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Research Papers

# Gender Differences in Capitate Kinematics are Eliminated After Accounting for Variation in Carpal Size

[+] Author and Article Information
Michael J. Rainbow, Douglas C. Moore

Department of Orthopaedics, The Warren Alpert Medical School of Brown University∕Rhode Island Hospital, 1 Hoppin Street, CORO West, Suite 404, Providence, RI 02903

Joseph J. Crisco1

Department of Orthopaedics, The Warren Alpert Medical School of Brown University∕Rhode Island Hospital, 1 Hoppin Street, CORO West, Suite 404, Providence, RI 02903josepẖcrisco@brown.edu

Scott W. Wolfe

Hand and Upper Extremity Center, Hospital for Special Surgery, Weill Medical College of Cornell University, 523 East 72nd Street, New York, NY 10021

1

Corresponding author.

J Biomech Eng 130(4), 041003 (May 23, 2008) (5 pages) doi:10.1115/1.2913332 History: Received April 20, 2007; Revised November 12, 2007; Published May 23, 2008

## Abstract

Previous studies have found gender differences in carpal kinematics, and there are discrepancies in the literature on the location of the flexion∕extension and radio-ulnar deviation rotation axes of the wrist. It has been postulated that these differences are due to carpal bone size differences rather than gender and that they may be resolved by normalizing the kinematics by carpal size. The purpose of this study was to determine if differences in radio-capitate kinematics are a function of size or gender. We also sought to determine if a best-fit pivot point (PvP) describes the radio-capitate joint as a ball-and-socket articulation. By using an in vivo markerless bone registration technique applied to computed tomography scans of 26 male and 28 female wrists, we applied scaling derived from capitate length to radio-capitate kinematics, characterized by a best-fit PvP. We determined if radio-capitate kinematics behave as a ball-and-socket articulation by examining the error in the best-fit PvP. Scaling PvP location completely removed gender differences $(P=0.3)$. This verifies that differences in radio-capitate kinematics are due to size and not gender. The radio-capitate joint did not behave as a perfect ball and socket because helical axes representing anatomical motions such as flexion-extension, radio-ulnar deviation, dart throwers, and antidart throwers, were located at distances up to $4.5mm$ from the PvP. Although the best-fit PvP did not yield a single center of rotation, it was still consistently found within the proximal pole of the capitate, and rms errors of the best-fit PvP calculation were on the order of $2mm$. Therefore, the ball-and-socket model of the wrist joint center using the best-fit PvP is appropriate when considering gross motion of the hand with respect to the forearm such as in optical motion capture models. However, the ball-and-socket model of the wrist is an insufficient description of the complex motion of the capitate with respect to the radius. These findings may aid in the design of wrist external fixation and orthotics.

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## Figures

Figure 1

The capitate coordinate system based on inertia: I1, I2, and I3=first, second, and third principal inertial axes. L1 is the length of the capitate from the proximal surface to the distal surface along I1. L2 and L3 are similarly defined but corresponding to I2 and I3, respectively.

Figure 2

There is a positive linear relationship between capitate long axis length and volume. Note that female capitates are smaller than males in both L1 and volume.

Figure 3

The smallest capitate’s PvP location was scaled up 16.2%, while the largest was scaled down 15.4%. Small changes in scaling factor correspond to large changes in volume.

Figure 4

Location of best-fit PvP from capitate centroid along I1. PvPI1 becomes more negative as it approaches the proximal pole of the capitate. (a) Although the R2 value is low, a trend can be seen that reveals a relationship between capitate size and PvP location; as the capitates get larger, the distance from capitate centroid to the PvP increases. (b) Scaled PvP locations. The relationship between capitate size and PvP location vanishes.

Figure 5

Capitate PvP for each subject visualized in a lateral view of the capitate inertial coordinates, male (black), female (white). (a) Unscaled PvPs visualized within the smallest female (inner) and largest male (outer) capitates (registered to the inertial coordinate system). (b) PvPs scaled to L1avg, visualized within the average subject’s capitate. Both male and female PvPs tend toward the center of the proximal pole.

Figure 6

The average of all scaled PvPs (black sphere). Representative HAMs for each motion bin. Refer to Table 1 for abbreviations.

## Errata

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