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Anatomy of Skull and Spine

= primary structural support of human body (Francis Denis, 1983)

  1. transmitting axial load of most of the body's weight
  2. restraining motion during flexion, extension, rotation, lateral bending
  1. Anterior column
    = anterior longitudinal ligament, anterior annulus fibrosus, anterior ⅔ of vertebral body
    Function: bearing axial load, resisting extension
  2. Middle column
    = posterior ⅓ of vertebral body, nucleus pulposus, posterior annulus fibrosus, posterior longitudinal ligament


    Function: bearing some axial load, resisting flexion
    • Integrity of the middle column is synonymous with stability!
  3. Posterior column
    = posterior elements (pedicles, facets, laminae) + ligaments (lig. flavum, interspinous ligament, supraspinous ligament)
    Function: resisting flexion, stabilizing rotation + lateral bending

Posterior Ligamentous Complex (PLC)!!navigator!!

Function: “tension band” of spinal column resisting compressive forces on vertebral bodies

  1. Supraspinous ligament
    = strong cordlike ligament connecting tips of spinous processes from C7 to sacrum
  2. Interspinous ligament
  3. Articular facet capsules
  4. Ligamentum flavum
    = thick broad structure connecting laminae of adjacent vertebrae

Posterior Longitudinal Ligament!!navigator!!

Function: contributes to stability of spinal column

Attachment: tethered to vertebral body via a central septum creating a left + right anterior epidural space

  1. superficial dorsal layer: 8–10 mm wide at disk space level separable from dura at dissection
  2. deep layer: 2–3 mm wide at disk space level

Thoracic Spine!!navigator!!

  • 12 load-bearing vertebrae
  • posterior arch (= pedicles, laminae, facets, transverse processes) handles tensional forces
  • vertebral bodies:
    1. height of vertebrae anteriorly 2–3 mm less than posteriorly → mild kyphotic curvature
    2. AP diameter: gradual increase from T1 to T12
    3. transverse diameter: gradual increase from T3 to T12

Thoracolumbar Spine (T12–L1)!!navigator!!

Functional unit: 2 vertebrae + interconnecting soft tissues

  1. anterior portion = 2 aligned vertebral bodies + intervertebral disk + anterior and posterior longitudinal ligaments
  2. posterior portion = vertebral arches + facet joints + posterior ligamentous complex

Transitional Vertebra!!navigator!!

= vertebra retaining partial features of segments below and above; total number of vertebrae in 92% unchanged with 24 (= 7 + 12 + 5) segments

Incidence: 3–21% of population

  • Bertolotti syndrome = back pain from transitional 5th lumbar vertebra resulting in partial sacralization
  • incidental finding

Location: thoracolumbar (4%) + lumbosacral (15%) junction

Variability of distribution (see table):

  1. variation from 12 thoracic + 5 lumbar segments maintaining together 17 presacral segments, eg, 11 thoracic + 6 lumbar OR 13 thoracic + 4 lumbar
  2. anomalous number of vertebrae (= 23 / 25 presacral segments)
  3. thoracolumbar / lumbosacral transitional vertebra

Thoracolumbar Transitional Vertebra

  • one side with a rib (= laterally downsloping osseous structure with articulation to vertebra)
  • other side with a transverse process (= horizontal osseous structure without central articulation to vertebra)

Lumbosacral Transitional Vertebra

  • The first non–rib-bearing vertebra = L1
  • “sacralized L5” = L5 incorporated into sacrum
  • “lumbarized S1” = S1 incorporated into lumbar spine
  • uni- / bilateral dysplastic / enlarged transverse processes of lumbosacral transitional vertebra ± uni- / bilateral contact / pseudarthrosis / fusion to adjacent sacral ala
  • “squared” morphology of transitional vertebra
  • decreased height of intervertebral lumbosacral disk
  • none / small residual / well-formed S1-2 disk
  • alteration of lumbosacral intervertebral disk angle

Cx: confusion over labeling / assignment of vertebral levels during treatment planning

  • Counting cephalad from presumed lumbosacral angle can lead to errors!

Numbering Vertebral Levels at Lumbar MR

Nota bene:

  1. The iliolumbar ligament (= low SI structure extending from transverse process to posteromedial iliac crest) identifies the lumbosacral junction (99%), but does NOT ALWAYS denote level of L5
  2. The magnitude of the lumbosacral intervertebral disk angle is not useful
  3. A thoracolumbar transitional vertebra is NOT associated with an anomalous number of presacral segments

    Distribution of Presacral Segments

    Segmentsthoracic + lumbar seg.
    235%3.5%*11 + 5
    1.5%*12 + 4
    2492%89.0%*12 + 5
    2.2%*13 + 4
    0.8%*11 + 6
    253%2.4%*12 + 6
    0.6%*13 + 5

    * The cervical spine is morphologically stable with 7 vertebrae

  4. A lumbosacral transitional vertebra is associated with an anomalous number of presacral segments:
    • Avoid wrong-level spine surgery and obtain whole-spine localizer image!

MR Report:

  • “This report assumes that there are five lumbar-type vertebrae, with the lowest lumbar vertebra identified by the iliolumbar ligament.”
  • “A lumbosacral transitional vertebra is present characterized by …. The lowest well-formed intervertebral disk is at ….”

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