The upper limb, or arm, is comprised of three bones: the humerus, radius, and ulna. The upper limb articulates with the axial skeleton at the shoulder joint. The humerus articulates with the radius and ulna at the elbow. Finally, the arm articulates with the carpal bones of the hand at the wrist.
Below are complete specimens of each of the listed bones.
The lateral margin of the humeral shaft bears the insertion of the deltoid muscle, the deltoid tuberosity, near midshaft. The shaft of the humerus is circular in cross section near the proximal end, but becomes more triangular (with a ventrally pointing apex) near the distal end.
The distal end of the humerus articulates with the radius and the ulna to form the elbow. The lateral and medial aspects of the distal humerus each possess a non-articulating epicondyle, which serve as muscle attachment sites. The lateral epicondyle serves as the insertion point for many forarm extensors, while the more robust medial epicondyle provides the insertion point for many forearm flexors. The distal aspect of the humerus bears a convex capitulum for receiving the circular head of the radius and a spool-shaped trochlea for receiving the hooked proximal end of the ulna. The trochlea is located medial to the capitulum. The coronoid fossa, located on the ventral aspect of the distal humerus just proximal to the margin of the trochlea, receives the coronoid process of the ulna during extreme elbow flexion. During extreme elbow extension the much larger olecranon fossa, located on the dorsal surface, receives the olecranon process of the ulna.
Siding the Humerus
Proximal humeri can usually be easily sided because the massive head faces medially, the lesser tuberosity is ventral, while the greater tuberosity is lateral. For distal humeri, the olecranon fossa is on the dorsal aspect, the medial epicondyle is much larger than the lateral epicondyle, and the trochlea is medial to the capitulum.
The proximal end of the ulna bears a large and flat olecranon process, that has includes the insertion of the triceps brachii muscle on the posterior aspect which extends the forearm. The olecranon process fits into the olecranon fossa of the humerus during elbow extension. The half-moon shaped trochlear notch is just distal to the olecranon. The notch articulates with the trochlea of the humerus. The articular surface of the notch has a characteristic wedge shape consisting of medial and lateral articular portions divided by a guiding ridge. The movements of the ulna are more restricted than that of the radius because the wedged shape of the trochlear notch interlocks with the trochlea to restrict rotation of the ulna about its long axis during elbow flexion. The distal margin of the trochlear notch forms a pointed coronoid process which is received by the coronoid fossa of the humerus during elbow flexion. The radial notch is located on the lateral border of the coronoid process where the radius articulates. Immediately distal to the radial notch is a rather sharp supinator crest. Just distal to the coronoid process is the roughened surface of the ulnar tuberosity, which provides the insertion point for the brachialis muscle.
The ulnar shaft tapers distally and is highly irregular in cross-section. The lateral margin of the shaft bears a long and sharp interosseous crest.
The ulnar shaft terminates distally in a round head on the lateral and distal margin, which articulates with the ulnar notch of the radius. The dorso-medial margin of the distal ulna possesses a distally projecting styloid process which attaches to the ulnar collateral ligament.
Siding the Ulna
For proximal ulnae, the radial notch is on the lateral margin, and the olecranon process is on the dorsal and distal margin. For the shaft, the interosseous crest is lateral. The ventral shaft surface is more concave than the convex dorsal shaft surface. For distal ulnae, the styloid process is dorsal and medial, while the head is ventral and lateral.
The radius is the lateral bone of the forearm. Along with the ulna, it articulates proximally with the humerus to form the elbow, and distally with the carpal bones to form the wrist. The radius also articulates with the ulna both proximally and distally.
The proximal end of the radius bears a round head with a concave depression, which articulates with the capitulum of the humerus. During flexion at the elbow, the cupped end of the radius slides superiorly along the capitulum. It is the circular head of the radius, rotating about the capitulum, which allows the distal end of the radius to cross medially over the ulna during pronation (see animation). The ring which borders the articular portion of the radial head is known as the articular circumference. The articular circumference is proximo-distally widest on the medial side where it articulates with the radial notch of the ulna.
The shaft of the radius is roughly circular in cross-section just distal to the head. This region is referred to as the neck of the humerus. Distal to the neck, the radial tuberosity bulges out ventro-medially to serve as the insertion for the biceps brachii muscle. The medial margin of the radial midshaft bears a very sharp interosseous crest. This crest serves as the attachment site for the interosseous membrane which divides the forearm musculature into ventral (flexor) and dorsal (extensor) compartments. The shaft flares medio-laterally as it approaches the distal end of the radius.
The dorsal aspect of the distal radius bears a large dorsal tubercle. The styloid process on the lateral aspect projects sharply distally. The medial aspect possesses a rounded ulnar notch which articulates with the distal ulna. The distal aspect of the distal radius bears two smooth articular surfaces for carpal bones. The medial surface articulates with the lunate, while the dorsal surface articulates with the scaphoid.
Siding The Radius
For proximal radii, the widest and flattest portion of the articular circumference is medial, as is the radial tuberosity. For shaft fragments, the interosseous margin is medial. Distinguishing dorsal from ventral is more difficult, but the ventral aspect of the shaft is more flattened, while the dorsal aspect is slightly concave as it approaches the rounded lateral shaft margin. For distal radii the styloid process is on the lateral side, the ulnar notch is on the medial side, and the dorsal turbercle is on the dorsal side.