The Optimal Location to Measure Scapholunate Diastasis on Screening Radiographs

Abstract

Background: Lack of a universally accepted location and normal value limits the utility of measuring scapholunate diastasis. The primary purpose of this study was to define the optimal location to measure the scapholunate gap throughout sequential ligament transections. Secondary purposes were to compare plain radiographs with fluoroscopy and to evaluate interrater reliability in measuring scapholunate diastasis.

Methods: Five cadaver forearms were imaged with intact carpal ligaments and after sequentially transecting the scapholunate, radioscaphocapitate, and scaphotrapezium-trapezoid ligaments. Plain radiographs and static fluoroscopic images were obtained with wrists in neutral and 30° ulnar deviation for each stage. Multiple reviewers performed measurements of the scapholunate interval at 3 separate locations. Mean distances were calculated and pairwise comparisons between groups were made. Intraclass correlation was calculated to determine interrater reliability.

Results: Overall, measurements made in the middle of the scapholunate joint had the smallest margins of error for all imaging modalities, ligament disruptions, and wrist positions. For normal wrists, the mean scapholunate measurements were all less than 2.0 mm at the middle of the joint, regardless of imaging modality or wrist position. Fluoroscopy detected significance between more stages of instability than plain radiographs at the middle of the joint.

Conclusions: Measurements in the middle of the scapholunate joint in neutral and 30° of ulnar deviation under fluoroscopic imaging best capture all stages of ligamentous disruptions. Measurements less than 2.0 mm at the middle of the scapholunate interval may be considered within normal range.

Keywords: carpal instability; dorsal intercarpal; dorsal radiocarpal; radioscaphocapitate; scapholunate; scaphotrapezium-trapezoid.

Figure 1.

Illustrations of cadaver positioning. Note. (a) Each specimen was fixed to a wooden base using headless pins in the radius, ulna, and third metacarpal as shown. (b) Ulnar deviation was achieved by translating the third metacarpal pin to a position in the base set at 30° to the radius shaft. A 2.6-cm Kirschner wire was placed within the field to calibrate digital measurements between all images.

Figure 2.

Illustrations of cadaver dissections. Note. (a) The standard dorsal approach to the wrist is performed with retraction of the extensor digitorum comminus (EDC) and extensor pollicis longus (EPL) tendons. A ligament-preserving dorsal capsulotomy is shown by the dashed lines,^, (b) elevation of the capsular flap and transection of the scapholunate interosseous ligament (SLIL) is performed, (c) the radioscaphocapitate (RSC) ligament was transected between the radius and scaphoid, and (d) finally, the scaphotrapezium-trapezoid (ST) ligament was transected. DIC = dorsal intercarpal ligament; C = capitate; Td = trapezoid; Tm = trapezium; L = lunate; S = scaphoid; DRC = dorsal radiocarpal ligament; R = radius.

Figure 3.

Plain radiograph demonstrating example locations for proximal (blue), middle (green), and distal (yellow) measurements in an intact scapholunate joint in neutral wrist position.

Figure 4.

(a-d) Graphs depicting mean measurements ± standard deviation for neutral and ulnar deviation radiographs and for neutral and ulnar deviation fluoroscopy. Note. Changes in measurements at each of the 3 locations are shown against each ligament dissection. SL = scapholunate interosseous ligament; RSC = radioscaphocapitate ligament; ST = scaphotrapezium-trapezoid ligament.