Anatomy of Arm in a Nutshell

Anatomy of Arm

• Distal humerus: remember, lesser tubercle is on anterior aspect of humerus → subscapularis inserts here (the only one that does).

• • Humerus flares out to form epicondyleà lateral and medial (associated with the ulnar nerve).
  • Above epicondyle, have ridge (both sides) = lateral / medial supracondylar ridge.
  • Capitulum = rounded area on anterior aspect.
  • Medial to the capitulum= the trochlea. Shaped similar to wheel of pully, acts as a hinge aspect of the elbow joint. Seen both anteriorly and posteriotly.
  • Proximal (ant and post) to trochlea = 1. coronoid fossa (anterior aspect above trochlea )which accepts coronoid process of another bone and 2. olecranon fossa (posterior aspect above trochlea) which accepts olecranon process.

• Elbow joint:

• • Radius is on lateral (thumb) side. On proximal end, have disc (head of radius) which articulates with capitulum
  • Ulna is on medial side. Has prominent process = olecranon process on proximal end. On this process, have large trochlear notch which will fit with trochlea of the humerus to form hinge joint.
  • On anterior aspect of ulna, have corocoid process which (when arm is flexed) will articulate with coronoid fossa.
  • Both ulna and radius have bony tuberosities (ulnar and radial tuberosities). These areas are for insertions of muscles.

Joint ligaments: joint capsule innermost

• On each side, medial and lateral, have collateral ligaments. On ulnar side = ulnar colalteral ligament / on radial = radial collateral ligaments.
• Prominent ring like ligament that wraps head of radius = anular ligament– holds the radius in place. Thus, radius can spin around long axis while being held steady at elbow.

• Motions:

• • Flexion and extension motion happens at elbow joint
  • Pronation = turn forearm palm down so that radius crosses over stationary ulna. Radius spins on long axis to allow this crossing.
  • Supination = turning forearm back into anatomical position, bones no longer crossed.

Muscles: Anterior compartment of arm. Innervated by musculocutaneous nerve.

• Brachialis: flexor of elbow joint.  Inserts on ulnar tuberosity.
• Biceps brachii: flexion and supination. Has two specializations at distal end. Wide flat tendon = bicepital aponeurosis→ inserts on deep fascia of proximal forearm and is involved with flexion.  Also has biceps tendon = normal cord tendon that attaches to radial tuberosity and is involved with flexion (esp. with supine forearm) and also with supination (esp. with prone forearm).

Muscles: Posterior compartment.

• Triceps brachii: action = extension of the elbow joint.  Inserts on the olecranon process. Medial head often hidden between long and lateral. (3 heads total).
• Anconeus: starts at lateral epicondyle and attaches to ulna. Also contributes to extension.

Both innervated by radial nerve.

Veins:

• Basilic vein: medial side, starts subcutaneous→ penetrates deep fascia ½ up the arm→ travels to inferior border of teres major to become axillary vein.  Brachial veins (2) join axillary after traveling with brachilal artery up the arm.
• Cephalic vein: passes up lateral aspect of arm / forearm.  Have connection between Basalic and cephalic = medial cubital vein.  This is the primary place for venipuncture.

Arteries:

Last 2 branches of Axillary Artery = anterior / posterior humeral circumflex (anterior smaller, posterior larger) branches.  Changes to brachial artery→ with many branches:

• Profunda brachii artery = goes onto posterior aspect.  One branch heads superiorly to contribute to collateral vasculature. Another branch heads toward elbow joint→ passes along posterior aspect of humerus and changes name→giving off middle and radial collateral arteries toward elbow.  Gives off a branch toward triceps as well.
• On ulnar side, have long collateral artery heading toward elbow joint.  Another branch comes off = ulnar collateral arteries (superior / inferior).
• Continues across forearm, passing deep to the bicepetal aponeurosis→ branches into 2 major branches = Radial and Ulnar arteries.
• Radial artery: Gives off radial recurrent artery.
• Ulnar artery: Gives off several ulnar recurrent arteries.  Recurrent arteries come back to the elbow joint.

Thus, elbow gets blood supply superiorly from collateral arteries (radial, middle, ulnar) and inferiorly from recurrent arteries (one radial, few ulnar).  These two sources form collateral circulation around the elbow.

Note: Triangular interval contains the profunda brachii artery and the radial nerve. Radial nerve is in the radial groove (spiral groove) on the humerus.

Innervation:

M shape = Musculocutaneous, median, ulnar nerves.

• Ulnar nerve: no major branches in the arm. Passes down the extent of the arm, posterior to the median epicondyle. Does nothing in the arm.
• Musculocutaenous nerve: penetrates the coracobrachialis muscle and passes between the biceps brachii and brachialisà sits right on the brachialis. Innervates these muscles as it passes, giving off motor fibers. Terminates as the lateral cutaneous nerve of the forearm.  This is sensory for lateral forearm. This change from musculocutanous to lateral cutaneous nerve occurs just after the last motor branch (because now, only sensory fibers are left)
• Middle part of M = median nerve. Does nothing in the arm, passes into the forearm.

Cross section of Arm:

• Intermuscular septa: medial and lateral.  Posterior to these = triceps, extensor compartment.  Anterior = flexor compartment with brachalis, biceps, coracbrachialis.
• Extensor compartment:

• • Contain profunda brachii artery with veins and radial nerve along humerus.

• Medial compartment:
  • Contains musculocutaneous N
  • Brachial artery (with 2 associated veins), median nerve, basilic vein, ulnar nerve
• Flexor compartment:
  • Biceps brachii

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