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Ischial spine

The ischial spine is a thin, pointed bony projection located on the posterior border of the ischium, one of the three bones that fuse to form the hip bone (os coxae) in the adult pelvis.^[1] It lies at the junction between the body and ramus of the ischium, superior to the lesser sciatic notch and inferior to the greater sciatic notch, thereby separating these two key foramina that permit the passage of neurovascular structures such as the pudendal nerve and internal pudendal vessels.^[2] This structure is essential for pelvic stability, serving as a primary attachment site for the sacrospinous ligament, which spans from the sacrum to the ischial spine and helps convert the greater sciatic notch into the greater sciatic foramen.^[1] In addition to ligamentous attachments, the ischial spine provides origins for several muscles critical to pelvic floor function and lower limb movement, including the superior gemellus, coccygeus, and components of the levator ani (such as the iliococcygeus).^[3] These connections support the maintenance of pelvic organ position and facilitate actions like hip rotation and stabilization during weight-bearing activities. Clinically, the ischial spine serves as an important anatomical landmark in obstetrics and gynecology, where its prominence and orientation influence pelvic dimensions—for instance, in the gynecoid pelvis type, the spines are relatively blunt, contributing to wider internal diameters that ease childbirth.^[1] It is also relevant in diagnosing conditions like piriformis syndrome, where irritation near the greater sciatic foramen may involve structures adjacent to the spine,^[4] and in imaging or surgical approaches to the pelvis, such as during pudendal nerve blocks.^[5] Variations in its development, such as accessory ossification centers, can occasionally lead to anatomical anomalies affecting nearby foramina.^[2]

Anatomy

Location and gross structure

The ischial spine is a thin, pointed, triangular bony projection that arises from the posterior border of the body of the ischium, one of the three bones forming the pelvis. This eminence extends backward from the ischium, serving as a distinct landmark in the pelvic architecture.^[6] Positioned on the posterior aspect of the ischium, the ischial spine lies between the greater sciatic notch superiorly and the lesser sciatic notch inferiorly, effectively separating these two notches along the posterior border. It projects posteriorly and medially, directed toward the central pelvic cavity and contributing to the contour of the posterior pelvic wall.^[7]^[8] The structure exhibits individual variations in length and prominence, being more or less elongated across subjects, which can influence its visibility and palpability in anatomical assessments.^[6]

Muscle and ligament attachments

The ischial spine provides attachment sites for several muscles and ligaments on both its external and internal surfaces, serving as a key bony anchor in the pelvic region. On the external (posterior) surface, the superior gemellus muscle originates directly from the spine, blending with the tendon of the obturator internus muscle to facilitate its passage through the lesser sciatic foramen.^[9] The internal (pelvic) surface of the ischial spine gives rise to the coccygeus muscle, which originates along the spine and extends to the lateral margins of the sacrum and coccyx, forming part of the posterior pelvic diaphragm. Additionally, the iliococcygeus component of the levator ani muscle complex originates from the ischial spine as well as the adjacent tendinous arch of the levator ani (arcus tendineus levator ani), which spans from the pubic bone to the spine, thereby integrating the muscle's attachment into the broader pelvic floor structure.^[10]^[11]^[12] At its pointed medial extremity, the sacrospinous ligament inserts onto the ischial spine, spanning from the lateral sacrum and coccyx to this site, and together with the sacrotuberous ligament, converting the lesser sciatic notch into the lesser sciatic foramen.^[13]^[10]^[14] The internal attachments on the pelvic surface are enveloped by layers of pelvic fascia, including the superior fascia of the pelvic diaphragm, which covers the origins of the coccygeus and levator ani muscles, providing structural support and continuity with the obturator fascia.^[15]^[16] These musculoskeletal and ligamentous connections collectively position the ischial spine as a critical anchor point for stabilizing attachments within the pelvis, linking bony, muscular, and fascial elements without direct involvement in broader pelvic dynamics.^[10]

Relations to neurovascular structures

The ischial spine maintains a close anatomical relationship with the pudendal neurovascular bundle, which emerges from the sacral plexus and exits the pelvis through the greater sciatic foramen before hooking around the spine.^[17] Specifically, after exiting the greater sciatic foramen, the pudendal nerve crosses the sacrospinous ligament near the ischial spine and enters the perineum via the lesser sciatic foramen.^[18]^[17] This positioning places the bundle in direct proximity to the spine's posterior projection, facilitating potential mechanical interaction during pelvic movements.^[19] Accompanying the pudendal nerve are the internal pudendal artery and vein, which form the complete neurovascular bundle and course together through the pudendal canal (Alcock's canal) adjacent to the ischial spine.^[17] The artery arises from the internal iliac artery within the pelvis, while the vein drains into the internal iliac vein, both running parallel to the nerve along the lateral wall of the ischiorectal fossa after passing near the spine.^[20] This bundled arrangement ensures coordinated supply to the perineal structures, with the spine serving as a key bony landmark delineating the transition from the greater to the lesser sciatic foramen.^[21] The sciatic nerve, the largest branch of the sacral plexus, exhibits a superior positional relationship to the ischial spine, exiting the pelvis through the greater sciatic foramen immediately above the level of the spine and sacrospinous ligament.^[22] Originating from the lumbosacral plexus (L4-S3), it descends inferolaterally across the posterior surface of the ischium, maintaining separation from the spine's apex while sharing the foraminal pathway with inferior structures like the pudendal bundle.^[23] The spine's location thus contributes to the spatial organization of these nerves, with the sciatic positioned more laterally and superiorly to avoid direct overlap.^[19] Branches of the sacral plexus, including those forming the pudendal and sciatic nerves, converge near the ischial spine on the posterior pelvic wall, where the plexus itself overlies the piriformis muscle.^[17] This proximity underscores the spine's role as a reference point for lower sacral outflow (S2-S4 for pudendal; L4-S3 for sciatic), with potential for positional compression or irritation of these branches due to the spine's projection into the foraminal spaces.^[4]

Development

Embryological development

The ischial spine arises from the inferior portion of the ischial anlage within the developing pelvic girdle, originating primarily from the lateral plate mesoderm in conjunction with contributions from somites at levels 26 through 35.^[24]^[25] This mesenchymal condensation occurs within the primitive limb buds by approximately the 6th week of gestation, marking the initial outlining of the ischium alongside the ilium and pubis.^[26] As part of the broader pelvic embryology, the ischial spine forms within the triradiate cartilage complex, a Y-shaped structure that interconnects the cartilaginous precursors of the ilium, ischium, and pubis around the acetabulum beginning at Carnegie stage 18 (approximately week 7 of gestation).^[24] This complex facilitates the eventual separation and differentiation of these elements, with the ischium growing medially and caudally from stage 19 onward, while the spine emerges as a distinct projection as the ischial body elongates.^[27] Chondrification of the ischial region initiates earlier, at Carnegie stage 18 (around weeks 5–6), establishing the foundational cartilage model for the spine's inferior positioning relative to the pubis and ilium.^[24]^[27] The morphological differentiation of the ischial spine becomes evident by Carnegie stage 22 (weeks 7–8), when it approximates the inferior ramus, and progresses through the early fetal period (by week 12), coinciding with the unification of the ischiopubic rami and initial formation of the obturator foramen.^[27] This timeline aligns with the overall pelvic girdle's separation into distinct bony elements, driven by patterned growth rates where the ischium expands at a pace comparable to the pubis but slower than the ilium.^[27] Genetic regulation plays a pivotal role in shaping the ischial spine's projection, with Hox genes (such as Hoxa, Hoxb, Hoxc, and Hoxd clusters) establishing anteroposterior patterning in the somatopleuric mesoderm, in coordination with co-factors like Pbx1, Pbx2, Emx2, and Islet1 that specify the inferior pubis-ischium domain.^[25]^[28] These processes are modulated by signaling pathways, including BMP and Wnt, which interact with retinoic acid to refine Hox expression and promote chondrogenic differentiation in the pelvic mesenchyme.^[25]^[29]

Ossification and postnatal changes

The ossification of the ischial spine commences in utero as part of the ischium's primary ossification center, which emerges during the 4th to 5th month of gestation (approximately 16-20 weeks) and expands through endochondral ossification inferiorly and anteriorly.^[30] Although initiated prenatally, the process extends into postnatal life, with the spine becoming distinctly prominent and well-developed by one year of age. This early postnatal maturation aligns with the overall shaping of the pelvic bones to support emerging weight-bearing functions. Further postnatal changes occur progressively through childhood and into adolescence. By ages 5 to 6 years, a nonarticular plaque develops in the acetabular depression on the posterior margin near the ischial spine's articulation site, contributing to the refinement of the acetabular region. The spine reaches its full prominence during adolescence, as the pelvic bones undergo rapid elongation and angular adjustments in preparation for adult morphology. The fusion of the ischium, including the spine, with the ilium and pubis at the acetabulum typically begins around ages 15 to 17 years during puberty, completing the formation of the os coxae and stabilizing the pelvic ring. This triradiate cartilage fusion marks the end of significant longitudinal growth in the ischial spine. Growth hormones play a key role in postnatal elongation of the ischial spine by stimulating chondrocyte activity in the growth plates, while mechanical stresses from locomotion and weight-bearing promote modeling through adaptive bone deposition, as per principles of mechanotransduction in skeletal development. In adulthood, the ischial spine experiences ongoing remodeling in response to chronic weight-bearing loads, involving osteoclast-mediated resorption and osteoblast-driven formation to preserve density and architecture, though age-related shifts may include gradual periosteal apposition and minor density variations.

Function

Role in pelvic stability

The ischial spine serves as a critical anchor for the sacrospinous ligament, which spans from the sacrum to the spine and helps resist excessive nutation of the sacrum, thereby preventing downward displacement of the pelvic floor during upright postures.^[31] In conjunction with the sacrotuberous ligament, which attaches nearby on the ischial tuberosity, this arrangement enhances overall pelvic ring stability by limiting rotational movements at the sacroiliac joint and supporting the transmission of forces across the pelvis.^[32] The ischial spine also provides key origins for components of the pelvic floor musculature, including the coccygeus muscle and the posterior fibers of the iliococcygeus portion of the levator ani, which together form the pelvic diaphragm.^[33] These attachments enable tonic contraction to maintain pelvic floor tone, supporting the visceral contents such as the bladder, rectum, and reproductive organs while resisting descent under gravitational and intra-abdominal forces.^[34] Weakness in these muscles, often linked to their origins at the ischial spine, can contribute to pelvic organ prolapse, affecting up to 50% of women.^[35] By forming a bony projection between the greater and lesser sciatic notches, the ischial spine contributes to the division of the sciatic foramina, with the overlying sacrospinous ligament further delineating these openings to compartmentalize pelvic contents.^[36] This structural role separates the gluteal region from the perineum, organizing the passage of neurovascular structures like the sciatic nerve and pudendal vessels while maintaining the integrity of pelvic compartments against herniation or displacement.^[36] As part of the ischium, the ischial spine indirectly facilitates weight transmission from the trunk to the lower limbs, particularly during standing, by stabilizing the posterior pelvis through its ligamentous and muscular attachments.^[32] In the sitting position, the broader ischial framework bears upper body weight, with the spine's projections ensuring balanced load distribution to prevent undue stress on the sacroiliac joint.^[32] Through its role in anchoring the pelvic diaphragm, the ischial spine supports the regulation of intra-abdominal pressure by allowing coordinated contraction of the levator ani and coccygeus during activities like coughing or lifting, which counteracts pressure spikes to protect visceral support and spinal alignment.^[34] This interaction helps distribute forces evenly across the core, enhancing overall postural stability without excessive strain on surrounding structures.^[31]

Biomechanical contributions

The ischial spine serves as a critical attachment point for the sacrospinous ligament, which generates tension that converts vertical compressive forces from the upper body into horizontal stability components during bipedal posture. This mechanism helps maintain sacroiliac joint integrity by resisting excessive nutation and facilitating load transfer across the pelvic ring under body weight. Biomechanical models demonstrate that variations in sacrospinous ligament stiffness nonlinearly influence pelvic kinematics, with softer states increasing innominate bone mobility by up to 900% in relative displacement under loads of 500 N, thereby enhancing overall postural equilibrium.^[37] In gait dynamics, the ischial spine contributes to resisting rotational forces on the pelvis through its role as the origin of the gemellus superior muscle, a short external rotator that provides leverage for stabilizing the hip and pelvis during the swing and stance phases. This muscle's action counters internal rotation torques generated by ground reaction forces, working in concert with other rotators to minimize pelvic tilt and obliquity, as evidenced by electromyographic studies showing sustained activity of external rotators during walking to support lumbopelvic coordination. The projection of the ischial spine amplifies this leverage, allowing efficient force application without excessive energy expenditure.^[38]^[39] The ischial spine's position shapes the pelvic outlet, influencing force distribution during positions like squatting, where it acts as a medial boundary that can shift laterally under muscular tension to widen the outlet by approximately 2.5 cm. This adaptation optimizes load-bearing by redistributing compressive forces from the sacrum and coccyx across the ischiopubic rami and ligaments, reducing localized stress during upright postures involving hip flexion. In squatting, opposing forces from the lower limbs and back extensors flex the sacroiliac joint, leveraging the spine's prominence to enhance outlet dimensions and improve biomechanical efficiency for weight support.^[40] Stress analysis reveals that the ischial spine endures both compressive and tensile loads during locomotion and posture maintenance, with dorsoventral compressive strains typically ranging from 200 to 600 µε under physiological conditions, indicating robust adaptation to repetitive loading. Under extreme stress, such as high-impact activities, these loads can lead to microfractures if bone remodeling fails to keep pace, as seen in rare cases of ischial region stress injuries where accumulated tensile forces from ligament and muscle attachments exceed material limits.^[41]^[42] Comparatively, the ischial spine's dorsal projection in humans and vertical clingers and leapers provides greater mechanical leverage for hip extensors compared to the flatter, less prominent structures in arboreal quadrupeds, enhancing propulsion and load resistance during bipedal gait. This feature scales with positive allometry (slope ≈1.87) across primates, correlating with locomotor demands where pronounced projections in bipeds and leapers increase moment arms for muscles like the hamstrings, outperforming the broader load distribution in taxa with elongated, flatter ischia suited to quadrupedal or suspensory locomotion.^[41]

Clinical significance

Use in pelvimetry and obstetrics

The ischial spines serve as critical anatomical landmarks in pelvimetry, particularly for assessing the interspinous diameter, which represents the transverse dimension of the midpelvis and is typically approximately 10 cm in adult women.^[43] This measurement is essential because the interspinous diameter is the narrowest transverse dimension through which the fetal head must pass during labor, influencing the feasibility of vaginal delivery.^[44] In clinical practice, the ischial spines are palpated during digital vaginal examinations to evaluate pelvic adequacy, especially in cases of suspected cephalopelvic disproportion or prolonged labor.^[45] These spines are readily identifiable as bony prominences approximately one finger-length inside the vagina at the 4 and 8 o'clock positions, allowing clinicians to gauge fetal station relative to the midpelvis and assess progress.^[46] Manual pelvimetry involving palpation of the ischial spines has been a standard component of obstetric evaluation since the 19th century, originating from early efforts to quantify pelvic dimensions for predicting labor outcomes, as documented in foundational works like those of Vaneuvel in 1841 and Pershing in 1889.^[47] Today, this technique is often supplemented by imaging modalities such as MRI or CT for more precise measurements in high-risk cases.^[43] A narrow interspinous distance, particularly less than 10 cm, is associated with increased risk of labor dystocia, where the fetal head fails to descend adequately, often necessitating interventions like operative vaginal delivery or cesarean section.^[48] Studies have shown that shorter midpelvic dimensions correlate with cephalopelvic disproportion, contributing to prolonged second-stage labor and higher cesarean rates, emphasizing the prognostic value of this measurement in obstetric decision-making.^[49] Gender differences in ischial spine prominence further highlight their obstetric relevance, as males exhibit more pronounced spines that project farther into the pelvic cavity, resulting in a narrower midpelvic diameter compared to females.^[50] In females, the relatively less prominent spines allow for a wider interspinous space, facilitating childbirth, which aligns with evolutionary adaptations in pelvic architecture.^[51]

Pudendal nerve procedures

The pudendal nerve block is a regional anesthesia technique that utilizes the ischial spine as a key anatomical landmark to target the pudendal nerve, which courses through the pudendal canal near the sacrospinous ligament. This procedure involves injecting a local anesthetic to temporarily interrupt nerve conduction, providing analgesia to the perineum, vagina, and anal regions. It can be performed via transvaginal or transperineal approaches, with the ischial spine palpated digitally through the vagina or rectum to guide needle placement.^[5] The technique was first described in 1916 for vaginal delivery analgesia, though it gained widespread adoption in the 1950s following refinements in obstetric practice. In the transvaginal method, the clinician inserts an index finger into the vagina to palpate the ischial spine, then advances a 20- to 22-gauge needle along the finger to the ligament's attachment, directing it approximately 1 cm medial and posterior toward the pudendal canal for anesthetic injection (typically 5-10 mL of 1-2% lidocaine or bupivacaine). The transperineal approach similarly relies on rectal palpation of the spine, with the needle inserted through the perineal skin and advanced to contact the ischial spine before slight withdrawal and redirection medially. Ultrasound or fluoroscopic guidance may enhance precision in modern variations.^[52]^[5]^[53] Indications for pudendal nerve blocks include perineal pain relief during childbirth, anesthesia for urological or gynecological procedures such as hypospadias repair or perineal surgeries, and management of chronic pelvic pain syndromes like pudendal neuralgia. These blocks are particularly valuable when pudendal nerve entrapment occurs near the ischial spine, as referenced in its relations to neurovascular structures.^[5]^[54] Potential risks include vascular puncture of the internal pudendal artery if the needle deviates from the ischial spine landmark, leading to hematoma; direct nerve injury causing temporary or prolonged paresthesia; infection at the injection site; and systemic effects from anesthetic toxicity if intravascular injection occurs. Contraindications encompass patient refusal, local infection, or known allergy to the anesthetic agent, with overall complication rates reported as low when performed by experienced practitioners.^[5]^[52]

Pathological conditions and variations

Anatomical variations of the ischial spine include accessory spines projecting from the ilioischial synchondrosis, which can separate the greater and lesser sciatic notches and alter pelvic dimensions.^[55] Prominent ischial spines, where the structure projects medially into the pelvis, occur in approximately 5% of the normal population and may influence the sciatic notch morphology.^[56] These variations can affect neurovascular structures, such as the pudendal nerve, potentially leading to compression-related symptoms like sciatica if the spine encroaches on adjacent spaces.^[57] Pathological conditions involving the ischial spine include fractures, which are uncommon but arise from high-impact trauma, such as motor vehicle accidents or falls, resulting in fracture-dislocation of the hip with impalement onto the spine.^[58] Avulsion fractures of the ischial spine occur primarily in adolescents during sports activities involving sudden muscle contractions, like those of the superior gemellus or coccygeus, leading to detachment at the apophysis.^[59]^[60] Stress fractures may also develop in athletes, presenting as non-union with chronic pain and requiring imaging for confirmation.^[42] In developmental dysplasia of the hip (DDH), posterior rotation or prominence of the ischial spine is frequently observed, contributing to acetabular retroversion and inadequate femoral head coverage, which increases the risk of instability and osteoarthritis.^[61] This rotation alters pelvic tilt and innominate bone alignment, as evidenced by three-dimensional computed tomography measurements showing internal rotation of the ischiopubic segment in affected individuals.^[62] Pregnancy-related ischial spine pain syndrome often stems from ligamentous laxity and compression of the pudendal nerve at the ischial spine, exacerbated by fetal head pressure during late gestation or delivery, resulting in perineal and sciatic pain.^[63] Hormonal changes increase pelvic floor muscle activity, promoting bony remodeling around the spine that entraps the nerve, a condition more pronounced postpartum in some cases.^[64] Diagnostic imaging plays a crucial role in evaluating these conditions; computed tomography (CT) excels at delineating bony variations and fractures, such as avulsions or prominence, while magnetic resonance imaging (MRI) assesses soft tissue involvement, including pudendal nerve compression or associated edema in DDH and pain syndromes.^[65] The ischial spine sign on anteroposterior radiographs indicates retroversion when the spine projects within pelvic borders, guiding further MRI or CT for confirmation.^[66]

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Pudendal Nerve Block - StatPearls - NCBI Bookshelf
Alternatively, the injection can be performed just medial to the ischial spine.[23] The nerve is located just behind the ligament in the loose areolar tissue.Anatomy and Physiology · Indications · Equipment · Technique or Treatment
[50]
Transvaginal Pudendal Nerve Block - Medscape Reference
Dec 9, 2020 · Technique · Place the end of the guide beneath the tip of the ischial spine. · Push the needle into the vaginal mucosa. · Aspirate to ensure that ...
[51]
A novel technique for pudendal nerve block - PubMed
A 25-gauge 3.5 cm needle is advanced to the tip of the ischial spine where the pudendal nerve transiently leaves the pelvis. The pudendal nerve block is ...
[52]
Pudendal Nerve Block: What It Is, Procedure & Side Effects
With your consent, the provider will gently insert their finger into your rectum or vagina to feel for your ischial spine. This helps them locate where your ...
[53]
Opus V: Skeletal System: Pelvis - Anatomy Atlases
An accessory ischial spine may project from the ilium/ischium synchrondrosis, separating the greater sciatic foramen into two compartments. An accessory spine ...<|control11|><|separator|>
[54]
The Ischial Spine Sign: Does Pelvic Tilt and Rotation Matter? - PMC
Depending on the size of the patient, the same size of the ischial spine can lead to different appearances of the retroversion.Missing: length | Show results with:length
[55]
Anatomic variations of pudendal nerve within pelvis and ... - PubMed
The median distance from the point of the pudendal nerve formation to the ischial spine was 27.5 mm (range, 14.5-37 mm). The width of the pudendal nerve in the ...
[56]
Irreducible Traumatic Fracture-Dislocation of the Hip with ... - PubMed
Apr 13, 2022 · Irreducible Traumatic Fracture-Dislocation of the Hip with Impalement onto the Ischial Spine: A Case Report. JBJS Case Connect. 2022 Apr 13 ...
[57]
Traumatic Obturator Internus and Quadratus Femoris Injury in a ...
Feb 5, 2021 · Magnetic resonance imaging demonstrated a quadratus femoris tear, obturator internus tear, and ischial spine avulsion fracture. Although a ...
[58]
The Ischial Spine in Developmental Hip Dysplasia
Aug 18, 2020 · Prominence of the ischial spine (PRIS) is an alternate radiographic sign for acetabular retroversion because in these hips, the whole hemipelvis ...
[59]
https://pubmed.ncbi.nlm.nih.gov/33541258/
[60]
Pudendal Nerve Entrapment Syndrome - StatPearls - NCBI Bookshelf
If the nerve is compromised at the ischial spine or the sacrospinous ligament, it causes pain medial to the ischium—similarly, tenderness over the greater ...
[61]
Anatomical basis of chronic pelvic pain syndrome: the ischial spine ...
Chronic pelvic pain is often caused by a compression neuropathy of the pudendal nerve. The bony remodeling as a result of the activity of pelvic floor muscles ...
[62]
Imaging of the Hip - Musculoskeletal Diseases 2021-2024 - NCBI
Apr 13, 2021 · Both CT and MRI are feasible for measuring femoral torsion, with MRI being the favored imaging method due to the lack of radiation. Key ...
[63]
Ischial spine sign | Radiology Reference Article - Radiopaedia.org
Jul 3, 2020 · The triangular projection of the ischial spine is visible medially to the pelvic inlet or iliopectineal line. The sign is supposed to have a ...Missing: posteromedially | Show results with:posteromedially