Assessment of bone structures and avascular necrosis
Diagnostics of fracture non-union and bone fragment viability requires a comprehensive approach. A classic example is the evaluation of scaphoid pseudarthrosis (scaphoid fracture non-union). Posteroanterior radiographs can reveal the non-union line and sclerosis of the proximal pole, which indirectly indicates avascular necrosis (avascular necrosis, AVN).
MRI provides a more detailed picture:
- On coronal T1-weighted images, the bone marrow cavity demonstrates low signal intensity, confirming the pattern of necrosis.
- The fat-suppressed T2 sequence allows for the visualization of bone marrow edema.
- The definitive examination is fat-suppressed T1 following contrast administration. If marked contrast enhancement is observed, this indicates the preservation of blood supply to the scaphoid bone, even if non-contrast series suggested necrosis.
A similar pathology is Kienböck's disease (Kienböck’s Disease), which represents necrosis of the lunate bone. Radiologically, the bone is reduced in size, sclerotic, and shows signs of early fragmentation. On MRI (T1), low signal intensity is visible due to osteosclerosis, while bone marrow edema may be absent on T2FS.
Types of instability and wrist dislocations
Wrist instability is frequently associated with bone malalignment, alteration of the scapholunate angle, and disruption of Gilula's arcs.
- Dorsal intercalated segmental instability (DISI): Associated with scapholunate ligament tear and scaphoid fractures. On lateral radiographs, the angle exceeds 60 degrees, and dorsal tilt of the lunate bone is noted. On the anteroposterior projection, scapholunate diastasis is visible.
- Volar intercalated segmental instability (VISI): Occurs with lunotriquetral ligament tears. The angle is less than 30 degrees, the lunate bone is rotated volarly, and a "step-off" between the lunate and triquetrum is visible on the anteroposterior radiograph.
Dislocations require special attention. In lunate dislocation, it shifts in the palmar direction, completely losing contact with the radius and the remaining carpal bones (acquiring a triangular shape on the anteroposterior view). In perilunate dislocation, the lunate remains articulated with the radius, while the other bones are displaced (most commonly dorsally). The lateral view is optimal for differentiating between them.
Injuries of ligaments and the fibrocartilage complex
MR arthrography and digital subtraction arthrography are actively used to detect ligamentous pathologies. Normally, contrast agent fills only the radiocarpal joint. Contrast leakage into the midcarpal joint indicates ligamentous tears, for example, through the lunotriquetral articulation.
A tear of the scapholunate ligament on T1 arthrograms manifests as its absence and diastasis. This can lead to secondary osteoarthritis with cartilage loss and subchondral cyst formation.
The triangular fibrocartilage complex (TFCC) normally has low MRI signal intensity. Its injuries are classified into:
- Degenerative: lead to central perforation. Frequently associated with a positive ulnar variance and ulnar impaction syndrome (impingement between the ulna and triquetrum).
- Traumatic: avulsions from the ulnar styloid process (contrast leaks between them), frequently causing distal radioulnar joint instability.
The ulnar collateral ligament of the thumb (“skier's thumb” injury) is also important. Stener lesion is of particular note—a tear with displacement, where the ligament turns over the aponeurosis of the adductor pollicis muscle. Such a condition does not heal on its own and requires surgery, while the displaced ligament is clearly visible on MRI and ultrasound.
Pathologies of tendons and nerves
Wrist tendons are divided into flexors and extensors. Extensors form 6 compartments, numbered from the radius to the ulna. De Quervain's tenosynovitis (affecting the abductor pollicis longus and extensor pollicis brevis) frequently develops in the first compartment. Tendinopathy of the extensor carpi ulnaris is frequently localized at the styloid process and can cause its erosion in inflammatory arthropathies.
Flexor tendons pass through the carpal tunnel (except for the flexor carpi radialis and flexor carpi ulnaris). Flexor tenosynovitis can increase pressure within the canal, causing carpal tunnel syndrome (CTS), a median neuropathy.
On MRI and ultrasound, signs of CTS include flattening of the nerve within the canal and its proximal thickening. If the cross-sectional area of the median nerve at the tunnel inlet exceeds 10 mm², it raises suspicion for CTS, and greater than 12 mm² is a highly prognostic sign.
Briefly: answers to topical questions
Which specific tendons pass through the carpal tunnel along with the median nerve?
The tendons of three flexor muscles pass through the carpal tunnel along with the median nerve (n. medianus).
- The flexor digitorum superficialis (m. flexor digitorum superficialis)—tendons for digits II–V occupy the medial part of the canal.
- The flexor digitorum profundus (m. flexor digitorum profundus)—tendons are located deeper and more medially.
- The flexor pollicis longus (m. flexor pollicis longus)—the tendon is located laterally to the flexor tendons of digits II–V.
For differential diagnosis: in perilunate dislocation, the lunate is attached to the radius, whereas in isolated lunate dislocation, it completely loses contact with all adjacent bones.
Class and exam questions
How does MRI help determine scaphoid viability in non-union?
The key examination is the fat-suppressed T1-weighted sequence following contrast administration. If the bone actively enhances with contrast, its blood supply is preserved.
What are the main radiographic signs of VISI?
These include volar rotation of the lunate on the lateral radiograph, a decrease in the scapholunate angle to less than 30 degrees, and the presence of a clear "step-off" between the distal articular surfaces of the lunate and triquetrum on the anteroposterior radiograph.
Why does a Stener lesion require surgical intervention?
In a Stener lesion, the torn ligament retracts and wraps around the adductor aponeurosis. Due to this anatomical barrier, spontaneous healing of the ligament becomes impossible.
What else to study on this topic
- Functional radiography and videofluoroscopy in occult instability
- Differential diagnosis of dissociative and non-dissociative wrist instability
- Anatomical structure of the six compartments of wrist extensor tendons
- Pathogenesis and imaging findings of ulnar impaction syndrome
- Specifics of performing MR arthrography in TFCC injuries
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