13.1 Humerus

The humerus is the largest bone in the upper limb. It articulates with the scapula (▶Fig. 13.1) at the glenohumeral joint proximally and the radius and the ulna distally at the elbow joint. The proximal end of the humerus consists of three parts: the articular head, lesser tuberosity, and greater tuberosity (▶Fig. 13.2). The hemispherical head articulates with the glenoid cavity of the scapula. The junction of the articular portion and the remaining head is called the anatomic neck, while the junction between the entire head fragment and the shaft, below the tuberosities, is called the surgical neck. The surgical neck is the most common site of fractures. The axillary nerve runs along the posterior aspect of the surgical neck.

The greater tuberosity is located lateral to the bicipital groove. It is the point of attachment of three rotator cuff muscles: the supraspinatus and infraspinatus (both innervated by suprascapular nerve) and the teres minor (innervated by axillary nerve). The lesser tuberosity, located medial to the bicipital groove, is the site of subscapularis insertion (innervated by the upper and lower subscapular nerves). The shaft of the humerus (▶Fig. 13.3) is tubular and has two important prominent features: the deltoid tuberosity laterally for attachment of the deltoid muscle and the oblique radial groove (spiral groove) posteriorly, in which the radial nerve and deep brachial artery of the arm cross. The distal end of the humerus fans out into two columns and then supports the elbow articular surface. This joint includes the spooled trochlea (articulates with the ulna medially), the capitellum (articulate with the head of the radius laterally), and three fossae (radial fossa and coronoid fossa anteriorly, and olecranon fossa posteriorly). The following areas of the humerus are clinically important, as they are in direct contact with the indicated nerves:

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  Surgical neck: axillary nerve

  
  
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  Radial groove: radial nerve

  
  
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  Medial epicondyle: ulnar nerve posterior to it

  
  
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  Lateral epicondyle: radial nerve anterior to it

13.2 Cutaneous Innervations of the Arm

Posterior surface of the arm:

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  Superior lateral cutaneous nerve of arm (branch of axillary nerve)

  
  
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  Posterior cutaneous nerve of arm (branch of radial nerve)

  
  
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  Inferior lateral cutaneous nerve of arm (branch of radial nerve)

Lateral surface of the arm:

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  Superior lateral cutaneous nerve of arm (branch of axillary nerve)

  
  
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  Inferior lateral cutaneous nerve of arm (branch of radial nerve)

Medial surface of the arm:

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  Intercostobrachial nerve (lateral cutaneous branch of T2)

  
  
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  Medial brachial cutaneous nerve of arm (from medial cord of brachial plexus)

  
  
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  Medial antebrachial cutaneous nerve of arm (from medial cord of brachial plexus)

13.3 Muscles of the Arm

The anterior compartment contains three flexor muscles, all innervated by the musculocutaneous nerve (▶Fig. 13.4).

Biceps brachii: It consists of a short head and a long head, each with different sites of origin. The short head is attached to the coracoid process of the scapula, while the long head originates from the supraglenoid tubercle of scapula. Distally, the biceps muscle terminates into one tendon and a fascial extension. The true tendon attaches to the bicipital tuberosity on the radius, while the fascial bicipital aponeurosis fans onto the ulnar part of the antebrachial fascia and medial epicondyle. The blood supply of the biceps brachii is via the brachial artery.

Brachialis: This muscle originates on the anterior surface of humerus and inserts on to the coronoid process of ulna. The brachialis is the main flexor of the elbow. The brachial artery and radial recurrent artery are the blood supply to the brachialis muscle (▶Fig. 13.5).

Coracobrachialis: Originating from the coracoid process of scapula, it inserts on to the middle third of medial surface of humerus. This muscle has two important anatomical landmarks. The musculocutaneous nerve pierces it 5 to 6 cm from its origin (▶Fig. 13.6a). Its distal attachment indicates the location of the nutrient foramen of the humerus, where the main nutrient artery penetrates the bone, and the location where the ulnar nerve courses from anterior to posterior and penetrates the intermuscular septum. It is supplied by the brachial artery.

The posterior compartment contents include two muscles innervated by the radial nerve.

Triceps brachii: The long head originates from the infraglenoid tubercle of scapula, the lateral head (strongest) originates from the posterior surface of humerus, and the medial head originates from the lower half of the posterior surface of humerus inferior to the radial groove (▶Fig. 13.7). The three heads converge to form one tendon, which inserts onto the proximal end of the ulna at the olecranon process. Its blood supply is the profunda brachii artery of the arm.

Anconeus: This small triangular muscle originates on the lateral epicondyle of humerus and insert to lateral surface of the olecranon. It is supplied by the interosseous recurrent artery.

13.4 Nerves of the Arm

The arm contains four main nerves (median, ulnar, musculocutaneous, and radial), two of which (median and ulnar) supply no branches to the upper arm. The majority of the nerves and arteries in the arm are on the medial aspect of the anterior compartment (▶Fig. 13.6b).

13.5 Musculocutaneous Nerve (C5–C6)

The musculocutaneous nerve arises from the lateral cord of the brachial plexus. Proximally, it is located lateral to the median nerve (▶Fig. 13.6c). As it descends to the arm, it pierces the coracobrachialis approximately 5 to 8 cm distal to the coracoid. ¹ The prevalence of cases of musculocutaneous nerve outside (not piercing) the coracobrachialis muscle is reported to be 0 to 8% in anatomical dissections. ^{2 – 4} In the proximal third of the arm, it is located between the biceps brachii and the coracobrachialis. It supplies motor innervation to the flexor muscles of the arm (coracobrachialis, biceps brachii, and brachialis). After supplying all three muscles of the anterior compartment of the arm, it terminates as the lateral cutaneous nerve (purely sensory nerve) of the forearm, which innervates the radial side of the forearm (▶Fig. 13.8). Variation exists with regard to communication between the musculocutaneous nerve and the median nerve. In an anatomical ⁵ dissection of 56 upper limbs, communications were seen between the musculocutaneous and median nerves in 53.6% of the arms.

13.6 Median Nerve in the Arm (C6–T1)

The median nerve, from the medial and lateral cords of the brachial plexus, runs lateral to the brachial artery on top of the coracobrachialis muscle until it reaches the middle of the arm, where it crosses to the medial side as it encounters the brachialis muscle (▶Fig. 13.9). It runs along the medial edge of the biceps brachii, with the brachial artery lying lateral to it. In the antecubital fossa, the median nerve lies deep to the bicipital aponeurosis; medial to the antecubital vein, the brachial artery, and the biceps tendon; and lateral to the common origin of the flexor and pronator muscles. ¹ The median nerve has no branches in the arm, but it does supply articular branches to the elbow joint. The median nerve may become entrapped proximally at two locations in the arm, the ligament of Struthers and the lacertus fibrosis. The ligament of Struthers is an abnormal structure that is located 5 cm proximal to the medial epicondyle. It is a fibrous band originating from an aberrant supracondylar process on the medial aspect of the distal humerus that connects to the medial epicondyle. ¹ The lacertus fibrosis is a normal fascial sleeve that extends from the biceps tendon to the medial epicondyle and covers the median nerve and brachial artery. The median nerve is lateral to the brachial artery in the upper part, anterior to it in the middle part, and medial to it in the lower arm. The median nerve supplies no muscular innervation in the upper arm.