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.
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.