What is the middle lamella of the eye?

The Middle Lamella of the Eye: Understanding Corneal Anatomy

The middle lamella refers to the lamellar interface or junction between corneal stromal layers in the eye, which is a critical anatomical structure targeted in lamellar keratoplasty procedures.

Anatomical Structure and Function

The cornea consists of several distinct layers, with the stroma making up approximately 90% of the total corneal thickness. The stroma is composed of multiple lamellae (sheets) of collagen fibrils arranged in an organized pattern that provides both transparency and mechanical strength to the cornea 1.

These stromal lamellae have specific characteristics:

• They are arranged in orthogonal layers
• Contain densely packed collagen fibrils (25-30 nm in diameter)
• Are organized in a regular pattern that minimizes light scattering
• Provide mechanical stability while maintaining transparency

Clinical Significance in Corneal Surgery

The middle lamella is particularly important in corneal surgical procedures, especially:

Anterior Lamellar Keratoplasty (ALK)

• Used for treatment of anterior stromal opacities
• Involves creating a lamellar interface between corneal layers
• Can be performed using manual techniques, microkeratome, or femtosecond laser 2
• The quality of the lamellar interface significantly impacts visual outcomes

Deep Anterior Lamellar Keratoplasty (DALK)

• Removes all or nearly all corneal stroma while preserving Descemet's membrane and endothelium
• Creates a lamellar interface at the deepest stromal level
• Provides significant advantages over penetrating keratoplasty including:
  • Reduced risk of endothelial rejection
  • Greater wound strength
  • Faster recovery time 2

Surgical Considerations

When operating at the middle lamella:

1. Interface quality is critical for visual outcomes:

   • Smooth interfaces result in better visual acuity
   • Irregular interfaces can cause scarring and reduced vision 2

2. Modern techniques have improved lamellar interface creation:

   • Femtosecond lasers create more precise lamellar cuts
   • Microkeratomes provide smoother interfaces than manual dissection
   • These technologies have significantly improved visual outcomes in lamellar procedures 2

3. Potential complications include:

   • Interface scarring or haze
   • Irregular astigmatism
   • Interface deposits or opacities 2

Pathological Implications

Disruption of the normal lamellar architecture can lead to:

• Corneal ectasia (as in keratoconus) due to biomechanical weakening 3
• Altered distribution of elastic microfibrils between lamellae
• Changes in corneal transparency and mechanical strength

Clinical Pearls and Pitfalls

• When performing lamellar procedures, achieving a smooth interface between lamellae is essential for optimal visual outcomes
• Excessive ablation beyond the anterior third of the corneal stroma risks corneal ectasia 4
• Application of masking agents during PTK helps create smoother post-ablation surfaces when treating irregular interfaces 2
• Mitomycin-C may be used to prevent scarring at lamellar interfaces, but carries risks including keratocyte depletion and endothelial toxicity 2

Understanding the middle lamella is crucial for corneal surgeons performing lamellar procedures, as the quality of this interface directly impacts visual outcomes and the success of corneal transplantation.