Dorsal apex curve of the proximal ulna

To analyze the apexes and angulations along the curvature in the posterior border of the proximal ulna-termed the "dorsal apex curve" (DAC)-using the computer-aided design software.

Eight pairs of normal cadaveric ulnae were analyzed. The ulnar Cartesian coordinate system was standardized using a user-defined coordinate systems feature. DAC was defined by the best-fit curvature of serial apexes in the posterior border in axial cross-sections along the ulna, whereas the best-fit curvature was obtained using polynomials interpolation method. DAC apexes in three different planes were determined by calculating the second derivative value of curve function, and subsequently, DAC angulations were calculated. Statistical analysis was performed using analysis of variance with repeated measures with significance level set as 0.001.

The average ulnar length was 246±15 mm (224-274 mm). The average ratios of dorsal apex, varus apex, and edge point were 23±3% (17-27%), 33±4% (27-43%), and 10±1% (8-13%), respectively, relative to the ulnar length. The average amount of varus, dorsal, hook, and torsion angulation were 167±4° (157-172°), 176±1° (175-178°), 90±0°, and 31±10° (15-49°), respectively. We found no significant differences between left and right ulnae or between male and female ulnae.

Dorsal apexes and angulations are important landmarks for surgeons when applying plates and attempting total elbow replacement surgery. This curve provides valuable information to medical manufacturers for modeling both plates and ulna components of artificial elbow joints.