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Lesser sciatic notch

The lesser sciatic notch is a shallow, U-shaped indentation on the posterior margin of the ischium, the posteroinferior component of the hip bone (os coxae), positioned inferior to the ischial spine and superior to the ischial tuberosity.^[1] This notch serves as a key anatomical landmark in the bony pelvis, forming the superior boundary of the lesser sciatic foramen when bridged by the sacrospinous ligament, which extends from the lateral sacrum to the ischial spine.^[2] Together with the sacrotuberous ligament, it delineates a passageway for critical neurovascular structures exiting the pelvic cavity toward the perineum and lower limb.^[3] The lesser sciatic foramen, resulting from the notch's enclosure, transmits the tendon of the obturator internus muscle, the nerve to the obturator internus, the pudendal nerve, and the internal pudendal artery and vein.^[1] These structures are essential for innervation and vascular supply to the perineal region, including the external genitalia and anal sphincter.^[3] Anatomically, the notch's configuration varies slightly among individuals but maintains its role in dividing the pelvic outlet and facilitating the transition of soft tissues from the pelvis to the gluteal region.^[2] Clinically, the lesser sciatic notch and its foramen are relevant in conditions involving pudendal nerve compression, which can lead to perineal pain, sensory disturbances, or dysfunction in pelvic floor muscles.^[1] Imaging modalities such as MRI or CT scans often visualize this structure to assess pelvic fractures, ligamentous injuries, or congenital variations that may impinge on passing neurovascular elements.^[1]

Anatomy

Location

The lesser sciatic notch is a concave indentation located on the posterior border of the ischium, which forms the lower and posterior portion of the hip bone (os coxae).^[1]^[4] This notch is positioned immediately inferior to the ischial spine and superior to the ischial tuberosity, delineating key bony landmarks on the ischium.^[2]^[5] It lies on the medial aspect of the pelvis, contributing to the boundary that separates the pelvic cavity from the gluteal region posteriorly.^[6]^[7] The lesser sciatic notch is typically smaller and shallower than the greater sciatic notch, serving as its inferior counterpart on the hip bone.^[1]^[4] It becomes fully defined during the postnatal ossification and fusion processes of the ischium, which generally complete around ages 15–16 years as secondary ossification centers integrate.^[8]^[1]

Structure

The lesser sciatic notch is a concave indentation on the posterior border of the ischium, situated between the ischial spine superiorly and the ischial tuberosity inferiorly, forming a smooth, cartilage-coated concavity in its fresh state that serves as a transitional surface for ligament attachment.^[9]^[10]^[11] This 'C'-shaped structure provides the anterior bony margin for the passage of structures exiting the pelvis.^[11] The notch is transformed into the lesser sciatic foramen by the spanning of the sacrospinous ligament along its superior border and the sacrotuberous ligament along its inferior border, resulting in a small, oval-shaped osteoligamentous opening.^[12]^[7] The boundaries of this foramen are defined superiorly by the sacrospinous ligament (extending from the ischial spine to the lateral sacrum or coccyx), inferiorly by the sacrotuberous ligament (spanning from the ischial tuberosity to the sacrum and coccyx), anteriorly by the ischial bone comprising the notch itself, and posteriorly by the ligamentous structures bridging to the sacrum and coccyx.^[7]^[1] Anatomical variations of the lesser sciatic notch include occasional bridging by osseous or ligamentous tissue, which may result in partial or complete closure of the foramen in affected individuals.^[13] On imaging, the lesser sciatic notch appears as a distinct concavity inferior to the ischial spine on computed tomography (CT) and magnetic resonance imaging (MRI), with the completed foramen best evaluated in coronal and sagittal planes to assess its boundaries and patency.^[12]^[14]

Function

Role in muscle support

The lesser sciatic notch, located on the posterior border of the ischium, is spanned by the sacrospinous and sacrotuberous ligaments, which convert the notch into the lesser sciatic foramen and play essential roles in stabilizing the sacrum and coccyx relative to the ischium and overall pelvic girdle. The sacrospinous ligament spans from the lateral margins of the sacrum and coccyx to the ischial spine adjacent to the notch, providing tensile support that resists anterior rotation of the sacrum during upright posture and movement. Similarly, the sacrotuberous ligament attaches from the sacrum, coccyx, and ilium to the ischial tuberosity near the notch, enhancing posterior pelvic stability by countering shear forces across the sacroiliac joint. These ligamentous connections collectively prevent excessive mobility of the sacrum against the ischium, maintaining the structural integrity of the pelvic ring under compressive loads. The notch indirectly facilitates the biomechanical function of the obturator internus muscle, a key deep rotator of the hip, by forming part of the lesser sciatic foramen through which its tendon passes. The tendon's path hooks around the ischial spine and traverses the foramen created by the notch and overlying ligaments, allowing the muscle to insert on the greater trochanter of the femur after exiting the pelvis. This arrangement enables the obturator internus to contribute to external rotation of the extended hip and abduction of the flexed hip, while also stabilizing the femoral head within the acetabulum during dynamic activities. The foramen's position ensures efficient force generation from the pelvic cavity to the lower limb without compromising the surrounding soft tissues. By delineating the inferior boundary of the pelvic outlet, the lesser sciatic notch and its associated ligaments contribute to the overall integrity of the pelvic floor, helping to compartmentalize the pelvis into the false (greater) and true (lesser) pelvis regions. The sacrospinous ligament, in particular, acts as a divider between the greater and lesser sciatic foramina, reinforcing the pelvic floor's diaphragmatic structure and supporting the visceral contents above while permitting selective passage of structures below. This configuration aids in maintaining intra-abdominal pressure gradients essential for continence and organ support. During locomotion and weight-bearing, the notch supports force transmission from the gluteal muscles to the perineum via the sacrotuberous ligament, which receives partial attachments from the gluteus maximus and biceps femoris. This ligamentous pathway helps distribute tensile forces from the lower limb musculature to the sacrum, preventing excessive caudal (posterior) displacement or nutation of the sacrum under gravitational and muscular loads. Such stabilization is crucial for efficient gait, as it minimizes energy loss at the sacroiliac joint and preserves pelvic alignment during heel strike and propulsion phases. Sexual dimorphism in the lesser sciatic notch contributes to pelvic adaptations for load-bearing versus parturition, with the notch typically narrower in males to enhance rigidity and force transmission in a more linear pelvic architecture. In females, a relatively broader notch accommodates greater ligamentous flexibility and pelvic outlet expansion, reflecting evolutionary trade-offs between bipedal locomotion and childbirth. These differences arise post-puberty and underscore the notch's role in gender-specific biomechanical demands.

Neurovascular passage

The lesser sciatic foramen, formed by the lesser sciatic notch, serves as a conduit for several key neurovascular structures transitioning from the pelvis to the gluteal region and perineum.^[7] The primary muscular structure passing through is the tendon of the obturator internus muscle, which originates from the internal surface of the pelvis, traverses the lesser sciatic foramen, and hooks sharply around the ischium to insert on the medial surface of the greater trochanter of the femur.^[15] Accompanying this tendon is the nerve to the obturator internus, a branch of the sacral plexus (L5-S2), which exits the pelvis via the greater sciatic foramen, winds around the ischial spine, and re-enters through the lesser sciatic foramen to innervate the obturator internus muscle on its pelvic surface.^[16] The pudendal nerve (S2-S4), originating from the sacral plexus, provides motor and sensory innervation to the perineum and urogenital structures, including the external anal sphincter, urethral sphincter, and perineal skin.^[17] It follows a similar trajectory, departing the pelvis through the greater sciatic foramen inferior to the piriformis muscle, crossing the sacrospinous ligament and ischial spine, and re-entering via the lesser sciatic foramen to reach the perineum.^[7] Paralleling the pudendal nerve are the internal pudendal artery and vein, which arise from the internal iliac artery and vein, respectively, and supply arterial blood and venous drainage to the perineal region, including the anal canal, rectum, and external genitalia.^[18] These vessels exit the greater sciatic foramen alongside the nerve before curving medially to pass through the lesser sciatic foramen and enter the pudendal canal.^[19] In general, these structures share a common course: they initially exit the pelvic cavity through the greater sciatic foramen into the gluteal region, then loop around the sacrospinous ligament and ischial spine to re-enter the pelvis via the lesser sciatic foramen, ultimately distributing to the perineal compartment without traversing the pelvic floor musculature.^[20] Anatomical variations in this passage are documented, particularly in the positioning of the pudendal nerve relative to surrounding structures, potentially altering its relationship to the lesser sciatic foramen and associated tissues.^[21]

Clinical significance

Pathologies

The lesser sciatic notch may be involved in pelvic or acetabular fractures, which are typically high-energy injuries from trauma such as motor vehicle accidents or falls from height. These fractures can disrupt the integrity of the ischium and affect the formation of the lesser sciatic foramen. Symptoms include severe pelvic pain, instability, and potential neurovascular compromise. Pudendal nerve entrapment (PNE), also known as pudendal neuralgia, involves compression of the pudendal nerve within the lesser sciatic foramen, often due to hypertrophy of the sacrospinous ligament, repetitive trauma from cycling, or direct injury.^[22] This condition causes chronic perineal pain exacerbated by sitting, along with symptoms such as genital numbness, urinary urgency, and sexual dysfunction including erectile difficulties in males or dyspareunia in females.^[22] Diagnosis is confirmed through clinical history, pudendal nerve blockade for temporary relief, and imaging such as MRI to visualize compression sites; incidence is estimated at approximately 1 in 100,000 overall, while the prevalence of genital symptoms reaches approximately 50–60% in endurance athletes like cyclists exceeding 400 km weekly.^[22]^[23] Infections and abscesses in the pelvic region can spread contiguously to involve structures near the lesser sciatic notch, such as from bacterial prostatitis or diverticulitis, leading to purulent collections in surrounding soft tissues. These complications present with systemic signs like fever and localized gluteal swelling, alongside severe pelvic pain and potential pudendal nerve irritation. Treatment typically requires percutaneous or surgical drainage combined with broad-spectrum antibiotics. Tumors near the lesser sciatic notch, such as metastatic lesions or primary sarcomas including soft-tissue sarcomas, can invade the foramen and obstruct neurovascular passages. These tumors may cause progressive neuropathy with symptoms such as perineal pain, sensory deficits, and dysfunction due to pudendal nerve compression.^[1] Imaging reveals mass effect on CT scans, confirming bony erosion or soft-tissue invasion, while MRI delineates the extent of neural involvement.^[24] Sciatic hernias protruding through the lesser sciatic foramen are uncommon pelvic floor defects, more frequent postoperatively following abdominoperineal resections or in cases of chronic increased intra-abdominal pressure.^[25] Risk factors include multiparity, obesity, and significant weight loss, which weaken the pelvic ligaments and facilitate herniation of bowel or omentum.^[26] Patients experience perineal bulging, chronic pain, and bowel obstruction symptoms if incarceration occurs.^[27] Diagnostic evaluation of lesser sciatic notch pathologies relies on multimodal imaging and electrophysiology: MRI excels in assessing soft-tissue involvement, nerve compression, and abscesses; CT provides detailed bony integrity for fractures or erosions; and electromyography (EMG) detects nerve dysfunction in entrapments or tumors.^[28]^[24]^[29]

Surgical considerations

The lesser sciatic notch serves as a critical landmark in pudendal nerve decompression surgeries for entrapment syndromes, where transgluteal and transperineal approaches are commonly employed to access the nerve as it traverses the lesser sciatic foramen while minimizing risks to adjacent structures like the sciatic nerve.^[22] The transgluteal approach is particularly suited for posterior entrapments between the sacrospinous ligament and the lesser sciatic foramen, involving gluteal muscle splitting to expose and release the nerve.^[30] These techniques achieve success rates of approximately 70-80% in pain relief and functional improvement at 12-24 months follow-up, though outcomes vary by entrapment site and surgical expertise.^[31] In obstetric and gynecologic procedures, the lesser sciatic notch is relevant during sacrospinous hysteropexy for uterine prolapse, where sutures are affixed to the sacrospinous ligament near the notch to support the uterus.^[32] This attachment site risks pudendal or posterior femoral cutaneous nerve damage due to suture proximity, with reported complication rates for nerve injury around 1-5% manifesting as perineal numbness or pain.^[33] Techniques emphasizing suture placement away from the ischial spine help mitigate these risks.^[34] For pelvic fracture repair involving ischial fractures that extend to the lesser sciatic notch, open reduction and internal fixation (ORIF) via posterior approaches is standard, often incorporating percutaneous screws to stabilize the posterior column while preserving neurovascular integrity.^[35] Postoperative monitoring is essential for compartment syndrome in the gluteal region, which can arise from hematoma or swelling post-fixation, requiring fasciotomy if pressures exceed 30 mmHg.^[36] Tumor resection involving the lesser sciatic notch, such as for sarcomas near the ischium, typically employs an en bloc posterior or infragluteal approach to achieve wide margins while preserving pudendal vessels. For metastatic lesions, adjuvant radiation therapy follows resection to control local recurrence, with doses of 45-60 Gy based on tumor histology and margins.^[37] The internal pudendal artery, passing through the foramen, remains a key structure at risk during these dissections.^[38] Anesthesia considerations include ultrasound-guided pudendal nerve blocks targeting the nerve at the lesser sciatic foramen for perineal procedures like hemorrhoidectomy or episiotomy repair, providing effective analgesia with reduced systemic opioid needs.^[39] This approach visualizes the ischial spine and sacrospinous ligament to ensure precise needle placement 1 cm medial and inferior to the ligament attachment.^[38]

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