Anatomy, Shoulder and Upper Limb, Distal Radio-Ulnar Joint

Structure and Function

Osteology

The distal ulna bears the ulnar styloid process and head, separated by a depression called the "fovea." The disc-like ulnar head articulates with the radial bone's sigmoid (ulnar) notch. The styloid process is an attachment site for the radioulnar ligaments (RULs) (see Image. Forearm Bone Markings).[2] The fovea, the sulcus for the extensor carpi ulnaris (ECU) tendon, the styloid process, and the cartilage surface define the shape of the distal ulna.

An anatomical study demonstrated that the length of the sigmoid notch increased from volar to dorsal. Ulnar length varied significantly and showed a negative correlation with the midcoronal length of the sigmoid notch. The study also identified a correlation between DRUJ obliquity and ulnar variance, a measure of the relative length disparity between the radius and ulna.[3] Patients with typical negative ulnar variance, where the ulna is shorter than the radius, had a sigmoid notch angled in a proximal-ulnar direction. This angle decreased and, in some cases, reversed as ulnar variance became more positive or when the ulna extended beyond the radius.

Although substantial variability existed between specimens, each displayed consistent bilateral anatomy. This finding supports comparing extremities within the same individual but warrants caution when comparing different individuals.[4]

Distal Radioulnar Joint Anatomy and Function

The DRUJ is an incongruent synovial, diarthrodial articulation between the distal radius and ulna, crucial for forearm pronation and supination. The DRUJ's normal motion range is 150° to 180°, with an additional 30° through the radiocarpal joint. DRUJ motion is primarily rotational but includes axial and translational components.

Typically, 80% of the force at the wrist is transmitted through the radius, with the remainder transmitted through the ulna. This amount varies with ulnar and radial deviation. Extreme pronation and supination may result in minimal articular contact, affecting less than 10% of the joint surface area.

Approximately 20% of the DRUJ's stability comes from its bony architecture, with the remainder provided by the surrounding soft tissues. The restraints to the joint's movement come from the TFCC, joint capsule, IOM, and musculotendinous units like the pronator quadratus and ECU.[5]

Triangular Fibrocartilage Complex

The TFCC is the DRUJ's main passive stabilizer (see Image. Triangular Fibrocartilage Complex).[6] This ligamentous complex is comprised of the articular disc, volar and dorsal RULs, the ulnocarpal ligaments, the ulnar collateral ligament (UCL), the ECU tendon sheath, the capsule, and the meniscus homologue.[7] 

The TFCC has superficial and deep components. The superficial portion connects with the UCL. The deep segment, known as the triangular ligament or ligamentum subcruentum, attaches to the ulnar fovea via volar and dorsal limbs, forming the volar and dorsal distal RULs. Previous anatomical and biomechanical research suggests that the TFCC's deep component contributes more significantly to DRUJ stability than its superficial counterpart.[8] TFCC disruption can produce ulnar variance alterations, evident on an axial-loaded wrist radiograph.[9]

The TFCC primarily extends the articular surface from the distal radius to the ulnar head, transmitting axial force across the ulnocarpal joint. This complex fibrous structure serves as a robust yet flexible connection between the distal radius and ulna, supporting the ulnar carpal region.

Articular disc

The triangular-shaped articular disc (triangular fibrocartilage proper or TFC proper) is a peripherally thickened fibrocartilaginous structure arising from the cartilage, covering the distal radius and inserting onto the fovea. Fibrous tissues connect the articular disc to the ECU tendon subsheath. The disc is surrounded by the palmar and dorsal radioulnar ligaments.[10] The articular disc is divided into the proximal and distal laminae at its ulnar side by the triangular ligament.[11] The TFC proper functions as a static stabilizer of the DRUJ.[12]

Radioulnar ligaments

The volar and dorsal RULs are proximal condensations of the TFCC that contribute the most to DRUJ stabilization. Superficial RULs connect with the ulnar styloid process, while deep RULs attach to the fovea's articular surface. The deep dorsal and superficial volar fibers resist the ulna's dorsal subluxation during maximum supination, while the deep volar and superficial dorsal fibers become taut during pronation, providing stability to the DRUJ.[13] The RULs support the DRUJ under dynamic loading, while the articular disc separates the DRUJ from the wrist joint cavity.[14]

The RULs significantly contribute to DRUJ stabilization. Evidence suggests that the dorsal RUL tightens while the volar RUL relaxes during pronation. The reverse happens during supination.[15] Several studies also found that the dorsal RUL is crucial for dorsal translational stability during pronation, while the volar RUL is important for volar translational stability during supination. Ward et al's biomechanical study showed that RUL disruption increased the rotational range of motion, underscoring these ligaments' importance in DRUJ rotational stability.[16]

Ulnocarpal ligaments

The ulnocarpal ligament complex consists of the ulnocapitate, ulnolunate, and ulnotriquetral ligaments. These fibrous structures are macroscopically confluent and indistinguishable without joint fluid or contrast material. The volar fibers of the ulnolunate and ulnotriquetral ligaments arise from the volar radioulnar ligament at the fovea and ulnar styloid base and extend distally to insert at the volar lunate and triquetrum. These ligaments are part of the deep extrinsic wrist ligaments contributing to both DRUJ and ulnocarpal stability.

Ulnar Collateral Ligament

The existence of the UCL is highly debated. However, Turker et al's 2019 study using high-resolution ultrasound, dissection, and histologic examination confirmed the anatomic location of this ligament. The UCL arises from the tip of the ulnar styloid process and inserts onto the ulnar aspect of the triquetrum. The UCL occupies the floor of the 6th extensor compartment. This ligament provides static stability to the DRUJ and wrist joint.[17]

Meniscus homologue

The meniscus homologue (ulnocarpal meniscus or ulnomeniscal homologue) is composed of synovial tissue and has 4 compartments: styloid, distal, collateral, and radioulnar, with the styloid being the main component. The meniscus homologue reinforces the medial-sided joint capsule along with the ECU subsheath and UCL, serving as the wrist's "functional ulnar collateral ligament." The meniscus homologue acts as a shock absorber rather than a direct stabilizer.

Joint capsule

The radioulnar joint cavity resembles an "L" shape, enclosed by a fibrous band forming the radioulnar joint capsule. This capsule has a sack-shaped recess to accommodate twisting during pronation. The IOM, joint capsule, and articular disc have recognized roles in stabilizing the DRUJ, particularly during pronation and supination.[18] However, variability exists in how these structures contribute to stability, and their correlation with gross joint instability has not been consistently demonstrated across studies.[19] While biomechanical sectioning sequences include these structures, their specific contributions under different conditions remain a topic of investigation.[20]

Interosseous Membrane

The forearm IOM is the fibrous connective tissue sheet bridging the radius and ulna (see Image. Radioulnar Joints). As mentioned, this membrane stabilizes the forearm ring. This fibrous structure also supports the DRUJ during supination and enhances the forearm's longitudinal stability. The forearm IOM is divided into the central, proximal, and distal bundles.[21] The radial bone's axial stability during proximal radial head fractures depends on both the IOM's central band and the TFCC. DRUJ fracture instability can manifest in various directions: anterior, posterior, or intermediate directional instability.[22]

Embryology

Although extensive embryological research focuses on the TFCC, studies examining other wrist ligaments are scarce. Findings indicate that ligamentous structures begin to develop in O’Rahilly stage 23 embryos and are fully formed by week 14 of gestation. The appearance of the interzone within the condensed mesenchyme marks the initiation of wrist joint formation. The interzone comprises 3 layers, with the intermediate layer considered crucial for wrist ligament formation.[23]

Blood Supply and Lymphatics

The DRUJ's arterial supply primarily derives from the palmar and dorsal branches of the anterior interosseous artery. These arteries branch at the proximal border of the pronator quadratus and form an anastomotic complex around the joint. Other arteries contributing to this network include the posterior (dorsal) interosseous artery connecting to the anterior interosseous artery's dorsal branch, ulnar artery joining the anterior interosseous artery's palmar branch, and collateral vessels from dorsal and palmar carpal arches.[24][25][26] 

The lymphatic pathways of the upper extremity have been poorly described, but various routes have been observed, including perforators accessing the wrist compartment. These pathways typically drain from distal to proximal and show little consistency in flow direction, whether from deep to superficial or superficial to deep. Primary lymph nodes in the upper extremity are typically found in the cubital and axillary regions.[27]

Nerves

The DRUJ receives innervation from the anterior interosseous nerve, a median nerve branch, and the posterior interosseous nerve, a radial nerve branch.[28][29][30] These nerves form articular branches that supply the region distal to the proximal border of the pronator quadratus.[31]

Muscles

Muscles around the wrist, hand, and fingers further stabilize the DRUJ. The pronator quadratus is particularly important, as biomechanical analysis shows that this muscle tightens the DRUJ capsule to prevent excessive pouching within the joint during forearm rotation. Both the deep and superficial heads of the pronator quadratus originate from the dorsoulnar aspect of the ulna and attach to the palmar side of the radius. The muscle's orientation prevents capsule interposition and joint diastasis.[32]

Physiologic Variants

As mentioned, ulnar length exhibits greater physiological variability than radial length, leading to significant variability in sigmoid notch shape. This variation should be taken into account when evaluating patients with forearm fractures or wrist pain.

Surgical Considerations

TFCC tears may be treated using either an arthroscopic or an open surgical approach. One repair technique uses the palmaris longus tendon.[33] Surgery is indicated for patients with isolated or associated injuries producing significant DRUJ instability.  

DRUJ surgery is contraindicated in patients with other evident pathology, including radial and ulnar length discrepancies, DRUJ arthritis, sigmoid notch incompetence, ECU tendon subluxation, and conditions that cause wrist pain, such as ulnar impaction syndrome. TFCC and DRUJ reconstruction often do not restore stability to the joint. Patients must thus be screened carefully and informed that these procedures are primarily adjunctive for pain management. Various surgical reconstruction procedures exist in the literature, including RUL reconstruction, extrinsic radioulnar tether placement, and radiocarpal sling or tenodesis procedure. These interventions have inherent limitations but have been found to help control symptoms.[34]

Wrist arthroscopy is part of the diagnostic process for pain on the ulnar side of the wrist, which is frequently misdiagnosed as a TFCC injury. Other recommended imaging tools include radiography and magnetic resonance imaging (MRI).[35]

Distal radial fractures can lead to DRUJ instability and TFCC tears. These conditions may be treated with an intramedullary nail.[36] Fractures of the distal radius or radial head may present with acute DRUJ dislocations, ulnocarpal abutment, ECU tendonitis, and ulnotriquetral ligament tears. Distal radial fracture malunion can cause DRUJ instability. Distal radial fractures may be repaired using a fixed-angle volar locking plate.[37]

Ulnar styloid process fractures can complicate distal radial fractures. However, Zyluk et al's study showed that an unrepaired ulnar styloid fracture associated with a distal radial fracture did not affect the outcomes after augmented Kirschner wire fixation.[38]

Clinical Significance

Distal Radioulnar Joint Instability

Acute traumatic injury and chronic degeneration may give rise to DRUJ instability. However, these conditions have distinct treatment considerations. Chronic tears more commonly affect older patients, whereas acute ones typically result from upper extremity trauma, which can occur at any age. Traumatic distal radial and ulnar styloid fractures often contribute to acute DRUJ instability. The clinical significance of these injuries is contentious, with functional outcomes likely more influenced by repairing the radial fracture rather than the ligamentous or ulnar styloid fracture. DRUJ instability evaluation should include a comprehensive history and physical examination alongside the appropriate diagnostic tests.[39]

Subluxation-related ulnar neuropathy syndrome is a possible complication of chronic DRUJ instability. The ulnar nerve may be compressed within the Guyon canal, potentially producing numbness and tingling of the 4th and 5th digits and intrinsic hand muscle weakness in more advanced cases. Evaluation of ulnar neuropathy symptoms should include stressing the DRUJ to verify the condition's etiology.[40] 

Diagnosis of distal radioulnar joint instability and triangular fibrocartilage complex tears

DRUJ instability and TFCC tears are often diagnosed clinically. Computed tomography (CT) and MRI may be performed in supination and pronation to help diagnose these conditions. DRUJ instability should be suspected if the distance between the radius and ulna is more than 6 mm on CT. Radiographs are useful for identifying possibly associated lesions, such as ulnar styloid pseudoarthrosis, osteoarthritis, distal radial malunion, ulnocarpal impaction, and ulnocarpal abutment.

Ulnar pain is usually seen in DRUJ instability. A click is appreciated in more severe cases due to a positional dislocation or relocation of the ulnar head.[41] Pain with pronation and supination against resistance may indicate TFCC damage.

Physical maneuvers 

Physical maneuvers that can help identify DRUJ instability and TFCC tears include the following, which must be performed on both the affected and unaffected limbs:

• Piano key sign: The examiner dorsally positions the head of the ulna (as if beginning to play the piano) while the forearm is pronated, then applies a volar force on the ulnar head. The test is positive if increased translation is observed.
• Press test: The patient rises from a seated position by pressing their hands on the chair's armrests. The presence of ulnar pain indicates a TFCC tear.
• Grind test: The examiner stabilizes the patient's distal forearm while holding the wrist with the other hand, supinating the limb in ulnar deviation. This action compresses the carpus against the TFCC, eliciting a response.[42]
• Fovea sign: The examiner compresses the interval between the styloid process and the ECU tendon. Tenderness suggests a DRUJ lesion.
• DRUJ ballottement test: The examiner stabilizes the radiocarpal joint with one hand while moving the ulna anteroposteriorly with the other hand. Pain or increased displacement indicates DRUJ instability. The test is conducted in both supination and pronation.

Potential complications and other key diagnostic considerations

DRUJ instability may result in weakness, pain, and degenerative arthritis, often from untreated distal forearm injuries. Tears isolated to the DRUJ, caused by falls onto outstretched hands, may be misdiagnosed as wrist sprains. Inflammatory arthritis can also contribute to DRUJ instability. TFCC disruption may result in ulnocarpal abutment, characterized by significant pain localized to the ulnar head impacted within the ulnar carpus.