The Holy Grail of cataract surgery is a postoperative outcome that approximates the visual performance of a healthy, young eye. Ideally, this would mean not only UCVA of 20/20 or better but also near-zero residual refraction, optimal contrast sensitivity, and low glare sensitivity. Over the course of the 60-year history of IOLs, researchers, cataract surgeons, and manufacturers have pursued a long, step-by-step quest to achieve this ultimate goal.
The spherical IOL, introduced by Sir Harold Ridley in 1950, revolutionized the treatment of cataract but left the postoperative refractive outcome uncertain. In the decades that followed, IOL manufacturers released many creative IOL designs in a search for ways to position and fixate the IOL in the eye. At the same time, early IOL surgeons built up the intuition needed to estimate appropriate IOL power based on ocular biometry. The next revolution in cataract surgery was the advent of IOL calculation formulas in the 1970s. These formulas greatly reduced the surgeon’s need to be intuitive and popularized cataract surgery among a larger group of surgeons by providing an objective framework to make the right choice of IOL power. Successive generations of IOL calculation formulas in the 1980s and 1990s led to more predictable postoperative refractions and better quality of vision for patients. In the 1990s, when surgeons had the ability to routinely achieve accurate postoperative spherical refraction, IOL developers focused on correcting corneal astigmatism and finding mechanical or optical methods to restore accommodation after cataract surgery. After the turn of the century, IOLs that correct or minimize postoperative spherical aberration and that protect the retina from highly energetic blue light were introduced.
Each of these innovations optimized a specific aspect of vision quality (eg, restoring accommodation using diffractive optics) but also affected other aspects of visual quality (eg, reducing contrast sensitivity). The key is finding a tradeoff in all these aspects of postoperative quality of vision that is acceptable for most patients. This is where the process of IOL design becomes key.
DESIGN VERSUS OCULAR PHYSIOLOGY
The concept for an IOL model can be tested using optical modeling software and a mathematical model eye. This allows the designer to perform virtual implantation of the IOL into an eye and to calculate many aspects of the postoperative visual quality, such as the wavefront, the modulation transfer function, positive dysphotopsia, and light scatter. Model eyes are wonderful tools for designers, but they are still only ideal representations of a real eye. IOL designs are optimized for implantation along the optical axis of an eye that is mostly rotationally symmetric. In other words, any physiologic tilts or decentrations of the ocular structures are not taken into account.
Most phakic eyes do not have significant lens tilt or decentration; however, pseudophakic eyes are different. At the moment of implantation, the IOL may be perfectly centered along the line of sight and assume the physiologic tilt of the capsular bag, which is the best postoperative situation for which any surgeon can hope. But after the IOL is implanted, it comes into contact with residual lens epithelial cells (LECs) on the capsular bag, and a strong immune reaction causes the LECs to start proliferating and differentiating.
The visual consequences of this reaction depend greatly on its severity, which can range from a few scattered clusters of LECs to strong anterior capsular contraction combined with posterior capsular opacification (PCO). The former will not result in significant visual effects for the patient; the latter will cause displacement of the IOL, resulting in considerable reduction in visual acuity and contrast sensitivity and an increase in glare effects.
Currently, we are unable to predict how strongly residual LECs will respond to the presence of an IOL. Although certain choices in lens material and edge designs can help reduce the effects of LEC reaction, an undesired lens dislocation is still a possibility for any in-the-bag IOL design, causing the lens to move to a suboptimal position and resulting in suboptimal vision quality.
For this reason, our department introduced the Morcher 89A Bag-in-the-Lens IOL (BiL; Morcher GmbH, Stuttgart, Germany; Figure 1), which requires both anterior and posterior capsulorrhexes for implantation. The edges of both capsulorrhexes are positioned into a groove between two flat elliptical haptics, much as a bicycle tire is put into the rim of a wheel. This moves the LECs away from the optical zone to an area where they can do no harm, resulting in a 0% PCO rate (Figure 2). Moreover, the double rhexis method causes the BiL to assume the physiological tilt of the capsular bag and to have good postoperative rotational1 and translational stability.2 As a result, a toric version of the BiL was successfully introduced. This experience suggests that the BiL is an ideal platform to implement many of the innovations described above without having to worry about undesired postoperative IOL repositioning.
TOWARD OBJECTIVE BENCHMARKING
Patient satisfaction is a subjective concept. Postoperative quality of vision can be satisfactory for one patient, but the next patient could be unhappy with exactly the same result. Therefore, it is imperative that reliable benchmarks of normal ocular biometry and normal quality of vision be made available to physicians to provide references as to what most people with normal ocular physiology find to be acceptable.
In the future, reimbursements by national health insurance could be linked to postoperative performance based on such benchmarks. Another application of these benchmarks would be to redefine the minimum visual acuity required to drive a car or to pilot an aircraft after cataract or refractive surgery. This could include currently untested parameters, including contrast and glare sensitivity.3
As such benchmarks are currently available only in limited form, our center has begun a benchmarking study called Project Gullstrand. The goal of our large-scale European multicenter study is to create an online database that contains the biometry and the associated quality of vision of thousands of healthy individuals. This database will become an indispensable tool for researchers and physicians alike as new-concept IOLs that take quality of vision as their starting point are developed.
Marie-José Tassignon, MD, PhD, is Head of the Department of Ophthalmology at the Antwerp University Hospital, Edegem, Belgium, and Full Professor in the Deptartment of Medicine, Antwerp University, Wilrijk, Belgium. Professor Tassignon states that she has a patent ownership with Morcher GmbH. She may be reached at tel: +32 3 821 33 77; fax +32 3 825 19 26; e-mail: Marie-Jose.Tassignon@uza.be.
Jos J. Rozema, MSc, PhD, is a senior staff member in the Department of Ophthalmology at the Antwerp University Hospital, Edegem, Belgium, and an Assistant Professor in the Deptartment of Medicine, Antwerp University, Wilrijk, Belgium. Dr. Rozema states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: firstname.lastname@example.org.
- Rozema JJ,Gobin L,Verbruggen K,Tassignon MJ.Changes in rotation after implantation of a bag-in-the-lens intraocular lens. J Cataract Refract Surg.2009;35(8):1385-1388.
- Verbruggen KHM,Rozema JJ,Gobin L,Coeckelbergh T,De Groot V,Tassignon MJ.Intraocular lens centration and visual outcomes after bag-in-the-lens implantation.J Cataract Refract Surg.2007;33(7):1267-1272.
- Bal T,Coeckelbergh T,Van Looveren J,Rozema JJ,Tassignon M-J.Influence of cataract morphology on straylight and contrast sensitivity and its relevance to fitness to drive.Ophthalmologica.2011;225(2):105-111.