Avulsion

Avulsion injuries occur when abnormal stress on certain structures, such as ligaments and tendons, leads to disruption at their sites of attachment. Many bones may be subject to this type of injury, including the calcaneus (Fig.3), patella and ulnar olecranon. In children, an entire apophysis may sometimes be torn away. Avulsion injuries about the hip and pelvis in children and adults occur at characteristic locations, including those of the anterior superior iliac spine (Fig.4), anterior inferior iliac spine, the lesser trochanter (Fig.5), the ischial tuberosity, greater trochanter, apophysis of the iliac crest and symphysis pubis. In some cases the fragment may later become incorporated into the parent bone and lead to skeletal deformity (Fig.6).

Avulsion of the posterior ring apophysis of the lumbar vertebrae may occur after trauma in children and adolescents. Disc herniation may be associated with this lesion, but whether either of these entities causes the other is unclear. The lesions are difficult to visualize radiographically, but findings on lateral films include narrowing of the intervertebral disc space, irregularity of the posterior vertebral corner, and an irregular wedge-shaped bone fragment in the spinal canal. The latter fragment may appear as a crescentic structure on CT scans, but the fragment cannot be seen as well on MR images owing to difficulty in distinguishing it from the posterior longitudinal ligament, which is of similar low signal intensity.

DR/RB