The latest laser presbyopia correction
In the settings of the Alcon WaveLight EX500 device to run protocols similar to READ, the main key lies in manipulating the Q-Value (or corneal asphericity). The goal is to hyper-prolate the cornea to increase depth of field.
Here are the technical details that surgeons and specialists use on this platform:
1. Concept of changing Q-Value in READ
Naturally, \bm{Q}-Value of human cornea is about \bm{-0.20}. In conventional LASIK surgery, the goal is to maintain this value. But in the READ method:
• By making \bm{Q} more negative (for example, towards \bm{-0.60} or more), negative spherical aberration (Negative Spherical Aberration) is induced.
• This change causes the light rays entering from the periphery of the pupil to focus at a different point from the central rays and create a "long focus area".
2. Common numerical settings (Custom Q)
In the Custom Q menu of the Elkan device, the surgeon usually applies the following changes:
• Target Q: usually the target \bm{Q} is set between \bm{-0.50} to \bm{-0.70}.
• Amount of change (\bm{\Delta Q}): The surgeon may take the patient's current \bm{Q} value and change it to about \bm{0.2} to make \bm{0.4} unit more negative.
• Note: If \bm{Q} becomes too negative (for example, beyond \bm{-1.0}), the patient may experience "Halo" at night and a sharp decrease in contrast sensitivity. Therefore, balance is vital here.
3. Combining with Micro-Monovision
In the READ method with Elkan devices, the setting does not end only with \bm{Q}-Value; Usually, a different Target Refraction is considered for two eyes:
1. Dominant Eye (Distance): Target on \bm{0.00} (Planus) with a slight adjustment in \bm{Q} to improve vision quality.
2. Non-dominant eye (Near/Intermediate): The target is set to \bm{-1.25} to \bm{-1.50} diopters.
An important experimental point: for the success of this procedure, the surgeon must perform a spherical aberration tolerance test with a contact lens for the patient before surgery, because some people are very sensitive to the decrease in contrast caused by the change of \bm{Q}.