What is the external nasal anatomy and its clinical significance?

External Nasal Anatomy and Its Clinical Significance

The external nasal anatomy consists of bony and cartilaginous structures covered by soft tissues of varying thickness, with critical neurovascular elements that must be preserved during surgical procedures to minimize complications such as nasal tip numbness and functional deficits.

Anatomical Components

Bony Framework

• The upper third of the nose consists of paired nasal bones that form the nasal bridge
• These bones articulate with the frontal bone superiorly and the maxilla laterally 1

Cartilaginous Framework

• The middle third contains the upper lateral cartilages that connect to the nasal bones
• The lower third consists of:
  • Paired lower lateral (alar) cartilages that form the nasal tip and alae
  • Septal cartilage that provides midline support 2, 1

Soft Tissue Layers

• Skin
• Subcutaneous fat (variable distribution)
• Nasal SMAS (superficial musculoaponeurotic system)
• Deep fatty layer
• Perichondrium/periosteum 3, 2

Key Anatomical Variations

1. Soft Tissue Thickness:

   • Varies significantly between individuals
   • Subcutaneous fat distribution is highly variable:
     • 50% of individuals have continuous fat from root to tip
     • Others have fat concentrated at either the root or tip 2

2. Septal Cartilage Position:

   • The superior border of septal cartilage does not form a linear extension of the nasal bones
   • It angles downward, creating a discrepancy between the skeletal structure and external profile 2

3. Muscular Variations:

   • Nasalis muscle was identifiable in 75% of specimens
   • In half of cases, it completely traverses the nose
   • In 25% of cases, only the lateral portion was identifiable 2

Neurovascular Anatomy

External Nasal Nerve

• Critical sensory nerve supplying the nasal tip
• Exit point: 7.3 ± 0.6 mm lateral to nasal midline
• Average diameter: 0.35 ± 0.036 mm
• Course: Passes through deep fatty layer under nasal SMAS
• Branching patterns:
  • Type I (50%): Single nerve without branches
  • Type II (30%): Single nerve splitting into two branches at intercartilaginous junction
  • Type III (20%): Two main branches from exit point 3

Clinical Significance

Surgical Considerations

1. Rhinoplasty Precautions:

   • Avoid deep intercartilaginous/intracartilaginous incisions to prevent external nasal nerve injury
   • Limit dissection to within 6.5 mm from midline on each side
   • Use implants less than 13 mm wide at rhinion level for dorsal augmentation 3

2. Nasal Valve Function:

   • The relationship between cartilages and muscles affects the nasal valve area
   • Understanding these relationships is crucial for functional rhinoplasty outcomes 4

3. Dermatological Surgery:

   • The nose is a common site for skin pathology and skin cancer
   • Knowledge of the layered anatomy is essential for appropriate depth of excision and reconstruction 5

Diagnostic Relevance

• Variations in external nasal anatomy may influence airflow dynamics
• Understanding the relationship between internal structures and external appearance aids in preoperative planning 2, 1

Common Pitfalls and Caveats

• Assuming the external nasal profile directly reflects the underlying skeletal structure
• Failing to account for individual variations in soft tissue thickness
• Performing deep dissections beyond 6.5 mm from midline, risking external nasal nerve injury
• Neglecting the variable course of the external nasal nerve during surgical approaches
• Using oversized implants (>13 mm) for dorsal augmentation, which may compress neurovascular structures