Morphologic analysis of the proximal ulna with special interest in elbow implant sizing and alignment.

A better understanding of the morphology of the proximal ulna should permit the development of ulnar component designs which have an improved fit to the native bone, thus leading to more accurate implant positioning. Computed-tomography (CT) scans of 31 cadaveric proximal ulnae were analyzed using computer aided design software to determine the shape of the medullary canal relative to the articular surface. The diameter, curvature, cross-sectional centroid position, and coronal and sagittal angulation of the ulnar canal were all calculated with respect to the center of the greater sigmoid notch. Posterior and lateral offsets increased distally from the articulation center, and the mean diameter of the canal was larger in males than in females (P < .05). The average valgus angulation was 8.0 +/- 4.0 degrees for males and 7.2 +/- 3.1 degrees for females (P = .6). Longer stemmed ulnar implants may require a modular design to meet anatomic constraints during implant positioning.