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

C>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

PhysesPhyses

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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 Second>Five Secondary Ossification Centres

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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 G>Vertebral Growth

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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 Proc>Costal Processes

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Thoracic – Rib

Cervical – Anterior tubercles of transverse process

Lumbar – Transverse processes

Sacral – Part of ala & all sacrum lateral to lateral crest

Transverse Processe>Transverse Processes

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Contribute to different parts of vertebral body

Cervical – Posterior tubercles of transverse process

Thoracic – Transverse process

Lumbar – Mamillary & Accessory bodies

Sacral – Ala

Specialized Vertebrae

>Specialized Vertebraeing">Atlas

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

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• 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