EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/274531
E W CATARACT 40 February 2014 by George H.H. Beiko, BM, BCh, FRCS(C) Impact of lens design and materials on cataract surgery outcomes S ynergy is the value and performance of any ele- m ents which, when com- bined, are greater than the sum of the separate indi- vidual parts. Achieving optical syn- ergy in visual outcomes following cataract surgery is accomplished by selecting a high performance lens design produced with proven, high caliber materials. Proper lens selec- t ion has been shown to improve spherical aberration correction, chromatic aberration correction, light transmission, material clarity, as well as limit lens epithelial cell (LEC) migration. While each of these attributes provides an individ- ual enhancement, the total benefit of these combined attributes creates a significant visual improvement. Spherical and chromatic aberration Zero spherical aberration and cor- rected chromatic aberration lay the foundation for sharp, clear cataract surgery outcomes. Spherical aberra- tion increases with age, causing a decrease in contrast sensitivity. Targeting zero spherical aberration is the means to achieve maximum contrast sensitivity. Selection of an IOL with a prolate profile, which is intended to compensate for the positive aberration of the cornea and results in negative spherical aberra- tion, improves the contract sensitiv- ity under both mesopic and photopic conditions. 1 Chromatic aberration is the uneven focusing of an optical system that causes the different wavelengths in white light to have different focal points. 2 Chromatic aberration of optical materials can be expressed by Abbe number. A higher Abbe number is associated with less chromatic aberration and sharper focus for better optical per- formance. 3,4 Selecting lens materials with low Abbe numbers and high chromatic aberration negatively im- pacts contrast sensitivity, producing a lower image quality. 3 The beneļ¬ts of blue light Blue blocking IOLs were developed before we understood how healthy levels of blue light affect overall health. Today, there is increasing ev- idence that blocking blue light does not provide any proven benefit, and multiple peer-reviewed studies have failed to find a link between age-re- lated macular degeneration (AMD) and blue light exposure. 3 In the eld- erly, pupillary miosis and yellowing of the crystalline lens limit the amount of blue light exposure, lead- ing to less melatonin suppression and circadian dysfunction. Cataract surgery could be a viable option to restore vision, overall circadian health, and improve mental health. Scotopic vision declines with age, even in healthy eyes without cataract or retinal disease. 5 Driving, mobility, and peripheral vision prob- lems have all been associated with reduced scotopic vision. 6 An IOL permitting a healthy level of blue light transmission has been shown to contribute to optimal scotopic vision. Blue light provides 35% of scotopic sensitivity, 5 while blue blocking IOLs reduce scotopic sensi- tivity up to 21%, 5 thus exacerbating the symptoms of circadian dysfunc- tion such as insomnia, depression, and memory problems. Blue light is essential for healthy circadian rhythms, 5 the normal 24- hour cyclic activities in the body that affect sleep pattern, mood, memory, alertness, and systemic health. Multiple studies indicate cataract surgery with a UV-only blocking IOL decreases insomnia and sleepiness. 7,8 Blue light aids in the regulation of melatonin levels and enhances alertness, even in blind people. 9 The sensitive retinal ganglion cells provide photic input to the suprachiasmatic nuclei, the human body's master pacemaker and clock, which influences circa- dian rhythms. Blue light suppression of melatonin during the day pro- motes wakefulness; conversely, max- imal secretion of melatonin at night promotes a good night's sleep. Blue light has been shown to improve mental and memory func- tion. The photoreceptive retinal ganglion cells also play a role in distinguishing patterns or tracking overall brightness levels. These cells seem to enable ambient light to in- fluence cognitive processes such as learning and memory. 10 Individuals exposed to blue light demonstrated faster reaction times and fewer at- tention lapses than those who were exposed to green light when they were asked to report when they heard a sound. 11 Selecting a lens that permits healthy levels of blue light while of- fering protection from harmful UV rays may offer a variety of benefits contributing to the overall well being and health of cataract surgery patients. Lens designs and materials The biomaterial composition and design of the IOL influences the clinical outcomes of cataract surgery, and several studies have been con- ducted comparing hydrophilic and hydrophobic IOLs. A higher inci- dence of opacification or discol- o ration of the optic component, both superficially and within the substances of the lenses, has been shown with hydrophilic IOLs in clinical studies. 12 In particular, this type of complication has been seen following lamellar and full thickness corneal transplant surgery, usually appearing three to 51 months after i mplant. The common factor in corneal surgery seems to be the injection of air into the anterior chamber. 13 Studies conducted on rabbits indicate distinct calcium and phos- phorus peaks, as well as surface delamination and pitting for the hydrophilic acrylic IOLs, but not for hydrophobic acrylic or silicone IOLs. 14 Additionally, patients who re- ceived a hydrophobic IOL with a sharp posterior optic edge required half the number of capsulotomies five years after implantation com- pared to patients with a hydrophilic lens. 15 Within the realm of hydropho- bic lens options, caution should be exercised to select a lens manufac- tured of materials resistant to capsu- lar opacification. Disabling glare symptoms resulting from glistening and whitening have been shown to occur in certain hydrophobic acrylic lenses. These glistenings, or white spots, are distributed throughout the IOL and are responsible for forward light scattering onto the retina. 16 This light scattering continuously increases postoperatively and is a risk factor for decreased visual func- tion. 17 Complaints include hazy vi- sion, increased glare hindrance, loss of contrast and color, halos around bright lights, and difficulties with against-the-light face recognition. Patients are also more likely to ex- hibit nighttime driving avoidance. Conclusion The selection of a lens, beyond power and type, has a significant impact on both surgeon and patient Analyzing the spectrum of light used for scotopic vision shows that blue light provides 35% of scotopic sensitivity. Source: George H.H. Beiko, BM, BCh, FRCS(C) continued on page 42 38-45 Cataract_EW February 2014-DL2_Layout 1 1/30/14 10:18 AM Page 40