Paragon CRT Dual Axis - Paragon CRT Dual Axis Options for Peripheral Corneal Elevation Differences

DUAL
AXIS
®
CRT
PARAGON CRT DUAL AXIS
®
OPTIONS TO COMPENSATE
FOR PERIPHERAL CORNEAL ELEVATION DIFFERENCES
Most eyes with corneal astigmatism manifest a signicant dierence in
elevation between the steep and the at corneal meridians (Image A).
Some corneas that appear spherical may also manifest a peripheral corneal
elevation dierence. e Paragon CRT® lens design has a xed chord length
of 8 mm (6 mm treatment zone surrounded by a 1 mm return zone width).
When the elevation dierence exceeds approximately 30 microns at a chord
diameter of 8 mm, it may not be possible for the lens to reach the cornea
circumferentially in the landing zone (Image B). is creates a weakened
compressive force in the steep meridian and results in poor centration,
lens exure, and/or undertreatment. An example of an elevation map with
elevation values at 4 mm from reference center in each meridian is shown
in Image C.
A common rule in contact lens tting holds that spherical lenses will always
move freely along the steep meridian and exhibit restricted movement
along the at meridian. e clinician will observe that spherical lenses will
touch rst in the attest meridian peripherally and rock, ex or tilt over the
steepest meridian. In conventional corneal reshaping designs, the lens can
only reach the peripheral steep meridian by compressing or applanating the
at meridian or by way of lens exure. Oval treatment zones are commonly
observed with peripheral corneal elevation dierences.
Aptical Astigmatism
Image A Image B Image C
Limbus to Limbus
Astigmatism
8mm Chord Values