Normal Chamay's Translation Index Size on Radiography
Chamay's Translation Index is a radiographic ratio used to quantify ulnar translation of the carpus relative to the distal radius on posteroanterior wrist radiographs. It is calculated by dividing the distance from the lunate midpoint to the longitudinal axis of the radius by the width of the capitate, providing a standardized, size-independent measure. Accurate assessment of this index is clinically important for identifying pathological carpal instability patterns, particularly in inflammatory arthropathy and post-traumatic conditions.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Posteroanterior | 0.25-0.31 |
Clinical Significance
Under normal biomechanical conditions, the carpus is maintained in a neutral position relative to the distal radius, reflected by a Chamay's Translation Index of 0.25–0.31. Values exceeding this range indicate ulnar translation of the carpus, a form of carpal instability in which the proximal carpal row shifts ulnarward, compromising wrist function and grip strength.
Ulnar carpal translation is most commonly encountered in the setting of rheumatoid arthritis, where progressive ligamentous laxity and synovial destruction destabilize the radiocarpal joint. It may also occur following distal radius fractures, scapholunate ligament disruption, or inadequate surgical reconstruction. Accurate measurement guides surgical planning, including decisions regarding soft-tissue reconstruction or radiocarpal arthrodesis.
- Rheumatoid arthritis with radiocarpal ligament destruction
- Post-traumatic carpal instability following distal radius fracture
- Scapholunate ligament insufficiency
- Congenital or developmental ligamentous laxity
- Failed wrist arthroplasty or ligament reconstruction
Reference: Schmitt R, Lanz U. Diagnostic Imaging of the Hand. TIS. (2008).
Imaging Notes
Chamay's Translation Index is assessed exclusively on a true posteroanterior (PA) wrist radiograph obtained with the shoulder abducted 90°, elbow flexed 90°, and the wrist in neutral rotation and deviation. Proper patient positioning is essential, as even slight forearm rotation or wrist deviation will alter the apparent position of the carpus and introduce measurement error. The index is derived by measuring the perpendicular distance from the midpoint of the lunate to the longitudinal axis of the radius, then dividing by the transverse width of the capitate at its widest point.
When interpreting serial radiographs in patients with inflammatory arthritis, consistent technique and identical landmark identification are critical to detecting progressive ulnar drift. Digital radiography with dedicated measurement tools improves reproducibility; however, the examiner should always confirm true neutral positioning before applying the index clinically.