Negative Image Shell

an explanation from Cary M. Herzberg, OD on the mechanism of ortho-k myopia control:

Image Shells and Ortho-k: Preventive Care With the discovery of the phenomenon of form deprivation myopia, research has demonstrated that ocular growth and refractive development are regulated by visual feedback associated with the eye's refractive status. The conclusions of Dr. Earl Smith and colleagues (2009) point to the peripheral retina in primates as being more influential for controlling eye growth than previously thought possible. When susceptible visual systems are corrected with myopic lenses, it creates an image shell described as a positive curvature of field. These visual systems, while being corrected centrally, suffer from a relative hyperopia peripherally that causes conflicting visual stimulus. The result is elongation of the eye to correct the peripheral hyperopia. If, however, the peripheral retina is experiencing a relative myopia effect (negative curvature of field), then elongation may slow. The measurement of this discrepancy between peripheral and central retina is performed via peripheral refraction (Mutti, 2007).

Unlike conventional methods of correcting myopia such as glasses or regular contact lenses, myopia-correcting corneal reshaping lenses create a negative image shell on the retina of a treated patient (Figure 1). It is not yet apparent what ortho-k design is optimal to accomplish the best image shell. It may also be possible to track changes in peripheral refraction through corneal topography. Future research will include the optimization of orthokeratology to correct peripheral refractive error and the development of testing equipment that can measure a patient's susceptibility to develop myopia based on peripheral refraction (Mutti, 2007). This could lead to early treatment with corneal reshaping of those patients most susceptible to axial elongation of the eye (Cross, 2008). It is with great optimism that I look forward to a future of improved, efficient, and efficacious delivery of corneal reshaping products to solve our patients' corneal and refractive conditions.

excerpt from THIS ARTICLE


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