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Minneapolis?Refractive surgeons should incorporate rather than dismiss corneal refractive therapy (orthokeratology) and contact lens fitting because those techniques can have a synergistic benefit rather than play a competitive role in a refractive surgery practice, according to David R. Hardten, MD.
"Our goal as clinicians is to address our patients' needs, and for certain patients who are not ready for surgery, orthokeratology or contact lenses can provide an appropriate and effective solution. Keep in mind as well that these individuals may become more interested in refractive surgery in the future, and so it is to the benefit of your practice if you can retain them under your care by having options available," said Dr. Hardten, adjunct associate professor of ophthalmology, University of Minnesota, Minneapolis, and director of clinical research, Minnesota Eye Consultants, Minneapolis.
He noted there have been advances in design and materials for contact lenses and orthokeratology products. As a result, those modalities are now much safer than in the past. In addition, with orthokeratology, the treatment effect can now be achieved much faster (usually within a month) using just one or two pairs of high-Dk rigid gas-permeable lenses worn overnight. In the past, when orthokeratology involved daily wear of polymethylmethacrylate (PMMA) lenses, patients used an average of eight pairs of lenses in a process that took 9 to 12 months.
A temporary fix
"Patients need to understand the limitations of these lens-based modalities, including the temporary nature of the vision correction and the inability of orthokeratology to correct astigmatism predictably, along with the risks. In our consultation discussions, it is also important to share the outstanding results that can now be achieved with customized wavefront laser surgery, including the benefits of iris registration for improving outcomes in patients with higher levels of astigmatism, and the opportunity to achieve more accurate and longer-lasting vision correction with the surgical approach," Dr. Hardten said.
Another consideration is that orthokeratology can only achieve a limited amount of spherical correction. Its mechanism of action is thought to involve compression-induced migration and redistribution of corneal epithelial cells that results in central thinning and midperipheral thickening. Considering that only about 20 μm of epithelium can be displaced and applying that value in Munnerlyn's formula, only about 1.75 D of refractive change can be achieved when maintaining an optical zone of 6.0 mm. For an optical zone of 4 mm, the amount of refractive change attainable is about 4 D.
Greater amounts of correction would necessitate the use of smaller optical zones that can introduce significant problems with halos. Halos associated with orthokeratology might also be explained by their effects on the wavefront profile, Dr. Hardten said.
"In a study where measurements were performed with a 6-mm pupil in eyes with 3 D of change, higher-order aberrations increased 2.5 times compared with preoperatively," he reported.
Orthokeratology also does not achieve the same high levels of excellent visual acuity that are associated with wavefront-guided laser correction. Results from various studies report that up to two-thirds of patients achieve 20/20 uncorrected visual acuity (UCVA). In one study of carefully selected candidates, 100% were seeing 20/25 or better uncorrected.
"However, that still does not match the 90% to 95% rates of 20/20 or better UCVA that can be achieved with custom wavefront LASIK, especially in these low levels of spherical corrections. In addition, the good acuity with orthokeratology is not stable throughout the day for all patients, and that fluctuation is a major reason for discontinuing orthokeratology," Dr. Hardten said.