Ocular Wavefront-Guided Treatment
Efforts to correct refractive errors have led to the development of customized topography guided (or corneal wavefront-guided) (TG or CWF) and ocular (or whole-eye) wavefront guided (WFG) ablation. The possibility of achieving supernormal vision in terms of acuity and contrast has fuelled the imagination and creativity of vision researchers to pursue the goal of customized wavefront refractive surgery. This goal is achieved by generating an optimal ablation pattern based on individual anatomical and functional characteristics of the treated eye. The wavefront sensor allows the clinician not only to measure the defocus and astigmatism that are the most important determinants of refractive error, but also “higher order aberrations” (HOAs) as well. Defocus and astigmatism are referred to as lower order aberrations (LOAs). HOAs, such as coma and spherical aberration, refer to aberrations other than defocus and astigmatism.
The visual benefit in some eyes in the normal population is considerable. These eyes have substantial amounts of HOAs just as some normal eyes have a large amount of astigmatism. In these patients, wavefront sensing is a powerful tool in characterizing their optical abnormality, which was previously difficult to describe. It is noteworthy that visual acuity is a far less sensitive measure of the benefits of correcting HOAs than contrast sensitivity. This is because the contrast sensitivity function decreases quickly at the acuity limit and a significant increase in contrast sensitivity increases visual acuity only minimally. Thus, the greatest gains in correcting HOAs are noted in improved contrast particularly under low light conditions.
WFG Laser in Situ Keratomileusis (LASIK) is safe and effective for the correction of primary myopia or primary myopic astigmatism and that there is an increased level of patient satisfaction. The WFG procedure seems to have similar or better refractive accuracy and uncorrected visual acuity outcomes compared with conventional LASIK. Likewise, there is evidence of improved contrast sensitivity and fewer visual symptoms, such as glare and halos at night, compared with conventional LASIK. Even though the procedure is designed to measure and treat both LOAs and HOAs, the latter are generally increased after WFG LASIK. The reasons for the increase in HOAs are likely multifactorial, but the increase is typically less than that induced by conventional LASIK.
Wavefront sensors can also be used to diagnose and possibly treat a variety of conditions including corneas with “irregular astigmatism” from corneal transplantation, radial keratotomy (RK), decentred or irregular ablations, and central islands. It can give an objective measure to the patient’s own subjective symptoms of glare and haloes.
Currently, the use of WFG surface ablation is a breakthrough in the management of mild cases of keratoconus (KC). It’s used to manage the spherocylindrical error and the HOAs after halting the progression by corneal cross linking (CXL).
In the average, nonsurgical eye, the blurring caused by HOAs is not particularly large. It is equivalent to only about 0.3 diopter (D) of defocus.
There are many factors that limit how much we can optimize human vision.
These include: Pupil diameter, chromatic aberration, dependence of HOAs on accommodative state, accommodative lag, rapid changes in wave aberration over time, changes in wave aberrations with age, depth of field, photoreceptor sampling and neural factors, biomechanical effects in the cornea, accuracy of centration of correction.
When the pupil diameter is about 3 mm or smaller, HOAs are greatly reduced and the optical quality of the eye is determined mainly by blurring due to the diffraction of light at the pupil. Clearly, customized correction cannot undo the blur caused by diffraction. However, in young eyes, which tend to have large pupils, dim conditions such as night driving, and eyes with especially large amounts of HOAs, customized correction may be valuable.
The aberrometer measurement is one of the most critical elements of the WFG LASIK procedure. The precision of the laser ablation depends on obtaining an accurate assessment of the aberrations of the eye. A variety of aberrometers is currently available, but those most commonly used for WFG LASIK are based on a Hartmann-Shack sensor.
Many studies reported superior results of myopic WFG compared to wavefront-optimized (WFO) LASIK and standard non-wavefront treatments.
Although the UDVA and refractive outcomes were similar between conventional and WFG LASIK with the AMO-VISX Star 4 platform, it was shown that WFG treatment had significantly better outcomes than conventional LASIK in terms of contrast sensitivity, glare under mesopic conditions and subjective complaints. No correlation with pupil size was found.
Several studies have evaluated the safety and efficacy of WFG enhancements with LASIK or surface ablations in treating residual refractive errors, postoperative HOAs and refractory LASIK flap striae in symptomatic patients after previous keratorefractive procedures. Those studies found that WFG treatments were most beneficial in patients with highly aberrated corneas.
As the creation of a LASIK flap itself increases HOAs, some surgeons support the idea that customized ablation is best performed using surface ablation such as PRK or LASEK. Some studies have demonstrated that the degree of aberration increases with the level of attempted refractive correction. The largest increase occurred in spherical aberration, possibly due to the transitional zone from the treated to the untreated cornea, but subclinical decentrations and biomechanical effects may also have contributed.
KeywordsWavefront Wavefront-guided Wavefront-optimised Aberrations Coma Spherical aberration Improved contrast sensitivity Wavefront sensing Aberrometers Aberrometry Customised
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