD, Palmer IJ, Hatem M. Monopolar radiofrequency use in deep gluteal space endoscopy: sciatic nerve safety and fluid temperature. Arthroscopy. 2014;30(1):60-4. 19. Hu MH, Wu KW, Jian YM, Wang CT, Wu IH, Yang SH. Vascular compression syndrome of sciatic nerve caused by gluteal varicosities. Ann Vasc Surg. 2010;24(8):1134e1-4. Page 32 of 32