Anatomy and clinical relevance of the ulnocarpal ligament.

The ulnocarpal ligament, or ulnocarpal ligamentous complex (UCLC),1 is anatomically divided into the ulnocapitate, ulnotriquetral, and ulnolunate ligaments, though, macroscopically, these three ligaments are often confluent and indistinguishable from each other. The ulnotriquetral, ulnocapitate, and ulnolunate ligaments originate together at the fovea of the ulnar head and the base of the ulnar styloid. These ligaments, along with the palmar radioulnar (RU) ligament, extend distally like a fan in the coronal plane and insert at the palmar aspects of the triquetrum, capitate, and lunate (Figs. 1, ​,2).2). The three ligaments of the UCLC merge firmly with the palmar RU ligament. The palmar RU ligament also merges with the dorsal RU ligament on the ulnar side, and they attach together on the ulnar fovea and the ulnar styloid process. This one compound ligamentous complex (UCLC and two RU ligaments), which extends in two perpendicular planes to attach the carpus (in the coronal plane) and the radius (in the transverse plane) to the ulna, is essential for coordination of the distal radioulnar joint (DRUJ) and the ulnocarpal joint.2 Fig. 1 Dissection of a human right wrist demonstrating the ulnocarpal ligament complex viewed from the dorsal side (a) and the palmar side (b). The ulnotriquetral, ulnocapitate, and ulnolunate ligaments originate together at the fovea of the ulnar head and ... Fig. 2 Anatomic relationship between ECU sheath floor, prestyloid recess, and triangular fibrocartilage complex (TFCC), which comprises ulnocarpal ligamentous complex (UCLC), disk, and palmar and dorsal radioulnar (RU) ligaments. The ulnocapitate ligament is the most superficial of the three ulnocarpal ligaments.3 It attaches not directly to the triangular fibrocartilage complex (TFCC) but to the palmar portion of the fovea region of the ulnar head, which is a slight depression at the base of the ulnar styloid process. At the fovea, the ulnocapitate ligament merges with the deep fibers of the palmar RU ligament.3 From the fovea, the ulnocapitate ligament passes distally, just anterior to the junction between the other ulnocarpal ligaments, and inserts on the ulnopalmar aspect of the capitate. The ulnotriquetral ligament originates mainly from the palmar RU ligament, and some fibers originate from the palmar radial aspect of the ulnar styloid process.3 It inserts at the proximal and ulnar surfaces of the triquetrum. The ulnolunate ligament originates proximally from the palmar RU ligament, thus attaching indirectly to the ulna. The ulnolunate ligament is continuous with the short radiolunate ligament and attaches to the palmar cortex of the lunate just as the short radiolunate ligament does. The extensor carpi ulnaris (ECU) sheath floor is a part of the infratendinous extensor retinaculum, which originates from the dorsal aspect of the triquetrum and inserted on the dorsal and ulnar aspects of the ulnar styloid (Fig. 2). The ECU sheath floor firmly blends with the dorsal RU ligament on the dorsal ulnar aspect of the ulnar fovea and the ulnar styloid. Therefore, the ECU sheath floor, the dorsal and palmar RU ligaments, the articular disk, and the UCLC together form a u shape when viewed from the ulnar side (Fig. 3a). Fig. 3 Ulnar view (a) and palmar views (b) of a right cadaver wrist. a The ECU sheath floor, the dorsal and palmar RU ligaments, the articular disc, and the UCLC together form a u shape. The ulnar head and the ECU are removed. b There is a “soft spot” ... There is a “soft spot” between the ulnotriquetral ligament and ECU sheath floor that is easy to identify by palpation (Fig. 3b). This portion is covered by a thin capsule that forms the anterior wall of the prestyloid recess, which is an area where the meniscus homologue does not cover the ulnar styloid process, thereby creating a pouch, variable in size and shape.4 This soft spot is used as a 6U portal in wrist arthroscopy. In the ulnopalmar view of a three-dimensional (3D) computed tomography (CT) arthrogram, the prestyloid recess appears as a small bulge that is anterodistal to the ulnar styloid and distinguishable from the palmar bulge of the DRUJ capsule (Fig. 4). The narrow area between the two bulges is the ulnar insertion of the UCLC and palmar RU ligament. It is worth noting that this portion is the common lesion of a foveal avulsion of the TFCC. In a foveal TFCC avulsion, especially in cases with a positive fovea sign (a localized tenderness on the palmar aspect of the fovea and ulnar styloid), this portion is often disrupted and filled with loose scar tissue. We have reported open repair of a foveal tear of the TFCC using a palmar approach.5 The advantage of this approach, compared with the dorsal approach, is that iatrogenic damage to the dorsal RU ligament and ECU sheath floor is avoided, while it provides satisfactory views of the disrupted foveal insertions, UCLC and palmar RU ligament. Fig. 4 The ulnopalmar view of a 3D CT arthrogram of a right wrist in which dye was injected in the radiocarpal and distal RU joints. The prestyloid recess is depicted as a small bulge anterodistal to the ulnar styloid and distinguishable from the palmar bulge ... In vivo 3D studies on the ulnocarpal ligaments have shown that carpal movements that occur during wrist hyper–radial extension or hyperextension result in substantial strain to the ulnocapitate and ulnotriquetral ligaments and on the foveal and palmar styloid insertions of the TFCC.6 Considering that the ulnotriquetral ligament has another origin on the palmar aspect of the ulnar styloid, which is eccentric from the center of forearm rotation (fovea), it is easily seen that the change in length of this fascicle would be increased by forearm supination. Strain of this fascicle during forearm supination may thus affect the tension of the foveal insertion. Based on these findings, one major injury mechanism of a TFCC foveal avulsion can be excessive traction of the UCLC, especially the ulnocapitate ligament, caused by wrist hyper–radial extension or hyperextension with forearm supination (Fig. 5). This theory agrees with the fact that more than half of the patients with traumatic TFCC foveal avulsion were caused by a fall on the outstretched hand and had a positive fovea sign.5 Fig. 5 A simulated TFCC foveal avulsion by excessive traction of the UCLC, especially the ulnocapitate ligament, caused by wrist hyper–radial extension with the forearm supination. A right cadaver wrist is viewed from the distal and palmar side. In cases with nonreparable TFCC disruption, TFCC reconstruction is considered. We made use of the UCLC to reconstruct the TFCC because a remnant of UCLC is often retained even in cases with severe ulnar disruption of TFCC (Fig. 6). In our preceding cadaver study, our palmar reconstruction restored stability despite only the palmar portion of the TFCC being reconstructed.7 In our technique, we focused on reconstructing the palmar deep RU ligament rather than the dorsal deep RU ligament, because the most common form of instability is dorsal displacement of the distal ulna with respect to the radius.8 Stuart et al reported in a cadaveric study that the palmar RU ligament provided the greatest restraint to dorsal ulnar translocation.9 Moreover, one previous cadaver study demonstrated that volar dislocation of the ulna relative to the radius did not occur even when the RU ligaments were totally excised, as long as the interossous membrane was intact.10 Fig. 6 Illustrations of palmar reconstruction of TFCC that used a UCLC remnant, which is viewed from the palmar and ulnar side. The palmaris longus tendon graft was passed through slits made in the UCLC. The graft was then sutured to the UCLC and the palmar ... The UCLC an important role in stabilizing the ulnocarpal joint. The UCLC firmly merges with the palmar RU ligament, which provides the greatest restraint to dorsal ulnar translocation. Knowing the anatomy and function of the UCLC is worthwhile for understanding the pathomechanism of foveal TFCC avulsion and the palmar surgical approach for TFCC repair or reconstruction.