Severity of scapholunate instability is related to joint anatomy and congruency

Abstract

Purpose: To determine whether the bony architecture of the distal radius and proximal scaphoid have a role in stabilizing the scaphoid, and to determine whether a relationship between the bony geometry measurements and the amount of wrist constraint could be determined.

Methods: Eight cadaver wrists were tested in a wrist joint motion simulator. The level of scapholunate instability after sectioning the scapholunate interosseous, radioscaphocapitate, and the scaphotrapezium ligaments was determined and related to radiographic measurements of volar tilt, lateral tilt (ulnar tilt of the radioscaphoid fossa), the depth of the radioscaphoid fossa, and 6 radii of curvature measurements of the proximal scaphoid and distal radius. The force to dorsally dislocate the scaphoid out of the radioscaphoid fossa was computed.

Results: The radioscaphoid fossa and scaphoid curvatures were larger in those wrists that did not show gross instability after ligamentous sectioning in the wrist simulator. Similarly, those wrists with a deeper radioscaphoid fossa and greater volar tilt were also more stable. The force required to dislocate these wrists was greater than in those wrists that showed gross carpal instability.

Conclusions: This study suggests that the bony anatomy of the radius and scaphoid have a role in stabilizing the carpus after ligament injury. The effect of ligament sectioning on producing carpal instability may be moderated by the bone geometry of the radiocarpal joint. This may explain why some people may have a tear of the scapholunate interosseous ligament but not present with clinical symptoms.

Figure 1

Scaphoid clunk. (A) Lateral view of the scaphoid at 20° of wrist extension with the wrist moving from neutral to extension, with all ligaments intact. (B) Corresponding transverse plane view of the scaphoid shown in (A). (C) Lateral view of the same scaphoid with the wrist at 20° extension, but with the SLIL, RSC, and ST ligaments sectioned. The scaphoid has flexed, moved dorsally, and radially compared to the intact state, and is now on the dorsal rim of the radius. (D) Corresponding transverse view of the scaphoid shown in (C). As the wrist and scaphoid continue to extend, the scaphoid “snaps” back into the radioscaphoid fossa, approximately 0.04 seconds later in the cadaver wrist motion: (E) lateral view; (F) corresponding transverse view.

Figure 2

The RS fossa radius of curvature computation. (A) Three-dimensional CAD model showing a virtual sagittal plane cut in the distal radius though the RS fossa. (B) Sagittal plane section showing the RS fossa surface and points located on the surface from which the radii of curvature is determined.

Figure 3

Scaphoid sagittal plane curvature computation. Sagittal plane section showing the result of a virtual cut through the scaphoid and the points located on the surface from which the radii of curvature is determined.

Figure 4

Determination of the dislocation force of the scaphoid. (A) Sagittal view of the radius and scaphoid under no compressive or dorsal loading. (B) Scaphoid has dislocated and is sitting on the dorsal rim of the radius. A 200 N compressive force was applied to simulate wrist grip.

Figure 5

Determination of the dorsal/volar constraint of the scaphoid. After a 200 N compressive force was applied to simulate wrist grip, first a 50 N dorsal force and then a 50 N volar force was applied. The displacement of the scaphoid as a result of the 50 N dorsal force was summed with the displacement of the scaphoid as a result of the 50 N volar force to compute the dorsal/volar constraint of each scaphoid.