Carpal height is used to diagnose and assess the severity of carpal collapse. It is defined as the distance between the base of third metacarpal and the subchondral bony cortex of the distal radius. Due to the wide range of normal variations between individuals, several measurement techniques have been developed and studied.
Measurement
On a neutral PA radiograph of the wrist, three methods can be used to determine the carpal height ratio. Each has it's advantages and disadvantages; however, several studies have shown that the Youm and Natrass methods have poor correlation with each other, and likely the Capitate-Radius index provides the highest accuracy and reliability for detecting carpal collapse4,5.
Carpal Height Ratio (Youm Method)
It is calculated by dividing the carpal height by the length of the third metacarpal 1.
Normal range: 0.51 - 0.57
Revised Carpal Height Ratio (Natrass Method)
This method can be used when the radiograph does not fully include the third metacarpal.
It is calculated by dividing carpal height (measured in line with the third metacarpal axis) by the greatest length of the capitate 2.
The capitate length is measured from the subchondral cortex adjacent to the bases of the second and third metacarpal to the proximal cortex along a line that passes through the center of the capitate.
Normal range: 1.57 ± 0.05 3
Capitate-Radius Index
This single measurement has been shown in multiple studies to be the most accurate of the three methods for the detection of carpal collapse and provides a higher inter-rater reliability4,5.
The value is obtained by simply measuring the shortest distance between the cortices of the capitate and radius on PA projection.
Normal: ≥ 0.92 cm
Interpretation
These values are decreased in diseases that cause carpal collapse, such as rheumatoid arthritis, Kienböck disease, traumatic injuries, and scapholunate advanced collapse.
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