What is the Humerus Bone
Humerus is the only bone of the upper arm, extending from the shoulder to the elbow, anchoring many powerful muscles. It is the largest arm bone, and helps with almost all activities of the arm, like writing, lifting, and throwing.
Being one of the longest bones in the body, it is more prone to fractures upon impact.
Where is the Humerus Located
The humerus is located in the upper arm, between the shoulder joint and the elbow joint.
|Length||14.4 inches (on average in adults)|
|How many are there in the human body||2 (1 in each arm)|
|Articulates with||Shoulder blade or scapula (proximal side), Ulna and radius (distal side)|
- It helps in the functioning of the upper limb by providing structural support, and serving as an attachment site to 13 muscles that aid in the movements of the hand and elbow.
- The humeral head makes up a portion of the ball-and-socket shoulder joint, which is the insertion point for muscles making up the shoulder girdle.
- Several ligaments present in this area assist with securing the musculature. They also provide motion to the shoulder joint.
- The basilic vein, traveling close to the humerus, helps drain parts of the hand and forearm.
- The brachial plexus lying across the front portion of the bone provides sensation and motion to every muscle in the arm, and certain portions of the neck and spinal cord.
Development and Ossification
The humerus is one of the structures that begin to ossify during embryonic development. Its ossification starts from the shaft of the bone.
The adult humerus develops from three bones in the fetus: two epiphyses that form the end caps of the bone and a diaphysis, or central shaft. A thin layer of hyaline cartilage, called the epiphyseal plate or growth plate, exists between these three bones. Cartilage in the epiphyseal plate grows throughout childhood and adolescence to elongate the humerus, thus helping the arm’s growth.
Later, the cartilage begins to get replaced by bony tissues to enable the humerus to increase significantly in length while the epiphyseal plate remains relatively thin. Eventually, at the end of puberty, the cartilage stops growing and gets entirely replaced by bone to form a single, unified humerus. The bony region between the epiphysis and diaphysis in the mature bone is known as the metaphysis.
There are eight ossification centers, and their order of ossification is as follows: body (8th week of fetal life), head (1 year of age), capitulum (1 year), greater tubercle (3 years), lesser tubercle (5 years), medial epicondyle (5 years), trochlea (10 years), and lateral epicondyle (10 years).
Structure and Anatomy
The humerus comprises a proximal region, a shaft, and a distal region. All of these are important anatomical landmarks.
The detailed anatomy of each part is discussed below:
1. Proximal Landmarks of Humerus
Head: The proximal end of the humerus forms a smooth, spherical structure known as the head. It forms the ball-like region of the ball-and-socket shoulder joint, with the glenoid cavity of the scapula acting as the socket. The head faces medially, upwards, and backward. Its round shape allows it to move in a complete circle and rotate around its axis at the shoulder joint.
Anatomical Neck: Just below the head, the humerus narrows into the anatomical neck. It separates the head from the other two regions: greater and lesser tuberosities.
Greater Tuberosity: It is located laterally on the humerus and bears anterior and posterior surfaces. It acts as an attachment site for three rotator cuff muscles – the supraspinatus, infraspinatus, and teres minor. These muscles attach to superior, middle, and inferior facets (respectively) on the greater tuberosity.
Lesser Tuberosity: It is much smaller than the greater Tuberosity, and is located inferior to the head, bearing an anterior surface only. It provides an attachment site for the last rotator cuff muscle – the subscapularis.
Intertubercular Sulcus: Separating the two tuberosities is a deep groove known as the intertubercular sulcus or bicipital groove. The tendon of the biceps emerges from the shoulder joint and runs through this groove. The edges of the intertubercular sulcus are known as lips, where pectoralis major, teres major, and latissimus dorsi get inserted.
Surgical Neck: It is the part between the tuberosities and the shaft. The humerus narrows below the tubercles again to form the surgical neck before extending toward the elbow joint. Here, circumflex humeral vessels and the axillary nerve lie against the bone.
2. The Shaft
A long, cylindrical shaft (body) composes the middle part of the humerus. It bears a roughened surface on the lateral side, known as the deltoid tuberosity, as the deltoid muscle gets attached over there. Below the deltoid tuberosity, the humerus gradually widens, doubling its width as it approaches the elbow.
A shallow depression called the radial (or spiral) groove, runs diagonally down the posterior surface of the humerus, parallel to the deltoid tuberosity. This groove contains the radial nerve and profunda brachii artery.
The following muscles attach to the humerus along its shaft:
Anterior Side: Coracobrachialis, deltoid, brachialis, brachioradialis
Posterior Side: Medial and lateral heads of the triceps (the spiral groove demarcates their respective origins)
3. Distal Region of Humerus
The lower end of the humerus is the distal humerus, containing two joint-forming processes, the capitulum, and the trochlea. Towards the center of the arm, the trochlea tightly hinges with the ulna of the forearm, forming half of the elbow joint. On the other hand, the convex capitulum articulates with the concave head of the radius on the lateral side of the arm. The joint thus formed allows the forearm and hand to rotate and bend at the elbow.
A small cavity, called the olecranon fossa, located on the posterior side of the humerus, locks the tip of the ulna (alternatively known as the olecranon) into the bone. This locking prevents the extension of the elbow beyond 180 degrees. Despite this, the distal portion of the humerus also contains two other depressions, known as the coronoid and radial fossae. They lodge the forearm bones during flexion or extension of the elbow.
The medial and lateral borders of the distal humerus form the medial and lateral supracondylar ridges. Among the two ridges, the lateral supracondylar ridge is more roughened, providing the common origin site of the forearm extensor muscles.
The extracapsular projections of the bone, the lateral and medial epicondyles, are found immediately distal to the supracondylar ridges. Both of the epicondyles can be felt at the elbow from outside. Among the two, the medial is larger, extending more towards the elbow joint. The ulnar nerve passes through a groove, present on the posterior side of the medial epicondyle.
Proximally: The proximal portion of the humerus articulates with the glenoid cavity or glenoid fossa of the scapula, forming the shoulder (or glenohumeral) joint.
Distally: At the distal region, i.e., at the elbow joint, the trochlea articulates with the trochlear notch of the ulna. On the other hand, the capitulum articulates with the head of the radius.
Left vs. Right Humerus
Here is how you can identify if a specific humerus bone comes from the left or right arm:
First, orient the bones in a way so that the rounded head is up. Then, look for the deep olecranon fossa on the lower (distal) end. Observe which side the capitulum is facing.
For that, first rotate the bone in a way that the both the capitulum and trochlea are facing you. Then, if the capitulum lies on the left side of the bone, it is a left humerus. On the contrary, if the capitulum is on the right side, it is a right humerus.
Ans. The muscles that move the humerus include both axial and scapular muscles.
Axial muscles: pectoralis major, and latissimus dorsi
Scapular muscles: coracobrachialis, deltoid, subscapularis, supraspinatus, infraspinatus, teres minor, and teres major
Ans. When the ulnar nerve gets bumped against the humerus, one feels a strange (funny) sensation. For this reason, the humerus is called the funny bone, though it is actually the ulnar nerve that is responsible.
Ans. Being one of the longest bones in the body, it is more prone to fractures upon impact. At the proximal end, most fractures occur at the surgical neck and are common in the elderly, especially those with osteoporosis.
Other medical issues associated with the humerus include metastatic bone disease, radial nerve injury, osteochondroses, Charcot arthropathy, humerus Varus.