Contact Lens Design & Materials
Current Ortho-K Options
By Ronald K. Watanabe, O D, FAAO
Orthokeratology has evolved to its current status as a well-accepted, scientific part of lens practice. It is a relatively predictable option to correct myopia, hyperopia, and astigmatism. There is increasing literature supporting orthokeratology as an effective method for myopia control in children (Walline, 2009; Lotoczky, 2009; Swarbrick, 2010; Kakita, 2011) and there is a growing list of FDA-approved designs.
First FDA-approved 10 years ago, the Paragon CRT system has a seamless reverse geometry back surface design. The basic design has three primary zones: base curve/optic zone, Return Zone, and Landing Zone.
The Return Zone is a sigmoid curve that allows you to control the sagittal height of the optic zone over the central cornea. The Landing Zone is a planar zone that provides centration and stability to the lens. Both Return Zone and Landing Zone can be adjusted to change the overall sagittal depth of the lens to achieve optimal visual results.
Paragon also introduced a Dual-Axis design that incorporates different Return Zones and/or Landing Zones in the principal meridians of the lens. In simple terms, it has a toric periphery. fThis design is useful on moderately toric corneas when the standard design does not center well enough or does not correct enough astigmatism.
Another variation is the CRT for Hyperopia. It also has a Return Zone and Landing Zone, but the geometry is designed differently to steepen the central cornea (Figure 1).
Figure 1.CRT for Hyperopia with apical clearance fitting relationship.
Bausch + Lomb VST
Bausch + Lomb offers a series of designs that comprise its Vision Shaping Treatment system: Emerald, DreamLens, Contex OK E, BE Retainer, CKR, NightMove, MiracLens, Orthofocus, Vipok, and Wave. All are made by different independent GP laboratories and have reverse geometry designs that create central corneal flattening to reduce myopia (Figure 2). Some have seamless aspheric curves while others have spherical curves. Some are available in toric periphery designs for moderately astigmatic corneas. One has a high myopia design for up to –10.00D of correction.
Figure 2.DreamLens for myopia correction.
Each has a different fitting process. The Emerald only requires keratometry and refraction data to determine initial lens parameters, while the Wave design is determined entirely by corneal topography. The DreamLens can be designed, customized, and ordered through its proprietary website. The CKR lens can be ordered through an online connection between a topographer and the lab or by using a fitting set. The BE Retainer uses custom software that incorporates topography data and results from one-to two-day follow up with diagnostic lenses from the fitting set.
The Bottom Line
Whatever the system, it is important to know the fitting process and the theories behind the designs. Each lens requires certification to ensure that practitioners know how to fit them. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #196.
|Dr. Watanabe is an associate professor of optometry at the New England College of Optometry. He is a diplomate in the American Academy of Optometry's Section on Cornea and Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass. You can reach him at firstname.lastname@example.org.|