The flat spot of the proximal ulna: a useful anatomic landmark in total elbow arthroplasty.

The relationship between the flat subcutaneous dorsal surface (flat spot) of the ulna and the anatomic flexion axis of the elbow was investigated in 68 cadaveric ulnae. The flat spot and the guiding ridge of the greater sigmoid notch were digitized with an electromagnetic tracking device. The area of the flat spot, as well as its angle with the plane of the greater sigmoid notch, was calculated. The size of the flat spot was variable but was found to be nearly perpendicular to the plane of the greater sigmoid notch (mean, 89.8 degrees +/- 6.5 degrees ). This indicates that the subcutaneous dorsal surface of the proximal ulna is a useful landmark to assist the surgeon intraoperatively to orient the ulnar component axially in total elbow arthroplasty.