The Watershed Line of the Distal Radius: Cadaveric and Imaging Study of Anatomical Landmarks

Abstract

Background Placement of volar plates remains a challenge as the watershed line may not be an easy-identifiable distinct line intraoperatively. Objectives The main objective of this article is to define how anatomical landmarks identifiable upon the volar surgical approach to the distal radius relate to the watershed line. Methods We identified anatomical landmarks macroscopically upon standard volar approach to the distal radius in 10 cadaveric forearms and marked these with radiostereometric analysis (RSA) beads in cadaveric wrists. The RSA beads were then referenced against the volar osseous structures using quantification of three-dimensional computed tomography and advanced imaging software. Results The mean measurements were the radial and ulnar prominences 11.1 mm and 2.1 mm proximal to the joint line of the distal radius, respectively. The interfossa sulcus was 0.3 mm proximal and 3 mm dorsal to the ulnar prominence. The watershed line was between 3.5 (minimal) and 7.6 (maximal) mm distal to the distal line of insertion of the pronator quadratus. Conclusion The watershed line is situated distal to the pronator quadratus, but with a wide variability making it an impractical landmark for plate position. The osseous ulnar prominence is a good anatomical reference for safe plate positioning, as it is located on the watershed line and easily palpated at surgery. One should keep in mind the sulcus-the point on the watershed line where the flexor pollicis longus runs-can be situated just proximal to the ulnar prominence. Clinical Relevance To provide anatomical landmarks that are easy to identify upon surgical approach without the direct need for intraoperative imaging.

Keywords: Q3DCT imaging; anatomy; distal radius fracture; volar approach; volar plating; watershed line.

Fig. 1

Nelson and Orbay coined the watershed line (WS) as “a theoretical line marking the most volar aspect of the volar margin of the radius,” to serve as the distal margin for volar plating to minimize tendon injuries. PQ, pronator quadratus line, or PQ line; VR, volar radial ridge; WS, watershed line; X, volar radial tuberosity; (with permission from http://eradius.com/AnatomyOfDistalRadius.htm 3 ).

Fig. 2

Anatomical landmarks of the volar cadaveric distal radius marked with radiostereometric analysis (RSA) beads ( A ). Cadaveric specimen with subsequent RSA-marked anatomical landmarks ( B ) was referenced using advanced imaging software ( C ) with the volar osseous references of the watershed line (small dots).

Fig. 3

Three-dimensional polygon mesh reconstructions of the radius. ( A ) The most volar aspect of the radius (small dots) and the radial and ulnar osseous prominences (dark vague dots), and interfossa sulcus (light vague dot) are marked. In ( B ) the vertical line shows proximal–distal measurements performed parallel to the x-axis.

Fig. 4

The x-axis represents proximal to distal, the y-axis radioulnar, and the z-axis dorsal-to-volar. The x-axis is aligned with the longitudinal axis of the radius as defined by the line through the centroid of the five circumferences along the longitudinal axis of the radial shaft ( A ). The y-axis was from the tip of the radial styloid (gray dot) and the most radial aspect of the ulnar notch (white dot) ( B ). The z-axis resulted from these x- and y-axes ( C ).

Fig. 5

Distal radius landmarks : ( A ) The ulnar and radial prominences are the two most prominent volar points. ( B ) The bone of the interfossa sulcus was defined by drawing a line between the ulnar and radial prominence and determining the point of the volar margin of the distal radius that was furthest away from this line. ( C ) The point on the joint line directly distal to the radial prominence was defined using the vertex and intersection functions. ( D ) In anteroposterior view, perpendicular to the x-axis, the most ulnar point of the volar margin of the distal radius was marked.

Fig. 6

Proximal–distal distances soft tissue—osseous anatomy ( A ) and osseous anatomy ( B ). Ulnar–radial distances ( C ). Volar–dorsal distances ( D ). 1. Radial prominence—PQ 2. Min. Watershed—PQ 3. Interfossa sulcus—PQ 4. Ulnar prominence—PQ 5. Max. Watershed—PQ 6. Sulcus—radial prominence 7. Ulnar prominence—sulcus 8. Ulnar notch—ulnar prominence 9. Joint line—radial prominence 10. Ulnar prominence—sulcus 11. Joint line—Ulnar prominence 12. Ulnar prominence—Radial prominence 13. Ulnar prominence—Sulcus. 14. Sulcus—radial prominence.

Fig. 7

Distal radius with mapping of the watershed line (light lines) and the pronator quadratus (PQ) (dark lines) of all 10 specimens. Note the wide range, which makes the PQ position an unreliable surgical reference.