The anteromedial facet of the coronoid process of the ulna.

Fracture of the anteromedial facet of the coronoid process has been recognized as an important type of coronoid fracture. We performed a quantitative analysis of 21 3-dimensional computed tomography scans to evaluate the degree to which the anteromedial facet protrudes as a distinct process separate from the proximal ulnar metaphysis. The distance between the center axis of the trochlear notch and the most medial edge of the anteromedial facet averaged 12.5 mm (range, 8.7-20.1 mm). The part of the maximum anteromedial facet width that was supported by the proximal ulnar metaphysis and diaphysis averaged 5.4 mm (range, 1.7-11.5 mm). On average, 58% of the anteromedial facet (range, 26%-82%) was unsupported by the proximal ulnar metaphysis and diaphysis. It is not surprising that this relatively vulnerable protrusion from the anteromedial facet of the coronoid is frequently a separate fracture fragment in complex traumatic elbow instability.