AAO-CE 2010 Course:
Case Studies: Fitting Keratoconus patients with the use of VKE to determine the BOZD
and LD of corneal contact lenses
by L. Sorbara, OD, MSc, FAAO, Dipl C&CL, University of Waterloo, School of
Optometry, Waterloo, ON Canada
Learning Objectives
1. To review video-keratoscopic principles of measurement of corneal contour.
2. To understand and interpret topographical maps produced by the corneal
topographers.
3. To identify the type of cone that each case has and how to select the appropriate
contact lens based only on this information.
4. To review contact lens design options appropriate for fitting the Keratoconic
patient and how to select the initial lens parameters that is the base curve, lens
diameter and back optic zone diameter using the maps.
Course outline
Video-keratoscopy (VKE) has become a standard instrument in private practice, to
determine the shape of the cornea. The keratometer which is most widely used to
determine corneal curvature is limited to a central 3 mm zone and provides little
information on the asphericity of the cornea, which defines its shape. Electronic devices
based on the Placido disc (providing radius of curvature) and slit scanning devices
(providing elevation data) can help define the shape of the cornea essential for contact
lens fitting of normal and keratoconic patients, pellucid marginal degeneration (PMD),
orthokeratology and corneal surgical procedures as LASIK and penetrating
keratoplasty. Apically decentred corneal shapes are more easily identified with the use
of video-keratoscopy with the use of tangential radius of curvature maps.
Lens movement, centration and comfort are determined by the contact lens parameters
and in particular: the back optic zone diameter (BOZD), the lens diameter (LD) and the
relationship of the back optic zone radius (BOZR) to the corneal curvature. Balancing
these parameters to achieve excellent centration, movement and comfort can be difficult
especially with advanced cases of keratoconus and PMD where the cone diameters can
increase in size and apically decentre. Matching the lens design with the corneal shape
is a step towards achieving the goal of centred, moving with the blink and comfortable
contact lenses.
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This CE course will examine the use of video-keratoscopy, using maps from the
Orbscan II and Atlas corneal topographers, in fitting corneal contact lenses to four
keratoconic patients. Each case has been selected to represent a type of cone (centred,
oval or pellucid marginal degeneration) and range in severity from mild to severe. From
each topographic map data needed to fit the correct lens diameter (LD) and back optic
zone diameter (BOZD) will be extracted and related to the type and centration of the
cone, along with its diameter.
This course will attempt to increase the practitioners’ awareness and ease in managing
these patients using a case study format.
Topographic data to be collected:
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Simulated K readings/Average K readings
Steepest K readings
Centration/position of the cone
Eccentricity value(s)
Diameter of the steepest portion of the cone
Diameter of the entire cone area
Contact Lens Variable related to Topographic data:
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K readings relate to back optic zone radius (BOZR) i.e. cone severity
Type of cone relates to type of lens (floating of fixed BOZD lenses)
E values relate to axial edge lift of the lens periphery
Diameter of the cone related to lens back optic zone diameter/overall diameter
Once the diameters are determined, adjust back optic zone radius to compliment
lens sagittal depth
Reading References:
Sorbara L, Chong T, Fonn D, (2000) Visual Acuity of Keratoconic Eyes as a Function of
Rigid Gas Permeable Contact Lens Base Curves, Contact Lens and Anterior Eye,
23:2,48-52.
Sorbara L, Luong J. (1999) Contact Lens Fitting Guidelines for the Keratoconic Patient
Using Videokeratography Data, Practical Optometry, Dec. 10:6,238-243.
Mandell RB. The enigma of the corneal contour. CLAO J 1992;18:267-72.
Roberts C. A Practical Guide to the Interpretation of Corneal Topography. Contact Lens
Spectrum 1998;13:25-33.
Moezzi A, Fonn D, Simpson T, Sorbara L. Contact lens induced corneal swelling
measured with the OrbscanTM II Corneal Topographer. OVS 2004;81(3).
Sorbara L, Richter D, Chong T, (1998) Evaluation and Comparison of Video-Keratoscopic
Simulated Fluorescein Programs, CJO Vol 60, No. 3; 158-163.
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