Morphometric and curvature CT-based study of the distal radius watershed line.

Purpose: Fixation of distal radius fractures involving the volar rim is technically demanding and often complex. In most cases, it requires the use of so-called "specific" plates. Although these plates have been developed using morphometric databases, proper application can still be imperfect-even when the plate appears to be correctly positioned. This mismatch may result in secondary displacement of the fragment, tendon irritation, or even tendon rupture. We hypothesized that anatomical variations in the radius, particularly in the shape of the watershed line, may explain the difficulty in achieving optimal plate adaptation in some patients.

Methods: Nineteen distal radius were analyzed using Computed Tomography-scan segmentation and curvature analysis to assess the shape of the watershed line. K-means clustering was then performed to identify distinct groups based on volar rim curvature patterns.

Results: Clustering analysis revealed two distinct anatomical groups based on volar rim curvature. The first group exhibited a mean curvature of 0.07 ± 0.03 mm^-¹, while the second group had a significantly higher curvature of 0.23 ± 0.06 mm^-¹ (mean ± SD). A Student's t-test confirmed a statistically significant difference between the two groups (p < 0.001).

Conclusions: Our findings suggest the existence of at least two anatomical variations in volar rim shape at the watershed line, forming a spectrum between flatter and more sharply curved forms. These anatomical differences may explain inconsistencies in plate adaptation and should be taken into account by surgeons when selecting and positioning fixation hardware.

Level of evidence: Diagnostic study (IIIb).