Comprehensive Anatomy of the Arm and Humerus

Overview of the Arm

• Regional Definition: The arm is defined as the region of the upper limb located between the shoulder (glenohumeral) joint and the elbow joint.

• Primary Bone: The humerus serves as the structural foundation of the arm.

• Compartmentalization: The arm is divided into two distinct functional compartments:

  • Anterior Compartment: Consists of three muscles primarily responsible for flexion. These muscles share a common nerve supply via the musculocutaneous nerve.

  • Posterior Compartment: Consists of two muscles primarily responsible for extension. These muscles share a common nerve supply via the radial nerve.

Osteology: Bony Landmarks of the Humerus

• Proximal Landmarks:

  • Head: The rounded proximal articular surface that fits into the glenoid cavity.

  • Anatomical Neck: Located at the junction of the head and the tubercles. Fractures in this region are considered intra-articular.

  • Surgical Neck: Situated inferior to the head and tubercles. It is a common site for fractures and is considered extra-articular. The axillary nerve is particularly vulnerable to injury at this location.

  • Greater and Lesser Tubercles: Prominent projections serving as muscle attachment sites.

  • Intertubercular Groove (Sulcus): A deep furrow located between the greater and lesser tubercles. It is bordered by the crests of the greater and lesser tubercles.

  • Deltoid Tuberosity: A roughened surface on the lateral aspect of the humeral shaft for the attachment of the deltoid muscle.

• Shaft Landmarks:

  • Radial (Spiral) Groove: Located on the posterior aspect of the shaft. It transmits the radial nerve and profunda brachii artery. This area is vulnerable during midshaft fractures; the groove is partially formed by the posterior edge of the deltoid tuberosity.

• Distal Landmarks:

  • The distal end of the humerus is flattened and widened in the medial-lateral direction.

  • Medial and Lateral Supracondylar Ridges: Sharp borders located above the epicondyles.

  • Medial and Lateral Epicondyles: The medial epicondyle has a larger projection than the lateral epicondyle and contains a groove for the protection of the ulnar nerve.

  • Anterior Fossae:

    • Coronoid Fossa: Accommodates the coronoid process of the ulna during flexion.

    • Radial Fossa: Accommodates the head of the radius during flexion.

  • Posterior Fossa:

    • Olecranon Fossa: Accommodates the olecranon process of the ulna during extension.

Joint Articulations of the Elbow

• Medial Articulation: Formed between the trochlea of the humerus and the trochlear notch of the ulna.

• Lateral Articulation: Formed between the capitulum of the humerus and the head of the radius.

• Motions: The primary motions permitted at these articulations are flexion and extension.

Vasculature of the Arm

• Arterial Supply:

  • Brachial Artery: The continuation of the axillary artery starting at the inferior border of the teres major muscle. It is the primary source of blood for the arm.

  • Profunda Brachii Artery (Deep Brachial Artery): A branch of the brachial artery that supplies the posterior compartment of the arm.

  • Distal Division: Near the elbow, the brachial artery bifurcates into the radial and ulnar arteries.

• Venous Drainage:

  • Cephalic Vein: Ascends the arm in the lateral bicipital groove and then transitions through the deltopectoral groove. It empties into the axillary vein after piercing the deep fascia at the clavipectoral (or deltopectoral) triangle.

  • Basilic Vein: Ascends in the medial bicipital groove. It pierces the deep brachial fascia and is continuous with the axillary vein.

  • Median Cubital Vein: A significant superficial vein connecting the cephalic and basilic veins; it is the common site for venipunctures.

The Anterior Compartment of the Arm

• Functional Overview: The muscles in this compartment are predominantly elbow flexors. Some also function as shoulder flexors or perform both actions.

• Common Innervation: Musculocutaneous nerve.

• Muscles:

  • Coracobrachialis:

    • Proximal Attachment: Coracoid process.

    • Distal Attachment: Medial side of the humeral shaft, just proximal to the mid-point.

    • Action: Shoulder flexion.

  • Biceps Brachii:

    • Proximal Attachment (Long Head): Supraglenoid tubercle (blends with the glenoid labrum). The tendon runs through the intertubercular groove, held by the transverse humeral ligament.

    • Proximal Attachment (Short Head): Coracoid process.

    • Distal Attachment: Radial tuberosity via the bicipital aponeurosis.

    • Actions: Both heads perform elbow flexion and powerful forearm supination. The long head also contributes to shoulder flexion.

  • Brachialis:

    • Proximal Attachment: Anterior surface of the distal half of the humerus.

    • Distal Attachment: Tuberosity of the ulna.

    • Action: Elbow flexion.

The Posterior Compartment of the Arm

• Functional Overview: The muscles in this compartment act predominantly as elbow extensors. The triceps can also complete shoulder extension.

• Common Innervation: Radial nerve.

• Muscles:

  • Triceps Brachii:

    • Long Head Proximal Attachment: Infraglenoid tubercle.

    • Lateral Head Proximal Attachment: Posterior humeral shaft, superior to the radial groove.

    • Medial Head Proximal Attachment: Posterior humeral shaft, inferior to the radial groove.

    • Distal Attachment: Olecranon of the ulna.

    • Actions: Elbow extension; the long head also assists in shoulder extension.

  • Anconeus:

    • Proximal Attachment: Lateral epicondyle of the humerus.

    • Distal Attachment: Olecranon and proximal ulna.

    • Action: Elbow extension.

Nerve Pathways and Clinical Correlates

• Musculocutaneous Nerve:

  • Branch of the lateral cord of the brachial plexus.

  • Pierces the coracobrachialis muscle.

  • Runs distally between the biceps brachii and brachialis muscles.

  • Terminates as the lateral cutaneous nerve of the forearm.

• Median Nerve:

  • Descends the medial side of the biceps brachii and brachialis.

  • Positions relative to the brachial artery: Proximally, it is lateral to the artery; distally, it is medial to the artery.

  • Note: Does not supply any muscles within the arm.

• Ulnar Nerve:

  • Descends medial to the brachial artery.

  • Passes behind the medial epicondyle into the forearm.

  • Note: Does not supply any muscles within the arm.

• Radial Nerve:

  • Terminal branch of the posterior cord.

  • Passes inferior to the teres major to reach the posterior arm.

  • Enters the triangular interval along with the profunda brachii artery.

  • Descends in the radial groove between the medial and lateral heads of the triceps brachii.

  • Clinical Correlate: Radial nerve impingement is a common complication of humeral shaft fractures. It is estimated that up to $18\,\%$ of shaft fractures involve damage to the radial nerve. Injury results in radial nerve palsy, characterized by the inability to extend the wrist and fingers.

Muscle Attachment and Functional Summary