Table of Contents
Arm in Cross Section
The cross section view of the arm shows two fascial compartments along with the following structures:
Arteries: brachial artery, profunda vessels
Veins: basilic vein, cephalic vein, brachial vein
Nerves: radial nerve, median nerve, ulnar nerve, musculocutaneous nerve, medial cutaneous nerve of the arm
Muscles: brachialis, biceps, triceps
Humerus is the long bone of the arm that gives the arm its structure. In cross section, humerus has a prismatic shape. It articulates with the scapula proximally, to make the shoulder joint and with radius and ulna distally, to make up the elbow joint.
- Parts of the humerus involved in the shoulder joint: head of the humerus articulates with the gleinoid cavity of the scapula to make up the glenohumeral joint. Greater and lesser tubercles, found on the proximal humerus, provide attachment sites for rotator cuff muscles of the shoulder joint. Deltoid tuberosity serves as an insertion point for deltoid muscle.
- Parts of the humerus involved in the elbow joint: the condyles, consisting of the capitulum, which articulates with the radius bone of the forearm and trochlea, which articulates with the ulna bone of the forearm. Closely superior to the trochlea and capitulum lie the medial and lateral epicondyle. Three other important structures also laying superior to the trochlea and capitulum are the radial fossa, coronoid fossa and olecranon fossa. These accommodate different parts of the radius and ulna to form the elbow joint.
- Other important features of the humerus worth mentioning are the anatomical neck, in other words, surgical neck. This is a constriction that lies inferior to the greater and lesser tubercles and is called the surgical neck due to its susceptibility to fracturing. Spiral groove carries the radial nerve as it makes its way anteriorly on the shaft of the humerus. Radial nerve injury can result if fracture occurs on the shaft of the humerus sporting the spiral groove.
- Clinical considerations related to nerve injury:
- Ulnar nerve passes posteriorly to the medial epidocondyle, hence susceptible to damage in elbow injuries. “Funny” feeling or intense pain is reported if the ulnar nerve is disturbed.
- Axial nerve and artery can both damaged be if the glenohumeral joint is dislocated. This is due to the close proximity of both of the structures to the proximal part of the humerus.
Movements of the Arm
- Flexion: the act of decreasing the angle between the arm and the shoulder joint
- Extension: the act of increasing the angle between the arm and the shoulder joint
- Abduction: moving the arms away from the midline
- Adduction: moving the arms towards the midline
- Internal or medial rotation: rotating the arms inwards, that is, towards the midline while the elbows are at 90° to the ground
- External or lateral rotation: rotating the arms outwards, that is, away from the midline, while the elbows are at 90° to the ground.
The upper arm is divided into the anterior and posterior fascial compartments by medial and lateral intermuscular septa. Both medial and lateral intermuscular septa extend from the humerus to the deep fascia. Both compartments’ functions are antagonist of each other, i.e. one flexes while the other extends the elbow joint.
The anterior compartment of the arm contains three muscles, all of which function to flex the arm and the forearm. Hence it is a flexion compartment.
Muscles, along with their functions, attachments and innervation, belonging to the anterior compartment are as follows:
- Coracobrachialis: originates from the apex of the coracoid process (a hook-like protrusion on the anterior-superior part of the scapula) and inserts in the medial side of the mid-shaft of the humerus. Its function is to flex the arm on the glenohumeral joint. Innervation is by musculocutaneous nerve (C5, C6, C7)
- Biceps brachii: this muscle originates from two separate locations as two heads; long head and short head. The long head originates from the supraglenoid tubercle of the scapula whereas the short head originates from the coracoid process. They both insert on the radial tuberosity on the radius. They work to cause flexion of the forearm at the elbow joint as well as helping in supination. They also assist coracobrachialis in the flexion of the arm on the glenohumeral joint. Innervation is by musculocutaneous nerve (C5, C6)
- Brachialis: this muscle originates from the lateral and medial surface of the anterior part of the humerus and attaches on the tuberosity of the ulna. It, along with Biceps brachii, flexes the forearm. Innervation by musculocutanous nerve (C5, C6)
Posterior compartment contains only a singular muscle which works to extend the forearm.
This muscle is called the Triceps brachii. It originates as three separate heads known as the long head, medial head and the lateral head. The long head takes origin from the infraglenoid tubercle on the scapula, whereas both medial and lateral heads originate from the posterior surface of the humerus, inferior to the radial groove. All three heads insert at the olecranon.
The Triceps brachii functions to extend the forearm at the elbow joint, as well as extending and adducting the arm at the shoulder joint. It is innervated by the radial nerve (C6, C7, C8).
Cubital fossa is the region anterior to the elbow. It is seen as triangular depression between the brachioradialis and pronator teres muscles. Except ulnar nerve (which goes posteriorly), most of the major neurovascular structures transition from the arm to the forearm via the cubital fossa.
Three most important contents of the cubital fossa are (lateral-medial):
- Tendon of the Biceps brachii muscle
- Brachial artery
- Median nerve.
The radial nerve, although not considered to be a part of the cubital fossa, passes deep to the brachioradialis muscle (lateral boundary of the fossa)
Boundaries of the cubital fossa:
Boundaries of the cubital fossa are determined once an imaginary line is considered to be drawn between the lateral and the medial epicondyles. This imaginary horizontal line also makes up the superior margin of the cubital fossa.
Medial or the ulnar boundary: lateral border of the Pronator teres muscle
Lateral or the radial boundary: medial border of the Brachioradialis muscle
Apex of this triangle is where the medial and lateral boundary meet. This is also the point in most people where the brachial artery is branched into radial and ulnar arteries, which then enter the forearm.
Floor of the cubital fossa is made by the brachialis and supinator muscles, whereas the roof is made by a number of structures including skin, superficial and deep fascia and bicipital aponeurosis.
Clinical significance of the cubital fossa: blood pressure measurement and phlebotomy.
During measurement of blood pressure, using sphygmomanometer and stethoscope to get the pulse of the brachial artery, the stethoscope is kept on the cubital fossa. This is because the brachial artery can be located on the apex of the cubital fossa before bifurcating into radial and ulnar arteries.
Due to the shallow location of the numerous veins passing through the cubital fossa, it is also considered easily accessible for insertion of central catheter for blood collection.
Three major veins that pass through the cubital fossa are:
- Median cubital vein
- Cephalic vein
- Basilic vein
- Paired brachial veins.
Arterial Supply and Nerves of the Arm
Brachial artery is the main artery of the arms. It is the continuation of the axillary artery from the teres major muscle. It keeps a medial position initially around the proximal part of the arm; around the distal part it takes the middle position before entering the cubital fossa and bifurcating.
Profunda brachii artery is a branch of the brachial artery that supplies to the posterior compartment of the arm.
The four major nerves of the arm are the musculocutaneous nerve, median nerve, ulnar nerve and radial nerve.
The origin, course, functions and clinical correlates of each are as follows:
Origin: brachial plexus
Course: exits axilla and enters the coracobrachialis muscle. Descends down diagonally between Biceps brachii and brachialis and gives motor branches to all the muscles of the anterior compartment of the arm. On reaching the tendon of the Biceps brachii, it enters the deep fascia and turns into lateral cutaneous nerve of the arm which supplies to the skin.
Function: motor supply of all three muscles of the anterior compart of the arm and sensory innervation of the skin of the lateral forearm.
Clinical correlates: musculocutaneous nerve compression or injury may occur due to entrapment between the structures such as biceps aponeurosis and brachialis fascia, upper brachial plexus palsy, fracture or surgery. As a result, we notice weakness in flexion, supination and bicep reflex.
Origin: brachial plexus
Course: exits axilla and enters the arm by travelling at the inferior margin of the teres major muscle. It follows descends down along the brachial artery hence following the same route. It gives no significant branch in the upper arm. However, it does give an important branch to pronator teres in the forearm.
Clinical correlates: Median nerve injury occurs due to supracondylar fracture and carpal tunnel syndrome, compression of the median nerve to overworking the wrist in certain activities is also a common mechanism of median nerve injury.
Origin: brachial plexus
Course: ulnar nerve emerges from the axilla and all through the proximal region of the arm stays medial to the axillary artery. At reaching the middle, the ulnar nerve penetrates through the intermuscular septum and enters the posterior compartment of the arm, through which it goes posterior to the medial epicondyle and enters the forearm.
Function: ulnar nerve gives no significant branches in the upper arm.
Clinical correlates: cubital tunnel syndrome.
Origin: posterior cord of the brachial plexus
Course: it emerges from the axilla and passes by the inferior margin of the teres major muscle. In the arm, it initially stays posterior to the brachial artery and then, along with profunda brachii artery, enters the posterior compartment of the arm, where it stays in the radial groove. It gives off motor branches in the posterior compartment and then moves laterally to pierce the intermuscular septum and enters the anterior compartment where it attaches to the lateral supraepicondylar ridge.
Functions: gives motor branches to the Triceps brachii and brachioradialis. It also gives subcutaneous branches through the inferior lateral cutaneous nerve of the arm and posterior cutaneous nerve of the forearm.
Clinical correlates: wrist drop – patients are unable to extend their wrist.