What findings should be assessed on an X‑ray of the right thumb base (first metacarpal and trapezium)?

X-Ray Views for Right Thumb Base Assessment

For evaluating the right thumb base (first metacarpal and trapezium), obtain a minimum 2-view radiographic examination (posteroanterior and lateral), with the addition of an oblique projection providing increased diagnostic yield 1.

Standard Radiographic Protocol

The ACR Appropriateness Criteria clearly establishes that most fractures of the thumb are visible on a 2-view radiographic examination, although there is a slight increase in diagnostic yield with the addition of an oblique projection, which can be obtained along with a PA examination of the whole hand 1.

Specific Views to Order:

• Posteroanterior (PA) view of the entire hand
• Lateral view of the thumb
• Oblique projection (increases diagnostic sensitivity)

Key Findings to Assess

When interpreting thumb base radiographs, systematically evaluate the following structures and pathologies:

Osseous Structures:

• First metacarpal base fractures - look for cortical disruption, displacement, and intra-articular extension
• Trapezium fractures - assess all four articulating surfaces (with first metacarpal, scaphoid, trapezoid, and second metacarpal base)
• Carpometacarpal (CMC) joint alignment - evaluate for subluxation or dislocation

Articular Assessment:

• Joint space narrowing at the trapeziometacarpal (TMC) joint
• Osteophyte formation - more commonly seen on trapezium than metacarpal 2
• Subchondral sclerosis - visible in approximately 84-90% of arthritic cases 2
• Subchondral cysts
• Erosive changes (in erosive osteoarthritis)

Alignment Parameters:

• Metacarpal subluxation - dorsal subluxation visible on lateral view (98% sensitivity), radial subluxation on PA/oblique views (77% sensitivity) 2
• Thumb metacarpal adduction deformity - present in 23-32% of arthritic thumbs 2
• Pantrapezial arthritis - evaluate scaphotrapeziotrapezoid (STT) joint involvement (26% of cases) 2

Advanced Imaging Considerations

When Initial Radiographs Are Negative or Equivocal:

If clinical suspicion remains high despite negative radiographs, CT without IV contrast is the next appropriate study 1. CT excels at:

• Detecting occult fractures
• Evaluating carpometacarpal joint fracture-dislocations
• Preoperative planning for complex articular injuries with 3D reconstructions

MRI without IV contrast should be reserved for suspected soft-tissue injuries including collateral ligaments, volar plates, tendons, and pulleys 1. However, for acute metacarpal and digital fractures, MRI is usually not indicated 1.

Clinical Pitfalls to Avoid

1. Don't rely on 2 views alone - The ACR explicitly states that examinations with only 2 views are inadequate for detecting fractures in hand and wrist joints 1

2. Don't miss carpometacarpal fracture-dislocations - These are notoriously difficult to recognize on standard radiographs and may require CT 1

3. Assess for associated injuries - Trapezium fractures may be associated with first metacarpal base fractures and CMC joint dislocations 3

4. Consider specialized views for specific pathology:

   • Robert view (hyperpronation) - superior for detecting trapezial osteophytes and radial metacarpal subluxation 2
   • Stress views - useful for evaluating TMC joint laxity and hypermobility 4
   • Peritrapezial view - provides better visualization of all four trapezial articulations with higher inter-observer reliability than Robert view 5

Surgical Decision Points

For fractures requiring operative consideration, radiographs must demonstrate:

• Articular involvement - fragments involving >1/3 of articular surface typically require fixation
• Displacement - palmar displacement or interfragmentary gap >3mm indicates need for surgery 1
• Residual step-off - <2mm is necessary to avoid long-term osteoarthritis 1