In the forearm, reference to normal anatomic relationships begins with the radius and ulna, proximal to distal. The distal radioulnar joint is a critical articulation in forearm pronation and supination. This joint has a separate synovial compartment and bordered distally by the triangular fibrocartilage (May, 2002). Generally, the radial styloid process extends beyond the ulnar styloid process by approximately 9 to 12 mm. At the proximal articular surface of the lunate, however, the two bones are of approximately of the same level. An ulna of less length provides for negative ulnar variance, and a longer ulna is described by a positive ulnar variance. The normal arrangement of the distal ulna and radius provides for the radial angle (also known as the ulnar slant or ulnar inclination), which is usually measured at 15 to 25 degrees. The distal surface of the radius also demonstrates an orientation toward the palmar or volar surface of approximately 10 to 25 degrees (Greenspan, 2000; May, 2002). These anatomic relationships are critical in imaging assessment for the orthopedist, both in assessing for pathology and in planning the appropriate course of intervention (Figures 7–1 and 7–2).
A normal-appearing PA radiograph of the hand and wrist.
A normal-appearing lateral view radiograph of the hand and wrist.
At the articular surface of the ulna is the triangular fibrocartilage complex (TFCC), which functionally extends the distal ulna to be approximately the same length of the radius. The TFCC consists of the triangular fibrocartilage, dorsal and volar radioulnar ligaments, sheath of the extensor carpi ulnaris tendon, ulnocarpal ligaments, ulnar collateral ligament, and ulnomeniscal homologue. Of significance is the triangular fibrocartilage's tendency to be thicker peripherally and it may attenuate centrally to have a small opening. In addition to contributing to stability of the wrist, the TFCC also provides for a cushion between the proximal carpals and distal radius. The proximal row of carpals, forming an arc, consists of the scaphoid, lunate, and triquetrum, connected by their interosseous ligaments. The pisiform is also included in the proximal row, but as a sesamoid bone is more loosely connected than the others and is located in the flexor carpi ulnaris tendon anterior to the triquetrum. Another anatomic alignment consideration is the palmar orientation of the scaphoid of approximately 45 degrees. This proximal row functions as a linkage between the distal radius and the distal row of carpals. The distal row of the trapezium, trapezoid, capitate, and hamate also forms an arc in articulating with the bases of the metacarpals. The lunate and capitate form a central carpal column, which is functionally important for force transmission. The concave volar surface of the carpals, covered by the wrist joint capsule, forms the dorsal boundary of the carpal ...