What is Cornea?

The CORNEA is located at the front of the eyeball. It not only protects the eye from trauma and infection, but is also the first and strongest refractive medium to allow light to enter into the eye. Besides the opacified lens, cornea is the only other tissue in the eye that can be replaced. As deceased corneal donation is not widely accepted, not many patients can receive corneal transplant surgery to restore their sight. Cornea is very crucial for our sight and we have to protect it carefully.

Cornea is divided into five layers, in order from the outside to the inside as below:

Epithelium Structural feature:
- Consists of 5 to 6 layers of epithelial cells, approx. 0.05mm thick

Physiological characteristics:
- Has the ability to regenerate, renewing every seven days (differentiated from the limbal stem cells)
Bowman’s membrane Structural features:
- Located below the epithelium, around 0.01 mm thick
- No cell in the structure, is a transparent film mainly composed by collagen fibers

Physiological characteristic:
- Cannot regenerate and self-repair after injury, will scar and affect the vision
Stroma Structural features:
- The thickest and strongest layer of the cornea. Its thickness is about 0.5mm, which is 90% of the total tissue.
- It is mainly compromised of collagen fibers type I.

Physiological characteristics:
- Cornea has a high transparency because of the regular arrangement of collagen fibers with a specific diameter and spacing. Injuries can cause scarring and affect vision.
- Cannot protect the eyeball if it becomes too thin or perforated after a serious injury
Descemet’s membrane Structural feature:
- Consists of collagen secreted by corneal endothelial cells, around 0.1mm thick for adult.
Endothelium Construction features:
- Single layer of hexagonal cells
- Limited ability to regenerate

Physiological characteristic:
- Maintaining the cornea in a dehydrated state, which is one of the main factors for maintaining cornea transparency
.

What is Corneal Blindness?

Conditions that cause the cornea to no longer be transparent, or may even cause pain. These conditions may require corneal transplant surgery to restore the vision for patients.

External problems: infections, inflammations, accidents (hit by a hard object or splashed with a corrosive liquid) can perforate or leave a scar on the cornea.

Structural problems: some genetic conditions or frequent eye rubbing may loosen the corneal structures and cause a conical protrusion in the cornea (keratoconus), where light rays cannot be properly focussed.

Congenital problems: such as when cornea endothelial cells are lost too quickly and fluid (edema) is trapped in the cornea, or there is abnormal deposition of material in the cornea, causing clouding of the cornea.

Other ophthalmic surgery problems: if the patient has undergone other complex or multiple eye surgeries, too many cornea endothelial cells may be lost and fluid (edema) may be trapped and cannot be pumped out of the cornea.

Types of Cornea Transplant

Penetrating Keratoplasty (PKP)

Removed tissue:
Full-thickness

Wounds and stitches:
At least 16 stitches, large wound

Astigmatism after surgery:
High in the initial stage and gradually decreases after stitch removal

Risk of rejection: Small

Recovery time:
Stitch may be removed after 12 months, and will have residual refractiv​e and astigmatic errors; wound not as strong and need ocular protection

Deep Anterior Lamellar Keratoplasty (DALK)

Removed tissue:
The diseased front layer

Wounds and stitches:
16 stitches, less invasive than PKP

Astigmatism after surgery:
High in the initial stage and gradually decreases after stitch removal

Risk of rejection: Very small

Recovery time:
Stitch may be removed after 6-12 months, and will have some residual astigmatism

Endothelial keratoplasty (EK)


Removed tissue:
The diseased endothelial layer

Wounds and stitches:
No stitches on the transplanted cornea

Astigmatism after surgery:
About 1-2 dioptres, similar as before surgery

Risk of rejection: Minimal

Recovery time:
70-80% of the eyesight can be restored in 1 month and fully recover in 6 months