Upper Cervical Spine Biomechanics
Rotation
Nearly 50% of the total amount of axial rotation that is attributed to the cervical spine takes place at the occipitoatlantoaxial complex. Axial rotation is limited to approximately
+47 degrees from the mid frontal position to the extreme lateral positions. Most of the rotational component occurs at the C1/C2 motion segment, with a small but important component occurring at the C0/C1 segment.
Click here to display a table that summarizes these findings.
C0/C1 rotation is constrained by: 1) the ligamentous attachments of the occiput to the atlas and the axis; 2) the characteristic shape of both the occipital condyles and the superior articular facets of the atlas. Rotation of the occiput results in stretching of the lateral atlanto-occipital ligament as it wraps around the odontoid. Tension developed in this ligament causes the occipital condyles to slide approximately 2 to 3 mm to the left and to the right with rotation of the occiput to the right and to the left respectively.
Rotation of the occiput is always associated with coupled sidebending to the opposite side (
Type I Mechanics). Loss of these movements may result in conditions that are clinically significant.
Click here to view the occiput as it rotates. Notice the small amount of left sidebending that accompanies right rotation, and the small amount of right sidebending that accompanies left rotation.
C1/C2 rotation is constrained at the extreme positions by a combination of the alar ligament, the posterior atlantoaxial membrane, and the atlantoaxial joint capsules. The motion that occurs as C1 rotates upon C2 provides a classic example of a type of
coupled motion that is called helical motion. Helical motion refers to motion in which rotation of an object about one axis is consistently associated with simultaneous translation along the same axis. For example, when you twist the cap on a jar (the primary motion), the threads on the jar cause the cap to either tighten or loosen. Likewise, when you turn a screw or a bolt (the primary motion), it moves in and out (the secondary motion) of a threaded hole. To see this illustrated with the lid of a jar,
click here. The inferior facets of the atlas are covered by articular cartilage that is convex anteroposteriorly and from side to side. Because of the oblique orientation of the lateral
facets, together with the slight convexity of the
condylar surfaces on C2, rotation of the head is accompanied by a slight vertical ascent/descent (4 - 6 mm) of the head. As the atlas rotates from a neutral position to the right on the axis, the right facet of the atlas slides downward on the posterior aspect of the right facet of the axis, and the left inferior facet of the atlas slides downward on the anterior aspect of the left facet of the axis. In this posterior view (
click here) the vertical motion (the secondary motion) is best seen by focusing upon the anterior aspect of C1. Notice that as C1 rotates around the vertical (superior/inferior) Y axis, it also translates along the Y axis. The atlantoaxial joint is the
only vertebral motion segment in which rotation is
not coupled with sidebending.
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