In a normal, untreated eye, the image of what the eye is looking at will be focused on the retina (central), but the peripheral images may be focused behind the retina leading to “hyperopic peripheral defocus”. Research studies proposed that this hyperopic peripheral defocus may be the cause of the myopic progression. 

The cornea is reshaped in ortho-k to effect myopic reduction (i.e. temporary reduction of myopia such that ortho-k wearers may not need to wear glasses during the day time). The reshaped cornea also leads to a reduction in hyperopic peripheral defocus and researchers have proposed this to be the mechanism by which ortho-k lens wear effects slowing of myopic progression.