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The Scleral Lenses
Solution - Onefit |
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FUNDAMENTALS
The
geometry of the Onefit P+A is specifically designed for corneas with a
normal prolate profile, astigmatism, post grafts ( prolate profile ) or
moderately irregular corneas such as emergent or fruste keratoconus
cases. It also is available with multifocal optics, delivering superior
visual performance for today’s active presbyopes.
Onefit is
designed to vault a given topography with an optimal sag height, and is
specified by the value of the base curve in mm; simplifying the fitting
process and making it more user-friendly.
FITTING
PHILOSOPHY
Onefit 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, 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 170 to 185 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.
Based on clinical
trials, every 0.10 mm change in the base curve value results in an
average variation in central clearance of 30 to 35 microns. For example,
if the base curve is steepened by 0.10 mm the apical clearance will
increase by 30 to 35 microns. On the other hand, if the base curve is
flattened by 0.10 mm the apical clearance will be reduced by
approximately 30 to 35 microns. |
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