The Capitate-to-Axis-of-Radius Distance (CARD): A New Radiographic Measurement for Wrist and Carpal Alignment in the Sagittal Plane

Abstract

To determine the reliability of a new radiographic index evaluating sagittal radiocarpal alignment, the capitate-to-axis-of-radius distance (CARD). A secondary purpose was to validate this index by comparing values between normal wrists and those with distal radial fractures (DRFs) and rheumatoid arthritis (RA). The CARD is defined as the perpendicular distance from the center of the capitate head to the axis of the radius. Inter- and intraobserver reliability was tested. Cronbach alpha was calculated, and 2 methods of measurement were compared. The superior one (volar border of radial shaft) was used in the second part of the study. The normal CARD was then compared with unilateral DRFs with dorsal displacement DRF (n= 25) and RA (n= 25). Correlations between the CARD and other radiographic parameters (dorsal angulation, radial inclination, and ulnar variance) were calculated as well as between the CARD and the severity of disease or fracture displacement (mild/moderate/severe). The CARD showed excellent intra- and interobserver reliability. The volar radius measurement method was superior to the midaxis method and was, therefore, used for the second portion of the study. The mean CARD for normal, fractured, and RA wrists was significantly different (2.2 ± 2.5 mm, 15.7 ± 6.5 mm and 0.2 ± 4.4 mm, respectively). There was a strong side-to-side correlation in normal wrists (r= 0.77) and a significant correlation between the CARD (mm) and the severity of deformity (RA, r= -0.7; DRF, r= 0.8). The CARD is a reproducible, easy-to-use measurement of sagittal carpal alignment with a strong side-to-side correlation. The CARD increases with dorsal angulation of the distal radius and decreases as severity of deformity with RA increases. The correlation of the CARD with severity of deformity in DRFs and RA makes it a useful method of assessing deformities in the sagittal plane. The normal wrist can be used as a comparison when evaluating the CARD in the setting of unilateral wrist disease.