Embryology Genetics homeobox (Hox) genes direct and regulate processes of embryonic differentiation and segmentation along the craniocaudal axis Osteology The cervical spine contains 7 vertebral bodies C1 (atlas) C2 (axis) C1 to C7 have a transverse foramen vertebral artery travels through the transverse foramen of C1 to C6 C2 to C6 have bifid spinous processes C7 despite having a transverse foramen, the vertebral artery does NOT travel through it in the majority of individuals there is no C8 vertebral body, but there is a C8 nerve root Alignment Normal sagittal lordosis (measured from C2 to C7) Spinal Canal Spinal canal normal diameter is 17 mm <13 mm indicates possible cord compression Atlas (C1) Has no vertebral body and no spinous process Embryology three ossification centers one for each lateral mass (come together posteriorly to form posterior neural arch) lateral masses fuse to the body at age 7 one for the anterior arch anterior arch does not appear until 1 year of age Articulations occiput-C1 two superior concave facets that articulate with the occipital condyles makes up 50% of neck flexion and extension Axis (C2) Osteology has an odontoid process (dens) and body embryology develops from five ossification centers subdental (basilar) synchondrosis is an initial cartilaginous junction between the dens and vertebral body that does not fuse until ~6 years of age the secondary ossification center appears at ~3 years of age and fuses to the dens at ~12 years of age Kinematics C1-C2 (atlantoaxial) articulation is a diarthrodial joint that provides 50° (of 100°) of cervical rotation 10° (of 110°) of flexion/extension 0° (of 68°) of lateral bend C2-C3 joint participates is subaxial (C2-C7) cervical motion that provides 50° of cervical spine flexion/extension 50° of cervical spine rotation 60° of lateral bending C2 blood supply a vascular watershed exists between the apex and the base of the odontoid apex is supplied by branches of the internal carotid artery base is supplied by branches of the vertebral artery the limited blood supply in this watershed area is thought to affect healing of type II odontoid fractures Occipital-C1-C2 Ligamentous Complex Provided by the odontoid process and its supporting ligaments transverse ligament limits anterior translation of the atlas apical ligaments limit rotation of the upper cervical spine alar ligaments limit rotation of the upper cervical spine Subaxial Cervical Spine (C3 to C7) C1 to C7 have a transverse foramen vertebral artery travels through transverse foramen of C1 to C6 C2 to C6 have bifid spinous processes C6 contains palpable carotid tubercle, which is a valuable landmark for the anterior approach to the cervical spine C7 non-bifid spinous process despite having a transverse foramen, the vertebral artery does NOT travel through it in the majority of individuals there is no C8 vertebral body, but there is a C8 nerve root The superior articular facets of the subaxial cervical spine (C3-C7) are oriented in a posteromedial direction at C3 and posterolateral direction at C7, with variable transition between these levels