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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 the three essential projections for sacrum and coccyx imaging: the AP sacrum (15° cephalad), the AP coccyx (10° caudad), and the lateral sacrum/coccyx. We'll cover Clark's-recommended centering points, evaluation criteria, and the most common positioning errors — plus ARRT exam tips to help you lock in these concepts for the registry.

Anatomy You Need to Know

The sacrum is a wedge-shaped bone formed by five fused sacral vertebrae (S1–S5). It's curved with an anterior concavity (sacral kyphosis) — this curvature is the reason the AP sacrum requires a 15° cephalad angle to open the sacral foramen and clearly visualize the body of the sacrum.

Below the sacrum, the coccyx (tailbone) consists of four fused coccygeal vertebrae (Co1–Co4). It articulates with the sacrum at the sacrococcygeal junction, which can be a site of post-traumatic pain or hypermobility.

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 Sacrum (Clark's)

The AP sacrum is the primary projection for evaluating sacral fractures, SI joint pathology, and sacral tumors. The key technical consideration is the 15° cephalad angle, which compensates for the anterior concavity of the sacrum.

Patient Position

• Supine with arms at sides
• Legs extended, feet slightly internally rotated to open the femoral necks
• Midsagittal plane centered to the grid and perpendicular to the IR
• ASIS must be equidistant from the table — check for rotation

Central Ray (Clark's, p160–161)

• Angled 15° cephalad (toward the head)
• Centering point: At the MSP, midway between the ASIS and the symphysis pubis
• The cephalad angle allows the CR to pass perpendicular to the sacral curvature, "opening" the sacrum

Technical Factors

Parameter	Value
IR size	24 × 30 cm (10 × 12 in), portrait
SID	100–110 cm (40 in)
Grid	Yes
kVp	70–80
mAs	20–30
Breathing	Suspend expiration (depresses diaphragm, reduces bowel gas)

Evaluation Criteria

• Entire sacrum visible from S1 to S5
• Sacral foramen clearly open and visible — if not, the angle was insufficient or the technique too low
• SI joints symmetrical — asymmetric = rotation
• Iliac crests level and symmetrical
• No rotation — sacrum should be midline between femoral heads
• L5/S1 articulation included in collimation

Projection 2: Lateral Sacrum and Coccyx (Clark's)

The lateral view is the single most informative projection for the sacrum and coccyx. It evaluates anterior-posterior displacement in fractures, sacral alignment, and coccygeal mobility. It's usually performed as a combined sacrum/coccyx lateral in a single image.

Patient Position

• Lateral recumbent (left or right, whichever is more comfortable for the patient)
• Knees flexed for stability and comfort
• Place a small pillow or cushion under the waist to maintain a true lateral position
• 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 (Clark's)

• Perpendicular (0°) to the IR
• Centering point: 8–10 cm (3–4 in) posterior to the ASIS
• This centering places the CR at approximately the S2–S3 level

Technical Factors

Parameter	Value
IR size	24 × 30 cm (10 × 12 in), portrait
SID	100–110 cm (40 in)
Grid	Yes
kVp	80
mAs	30–40 (higher than AP due to increased tissue density)
Breathing	Suspend expiration

Evaluation Criteria

• Entire sacrum and coccyx visible from L5/S1 to the distal coccyx tip
• Greater sciatic notches superimposed — confirms true lateral (no rotation)
• Femoral heads superimposed — additional rotation check
• Adequate penetration to visualize the sacral cortex and coccygeal segments
• L5/S1 articulation clearly demonstrated

Projection 3: AP Coccyx (Clark's)

The AP coccyx uses a 10° caudad angle to project the coccyx below the pubic rami, where it can be clearly visualized. This view is often performed when there's focal coccyx pain or suspected fracture after a fall — like slipping on ice or landing directly on the tailbone.

Patient Position

• Supine with arms at sides, legs extended
• Same basic setup as the AP sacrum, but the CR angle is critically different

Central Ray (Clark's)

• 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 projects the coccyx below the pubic rami, eliminating superimposition

Technical Factors

Parameter	Value
IR size	24 × 30 cm (10 × 12 in), portrait
SID	100–110 cm (40 in)
Grid	Yes
kVp	80
mAs	15
Breathing	Suspend expiration

Evaluation Criteria

• Coccyx free of pubic rami superimposition — this is the most critical check
• Lateral margins of the coccyx equidistant from the pelvic brim (no rotation)
• Adequate penetration to visualize coccygeal segments
• Close collimation to the area of interest (gonadal protection)

Positioning Errors: Quick Reference

Error	Likely Cause	Correction
Sacral foramen not open (AP sacrum)	Insufficient cephalad angle (<15°)	Increase angle to 15° — CR should be perpendicular to sacral curve
Coccyx superimposed by pubic rami (AP coccyx)	Angle too shallow or wrong direction	Use 10° caudad — NOT cephalad
Sacrum foreshortened (AP sacrum)	Excessive cephalad angle (>15°)	Reduce to exactly 15°
Greater sciatic notches not superimposed (Lateral)	Patient rotation	True lateral — check PSIS alignment, adjust waist cushion
Coccyx cut off inferiorly (Lateral)	Collimation too tight or centering too high	Include distal coccyx; center 8–10 cm posterior to ASIS
L5/S1 not visible (any view)	Insufficient superior collimation	Expand collimation to include L5/S1 junction
Overall underexposed	kVp too low	Increase to 70–80 kVp to penetrate sacral density
Overlying bowel gas obscures anatomy	No patient prep or expiration pause	Consider abdominal prep for elective exams; use expiration pause

ARRT Exam Prep: Sacrum & Coccyx Quick Review

The ARRT registry typically includes 3–5 questions on the sacrum and coccyx, often mixed in with the pelvis and spine sections. Here are the highest-yield facts:

1. AP sacrum = 15° cephalad — the CR angle compensates for the anterior sacral concavity to open the sacral foramen
2. AP coccyx = 10° caudad — projects the coccyx below the pubic symphysis
3. Lateral sacrum/coccyx — centering point is 8–10 cm posterior to the ASIS with a perpendicular (0°) CR
4. Lateral evaluation — check superimposition of the greater sciatic notches (rotation = not superimposed)
5. Sacral foramen visualization = adequate penetration and correct CR angle
6. Coccyx on AP must be free of pubic rami superimposition
7. SI joints are assessed for symmetry on the AP sacrum view
8. Collimation must include L5/S1 superiorly and the distal coccyx inferiorly
9. Lead shielding behind the sacrum on the lateral view reduces scatter radiation
10. Bowel gas is the most common source of non-diagnostic sacrum images — use expiration pause and consider prep for elective studies

Clinical Pearls: Tips from the Techs

• Always palpate the ASIS — the centering for both AP and lateral views depends on this landmark. Don't guess; take the three extra seconds to feel for it.
• Expiration pause is your friend — the sacrum sits posterior to the bowel. Having the patient suspend at end-expiration displaces bowel gas upward, giving you a cleaner image of the sacrum.
• Higher technique than you think — the sacrum is dense bone surrounded by thick soft tissue. Don't be afraid to use 75–80 kVp with appropriate mAs. An underexposed sacrum is non-diagnostic.
• 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.
• When in doubt, do the lateral first — the lateral sacrum/coccyx gives you the most diagnostic information per projection. If the patient is in significant pain and can only tolerate one view, make it the lateral.

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 stress views require patient cooperation and are typically ordered by a spine specialist or pain management physician.

Sacral Insufficiency Fractures

In elderly patients with osteoporosis, sacral insufficiency fractures are an important and often missed diagnosis. These fractures are frequently not visible on AP radiographs alone — the lateral view and CT provide better sensitivity. Look for vertical fracture lines through the sacral ala running parallel to the SI joint on the AP view.

Summary

Projection	Patient Position	CR Angle	Centering Point	Primary Purpose
AP Sacrum	Supine	15° cephalad	MSP midway ASIS & symphysis pubis	Sacral fractures, SI joints, sacral foramen
AP Coccyx	Supine	10° caudad	5 cm superior to pubic symphysis at MSP	Coccyx fractures, coccydynia
Lateral Sacrum/Coccyx	Lateral recumbent	0° (perpendicular)	8–10 cm posterior to ASIS	AP displacement, alignment, hypermobility