The proximal epiphysis (proximal end or extremity) of the ulna is easily distinguished by its large olecranon process. The proximal end forms two joints. It articulates with the trochlea of the humerus, forming the humeroulnar joint and with the head of the radius, forming the proximal radioulnar joint.

The surface of the proximal ulna is irregular and has several bony prominences for muscle attachments, which enable movements at the elbow joint. The proximal end is larger than the distal, and it tapers towards it (opposite to the radius).

Overall, the proximal epiphysis of the ulna features the following landmarks:

Olecranon - a large, thick, curved bony projection that extends proximally behind the elbow, forming a part of the trochlear notch; it is bent forward at the summit, thus presenting a prominent lip, which is received into the olecranon fossa of the humerus during the extension of the forearm; the olecranon can be palpated as the most pointed portion of the elbow; the superior surface of the olecranon serves as an attachment site for the triceps brachii muscle; overall, the olecranon and the olecranon fossa of the humerus are involved in the flexion and extension of the forearm;
Trochlear notch (semilunar or greater sigmoid notch) - a large depression that is formed as a bridge between the olecranon and the coronoid process; it is wrench-shaped, and the trochlea of the humerus articulates and fits into it, forming a "hinged" humeroulnar component of the elbow joint;
Guiding ridge - a vertical rough line that separates the trochlear notch into the medial and lateral portions;
Coronoid process - a beak-shaped bony protrusion that projects forward from the anterior proximal portion of the ulna at the base of the trochlear notch; the apex of this process is received into the coronoid fossa of the humerus during the flexion of the forearm; its medial surface serves for the attachment of the ulnar collateral ligament; it is also the origin site of the humeroulnar head of the flexor digitorum superficialis and the ulnar head of the pronator teres muscle; finally, it also serves as an attachment site for the brachialis muscle;
Ulnar tuberosity (brachial tuberosity) - a roughened eminence that is found immediately below the coronoid process; a part of the brachialis muscle inserts here;
Radial notch (lesser sigmoid notch) - a narrow, oblong depression located on the lateral aspect of the proximal epiphysis; it is found below the trochlear notch and lateral to the coronoid process; it receives the articular circumference of the head of the radius at the proximal radioulnar joint;
Sublime tubercle - a triangular-shaped small protuberance on the medial aspect of the coronoid process; some fibers of the ulnar collateral ligament insert into this tubercle.

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Ulna (proximal epiphysis)