What is the normal width between the scaphoid and lunate bones?

Normal Width Between Scaphoid and Lunate

The normal radiographic width between the scaphoid and lunate bones should not exceed 2 mm when measured at the midportion of the joint space. 1

Measurement Technique and Considerations

Optimal Measurement Location

• The scapholunate interval should be measured at the midportion of the flat lateral facet of the scaphoid 2
• The midportion of the scapholunate joint shows the least absolute measurement variance in width and should be the most precise part of the joint to measure 3
• Measurements at articular margins (dorsal-palmar and proximal-distal) are less reliable due to greater variation

Optimal Imaging Technique

• Standard posteroanterior (PA) radiographs may not accurately demonstrate the scapholunate space due to bone overlap 2
• A PA radiograph with 10 degrees of tube angulation from the ulna toward the radius best demonstrates the scapholunate space 2
• MRI studies show the normal scapholunate joint interval measures less than 2 mm 3

Clinical Significance

Diagnostic Implications

• Scapholunate diastasis (widening) of >4 mm on radiographs suggests dorsal intercalated segmental instability (DISI) 1
• Widening of the scapholunate interval is a key radiographic sign of scapholunate ligament injury
• CT arthrography has demonstrated sensitivity, specificity, and accuracy between 80% and 100% for intrinsic ligament tears located within the proximal carpal compartment 1

Patterns of Joint Morphology

• Three patterns of midjoint space cortical conformation have been observed 3:
  1. Parallel congruent (78%)
  2. Inverted Y (15%)
  3. Point-like (8%)

Lunate Morphology Considerations

• Lunate morphology (Type I vs Type II) can affect scapholunate relationships 4, 5
• Type II lunates (with a hamate facet) are associated with a lower incidence of DISI deformity 5
• Lunate type can be determined using capitate-triquetrum (C-T) distance:
  • Type I lunate: C-T distance ≤2 mm
  • Type II lunate: C-T distance ≥4 mm

Age-Related Considerations

• In pediatric patients, the normal scapholunate interval varies with age and decreases as age increases 6
• Adult criteria (>2 mm widening) cannot be applied to skeletally immature individuals due to ongoing ossification of carpal bones 6
• The average values for the radiographic SL interval in children range from 9.07 mm (younger children) to 1.57 mm (adolescents) 6

Advanced Imaging Considerations

• When conventional radiographs are normal but clinical suspicion is high, MRI without contrast is recommended 7
• CT arthrography is considered the gold standard for diagnosing scapholunate ligament tears with sensitivity, specificity, and accuracy approaching 100% 1
• MR arthrography provides better diagnostic accuracy for scapholunate interosseous ligament tears than conventional MRI 1

The 2 mm threshold for normal scapholunate width is a critical diagnostic parameter in wrist injury assessment, and proper measurement technique at the midportion of the joint is essential for accurate evaluation.