Scleral Contact Lenses
Scleral contact lenses, as the name implies and in contrast to corneal contact lenses, bear their weight on the portion of the eye called the sclera, or the "white of the eye". These lenses are typically even larger than soft contact lenses. While not restricted to being used only with corneas that have been damaged or distorted by ectatic conditions like keartoconus, keratoglobus and post lasik ectasia, the greater unit cost and more challenging handling characteristics of the lenses has made them most appreciated by people with compromised best-corrected vision.
In the above header picture, a nice detail of a scleral lens shows the vaulting of the lens over the distorted cornea, filled in behind the lens and in front of the cornea with a protective layer of saline and the peripheral distribution of the weight of the lens on the sclera. Soft contact lenses, by contrast, 'drape' the cornea and sclera, adhering via capillary attraction, while corneal lenses, typically rigid gas-permeable lenses, bear their weight on the much more acutely sensitive cornea. In spite of their much greater size, scleral lenses are most often considered more comfortable than corneal lenses, approaching the quality of comfort of a soft contact lens.
Scleral contact lenses require more sophisticate evaluation of the bearing characteristics of the lens than do rigid corneal lenses or soft contact lenses. While it is possible to accomplish this via slit lamp biomicroscopy, ocular coherence tomography (OCT) provides distinctly superior analysis of the magnitude of vault over the ectatic cone of the cornea. Scleral lenses are only available as a custom ordered lens. Multiple visits to confirm the quality of the fit are required to confirm the maintenance of the 'vault', the absence of compression on the scleral conjunctiva or lens bearing on the limbus. Special, "non-preserved" saline is necessary to fill the space between the lens and cornea and is used at every insertion of the lens.
Scleral contact lens designs are not new. However, it has only been with the introduction of rigid lens materials that combine silicone and fluorine into the plastic that allow the lenses to provide oxygen transmissibilty sufficient to maintain the high metabolic rate of the cornea. Computer controlled lathes are able to fabricate lenses in designs that are customizable enough to meet the variable anatomical features of the human eye, made only more complex by ectatic corneal conditions.