Contact Lens Case Reports
Orthokeratology Optics for Specific Patient Types
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
Today, orthokeratology (OK) is used primarily in two groups of patients: 1) in children and adolescents for myopia control; and 2) in adults to eliminate or lessen the need for daytime optical correction. For myopia control, we need an optic that presents central minus power throughout the pupil to correct the foveal refractive error and significant peripheral plus power to form a myopic defocus image shell within the eye.
These optics create high levels of spherical aberration (as well as other higher-order aberrations), which historically have been well tolerated by our younger patients. On the other hand, these optics are not well tolerated by the adult visual system.
Tailoring OK Designs
Because of the radically different optical requirements for pediatric/adolescent OK versus adult OK, we believe two different reverse geometry lens designs are needed to optimize the optics for each group. One way to achieve this is to vary the size and profile of the posterior optical zone.
Figure 1 shows a proposed tear lens profile for a –4.00D correction. The top image shows the profile for a proposed myopia control OK design with a 5.0mm posterior optical zone, 8 microns of apical clearance, and a tear film reservoir depth of 54 microns. The lower image is that of a proposed adult OK lens design with a 7.0mm posterior optical zone, 8 microns of apical clearance, and a tear reservoir depth of 34 microns. The fluorescein patterns clearly demonstrate the differences in the two lens designs.
Figure 1. Post-lens tear film profiles; myopia control lens design (top) and the adult OK lens design (bottom).
The two lenses were placed on the right eye (myopia control OK design) and left eye (adult OK design) of a single subject and worn overnight. Figure 2 shows the axial display (dioptric power) maps of the subject’s right and left eyes showing bilateral central treatment patterns, but significant differences in the size of the treatment zone. Figure 3 shows the same maps in the tangential (shape) display mode.
Figure 2. Axial display (dioptric power) maps of the two designs following one episode of overnight lens wear. Note the size differences of the treatment zones.
Figure 3. The tangential display (surface shape) maps of the two designs showing the well-centered treatment patterns.
Figure 4 shows the sectional power display maps of the myopia control OK design (upper image)and the adult OK lens design (lower image). The myopia control design has central foveal correcting minus power and increasing plus power across the 5.0mm pupil. There is less peripheral aberration with the adult OK lens. CLS
Figure 4. The power profile view maps illustrating the myopic and hyperopic powers across the subject’s pupil.
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Contamac. Mark André is an associate professor of optometry at Pacific University. He is also a consultant to CooperVision.