Assessment of axes and carpal bone dislocations
The basis of wrist radioanatomy on a lateral view is checking for correct bone alignment. Normally, the radius (radius), lunate (lunate), and capitate (capitate) should form a strict straight line. Disruption of this geometry is a primary indicator of severe trauma.
Lunate Dislocation In this pathology, the lunate is displaced, which drastically alters the radiographic picture:
- On the anteroposterior (PA) view, it loses its normal shape and acquires a triangular configuration. The smooth arc that normally runs along the distal contour of the scaphoid, lunate, and triquetrum is interrupted.
- On the lateral view, alignment disruption is confirmed: palmar (volar) dislocation of the lunate relative to the axes of the capitate and radius is observed.
Trans-Scapho Perilunate Dislocation This is a more complex combined injury. It is accompanied by a fracture of the scaphoid (scaphoid). On the lateral radiograph, the radiolunate relationship remains normal, but all other carpal bones (carpus) are dislocated dorsally.
Scapholunate dissociation
This condition occurs when the ligaments between the scaphoid and lunate rupture, leading to diastasis—an abnormal widening of the interosseous space.
Key radiological signs:
- "Terry Thomas" sign: visualization of a pronounced gap between the scaphoid and lunate bones. The sign is named after the British comedian who had a noticeable gap between his front teeth (diastema).
- The scaphoid bone itself may appear pathologically shortened on the radiograph. This occurs due to its rotational subluxation and hyperflexion.
To better visualize dissociation and joint space widening, the patient is asked to make a fist when taking the radiograph. This provocative maneuver makes diastasis much more evident to the diagnostician.
Injuries of the metacarpus and phalanges
Injuries to the metacarpals and phalanges often require careful examination of the articular surfaces, as treatment tactics depend on their preservation.
Carpometacarpal joints Their injuries can be either isolated or combined with fractures of the metacarpal base or adjacent carpal bones. Radiographically, on the anteroposterior (PA) and oblique views, the normal joint space between the distal row of carpal bones and the bases of the metacarpals disappears. The lateral view shows divergence of the longitudinal axes of the metacarpals and displacement of their base dorsally. The 5th ray is the most vulnerable: it accounts for 50% of all such dislocations.
Typical metacarpal fractures:
- Punch injury: a classic fracture of the 5th metacarpal head, which is usually accompanied by moderate angular displacement.
- Bennett fracture: an intra-articular fracture of the base of the 1st metacarpal. The complexity of the pathomechanics lies in the fact that the tendon of the abductor pollicis longus muscle (m. abductor pollicis longus) attaches to the larger bone fragment. This muscle constantly pulls the fragment toward itself, causing persistent displacement, which is why the injury requires mandatory surgical fixation.
Most transverse and spiral fractures are clearly visible on radiographs. The decision to operate always depends on how much the articular surface is involved and the size of the fractured fragment.
Condylar Fractures Usually involve the articular surface of the distal phalanx. These are complex injuries that often require surgical treatment, so patients with such a diagnosis should be immediately referred to a specialist (hand surgeon).
To memorize the normal lateral view, imagine an apple (capitate) resting in a cup (lunate), which sits on a saucer (radius). Their centers should form a perfect straight line.
Class and exam questions
How to distinguish lunate dislocation from perilunate dislocation on a lateral view?
In lunate dislocation, it is displaced in the palmar direction, disrupting the axis with the radius. In perilunate dislocation, the radiolunate axis is preserved, but all other carpal bones are dislocated dorsally.
Why is a wrist radiograph performed with a clenched fist?
This is a provocative test that increases stress on the ligamentous apparatus. It makes diastasis (widening of the interosseous space) in scapholunate dissociation more pronounced and noticeable on the radiograph.
Why does a Bennett fracture strictly require fixation?
Due to the pull of the abductor pollicis longus tendon (m. abductor pollicis longus). The muscle pulls the bone fragment, preventing it from being held in the correct position without reliable fixation.
What else to study on this topic
- Criteria for surgical treatment of phalangeal condylar fractures
- Evaluation of bone fragment size in metacarpal fractures
- Mechanism of scaphoid hyperflexion in ligament rupture
- Combined injuries of the carpometacarpal joint
- Features of angular displacement in punch injuries
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