Spine and Pelvis
32 Sacrum and Coccyx
Anatomy & Pathology
Projections
AP Axial Sacrum
CR Location & Positioning
- SID: 40 inches
- Patient position: Supine, knees flexed to reduce lumbar lordosis
- Adjustments: Center midsagittal plane to midline of IR; ensure both ASIS are equidistant from the table
- CR: 15° cephalad angle, centered 2 inches superior to the pubic symphysis
- Patient instructions: Suspend respiration
- Exposure: Moderate kVp for balanced penetration and contrast across dense and trabecular bone
Evaluation Criteria
- Coverage: Entire sacrum centered and included from sacral promontory to coccygeal base
- Rotation checks: Symmetric alae indicate no rotation; sacrum aligned with pubic symphysis
- Motion checks: Sharp bony margins and clear foramina show absence of patient motion
- Technique checks: Proper angle straightens the sacral curvature; adequate contrast shows trabecular detail without burnout
- Clinical aim: Demonstrate sacral morphology free of superimposition by pubic bones
AP Axial Coccyx
CR Location & Positioning
- SID: 40 inches
- Patient position: Supine, knees flexed to flatten lumbar curve
- Adjustments: Align midsagittal plane to IR; ensure pelvis is not rotated
- CR: 10° caudad angle, centered 2 inches superior to the pubic symphysis
- Patient instructions: Suspend respiration
- Exposure: Slightly lower kVp than sacrum to enhance visibility of fine cortical margins
Evaluation Criteria
- Coverage: Entire coccyx visible and centered above the pubic symphysis
- Rotation checks: Coccyx aligned with pubic symphysis; equal pelvic margins on both sides
- Motion checks: Sharp separation of coccygeal segments (if unfused) confirms no motion
- Technique checks: Correct caudad angle projects coccyx free of pubic bones; optimal exposure shows subtle trabecular pattern
- Clinical aim: Evaluate alignment and integrity of coccygeal segments
Lateral Sacrum and Coccyx
CR Location & Positioning
- SID: 40 inches
- Patient position: True lateral, hips and knees flexed; arms at right angles to body
- Adjustments: Place support under waist to align long axis of spine horizontally; superimpose knees and ankles
- CR: Perpendicular to 3.5 inches posterior to ASIS (for sacrum) or 3.5 inches posterior and 2 inches inferior to ASIS (for coccyx)
- Patient instructions: Suspend respiration
- Exposure: High enough mAs to penetrate pelvis; use lead shield behind patient to reduce scatter
Evaluation Criteria
- Coverage: Sacrum and coccyx fully included in lateral profile
- Rotation checks: Superimposed posterior margins of ischia and ilia confirm no rotation
- Motion checks: Sharp cortical outlines and foramina edges show absence of movement
- Technique checks: Proper exposure demonstrates uniform brightness with good contrast; lead backing prevents scatter fog
- Clinical aim: Show lateral morphology and alignment of sacrum and coccyx in a single projection
Memory Aids
Top bone → angle up. Bottom bone → angle down. (cephalad for sacrum, caudad for coccyx)
Big bone = bigger angle (15° cephalad for sacrum).
Small bone = smaller angle (10° caudad for coccyx).
Flex knees to flatten the back and straighten the sacrum.
Lateral = both bones, one image, less dose.
Hows & Whys of Sacrum & Coccyx Radiography
Anatomy
- How many segments originally form the sacrum?
Five vertebral segments fuse to form the sacrum. - How many segments make up the coccyx?
Usually four (but may be three to five). - What do the sacral foramina transmit?
Nerves and blood vessels pass through the anterior and posterior sacral foramina. - What forms the median sacral crest?
The fused spinous processes of the sacral vertebrae. - What are the horns (cornua) of the sacrum and coccyx?
They are remnants of the inferior and superior articular processes that articulate between the sacrum and coccyx. - The auricular surfaces of the sacrum form the medial walls of what joint?
Sacroiliac. - The sacral promontory forms the posterior wall of what opening?
The inlet of the true pelvis.
Positioning
- Why are the patient’s knees flexed for AP sacrum and coccyx imaging?
To reduce lumbar lordosis so the sacrum and coccyx lie more parallel to the IR. - Why is the CR angled 15° cephalad for the sacrum?
To project the sacrum free of the pubic symphysis and reduce foreshortening. - Why is the CR angled 10° caudad for the coccyx?
To elongate the coccyx and project it above the pubic bones. - Why is the CR angled cephalad for the sacrum and caudad for the coccyx?
The sacrum curves posteriorly, requiring a cephalad angle to project it free of the pubic symphysis, while the coccyx curves anteriorly and needs a caudad angle to elongate it. - Why should respiration be suspended?
To prevent motion blur and ensure sharp bone detail.
Technique & Image Evaluation
- How can you tell if the sacrum is rotated?
Unequal sacral alae or a shifted midline compared to the pubic symphysis. - How can you tell if the sacrum angle is too shallow?
The pubic symphysis overlaps the sacrum. - How can you tell if the coccyx angle is too steep?
The coccyx appears foreshortened or projected below the pubic symphysis. - How can you tell if the lateral is positioned correctly?
Posterior margins of the pelvis and greater sciatic notches are superimposed.
Clinical Applications
- Why is the sacrum angled differently from the coccyx on radiographs?
The sacrum and coccyx curve in opposite directions; matching the beam angle to their natural curvature ensures full visualization without foreshortening. - Why is one lateral used for both bones?
It reduces gonadal dose and still includes the entire region. - Why use lead shielding behind the patient for laterals?
It absorbs scatter and improves image contrast.
