Imaging description

The capitellum is a rounded protuberance that projects off the distal end of the humerus both medial and anterior to the longitudinal axis of the humerus. The width of the articular surface of the capitellum is not uniform. The width tapers as the capitellum curves inferiorly and posteriorly, with the width of the posterior aspect of the capitellum being only half as wide as the anterior aspect. Therefore, when one looks at the capitellum from below, it appears roughly twice as wide anteriorly as posteriorly (Figure 19.1).

Additionally, the lateral edge of the smooth articular surface of the capitellum demonstrates a sharp contrast to the adjacent rough non-articular surface of the lateral epicondyle of the humerus. This articular margin overhangs this adjacent rough bone. This overhanging edge with smooth bone and articular cartilage with the rough lateral epicondylar bone being subjacent gives the false appearance of a defect or notch on MR images (Figure 19.2). This pseudodefect is even more pronounced by the typical positioning of the elbow in extension on MR imaging. The radial head is positioned slightly lateral and posterior to the central articular surface of the capitellum when the elbow is extended. That is, in extension, the anteromedial aspect of the radial head appears to articulate with the smooth surface capitellum; however, the posterolateral aspect of the radial head projects by the rough non-articular surface of the distal humerus. This tends to accentuate the pseudodefect on the capitellum (Figure 19.3).