The ulnocarpal ligaments serve an important role in stabilizing the ulnar carpus and helping prevent supination and volar translocation of the carpus on the distal radius and ulna [17]. Stuart et al. found in their biomechanical study testing the relative contributions to stability of the DRUJ that the ulnocarpal ligament complex was not a significant contributor [3]. Although not always included as a part of the TFCC, the ulnocapitate ligament originates from the palmar aspect of the fovea, where it blends with the palmar radioulnar ligament and then runs superficial to the other ulnocarpal ligaments before inserting onto the capitate [18]. The ulnotriquetral ligament arises from the palmar aspect of the fovea and the meniscus homologue and the palmar radioulnar ligament before inserting on the triquetrum. The pisotriquetral orifice may be seen in the distal aspect of this ligament during arthroscopy. The ulnolunate ligament originates from the palmar aspect of the fovea and the articular disc, with a few fibers from the radius, and is intertwined with the palmar radioulnar ligament before inserting distally on the lunate (Fig. 4.4) [14, 19].

Moritomo et al. found that the length of the ulnotriquetral and ulnocapitate ligaments increased the greatest amount with wrist radial extension, whereas the ulnolunate increased the most with wrist extension, while the palmar radioulnar ligament had minimal change in length with any motion. This stress imparted through the ligaments might explain how a fall on an outstretched hand could lead to a foveal tear of the TFCC from excessive traction imparted through the ulnocarpal ligaments. Morimoto goes on to say, however, that pure wrist hyper-radial extension or hyperextension would not be adequate by itself to cause these injuries and that additional forces would be required to initiate a tear [19].

The meniscus homologue is a C-shaped vascular tissue located between the superficial portion of the radioulnar ligaments, the capsule, and the triquetrum in the ulnar aspect of the radiocarpal joint. Some debate has occurred over whether this is a true component of the TFCC, as it differs histologically by being composed of loose connective tissue instead of dense collagen and also by being lined by synovial cells on its inner surface [18]. Ishii et al. found in their study that the meniscus homologue, which they classified into three types as related to the prestyloid recess, was not identifiable as the separate structure that was previously described by Bowers and Taleisnik [14, 20, 21].

The prestyloid recess is a pouch often found anterior to the ulnar styloid where the meniscus homologue does not cover the ulnar styloid, and should not be confused with a tear of the TFCC [18]. It can be found at the apex of radioulnar ligaments and is typically lined with synovial villi (Fig. 4.5) [22]. Ishii described three different variations in the prestyloid recess: narrow, wide, and no-opening types [14]. The narrow opening type was found in 74 % of specimens and consisted of the meniscus homologue being attached to the ulnar styloid proximal to its radial, dorsal, and palmar aspects and distal to the ulnar styloid tip circumferential surface [14]. The prestyloid recess was found to communicate with the ulnocarpal space by a long narrow tunnel from the palmar aspect of the styloid [14]. The wide opening type was found in 11 % and consisted of the attachments similar to the narrow opening, but without attachment to the tip of the styloid [14]. A short, wide opening was found between the prestyloid recess and the ulnocarpal space [14]. The no-opening type was found in 15 % of patients, and the prestyloid recess was found to come from the radial aspect of the styloid and to communicate with the distal radioulnar joint instead of the ulnocarpal space [14]. The ligamentum subcruentum, as it was originally described, was not found in this type, but was identified in the narrow and wide opening variations [14].

The dorsal and palmar radioulnar ligaments are intracapsular intrinsic stabilizers that measure 4–5 mm in thickness and impart the majority of stability to the DRUJ in pronation and supination as translational motion occurs [1–3, 12]. The histologic makeup of these ligaments demonstrates longitudinally-oriented parallel fibers, which suggest a tissue that experiences tensile loads, a premise which supports their role as stabilizing structures [23]. The palmar radioulnar ligament forms the proximal attachment of the ulnocarpal ligaments, while the dorsal radioulnar ligmanent splits ulnarly to form the subretinacular sheath of the extensor carpi ulnaris [15]. These ligaments have both a superficial and deep portion with the insertions on the ulnar styloid and fovea, respectively (Fig. 4.6) [1, 24]. At their broad radial attachment at the distal rim of the sigmoid notch, the radioulnar ligaments are conjoined, but then split prior to reaching the ulnar styloid or fovea into a superficial and a deep portion (Fig. 4.7) [2, 14]. The insertion of the deep portion, commonly called the ligamentum subcruentum, is more lateral and proximal into the fovea compared to the more central and distal insertion of the superficial ligament. The palmar and dorsal portions of the deep ligament converge and intertwine as they insert into the fovea (Fig. 4.8) [15]. The original description of the ligamentum subcruentum was of a vascularized space separating the proximal deep and distal superficial portions of the TFCC, but this term has changed over time to describe the insertion of the deep component of the ligaments [1, 24]. Ishii, in a cadaveric study, found that the deep portion inserts on the fovea, but that there was not a clearly distinct insertion site of the superficial ligament on the styloid [14].

The insertion of the deep component of the ligament provides a better mechanical advantage for controlling rotation as its angle of insertion is less acute than that of the superficial ligament [1]. To demonstrate this concept, Kleinman uses the analogy of a team of horses, a buckboard, and a driver, in which the radius is the team of horses, the ulna is the buckboard, and the different angles of the reins going to the driver represent the deep and superficial ligaments [1]. This analogy shows how the less-acute angle of the reins, which represents the deep ligament, is more advantageous in controlling the rotation of the horses, which represents the radius [1]. The radioulnar ligaments have a spiral configuration as they insert onto the ulnar head, and this rotational insertion, along with the differing locations of insertion of the deep and superficial ligaments, allows continuous shifts in tension and compression and is the core of DRUJ stability [7].

The superficial radioulnar ligaments also provide stability to the DRUJ, but their action is the opposite of the deep ligaments, with the dorsal fibers giving stability in pronation and the palmar fibers providing stability in supination. While the superficial fibers do provide stability, they have a more acute angle of insertion and thus less mechanical advantage for rotational control. The second reason the superficial ligaments play a lesser role is that in maximum pronation or supination, the majority of the ulna has escaped out from under the superficial fibers, rendering them ineffective in these extremes of motion (Fig. 4.9a, b). In this position, the deep portion of the radioulnar ligament provides stability by providing a tethering action, thus preventing dislocation of the DRUJ [1, 2].