This cadaver and volunteer biomechanics study defines the normal kinematic axes of the wrist during radial-ulnar deviation and flexion-extension. It derives two radiographic ratios that quantify carpal collapse and ulnar translation independent of hand size or X-ray magnification. Subjects included 6 cadaver wrists, 6 normal volunteers, and 100 wrists of normal subjects.
Two numbers from this paper belong in every hand surgery trainee's memory: carpal height ratio 0.54 and carpal-ulnar distance ratio 0.30.
When you pull up a PA wrist radiograph on a patient with suspected carpal instability, SLAC wrist, or rheumatoid wrist deformity, measure carpal height and carpal-ulnar distance and divide each by the third metacarpal length. A carpal height ratio below 0.54 documents carpal collapse. A carpal-ulnar distance ratio above 0.30 documents ulnar carpal translation. Because these are ratios, magnification and hand size are already controlled for.
Before Youm et al., clinicians used the scapholunate angle (normal ~46 degrees) to assess carpal collapse — but this paper showed that measurement varies with hand position, limiting its reliability as a standalone tool. The ratio-based approach solved that problem.
The kinematic finding that radial deviation occurs purely at the intercarpal joint (proximal row fixed) while ulnar deviation involves both rows is directly relevant to planning partial wrist fusions. An intercarpal fusion will affect radial deviation more than ulnar deviation.
This cadaver and volunteer biomechanics study defines the normal kinematic axes of the wrist during radial-ulnar deviation and flexion-extension. It derives two radiographic ratios that quantify carpal collapse and ulnar translation independent of hand size or X-ray magnification. Subjects included 6 cadaver wrists, 6 normal volunteers, and 100 wrists of normal subjects.
Two numbers from this paper belong in every hand surgery trainee's memory: carpal height ratio 0.54 and carpal-ulnar distance ratio 0.30.
When you pull up a PA wrist radiograph on a patient with suspected carpal instability, SLAC wrist, or rheumatoid wrist deformity, measure carpal height and carpal-ulnar distance and divide each by the third metacarpal length. A carpal height ratio below 0.54 documents carpal collapse. A carpal-ulnar distance ratio above 0.30 documents ulnar carpal translation. Because these are ratios, magnification and hand size are already controlled for.
Before Youm et al., clinicians used the scapholunate angle (normal ~46 degrees) to assess carpal collapse — but this paper showed that measurement varies with hand position, limiting its reliability as a standalone tool. The ratio-based approach solved that problem.
The kinematic finding that radial deviation occurs purely at the intercarpal joint (proximal row fixed) while ulnar deviation involves both rows is directly relevant to planning partial wrist fusions. An intercarpal fusion will affect radial deviation more than ulnar deviation.