Normal Youm's Carpal Height Index Size on Radiography
Youm's Index of Carpal Height is a dimensionless ratio used to quantify the overall height of the carpal column relative to the length of the third metacarpal on posteroanterior (PA) wrist radiographs. It provides an objective, reproducible measure of carpal architecture that is independent of radiograph magnification. Accurate assessment of this index is clinically important for identifying carpal collapse, a hallmark of advanced wrist pathology.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Posteroanterior | 0.51-0.57 |
Clinical Significance
A normal Youm's carpal height ratio falls between 0.51 and 0.57 on PA radiography. Values falling below this range indicate loss of carpal height, reflecting progressive carpal collapse. This finding is a reliable indicator of structural compromise within the proximal carpal row and is particularly valuable in staging wrist conditions over time.
Reduction in carpal height is most commonly encountered in the following clinical scenarios:
- Scapholunate advanced collapse (SLAC) — degenerative arthritis following chronic scapholunate dissociation
- Scaphoid nonunion advanced collapse (SNAC) — progressive carpal collapse after untreated scaphoid nonunion
- Kienböck's disease (avascular necrosis of the lunate) — lunate fragmentation leading to progressive carpal shortening
- Rheumatoid arthritis — synovitis-driven ligamentous destruction and carpal settling
- Perilunate instability — acute or chronic ligamentous injury disrupting normal carpal alignment
A key pitfall is poor radiographic positioning; even mild forearm rotation can artificially alter carpal column geometry, leading to erroneous ratios. Serial comparisons are most meaningful when obtained with standardized, true PA projections under identical conditions.
Reference: Brown DE, Neumann RD. Orthopedic Secrets. Elsevier Health Sciences. (2004).
Imaging Notes
Youm's Index is calculated exclusively on a true posteroanterior (PA) radiograph of the wrist obtained in neutral forearm rotation, with the elbow flexed to 90° and the shoulder abducted to 90°. The ratio is derived by dividing the carpal height (measured from the proximal articular surface of the capitate to the base of the third metacarpal) by the length of the third metacarpal. Strict adherence to this landmark selection is essential, as small measurement errors are amplified in the final ratio.
Digital radiography with calibration tools improves measurement precision. When evaluating serial studies for progression of collapse, ensure consistent positioning and use the same anatomical landmarks throughout to maintain inter-study comparability.