The holy grail of refractive procedures is to have excellent far, intermediate, and near distance vision, as in one's prime. However, once the eye loses its ability to accommodate, the only way to achieve that goal is by increasing depth of focus. The shape and structure of the cornea influences depth of focus, and correction may be achieved in different ways. Recently, my colleagues and I devised a new strategy—intentionally changing the shape of the cornea to increase depth of focus.
We now create a hyperprolate cornea as the preferred treatment in 25% of our presbyopic patients. The prolate cornea is steeper in the central region and gets flatter toward the periphery. Patients with this corneal shape typically continue to have good reading ability as they age. We must also increase the spherical aberration in this system to create better intermediate distance vision.
HOW DOES IT WORK?
One strict guideline is that the concept of creating a hyperprolate cornea should be used only with an enhanced monovision strategy. Therefore, we correct the dominant eye for far distance—without any modification and zero aberrations if possible—and create micro-monovision (approximately 1.00 D) in the nondominant eye. We create the hyperprolate cornea only in the nondominant eye.
Pupil mobility is also an important consideration when using a hyperprolate cornea to increase the depth of focus. During far-distance vision, when the pupil is between 4 and 6 mm, light rays travel through the periphery of the cornea, and the image is dominated by these marginal rays. Pseudo-accommodation occurs as a result of the near reflex and the amount of spherical aberration. During accommodation for near distance, the pupil shrinks to 2 to 4 mm, causing light to pass through only the center of the cornea. The image is dominated by central rays. Therefore, the refractive power should be greater in the central part of the cornea.
The accommodative effect we are creating—provided by the near reflex and the Q-value (ie, asphericity) of the cornea—does not improve accommodation; it merely creates spherical aberration and, therefore, depth of focus.
INDICATIONS AND LIMITATIONS
We usually choose to create the hyperprolate cornea in patients aged 40 to 55 years who have only residual accommodation and a refraction of between 3.00 to -1.75 D. Good candidates have a good near reflex and a balance between postoperative spherical aberration and defocus. The ideal combination (ie, sphere plus spherical aberration) may be determined with topography, wavefront, and biometry.
MONOFOCAL VS ENHANCED MONOVISION
With monofocal monovision, a large pupil does not create much depth of focus. (That is why we start to feel the effects of presbyopia as we age.) How much effect does the hyperprolate shape have on depth of focus? Figure 1 depicts the accommodative effect of pupil sizes ranging from a radius of 1.5 to 3 mm. As the pupil increases, depth of focus decreases. For a 1.5-mm pupil radius, at half-maximum, the depth of focus ranges from -1.00 to 1.00 D. When the range increases, the diameter of depth of focus decreases to -0.50 to 0.50 D.
When we create enhanced monovision with spherical aberration (Figure 2), we see the opposite effect. By increasing the pupil size, we broaden the depth of focus. In this case, the 1.5-mm pupil radius, at half-maximum, is -1.50 to 1.50 D. However, maximal visual acuity is reduced compared with monofocal monovision, and therefore some surgeons are slow to accept hyperprolate corneas for pseudo-presbyopia correction. We still are unsure how much spherical aberration a single person can accept for this enhanced monovision to be beneficial. Other research groups are attempting to answer this question with the use of adaptive optics.
By emulating the depth of focus in diopters for monofocal and enhanced monovision (Figure 3), we noticed the pupil dependence for near distance provides the same performance. However, for far distance, visual performance is better with enhanced versus monofocal monovision.
Spherical aberration and the patient's near reflex are the driving forces of pseudoaccommodation. When we create enhanced monovision by adding spherical aberration to increased depth of focus, we also increase far distance vision compared with monofocal monovision. Because a hyperprolate cornea with a Q-value less than -0.6 may increase the positive spherical aberration, and therefore depth of focus, I consider this treatment to be a viable option for presbyopia correction. Our rate of retreatment is mainly due to correction in the dominant eye for far distance vision. We have not had to re-treat the hyperprolate shape of the nondominant eye.
Creating a hyperprolate shape on the cornea may have advantages compared with other methods of presbyopia correction. First, we create the monovision with monofocality, meaning that we can still improve a patient's vision with spectacles. Second, the hyperprolate shape is less dependent on centration than other corneal laser surgery approaches to presbyopia correction.
Michael Mrochen, PhD, is a Senior Scientist and the Director of the Institute of Refractive and Ophthalmic Surgery, Zürich, Switzerland. Professor Mrochen states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +41 43 488 38 00; e-mail: email@example.com.