11.2 Radiographic Positioning and Normal Anatomy: Spine and Pelvis
Key Takeaways
- The standard cervical spine series includes AP lower cervical, open-mouth odontoid, neutral lateral, and bilateral obliques; flexion-extension laterals are reserved for instability assessment after fracture is excluded.
- On a true lateral cervical radiograph, the spinous processes should be equidistant from the pedicles, C1-C7 should be visible, and the C4 vertebral body should align with the chin and the C7 spinous process with the shoulder.
- Lumbar oblique views at approximately 45 degrees profile the pars interarticularis — the 'Scotty dog' sign — and are added when spondylolysis is suspected.
- The AP pelvis view visualizes the hip joints, sacrum, and sacroiliac joints; the frog-leg lateral is contraindicated when a hip fracture is suspected.
- Positioning faults such as rotation, flexion/extension tilt, and inadequate mouth opening produce predictable anatomy distortion that can mimic or mask pathology.
11.2 Radiographic Positioning and Normal Anatomy: Spine and Pelvis
Quick Answer: Part II tests whether you know the standard view series for each spinal region and the pelvis, the anatomic landmarks that confirm correct positioning, and the positioning errors that create false pathology or mask real findings. Master the cervical open-mouth and lateral criteria, lumbar oblique pars visualization, and pelvis/hip positioning rules.
Cervical Spine
Standard View Series
| View | Purpose |
|---|---|
| AP lower cervical (C3–T1) | Vertebral body height, intervertebral disc spaces, uncovertebral joints, spinous process alignment |
| AP open-mouth (odontoid) | Dens (odontoid process), lateral masses of C1, C1-C2 articulation, atlantoaxial alignment |
| Lateral (neutral) | Overall cervical lordosis, vertebral body heights, disc spaces, spinous processes, prevertebral soft tissues |
| Bilateral obliques (45°) | Intervertebral foramina, facet joints, pedicles |
| Flexion-extension laterals | Dynamic segmental stability — added only when instability is suspected and acute fracture has been excluded on neutral views |
Positioning Criteria and Landmarks
Lateral cervical is the most commonly repeated view due to positioning difficulty. Criteria for a true lateral:
- Spinous processes are equidistant from the pedicles on every vertebral level (no rotation)
- The mandible does not superimpose C1–C3 (patient chin elevated)
- C1 through C7 (and ideally T1) are visible
- The C4 vertebral body aligns with the chin; the C7 spinous process aligns with the shoulder
- Prevertebral soft tissue stripe: normal width is less than 7 mm at C2 and less than 22 mm at C6
Open-mouth odontoid requires the patient to open the mouth as wide as possible with the head positioned so the upper incisors and base of the skull are parallel to the receptor. The dens should be centered between the lateral masses of C1. If the mandible superimposes the upper cervical spine, the mouth is not open enough or the head is tilted.
Oblique cervical views rotate the patient approximately 45° so the intervertebral foramina on the side farther from the receptor are profiled. The pedicle appears as a round density near the center of the vertebral body on each oblique.
Normal Anatomy Highlights
- Cervical lordosis: a smooth anterior curve; loss may indicate muscle spasm or disc pathology
- Uncovertebral joints (of Luschka): appear as small lateral projections at C3–C7 on AP views
- Atlantodental interval (ADI): the space between the anterior arch of C1 and the dens; greater than 3 mm in adults suggests atlantoaxial instability
Thoracic Spine
Standard View Series
| View | Purpose |
|---|---|
| AP | Vertebral body heights, disc spaces, pedicle alignment, costovertebral joints, paraspinal lines |
| Lateral | Vertebral body heights, disc spaces, thoracic kyphosis, spinous processes, posterior elements |
Positioning Criteria and Landmarks
Thoracic positioning is challenging because of overlapping ribs and shoulders. For the lateral thoracic:
- Arms raised or hands placed on hips to lift scapulae off the thoracic spine
- Shoulders and hips at equal distance from the receptor (no rotation)
- Full inspiration and breath-hold to spread ribs and improve lung aeration behind the spine
- The sternum should superimpose the thoracic bodies evenly (confirms no rotation)
On the AP thoracic, spinous processes should be equidistant from the pedicles. The pedicles appear as paired round densities lateral to the vertebral bodies.
Normal Anatomy Highlights
- Thoracic kyphosis: normal range is approximately 20–45°; excessive kyphosis may indicate compression fractures or Scheuermann disease
- Pedicles: intact pedicles on AP view are important for ruling out destructive lesions
- Costovertebral and costotransverse joints: visible at the rib attachments on AP views
Lumbar Spine
Standard View Series
| View | Purpose |
|---|---|
| AP (or AP pelvis-inclusive) | Vertebral body heights, disc spaces, pedicle alignment, sacroiliac joints, hip joints |
| Lateral | Vertebral body heights, disc spaces, lumbar lordosis, spondylolisthesis, pars defects |
| L5–S1 spot lateral | Dedicated visualization of the L5–S1 disc space and lumbosacral junction |
| Bilateral 45° obliques | Pars interarticularis (Scotty dog profile), facet joints — added when spondylolysis is suspected |
Positioning Criteria and Landmarks
Lateral lumbar: the posterior vertebral body lines, spinous processes, and pedicles should be superimposed (no rotation). The iliac crests should be at equal height. The L5–S1 disc space must be visible — if not, a spot lateral or increased cephalad angulation is needed.
Lumbar obliques are taken with the patient rotated approximately 45° from true lateral. The Scotty dog sign appears on the oblique view:
- Ears: superior articular process
- Nose: transverse process
- Eye: pars interarticularis (the region most prone to spondylolysis)
- Leg: inferior articular process
- Tail: spinous process
A defect in the pars (the "collar" of the Scotty dog) indicates spondylolysis.
Normal Anatomy Highlights
- Lumbar lordosis: normal range approximately 40–60°; loss may indicate disc degeneration or muscle spasm
- Lumbosacral angle: the angle between the L5 vertebral body and the sacrum; relevant for spondylolisthesis grading
- Sacralization or lumbarization: transitional vertebrae at the lumbosacral junction are common anatomic variants
Pelvis and Sacroiliac Joints
Standard View Series
| View | Purpose |
|---|---|
| AP pelvis | Hip joints, femoral heads/necks, acetabula, sacrum, sacroiliac joints, pubic symphysis |
| Frog-leg lateral (bilateral) | Femoral heads and necks in profile — assesses for slipped capital femoral epiphysis or hip joint effusion |
| Cross-table lateral | Preferred when hip fracture is suspected — avoids hip flexion/abduction that could displace fragments |
Positioning Criteria and Landmarks
On the AP pelvis, the patient is supine with legs internally rotated 15–20° (so the femoral necks are parallel to the receptor and not foreshortened). Landmarks of correct positioning:
- The coccyx is approximately 2 cm superior to the pubic symphysis (if the coccyx overlaps the symphysis, the pelvis is over-rotated)
- The obturator foramina are symmetric
- The sacroiliac joints are visible bilaterally
- The greater trochanters are equidistant from the midline
Frog-leg lateral is contraindicated when a femoral neck or intertrochanteric fracture is suspected because hip abduction and internal rotation can displace fracture fragments.
Normal Anatomy Highlights
- Sacroiliac joints: appear as thin radiolucent lines between the sacral ala and iliac wings; asymmetry alone is not diagnostic of SI joint pathology
- Acetabular roof: should form a smooth arc over the femoral head on AP view
- Shenton's line: a continuous curved line from the medial femoral neck to the inferior pubic ramus; disruption suggests hip fracture or dislocation
Common Positioning Faults: Spine and Pelvis
| Fault | Effect | Prevention |
|---|---|---|
| Rotation on lateral spine | Pedicles and spinous processes not superimposed; false scoliosis or facet asymmetry | Equal shoulder and hip contact with receptor; confirm spinous process midline |
| Inadequate mouth opening (odontoid) | Mandible superimposes C1–C2; dens not visualized | Coach patient to open wide; check head tilt |
| Foreshortened femoral necks (AP pelvis) | Legs not internally rotated; necks appear short, Shenton's line unreliable | Tape feet together with toes pointed inward 15–20° |
| Flexion/extension tilt on cervical lateral | False loss of lordosis or abnormal ADI | Ensure patient neck is neutral, not flexed or extended |
| Insufficient inspiration (thoracic lateral) | Ribs obscure vertebral bodies | Coach full inspiration and breath-hold |
On a properly positioned open-mouth odontoid view, which structure should be centered between the lateral masses of C1?
The 'Scotty dog' sign is used to evaluate which anatomic structure on lumbar oblique radiographs?
Which view is contraindicated when a femoral neck fracture is suspected?
On a true lateral cervical radiograph, which landmark confirms the absence of rotation?