Distal radius plate: correlation between the length of distal epiphyseal screws and the width of the watershed line.

BACKGROUND: Accurate distal screw length selection during volar plating of distal radius fractures is important to provide subchondral support while reducing the risk of dorsal cortical penetration. The width of the watershed line may provide an anatomical guide for screw selection. METHODS: Fifty-one wrist CT scans (35 men, 16 women) were retrospectively analyzed. Watershed line width and distal epiphyseal screw lengths (B1-B5) were measured after virtual placement of fixed-angle volar plates. The relationship between watershed line width and screw length was assessed using linear regression. Conservative screw-length recommendations were derived from the 10th percentile of the observed distributions and rounded down to the nearest available 2-mm screw length. RESULTS: Mean watershed line width was 28.6 ± 2.6 mm. Mean screw lengths ranged from 18.5 ± 2.2 mm for B4 to 21.9 ± 1.6 mm for B3. Significant positive correlations were found between watershed line width and screw length for B1 to B4 (r = 0.46-0.65; all p ≤ 0.001), but not for B5 (r = 0.13; p = 0.65). Screw-length distributions differed between standard and large plate subgroups (all p < 0.005). Proposed conservative screw lengths were 18, 18, 20, and 16 mm for standard plates, and 20, 20, 22, 20, and 18 mm for large plates. CONCLUSION: Watershed line width is correlated with distal epiphyseal screw length in fixed-angle volar distal radius plates and may assist conservative initial screw-length selection according to plate size. LEVEL OF EVIDENCE: IV.