The Bones and Joints of the Elbow and Forearm
The elbow and the forearm are made up of only three bones and two joints. Half of the elbow is formed by the humerus, the lone bone of the arm. The forearm contains two bones; the radius is on the lateral side of the forearm and the ulna is on the medial side.
The humerus: The head of the humerus forms the glenohumeral joint of the shoulder with the scapula. The distal end of the humerus (at the elbow) has two prominent bumps: the medial and lateral epicondyles. Medial and lateral supraepicondylar ridges follow the shaft of the humerus down to the epicondyles.
The condyle of the humerus forms the elbow joint. The condyle has a capitulum that articulates with the head of the radius, and the trochlea articulates with the trochlear notch of the ulna. The coronoid fossa is an indentation on the anterior part of the condyle, which leaves room for the coronoid process of the ulna when you flex the elbow. The radial fossa is a shallow indentation that accommodates the head of the radius. The olecranon fossa is a similar indentation located on the posterior part of the condyle, which leaves room for the olecranon of the ulna when the elbow is straightened out.
The radius: The head of the radius has a concave top that articulates with the capitulum of the humerus. It also articulates with the radial notch of the ulna. The neck of the radius is the narrow portion between the head of the radius and the shaft of the radius. The shaft widens as it gets closer to the wrist. The radial tuberosity is distal to the neck of the radius.
The distal radial styloid process articulates with the wrist.
The ulna: The proximal end of the ulna has two large projections: The olecranon is on the posterior surface, and the coronoid process is on the anterior surface. The space between the two projections is called the trochlear notch. The trochlea of the humerus fits into the trochlear notch.
The tuberosity of the ulna is located distal to the coronoid process. The radial notch is a shallow depression found on the lateral side of the coronoid process. The supinator crest (prominent ridge) and supinator fossa (indentation) are located distal to the coronoid process. The shaft of the ulna is thicker near the elbow and gets thinner as it moves toward the wrist.
The three bones of the elbow and forearm form two joints in the proximal forearm. The elbow is the main joint that most people can easily recognize. It involves all three bones. The lesser known and somewhat hidden second joint is the radioulnar joint, which involves (can you guess?) the radius and ulna.
The elbow: The elbow joint is a synovial hinge joint with two articulations. The trochlea of the humerus articulates with the trochlear notch of the ulna, and the capitulum of the humerus articulates with the head of the radius. The elbow joint is lined by a synovial membrane and is surrounded by a fibrous joint capsule. The joint capsule is supported by collateral ligaments:
Radial collateral ligament: This ligament runs from the lateral epicondyle of the humerus to the anular ligament of the radius.
Ulnar collateral ligament: This ligament runs from the medial epicondyle of the humerus to the coronoid process and the olecranon of the ulna. It has three parts: the anterior, posterior, and oblique bands.
The collateral ligaments and the hinge shape of the elbow joint allow you to flex and extend the elbow. The elbow joint is innervated by the musculocutaneous, radial, and ulnar nerves.
The radioulnar joints: Two separate joints exist between the radius and the ulna. The distal radioulnar joint is down by the wrist. The proximal radioulnar joint is a synovial pivot joint that allows the head of the radius to move as it articulates in the radial notch of the ulna. It’s lined with a synovial membrane and covered with a fibrous joint capsule. The anular ligament of the radius surrounds the head of the radius and holds it in the radial notch. It allows for pronation and supination of the forearm.