HM

Lumbar, Sacrum & Coccyx – Anatomy and Radiographic Techniques

Anatomy and Radiographic Techniques: Lumbar, Sacrum & Coccyx

  • Source content covers lumbar spine anatomy, sacrum, coccyx, and detailed radiographic techniques for lumbar region imaging.
  • Plates (visual references) noted throughout (e.g., Plate 1–60) to illustrate specific anatomy or projection views.

Anatomy: Lumbar Spine

  • The five lumbar vertebrae are the largest in the vertebral column; they are the strongest because body weight load increases toward the inferior end of the column.
  • Cartilaginous intervertebral disks between inferior lumbar vertebrae are common sites of injury and pathology.
  • Lumbar vertebral bodies are larger than thoracic and cervical; L5 is the largest.
  • Transverse processes are fairly small; spinous process is bulky and blunt.
  • Palpable landmark: the lower tip of each lumbar spinous process lies at the level of the intervertebral disk space inferior to each vertebral body (Plate 1).

Vertebra anatomy (lateral view, Plate 1)

  • Spinous process, superior and inferior articular processes (facets), pedicles, transverse processes, vertebral foramen, and body.
  • Intervertebral foramina are formed by the notches on adjacent pedicles; the superior vertebral notch is on the upper surface of the pedicle, the inferior vertebral notch on the lower surface. When vertebrae stack, these notches align to form the intervertebral foramina.

Anatomy: Intervertebral Foramina

  • Foramina are openings between pedicles through which spinal nerves and vessels pass (two foramina per adjacent vertebrae, one on each side).
  • Superior vertebral notch (roof of foramen) and inferior vertebral notch (floor) form the intervertebral foramen when stacked.
  • Best demonstrated on a lateral lumbar radiographic image.

Anatomy: Zygapophyseal Joints and Facets

  • Each typical vertebra has four articular processes at the junction of pedicles and laminae: superior articular processes project upward; inferior articular processes project downward.
  • The articular surface is called a facet; the joint is the zygapophyseal joint.
  • Zygapophyseal joints form an angle open from $30^{\u00b0}$ to $50^{\u00b0}$ to the MSP (Plate 2).
  • Proximal lumbar vertebrae are nearer the $50^{\u00b0}$ angle; distal lumbar vertebrae nearer $30^{\u00b0}$.
  • Demonstration of zygapophyseal joints is achieved by rotating the patient’s body an average of $45^{\u00b0}$ (Plate 2).
  • Laminae form a bridge between the transverse processes, lateral masses, and spinous process.
  • Pars interarticularis: the portion of the lamina between the superior and inferior articular processes; radiographically demonstrated on oblique lumbar images (Plate 3).

Anatomy: Lumbar Spine Projections and Views

  • Anterior and posterior views (Plate 3) show vertebral body anatomy, spinous processes, and transverse processes protruding laterally.
  • AP or PA projections demonstrate spinous processes superimposed on vertebral bodies.
  • Lateral projection demonstrates intervertebral foramina clearly.

Anatomy: Oblique Lumbar Vertebrae – Scottie Dog (Plate 34–36, 37)

  • Oblique views produce a radiographic appearance of a “Scottie dog” formed by the anatomy of the lumbar vertebrae.
  • The parts of the Scottie dog:
    • Head and neck: pars interarticularis (part of the lamina forming the neck/shoulder area).
    • Ear: superior articular process.
    • Eye: pedicle (seen end-on).
    • Nose: transverse process.
    • Front leg: inferior articular process.
    • Front leg’s intersection with the transverse process and superior articular process creates the “front legs” and other features (Plate 14).
  • Oblique projection angles: typically $45^{\u00b0}$; rotation assessed by open zygapophyseal joints and the pedicle (eye of Scottie dog).
  • Upward vs downward zygapophyseal joints are demonstrated depending on oblique side (RPO/LPO show downside joints; RAO/LAO show upside joints) (Plate 16–20).

Anatomy: Lumbar Joints and Intervertebral Disks

  • Zygapophyseal joints: synovial, diarthrodial joints with synovial membrane and plane (gliding) movement.
  • Intervertebral joints: bodies connected by intervertebral disks made of fibrocartilage; slightly movable (amphiarthrodial, symphysis subclass).
  • Disks act as cushions and facilitate movement; degeneration or herniation affects mobility and nerve roots.

Anatomy: Sacrum

  • Situated inferior to L5; anterior surface concave; originally five segments fuse into a single bone in the adult.
  • Sacrum is shovel-shaped with apex pointing inferiorly and anteriorly.
  • Four anterior sacral foramina transmit nerves and vessels; sacral foramina are similar to superior foramina in higher spine sections.
  • Ala (wings) of the sacrum are large lateral masses; superior articular processes form zygapophyseal joints with L5.
  • Promontory (anterior edge of S1 body) helps form the pelvic inlet; best seen in lateral view (sacral promontory).
  • Median sacral crest formed by fused spinous processes (Plate 5).
  • Sacroiliac joint: the auricular surface (resembles the auricle of the ear) articulates with the ilium.
  • Sacral horns (cornua) are small tubercles projecting inferiorly from the sides of S5; they articulate with coccygeal cornua.
  • Lateral view shows the posterior concavity of the sacrum and the coccyx projecting anteriorly; the angle requires cranial angulation on AP projections of the sacrum or coccyx (Plate 5–9).

Anatomy: Sacrum Details (continued)

  • Auricular surface articulates with ilium to form the sacroiliac joint; sacroiliac joints open obliquely posteriorly at about $30^{\u00b0}$ (posterior oblique views clarify the joints).
  • Eight posterior sacral foramina (four on each side) correspond to anterior foramina.
  • Sacral canal contains sacral nerves; remnants of the sacral canal may be seen on radiographs.

Anatomy: Coccyx

  • Typically four fused segments; sometimes the second segment does not fuse solidly with the first.
  • Coccyx has apex (distal tip) and base (broader superior part).
  • Radiographic plates show coccyx anatomy in anterior (Plate 7) and posterior (Plate 8) views.
  • Coccyx curvature: normally anteriorly; more pronounced in men than in women; sometimes curves into the birth canal in women, which can impede birth.
  • Injuries commonly result from a direct blow while sitting or from a fall backward onto the buttocks.

Anatomy: Lateral View of Sacrum & Coccyx (Plate 9)

  • Sacrum appears as a large solid bone; coccyx is smaller.
  • Sacrum axis shows posterior tilt; this requires cranial angulation on AP projections.
  • Coccyx curvature is forward (anterior) and more pronounced in men.

Plate References and Visual Cues (Summary)

  • Plate 1: Lumbar vertebra – lateral view: landmarks such as palpable spinous tips.
  • Plate 2: Lumbar vertebra – axial view: intervertebral foramina anchor points and vertebral notches.
  • Plate 3: Lumbar vertebra – posterior view: relationships of articular processes and pars interarticularis.
  • Plate 4: Sacrum anterior view: alae and SI joints.
  • Plate 5–6: Sacrum lateral views and posterior foramina features.
  • Plate 7–8: Coccyx anterior and posterior views.
  • Plates 9–12: Various sacrum and coccyx views and spine-to-pelvis relationships.
  • Plates 14–15: Scottie dog anatomy on oblique lumbar radiographs (RPO/LPO/RAO/LAO).
  • Plate 27–33: AP/PA and lateral lumbar radiographs, incl. L5-S1 specific views and centering cues.
  • Plates 34–37: Additional Scottie dog demonstrations and oblique radiographs.
  • Plates 38–42: Zygapophyseal joints and oblique radiographs (RPO/LPO, RAO/LAO) and evaluation criteria.

Radiographic Techniques: Overview

  • Goals: demonstrate lumbar vertebrae, disk spaces, spinous processes, facets, intervertebral foramina, SI joints, sacrum, coccyx.
  • Projections include AP (or PA), lateral, oblique (RPO/LPO and RAO/LAO), and specialized views (coned-down, scoliosis series, flexion/extension, sacral/coccyx AP axial, sacroiliac joints AP axial, RPO/LPO for SI joints).
  • Projections chosen depend on clinical indication (spondylosis, spondylolisthesis, scoliosis, metastasis, TB Pott’s, herniated disk, fractures).

Surface Landmarks for Positioning (Paired with Radiographs)

  • A: Symphysis pubis (superior margin level used for landmarks such as L5-S1 and general pelvis alignment).
  • B: Anterior superior iliac spine (ASIS) corresponds to the S1–S2 region.
  • C: Highest point of iliac crest (level with L4–L5 junction).
  • D: Lowest margin of the ribs or lower costal margin (approx. L3).
  • E: Xiphoid tip (approx. T9–T10).
  • Plate 23 shows surface landmarks.

Patient Protection and Positioning Practices

  • Radiation protection: tight collimation; gonadal shielding for males (placed at lower margin of the symphysis); apply a ten-day rule for females of reproductive age.
  • Patient comfort and motion reduction: knee/hip flexion using a triangular form or pillow; leg positions to reduce lordosis and improve disk space visibility.
  • Breathing instructions: suspend expiration for certain projections to minimize motion and improve image clarity.

Technical Parameters (Representative Values)

  • kVp: AP lumbar $70$–$85$ kVp; lateral lumbar $80$–$100$ kVp; higher for obliques or erect positions as needed.
  • mAs: AP $25$–$36$; lateral $36$–$46$ (depends on system and clinical factors).
  • FFD (distance): $100$–$120 ext{ cm}$.
  • Grid: Yes (use when body habitus dictates).
  • Tube angulation: generally small or none for some projections; coned-down and obliques require specific angulations.
  • Cassette sizes: varied; common sizes include $30 imes 40 ext{ cm}$ (14"×16"), $35 imes 43 ext{ cm}$, and $18 imes 24 ext{ cm}$ (portrait).
  • Breathing: suspended expiration (for stability and to reduce motion).
  • Table top lead masking: Yes for lat, obese patients when feasible.

Patient Alignment and Centering Points (Key Projections)

  • AP lumbar:
    • Patient supine with the MSP aligned to table midline.
    • ASIS equidistant from tabletop.
    • Hips/knees flexed; plantar aspect on table to reduce lumbar lordosis.
    • Upper limbs by the body.
    • Central beam centered at midline at the level of the lower costal margin (
      L3).
    • Plate 27 demonstrates AP view.
  • PA projection: more parallel intervertebral spaces to X-ray divergence; erect PA may reflect weight-bearing alignment.
  • Lateral lumbar: patient on side with head on a pillow; knees flexed; 90° coronal plane to midline; spacers/bolsters as needed to keep spine parallel to IR; centering at a point 7.5 cm anterior to the L3 spinous process at the lower costal margin; in wider pelvis, consider 5°–8° caudal angulation.
  • Lateral coned-down view (L5–S1): no tube angulation if supported; or 5°–80° caudal angulation depending on support; center 5 cm below iliac crest and 5 cm posterior to ASIS on the elevated side.
  • RPO/LPO and RAO/LAO (L5–S1): 45° rotation; MSP to 45° to the table; centering: 2.5 cm medial to upside ASIS; ensure Scottie dog configuration of L5 elements.
  • Scoliosis series (PA erect and lateral erect): erect and recumbent comparisons; lower IR edge placed 3–5 cm below iliac crest; arms at sides; MSP to CR; compensation filters for protection.
  • AP/PA axial sacrum: various centering depending on projection; AP: 15°–20° cranial angulation; PA: 15°–20° caudal; center to midline of sacrum with CR directed accordingly; patient may be supine or prone as clinical scenario dictates.
  • Coccyx AP axial: supine with 5° caudal angulation; can increase to 10°–15° caudal for patients with lesser anterior curvature; PA approach with 10° cephalad angle also possible; prone alternative with 10° cephalad angle can be used.
  • Sacroiliac joints (AP axial): supine with MSP at 90° to table; central ray to a point 5 cm inferior to ASIS with $30^{\u00b0}$–$35^{\u00b0}$ upward angulation; males ~$30^{\u00b0}$; females ~$35^{ 0}$ with lumbo-sacral curve considerations; if needed, PA projection with 30°–35° downward angulation in prone position.
  • RPO/LPO sacroiliac joints: 25°–30° posterior oblique; elevated side side joint closer to midline; use foam pad under elevated hip and flex elevated knee; central ray 2.5 cm medial to upside ASIS; ensure correct angles with an angle-measuring device (Plate 60).

Projections and Their Clinical Indications

  • AP lumbar: general overview of lumbar spine; central ray at L3; assess disk spaces and SI joints (Plate 28).
  • PA lumbar: better parallelism of intervertebral spaces to divergence; useful if erect positioning is needed (Plate 28).
  • Lateral lumbar: assess intervertebral foramina; evaluate disk spaces, vertebral bodies, and joints; coned-down views for L5–S1 to reduce gonadal dose (Plate 31, Plate 33).
  • Oblique lumbar (45° obliques): evaluate pars interarticularis and facet joints; 45° rotation standard; assess Scottie dog anatomy (Plate 34–37).
  • RPO/LPO (L5–S1): demonstrate downside joints; patient rotated so MSP at ~45°; visualize pars interarticularis.
  • RAO/LAO: visualize upside zygapophyseal joints; patient rotated accordingly; 45° oblique angle.
  • Lateral flexion/extension (hyperflexion/hyperextension): assess stability and mobility around fusion sites; upright or recumbent positions; two exposures; evaluate movement; plate 43–44 demonstrate lateral views (Plate 43).
  • PA erect scoliosis series: determine degree/severity of scoliosis; erect and recumbent comparisons; IR edge below iliac crest; protect both breasts and gonads with shielding and compensation filters (Plate 44).
  • Lateral erect scoliosis series: evaluate kyphosis or lordosis; convex side against IR; ensure no rotation; lower IR below iliac crests (Plate 46).
  • AP supine with left and right bending scoliosis: assess range of motion; patient bending laterally; supine recumbent options (Plate 46–47).
  • Lateral coned-down view for L5–S1: focused image for listhesis or other L5–S1 pathology; center 5 cm below iliac crest; 5 cm posterior to ASIS on elevated side (Plate 33–34).
  • Sacrum AP axial and PA axial: evaluate sacral pathology and fractures; ensure bowel prep and bladder empty; use appropriate cranial angulation ($15^{\u00b0}$ to $20^{ 0}$) depending on projection; center to sacrum midline (Plate 48–52).
  • Coccyx AP axial and lateral: evaluate coccygeal fractures and pathology; center 5 cm superior to symphysis with ~10° downward angulation (or 15° downward if posterior curvature is greater); lateral projection with centered beam to include coccyx and sacrum (Plate 52–56).
  • SI joints AP axial and RPO/LPO: evaluate sacroiliac joints for dislocation or subluxation; bilateral study for comparison; ensure marks and centering for symmetry (Plates 58–60).

Clinical Indications for Lumbar Radiography

  • Spondylosis, spondylolisthesis, ankylosing spondylitis, spina bifida, scoliosis, lordosis, metastasis, TB of the spine (Pott’s disease), herniated disk, and fractures.

Special Considerations by Patient Population

  • Paediatrics: emphasize communication, immobilization, and shorter exposure times; radiation protection considerations.
  • Geriatrics: address potential osteoporosis; adjust exposure factors downward and minimize motion; ensure comfort and immobilization.
  • Obese patients: palpation of landmarks can be challenging; may require increased exposure factors; increased scatter reduces image quality; plan for oblique consistency and longer exposure times if needed.

Key Propositions and Practical Tips

  • Correct alignment ensures that the beam passes through intervertebral disk spaces, particularly in the lower lumbar region.
  • Rotation assessment is critical in oblique studies; open zygapophyseal joints and pedicle position indicate proper rotation; under-rotation or over-rotation can obscure structures.
  • For scoliosis imaging, erect positions capture weight-bearing and curvature; use compensatory filtration and minimize rotation when possible.
  • Coned-down views (L5–S1) reduce gonadal dose while focusing on critical joints.
  • The Scottie dog appearance is a practical mental model for recognizing proper oblique lumbar anatomy and verifying rotation.

Quick Reference: Key Angles and Landmarks (LaTeX-formatted)

  • Zygapophyseal joints open angle: $30^{\u00b0}$ to $50^{ 0}$ to the MSP.
  • Oblique lumbar rotation standard: $45^{\u00b0}$.
  • Lateral lumbar centering: approximately $7.5\text{ cm}$ anterior to L3 spinous process at the lower costal margin;
    adapt to patient habitus (widest pelvis may require slight angulation changes).
  • Sacrum AP axial: central ray angulation typically $15^{ 0}$–$20^{ 0}$ cranial;
    males around $30^{ 0}$; females around $35^{ 0}$ for Sacroiliac joint visualization in a broader lumbar curve context.
  • Coccyx AP axial: 5° caudal angle (can increase to 10°–15° caudal for pronounced anterior curvature).
  • L5–S1 coned-down: center 5 cm below iliac crest; 5 cm posterior to ASIS on elevated side (Plate 33).
  • Lateral coned-down: no tube angulation if supported; otherwise 5°–8° caudal angle may be used for wider pelvis patients (Plate 33).
  • AP/PA sacrum: $15^{ 0}$–$20^{ 0}$ angle changes based on projection; center at sacrum midline (Plate 48–52).

Plate-Based Visual Cues (What to Look For in Images)

  • Plate 27: AP view – verify spacing and SI joint visualization.
  • Plate 28: PA view – better SI joint visualization; ensure open disk spaces.
  • Plate 31–33: Lateral lumbar – confirm open vertebral foramen, foramina, and L5–S1 alignment; check for scoliosis or listhesis.
  • Plate 34–37: Oblique lumbar – confirm the Scottie dog anatomy and proper rotation.
  • Plate 39–42: Oblique views – assess pars interarticularis and apophyseal joints on the downside or upside depending on projection.
  • Plate 43–46: Lateral flexion/extension – demonstrate mobility around fusion sites and scoliosis curves.
  • Plate 58–60: SI joint views – evaluate sacroiliac joints in AP axial and oblique projections.
  • Plate 52–56: Coccyx and sacrum lateral views – ensure alignment and centering for combined sacrum and coccyx imaging.

Note: All numeric values, angles, and measurements are referenced as shown in the transcript and have been converted to LaTeX format in this summary for precision and exam-ready clarity.