Embryology & Development of the

Craniovertebral Junction

Shashank Gandhi, MD

Craniovertebral Junction

Divided into 2 portions:● Central pivot

o Dens, C2 vertebral body, basiocciput

● 2 ringed structures surrounding central pivoto Foramen magnum ring

clivus, exocciput, occipital condyles, opisthiono Atlantal ring

Embryology

● Notochord forms early axial skeleton● During 4th week gestation, 42 somites

formedo 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5

sacral, 8-10 coccygeal● Each somite differentiates

o outer dermatomeo inner myotomeo medial sclerotome - form vertebrae around notocord

4 occipital sclerotomes● 4th Occipital Sclerotome - Proatlas - important for CVJ

development

Formation of CVJ

Sclerotomes in Formation of CVJ

**DO NOT RESEGMENT

Sclerotome Origins of CVJ

Posterior Fossa

● Expansion occurs by endochondral reabsorption, sutural growth, & boney accretion until 16-18 years of age

● Growth of basion elongated basiocciput & lowers frontal margins of foramen magnum

● Downward displacement of cerebellum & rotation of occipital and temporal lobes causes resorptive drift down and backward of opisthion

Development of C2

● Apical dens from proatlas● Dens from 1st cervical sclerotome● VB of C2 from 2nd cervical sclerotome● Odotoid separate from VB at birth by

cartilaginous bands - 2 synchondroses● 3 phases of synchondrosis & ossification

o 1st - 4mo fetus at VB/neural archeso 2nd - 6mo fetus basal dental segment

ossification of lower syndrondrosis at birtho 3rd - 3-5 yrs at apical dental segment

not completely fused to basal dens until 6-9 yrso Fully completed by age 16-18

Developmental Phases of C2

Developmental Abnormalities

● In general abnormalities result in:o Central pivot

instability however, basilar impression & retroflexed

odontoid can cause neural compression

o 2 rings (atlas & axis) deformity & crowding however, hypoplasia/aplasia can cause

weakening/loss of ligamentous structures - instability

Congenital Anomalies & Malformations

● Occipital boneo Manifestations of occipital vertebra

clivus segmentations remnants around foramen magnum atlas variants dens segmentations anomalies

o Basilar invaginationo Condylar hypoplasiao Assimilation of atlas

Congenital Anomalies & Malformations

● Atlaso Assimilation of atlaso Atlantoaxial fusiono Aplasia of atlas arches

● Axiso Irregular atlantoaxial segmentationo Dens dysplasias

Ossiculum terminale persistens Os odontoideum Hypoplasia-aplasia

o Segmentation failure of C2/3

Developmental & Acquired Abnormalities

● Foramen Magnumo Secondar basilar invagination (Paget’s dz,

osteomalacia, rheumatoid cranial settling, Rickets)o Foraminal stenosis (achondroplasia)

● Atlantoaxial Instabilityo Errors in metabolism (Morquio’s syndrome)o Down syndromeo Infection (Grisel’s syndrome)o Inflammatory (rheumatoid arthritis)o Traumatic OA & AA dislocation, os odontoideumo Tumors (NF)