The Spine 77paired atlanto-occipital joints. Although these are
condyloid joints, their parallel arrangement limits
their motion to primarily give rise to the “yes” move-
ment, or nodding movement of the head, allowing
about 10° to 20° of flexion and 25° of hyperextension
(Hay and Reid, 1982; Magee, 1997). Moderate lateral
flexion and very limited rotation are also allowed at
this joint (Levangie and Norkin, 2001).
The second cervical vertebra has a unique large,
peg-like projection, termed the dens (L. tooth) or
odontoid process, as seen in figure 3.7B. This process
projects superiorly from its body to form a pivot for
C1 (upon which the skull sits). Hence C2 is called
the axis, and the joint between C1 and C2 is called
the atlantoaxial joint (figure 3.7C). It is classified as
a pivot joint, allowing about 45° to 50° rotation and
giving rise to the “no” movement of the head (Hay
and Reid, 1982; Magee, 1997).
The extent of movement allowed at both of the
craniovertebral joints is much greater than allowed
at the other intervertebral articulations, permit-
ting freer movements of the head but leaving the
cervical spine vulnerable for injury. These motions
are normally constrained by various ligaments of
these craniovertebral joints. However, in trauma
or combat, the large ligament of the atlas, which
normally separates the dens from the spinal cord,
can be ruptured, allowing the dens to be driven into
the upper spinal cord or lower brainstem, often
resulting in paralysis or death (Moore and Agur,
1995).Joints Between
Thoracic Vertebrae and Ribs
In addition to the intervertebral discs and facet joints
between adjacent vertebrae, most thoracic vertebrae
join with a rib via two gliding joints termed the cos-
tovertebral joint and the costotransverse joint. The
costovertebral joint (L. costa, rib) is formed between
the head of a rib and flattened areas (called facets
or demi-facets) on the side of the body of the corre-
sponding vertebrae or two adjacent vertebrae and the
interposed disc. The costotransverse joint is formed
between another facet on the distal portion of the
transverse processes and a small projection (tuber-
cle) found on each of the first 10 ribs (figure 3.8).
These 10 upper ribs progress from their attachment
onto the spine laterally and then course anteriorly to
attach to the sternum either directly (“true ribs”) or
indirectly (“false ribs”) via a segment of cartilage (the
costal cartilages) as seen in figure 3.8A. The lower
two rib pairs are shorter—they do not join onto the
sternum—and because their lateral ends are free,
they are called “floating ribs.”
The 12 thoracic vertebrae, 12 ribs (and associated
costal cartilage), and sternum make up the thoracic
cage, or rib cage. This arrangement provides an
important protective “cage” for vital structures
such as the lungs and heart, as well as stability for
upright stance and movement, yet allows for small
but important motions of the ribs that accompany
breathing. During vigorous inhalation, the ribs are
elevated, while during exhalation the ribs return to
their normal position. This motion of the lower ribs
is referred to as the “bucket handle motion” of the
ribs (figure 3.8C), since it is like the motion in which
a bucket handle is slightly lifted and lowered. DueFIGURE 3.7 Specialized cervical vertebrae: (A) The
atlas and (B) the axis articulate to form (C) the atlanto-
axial joint.