Changes Due to Trauma in the Fat Plane Overlying the Pronator Quadratus Muscle: A Radiologic Sign

The radiologist usually has no difficulty identifying fractures of the lower ends of the radius and ulna. There are instances, however, in which there is no displacement of fragments and subtle fractures may be overlooked. It has been observed that a fat plane just anterior to the lower ends of the radius and ulna consistently undergoes alterations from trauma, and sometimes it is easier to appreciate the changes in this fat plane than in the bones themselves. This report concerns identification of the fat plane and a review of the author's experiences in children and adults. The Pronator Plane Radiologic Sign Figure 1, A illustrates a normal lateral projection of the wrist of a twelve-year-old boy, with arrows outlining a linear fat plane. This fat has a gentle convex curve just anterior to the lower one-sixth of the radius and ulna and is seen in true lateral projections of the wrist in almost all normal subjects from premature infants to persons of advanced age. The right and left sides are usually symmetrical, and there is no relationship to handedness. Figures 1, B and C show a transverse fracture of the lower end of the radius caused by a fall on the outstretched hand the day before. In comparison with the normal lateral view (Fig. 1, A), the fat plane is displaced anteriorly and is more bowed. Furthermore, the fat is less well defined, with blurring and irregularity. It is postulated that the fracture has led to bleeding into and swelling of the pronator quadratus muscle. These cause it to expand and displace the overlying fat. Also, edema and∕or hemorrhage in the fat plane itself have increased its radiographic density, giving it a blurred and irregular appearance on the film. No operative findings or postmortem studies of recent injuries were available to test the observation, but the limb of a cadaver was dissected and radiologic and clinical observations suggest that the postulate is true. Anatomical Description The forearm of a cadaver was obtained for study and it is illustrated before dissection in Figure 2, A. What appears to be the fat plane (arrow) is actually fat, plus a small amount of air which has tracked down from the elbow region where the forearm was disarticulated. Dissection revealed the plane of the fat to lie just over the pronator quadratus muscle. Another fat plane just anterior to this was found to be between the tendons of the flexor digitorum profundus muscle and the tendons and muscle masses of the flexor digitorum sublimis muscle. This thin fat plane is seen in Figure 1, A beneath the heads of the arrows. A photograph was then made with the flexor digitorum profundus elevated and rolled off the surface of the pronator quadratus (Fig. 2, B). The pronator quadratus muscle is attached to the lower one-sixth of the radius and ulna.