Normal McMurtry's Translation Index Size on Radiography
McMurtry's Translation Index is a radiographic ratio used to quantify the degree of ulnar or radial carpal translocation relative to the radius, measured on posteroanterior (PA) wrist radiographs. It provides an objective, reproducible assessment of carpal alignment that is critical in evaluating wrist instability and ligamentous injury. Accurate measurement guides both surgical planning and post-treatment monitoring.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Posteroanterior | 0.27-0.33 |
Clinical Significance
A normal McMurtry's Translation Index falls between 0.27 and 0.33 on PA radiographs. Values outside this range suggest pathological carpal translocation — most commonly ulnar translocation, where the carpus shifts medially relative to the distal radius. This pattern is frequently encountered in rheumatoid arthritis, chronic ligamentous insufficiency, and post-traumatic wrist instability.
Ulnar translocation (index below the normal range) indicates progressive carpal collapse and may necessitate surgical reconstruction or arthrodesis. Radial translocation, though less common, can occur following specific perilunate injuries or radial styloid fractures. Clinicians should interpret the index alongside clinical findings and additional radiographic parameters — such as radial inclination, carpal height ratio, and scapholunate angle — to avoid over- or under-diagnosis.
- Rheumatoid arthritis with ulnar drift and carpal translocation
- Perilunate dislocation or ligamentous dissociation
- Post-traumatic instability following distal radius fracture
- Chronic scapholunate dissociation with secondary carpal collapse
- Congenital or dysplastic carpal alignment variants
Reference: Schmitt R, Lanz U. Diagnostic Imaging of the Hand. TIS. (2008).
Imaging Notes
McMurtry's Translation Index is measured exclusively on a true posteroanterior (PA) wrist radiograph obtained in neutral rotation, with the forearm pronated, elbow at 90°, and shoulder abducted. Patient positioning is critical — any rotation introduces error by altering the apparent relationship between the carpus and the distal radius. The index is calculated as the ratio of the distance from the long axis of the radius to the midpoint of the capitate head, divided by the width of the carpal bones at a defined reference level.
Consistent beam centering over the radiocarpal joint and standardized wrist positioning are essential for reproducibility, particularly when serial studies are compared over time in chronic conditions such as rheumatoid arthritis. Digital radiography with post-processing measurement tools improves precision compared to conventional film-based methods.