Page 358 - Kinesiology of the musculoskeletal system foundations for physical rehabilitation

334 Section III Axial Skeleton Atlanto-axial Joint Complex
The concave-convex structure of the atlanto-occipital The atlanto-axial joint complex has two articular compo-
nents: a median joint and a pair of laterally positioned apoph-
joints permits angular rotation in two degrees of freedom. The yseal joints. The median joint is formed by the dens of the
primary motions are flexion and extension. Lateral flexion is axis (C2) projecting through an osseous-ligamentous ring
slight. Axial rotation is severely restricted and not considered created by the anterior arch of the atlas and the transverse
as the third degree of freedom. ligament (Figure 9-43). Because the dens serves as a vertical
axis for horizontal plane rotation of the atlas, the atlanto-axial
BOX 9-1. Organization of the Joint Anatomy and joint is often described as a pivot joint.
Regional Kinematics at the Craniocervical Region
The median joint within the atlanto-axial joint complex has
ANATOMY OF THE JOINTS WITHIN THE CRANIOCERVICAL REGION two synovial cavities. The smaller, anterior cavity is formed
••• between the anterior side of the dens and the posterior border
Atlanto-occipital joint to C7) of the anterior arch of the atlas (see Figure 9-43). A small
Atlanto-axial joint complex anterior facet on the anterior side of the dens marks this
Intracervical articulations (C2 articulation (see Figure 9-20, A). The much larger posterior
cavity separates the posterior side of the dens and a cartilage-
SAGITTAL PLANE KINEMATICS lined section of the transverse ligament of the atlas. This strong,
•• 2-cm long ligament is essential to the horizontal plane stabil-
• Osteokinematics of flexion and extension ity of the atlanto-axial articulation.33 Without its restraint, the
A••• rAIAtnhtttrllroaaannkcittenoore--vaomixcciaacatlilpicajisrottaiiolncfutjoflclaieontximtoionpnsle(axCnd2 extension atlas (and articulated cranium) can slip anteriorly relative to
to C7) the axis, possibly damaging the spinal cord.68,172
Osteokinematics of protraction and retraction
The two apophyseal joints of the atlanto-axial joint are
HORIZONTAL PLANE KINEMATICS formed by the articulation of the inferior articular facets of
•• the atlas with the superior facets of the axis (see exposed right
Osteokinematics of axial rotation joint in Figure 9-41). The surfaces of these apophyseal joints
C7) are generally flat and oriented close to the horizontal plane,
A•• rIAtnhttrlroaankcitenore-vamixciaaatllicajsrotiioncfut alcaxotiiamolnprsolet(xaCti2onto a design that maximizes the freedom of axial rotation.

FRONTAL PLANE KINEMATICS The atlanto-axial joint complex allows two degrees of
•• freedom. About 50% of the total horizontal plane rotation
Osteokinematics of lateral flexion within the craniocervical region occurs at the atlanto-axial
C7) joint complex. The second degree of freedom at this joint
A•• rIAtnhttrlroaankcitenore-vomiccaactlipicaisrttaiolcfujollaaintteitoranlsfl(eCx2iotno complex is flexion-extension. Lateral flexion is very limited
and not considered a third degree of freedom.
SPINAL COUPLING BETWEEN LATERAL FLEXION
AND AXIAL ROTATION Tectorial Membrane and the Alar Ligaments
A review of the anatomy of the atlanto-axial joint complex
must include a brief description of the tectorial membrane
and the alar ligaments, connective tissues that help connect

Posterior atlanto-occipital Posterior view Occipital condyle
membrane (cut) Occipital bone

Foramen
magnum

Atlanto-occipital Superior articular
joint capsule (cut) facet

Transverse Atlas Tectorial
process Ligamentum flavum membrane
Transverse
Atlanto-axial Axis foramen
(apophyseal)
joint capsule

Spinous process
FIGURE 9-40. A posterior view of exposed atlanto-occipital joints. The cranium is rotated forward to expose
the articular surfaces of the joints. Note the tectorial membrane as it crosses between the atlas and the cranium.