Humerus (The Upper Arm)
The bone forming the upper arm is the humerus.
Describe the anatomy of the humerus
- The humerus articulates with the scapula at the shoulder and with the radius and ulna at the elbow.
- The humerus is typically described as having a proximal region, shaft, and distal region.
- Both the anatomical and surgical neck of the humerus are highly susceptible to fracture.
- humerus: The bone of the upper arm.
- surgical neck: A constriction below the tubercles of the greater tubercle and lesser tubercle.
- capitulum: At the distal head of the humerus, it articulates with the radius of the forearm.
- trochlea: At the distal head of the humerus, it articulates with the ulna of the forearm.
- anatomical neck: A constriction adjacent to the humeral head that increases the range of movement possible at the shoulder joint.
The humerus is a bone in the upper arm. It runs from the shoulder to the elbow. Proximally it articulates with the scapula to form the shoulder joint, or glenohumeral joint. Distally, the humerus articulates with the radius and ulna to form the elbow joint.
The proximal portion of the humerus can be divided into three parts.
- The rounded humeral head projects medially and articulates with the glenoid cavity of the scapula.
- Immediately adjacent to the head is the narrower anatomical neck, which allows for a wider range of movements of the head within the shoulder joint.
- Finally, the greater and lesser tubercles are found at the most superior end of the main shaft of the humerus.
The four rotator cuff muscles attach to these tubercles, strengthening and maintaining the shoulder joint. Between the two tubercles lies a deep grove called the intertubercular sulcus, through which the tendon of the long head of the biceps brachii runs.
Below this proximal region lies the shaft, which is separated from the proximal region by the surgical neck, so termed as this in an area of frequent fracture. A major feature of the shaft is the deltoid tuberosity located laterally to which the deltoid muscle attaches.
The deltoid, corocobrachialis, brachialis, and brachioradialis muscles attach to the anterior surface, with the triceps brachii attaching to the posterior.
Distally, the humerus flattens to articulate with the ulna and radius at the elbow joint. The medially located trochlea articulates with the ulna. Located laterally to this is the capitulum that articulates with the radius.
Several muscles of the forearm responsible for extension at the wrist attach to the humerus immediately above the capitulum and trochlea.
Ulna and Radius (The Forearm)
The forearm contains two bones, the radius and the ulna.
Discuss the radius and ulna
- The radius and the ulna are long, slightly curved bones that lie parallel from the elbow, where they articulate with the humerus, to the wrist, where they articulate with the carpals.
- The radius is located laterally, near the thumb, and the ulna medially, near the little finger. The radius and the ulna have a styloid process at the distal end; they are also attachment sites for many muscles.
- The radius is smaller than the ulna.
- radius: One of two forearm bones, it is located laterally to the ulna.
- ulna: One of two forearm bones, it is located medially to the radius.
The forearm contains two bones—the radius and the ulna—that extend in parallel from the elbow, where they articulate with the humerus to the wrist, where they articulate with the carpals. The space between the two bones is spanned by the interosseous membrane.
Anatomically, the ulna is located medially to the radius, placing it near the little finger. The ulna is slightly larger than the radius.
Proximally, there are five key regions of the ulna:
- The olecranon is a projection of bone that extends proximally from the ulna.
- The triceps brachii muscle attaches to the ulna superiorly.
- The cornoid process, together with the olecranon, forms the trochlear notch where it articulates with the trochlea of the humerus.
- Laterally to the trochlear notch lies the radial notch, which articulates with the head of the radius to form the proximal radioulnar joint.
- Immediately distal to the coronoid process is the tuberosity of ulna, to which the brachialis muscle attaches.
The shaft of the ulna is triangular and numerus muscles involved in pronation and flexion of the forearm attach to its surface.
Distally, the ulna is much smaller and terminates with a rounded head that articulates with the ulnar notch of the radius to form the distal radioulnar joint. The styloid process of the ulna extends distally and is the site of attachment for ligaments found in the wrist.
Anatomically, the radius is located laterally to the ulna placing it near the thumb. The radius is slightly smaller than the ulna and pivots around the ulna to produce movement at the proximal and distal radioulnar joints.
Proximally, the radius terminates with a disk-shaped head that articulates with the capitulum of the humerus and the radial notch of the ulna. Immediately below the head lies the radial tuberosity to which the biceps brachii attaches. As with the ulna, the shaft of the radius is triangular in shape and numerous muscles, including the protonator teres, attach to it.
Distally the radius expands, medially the ulnar notch articulates with the head of the ulnar. Immediately adjacent to the ulnar notch, the radius articulates with the scaphoid and lunate carpal bones to form part of the wrist.
Carpals, Metacarpals, and Phalanges (The Hand)
Each hand consists of 27 bones, divided between the wrist bones (carpals), the palm bones (metacarpals), and the finger bones (phalanges).
Describe the types of bones in the hand
- There are eight carpal bones in each wrist.
- There are five metacarpal bones in each hand.
- There are proximal, intermediate, and distal phalanges in each digit except for the thumb, which lacks an intermediate phalange.
- metacarpal: Any of the bones of the palm.
- carpal: Any of the eight bones of the wrist.
- phalange: One of the bones of the digits.
The hand contains 27 bones. Each one belongs to one of three regions: the carpals, (wrist), the metacarpals, (the palm), and the phalanges (the digits).
The eight, irregularly shaped carpals are the most proximal bones of the hand. The carpals are often split into two rows, the proximal row containing the scaphoid, lunate, triquetrum, and pisiform, moving lateral to medial.
The scaphoid and lunate articulate with the radius, and the lunate and triquetrum articulate with the articular disk of the wrist. The pisiform carpal is a sesamoid bone, located within a tendon and is not involved in movement at the wrist.
The distal row contains the trapezium, trapezoid, capitate, and hamate, moving lateral to medial. The trapezium articulates with the scaphoid proximally and the first, thumb, and second metacarpal distally. The trapezoid articulates with the scaphoid proximally and the second metacarpal distally.
The capitate articulates with the scaphoid and lunate proximally and the third and fourth metacarpal. Finally, the hamate articulates with the lunate and triquetral proximally and the fourth and fifth, little finger, metacarpals distally.
The hand contains five metacarpal bones that articulate proximally with the carpals and distally with the proximal phalanges. They are numbered moving lateral to medial, and start with the thumb, which is metacarpal I, and end with metacarpal V, the little finger.
Each metacarpal consists of a base, shaft, and head, with the concave lateral and medial borders of the shaft allowing attachment of the interossei muscles.
The digits are named in a similar fashion to the metacarpals, moving lateral to medial, and starting at the thumb. With the exception of the thumb, each digit contains a proximal, intermediate, and distal phalange; the thumb lacks an intermediate phalange. The length of the phalanges decreases distally.