8.1 Introduction

Patients who have undergone corneal refractive surgery and, subsequently, present for cataract surgery, anticipate excellent visual outcomes and spectacle independence. These patients are usually seeking early cataract surgery not only as a rehabilitative procedure but also as a refractive procedure to restore their quality of vision and lifestyle.

Continuous innovations in cataract surgery, with the incremental improvement of femtosecond lasers, phacoemulsification fluidics, viscoelastic devices, anterior segment imaging and diagnostic instruments, and modern intraocular lens (IOL) designs and formulas, have all led to enhanced clinical outcomes.

As with any patient, selecting the appropriate IOL, accurate biometry, and target refraction are key components of successful cataract surgery. In refractive surgery patients, even before these considerations, however, the most important factor is to define specific refractive goals to manage unrealistic expectations. Today, refractive cataract surgery can accomplish significantly reduced spectacle dependence for most patients, which helps to meet patient’s needs and expectations, by using the best technology of both cataract and refractive worlds, including topography, tomography, spectral-domain optical coherence tomography (OCT), wavefront analysis, light-scattering measurements, and intraoperative aberrometry.

8.2 Modern Intraocular Lens Design and the Increased Demand for Emmetropia

Modern IOL designs include spheric and aspheric monofocal lenses, toric IOLs for the correction of astigmatism, multifocal IOLs with refractive or diffractive designs, accommodative IOLs, light-adjustable IOLS, and multicomponent IOLs for correction of residual postoperative refractive error. When used appropriately these various IOLs can provide high-quality postoperative acuity in patients with previous laser vision correction (LVC)and radial keratotomy (RK). However, additional considerations must be addressed when selecting the appropriate IOL after LVC, including unique IOL calculation methods for optimizing IOL power, higher-order aberration profiles, and special measurements for the consideration of toric IOLs. Table 8-1 provides an overview of the relative importance of various factors to consider in the IOL selection process after corneal refractive surgery.

8.3 The Role of Higher-Order Aberration Profiles in Monofocal IOL Selection after Corneal Refractive Surgery

The fields of wavefront technology in ophthalmology grew rapidly in response to needs in cataract, corneal, and refractive surgery, making it possible to measure, correct, or alter the aberration structure of the eye. Deficiencies in optical quality of vision not detected by visual acuity measurement can be effectively evaluated by wavefront analysis, stray light measurement, and contrast sensitivity tests. High-resolution imaging in ophthalmic optics can be affected by high-order aberrations such as coma, secondary astigmatism, and spherical aberration.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^{,​ 6} Currently, wavefront technology has been applied to several areas of ophthalmology, including wavefront-guided ablations, customization in IOL selection, glasses, contact lenses, or even retinal imaging with cameras that use the adaptive optics technology.

Higher-order aberrations (HOA) impact functional visual acuity, especially spherical aberration and coma. The unoperated cornea has natural low amounts of positive spherical aberration. Early in life, the crystalline lens compensates for the corneal positive spherical aberration. ⁷ Age-related changes in the higher-order aberration profile over time can reduce visual quality that cannot be assessed by high-contrast Snellen/Early Treatment Diabetic Retinopathy Study (ETDRS) visual acuity tests alone. Increasing positive spherical aberration is related to internal optical changes, presumably lenticular opacification, whereas the increase in coma is likely due to the age-related changes in the cornea. ^{7 ,​ 8 ,​ 9} The mean corneal values for the normal adult population are approximately K = 43.8 diopters (D), Q = – 0.26 µm, and spherical aberration = + 0.27 µm over a 6 mm pupil. ^{10 ,​ 11 ,​ 12}

Even in patients without a history of LVC, conventional spherical monofocal IOLs may degrade imaging quality by increasing total spherical aberration of the optical system. The light rays at the peripheral zones of a positive lens are refracted with larger angles and intersect the optical axis closer to the lens than the paracentral rays, producing positive spherical aberration (Fig. 8.1). Aspheric IOL designs can optimize image quality by limiting diffraction. The use of aspheric IOLs aiming for the reduction of the preoperative normal corneal positive spherical aberration improved visual function and contrast sensitivity in mesopic conditions.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^{,​ 14}

Several aspheric IOLs with varying amounts of spherical aberration correction are available (Table 8-2). These include the Tecnis (Advanced Medical Optics, Inc.), AcrySof IQ (Alcon), and Akreos AO/SoftPort AO/enVista (Bausch & Lomb, Inc.). IOL position and centration can affect higher-order aberrations, such as tilt and coma, which can compromise aspheric, toric, and multifocal IOL performance. Loss of image quality at 0.4 mm misalignment and tilt > 7° are superior for aspheric IOLs with negative spherical aberration compared to neutral IOLs.s. Literatur ^,​ s. Literatur ^{,​ 15} Preliminary studies have indicated the potential benefits of laser-assisted cataract surgery on IOL centration and internal vertical tilt. ^{16 ,​ 17}

Customization of the IOL selection by estimating the optimum amount of spherical aberration within the eye and IOL to optimize optical quality is now possible. ¹⁸ Wavefront devices measure low- and high-order aberrations of the entire optical system, whereas corneal asphericity (Q values) and corneal higher-order aberrations (Zernike coefficients) can be calculated using specially designed software, such as VOL-CT (CTView; Sarver and Associates); iTrace (Tracey Technologies), and OPD Scan-III Wavefront Aberrometer (NIDEK, Inc.). Custom information from the cornea alone is helpful in estimating the optimum amount of spherical aberration within the eye and IOL to optimize optical quality. In patients with a history of LVC, these considerations are of increased importance.

8.4 Correcting Higher-Order Aberrations Induced by Corneal Refractive Surgery

The demand for wavefront-enhanced IOLs is gradually increasing because patients who underwent wavefront-guided, wavefront-optimized, and topography-guided LVC in the past are entering their cataract years. These patients anticipate vision quality similar to that obtained with the original surgery.

Table 8-3 lists IOL options for various situations that may arise. Myopic laser in situ keratomileusis (LASIK) increases positive spherical aberration, more so with conventional myopic ablation profiles compared with wavefront-enhanced (wavefront-optimized or wavefront-guided) procedures (Fig. 8.2). ^{19 ,​ 20 ,​ 21 ,​ 22} Conventional monofocal spherical IOLs may exacerbate the patients’ symptoms by increasing the total spherical aberration of the optical system. The best option in these cases is to choose a monofocal aspheric IOL with negative spherical aberration to compensate for the excessive amount of corneal positive spherical aberration (Fig. 8.3).

On the contrary, a hyperopic ablation creates a hyperprolate cornea with increased negative spherical aberration (Fig. 8.4). The use of an aspheric IOL with negative spherical aberration in a patient with prior hyperopic LASIK can increase the patient’s ocular aberrations and be responsible for decreased contrast sensitivity. Standard monofocal spheric IOLs and aspheric neutral IOLs are good options for these patients.

The ultimate goal is to customize IOL selection based on direct measurement of the corneal asphericity and higher-order aberrations. Ideally, the possibility of assessing quality of vision and depth of focus by manipulating and correcting ocular aberrations using adaptive optics technology provides a unique tool for predicting the potential benefits of different IOLs.