Sacral Hiatus Anatomy for Caudal Epidural Blocks

The sacral hiatus is a natural opening at the bottom of the sacrum, the triangular bone at the base of your spine. It forms because the bony arch of the lowest sacral vertebra (usually the fifth) never fully closes during development, leaving a gap covered by a tough membrane. This opening is the main doorway clinicians use to access the sacral epidural space for pain-relieving injections and certain surgical procedures, and its size, shape, and exact position vary considerably from person to person.

How the Sacral Hiatus Forms

The sacrum is made of five vertebrae that fuse together during adolescence and early adulthood. Each vertebra has a bony arch (called a lamina) that normally closes around the spinal canal. At the lowest sacral level, this arch stays incomplete. The result is a U- or V-shaped gap on the back surface of the sacrum, flanked on each side by small bony bumps called the sacral cornua. A dense fibrous sheet, the sacrococcygeal membrane, stretches across this gap and keeps the underlying epidural space sealed. That membrane is made of parallel collagen fibers reinforced by elastic fibers on its deeper surface, and small nerve branches run along or occasionally through it.

Shape and Size Variations

If you lined up a hundred sacral bones, no two hiatuses would look identical. The opening comes in several recognized shapes. In a study of Ethiopian dry sacra, the most common shape was inverted-V (about 41%), followed by inverted-U (roughly 38%), with the rarest variant being complete spina bifida of the sacrum (under 2%).¹ An Indian study found the inverted-U shape most common at about 40%, while an Egyptian study reported that the dominant shape differed between men (inverted-U and inverted-V) and women (inverted-V and dumbbell-shaped).²³ The Egyptian study also noted that a completely absent hiatus occurred only in males.

The apex of the hiatus, its highest point, usually sits at the level of the fourth sacral vertebra. The Ethiopian study placed the apex there in about 61% of specimens, while the base of the hiatus was at the fifth sacral vertebra in nearly 79% of specimens.⁴ But some people have an apex as high as the third sacral vertebra or as low as the fifth, and this variability is one of the reasons clinicians cannot always assume a standard location.

In terms of raw dimensions, the hiatus averages roughly 22 to 28 mm in length depending on the population studied, with a transverse width at the base of about 13 to 18 mm. The front-to-back depth of the sacral canal at the apex averages around 4.5 to 6.5 mm.⁵⁶ That front-to-back depth matters a great deal: if it is less than about 3 mm, a needle may not fit through, and the procedure becomes impractical.

Do Men and Women Have Different Anatomy Here?

Most studies find that men tend to have a slightly longer sacral hiatus than women, which makes sense given that male sacra are generally larger. An Indian morphometric study found average hiatus lengths of about 23 mm in men versus 22 mm in women, a difference that was not statistically significant.⁷ A Thai study, however, found a larger and statistically significant gap: roughly 20 mm in men versus 16 mm in women.⁸ Width and depth measurements show similar inconsistency across populations: some studies find small sex differences while others find none worth noting. The practical takeaway is that population matters at least as much as sex. A clinician preparing for a caudal block benefits more from imaging the individual patient than from assuming dimensions based on sex alone.

Finding the Sacral Hiatus From the Outside

Because the hiatus is buried under skin, fat, and the sacrococcygeal membrane, clinicians need reliable surface landmarks. The classic approach uses the two posterior superior iliac spines (the bony bumps you can feel at the top of your buttocks on each side). A line between them sits roughly at the level of the second sacral vertebra. From there, the sacral hiatus lies further down, typically at the bottom of the sacrum.

An anatomical study found that the two bony ridges overlying the posterior superior iliac spines and the apex of the sacral hiatus form an approximately equilateral triangle, with sides averaging about 67 mm.⁹ A separate study confirmed that surface landmarks can be used to form an equilateral triangle whose lower apex corresponds to the sacral hiatus, and that this relationship holds despite individual anatomical variation.¹⁰ In practice, a clinician palpates the two sacral cornua, the small bony horns flanking the hiatus, and feels for the soft depression of the membrane between them. This “pop” through the membrane is the traditional confirmation of correct needle placement.

Other important landmarks include the sacral cornua themselves, the lateral sacral crests, and the distance from the hiatus apex to the second sacral foramina.¹¹ Knowing the distance from the hiatus to the dural sac (the fluid-filled membrane surrounding the spinal cord’s lower end) is critical for safety: if a needle advances too far, it can puncture the dural sac and cause a spinal headache or inject medication into the spinal fluid rather than the epidural space.

The Dural Sac and Safe Needle Distance

The dural sac, which contains cerebrospinal fluid and the lower nerve roots, usually ends at the first or second sacral vertebra level. Since the sacral hiatus sits at about the fourth or fifth sacral vertebra, there is normally a comfortable buffer zone between the needle entry point and the dural sac. An imaging study measured this needle distance at an average of about 58 mm, with the sacral curvature between those levels averaging about 25 degrees.¹² That curvature is worth noting: because the sacral canal is not a straight tube but a gently curved one, the needle must follow an angled path. Pushing straight will hit bone.

Knowing the exact termination level of the dural sac, along with the distances between it and the hiatus apex, helps clinicians plan both caudal epidural anesthesia and more specialized procedures that deliberately target the lumbar cistern through the hiatus.¹³

Caudal Epidural Blocks in Children

The sacral hiatus is probably best known for its role in caudal epidural anesthesia, and nowhere is this technique used more commonly than in pediatric surgery. In young children, the sacrum is still partly cartilaginous, making it easier for ultrasound waves to penetrate and for clinicians to visualize the epidural space. A caudal block provides excellent pain relief for surgeries below the belly button, including hernia repairs, circumcisions, and orthopedic procedures on the lower limbs.

Clinical reviews have found that the technique can be used safely in patients ranging from extremely premature infants up to children weighing about 50 kg.¹⁴ One practical challenge is that the sacral hiatus is harder to feel in roughly 11% of children aged seven and under, sometimes because of excess subcutaneous fat or anatomical variations in the cornua.¹⁵ In children, the dural sac sits lower relative to the hiatus than in adults, meaning there is less margin for error. A radio-anatomic study found that the needle should be inserted below the S2 level, about 1.4 to 2.2 cm below the midpoint between the posterior superior iliac spines, to avoid dural puncture in children aged one to nine.¹⁶

Caudal Epidural Injections in Adults

In adults, the sacral hiatus serves a somewhat different clinical purpose. The bones are fully fused, the hiatus is smaller relative to body size, and the overlying tissue is thicker. Adult caudal epidural injections are used primarily for managing chronic lower back and leg pain, particularly lumbar radiculopathy caused by disc herniations or spinal stenosis. Corticosteroid mixed with local anesthetic is injected through the hiatus into the epidural space, where it bathes the inflamed nerve roots.

Image guidance is recommended to ensure the needle tip sits below the third sacral vertebra, avoiding the dural sac.¹⁷ The failure rate of blind (landmark-only) caudal blocks in adults is considerably higher than in children, largely because of the wide anatomical variation in hiatus size and shape. Fluoroscopy and ultrasound have markedly improved success rates.¹⁸

When the Anatomy Makes Procedures Difficult or Impossible

Not everyone’s hiatus cooperates. A study looking specifically at variations that could cause caudal block failure, including an elongated hiatus, a completely absent hiatus, total agenesis of the sacral dorsal wall, or a canal narrower than 3 mm at the apex, found these problematic variants in about 17% of specimens overall, with a higher rate in women (about 25%) than men (about 11%).¹⁹ In the Egyptian morphometric study, a completely absent hiatus was observed only in males.²⁰ These seemingly contradictory findings across populations highlight a consistent theme: variation is the rule, not the exception, and any individual may have anatomy that defies the textbook description.

When a clinician cannot find the hiatus by palpation, or when the canal is too narrow for a needle, the usual alternative is a lumbar epidural or a targeted nerve root block at a different spinal level.

Ultrasound Versus Blind Technique

The traditional way to perform a caudal block relies entirely on feeling bony landmarks and detecting the subtle “give” of the sacrococcygeal membrane. This works well most of the time in children but runs into trouble in adults and in patients with difficult anatomy. Ultrasound has changed the picture considerably.

In a randomized trial involving children, ultrasound-guided caudal injection achieved a first-puncture success rate of 80% compared with 63% for the conventional approach. Overall block success was similar between the two methods (97% versus 93%), but the ultrasound group had far fewer complications: only one case of vascular puncture versus eight in the conventional group, and no cases of subcutaneous bulging versus eight.²¹ A separate randomized trial that specifically compared ultrasound-guided needle entry at the sacral hiatus with conventional sacral canal injection found similar overall success rates but a significantly higher first-puncture success rate with ultrasound (about 93% versus 60%) and lower rates of bloody puncture (about 6% versus 19%).²²

In adults, ultrasound can visualize the sacrococcygeal membrane, the sacral cornua, and even the spread of injected fluid within the epidural space in real time. Fluoroscopy remains the reference standard for confirming correct needle placement, particularly in adult pain management. However, ultrasound has been shown to be highly effective in guiding the needle into the caudal epidural space with treatment outcomes comparable to fluoroscopy.²³ Ultrasound carries the advantage of no radiation exposure, portability, and lower cost.

Accidental Intravascular Injection

One of the more concerning complications of any caudal epidural injection is accidentally placing the medication into a blood vessel instead of the epidural space. The sacral epidural space contains a rich venous plexus, and a needle tip can easily nick or sit inside a small vein. A study comparing ultrasound-guided needle depths found that when the needle was inserted deeper into the sacral canal, intravascular injection rates reached 24%, whereas a shallower insertion depth under ultrasound guidance eliminated intravascular injections entirely.²⁴ Aspiration of blood before injecting is the traditional safety check, but that same study found that only one of six patients with fluoroscopy-confirmed intravascular needle placement had shown positive blood aspiration beforehand, making aspiration alone an unreliable safeguard.

Patient factors also play a role. A logistic regression analysis found that people with chronic radicular symptoms (pain radiating down the leg from a pinched nerve) and longer symptom duration had a higher risk of accidental intravascular injection during caudal procedures.²⁵ The suspected reason is that chronic nerve irritation may promote engorgement of the epidural venous plexus, making blood vessels more prominent and easier to puncture.

The Sacrococcygeal Membrane Up Close

The fibrous sheet that seals the sacral hiatus from the outside is more complex than it might appear. Anatomical dissections have shown that when the sacrum and coccyx have not fused together, the gap between the cornua averages about 7 mm, bridged by the intercornual ligament.²⁶ This ligament is composed of vertically oriented collagen fibers with elastic fibers on its front surface, giving it a firm but slightly springy feel. That texture is what produces the characteristic “pop” or “click” that clinicians feel when a needle passes through it. Small nerve branches run along the front surface of this ligament and in rare cases pass directly through it, which may explain the brief sharp discomfort some patients report during caudal needle insertion even before any injection takes place.

Newer Procedures Through the Sacral Hiatus

Beyond standard epidural injections, the sacral hiatus has become the entry point for a growing range of minimally invasive spinal procedures. Trans-sacral epiduroscopy, for example, involves threading a tiny flexible endoscope through the hiatus, up the sacral canal, and into the lumbar epidural space. The clinician can then directly visualize adhesions, inflamed tissue, or disc herniations. In some cases, a laser fiber is passed through the scope to shrink protruding disc material. One comparative study found that patients who underwent trans-sacral epiduroscopic laser decompression (SELD) showed a significant reduction in protruding disc volume, from an average of about 453 cubic millimeters before the procedure to about 362 cubic millimeters afterward.²⁷

These procedures rely on the hiatus being large enough to accommodate the endoscope, which is why preoperative imaging to assess hiatus dimensions is standard practice. Patients with a narrow or absent hiatus may not be candidates.

Training and Simulation

Learning to locate the sacral hiatus and pass a needle through it is one of those skills that improves dramatically with practice but can feel uncertain at first. The tactile landmarks are subtle, and the anatomy varies enough that the first few dozen attempts can be humbling. Three-dimensional printed phantom models have been developed to let trainees practice ultrasound-guided caudal blocks on a realistic anatomical replica before working with patients. In one evaluation, every respondent expressed interest in using the 3D-printed model for educational and training purposes.²⁸ These models can replicate different hiatus shapes and sizes, letting trainees experience the full range of normal variation rather than just one “standard” anatomy.

For ultrasound-guided approaches specifically, real-time simulation helps trainees recognize key sonographic landmarks like the sacrococcygeal membrane, the two cornua, and the epidural space before they ever hold a probe against a patient’s skin. Given that ultrasound use during caudal blocks was documented at only about 2.4% of pediatric cases in a large-scale database survey, there is clearly room for more widespread adoption, and better training tools could help close that gap.²⁹