Standard, epithelium-off riboflavin/UVA corneal cross-linking is usually not recommended when patients have:

• A corneal thickness of less than <400 µm (due to the risk of endothelial damage). However, alternative procedures, such as, trans-epithelial corneal cross-linking or the use of a hypotonic riboflavin solution (to swell the cornea prior to cross-linking) may be used to treat very thin corneas.

• A history of incisional refractive surgery such as radial keratotomy or astigmatic keratotomy (because it may cause keratotomy incisions to rupture).

• Central corneal opacities and associated poor best-corrected visual acuity.

• Severe dry eye (because it may hinder reepithelialisation and thereby increase the risk of post-surgery infection).

• Herpes simplex keratitis (as corneal melting can occur).

• Severe keratoconus (with a preoperative keratometry reading of >58 D – as this presents a greater risk of continued keratoconus progression and permanent post-operative stromal haze).

• Systemic collagen diseases and/or are pregnant or nursing mothers (because insufficient investigations into the safety of the treatment in these populations has been carried out).

• A preoperative CDVA of better than 20/25 and are over the age of 35 years old