Why is a pterygium (a growth on the eye) typically triangular in shape?

Why Pterygium Has a Triangular Shape

The triangular shape of pterygium is primarily due to its growth pattern from the bulbar conjunctiva across the limbus onto the cornea, with the apex pointing toward the center of the cornea and the base at the conjunctiva.

Anatomical and Pathophysiological Basis

Pterygium develops as a fibrovascular growth that follows a specific pattern:

• Origin and Direction: Pterygia typically originate from the nasal bulbar conjunctiva and grow horizontally toward the cornea 1
• Growth Pattern: As the pterygium extends from its conjunctival base across the limbus, it naturally forms a triangular shape with:
  • Base: Wider portion anchored at the conjunctiva
  • Body: Narrowing as it crosses the limbus
  • Apex/Head: Pointed end extending onto the corneal surface

Contributing Factors to Triangular Morphology

Several factors contribute to this characteristic triangular shape:

1. Ultraviolet Radiation Exposure

   • UV light is the primary trigger for pterygium development 2, 3
   • The nasal limbus receives more concentrated UV exposure due to the cornea's curvature and light reflection patterns
   • This focused exposure creates a triangular pattern of cellular damage and growth

2. Limbal Stem Cell Alteration

   • UV exposure causes molecular genetic abnormalities in limbal stem cells 4
   • These altered cells proliferate in a directional pattern toward the cornea
   • The growth follows the path of least resistance, naturally forming a triangular shape

3. Fibrovascular Growth Dynamics

   • Pterygium involves proliferation, inflammatory infiltrates, fibrosis, and angiogenesis 5
   • The fibrovascular tissue grows more densely at the leading edge (apex)
   • This creates the characteristic triangular or wing-shaped appearance (the term "pterygium" derives from the Greek word "pterygion" meaning "small wing")

4. Tear Film Dynamics

   • Tear flow patterns across the ocular surface influence the growth direction
   • The nasal-to-temporal flow of tears helps shape the growth into a triangular pattern

Clinical Implications

The triangular shape has important clinical implications:

• Visual Impact: As the triangular growth extends toward the pupillary area, it can cause astigmatism and visual disturbances 2
• Surgical Considerations: The triangular shape influences surgical approaches, particularly when considering conjunctival autografts to cover the defect after excision 1
• Recurrence Patterns: Recurrent pterygia tend to follow the same triangular growth pattern but may be more aggressive 4

Management Considerations

Understanding the triangular morphology helps in treatment planning:

• Surgical excision remains the primary treatment, with various techniques available 2
• The triangular defect created after excision must be addressed to prevent recurrence
• Adjuvant therapies like beta-irradiation, mitomycin C, and anti-VEGF agents may help reduce recurrence rates 6

The triangular shape of pterygium is not merely a coincidental finding but reflects the underlying pathophysiology and growth dynamics of this common ocular surface condition.