The cornea is an amazing piece of tissue. Located at the very front surface of the eye as the clear dome over the coloured iris, the cornea is comprised of five layers of tissue, all designed to be optically transparent. The average cornea is approximately 550 microns thick. Light must be able to pass through this structure unhindered – as well as other structures such as the crystalline lens – in order to reach the sensory retina at the back of the eye to provide vision.
The cornea also provides a surface for the tear film, the layer of moisture with multiple functions, to adhere; the cornea and the tear film provide the majority of the refractive power of the eye., meaning that light is bent by the greatest degree when passing through these structures in an effort to come to a clear and focused point on the retina; the crystalline lens accounts for the remaining third of refractive power.
Refractive error arises when the power of these anatomical components – the cornea and the crystalline lens – are not well coordinated with the physical length of the eyeball. In long-sightedness (hyperopia) the eyeball is too short for its refractive power and in short-sightedness (myopia) it is too long.
Traditional refractive surgery techniques, such as LASIK and PRK surgery, have sought to address this mismatch of refractive power to eyeball length by reshaping the cornea and changing its refraction. However, difficulties arise with a thin cornea as treatment options become much more limited when there is less corneal tissue to work with.
Thin Cornea Treatment
Some refractive procedures are better suited than others for addressing thin corneas. Treatment suitability is also subject to other eligibility criteria, such as pre-existing corneal disease and even lifestyle factors, and carefully choosing the appropriate candidates for each type of procedure is important for success.
The minimum corneal thickness required for refractive surgery is dependent on the degree of refractive error that needs to be corrected – the higher the prescription, the more corneal tissue that needs to be removed. If too little residual corneal tissue is left after surgery there is the risk of a condition known as keratectasia, which refers to warping and distortion of a structurally weakened cornea.
As the LASIK procedure involves the creation of a flap of corneal tissue, this already reduces the amount of tissue available for reshaping.
PRK surgery circumvents this problem by simply removing the very top layer, known as the epithelium, which leaves the rest of the corneal tissues available for sculpting by excimer laser.
For patients whose corneal thickness and prescriptions exclude them even from PRK surgery, there are still other treatment options available in the form of refractive surgery techniques that do not involve the typical laser reshaping of the cornea.
Intraocular contact lenses, or implantable contact lenses (ICL), involve an artificial lens made of biosynthetic materials inserted between the iris and the crystalline lens.
The only disruption to the cornea is in the form of a small incision in order to position the intraocular contact lens inside the eye. Because no corneal tissue is removed during this procedure, there is no minimum corneal thickness requirement, and intraocular contact lenses are able to correct very high degrees of myopia, hyperopia, and astigmatism that would otherwise be unachievable with corneal refractive techniques such as LASIK or PRK.
Refractive lens exchange (RLE) is a procedure akin to cataract surgery but just performed in the absence of a cataract (that is, while the crystalline lens is still clear). It involves removal of the eye’s natural crystalline lens and replacing it with an artificial lens implant, known as an intraocular lens, which is calculated to account for the refractive error of the eye.
Similar to the intraocular contact lens technique, refractive lens exchange does not require any removal of corneal tissue and involves minimal disruption to this structure. Patients with thin corneas and who have passed the age of presbyopia (the natural age-related decline of near vision) are best suited to this procedure.
Although several other eligibility criteria apply to ensure that any refractive surgery technique is safe and effective for you, there are options in the form of both traditional and newer surgical procedures to allow patients with thin corneas the freedom from glasses and contact lenses.
To discuss which treatments are best for you, contact Eye Laser Specialists now to organise your initial consultation.
Note: Any surgical or invasive procedure carries risks. Before proceeding, you should seek a second opinion from an appropriately qualified health practitioner.