Custom Manipulation of Corneal Asphericity (The Q Factor)

When ablating the cornea with a myopic wavefront optimized (WFO) or aberration-free profile, the cornea is always going to receive more ablation in the centre than in the periphery of the optical zone and this will lead to the induction of spherical aberration. With manipulation of corneal asphericity, the final corneal asphericity can be influenced to preserve more natural prolateness than would otherwise occur. Asphericity-guided (Q-adjusted) procedures can also be used to induce corneal asphericity that enhances depth of field (hyper-prolate cornea). This is sometimes used in procedures called “Advanced monovision” due to the fact that the reading eye has better distance vision than would be the case in a patient with straight-forward monovision. Additional reasons for using an asphericity-guided ablation profile would be that for higher myopic corrections, the ablation depth is usually less than with a wavefront-optimised profile. Additionally, the refraction can be refined to 0.01D and the optical zone can be increased in 0.1 mm increments as opposed to 0.5 mm increments in WFO treatments. The target-Q can also be used to either attempt to preserve the preoperative Q-value (that is, not inducing any change) or a different Q-value could be targeted to create negative asphericity and increased depth of field for advanced monovision treatments.