Dalley, 10th Edition

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114

(A) Anterior views

BA D

Body of axis Dens of axis

Fractures

(D) Coronal CT

C3 C4

(B) Open mouth radiograph (compare with Fig. 2.8E)

(E) Median CT

FIGURE B2.19. Fractures of dens of axis (C2).

results in avascular necrosis (G., death). Almost as common are fractures of the vertebral body inferior to the base of the dens (Fig. B2.19B–E). This type of fracture heals more readily because the fragments retain their blood supply. Other frac tures of the dens result from abnormal ossi cation patterns. Rupture of Transverse Ligament of Atlas When the transverse ligament of the atlas ruptures, the dens of the axis is set free, resulting in atlanto axial subluxation—incomplete dislocation of the median atlantoaxial joint (Fig. B2.20A). Pathologi cal softening of the transverse and adjacent ligaments, usu ally resulting from disorders of connective tissue, may also cause atlantoaxial subluxation (Bogduk & Macintosh, 1984); 20% of people with Down syndrome exhibit laxity or agen esis of this ligament. Dislocation owing to transverse liga ment rupture or agenesis is more likely to cause spinal cord compression than that resulting from fracture of the dens (Fig. B2.20B). In this fracture, the dens fragment is held in

place against the anterior arch of the atlas by the transverse ligament, and the dens and atlas move as a unit. In the absence of a competent ligament, the upper cervi cal region of the spinal cord may be compressed between the approximated posterior arch of the atlas and the dens (Fig. B2.20A), causing paralysis of all four limbs (quadri plegia), or into the medulla of the brainstem, resulting in death. Steel’s Rule of Thirds (Steel, 1968): Approximately one third of the atlas ring is occupied by the dens, one third by the spinal cord, and the remaining third by the uid- lled space (see Fig. 2.49, CSF in subarachnoid space) and tissues surrounding the cord (Fig. B2.20C, D). This explains why some people with anterior displacement of the atlas may be relatively asymptomatic until a large degree of movement (greater than one third of the diameter of the atlas ring) oc curs. Sometimes in ammation in the craniovertebral area may produce softening of the ligaments of the craniover tebral joints and cause dislocation of the atlantoaxial joints. Sudden movement of a patient from a bed to a chair, for example, may produce posterior displacement of the dens of the axis and injury to the spinal cord.

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