Growth arrest lines and recurrent patellar dislocation: a new sign.

The phenomenon of growth arrest lines has been widely described in the medical literature. They are usually found at the metaphysis of growing long bones and are the result of short periods of partial growth arrest. Recurrent dislocation of the patella is a well-recognised problem, particularly in adolescents. Several radiological features have been reported in association with patellar dislocation or instability. We have reported a hitherto undescribed radiological sign of patellar growth arrest lines on the skyline radiographs of two patients with this condition. The shape of the patella when symptoms were at their worst corresponded remarkably closely to the outline of the subsequent growth arrest line. We postulated that repeated dislocations adversely affect the process of normal maturation of the patella. With the resolution of symptoms, patella ossification resumes, leaving the telltale sign of previous injury in the form of a growth arrest line and an improvement in bone density once the patella has been stabilised and tracks normally.