Humerus bone

Introduction

The humerus is the long bone of the upper arm that connects the shoulder to the elbow. It plays a crucial role in the mobility and strength of the upper limb, serving as a site for muscle attachment and as a lever for movement. Understanding its anatomy is essential for clinical practice, particularly in trauma and orthopedic surgery.

Anatomy of the Humerus

Proximal End

The proximal end of the humerus articulates with the scapula at the shoulder joint. Key features include:

• Head: Rounded and smooth, articulates with the glenoid cavity of the scapula.
• Anatomical Neck: Constriction just below the head, separating it from the tubercles.
• Surgical Neck: Region below the tubercles prone to fractures.
• Greater and Lesser Tubercles: Prominent projections for attachment of rotator cuff muscles.
• Intertubercular (Bicipital) Groove: Groove between the tubercles that houses the tendon of the long head of the biceps brachii.

Shaft

The shaft, or diaphysis, is the long, cylindrical portion of the humerus and serves as the main site of muscle attachment and structural support. Its features include:

• Deltoid Tuberosity: Lateral projection where the deltoid muscle attaches.
• Radial Groove: Posterior groove for the radial nerve and profunda brachii artery.
• Medial and Lateral Borders: Define the edges of the shaft and provide attachment for muscles.
• Anterior and Posterior Surfaces: Serve as areas for muscle attachment and passage of neurovascular structures.

Distal End

The distal end of the humerus forms the upper part of the elbow joint and articulates with the ulna and radius. Important anatomical features include:

• Trochlea: Medial, spool-shaped structure that articulates with the ulna.
• Capitulum: Lateral, rounded structure that articulates with the head of the radius.
• Medial and Lateral Epicondyles: Projections for attachment of forearm muscles and ligaments.
• Coronoid and Olecranon Fossae: Posterior and anterior depressions that accommodate the ulna during flexion and extension.
• Radial Fossa: Shallow anterior depression above the capitulum for the head of the radius during elbow flexion.

Relations and Surrounding Structures

The humerus maintains close anatomical relationships with muscles, nerves, and blood vessels, which are crucial for upper limb function and clinical considerations.

• Muscular Attachments: Includes the rotator cuff muscles, biceps brachii, triceps brachii, brachialis, and brachioradialis.
• Nerve Relationships: Radial nerve courses along the radial groove, median and ulnar nerves are related to the distal humerus, important in fracture management.
• Vascular Relations: The brachial artery and its branches run along the humeral shaft and distal end, providing blood supply to the upper limb.

Development and Ossification

The humerus develops through a process of endochondral ossification, with distinct primary and secondary centers contributing to bone growth and maturation.

• Primary Ossification Center: Located in the shaft, appears during the eighth week of fetal development.
• Secondary Ossification Centers: Present in the proximal and distal ends, including the head, greater and lesser tubercles, and the epicondyles; appear postnatally and fuse during adolescence.
• Clinical Significance: Knowledge of ossification timelines is important for interpreting pediatric radiographs and assessing growth plate injuries.

Function

The humerus serves as a critical component of the upper limb, enabling a wide range of movements and providing leverage for muscle action.

• Shoulder Joint Movements: Facilitates flexion, extension, abduction, adduction, and rotation through articulation with the scapula.
• Elbow Joint Movements: Allows flexion and extension through articulation with the radius and ulna.
• Lever for Muscle Action: Acts as a lever arm for muscles to generate force, contributing to strength and dexterity of the arm.
• Stability and Mobility: Provides structural support while allowing functional mobility essential for daily activities.

Clinical Significance

Fractures

Fractures of the humerus are common and vary based on location, mechanism of injury, and patient age.

• Proximal Humeral Fractures: Often occur in elderly patients due to falls; may involve the surgical neck or tubercles.
• Mid-Shaft Fractures: Typically result from direct trauma or falls on an outstretched hand; radial nerve injury is a concern.
• Distal Humeral Fractures: Involve the supracondylar region or epicondyles; common in children and may affect the elbow joint.

Dislocations

• Shoulder Dislocation: The humeral head may dislocate anteriorly or posteriorly from the glenoid cavity; often associated with soft tissue injury.
• Elbow Dislocation: Involves displacement of the ulna and radius relative to the distal humerus; may occur with fractures of the distal humerus.

Common Pathologies

• Osteoporosis leading to increased risk of humeral fractures
• Bone tumors, including benign lesions like osteochondromas and malignant tumors such as osteosarcoma
• Osteomyelitis, an infection of the humeral bone

Imaging and Diagnostic Approaches

Accurate assessment of the humerus is essential for diagnosing fractures, pathologies, and developmental abnormalities.

• X-ray Imaging: Standard tool for detecting fractures, alignment, and bone integrity.
• CT Scan: Provides detailed cross-sectional images for complex fractures and preoperative planning.
• MRI: Evaluates bone marrow, soft tissue attachments, and suspected tumors or osteomyelitis.
• Ultrasound and Nuclear Imaging: May be used for assessing vascularity, soft tissue involvement, and metabolic activity.

Comparative Anatomy

The humerus exhibits variations among different species, reflecting adaptations to diverse modes of locomotion and upper limb function.

• Mammalian Variations: In quadrupeds, the humerus is often more robust and adapted for weight-bearing and locomotion, whereas in primates it allows for a greater range of motion and manipulation.
• Functional Adaptations: Animals that climb, fly, or burrow have humeral adaptations such as elongated shafts, modified muscle attachment sites, or reinforced cortical bone to support specialized movements.

References

1. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
2. Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 9th ed. Philadelphia: Wolters Kluwer; 2020.
3. Marieb EN, Hoehn K. Human Anatomy & Physiology. 11th ed. Boston: Pearson; 2019.
4. Rogers LF, White AA. Diagnostic Imaging of the Musculoskeletal System. 2nd ed. Philadelphia: Saunders; 2015.
5. Beaty JH, Kasser JR. Rockwood and Wilkins’ Fractures in Children. 9th ed. Philadelphia: Wolters Kluwer; 2020.
6. Skandalakis JE, Gray SW. Surgical Anatomy: The Embryologic and Anatomic Basis of Modern Surgery. 3rd ed. Baltimore: Williams & Wilkins; 2004.
7. Hoppenfeld S, deBoer P. Surgical Exposures in Orthopaedics: The Anatomic Approach. 4th ed. Philadelphia: Lippincott Williams & Wilkins; 2012.
8. Canale ST, Beaty JH. Campbell’s Operative Orthopaedics. 14th ed. Philadelphia: Elsevier; 2021.