Conformational changes in the carpus during finger trap distraction

Abstract

Purpose: Wrist distraction is a common treatment maneuver used clinically for the reduction of distal radial fractures and midcarpal dislocations. Wrist distraction is also required during wrist arthroscopy to access the radiocarpal joint and has been used as a test for scapholunate ligament injury. However, the effect of a distraction load on the normal wrist has not been well studied. The purpose of this study was to measure the three-dimensional conformational changes of the carpal bones in the normal wrist as a result of a static distractive load.

Methods: Using computed tomography, the dominant wrists of 14 healthy volunteers were scanned at rest and during application of 98 N of distraction. Load was applied using finger traps, and volunteers were encouraged to relax their forearm muscles and to allow distraction of the wrist. The motions of the bones in the wrist were tracked between the unloaded and loaded trial using markerless bone registration. The average displacement vector of each bone relative to the radius was calculated, as were the interbone distances for 20 bone-bone interactions. Joint separation was estimated at the radiocarpal, midcarpal, and carpometacarpal joints in the direction of loading using the radius, lunate, capitate, and third metacarpal.

Results: With loading, the distance between the radius and third metacarpal increased an average of 3.3 mm +/- 3.1 in the direction of loading. This separation was primarily in the axial direction at the radiocarpal (1.0 mm +/- 1.0) and midcarpal (2.0 mm +/- 1.7) joints. There were minimal changes in the transverse direction within the distal row, although the proximal row narrowed by 0.98 mm +/- 0.7. Distraction between the radius and scaphoid (2.5 mm +/- 2.2) was 2.4 times greater than that between the radius and lunate (1.0 mm +/- 1.0).

Conclusions: Carpal distraction has a significant (p < .01) effect on the conformation of the carpus, especially at the radiocarpal and midcarpal joints. In the normal wrist, external traction causes twice as much distraction at the lunocapitate joint than at the radiolunate joint.

Figure 1

The vectors (light red) representing the centroid (red sphere) displacement for all subjects with distraction loading. The vectors representing the average displacement with loading are shown in green and are also visualized separately in Figure 2. The radius, ulna, scaphoid, lunate and capitate are those of a single subject's wrist in a volar (left) and radial (right) view.

Figure 2

The average displacement vectors (green) of the centroids (red sphere) after loading. The displacement vectors and centroids are visualized with the bones of a single subject in volar (left) and radial (right).

Figure 3

Volar view of a single wrist in the unloaded (left) and loaded (right) state in which separation at the radiocarpal and midcarpal joints is visible. The centroid of each bone is shown (red sphere). The centroid distances that were analyzed are represented as lines connecting the two respective centroids.

Figure 4

Distractive loading caused a significant (*) increase in the centroid distances at the radioscaphoid, radiolunate and lunocapitate joints. There was a non-significant increase in the interbone distance at the 3^rd carpal-metacarpal joint.