Anatomy, Shoulder and Upper Limb, Forearm Triangular Fibrocartilage Complex

Structure and Function

This complex (TFCC) includes the triangular fibrocartilage disc proper (TFC), a meniscus homolog (MH), radioulnar ligaments (RUL), the ulnar collateral ligament (UCL) and the sheath of the extensor carpi ulnaris (ECU).[1]

Triangular Fibrocartilage Disc Proper

Being its main component, the horizontal triangular fibrocartilage disc proper is a disc-shaped fibrocartilaginous structure that extends from the sigmoid notch of the radius to the ulnar styloid. It is a single band in its radial attachment, which separates into a proximal and distal lamina. It is called the triangular ligament. The ligamentum subcruentum is a fibrovascular structure that separates these two laminae. While the proximal lamina attaches to the styloid fovea, the distal lamina connects to the ulna styloid at its distal portion.[2]

The triangular fibrocartilage disc proper has a biconcave shape, thinner in its center, and while it does not show fenestrations in the early decades of life, these will appear later.[3]

Radioulnar ligaments

Two thick ligamentous structures border the triangular fibrocartilage disc proper in its dorsal and volar aspect. These are the dorsal and volar radioulnar ligaments (RUL).[1] While they attach directly to the radius bone, the triangular fibrocartilage disc proper attaches into hyaline cartilage.[4] This hyaline attachment is weaker than the opposite bony attachment to the ulna.

Reinforcement of the radioulnar ligaments is by fibers attaching to the lunate; the ulnolunate ligament, triquetrum, ulnotriquetral ligament, and the hamate and base of the fifth metacarpal as the ulnar collateral ligament. The ulnar collateral ligament is superficial to the meniscal homolog and of limited importance.[2]

Meniscal Homologue

As opposed to the triangular fibrocartilage disc proper, it is primally constituted of fibrous tissue. It spans from the tip and lateral side of the ulnar styloid to the ulnar aspect of the triquetrum and base of the five metacarpal.[5] It becomes almost indistinguishable from the lateral ulnar collateral ligament and dorsal extensor carpi ulnaris tendon. It creates a smooth arc that fills the distal ulnocarpal gap creating a small synovial space between itself and the triangular fibrocartilage disc proper, the pre-styloid recess.[2]

Extensor Carpi Ulnaris Tendon

The extensor carpi ulnaris tendon sheath runs in an ulnar groove and appears to be an essential stabilizer of the ulnar side of the TFCC.[3] There are suggestions that the TFCC is a vital pulley component to the extensor carpi ulnaris and that TFCC injury can lead to abnormal loading.[6]

Function

TFCC serves three crucial roles. Most importantly, it stabilizes the distal radius ulnar joint. It also cushions the ulnar carpus, being responsible for 20% of axial wrist load. And it acts as a stabilizer of the ulnar carpus.[1]

While there is consensus that the dorsal and volar radioulnar ligaments primarily maintain distal radius ulnar joint stability, there is conflicting evidence as to which provides stability during pronation and supination.[7][8][9] During pronation and supination, there is the movement of the radius around the ulna. While the radial portion of the TFCC maintains is rigid, the ulnar TFCC and ligaments (extensor carpi ulnaris and ulnar collateral ligament) show dynamic changes that contribute to maintaining distal radius ulnar joint stability.[3]

Embryology

TFCC is closely related to wrist and carpal development. A thick mesenchymal organization at the TFCC site begins to form at week 8. This organization is complete by week 14.[10]

Blood Supply and Lymphatics

Three branches supply the TFCC. The ulnar artery itself via small dorsal and volar radiocarpal branches and the palmar and dorsal branches of the anterior interosseous artery, a branch of the ulnar artery.[11]

Blood supply originates from the surrounding joint tissues and variably penetrates the TFCC. Although there are different degrees of vascular penetration observed, the inner portion of the triangular fibrocartilage disc proper is considered avascular. The blood supply is limited to its peripheral zone and resembles the knee meniscus vasculature pattern.[12]

Physiologic Variants

Central fenestrations in the triangular fibrocartilage disc proper may be present from the third decade of life and, unless bidirectional, are to be considered physiological. Bidirectional defects lead to communication between distal radioulnar and the radiocarpal compartment.[3]

Ulnar variance affects TFCC thickness. Ulnar variance is the relative length of the distal articular surfaces of the radius and the ulna. It can be neutral (same length), positive (ulna longer over 2 mm), or negative (ulna shorter).[13] Positive variance is inversely associated with TFCC thickness. It also correlates with Kienbock disease and ulna-lunate impaction syndrome.[14]

Surgical Considerations

While non-operative management is to be considered initially in stable injuries, surgical intervention should be a consideration in cases of instability and persistent pain of the distal radial-ulnar joint. Reports indicate positive outcomes in most patients following surgical management.[15]

Clinical Significance

The TFCC undergoes progressive degenerative changes from the third decade of life. These changes more frequently occur in the ulnar portion of the triangular fibrocartilage disc proper. These are due to reduced cellularity, loss of elastic fibers, and erosion, which culminate in degeneration and fenestrations of the central part of the disc.[16]

TFCC injuries can occur in three areas. In the horizontal section, it is at the periphery or its attachments, which can result from trauma or degenerative changes. Trauma is usually associated with a fall on a pronated outstretched forearm, axial load, or distracting injury to the ulnar part of the forearm. Repeated axial loading seems to be associated with degenerative injuries. A classification has been described by Palmer et all. It distinguishes between traumatic (Class 1) and degenerative injuries (Class 2).[1] However, injuries to the TFCC are often complex and involve a pattern not described in the Palmer classification.

Clinically the ‘ulnar fovea sign’ is a useful bedside test for disruption of the DRL and ulnotriquetral ligament injuries. With a sensitivity of 95.2% and specificity of 86.5% in a study of 272 patients.[17]

Imaging of TFCC is best performed with MRI in a neutral forearm position with axial, coronal, and sagittal views. MRI arthrogram appears to be a superior method to identify TFCC injuries.[18] However, the additional steps to perform an arthrogram do not seem to justify an only slightly superior imaging technique.[19]

Although distal radius fractures are not predictive of TFCC injury, an ulnar styloid fracture is predictive of a TFCC injury.[20][21]

Given the lack of blood supply, lesions to its central portion are unlikely to heal. On the other hand, injuries to its vascularized peripheral area may be amenable to surgical repair, as there is a higher chance of healing.[12]