Ophthalmologists are keenly aware of the marvels of modern refractive cataract surgery, as we have the privilege to witness and experience the results daily with our most appreciative patients. The ability to resolve all levels of spherical ametropia, reduce—if not eliminate—astigmatic refractive error, and resolve the visual limitations of presbyopia for most patients is a routine accomplishment for refractive surgeons around the globe. The tools and devices innovated for modern refractive cataract surgery are astounding.
THE QUEST FOR BETTER RESULTS
Patients who undergo routine cataract surgery today are markedly better off than those of 30, 20, and even 10 years ago. The need for aphakic spectacles and the inability to correct astigmatism or to address inherent presbyopia associated with monofocal pseudophakic IOLs are issues of the past. Today, refractive surgeons can feel confident about what they provide to patients. However, refractive predictability with modern IOL implant surgery is still profoundly mediocre when compared with the refractive predictability of excimer laser corneal refractive surgery. For excimer laser corneal surgery, acceptable outcomes are as follows: 90% to 95% of eyes achieve at least 20/20 distance UCVA, greater than 90% of eyes are within ±0.50 D of the spherical equivalent target, and greater than 90% of eyes have no more than 0.50 D of astigmatism. Currently, the best outcomes with IOL placement are at half these levels. Clinical practice generally reveals that patients with greater than 0.50 D of spherical equivalent refractive error and greater than 0.50 D of astigmatism postoperatively will want better unaided visual function. To that end, the status quo must change to achieve a higher level of refractive outcomes with IOL implantation.
So what do we need to achieve better results? Do we need better keratometry? I do not believe so. Data from a properly calibrated manual keratometer with a zeroed eyepiece or keratometry data achieved by partial coherence interferometry provides accurate input data to achieve refractive level outcomes. Is better biometry for axial length required? I believe noncontact ultrasound with a frequency of 10 MHz or greater or an optical biometer (partial coherence interferometry) are more than accurate and precise enough in their current forms to achieve the data inputs we need for calculation.1
PREDICTING EFFECTIVE LENS POSITION
So what are we missing? I believe we need a device capable of a highly predictable estimation of the postoperative effective lens position. If cataract and refractive surgeons had the ability to predict preoperatively where any given lens optic would rest postoperatively, our problem would be solved. We would have refractive outcomes with IOLs on the same order as are seen with corneal laser vision correction. When this occurs, IOL surgery will have entered a new level of achievement, of which we can only dream at present. But I believe we are almost there.
Depending on the specific biometric device used, surgeons can obtain accurate and precise information about the distances between key anatomic structures: the cornea, the crystalline lens, and the retinal surface along the visual axis. Specific data on the front and back surface of the cornea, the front and back surface of the crystalline lens, and the vitreoretinal interface or the retinal pigment epithelium layer as well as the distances between these key structures are obtainable. Yet there are several essential anatomic points that diagnostic devices cannot currently quantify. These points include specific anatomical points of the ciliary body, the points of connection of the middle zonular fibers to the ciliary muscle and the equatorial capsular bag, the anatomic position of the capsular bag sulcus, and the exact diameter of the capsular sulcus-to-sulcus dimension. With this information, we would have the foundational data to provide refractive level outcomes.
Accurate information on these points could be available with ultrahigh frequency ultrasound, as provided by the Artemis 3 High-Resolution Ultrasound Eye Scanner (Arcscan, Inc., Morrison, Colorado; Figure 1). This device is not anticipated to be commercially available in the United States until the end of 2011. It is the only device that accurately images and measures behind the iris. The Artemis 3 provides 3-D angle-to-angle and sulcus-to-sulcus measurements as well as measurements of IOL position. The accuracy of the Artemis 3 is approximately 1 μm.
Daily use of the Artemis 3 in a clinical setting has not yet been established. An eyecup filled with a salinebased interface fluid couples the ultrasound signal to the eye. Complete scanning of an eye at 40 to 50 Hz and digitization of the information occurs in less than 2 minutes.
The next step for the ophthalmic field will be to tabulate population norms based upon axial length, anterior chamber depth, the frontal plane distance between the ciliary muscle, the capsular sulcus-to-sulcus diameter, and the crystalline lens thickness. A correlation of these data points preoperatively with resultant postoperative data points for the IOL and the refractive outcome could be used with regression analysis to create predictive formulas. This process is intricate and complex, but it is the only way an accuracy on the level of laser refractive surgery can be obtained with IOLs in a single surgical procedure. This process will be easiest with a monofocal IOL, where the optic diameter, optic haptic angulation, and maximal haptic-to-haptic diameter can be correlated. The process becomes more complex with accommodating IOLs, where there is a one-size-does-not-fit-all situation due to capsular bag variation and the need to achieve a desired resting position of the hinge angle or separation of dual-optic IOL platforms. Hence, more than one size of accommodating IOLs will be required to fit small, medium, and large anatomic sizes determined preoperatively with the aforementioned diagnostic capabilities.
Our field faces the challenge to achieve better refractive outcomes with IOL implant technology. One could avoid all the work and count on sequential surgery for the refractive misses with excimer laser corneal treatments or await the availability of light-adjustable technology to change the refraction of the individual eye after lens implantation. These routes work, but they require two interventions and the intervening visual morbidity of the patient. If it is our desire to implant a lens and have a single operative experience for our patients, then a more thorough understanding of the variables discussed above preoperatively and a system that takes them into account and accurately predicts the refractive outcome will be required.
John F. Doane, MD, is in private practice at Discover Vision Centers, Kansas City, Missouri, and he is a Clinical Assistant Professor in the Department of Ophthalmology, Kansas University Medical Center. He states that he has no financial interest in the product or company mentioned. Dr. Doane may be reached at tel: +1 816 478 1230; e-mail: email@example.com.
- Zeiss IOLMaster.Available at www.doctor-hill.com/iol-master/iolmaster_main.htm.Accessed April 27,2011.