Relative Contributions of the Midcarpal and Radiocarpal Joints to Dart-Thrower's Motion at the Wrist

Abstract

Purpose: To identify the relative contributions of the radiocarpal (RC) and midcarpal (MC) joints to dart-thrower's motion (DTM) of the wrist.

Methods: Six cadaveric upper extremities were fixed to a custom-designed loading jig allowing for pure moment-rotation analysis in 24 different directions of wrist motion. Each specimen was tested in 3 states: intact, simulated radiocarpal fusion (sRCF) and simulated pancarpal fusion (sPCF). Moments of ± 1.5 Nm were applied at each of 24 directions for each state and the resulting wrist rotation recorded. Data from each specimen were reduced to compute the range of motion (ROM) envelopes and the orientation of the ROM for the 3 different states.

Results: The ROM was significantly decreased in the sRCF and sPCF groups compared with the intact group in the directions of the pure extension, radial extension, ulnar flexion, and ulnar deviation. No significant difference in ROM was detected between the sRCF and sPCF groups in any direction. The ROM envelopes for the intact, sRCF, and sPCF groups were all oriented obliquely to the axis of pure wrist flexion-extension near a path of ulnar flexion-radial extension, consistent with prior reports on DTM.

Conclusions: Although both simulated fusion types decreased ROM compared with the intact wrist, the principal direction of wrist motion along the path of DTM was not significantly altered by simulated RCF or PCF.

Clinical relevance: These findings suggest that the RC and MC joints can each contribute to a similar mechanical axis of motion located along the path of DTM when the other joint has been eliminated via fusion. Surgical options such as partial wrist fusions may maintain the native wrist's mechanical axis if either the RC or the MC joint is preserved, despite significant reduction in overall ROM.

Keywords: Dart-thrower’s motion; midcarpal joint; radiocarpal fusion; wrist biomechanics.

Figure 1

Specimen jig that interfaced to a standard materials testing system. The forearm, which was fixed in neutral rotation, was mounted to the jig horizontally. The application of 1.5 Nm moments in 24 different directions of wrist motion was achieved by rotating the specimen about the forearm axis and then locking the potting fixtures to the jig in 15° increments. Two orthogonal linear slides (depicted as x and y) allowed the specimen to travel freely in the horizontal plane, providing an unconstrained application of the moments and allowing the wrist to rotate about its physiological axes. Four constant 4.45 N force springs attached the flexors and extensors to the potting. (Reprinted from Crisco, JJ, Heard, WMR, Rich, RR, Paller, DJ, Wolfe, SW: The Mechanical Axes of the Wrist Are Oriented Obliquely to the Anatomical Axes. J Bone Joint Surg Am 2011;93:169–177 with permission from Wolters Kluwer Health, Inc.)

Figure 2

Polar coordinate system to illustrate relationship between anatomic and mechanical axes relative to wrist position. This system is based on the assumption that wrist flexion-extension and radioulnar deviation axes are orthogonal to each other, with each direction comprising the four major directions of wrist motion. The remaining 20 directions, depicted as lines emanating from the center of the plot, are combinations of the two of the four major directions. The named directions (purple dotted lines) are equidistant (45°) from their two major direction components. For example, ulnar flexion is 45° from both pure flexion and ulnar deviation. The solid orange line depicts the plane of motion 15° from the ulnar flexion-radial extension axis, and 30° from the pure flexion-extension axis, that approximates the Dart-Throwers’ Motion plane from our previous study.

Figure 3

Steinmann pins placed in an oblique fashion across the carpal bones to simulate a pancarpal fusion (sPCF).

Figure 4

Steinmann pins placed across the radiocarpal (RC) joint to simulate a radiocarpal fusion (sRCF).

Figure 5

Bar graph a graph depicting comparison between intact, sRCF and sPCF groups with respect to each of the 6 major directions of motion: pure extension, ulnar deviation, 15° to ulnar flexion, pure flexion, radial deviation and 15° to radial extension. Note that the 15° to ulnar flexion-radial extension correspond to motion along the plane depicted by the orange line in Figure 2. A red asterisk (*) denotes significant difference between the intact group and the sRCF and sPCF groups. No significant differences were observed between the sRCF and sPCF groups for any of these directions. Error bars represent one standard deviation of the mean.

Figure 6

Mean values for wrist motion for the intact, sRCF and sPCF groups are plotted relative to the directions of wrist motion to depict the overall motion envelope for each group. Note the substantial overlap of envelopes between the sRCF and sPCF groups. Colored arrows depict the mechanical axes of the intact (red), sPCF (blue), and sRCF (green) groups, oriented 26.6°, 22.2° and 20.4° from the anatomic flexion-extension axis, respectively (solid colored lines).