Normal Bouman's Index No 1 Size on Radiography
Bouman's Index No 1 is a standardized radiographic ratio used to quantify ulnar translation of the carpus relative to the distal radius on posteroanterior wrist radiographs. Accurate measurement of this index is clinically important because ulnar carpal translation is a recognized pattern of wrist instability, particularly in inflammatory arthropathies such as rheumatoid arthritis. Early detection allows timely intervention to prevent progressive deformity and functional decline.
Normal Reference Values
| Orientation | Location | Measurement |
|---|---|---|
| Posteroanterior | Ulnar Translation | 0.83-0.91 |
Clinical Significance
A Bouman's Index No 1 value below the normal range of 0.83–0.91 indicates pathological ulnar translation of the carpus — a displacement of the proximal carpal row toward the ulna. This deformity is most commonly encountered in rheumatoid arthritis, where synovitis progressively disrupts the radiocarpal ligaments, but may also result from trauma or other causes of ligamentous laxity.
Ulnar carpal translation compromises grip strength, alters force transmission across the wrist, and predisposes to further instability and joint destruction if untreated. Serial measurement of Bouman's Index No 1 is valuable for monitoring disease progression and evaluating surgical or medical treatment outcomes. A key pitfall is ensuring consistent posteroanterior positioning with the wrist in neutral rotation, as forearm rotation can artificially shift index values.
- Rheumatoid arthritis with radiocarpal ligament destruction
- Post-traumatic carpal instability
- Scapholunate dissociation with secondary carpal shift
- Ligamentous laxity syndromes (e.g., Ehlers-Danlos)
- Post-surgical or post-infectious radiocarpal remodeling
Reference: Borisch N, Lerch K, Grifka J et al. A comparison of two indices for ulnar translation and carpal height in the rheumatoid wrist. J Hand Surg Br. 2004;29(2):144–7.
Imaging Notes
Bouman's Index No 1 is measured on a standard posteroanterior (PA) radiograph of the wrist obtained with the shoulder abducted 90°, elbow flexed 90°, and the wrist in neutral flexion-extension and radio-ulnar deviation. The index is calculated as a ratio of carpal position relative to defined landmarks on the distal radius, allowing normalization across varying wrist sizes. Strict adherence to neutral positioning is essential, as even mild rotation introduces measurable error in the calculated ratio.
When performing serial assessments — for example, in rheumatoid arthritis follow-up — identical projections and consistent landmark identification are critical to ensure comparability between studies. Digital radiography with calibration markers is preferred where available to minimize magnification variability.