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Prolate, Oblate and Not-so-Great: GP Contact Lens Management of Irregular Corneas
I.
Goals of GP Lens Fit
A. A well centered lens
B. No areas of harsh corneal bearing or excessive clearance
C. Patient comfort
D. Good visual rehabilitation
E.
Proper tear exchange
II. Base Curve Determination
A. By keratometry
1) Readings are difficult secondary to corneal distortion
2) Range extension with trial lenses (+2.25,+1.25,-1.00 diopters)
B. By topography
1) Provides information of steep/flat areas
2) Provides information on where GP lens will probably position
3) Provides a starting point for the selection of diagnostic lenses
C. By fluorescein pattern analysis
1) Use to select subsequent diagnostic lenses
2) Balance between minimal corneal bearing and minimal corneal clearance
3) Aim for adequate peripheral clearance
III. Lens Parameter Determination
A. Base curve radius
1) Final base curve determination based on:
a.
Fluorescein pattern analysis
b.
Lens position
c.
Lens movement
2) Make changes during follow-up visits based on:
a.
Fluorescein pattern analysis
b.
Corneal staining
c.
Patient’s symptoms
i.
Excessive lens movement
ii.
Lens edge awareness
B. Contact lens power
1) Determine by over-refracting the endpoint diagnostic contact lens and adding
the vertexed equivalent diopter sphere to the power of the diagnostic lens
2) SAM-FAP
3) Residual astigmatism
a.
Bitoric
b.
Front surface toric
c.
Spectacle over-correction
C. Overall diameter
1) Generally larger (10.5mm – 11.2mm) and beyond
a.
To improve lens centration and stability (for example, post-PKP & post-RK
cases)
b.
To place the OZD over the visual axis in pellucid marginal degeneration
cases
D. Optic zone diameter
1) Dependent on lens positions and pupil size
2) Large enough to vault corneal graft
3) Slightly larger than graft diameter in reverse geometry lens designs
E.
Peripheral curve(s)
1) Aspheric design
2) Reverse geometry design for oblate corneas
F.
Material selection
1) Post-surgical corneas should be prescribed with high Dk materials, for example
Dk 100
IV. Piggyback lens systems
A. Silicone hydrogels
B. To enhance GP lens centration (moderate plus power, for example +3.00)
C. To improve optics
D. To enhance patient comfort (low minus power, for example -0.25 or -0.50)