Normal Nattrass's Carpal Height Index Size on Radiography
Nattrass's Index of Carpal Height is a dimensionless radiographic 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 means of assessing carpal architecture that is independent of magnification. Accurate measurement is clinically important because loss of carpal height is a sentinel indicator of carpal instability, ligamentous disruption, or avascular necrosis.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Posteroanterior | 1.52-1.62 |
Clinical Significance
A normal Nattrass's carpal height index falls between 1.52 and 1.62. Values below this range indicate carpal collapse, reflecting a reduction in the vertical height of the proximal and distal carpal rows. Serial measurements are particularly valuable for monitoring disease progression over time.
Carpal collapse detected by a reduced index carries important therapeutic implications. It may guide surgical decisions such as partial or total wrist arthrodesis, proximal row carpectomy, or ligamentous reconstruction. A key pitfall is ensuring strict PA positioning with the wrist in neutral rotation, as even slight obliquity artificially alters the measured ratio.
- Scapholunate advanced collapse (SLAC) — most common cause of progressive carpal height loss
- Scaphoid non-union advanced collapse (SNAC) — secondary degenerative carpal instability
- Kienböck's disease — avascular necrosis of the lunate leading to collapse
- Rheumatoid arthritis — ligamentous laxity and erosive disease causing carpal settling
- Perilunate instability — acute or chronic ligamentous disruption altering carpal geometry
Reference: Brown DE, Neumann RD. Orthopedic Secrets. Elsevier Health Sciences. (2004).
Imaging Notes
The index is measured on a standard PA wrist radiograph obtained with the patient seated, elbow flexed to 90°, and the wrist in neutral flexion-extension and radio-ulnar deviation. The carpal height is measured as the perpendicular distance from the base of the third metacarpal to the proximal articular surface of the lunate (or the distal radius, depending on the specific method variant). This value is then divided by the length of the third metacarpal to yield the index.
Consistent neutral positioning and a true PA projection are essential; even minor supination or pronation rotates the carpus and introduces measurement error. Digital radiography with calibrated measurement tools improves reproducibility. Comparison with prior studies on the same patient is strongly recommended when monitoring for interval carpal collapse.