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Publication Exclusive: Contact lens-assisted cross-linking treats corneal ectatic disorders in thin corneas
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Collagen fibers in the human body are normally bonded together and stabilized by covalent cross-links. The tensile strength of mature collagen fibers is largely due to intermolecular covalent cross-links. Collagen cross-linking was described by Theo Seiler et al as a means to strengthen a weak and ectatic cornea. This works by utilizing ultraviolet A at a wavelength of 370 nm in the presence of the photosensitizer riboflavin to create an increased number of covalent bonds among corneal stromal collagen fibers.
Riboflavin is a photosensitizer and causes damage at a lower UVA irradiance level of 0.36 mW/cm2. The creation of oxygen free radicals leads to induction of collagen cross-links, which in turn leads to more compact interlamellar connections. Riboflavin also has an additional protective role by increasing the absorption coefficient, thereby decreasing final irradiance at the endothelial level down to 0.18 mW/cm2. However, 400 µm of riboflavin-saturated stroma above the endothelium is considered safe to avoid adverse effects. In patients with corneas thinner than this, conventional cross-linking cannot be performed. For patients with corneal stromal thickness between 350 µm to 400 µm after epithelial removal, we have performed a new technique called contact lens-assisted cross-linking (CACXL). This was described by Soosan Jacob.
A pre-corneal riboflavin film, a riboflavin-soaked UV barrier-free soft contact lens of negligible power and a pre-contact lens riboflavin film are used to decrease UV irradiance to safe levels at the level of the endothelium.
Technique
Preoperatively, lidocaine 2% and pilocarpine 2% are instilled to aid in epithelial removal and to promote miosis and reduce UVA exposure to the lens and retina. The central 9 mm of corneal epithelium is abraded. Iso-osmolar riboflavin 0.1% in dextran T500 is applied every 3 minutes for 30 minutes. At the same time, a SofLens daily disposable soft contact lens (Bausch + Lomb) made of hilafilcon without UV filter and of negligible power is immersed in isotonic riboflavin for 30 minutes. At the end of 30 minutes, adequate corneal saturation with riboflavin is confirmed by visualization of a green flare in the anterior chamber using a slit lamp. The riboflavin-soaked contact lens is then applied on the corneal surface, and thickness is measured again. Once confirmed to be more than 400 µm, treatment is continued.
The central 9 mm of the cornea is exposed to UVA light of 370 nm with an irradiance of 3 mW/cm2 for 30 minutes. During CACXL, riboflavin solution is generally reapplied once in 3 minutes. Any buckling of the contact lens is handled by reapplying sufficient riboflavin solution under and above the contact lens when required, allowing it to spread uniformly and encouraging the patient not to move his eyes or squeeze his lids. Riboflavin applied over the contact lens fills any persistent troughs on the contact lens surface and also gives a uniform layer over the lens (corresponding to standard CXL requirements). At the end of surgery, the eye is washed and a fresh bandage contact lens is applied until corneal epithelial healing takes place. Postoperatively, antibiotic drops are given until epithelial healing occurs followed by fluorometholone-tobramycin eye drops once epithelial healing is complete. The patient is also advised to wear UV-protective glasses.
Harvesting the PDEK graft is easy and can be done in donor corneas of any age. After achieving a type 1 big bubble, the donor graft is trephined along the margins of the bubble. The bubble is pierced at the extreme periphery, and trypan blue injected into the bubble to stain the graft. The PDEK graft is then cut around the trephine mark with a pair of Vannas scissors and placed in the tissue culture medium. The final size of the graft after cutting may be slightly larger than the trephine. The graft is loaded into an injector when ready for insertion.
Click here to read the full publication exclusive, Complications Consult, published in Ocular Surgery News APAO Edition, February 2016.