CONTACT LENS CASE REPORTS
HOW DOES THE CORNEA CHANGE BENEATH AN OK LENS?
PATRICK J. CAROLINE & MARK P. ANDRÉ
A fundamental feature of successful myopic orthokeratology (OK) is the creation of a “minus-shaped epithelial lens” (thin in the center, thick in the periphery) across the central and paracentral cornea (Figure 1). The epithelium thins and flattens over the central 2.0mm of the cornea with varying degrees of paracentral epithelial thickening that culminates at the apex of the reverse curve of the lens. The exact amount of thickening depends on the degree of myopic correction required; higher myopic corrections require greater amounts of thickening.
Figure 1. The optics of OK are the result of a minus-powered lens made of epithelium.
The epithelial changes result from an unequal tear film pressure force beneath the lens that is a positive (push) force in the center of the cornea and a negative (pull) force in the midperiphery. The perpetual pump that drives the fluid forces are the blink of the upper lid in the open-eye environment and the heartbeat-induced propulsion and retropulsion of the globe in the closed eye.
Our animal (cat) studies have shown that during the first week of OK, there is only slight central epithelial thinning of approximately 3 microns. However, there is significant paracentral thickening (38 microns) over the reverse curve of the lens (Figure 2). This appears to indicate that, in the initial stages, there is central cellular compression with intercellular fluid transfer. The corneal epithelial cells contain a multitude of structures and cytoplasmic components (i.e., water, ions, proteins, etc.). The topographical changes noted in OK may reflect the cell-to-cell transfer of these components. This process may be facilitated by the presence of gap junctions, small protein complexes that physically link the interior of one cell with the interior of adjacent cells. Compressive forces in the central epithelium may induce a movement of fluid from intercellularly toward the paracentral cornea, resulting in smaller cells in the center and enlarged cells in the midperiphery.
Figure 2. During the first week of OK, there is slight central epithelial thinning with significant “ballooning” of the paracentral cells over the reverse curve of the lens.
Following one week of lens wear, the paracentral cells display a less balloon-like shape. However, there is a dramatic increase in the number of paracentral epithelial cells due to possible: 1) cellular retention (i.e., the contact lens may act as a cap to slow down or prevent overnight epithelial cell sloughing); 2) alterations in natural cell apoptosis; and/or 3) increased cell mitosis (Figure 3).
Figure 3. After one week, the epithelial changes are extremely geographic, with the areas of greatest thinning adjacent to the peripheral alignment curve of the lens and the greatest areas of thickening adjacent to the reverse curve.
Our studies indicate that all of the optical changes in OK can be attributed to changes in the epithelial shape. However, due to the intimate interaction between the corneal epithelium and stroma, it is likely that long-term metabolic changes within the epithelium may slightly alter stromal anatomy and physiology—making this a wonderful area for further research. CLS
Patrick Caroline is an associate professor of optometry at Pacific University.
Mark André is an associate professor of optometry at Pacific University. He is also a consultant to CooperVision