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Sacrum and Coccyx X-Ray Positioning: AP, Lateral, and AP Coccyx Views

The sacrum and coccyx form the terminal end of the vertebral column, and imaging them presents unique challenges — their curved anatomy, overlying bowel gas, and proximity to the gonads make positioning technique especially important. Whether you're working up a patient with coccydynia (tailbone pain) after a fall or evaluating a suspected sacral insufficiency fracture, getting these views right the first time saves repeats and reduces patient dose.

In this guide, you'll learn a common three-image institutional protocol: the AP axial sacrum (15° cephalad), the AP axial coccyx (10° caudad), and a combined lateral sacrum/coccyx. Exact receptor size, centering, exposure, and permitted modifications remain department-specific; the ARRT content outline names these projections but is not a technique manual.

Anatomy You Need to Know

The sacrum is a wedge-shaped bone formed by five fused sacral vertebrae (S1–S5). Its anterior surface is concave. In a commonly taught adult AP axial method, a 15° cephalad central ray reduces foreshortening and better demonstrates the sacrum; this should not be described as literally “opening” every sacral foramen.

Below the sacrum, the coccyx (tailbone) typically has three to five segments (most often four); the degree of fusion is variable. It articulates with S5 at the sacrococcygeal joint, and its normal morphology and mobility vary substantially.

Key articulations to remember:

• L5–S1 (lumbosacral junction) — must be included in collimation
• S1–S5 (sacral segments) — fused
• Sacrococcygeal (S5–Co1) — potential site of fracture or subluxation
• Sacroiliac (SI) joints — bilaterally, sacrum articulates with ilia

Projection 1: AP Axial Sacrum

The AP axial sacrum demonstrates the sacrum in the frontal plane. It may be part of a local sacrum/coccyx series, but it is not a high-sensitivity test for subtle sacral fracture and is not a substitute for dedicated SI-joint projections or cross-sectional imaging when clinically indicated.

Patient Position

• Supine; flex the knees and support them if tolerated to reduce lumbar lordosis and improve comfort
• Do not internally rotate the legs merely to “open the femoral necks”; that is an AP pelvis maneuver, not a sacrum requirement
• Midsagittal plane centered to the grid and perpendicular to the IR
• ASIS must be equidistant from the table — check for rotation

Central Ray — Common Adult Method

• Angled 15° cephalad (toward the head)
• Centering point: At the MSP, approximately 5 cm (2 in) superior to the pubic symphysis in the cited University of Missouri clinical guide
• The angle reduces foreshortening; modify only under the local protocol for anatomy or habitus

Technical Factors

Parameter	Value
IR / SID example	24 × 30 cm (10 × 12 in), portrait; 100–110 cm (40–44 in), according to local protocol
Grid	Use when required by patient thickness and the department's validated technique chart
Exposure	Use the equipment- and patient-specific technique chart; fixed universal kVp/mAs values can promote noise or dose creep
AEC	Manual technique is often preferable for tightly collimated small anatomy; use AEC only when a validated protocol specifies chamber selection
Breathing	Suspend respiration to prevent motion; end-expiration may be used by local protocol but does not reliably displace bowel gas

Evaluation Criteria

• Entire sacrum visible from S1 to S5
• Sacral body, ala, foramina, and cortical margins demonstrated with appropriate contrast and without motion
• No rotation: sacrum centered to the image with symmetric pelvic landmarks; SI joints may be visible but are not expected to be open on this projection
• Include the portion of L5 required by the department's acceptance criteria and avoid unnecessary field size

Projection 2: Combined Lateral Sacrum and Coccyx

The lateral view demonstrates sacral and coccygeal alignment in profile and may show displacement. Dartmouth-Hitchcock's institutional protocol includes both structures on one lateral image. A static recumbent lateral does not measure coccygeal mobility; that requires specifically ordered, standardized standing-versus-seated lateral imaging.

Patient Position

• Lateral recumbent (left or right, whichever is more comfortable for the patient)
• Knees flexed for stability and comfort
• Use radiolucent support under the waist only if needed to make the spine/pelvis horizontal; excessive support causes tilt
• Ensure the patient's spine is parallel to the IR — not tilted anteriorly or posteriorly
• Arms folded in front or raised above the head

Central Ray — Combined Protocol

• Perpendicular (0°) to the IR
• Anterior-posterior landmark: approximately 9 cm (3.5 in) posterior to the ASIS is a common reference
• Select the superior-inferior center and collimation to include L5 through the distal coccyx. Dedicated lateral sacrum and lateral coccyx centers differ; follow the local combined-image protocol.

Technical Factors

Parameter	Value
IR / SID example	24 × 30 cm (10 × 12 in), portrait; 100–110 cm (40–44 in), according to local protocol
Grid	Use when required by patient thickness and the validated technique chart
Exposure / AEC	Use the local chart; lateral attenuation is usually greater than AP, but no fixed mAs ratio applies. Use AEC only if validated for this field and chamber.
Breathing	Suspend respiration

Evaluation Criteria

• Entire sacrum and coccyx visible from L5/S1 to the distal coccyx tip
• Posterior iliac/pelvic margins and greater sciatic notches closely superimposed — separation indicates rotation
• Adequate penetration to visualize the sacral cortex and coccygeal segments
• L5/S1 articulation clearly demonstrated

Projection 3: AP Axial Coccyx

A common AP axial coccyx method uses a 10° caudad angle to reduce superimposition of the coccyx by the pubic symphysis. Dedicated sacrum/coccyx radiographs have limited diagnostic value for uncomplicated coccydynia in many modern pathways, so perform them when clinically authorized rather than automatically after every fall.

Patient Position

• Supine; flex and support the knees if tolerated to reduce lumbar lordosis and improve comfort
• Same basic setup as the AP sacrum, but the CR angle is critically different

Central Ray — Common Adult Method

• Angled 10° caudad (toward the feet)
• Centering point: 5 cm (2 in) superior to the pubic symphysis at the mid-sagittal plane
• The caudad angle reduces, but does not guarantee elimination of, pubic-bone superimposition; anatomy varies

Technical Factors

Parameter	Value
IR / SID example	18 × 24 cm (8 × 10 in) or 24 × 30 cm (10 × 12 in), portrait; 100–110 cm (40–44 in), per local protocol
Grid	Use when required by patient thickness and the validated technique chart
Exposure / AEC	Use the local chart. Manual exposure is commonly suited to tight coccyx collimation; do not use AEC unless specifically validated.
Breathing	Suspend respiration

Evaluation Criteria

• Coccyx demonstrated with minimal pubic-bone superimposition and visible segmental detail
• Coccyx centered to the image without pelvic rotation
• Adequate penetration to visualize coccygeal segments
• Close collimation to the required anatomy for dose and scatter control

Positioning Errors: Quick Reference

Error	Likely Cause	Correction
Sacrum foreshortened / poorly projected	Angle, centering, habitus, or pelvic geometry	Verify the prescribed 15° cephalad baseline and center; modify only under local protocol
Coccyx obscured by pubic bone	Angle, centering, rotation, or individual curvature	Verify the prescribed 10° caudad baseline and MSP centering before any repeat
Posterior pelvic margins separated	Rotation	Correct shoulder/hip alignment without forcing a trauma patient
Coccyx cut off on combined lateral	Field too tight or center too high	Recheck full combined coverage before exposure
Required superior anatomy absent	Center/collimation mismatch	Apply that projection's local criterion; an AP coccyx need not include L5/S1
Excessive noise / poor penetration	Multiple technique or positioning causes	Check exposure indicator, thickness, grid, collimation, motion, and the validated chart—not kVp alone
Overlying bowel obscures anatomy	Normal anatomic overlap	Do not claim expiration moves gas; follow local prep policy and never delay urgent imaging for bowel preparation

ARRT Exam Prep: Sacrum & Coccyx Quick Review

The ARRT content outline places these projections within Head, Spine, and Pelvis Procedures, but does not assign a public question count specifically to sacrum/coccyx. Review these protocol-aware facts:

1. AP axial sacrum = 15° cephalad and approximately 5 cm superior to the symphysis for the cited common adult method
2. AP axial coccyx = 10° caudad to the same common surface level
3. Combined lateral uses a perpendicular CR; center and collimate to include L5 through the distal coccyx under local protocol
4. Lateral evaluation: check close superimposition of posterior pelvic margins and greater sciatic notches
5. Foraminal visibility alone does not dictate a repeat; use all image-quality criteria and the exposure indicator
6. Collimation requirements differ by projection: a dedicated AP coccyx need not include L5/S1
7. Routine gonadal/fetal contact shielding is not recommended because it can obscure anatomy, affect AEC, and cause repeats; follow institutional policy
8. Suspended respiration limits motion. It does not reliably move bowel gas, and bowel preparation is protocol-specific

Clinical Pearls: Tips from the Techs

• Use the landmarks specified by the projection: the pubic symphysis/MSP is used for the cited AP centers, while the ASIS is a useful lateral reference. Palpate only with consent, explain the contact, preserve draping, and use an alternative localization method when direct palpation is inappropriate.
• Prevent motion — give a clear suspend-breathing instruction. Do not tell patients that expiration predictably displaces bowel gas.
• Use the validated technique chart — select factors for measured thickness, detector, grid, projection, and exposure target; monitor the exposure indicator.
• Lateral recumbent, not decubitus — the lateral sacrum/coccyx is performed with the patient lying on their side, not standing. Make sure the patient is comfortable — a pillow between the knees and a small lumbar support go a long way toward preventing motion.
• Do not force trauma positioning — maintain spinal/pelvic precautions and immobilization. Obtain only safely tolerated views and consult the radiologist/trauma team when the routine series cannot be completed.

Related Conditions and Advanced Applications

Stress Views for Coccygeal Hypermobility

For patients with chronic coccydynia, a stress lateral (seated vs standing comparison) can demonstrate abnormal mobility at the sacrococcygeal or intercoccygeal joint:

• Normal: 5–25° of flexion on sitting compared to standing
• Hypermobile: >25° (marked if >35°)
• Subluxation: Posterior displacement on sitting — not normal, indicates ligamentous laxity

These published mobility ranges are used in specialist coccydynia evaluation, not as routine acceptance criteria for a static lateral. Dynamic views require a specific order, standardized technique, patient cooperation, and clinician interpretation.

Sacral Insufficiency Fractures

In older adults with osteoporosis, sacral insufficiency fractures are important and often radiographically occult. Negative AP or lateral radiographs do not exclude the diagnosis. MRI without contrast is usually the most sensitive next test for suspected stress/insufficiency fracture after negative or indeterminate radiographs; CT can better define cortical fracture detail. A visible vertical alar line may raise suspicion, but technologists should not reassure from a negative projection study when clinical concern persists.

Summary

Projection	Patient Position	CR Angle	Centering Point	Primary Purpose
AP Sacrum	Supine	15° cephalad	MSP, about 5 cm (2 in) superior to pubic symphysis in the cited method	Frontal sacral anatomy; limited sensitivity for subtle fracture
AP Coccyx	Supine	10° caudad	5 cm superior to pubic symphysis at MSP	Coccyx fractures, coccydynia
Lateral Sacrum/Coccyx	Lateral recumbent	0° (perpendicular)	About 9 cm posterior to ASIS; vertical center set for full combined coverage	Profile and alignment; static view does not measure mobility