BACKGROUND

The vault and peripheral system of this mini scleral lens is specifically designed to accommodate corneas with a normal prolate profile, corneas with astigmatism, post grafts (prolate profile) or moderately irregular corneas such as emergent or fruste keratoconus cases. Although the lens is designed with the purpose of vaulting a given corneal topography with an optimal sag height, the lens is specified by the value of its base curve in mm to make the fitting process simple and user friendly.

NOTE: For highly irregular corneas such as keratoconus and ectasias, the Onefit Cone Series is preferred. Some post-refractive surgery patients can also be fitted with Onefit Cone lenses, or with a reverse geometry smaller gas permeable lens, such as RSS from Blanchard Contact Lens.

FITTING PHILOSOPHY

Onefit P+A series is supported by the conjunctiva and the fluid layer under the lens, rather than by the cornea. It is designed to vault the entire corneal surface including the limbal area. However, the clearance over the cornea varies from center to the periphery to optimize oxygen transmission to the tissue, especially over the limbus where the stem cells are located. Current published work indicates that fluid layer thickness is an important concern with scleral lenses. With an estimated DK value of 80, an excessively thick fluid layer under the lens can deprive the cornea and stem cells of sufficient oxygenation. Keeping the Harvitt-Bonanno and Holden-Mertz criteria in mind, Onefit was designed to maximize oxygen transmission when combining the lens and tear layer thickness. For this reason the lens is thinner over the limbus compared to other designs. Its unique geometry reduces the tear layer from the center out to the limbal area. Optimum results in fit and corneal health are achieved with a clearance of 150 to 175 microns at the apex of the cornea, or in the area where the cornea is steepest (e.g. host-graft corneal junction) with limbal clearance not exceeding 40 microns. The lens peripheral edge should align with the conjunctiva. Onefit is manufactured in materials offering a minimum permeability of 100 DK.