Internal Derangement of the Wrist

Central Disk or Triangular Fibrocartilage

An understanding of the anatomy of the cartilaginous central disk is important so as to not mistake normal anatomy for pathology. The TFCC has five components composed of ligamentous and fibrocartilaginous connective tissues that include (1) the central disk (TFC), (2) dorsal and volar radioulnar ligaments, (3) the ulnolunate and (4) ulnotriquetral ligaments, and (5) the ulnomeniscal homologue (ulnar collateral ligament complex) ( Fig. 15-1 ).

The central disk, also known as the triangular fibrocartilage (TFC), is the portion of the complex best seen at imaging and has received the most attention in the literature. The fibrocartilaginous disk separates the distal radial ulnar joint from the proximal radial carpal joint and is a relatively avascular structure. On MRI, this band of normal intermediate-signal hyaline cartilage should not be mistaken for a tear of the radial aspect of the TFC ( Fig. 15-2 ). Along the ulnar aspect of the disk, it blends with the dorsal and volar radioulnar ligaments. These ligaments span from the ulna to the radial cortex, securing the disk.

The thickness of the TFC is inversely proportional to the relative length of the ulna with respect to the radius. Ulnar variance is the term used to describe the relationship between the distal ulna and the distal radius. When the distal ulnar head projects beyond the distal radius, that is called ulnar-positive variance, or simply ulnar plus. Conversely, when the ulna is shorter than the radius, it is called ulnar-negative variance, or ulnar minus ( Fig. 15-3 ).

Dorsal and Volar Radioulnar Ligaments

The radioulnar ligaments blend inseparably with the central disk along its dorsal and volar surfaces and are composed of lamellar collagen. Unlike the central disk, which attaches to hyaline cartilage on the radius, the radioulnar ligaments originate from the distal radial cortex. Much of the radial strength of the TFCC comes from these secure attachments. They span from the dorsal and volar radial cortex along the margin of the sigmoid notch to the ulna. The ulnar attachment of these ligaments is known as the peripheral attachment. These ligaments are important stabilizers of the distal radioulnar joint (DRUJ). Cadaveric studies show that there are both superficial (distal) and deep (proximal) fibers that constitute the radioulnar ligaments. The deep fibers of the radioulnar ligaments (also known as proximal lamina of the triangular ligament) attach to the ulna at the base of the ulnar styloid within the fovea. The superficial fibers (also known as the distal lamina of the triangular ligament) attach to the styloid process. Between the two bands is a highly vascular loose connective tissue called ligamentum subcruentum, which appears of high signal intensity on gradient- and T2-weighted images and is not a true ligament. The intermediate or high signal in between the deep and the superficial bands can interfere with the evaluation of peripheral TFCC tears ( Fig. 15-4 ).

Ulnocarpal Ligaments

Two ulnocarpal ligaments, the ulnotriquetral ligament and the ulnolunate ligament, contribute to the TFCC. The ulnotriquetral ligament has both dorsal and palmar components, whereas the ulnolunate ligament has just a palmar component. They both are extrinsic ligaments of the wrist. Routine MRI poorly demonstrates the ulnocarpal ligaments. However, they are nearly always visible on high-resolution, thin-section 3D gradient imaging, best seen in the sagittal plane ( Fig. 15-5 ). Functionally, these two ligaments are important stabilizers of dynamic wrist motion and, if disrupted, may contribute to instability and wrist pain.

Ulnomeniscal Homologue (Ulnar Collateral Ligament Complex)

Descriptions of the ulnarmost components of the TFCC have previously been varied. * Much of the disagreement about the ulnar collateral ligament complex likely comes from the high degree of variability found in this portion of the wrist. Many names have been used to describe these structures by various authors: meniscal homologue, ulnar carpal ligament, ulnar collateral ligament, ulnocarpal ligament, ulnar capsule, and ulnocarpal ligament complex. † In an attempt to unify and simplify these various descriptions, we primarily discuss the ulnomeniscal homologue (UMH) and briefly mention the extensor carpi ulnaris tendon sheath and ulnar collateral ligament.

Normal Anatomy

* References .

† References .

The UMH is not a fibrocartilaginous structure like the meniscus in the knee or the TFC but rather is a synovioid membrane that can take on a meniscoid shape. The UMH has a heterogenous appearance on MRI because histologically it is composed of dense, irregular connective tissue and densely packed collagen fibers.

The UMH can be divided into three contiguous components as it runs proximal to distal: (1) a main styloid component at its proximal attachment, (2) a collateral component at the level of the triquetrum, and (3) its distal insertion, which is somewhat varied and has insertional components on the distal triquetrum, hamate, and base of the fifth metacarpal ( Fig. 15-6 ). A fourth supportive radioulnar component that runs with the dorsal radioulnar ligament is also briefly discussed.

Biomechanics of the Triangular Fibrocartilage Complex

The TFCC serves many functions: it acts as a cushion between the ulna and carpus during axial loading of the wrist; it is the major stabilizer of the DRUJ, preventing dorsal and volar subluxation of this joint ; and it provides ulnocarpal joint stability. The majority of the compressive force that is transferred across the wrist joint passes from the carpus to the radius. A minority of the axial loading forces from the hand are transferred across the TFCC from the carpus to the ulnar head. In the setting of neutral ulnar variance, the radius bears 82% of the force and the ulna bears 18%. Force distribution changes greatly with variance; for example, with 2.5 mm of ulnar-positive variance, the force transferred to the ulna increases to 42%. However, this does not appear to be the only factor causing ulnar impaction syndrome. Other variables that increase ulnar variance include pronation and a clenched fist and likely account for cases of ulnar impaction seen in the setting of ulnar-neutral variance.

Pathology

Based on anatomic and biomechanical considerations and available literature, Palmer devised a classification of TFC lesions in 1989. The Palmer classification splits the TFCC pathology into two categories: degenerative lesions and traumatic tears. Differentiation of traumatic versus degenerative lesions can be difficult and is largely based on history, presentation, and age of the patient. Traumatic tears are further subdivided by location, whereas degenerative perforations are subdivided by severity ( eTable 15-1 ).

The ability of MRI to detect lesions within the TFCC depends both on the techniques used and on the location of the lesion. Lesions involving the central disk are well demonstrated on MRI. On the other hand, lesions involving the peripheral TFCC are difficult to demonstrate without intraarticular contrast medium. Furthermore, little literature has addressed tears outside of the central disk of the TFCC. Tears of the radioulnar ligaments have been seen on MRI ( eFig. 15-1 ).

Abnormalities of the ulnar collateral ligament complex can be a cause of ulnar-side wrist pain. Ulnar detachment of the UMH has been described at arthroscopy and has been seen on MRI ( eFig. 15-2 ) but has not been described in the MR literature to our knowledge.

Tears of the ulnocarpal ligaments are not described in the MR literature. There is only mention of ulnocarpal tears in the arthrography literature.

Manifestations of the Disease

The Palmer Classification

Class 1: Traumatic Lesions of the TFCC

Traumatic lesions of the TFCC ( Fig. 15-7 ) are less common than degenerative lesions. The mechanism of injury usually involves falling on an outstretched hand, forearm rotational injury, or an axial-loading or distraction-type injury. Traumatic lesions can occur in both the normal and the degenerated TFCC.

Class 1A Lesions

These lesions (see Fig. 15-7A ) affect the central disk. Most commonly, they are located close to the radial attachment of the TFC, within 2 to 3 mm. Tears are usually linear and slitlike, rather than the large circular lesion seen with degenerative perforations, and appear as linear fluid signal on MRI. The tears also tend to be oriented in a sagittal plane running from the anterior to the posterior aspect of the disk such that they can be seen best on coronal imaging as high T2 signal, or high signal on T1 in the case of an MR arthrogram. Tears here tend to be small, on the order of 1 to 2 mm long, requiring small slice thickness, small field of view, and high matrix imaging to be demonstrated well ( Fig. 15-8 ). Despite the small size of the tears, noncontrast MR imaging and MR arthrography have shown good sensitivities and specificities, as illustrated by a recent meta-analysis.

Because the centralmost aspect of the disk is avascular and unlikely to heal, tears of the radial aspect of the central disk are usually treated by arthroscopic débridement. Although these lesions occur close to the articular cartilage within the sigmoid notch of the radius, it is important to differentiate them from the radial attachment of the TFCC into the hyaline cartilage (see Fig. 15-2 ).

Class 1B Lesions

These lesions (see Fig. 15-7B ) are avulsions of the TFCC from the ulna. These tears are also referred to as peripheral tears. Recalling the anatomy of the TFCC, multiple components of the TFCC may be torn during an avulsion from the ulna, which can lead to instability of the DRUJ. The deep fibers of the dorsal and volar radioulnar ligament attach to the ulnar fovea. Superficial fibers of the radioulnar ligament blend with the ulnar collateral ligament complex and attach to the ulnar styloid. Class 1B lesions can result in fractures of the ulnar styloid ( Fig. 15-9 ).

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