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Updated: Jun 30 2024

Lumbar Spine Anatomy

Images xr.jpg orientation.jpg vs l5 3.jpg
  • Alignment
    • Sagittal plane
      • lumbar lordosis
        • average of 60 degrees
          • normal range is 20 to 80 degrees
        • apex of lordosis at L3
        • disc spaces responsible for most of lordosis
  • Lumbar Osteology
    • Lumbar spine has the largest vertebrae bodies in the axial spine
    • Components of vertebral bodies
      • anterior vertebral body
      • posterior arch
        • formed by
          • pedicles
            • pedicles project posteriorly from posterolateral corners of vertebral bodies
          • lamina
            • lamina project posteromedially from pedicles, join in the midline
      • spinous process
      • transverse process
      • mammillary processes
        • separate ossification centers
        • project posteriorly from superior articular facet
      • pars interarticularis
        • mass of bone between superior and inferior articular facets
        • site of spondylolysis
    • Articulations
      • intervertebral disc
        • act as an articulation above and below
      • facet joint (zygapophyseal joint)
        • formed by superior and inferior articular processes that project from junction of pedicle and lamina
        • facet orientation
          • facets become more coronal as you move inferior
  • Lumbar Pedicle Anatomy
    • Landmarks
      • midpoint of the transverse process used to identify midpoint of pedicle in superior-inferior dimension
      • lateral border of pars used to identify midpoint in medial-lateral dimension
    • Pedicle angulation
      • pedicles angulate more medial as you move distal
        • L1: 12 degrees
        • L5: 30 degrees
        • S1: 39 degrees
    • Pedicle diameter
      • L1 has smallest diameter in lumbar spine (T4 has smallest diameter overall)
      • S1 has average diameter of ~19mm
  • Lumbar Blood Supply
    • Lumbar vertebral bodies supplied by
      • segmental arteries
        • dorsal branches supply blood to the dura & posterior elements
  • Lumbar Neurologic Structures
    • Nerve roots
      • anatomy
        • nerve root exits foramen under same numbered pedicle
          • central herniations affect traversing nerve root
          • far lateral herniations affect exiting nerve root
        • dorsal rami
          • supplies muscles, skin
          • medial branch supplies facet joints
        • ventral rami
          • supplies anteromedial trunk
      • key difference between cervical and lumbar spine is
        • pedicle/nerve root mismatch
          • cervical spine C6 nerve root travels under C5 pedicle (mismatch)
          • lumbar spine L5 nerve root travels under L5 pedicle (match)
          • extra C8 nerve root (no C8 pedicle) allows transition
        • horizontal (cervical) vs. vertical (lumbar) anatomy of nerve root
          • because of vertical anatomy of lumbar nerve root a paracentral and foraminal disc will affect different nerve roots
          • because of horizontal anatomy of cervical nerve root a central and foraminal disc will affect the same nerve root
    • Intervertebral Disk
      • Sinu-vertebral nerve is responsible for nociception and proprioception of disk
      • Nerve fibers present along periphery of annulus fibrosus only
    • Cauda equina
      • begins at ~L1
  • Lumbar-Pelvic Sagittal Alignment
    • Pelvic incidence
      • pelvic incidence = pelvic tilt + sacral slope
      • a line is drawn from the center of the S1 endplate to the center of the femoral head
      • a second line is drawn perpendicular to a line drawn along the S1 endplate, intersecting the point in the center of the S1 endplate
      • the angle between these two lines is the pelvic incidence (see angle X in figure above)
      • correlates with severity of disease
      • pelvic incidence has direct correlation with the Meyerding–Newman grade
    • Pelvic tilt
      • pelvic tilt = pelvic incidence - sacral slope
      • a line is drawn from the center of the S1 endplate to the center of the femoral head
      • a second vertical line (parallel with side margin of radiograph) line is drawn intersecting the center of the femoral head
      • the angle between these two lines is the pelvic tilt (see angle Z in figure above)
    • Sacral slope
      • sacral slope = pelvic incidence - pelvic tilt
      • a line is drawn parallel to the S1 endplate
      • a second horizontal line (parallel to the inferior margin of the radiograph) is drawn
      • the angle between these two lines is the sacral slope (see angle Y in the figure above)
  • Image-Guided Interventions
    • Overview
      • performed using CT or fluoroscopic guidance
      • 22G-25G needle usually used for injection of local anesthetic and corticosteroid
    • Selective Nerve Root Injections
      • indications
        • unilateral radicular symptoms
        • used for therapeutic and diagnostic purposes
      • technique
        • transforaminal (outside-in) technique usually used
    • Facet joint injection
      • indications
        • to confirm facet joint as pain generator (diagnostic)
        • also a therapeutic procedure
    • Epidural injection
      • indications
        • lumbar spinal stenosis
    • Discography
      • indications
        • very controversial
        • to prove that pain arises from the intervertebral disc ("concordant pain") rather than other sources ("discordant pain")
      • technique
        • small amount of dilute contrast injected into the disc and pain response is recorded
        • contrast helps assess disc morphology and diagnose annular tears
  • Surgical Approaches
    • Posterior
      • posterior midline approach
        • can be used for PLIF or TLIF
      • Wiltse paraspinal approach
    • Anterior Lateral
      • retroperitoneal (anterolateral) approach
        • aorta bifurcation found at L4/5
        • superior hypogastric plexus on L5 body
          • damage causes retrograde ejaculation
      • also referred to as
        • transpsoas approach
        • direct lateral
      • patient position
        • lateral
        • usually performed on left side due to increased strength of aorta to injury
      • target levels
        • ideal for access for
          • L1/2
          • L2/3
          • L3/4
        • less ideal access
          • L4/5
            • highest risk of iatrogenic nerve injury to lumbar plexus and resulting hip flexion and knee extension weakness
          • T12/L1
            • will need to remove rib and take down diagphragm)
      • anatomic risks
        • lumbar plexus
          • moves dorsal to ventral moving down the lumbar spine
        • ilioinguinal and iliohypogastric nerves
          • may be injured during retroperitoneal approach resulting in groin paresthesias and abdominal paresis
        • segmental arteries
          • need to be stabled or tied off corpectomy performed
        • aorta
          • important to place anterior retractors so damage to aorta is prevented
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