2. Joints

C1/C2 articulations: The **atlanto-axial articulation**is a complex of three synovial joints, which join the atlas (C1) to the axis (C2).

Articulations

* **paired lateral atlanto-axial joints:** classified as planar-type joint between the lateral masses of C1 (inferior articular facet) and lateral mass of C2 (superior articular facet). It is somewhat more complex in shape with concavity of the superior axial facets
	+ Inferior articular facet of C1: Concave mediolaterally, projects medially in coronal plane
	+ Superior articular facet of C2: Convex surface, projects laterally
* **median atlanto-axial (atlanto-dental) joint:** pivot joint with articulations of odontoid process/dens of C2 to the anterior arch of C1 and transverse ligament of C1
	+ Synovial cavities between transverse ligament/odontoid & atlas/odontoid articulations
* fibrous capsules of the lateral and medial joints are thin and provide only minimal support, therefore ligamentous attachments provide the majority of support

* Atlantooccipital joints
	+ Inferior articular facet of occipital condyle: Oval, convex surface, projects laterally
	+ Superior articular facet of C1: Oval, concave anteroposteriorly, projects medially

Capsule

The atlantoaxial articular capsules are thick and loose, and connect the margins of the lateral masses of the atlas with those of the posterior articular surfaces of the axis.

Each is strengthened at its posterior and medial part by an accessory ligament, which is attached below to the body of the axis near the base of the odontoid process, and above to the lateral mass of the atlas near the transverse ligament.

Attachments

* **Anterior atlantoaxial ligament (continuation of ALL):** thick fibrous band attaches from inferior border of anterior arch of C1 to the anterior axial body
* **Posterior atlantoaxial ligament (continuation of ligamenta flava):** attaches from lower border of posterior atlantal arch (and more superiorly to the occipital bone) and upper borders of lamina of C2
* **Cruciform/cruciate ligaments:** superior, transverse and inferior bands posterior to dens
	+ Transverse ligament of atlas is strongest portion which stabilises odontoid to the lateral masses of C1 limiting lateral motion of C1 over C2 and limiting subluxation of the joint
	+ Craniocaudal components (superior and inferior) bands run from transverse ligament to the foramen magnum and C2.
* **Tectorial membrane:**
	+ Continuation of posterior longitudinal ligament; attaches to anterior rim foramen magnum (posterior clivus)
* accessory atlanto-axial ligament: from posterior body of C2 to lateral mass of C1

A number of ligaments which attach the axis to the occipital bone including the tectorial membrane, the paired alar ligaments, median apical ligament and the longitudinal parts of the cruciform ligaments

Function

The atlanto-axial joint allows 10-15o of flexion/extension and 30o of axial rotation.

Variant anatomy

there are a number of congenital anomalies which can affect the atlas, axis and the joint between the two:

* aplasias, hypoplasias, and clefts of the atlas arches
* split atlas

**Normal articulations:**

Intervertebral Discs

* **Overview**
	+ Lie between thin horizontal hyaline/fibrocartilage end-plates on superior, inferior surfaces of vertebrae
	+ With ALL/PLL, link vertebrae from C2 → sacrum
	+ Comprise 1/3 of spinal column height
		- Thickness varies (thinnest in upper T, thickest in lower L)
		- Lumbar discs 7-10 mm thick, 4 cm diameter
	+ Components
		- Central nucleus pulposus
		- Peripheral annulus fibrosus
	+ Major function is mechanical
		- Transmit, distribute load from weight/activity
		- Allow flexion/extension, lateral bending, torsion
		- Discs loaded preferentially in flexion
* **Annulus fibrosus**
	+ Concentric series of 15-25 fibrous lamellae
		- Surround, constrain nucleus pulposus
		- Collagen fibers lie parallel within each lamina
		- Fibers oriented 60° to vertical
		- Type I collagen predominates in outer annulus
		- Type II predominates in inner annulus
	+ Inner annulus blends gradually with nucleus
	+ Outer annulus attaches to ALL, PLL and to fused epiphyseal ring of vertebral bodies by Sharpey fibers
	+ Innervation: Branch of ventral primary ramus
	+ Vasculature: Outer annulus supplied by capillaries from spinal branches of dorsal rami
* **Nucleus pulposus**
	+ Origin: Remnant of notochord
	+ Eccentric position within annulus
		- More dorsal compared to center of vertebral body
	+ Components
		- 85-95% water
		- Loose fibrous strands of collagen, elastin with gelatinous matrix
		- Scattered chondrocytes
		- Major macromolecular component = proteoglycans
		- Proteoglycans = protein core + attached glycosaminoglycan chains
		- Glycosaminoglycan chains have negatively charged sulphate, carboxyl groups
		- Cations attract anions → high osmotic pressure enables disc to absorb water
	+ Except for outer annulus, disc relies on nutrient diffusion from endplate vessels
		- Steep metabolic gradient between vessels, disc centrum
		- Centrum has ↓ glucose + oxygen, ↑ lactic acid
		- Carbohydrate utilization dominated by glycolysis

Facet Joints

* **Articular processes** (zygapophyses)
	+ Paired posterior lateral joints
		- Superior facet surface directed dorsally
		- Inferior facet surface directed ventrally
		- Facets joined by pars interarticularis
	+ True synovial joint
		- Hyaline cartilage surfaces, synovial membrane, fibrous capsule
	+ Orientation
		- Obliquely sagittal in lumbar spine (protects disc from axial rotation)
		- Coronal in cervical and thoracic spine (protects against shear)
	+ Innervation: Nociceptive fibers from medial branch of dorsal ramus
	+ Function: Load bearing in extension, rotation
* **Pars interarticularis**
	+ Lies between subatlantal superior/inferior articular facets
	+ C2 unique
		- Anterior relation of superior to posterior placed inferior facet
		- C2 pars interarticularis unusually elongated