S2 - Posterior Superior Iliac Spine

The posterior superior iliac spine (PSIS) is a prominent bony projection at the posterior end of the iliac crest and an important landmark in the surface anatomy of the lower back and pelvis. The right and left PSIS can often be identified by palpation and commonly correspond to visible or palpable skin depressions in the lower back.

A transverse line connecting the two posterior superior iliac spines is traditionally used to approximate the level of the second sacral vertebra (S2). This relationship is clinically useful because S2 is associated with several important deeper structures, including the approximate inferior termination of the dural sac and subarachnoid space in adults.^[1][2]

Like other surface landmarks, the PSIS-S2 relationship is an approximation rather than an exact correspondence in every individual. Nevertheless, it provides a useful connection between an easily identifiable feature of the bony pelvis and the deeper anatomy of the sacrum, sacroiliac joints, meninges, and sacral nerve roots.

Location

The posterior superior iliac spine is located at the posterior end of the iliac crest. It forms part of the posterior border of the ilium and lies immediately superior to the posterior inferior iliac spine.

On each side, the PSIS is positioned near the posterior aspect of the sacroiliac joint. The two spines lie on either side of the upper sacrum and provide bilateral landmarks that can be related to the midline sacral vertebrae.

The PSIS is covered by skin, subcutaneous tissue, fascia, and ligamentous attachments. Its prominence varies among individuals, but its position can frequently be estimated from the characteristic shallow depressions of the skin overlying the posterior pelvis.

Relationship to S2

A horizontal line connecting the right and left posterior superior iliac spines is commonly described as crossing approximately the S2 vertebral level. This provides a practical method for estimating the position of S2 from the body surface.

The second sacral vertebra lies within the fused sacrum and cannot be palpated as an isolated mobile vertebra in the same way as many cervical, thoracic, or lumbar vertebrae. The PSIS therefore serves as an indirect landmark for identifying its approximate level.

This relationship is especially useful because several important anatomical features are associated with S2. The dural sac usually terminates around this level in adults, and S2 also lies near the midpoint of the sacroiliac joint and within the region traversed by sacral nerve roots.^[1][3]

Palpation

The PSIS can be located by first identifying the iliac crest and following it posteriorly. As the crest approaches the sacrum, it terminates at the posterior superior iliac spine.

In many individuals, the PSIS corresponds approximately to a small skin depression on either side of the sacrum. These paired depressions are sometimes called the dimples of Venus. They can assist visual orientation, although the skin depression itself is not a precise substitute for direct palpation of the underlying bony landmark.

Once both posterior superior iliac spines have been identified, an imaginary transverse line between them provides an estimate of the S2 level. Soft tissue thickness, pelvic morphology, body position, and individual variation can affect the accuracy of surface identification.

Anatomy of the Posterior Superior Iliac Spine

The PSIS forms part of the posterior border of the ilium. The posterior border extends inferiorly from the iliac crest and includes several features associated with the sacroiliac region and greater sciatic notch.

From superior to inferior, important landmarks include:

• Posterior superior iliac spine: The posterior termination of the iliac crest and a major palpable landmark.
• Posterior inferior iliac spine: Located inferior to the PSIS.
• Greater sciatic notch: A large indentation below the posterior inferior iliac spine that contributes to the greater sciatic foramen after completion by the sacrospinous and sacrotuberous ligaments.

The PSIS is therefore not an isolated projection but part of a continuous posterior pelvic border with important relationships to the sacrum, sacroiliac joint, and ligamentous structures of the pelvis.

Relationship to the Sacroiliac Joint

The posterior superior iliac spine lies close to the sacroiliac joint, which connects the auricular surfaces of the ilium and sacrum. The joint transfers forces between the vertebral column and the pelvic girdle.

The sacroiliac joint extends across several sacral levels. Its posterior aspect is reinforced by strong posterior sacroiliac ligaments, which occupy the region between the sacrum and posterior ilium.

Because the PSIS is palpable and lies close to the joint, it is commonly used as a surface reference for the posterior sacroiliac region. The joint itself lies deeper and more medially than the palpable prominence, so the PSIS should not be considered equivalent to the joint space.

Ligamentous Attachments

The posterior iliac region provides attachment for strong ligaments that stabilize the sacroiliac joint and lumbopelvic junction. The posterior sacroiliac ligaments connect the posterior ilium with the sacrum and are closely related to the PSIS.

The sacrotuberous ligament also has broad attachments in the posterior pelvic region, including attachment to the posterior iliac spines and adjacent sacrum, before extending toward the ischial tuberosity.

These ligamentous connections help stabilize the pelvis during transmission of body weight from the vertebral column through the sacrum to the hip bones.

Muscular and Fascial Relations

The PSIS lies within a region of extensive muscular and fascial attachments. The thoracolumbar fascia attaches to the iliac crest and sacral region and provides an important connective tissue framework for muscles of the lower back.

The erector spinae and multifidus occupy the posterior lumbosacral region medial and superior to the PSIS. The gluteal musculature lies inferior and lateral to the iliac crest, while portions of the gluteus maximus arise from the posterior ilium, sacrum, and associated ligamentous structures.

These relationships explain why the PSIS remains a useful surface landmark even though the surrounding soft tissues can be substantial.

Anatomy of S2

The second sacral vertebra is one of five fused vertebral segments forming the sacrum. In the adult, the individual sacral vertebrae are united into a single wedge-shaped bone between the two hip bones.

The S2 level can be recognized on the sacrum by its relationship to the second pair of anterior and posterior sacral foramina. These foramina transmit the anterior and posterior rami of sacral spinal nerves.

Feature at or Near S2	Anatomical Significance
Second sacral segment	Part of the fused sacrum
Second posterior sacral foramina	Transmit posterior rami of S2 spinal nerves
Second anterior sacral foramina	Transmit anterior rami of S2 spinal nerves
PSIS level	Common surface approximation of S2
Dural sac termination	Usually near S2 in adults

Relationship to the Dural Sac

The S2 level has an important relationship with the spinal meninges. The dural sac, formed by spinal dura mater and containing the arachnoid mater and subarachnoid space, extends inferiorly beyond the termination of the spinal cord.

In adults, the spinal cord usually terminates around the L1-L2 vertebral level, while the dural sac and subarachnoid space continue inferiorly to approximately S2. The region between the conus medullaris and the inferior end of the dural sac forms the lumbar cistern.^[1][3]

Within the lumbar cistern are cerebrospinal fluid, the descending lumbar, sacral, and coccygeal nerve roots of the cauda equina, and the filum terminale internum.

The PSIS-S2 surface relationship therefore provides an external reference for the approximate inferior extent of the dural sac, although this relationship should not be treated as sufficiently precise for procedures requiring exact localization.

Relationship to the Filum Terminale

The filum terminale is a slender extension of pia mater that helps anchor the spinal cord and meninges inferiorly. Its internal portion, the filum terminale internum, extends from the conus medullaris through the lumbar cistern to approximately S2.

Near the inferior end of the dural sac, the filum acquires a covering from the dura mater and continues inferiorly as the filum terminale externum, also called the coccygeal ligament. It ultimately attaches to the coccyx.

S2 therefore marks an important transition in the meningeal and filum terminale anatomy of the vertebral canal.

Relationship to Sacral Nerves

The sacral nerve roots descend within the vertebral canal as part of the cauda equina before reaching their corresponding sacral foramina. At the S2 level, the second sacral spinal nerves divide into anterior and posterior rami.

The anterior rami emerge through the anterior sacral foramina and contribute to the sacral plexus. S2 fibers participate in several nerves of the pelvis, perineum, and lower limb. The posterior rami emerge through the posterior sacral foramina and supply structures of the posterior sacral and gluteal regions.

S2 also contributes to the pelvic splanchnic nerves, which arise from anterior rami of S2 to S4 and carry preganglionic parasympathetic fibers to pelvic and abdominal viscera.

Relationship to Other Surface Landmarks

The PSIS-S2 relationship belongs to a series of commonly taught surface landmarks used to estimate levels of the vertebral column.

Surface Landmark	Approximate Vertebral Level
Vertebra prominens	Usually C7
Root of scapular spine	Approximately T3
Inferior angle of scapula	Approximately T7 in the anatomical position
Highest points of iliac crests	Approximately L4 or L4-L5
Posterior superior iliac spines	Approximately S2

These relationships allow the examiner to estimate deeper vertebral levels from structures that can be seen or palpated externally. None should be interpreted as an invariant relationship because normal anatomical variation can shift the correspondence between surface and deeper structures.

Anatomical Variation

The morphology and prominence of the posterior superior iliac spine vary among individuals. Differences in pelvic shape, iliac orientation, subcutaneous tissue, muscular development, and body position influence how readily the landmark can be identified.

The level of a transverse line connecting the PSIS can also vary relative to the sacrum. Although S2 is the conventional anatomical approximation, an individual's actual relationship may lie somewhat above or below the expected level.

Variation also occurs in the inferior extent of the dural sac. S2 represents the typical adult level, but the exact termination is not identical in every person.

Clinical Significance

The posterior superior iliac spine is clinically useful primarily because it is an accessible bony landmark for orientation of the posterior pelvis and sacral region. Its relationship to S2 also connects surface anatomy with deeper neural and meningeal structures.

Surface Localization of S2

A line between the posterior superior iliac spines provides a practical estimate of the S2 vertebral level. This can help orient examination of the sacrum, sacroiliac region, and lower vertebral column.

Because the relationship is approximate, precise identification of a sacral vertebral level should rely on imaging when exact localization is clinically necessary.

Sacroiliac Region

The PSIS provides a convenient surface reference for the posterior sacroiliac region. Its close relationship to the sacroiliac joint and posterior sacroiliac ligaments makes it useful when describing the location of structures or findings around the posterior pelvis.

However, tenderness at or near the PSIS is not anatomically specific for a single structure because ligaments, fascia, muscle attachments, and the sacroiliac joint occupy the surrounding region.

Meningeal Anatomy

The approximate correspondence between the PSIS and S2 is useful when learning the inferior extent of the spinal meninges. The dural sac and subarachnoid space usually terminate near S2, substantially inferior to the termination of the spinal cord itself.

This distinction is important for understanding why the lumbar cistern can extend through the lower lumbar region while the spinal cord has already ended at a more superior vertebral level.

Posterior Pelvic Orientation

The paired posterior superior iliac spines provide visible and palpable reference points for the orientation of the posterior pelvis. They can be considered together with the iliac crests, sacral midline, and other pelvic landmarks when examining the topography of the lower back.

The PSIS is therefore valuable not because it identifies S2 with absolute precision, but because it provides an accessible bridge between surface anatomy and the deeper anatomy of the sacrum, sacroiliac joints, sacral nerve roots, and spinal meninges. A transverse line between the two posterior superior iliac spines remains a useful conventional guide to the S2 vertebral level.