**Question:** Intrastromal corneal ring segments (intacs) are used in all except ____________.

**Core Concept:** Intrastromal corneal ring segments (intacs) are small, semi-circular, plastic rings that are implanted into the cornea to modify its shape and improve vision. They are commonly used in the treatment of certain corneal ectasia and irregular astigmatism conditions.

**Why the Correct Answer is Right:** Intrastromal corneal ring segments are typically used to correct refractive errors, keratoconus, or other corneal ectasia-related conditions. They are not indicated for cases where the corneal stroma is diseased or damaged, as they would not be effective in these situations.

**Why Each Wrong Option is Incorrect:**

A. Corneal dystrophies: Intrastromal corneal rings are not indicated in cases with dystrophies affecting the corneal stroma, as they would be ineffective in these conditions.
B. Herpetic keratitis: Corneal stromal disease due to herpes simplex virus infection is contraindicated for intacs placement because the disease process would likely worsen the condition rather than improve vision.
C. Traumatic corneal ulcers: In cases of corneal ulcers caused by trauma, intacs placement would not be effective due to the damaged corneal stroma.
D. Epithelial diseases: Conditions affecting the corneal epithelium, such as chemical burns or infections, are not suitable for intacs placement, as the underlying corneal stroma would be affected.

**Clinical Pearls:**

Intrastromal corneal ring segments are a valuable tool for correcting refractive errors and managing specific corneal ectasia conditions. However, they are contraindicated in cases where the corneal stroma is diseased or damaged, as they would not be effective in these situations.

**Correct Answer:** D. Epithelial diseases

Intrastromal corneal ring segments are typically used in cases of corneal ectasia and irregular astigmatism, but they are contraindicated for conditions affecting the corneal epithelium. This includes epithelial diseases, such as chemical burns or infections, where the corneal epithelium is damaged.