Humerus

What is the Brachial Bone

The brachial bone is a lengthy bone in the human arm, extending from the shoulder to the elbow. It is the most extensive bone in the human arm, and the lone bone in the upper arm, sometimes known as the upper arm bone. The brachial bone serves as the attachment point for multiple robust muscles and aids in all arm activities, such as writing, lifting, and throwing.

As one of the longest bones in the body, it is more susceptible to fractures upon impact.

Where is the Brachial Bone Situated

The brachial bone is positioned between the shoulder and elbow joints in the upper arm.

Brachial Bone Facts

Functions

• It aids in the operation of the upper limb by delivering structural support and serving as an attachment site to 13 muscles that assist in the movements of the hand and elbow.
• The brachial head constitutes a fraction of the ball-and-socket shoulder joint, which is the insertion point for muscles building the shoulder girdle.
• Several ligaments found in this area aid in securing the musculature and also provide motion to the shoulder joint.
• The basilic vein, running close to the brachial bone, aids in draining portions 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.

Structure and Anatomy

The brachial bone consists of a proximal region, a shaft, and a distal region. All of these are important anatomical landmarks.

The comprehensive anatomy of each part is discussed below:

1. Proximal Landmarks of Brachial Bone

Head: The proximal end of the brachial bone forms a smooth, spherical structure known as the head. It is the ball in the ball-and-socket joint in the shoulder, where the glenoid cavity of the scapula acts as the socket. Due to its round shape, the head rotates around its axis at the shoulder joint and moves in all directions.

Anatomical Neck: Just below the head, the brachial bone narrows into the anatomical neck. It separates the head from the other two regions: greater and lesser tuberosities.

Greater Tubercle (Greater Tuberosity): It lies on the lateral side of the bone, with an anterior and a posterior surface. The three rotator cuff muscles, supraspinatus, infraspinatus, and teres minor, attach to the greater tubercle’s superior, middle, and inferior facets, respectively.

Lesser Tubercle (Lesser Tuberosity): It is much smaller than the greater tubercle located inferior to the head, bearing only an anterior surface. This is where the fourth and last rotator cuff muscle, the subscapularis, attaches.

Intertubercular Sulcus: Separating the two tuberosities is a deep groove known as the intertubercular sulcus or bicipital groove. The tendon of the bicep’s long head 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 corded.

Surgical Neck: It is the part between the tuberosities and the shaft where the brachial bone narrows down to form the surgical neck before extending toward the elbow joint. Here, circumflex brachial vessels and the axillary nerve lie against the bone.

2. The Shaft

A long, cylindrical shaft (body) composes the middle part of the brachial bone. It bears a roughened surface on the lateral side, known as the deltoid tuberosity, as the deltoid muscle gets attached there. The width of the bone gradually increases past the deltoid tuberosity, becoming double as it moves towards the elbow joint.

A shallow depression called the radial (or spiral) groove runs diagonally down the posterior surface of the brachial bone, parallel to the deltoid tuberosity. This groove contains the radial nerve and the profunda brachii artery.

The shaft serves as the surface for attachment of several muscles

Anterior Side: Coracobrachialis, brachialis, deltoid, brachioradialis

Posterior Side: The medial and lateral heads of the triceps. Their origin is marked by the spiral groove on the posterior side of the brachial bone.

3. Distal Region of Brachial Bone

The lower end of the brachial bone is the distal brachial bone, containing two joint-forming processes, the capitulum and the trochlea. The trochlea tightly hinges with the forearm’s ulna, forming half of the elbow joint. On the other hand, the convex capitulum articulates with the concave radial head on the lateral side of the arm. The joint thus formed is the elbow that allows the human arm to bend and fold in the middle.

A small cavity called the olecranon fossa on the posterior side of the bone locks the olecranon or the tip of the ulna into the bone. This locking prevents us from extending the elbow beyond 180 degrees. Despite this, the distal portion of the brachial bone 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 brachial bone form the medial and lateral supracondylar ridges. Among the two ridges, the lateral supracondylar ridge is relatively 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 the 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.

Articulations

Proximally: The shoulder (or glenohumeral) joint is formed where the brachial bone articulates with the scapula’s glenoid cavity.

Distally: The elbow joint is formed where it articulates with the trochlear notch of the ulna, and its capitulum articulates with the radial head.

Left vs. Right Brachial Bone

Here is how you can identify if a certain brachial bone comesfrom the left or right arm:

To begin, position the bones so that the rounded head faces upward and then identify the deep olecranon fossa on the lower (distal) end. Take note of the side on which the capitulum is situated.

To determine whether it is a left or right humerus, rotate the bone so that both the capitulum and trochlea are visible. If the capitulum is positioned on the left side of the bone, it is a left humerus. Conversely, if the capitulum is on the right side, it is a right humerus.

Development and Ossification

Ossification of the humerus commences during embryonic development, with the shaft being the starting point.

A total of 8 ossification centers contribute to the development of the humerus. Ossification of the body begins during the 8th week of fetal development. The remaining ossification occurs after birth, with the head and capitulum ossifying at 1 year, the greater tubercle at 3 years, the lesser tubercle and medial epicondyle at 5 years, and the trochlea and lateral epicondyle at 10 years.

FAQs

Q.1. What muscles move the humerus?

Ans. Muscles responsible for moving the humerus include axial and scapular muscles.
Axial muscles: pectoralis major, and latissimus dorsi
Scapular muscles: coracobrachialis, deltoid, subscapularis, supraspinatus, infraspinatus, teres minor, and teres major

Q.2. Why is the humerus called the ‘funny bone’?

Ans. The humorous is termed as the ‘funny bone’ due to the strange sensation felt when the ulnar nerve is bumped against it, though it is actually the ulnar nerve responsible for this sensation.

Q.3. What are the common injuries and conditions associated with this bone?

Ans. As one of the longest bones in the body, the humerus is more susceptible to fractures upon impact. Fractures at the surgical neck are common at the proximal end, especially in the elderly with osteoporosis.
Other medical issues linked to the humerus include metastatic bone disease, radial nerve injury, osteochondroses, charcot arthropathy, and humerus varus.

References

1. Anatomy, Shoulder and Upper Limb, Humerus – Ncbi.nlm.nih.gov
2. Humerus – Sciencedirect.com
3. Humerus – Radiopaedia.org
4. Humerus – Osmosis.org
5. The Humerus – Teachmeanatomy.info