The ligament that stabilizes and encircles the head of the radius is the:

Question

BlackTom AI · Accepted Answer

The annular ligament encircles and holds the head of the radius in the radial notch of the ulna, so the correct choice is a. Annular ligament.

1) Relevant knowledge points and concepts
- The proximal radioulnar joint consists of the head of the radius articulating with the radial notch of the ulna.  
- The annular ligament attaches to the anterior and posterior margins of the radial notch of the ulna and forms a fibrous ring around the radial head.  
- This ring allows the radius to rotate within it during pronation and supination while preventing dislocation.  
- Other nearby ligaments (quadrate, lateral collateral, medial collateral) have different attachments and functions.

2) Step-by-step reasoning
- Identify which structure is described: “stabilizes and encircles the head of the radius” implies a ring-like ligament surrounding the radial head.  
- Recall which ligament forms a ring: the annular ligament is anatomically a loop around the radial head, securing it to the ulna’s radial notch.  
- Check alternatives against the anatomy (below) to confirm they do not match the description.

3) (No numerical calculations required) Anatomical intermediate steps
- Annular ligament: attaches to anterior/posterior borders of radial notch → forms ring → permits rotation, prevents displacement.  
- Quadrate ligament: small band from neck of radius to ulna, provides minor stability but does not encircle the head.  
- Lateral collateral (radial collateral) ligament: runs from lateral epicondyle of humerus to the annular ligament area, stabilizes lateral elbow, not a ring.  
- Medial collateral (ulnar collateral) ligament: from medial epicondyle to ulna, stabilizes medial elbow, not encircling the radius.

4) Why the correct answer is correct
- The annular ligament’s defining feature is that it literally encircles the radial head and stabilizes the proximal radioulnar joint while allowing rotation; that matches the question statement exactly.

5) Why the other options are incorrect (brief)
- Quadrate ligament: supports joint but is a short band, does not encircle the radial head.  
- Lateral collateral ligament: stabilizes the elbow laterally, not a circumferential ring around the radius.  
- Medial collateral ligament: stabilizes the medial elbow (ulnar side), unrelated to encircling the radial head.

6) Conclusion
The anatomy and function described identify the annular ligament as the correct structure. Final answer: a. Annular ligament.

The ligament that stabilizes and encircles the head of the radius is the:单项选择题

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