Volar Lunate Facet Fractures of the Distal Radius: Fracture Mapping Using 3D CT Scans

Abstract

Background Fractures of the distal radius involving the lunate facet at the volar articular surface are unstable injuries and are usually managed operatively. Management of these fractures is challenging as our understanding of the exact fracture characteristics and associated injuries to the carpus is poor. Purpose This study aims to define the anatomy and associated injuries of lunate facet fractures using three-dimensional computed tomography (CT) scans and fracture mapping techniques. Methods A consecutive series of CT wrists was analyzed to identify intra-articular fractures involving the lunate facet at the volar distal radius. Fractures were mapped onto standardized templates of the distal radius using previously described fracture mapping techniques. We also identified instabilities of the carpus including volar carpal translation, ulnar translocation, scapholunate diastasis, and distal radioulnar joint (DRUJ) instability. Results We present 23 lunate facet fractures of the distal radius. The lunate facet fragment displaces in a volar and proximal direction and the lunate always articulates with the displaced fragment. The smaller fragments displace a greater amount, in a volar direction, with pronation. The fracture tends to occur between the origin of the short and long radiolunate ligaments. Conclusion Lunate facet fractures are frequently comprised of osteoligamentous units of the distal radius involving the short and long radiolunate ligaments and the radioscaphocapitate ligament. Assessment and management of volar carpal subluxation, scapholunate instability, ulnar translocation, and DRUJ instability should be considered. Clinical relevance Our mapping of these fractures contributes to our understanding of the anatomy and associated instabilities and will aid in surgical planning and decision making.

Keywords: carpal instability; classification; distal radius; fracture mapping; lunate facet fracture.

Fig. 1

Mapping process—Process of the fracture mapping of the distal articular surface. The most distal axial computed tomography (CT) image is chosen through the distal radius. The fracture line of the lunate facet fragment is defined according to its percentile position at the sigmoid notch and volar articular surface (in this example the 25th percentile at the sigmoid notch and the 61st percentile at the volar cortex). The fracture is then traced onto the template. The remaining fracture lines are then traced onto the template. The translucent red is then applied to the fractured area of the distal radius. Finally, additional cases are superimposed to create the fracture map. The areas of translucent red become increasingly opaque as they are overlapped. On the final fracture map, the darker areas of red therefore indicate more frequently fractured areas.

Fig. 2

Displacement—Axial computed tomography (CT) image demonstrating the method of measurement for displacement at the sigmoid notch (white) and maximal volar displacement of the lunate facet fragment (white). The volar and dorsal distal radioulnar joint (DRUJ) intervals (yellow) were measured as the shortest distance from the ulna to the edge of the sigmoid notch. Percentiles—The location of the fracture lines of the lunate facet fragment were given as percentiles of the length of the sigmoid notch and the length of the volar cortex. At the sigmoid notch, the percentile was calculated as A/(A + B). At the volar cortex, the percentile was calculated as C/(C + D).

Fig. 3

Lunate facet subsidence—Sagittal computed tomography (CT) image demonstrating the method of measurement for lunate facet subsidence. The sagittal image demonstrating the maximal subsidence of the lunate facet was identified. A line is drawn along the longitudinal axis of the radius (A). A point is marked on the intact articular surface at the fracture (B) and on the lunate facet fragment at the fracture (C). The longitudinal distance between these points is measured (D) and defined as the lunate facet subsidence. Volar cortex length—Sagittal CT image demonstrating the method measurement for the length of the volar cortex. The sagittal image where the lunate facet fragment extends most proximally is used. The distance from the most proximal extent of the fragment to the most volar extent of the fragment is measured as the length of the volar cortex.

Fig. 4

Pronation angle—The lunate facet fragment is isolated and moved to represent its anatomical position. The angle between its anatomical position and fractured position is measured the pronation angle.

Fig. 5

Carpal translation—Sagittal computed tomography (CT) image demonstrating the measurement for carpal translation. A line is drawn along the volar cortex of the radius (A). The sagittal image is then chosen that identifies the most proximal point of the lunate (B). A perpendicular line is measured from A to B, this is defined as the volar carpal translation.

Fig. 6

Scapholunate interval—Axial and coronal computed tomography (CT) images demonstrating the measurement for S-L interval at the volar (yellow) and dorsal aspect (white). The method used was described by Sun et al.

Fig. 7

Lunate uncovering—Computed tomography (CT) images demonstrating the measurement for lunate uncovering. The coronal image through the mid-portion of the lunate is chosen and the most ulnar (A) and most radial (B) points of the lunate are identified. The distance between them is measured in the plane perpendicular to the long axis of the radius (C). Point A is then translated dorsally to a coronal image through the intact dorsal portion of the sigmoid notch. The distance from point A to the sigmoid notch is then measured (D). The lunate uncovering is calculated as the radio of D/C.

Fig. 8

Fracture maps showing all 23 cases superimposed onto the three standardized templates of the distal radius. Additional fracture maps show only the lunate facet fragment in each case.

Fig. 9

Fracture classification—Fracture maps divided into the four defined classification types. The fracture maps also demonstrate the position of radiocarpal ligaments around the articular surface (SRL, short radiolunate ligament; LRL, long radiolunate ligament; RSCL; radioscaphocapitate ligament; DRC, dorsal radiocarpal ligament). Type 1: Fractures that involved only the SRL attachment, that is, a single primary osteoligamentous unit. Type 2: Fractures that involved the attachments of the SRL and LRL, that is, two osteoligamentous units. Type 3: Fractures that involved the attachments of the SRL as one osteoligamentous unit and the LRL + RSCL as a separate distinct osteoligamentous unit. Type 4: Comminuted fractures that extended toward the dorsal rim incorporating the SRL, LRL, and RSCL.