Apparent Proximal Ulna Dorsal Angulation Variation Due to Ulnar Rotation.

OBJECTIVE

To investigate the effect of ulna rotation on the apparent proximal ulna dorsal angulation (PUDA).

METHODS

Computed tomography images of 59 ulnas were included in this study, 48 being bilateral specimens and the remaining 11 were unilateral. Three-dimensional models of the entire ulna were obtained, and the ulnas were rotated in 5-degree increments in internal rotation or external rotation from neutral. PUDA, PUDA apex, varus angulation, and varus apex were measured on each ulna.

RESULTS

With the ulna in neutral rotation, the mean (95% CI) PUDA was 3.7 (2.9-4.5) degrees, whereas the mean varus angle was 10.5 (9.8-11.1) degrees. The varus angle apex and PUDA apex were 28.9 (27.5-30.2)% and 19.6 (18.7-20.6)% along the total length of the ulna, respectively. As the ulna was rotated externally by 5, 10, and 15 degrees, the PUDA increased by 0.7 (0.5-0.9) degrees, 1.2 (0.9-1.4) degrees, and 1.4 (1.1-1.8) degrees, respectively. Conversely, with internal rotation of 5, 10, and 15 degrees, the PUDA decreased by 0.9 (0.8-1.1) degrees, 2.0 (1.8-2.3) degrees, and 3.3 (2.7-3.9) degrees, respectively.

CONCLUSIONS

This study demonstrates that small degrees of ulna rotation result in a statistically significant change in the apparent PUDA; however, this may not represent a clinically significant difference. Because of the anatomic variation between patients, it is important to obtain a contralateral film to determine the PUDA for anatomic reduction of the ulna in complex cases. When using a contralateral image, it is important to obtain a true lateral film or consider using 3-dimensional imaging for preoperative planning.