Kevin Jwo, MD; William F. Wiley, MD; Ji Won Kwon, MD, PhD; and Jimmy Lee, MD
The promise of premium intraocular lenses (IOL) is as heady as it is bold; spectacle independence at all distances, something that is a distant memory for many cataract patients. Premium IOL options are categorized by the type of additional vision correction they provide: toric IOLs treat astigmatism, whereas multifocal (either diffractive or refractive) and accommodating IOLs address presbyopia. Given their downsides and risks, these options must be appropriately chosen and evaluated based on the lifestyle and personality traits of the patient. A comprehensive preoperative history, examination, and testing are essential for successful outcomes. Discussion of patient expectations and motivations should be one of the first goals, as they need to understand the benefits, limitations, and risks of premium IOLs. Unrealistic hopes among patients should be addressed. A thorough social, medical, and ocular history; manifest and cycloplegic refractions; a complete ophthalmic evaluation including pupillary exam, motility, slit-lamp, and fundus examinations; and testing of ocular dominance to discuss possible monovision correction may all be important elements of the preoperative evaluation.
Assessing the type and degree of corneal astigmatism is important when considering toric IOL implants. Most studies that have shown superior postsurgical visual acuities following toric IOLs (compared with spherical IOLs) have been performed on patients with corneal astigmatism of at least 1.0 to 1.5 diopters (D), using IOL models with cylinder power of 1.5 D.1–4 As such, it has been suggested that a minimum of 1.25 D of corneal astigmatism be present when considering a toric IOL.5 Although patients with the classic “bowtie” astigmatism are ideal candidates, those with irregular astigmatism secondary to keratoconus,6 pellucid marginal degeneration,7 and post-keratoplasty8 have also shown to benefit from toric IOLs. As these implants require precise centration and alignment for efficacy, the surgeon needs to screen for trauma, pseudoexfoliation, or even less common systemic conditions such as Marfan syndrome, homocysteinuria, ectopia lentis, and Weill-Marchesani syndrome, situations where weak zonules can compromise the centration of the IOL-capsule apparatus.
Multifocal IOLs project multiple focal planes on the retina, from which the brain selects to process the clearest image. One tradeoff is that these lenses can create optical aberrations such as halos, especially under low-light conditions. Additionally, contrast sensitivity may be compromised. Therefore, it is paramount to gauge the patients’ personality. Patients with moderate to high hyperopic refractive errors tend to be the best candidates, as they are unaccustomed to having excellent uncorrected near vision. Patients who are hypercritical with type A personalities, obsessive about crisp distance vision, or whose occupation relies on intermediate or night vision, are less than ideal candidates.9
Studies have shown that residual postoperative astigmatism after multifocal IOL implantation leads to worse visual outcomes and greater halo effects.10 However, regular astigmatism can be addressed concurrently with manual or laser limbal relaxing incisions. Recently, multifocal toric platforms have been released outside the United States to address both astigmatism and presbyopia.
Irregular astigmatism remains a challenge, and significant amounts can be a relative contraindication. Ocular surface diseases such as dry eye syndrome and meibomian gland dysfunction can affect visual outcomes and should be aggressively treated preoperatively. Corneal scarring and dystrophies can also affect outcomes. For example, in Fuchs endothelial dystrophy, corneal edema and the light scattering effect of guttata can exacerbate glare and cause poor contrast sensitivity with multifocal IOLs.
Satisfaction with multifocal IOLs depends on proper implantation, centration, and pupil function. Patients with large, abnormal pupils or iris defects can have increased glare and photosensitivity. Iris colobomas and eccentric pupils can also lead to dissatisfaction. As with toric IOLs, conditions associated with zonular weakness, most commonly pseudoexfoliation, can lead to decentration and tilting, which causes increased aberrations, decreased contrast sensitivity, and poor visual acuity.11 It is also important to assess angle kappa between the center of the pupil and the first Purkinje image on the cornea (corneal light reflex), as it has been shown that patients with a larger angle kappa have worse outcomes with diffractive multifocal lens.12 As the multifocal IOL centers in the capsular bag, asymmetrically converging light rays of a patient with a large angle kappa may strike diffractive edges of the IOL at angles that cause light scatter and unwanted aberrations. Recently, Chang and Waring have described a new measure of centration to replace angle kappa and other indices, which are used inconsistently but thought to lack precision and specificity: the subject-fixed coaxially sighted corneal light reflex. This measurement relies on chord mu rather than angles: the 2-dimensional displacement of the entrance pupil center from the subject-fixed coaxially sighted corneal reflex13 (Figure 5-1). Although it has yet to be incorporated into biometric devices, its appeal and utility lies in standardizing centration measurements.
Finally, it is important to screen for any pathology that limits visual potential, such as maculopathies (macular degeneration, diabetic retinopathy), optic nerve dysfunction, glaucoma, uveitis, or amblyopia, before deciding to implant a multifocal IOL. The decreased contrast sensitivity from multifocal IOLs can be compounded by underlying ocular pathology that threatens contrast sensitivity, visual acuity, color perception, or field of vision. Moreover, another consideration for patients with retinal diseases is that multifocal IOLs have been shown to hamper intraoperative visualization during vitrectomy.14
Biometry for cataract surgery focuses primarily on three variables: axial length, corneal power, and estimated lens position (ELP).
Axial length traditionally has been measured by the A-scan ultrasound, in which a crystal oscillates to generate a high-frequency sound wave. There are 2 types of A-scan ultrasound biometry currently in use. The first is contact applanation biometry, in which an ultrasound probe is manually placed on the central cornea. Although this method is convenient and expedient, the operator may compress and indent the cornea. This can lead to a falsely low axial length, which can lead to an overestimation of IOL power. As the degree of compression is highly variable, this cannot be factored into a constant. The second method of A-scan ultrasound biometry is immersion A-scan biometry, in which a saline-filled scleral shell is placed between the probe and the eye, which avoids compression of the anterior chamber. Several studies have shown increased accuracy of immersion A-scan compared with contact A-scan, with an average of 0.25 to 0.33 mm shortening induced by contact.15 The main disadvantage of immersion A-scan biometry is that it is more involved and time-consuming.
In a development that has significantly changed eye biometry, Carl Zeiss in 1999 introduced a noncontact partial coherence laser interferometer (IOLMaster 500, Carl Zeiss Meditec). This was followed in 2008 by the introduction of the Lenstar LS 900 (Haag-Streit), which uses optical low-coherence reflectometry. Optical biometry in general measures the delay and intensity of infrared light reflected back from a diode to determine the axial length of the eye. Beyond being fast and easy to use, it has several major advantages over A-scan biometry. First, optical biometry measures from the cornea to the retinal pigment epithelium, whereas ultrasound biometry measures the distance from the anterior cornea to the inner limiting membrane. Second, ultrasound measures the longest axis to the posterior pole, whereas optical biometry measures to the fovea. In eyes that are highly myopic, or have staphylomas, ultrasound biometry can overestimate the axial length. Finally, optical biometry is superior to ultrasound in the measurement of pseudophakic and silicone oil-filled eyes. Ultrasound measurements are performed with an assumed average velocity of 1555 meters/second for the sound wave as it travels through the cornea, aqueous, vitreous, and lens. Due to differing indices of refraction for various media, the velocity of sound through each of those media is different, which can confound measurements for very long or short eyes. Although optical biometers also average indices of refraction across different media, the correction factor is smaller in comparison, with less error. In long eyes, however, it’s been shown that even optical biometry will overestimate axial length due to this assumption and lead to an overestimation of lens power.16
The main disadvantage of optical biometry relates to its need for a clear path for the infrared laser to travel from cornea to fovea. Opacities along the visual axis can block the infrared laser and interfere with proper measurement. Eyes with tear film abnormalities, corneal pathology, hypermature and posterior subcapsular cataracts, vitreous opacities, maculopathy, or retinal detachment are more difficult to measure. In addition, the patient must be able to maintain fixation. One group found that the Lenstar, IOLMaster, and immersion A-scan platforms are comparable in accuracy.17
For most cataract patients, corneal power (K) is measured reliably with either manual or automated keratometry, which is available as independent units or as part of biometry devices such as the IOLMaster or Lenstar. Manual and automated keratometry platforms share an underlying principle: a well-lit target is placed in front of the cornea, which acts as a convex mirror and produces a virtual image of the target, and the radius of curvature is calculated via a simple vergence formula by the size of the reflected image. Corneal power is then derived from the radius of corneal curvature. Manual keratometers read either 2 or 4 points, whereas the autorefractor in the IOLMaster uses 6 reference points in a hexagonal pattern. In contrast, the Lenstar calculates Ks by analyzing the anterior corneal curvature at 32 reference points oriented in 2 circles.
Placido corneal topographers also measure corneal power: reflected images of multiple concentric circles are digitally captured, and the curvature of the cornea is calculated based on the distance between adjacent mires. These topographers measure more than 5000 points over the entire cornea and more than 1000 points within the central 3.0, yielding a simulated keratometry (sim K) value that is comparable to K values obtained from a manual keratometer or autorefractor. Recently, a novel corneal topographer, the Cassini corneal shape analyzer (i-Optics), has been introduced. This device uses 700 red, green, and yellow light-emitting diodes, each positioned in a unique relationship to 4 of its neighbors, to project light onto the cornea. The multiple colors and the asymmetric positioning prevent errors if reflections are smeared or overlapped to obtain more accurate Ks for irregular corneas.18
In comparison to keratometry and Placido topographers, corneal tomography can obtain actual true net corneal power by measuring both the anterior and posterior surfaces of the cornea. The Pentacam (Oculus), Galilei Dual Scheimpflug Analyzer (Ziemer Ophthalmic Systems), and Orbscan (Bausch & Lomb) calculate the total corneal power and astigmatism based on direct measurements of anterior and posterior cornea. These devices measure the topography of both anterior and posterior corneal surfaces and corneal thickness through direct measure of elevation, allowing 3-dimensional reconstruction of the cornea from 2-dimensional cross sections.19 The Orbscan was the first developed; light is projected through vertical slits, 20 from the right and 20 from the left, at a fixed angle of 45 degrees, and a digital video camera analyzes the anterior and posterior edges of the slits to obtain true anterior and posterior elevation, corneal thickness, iris, and anterior capsule surface. Oculus’ Pentacam utilizes a rotating Scheimpflug camera along with a static camera. The rotating camera sweeps across the surface of the cornea along with a monochromatic slit-light source to obtain slit images and anterior and posterior topography from height data. The Galilei combines 2 Scheimpflug cameras, which are able to capture slit images from opposite sides, along with Placido disc topography. Anterior chamber ocular coherence tomography (OCT) devices use low coherence interferometry to provide detailed 2-dimensional, cross-sectional images of the anterior chamber and can also be used to calculate true corneal power.20 A listing of currently available biometric devices can be found in Table 5-1 (optical biometers) and Table 5-2 (corneal topographers).
* True keratometry: evaluates both anterior and posterior corneal surfaces and provides a net corneal power.