The forcible separation of the head of the radius from the capitulum of the humerus is mainly prevented by the:
High-Yield Explanation
The radial notch of the ulna and the annular ligament form a ring in which the head of the radius rotates at its aiculation with the capitulum of the humerus. The annular ligament narrows over the neck, cupping the head of the radius and preventing downward dislocation. Ref: Moon D.A., Foreman K.B., Albeine K.H. (2011). Chapter 31. Arm. In D.A. Moon, K.B. Foreman, K.H. Albeine (Eds), The Big Picture: Gross Anatomy.