The radius rotates around the ulna primarily at the proximal radioulnar joint, allowing for pronation and supination of the forearm.
Here's a breakdown of the rotation:
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Proximal Radioulnar Joint: This is a pivot joint located near the elbow where the head of the radius articulates with the radial notch of the ulna.
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Annular Ligament: The radial head is held in place against the ulna by the annular ligament, which encircles the radial head. This ligament allows the radius to rotate within it.
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Rotation Mechanism: During pronation (palm facing down), the radius crosses over the ulna. During supination (palm facing up), the radius and ulna lie parallel to each other. The radial head rotates within the annular ligament and against the radial notch of the ulna to facilitate this movement.
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Distal Radioulnar Joint: While the primary rotation occurs at the proximal joint, the distal radioulnar joint (near the wrist) also plays a crucial role. The ulna remains relatively fixed, while the radius rotates around it.
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Muscles Involved: Several muscles facilitate this rotation, including the pronator teres, pronator quadratus, supinator, and biceps brachii. These muscles contract and pull on the radius, causing it to rotate around the ulna.
In summary, the radial head's unique articulation with the ulna and annular ligament allows it to rotate within the proximal radioulnar joint, enabling pronation and supination of the forearm.
