A trigonometric analysis of distal patellofemoral realignment.

Patellofemoral malalignment is often treated successfully with realignment procedures. Although distal patellar realignment has been used widely with excellent results, there is no published equation to predict degrees of change in the Q angle with millimeters of medialization. The purpose of this study was to quantify this relationship. In 20 fresh cadaver knees, threaded Kirschner (K) wires were placed in the anterior superior iliac spine, center of patella, and tibial tubercle. Q angles and measurements from the center of the patella to tibial tubercle were obtained. After using Fulkerson's technique of anteromedialization, Q angles were measured with 5 mm and 10 mm of medialization in a plane parallel to the floor and perpendicular to the threaded K-wires. The triangle formed by the center of the patella, tibial tubercle (initial), and tibial tubercle (medialized) was in one plane and could be studied by trigonometric analysis. A formula derived from the Law of Cosines to predict the change in the Q angle with medialization was found to be highly accurate. The authors present an equation that can be used to ensure adequate correction of the Q angle in distal patellar realignment.