Learn the basics of design

In recent years, orthokeratology (ortho-k) has gained popularity for myopia control due to mounting evidence that children undergoing ortho-k have 37% to 63% less axial elongation than those in spectacles or single vision day-time contact lenses to correct their myopia.¹ Other benefits of ortho-k: freedom from spectacles or daytime contact lens wear; and a high-safety track record in both children and adults. In fact, an investigation comprised of 204 eyes of children and 392 eyes of adults, which ran from 1997 to 2015, shows an incidence of microbial keratitis of about seven cases per 10,000 patient years in adults, while no cases were reported in children.²

Given these advantages, those optometrists who remain in the dark on this close-to mainstream treatment should learn the basics of ortho-k design.

MATERIALS

Ortho-k lenses are designed in materials that deliver high-oxygen levels. Lenses manufactured in materials with a transmissibility (Dk/t) of 87 or higher meet the Holden and Mertz criteria for limiting overnight corneal swelling to levels consistent with no contact lens wear. Also, high-Dk materials improve clinical efficacy in ortho-k.³

ARCHITECTURE

An ortho-k lens consists of four to six zones. The overall diameter should be .8 mm to 1.2 mm smaller than the corneal diameter. The central or treatment zone, 5.0 mm to 6.8 mm in diameter and defined by the base curve, is determined by the amount of myopia.

The reverse zone, 0.5 mm to 1.0 mm wide, creates the desired amount of apical clearance. The relief zone, when present, is 0.5 mm to 0.7 mm wide, with a depth of 10 μm to 20 μm. It helps to stabilize the lens. The alignment curve is determined by the corneal eccentricity and is 0.5 mm to 1.0 mm wide; it is where the lens lands on the eye. The most peripheral curves, forming the lens edge with a depth of approximately 80 μm, prevent lens binding and further help the lens to properly align.

The relative peripheral myopic defocus created by ortho-k is believed to slow the progression of myopia. Thus, when designing lenses for myopia management in children, it is best to use a smaller optic zone, around 5.4 mm, to ensure delivery of the peripheral optics through the pupil.³

FITTING

Manufacturers provide contemporary ortho-k designs, some of which are fitted empirically; others fitted via diagnostic fitting sets. (See “Ortho-K Companies,” below to access information on their individual fitting guides.) As with any contact lens fitting, ODs should not rely solely on the computerized nomograms to gauge final fit and clinical performance. Knowing the design and fitting principles is essential to ensuring corneal integrity.

REWARDS ABOUND

Offering ortho-k is an impactful way to grow the pediatric demographic of clinical practice and differentiate the OD’s services. Patient satisfaction continues to fuel the clinical benefits and professional rewards. OM

References:1. Walline JJ, Jones LA, Sinnott LT. Corneal reshaping and myopia progression. 
Br J Ophthalmol. 2009; 93:1181-1185. doi: 10.1136/bjo.2008.151365. 

2. Gispets J, Yébana P, Lupón N, et al. Efficacy, predictability and safety of long-term orthokeratology: an 18-year follow-up study. Cont Lens Anterior Eye. 2022;45(1):101530. doi: 10.1016/j.clae.2021.101530.

3. Lum E, Swarbrick HA. Lens Dk/t influences the clinical response in overnight orthokeratology. Optom Vis Sci. 2011;88(4):469-75. doi: 10.1097/OPX.0b013e31820bb0db. PMID: 21270672. 

DR. RESNICK is president and managing partner at Drs. Farkas, Kassalow, Resnick, and Associates, a contact lens and anterior segment specialty practice in New York. She is a diplomate in the Cornea, Contact Lens, and Refractive Technology section of the AAO, a diplomate of the ABO, and a fellow of the Scleral Lens Education Society.