ORTHOKERATOLOGY
Objectives of this lecture:
1. To understand the applications and
design/fitting principles in orthokeratology
I. DEFINITION
A method - utilizing contact lenses -
for correction of refractive error, primarily myopia, by reshaping the
curvature of the cornea. The treatment consists of fitting a series of
specially designed, progressively flatter rigid contact lenses which are
intended to graduallt remold the cornea.
II. CHARACTERISTICS
III. PATIENT SELECTION
1. Ideal Patient
• High Motivation
• Availability
• Can Afford Costs
• Low Myopes
• Low Cylinder
2. Occupational Applications
• Airline Pilots
• Military
• Firemen/Police
• Bus Drivers
IV. ORTHOKERATOLOGY VERSUS REFRACTIVE
SURGERY
1. Benefits
• Non-Invasive
• Side Effects Not Permanent
2. Disadvantages
• Temporary Change
• Unpredictable
V. BENEFITS OF CORNEAL TOPOGRAPHY IN
ORTHOKERATOLOGY
1. Selecting Good Candidates
• No irregular corneas
• No undetected pathologies
2. Orthokeratology Lens Selection
• Based on central corneal curvature
and eccentricity
3. Follow Corneal Changes with Time
4. Correlate Vision with Lens Effects on Cornea
5. Corneal Topography Change as a Predictor
• Rate of flattening = eccentricity
• In orthokeratology the central
cornea flattens and the peripheral cornea steepens
• Eccentricity gradually approaches
zero (from .4 - .6)
• Mountford Equation: y = .20x
y =
eccentricity; x = refractive change (in D)
If y = .5; x
= 2.5D
If y = .4; x
= 2D
If y = .6; x
= 3D
VI. FITTING
1. Traditional Philosophies
• Much flatter than "K"
• Variability and WTR cylinder
induced
• 1 - 3D change over 1 year
2. Accelerated Orthokeratology
• Orthokeratology lenses with
secondary curve steeper than base curve (i.e., reverse geometry design)
• These lenses accelerate the
orthokeratology effect; typically achieve same effect in 3 months as 1 year of
traditional
orthokeratology
• Reverse Geometry lenses are
typically fit 1.5 - 2D flatter than "K". They achieve 4 - 5mm central
bearing with
2 - 3mm of
mid-peripheral pooling. Good centration with limited movement is the goal.
These lenses often have
a 6 - 7mm OZD.
• The OK Series from Contex
Series OZD SCR Use
OK-2 6.5 2D
Retainer
OK-3 6.0 3D
Starting Lens
OK-4 6.0 2D
Improve Centration of OK-2
OK-5P 6.5 3D
BD prism to lower lens
OK-6 6.5 3D
For large pupils
OK-7 7.0 3D
For large pupils
• Any reverse geometry lens can be
successful including so-called RK lenses (Plateau)
Lens Changes:
• With reverse geometry designs,
refractive changes occur within a few days
• Subsequent BCR should be 0.50D
flatter than previous lens
• When spherical corneal contour is
present, no change indicated
VII. RETAINER LENS WEAR
1. Typically last OK lens or a slightly
flatter one; worn a few hours/day
2. Overnight Wear
• High Dk lens worn EW
• Eliminates problems with
irritants, wind and dust
VIII. ADVERSE EFFECTS
1. Increased Corneal Astigmatism
• Less pressure on vertical meridian
• Greater with radically flat BCR
• Lens decentration
2. Peripheral Corneal Distortion
• Too flat too soon
• Flat peripheral cornea
3. Decreased Quality of Vision
• Haziness or "watery"
vision
• Due to corneal changes/induced
astigmatism
4. Visual Problems at Near
• Hyperactivity of accommodative
system
5. Regression Toward Pre-Fit Level
Berkeley Orthokeratology Study
(Polse)
• 80 treatment subjects, 40 control
• three phases
• Evaluation of safety &
efficacy
• Results:
a. OK can
reduce myopia by 1D which can be clinically significant
b. Change is
temporary; when CL wear is reduced to 4 hours; 45% of RE change is
lost. Complete return to baseline did not occur.
c. Vision is
variable without CLs
d. Safety
acceptable although monitoring important
IX. SUMMARY