Osteology of the Spine

Prenatal Vertebral Development

Mesodermal Somites

  • Sclerotome parts of Mesodermal Somites form
    • Bodies
    • Costal processes
    • & Arches
  • Around the Notochord & Neural tube
  • Lie in 44 paired segmental masses
  • Differentiates into
    • Loosely packed cells in cranial ½
    • Densely packed cells in caudal ½
  • Each body formed by 2 intersegmental halves
  • Vertebrae start as Chondroid rings

Comparative Anatomy

  • 1st 4 somites become Occipital somites
    • Basiocciput
    • 4th – Os Terminale
    • 4th – Alar ligament
  • Next 7 somites become Cervical somites
    • 1st & 2nd – Axis
    • Dense band of tissue ventrally called Hypochordal Bow
    • Triradiate rib attachment
    • Anterior Arch Atlas
  • Notochord – Apical Ligament & Nucleus Pulposis
  • Chondrification
    • Starts 6th embryonic week
    • At Cervico-thoracic junction
    • Four cartilaginous centres
      • Two in centrum fuse to one
      • Two for posterior arches
  • Primary Ossification
    • Starts 10th week
    • Three primary centres
      • Single centrum
        • Really starts as two
      • One per 1/2 arch
        • Also contribute to posterolateral body
    • Ossification occurs as in long bones
      • Cell hypertrophy
      • Matrix calcium
      • Bony cuff
      • Blood vessels penetration centre
      • Starts at thoracolumbar junction
    • Posterior arches fuse 2 years
    • Neurocentral junction 6 years

Physes

  • Ossification of centrum leaves 2 cartilage plates
  • At superior & inferior margins of vertebral body
  • Form physes
  • Produces ↑ in height of vertebral body
  • By typical enchondral ossification

Five Secondary Ossification Centres

  • Develop during in adolescence
    • Ring Apophysis x2
      • Line of ossification in end-plate epiphyses
      • Age 11-14 years
      • Extends to neurocentral synchondrosis
      • Doesn’t involve entire circumference of vertebral body (only centrum)
      • Doesn’t contribute to height of body
      • Fuse 14-15 years
    • Spinous process x1
    • Transverse processes x2

Vertebral Growth

  • Width
    • Centrum perichondrium
    • Membranous ossification
  • Height
    • Enchondral ossification at physis
  • Canal Growth
    • Occurs while 3 synchondroses are open
      • Via enchondral ossification
    • Canal is adult size at 6 years when neurocentral synchondrosis close
    • Posterior arch already fused at 2 years

Costal Processes

  • Thoracic – Rib
  • Cervical – Anterior tubercles of transverse process
  • Lumbar – Transverse processes
  • Sacral – Part of ala & all sacrum lateral to lateral crest

Transverse Processes

  • Contribute to different parts of vertebral body
  • Cervical – Posterior tubercles of transverse process
  • Thoracic – Transverse process
  • Lumbar – Mamillary & Accessory bodies
  • Sacral – Ala

Specialized Vertebrae

Atlas

  • Two Posterior Primary Ossification Centres
    • Expand posteriorly to form neural arch
    • Meet at Posterior Synchondrosis
    • Also expand anterolaterally
  • Single Anterior Secondary Ossification Centre
    • Forms ~ 1 year
    • Remnant of Hypochordal Bow
    • Expands towards posterior osseous centres
    • Forms 2 neurocentral synchondroses
    • Fuse by age 4
    • Canal at adult size at this stage

Axis

  • Five primary & Five secondary ossification centres
  • Dens develops two primary ossification centres
    • Usually join by age 3/12
    • Failure » Dens Bicornis
  • Dens separated from primary ossification centre of Centrum
    • By Dentocentral Synchondrosis
    • Failure to fuse » Os Odontoideum
    • Normally fuses ~ 6 years
  • Secondary ossification centre appears at dens tip
    • AKA Os Terminale
    • Appears at 3 years
    • Fuses at 12 years
  • 4 Other secondary ossification centres
    • Ring Apophysis
    • 2x Transverse Process
    • 1x Spinous Process