Scaphoid and triquetral fractures
The scaphoid (os scaphoideum) fractures frequently, but some fractures are occult. If the patient has symptoms and the radiograph is clear, we must manage them as having a fracture: wrist immobilization is prescribed, and the radiograph is repeated in 10 days. During this time, bone resorption in the injury zone makes the fracture line visible. If a quick decision is needed, MRI is considered the optimal method (coronal view, STIR sequence) — it excellently visualizes the low signal line and bone marrow edema.
The main problem with scaphoid injuries is the high risk of avascular necrosis. The blood supply here is retrograde: arteries enter distally and run toward the proximal pole. The probability of union directly depends on the level of injury:
- Distal pole — 100%
- Distal waist — 90%
- Middle waist — 84%
- Proximal waist — 57%
- Proximal pole — 27% (maximum risk of nonunion).
The triquetrum (os triquetrum) is the second most frequently injured carpal bone (20% of wrist injuries). On the anteroposterior view, changes are not visible. Pathology should be sought on the lateral radiograph, where an avulsed bone fragment is visualized on the dorsal surface — a consequence of tension on the radiolunotriquetral ligament.
Carpal dislocations and dissociations
Dislocations are most commonly caused by a fall on an outstretched hand with forced hyperextension. The key view for assessing alignment is the lateral view. Normally, the radius, lunate (os lunatum), and capitate (os capitatum) bones align in a straight line.
Main displacement patterns:
- Lunate dislocation. On the anteroposterior view, the bone loses its normal trapezoidal shape and becomes triangular, and the carpal arcs are disrupted. On the lateral view, the bone is dislocated in the palmar (volar) direction, and alignment is disrupted. It is always accompanied by rupture of intercarpal ligaments.
- Perilunate dislocation. Occurs more frequently. The lunate maintains its axis with the radius, while the capitate and the entire distal carpal row are displaced dorsally. Often combined with a scaphoid fracture.
- Midcarpal dislocation. Represents a partial spontaneous reduction of a perilunate dislocation. The lunate is in a state of volar subluxation, while the rest of the carpus is dorsally subluxated.
- Scapholunate dissociation. Occurs when intercarpal ligaments rupture. On the radiograph (especially with a clenched fist), widening of the gap between the bones > 4 mm is detected. This sign is known as the Terry Thomas sign (named after the British comedian with a gap between his front teeth). The scaphoid itself appears shortened due to rotational flexion subluxation.
Metacarpal and phalangeal injuries
Fractures and dislocations in the carpometacarpal joint often require attention to the lateral view: it clearly shows axis deviation and dorsal displacement of the base of the metacarpal bone. Half of all such dislocations involve the 5th ray.
Typical hand fractures:
- Bennett fracture — an intra-articular fracture of the base of the 1st metacarpal bone. It is absolutely unstable due to the pull of the m. abductor pollicis longus tendon, which constantly displaces the bone fragment. It requires mandatory osteosynthesis.
- Punch fracture — a fracture of the 5th metacarpal head with angular displacement.
- Volar plate avulsion — a small volar fragment in the area of the proximal interphalangeal joints, accompanied by severe soft tissue edema.
- Mallet finger deformity — occurs when the extensor tendon is avulsed from the dorsal surface of the distal phalanx (intra-articular fracture). An important rule: if the deformity is clinically present but no fracture is visualized on the radiograph, this indicates an isolated rupture of the tendon itself.
The "Terry Thomas sign" helps remember scapholunate dissociation (a wide gap between bones on the radiograph, resembling the diastema between the front teeth of the famous comedian).
Class and exam questions
Why is a patient with a suspected scaphoid fracture managed as having a fracture even if the radiograph is clear?
Due to the high risk of developing avascular necrosis. Over 10 days of immobilization, bone resorption occurs in the occult fracture zone, and on the repeat radiograph, the defect becomes visible.
How does a perilunate dislocation differ from a lunate dislocation on a lateral radiograph?
In a perilunate dislocation, the lunate maintains alignment with the radius, while the rest of the carpus is displaced. In a lunate dislocation, alignment is lost, and the lunate itself is displaced in the palmar direction.
How to interpret a mallet finger-type deformity if the bones are intact on the radiograph?
With high probability, this indicates an isolated rupture of the extensor tendon. It is impossible to artificially mimic such a deformity.
What else to study on this topic
- Radiographic anatomy of the carpal arcs in the anteroposterior view
- Transscaphoid perilunate dislocation: mechanism of combined injury
- Biomechanics of midcarpal dislocation
- Condylar fractures of the phalanges and indications for their surgical treatment
- Features of CT reconstruction in injuries of the small bones of the hand
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