How is the medial process of the first metacarpal bone identified on a wrist X-ray PA (posteroanterior) view?

Identifying the Medial Process of the First Metacarpal on Wrist PA X-ray

The medial process of the first metacarpal is best identified on a posteroanterior (PA) wrist X-ray by locating the medial projection at the base of the first metacarpal that articulates with the trapezium in the carpometacarpal joint. 1

Standard Radiographic Views for Evaluation

• A standard PA view of the wrist is essential for visualizing the first metacarpal and its medial process, but this should be part of a complete radiographic examination that includes multiple views 1
• For comprehensive wrist evaluation, a 3-view examination is recommended, including posteroanterior (PA), lateral, and 45° semipronated oblique views 1
• When specifically examining the thumb and first metacarpal, a 2-view radiographic examination is typically sufficient, though adding an oblique projection can slightly increase diagnostic yield 1

Anatomical Identification on PA View

• On the PA view, the first metacarpal appears shorter and broader than other metacarpals, with its base (proximal portion) articulating with the trapezium 2
• The medial process of the first metacarpal is visible as a bony prominence on the medial/ulnar aspect of the metacarpal base 2
• The first metacarpal's articular tilt angle (which ranges from -6.3° to 12.3°) affects the appearance and prominence of the medial process on PA view 2

Technical Considerations

• High-quality PA radiographs require proper positioning with the palm flat against the detector, fingers extended, and thumb slightly abducted 1
• Inadequate positioning can lead to rotational artifacts that may obscure the medial process of the first metacarpal 1
• For optimal visualization of the first metacarpal and its articulation with the trapezium, the wrist should be in neutral position without ulnar or radial deviation 1

Clinical Relevance

• The medial process of the first metacarpal is clinically important as it contributes to the stability of the first carpometacarpal joint 2, 3
• Morphological variations in this process can affect joint stability and may be associated with development of first carpometacarpal osteoarthritis 3
• In rheumatoid arthritis patients, asymmetrical damage to the wrist joint can induce lateralized cortical bone loss in the metacarpal diaphysis 4

Limitations and Additional Imaging

• Standard radiographs may miss subtle fractures or abnormalities of the first metacarpal; studies show radiography has limited sensitivity (57.8%) for detecting wrist fractures compared to CT 5
• When initial radiographs are equivocal but clinical suspicion remains high, CT without IV contrast is commonly used to confirm or exclude suspected wrist fractures 1
• For detailed evaluation of the first metacarpal's morphology, including the medial process, 3D reconstructions from CT can be particularly helpful 1

Common Pitfalls

• Inadequate number of views: A single PA view is insufficient for complete evaluation; multiple projections are necessary 1
• Poor positioning: Rotation of the wrist can significantly alter the appearance of the first metacarpal and its medial process 1
• Overlooking subtle findings: The sensitivity of radiography for carpal bone fractures (except scaphoid) ranges from 0-41.2%, highlighting the importance of careful evaluation 5
• Failure to correlate with clinical findings: Radiographic appearance should always be interpreted in the context of the patient's symptoms and physical examination 1