Anatomy of the Humerus: Proximal to Distal

1. Introduction

The humerus is the longest and largest bone of the upper limb. It articulates proximally with the scapula at the glenohumeral joint and distally with the radius and ulna at the elbow joint. Understanding its anatomy is vital for diagnosing fractures and associated nerve injuries.

2. Proximal Landmarks

• The Head: Half-spherical, it faces medially, upward, and backward to articulate with the glenoid cavity.
• Anatomical vs. Surgical Neck: The anatomical neck is a slight constriction just distal to the head. The Surgical Neck is a narrowed region distal to the tubercles. It is a common site of fracture and is closely related to the axillary nerve and posterior circumflex femoral artery.
• Greater and Lesser Tubercles: Sites for rotator cuff muscle insertions (Supraspinatus, Infraspinatus, Teres minor on the greater; Subscapularis on the lesser).

3. The Shaft and the Radial Groove

The humerus shaft has a unique feature on its posterior surface: the Radial (Spiral) Groove.

• The Radial Nerve and the Profunda Brachii Artery run in this groove.
• Mid-shaft fractures of the humerus often result in Radial Nerve Palsy, leading to "wrist drop" because the nerve cannot stimulate the extensor muscles of the forearm.
• On the lateral side, the Deltoid Tuberosity provides an insertion point for the deltoid muscle.

4. Distal Extremity (The Elbow Region)

• Trochlea: A pulley-shaped surface that articulates with the ulna.
• Capitulum: A rounded knob that articulates with the head of the radius.
• Epicondyles: The Medial Epicondyle is more prominent and protects the Ulnar Nerve (the "funny bone" sensation). The Lateral Epicondyle is the origin for forearm extensors.
• Olecranon Fossa: A deep posterior depression that receives the olecranon of the ulna during elbow extension.

5. Clinical Correlations

• Fracture Locations and Nerve Risks:
  1. Surgical Neck → Axillary Nerve.
  2. Shaft (Radial Groove) → Radial Nerve.
  3. Supracondylar region → Median Nerve and Brachial Artery.
  4. Medial Epicondyle → Ulnar Nerve.
• Supracondylar Fractures: Most common in children. If the brachial artery is compromised, it can lead to Volkmann’s Ischemic Contracture.

6. Review Questions

1. Which nerve is at risk in a mid-shaft humerus fracture?
2. What structure articulates with the trochlea of the humerus?
3. Where does the Subscapularis muscle insert on the humerus?
4. Explain the difference between the anatomical and surgical neck.