Normal Stahl's Lunate Index Size on Radiography
Stahl's lunate index is a radiographic ratio that quantifies the shape of the lunate bone by comparing its height to its width on standard wrist views. Accurate assessment of lunate morphology is clinically important because shape variants are closely associated with degenerative conditions of the wrist, particularly Kienböck's disease and ulnocarpal impaction syndrome. Establishing a normal reference range helps distinguish physiologic variation from pathologic change.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Posteroanterior, Lateral | 0.50-0.56 |
Clinical Significance
A Stahl's lunate index within the normal range of 0.50–0.56 indicates a lunate with proportionate height-to-width dimensions. A higher index (more square or tall lunate) has been associated with increased biomechanical loading across the radiocarpal joint and a predisposition to avascular necrosis of the lunate (Kienböck's disease). Conversely, a lower index may reflect a flatter lunate morphology, which can influence load distribution and carpal kinematics.
Lunate morphology should always be interpreted alongside ulnar variance, carpal height ratio, and clinical symptoms. Deviations from the normal index alone are insufficient for diagnosis but serve as important adjunctive data in the workup of wrist pain and instability.
- Kienböck's disease (avascular necrosis of the lunate)
- Ulnocarpal impaction syndrome
- Carpal instability dissociative (scapholunate or lunotriquetral)
- Post-traumatic lunate fracture or collapse
- Developmental lunate shape variant (Type I vs. Type II lunate)
Reference: Berger RA. Hand Surgery. Philadelphia: Lippincott Williams & Wilkins, c2004. (2004).
Imaging Notes
Stahl's lunate index is measured on standard posteroanterior (PA) and lateral wrist radiographs obtained with the wrist in neutral position, forearm pronated, and fingers extended. The index is calculated as the ratio of lunate height (proximal-to-distal dimension) to lunate width (radial-to-ulnar dimension) on the PA view. Consistent patient positioning is essential, as even mild wrist flexion or extension will alter apparent lunate dimensions and introduce measurement error.
Ensure adequate radiographic technique with the wrist flat on the detector to avoid parallax distortion. The lateral view complements assessment of lunate morphology in the sagittal plane and aids in identifying early collapse or dorsal intercalated segment instability (DISI) deformity associated with Kienböck's disease progression.