Impact of lens design and materials on cataract surgery

Impact of lens design and materials on cataract surgery

by George H.H. Beiko, BM, BCh, FRCS(C)outcomes






















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)














Comparison of transmission of blue light in the natural crystalline lens and different current IOLs18 Source: George H.H. Beiko, BM, BCh, FRCS(C)


Synergy is the value and performance of any elements which, when combined, are greater than the sum of the separate individual parts. Achieving optical synergy in visual outcomes following cataract surgery is accomplished by selecting a high performance lens design produced with proven, high caliber materials. Proper lens selection 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 individual enhancement, the total benefit of these combined attributes creates a significant visual improvement.

Spherical and chromatic aberration

Zero spherical aberration and corrected chromatic aberration lay the foundation for sharp, clear cataract surgery outcomes. Spherical aberration 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 aberration, improves the contract sensitivity 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 performance.3,4 Selecting lens materials with low Abbe numbers and high chromatic aberration negatively impacts contrast sensitivity, producing a lower image quality.3

The benefits of blue light 

Blue blocking IOLs were developed before we understood how healthy levels of blue light affect overall health. Today, there is increasing evidence that blocking blue light does not provide any proven benefit, and multiple peer-reviewed studies have failed to find a link between age-related macular degeneration (AMD) and blue light exposure.3 In the elderly, pupillary miosis and yellowing of the crystalline lens limit the amount of blue light exposure, leading 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 problems 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 sensitivity up to 21%,5 thus exacerbating the symptoms of circadian dysfunction 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 circadian rhythms. Blue light suppression of melatonin during the day promotes wakefulness; conversely, maximal secretion of melatonin at night promotes a good night's sleep.
Blue light has been shown to improve mental and memory function. 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 influence cognitive processes such as learning and memory.10Individuals exposed to blue light demonstrated faster reaction times and fewer attention 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 offering 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 conducted comparing hydrophilic and hydrophobic IOLs. A higher incidence of opacification or discoloration 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 implant. 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 phosphorus 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 received a hydrophobic IOL with a sharp posterior optic edge required half the number of capsulotomies five years after implantation compared to patients with a hydrophilic lens.15 Within the realm of hydrophobic lens options, caution should be exercised to select a lens manufactured of materials resistant to capsular 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.16This light scattering continuously increases postoperatively and is a risk factor for decreased visual function.17 Complaints include hazy vision, 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 exhibit nighttime driving avoidance.

Conclusion

The selection of a lens, beyond power and type, has a significant impact on both surgeon and patient satisfaction. Careful consideration and education is required to weigh the variations in design and material composition against the desired outcome. The key to premium outcomes is a comprehensive understanding of how the differences in biomaterials and design alterations factor into successful outcomes. For my patients, I prefer a hydrophobic acrylic IOL with a 360 posterior square edge on the optic, a negative spherical aberration value, and a high Abbe number. I select a lens free of glistenings with UV light blocking, but without blue light blocking. Currently, the Tecnis family of lenses (Abbott Medical Optics, Santa Ana, Calif.) is my first choice.

References
1. Beiko, George HH. Personalized correction of spherical aberration in cataract surgery. Cataract Refract Surg. 2007;33(8):1455-60.
2. Schwiegerling J. Theoretical limits to visual performance. Surv Ophthalmology. 2000;45(2):139-146.
3. Zhao H, Mainster MA. The effect of chromatic dispersion on pseudophakic optical performance. Br J Ophthalmol. 2007;91(9): 1225-1229.
4. Negishi K, Ohnumna K, Hirayama N, Noda T. Effect of chromatic aberration on contrast sensitivity in pseudophakic eyes. Arch Ophthalmol. 2001;119:1154-1158.
5. Mainster MA. Violet and blue light blocking intraocular lenses: Photoprotection vs. photoreception. Br J Ophthalmol. 2006;90:784-792.
6. Owsley C, McGwin G, Scilley K, Girkin CA, Phillips JM, Searcey K. Perceived barriers to care and attitudes about vision and eye care: Focus groups with older African Americans and eye care providers. Invest Ophth Vis Sci. 2006;47(4):2797-2802.
7. Asplund R, Lindblad BE. The development of sleep in persons undergoing cataract surgery. Arch Gerontol Geriatr 2002;35:179-187.
8. Asplund R, Lindblad BE. Sleep and sleepiness 1 and 9 months after cataract surgery. Arch Gerontol Geriatr 2004; 38:69-75.
9. Zaidl FH, Hull JT, Peirson SN, et al. Short-wavelength light sensitivity of circadian, papillary, and visual awareness in humans lacking an outer retina. Current Biology. 2007;17: 2122-2128.
10. Lok, C. Seeing without seeing. Nature. 2011;469:284-285.
11. Lockley SW, Evans EE, Scheer FA, Brainard GC, Czeisler CA, Aeschbach D. Short-wavelength sensitivity for the direct effects of light on alertness, vigilance, and the waking electroencephalogram in humans. Sleep. 2006:29(2):161–168.
12. Werner L. Causes of intraocular lens opacification or discoloration. J Cataract Refract Surg. Apr;33(4):713-726. 13. Fellman M, Werner L, Liu ET, et al. Calcification of a hydrophilic acrylic intraocular lens after Descemet-stripping endothelial keratoplasty: case report and laboratory analyses. J Cataract Refract Surg. 2013; 39:799-803. 14. Walker NJ. Calcification of hydrophilic acrylic intraocular lenses in combined phacovitrectomy surgery. J Cataract Refract Surg. 2010;36(8):1427-31.
15. Johansson B. Clinical consequences of acrylic intraocular lens material and design: Nd:YAG-laser capsulotomy rates in 3 x 300 eyes 5 years after phacoemulsification. Br J Ophthalmol. 2010;94(4):450-455.
16. Matsushima H. Whitening (sub-surface nano glistening). International Society for Intraocular Lens Safety (I.S.I.S). www.iolsafety.com/issues-under-discussion/glistenings/letters-of-opinion/146-hiroyuki-matsushima-md-phd-on-whitening.html. Accessed December 12, 2013.
17. Miyata K, Honbo M, Otani S, Nejima R, Minami K. Effect of visual acuity of increased surface light scattering in intraocular lenses. J Cat Refract Surg. 2012; 38(2):221-226.
18. Boettner EA, Wolter JR. Transmission of the ocular media. Invest Ophthalmol. 1962;1:776-783.

Editors' note: Dr. Beiko is an assistant clinical professor at McMaster University, Ontario, Canada, and a lecturer at the University of Toronto. He has financial interests with Abbott Medical Optics, Croma (Leobendorf, Austria), Bausch + Lomb (Rochester, N.Y.), and Novartis (Basel, Switzerland).

Contact information 
Beiko: george.beiko@sympatico.ca

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