Power Calculation Methods for the Post–Keratorefractive Procedure Eye
In 2002, Aramberri developed the double-K method, which uses the pre-LASIK corneal power (or, if unknown, 43.50 D) to calculate the ELP, and the post-LASIK (much flatter) corneal power to calculate the IOL power. These calculations can be performed automatically with computer programs.
Aramberri’s method is only one of more than 20 methods proposed over the years to either calculate the true corneal power or adjust the calculated IOL power to account for the errors discussed in the preceding sections. Some methods require knowledge of pre-refractive surgery values such as refractive error and K reading. Many of these methods have come in and out of favor on the basis of studies investigating their accuracy. It is up to the surgeon to keep abreast of the most accurate available methods.
It is not possible to describe all these methods in this chapter, but all of them are included in the Hoffer/Savini LASIK IOL Power Tool, which can be downloaded free of charge (see reference below). The tool requests the data needed to calculate each method, and the results are automatically calculated for every method for which complete data have been entered. The ultimate choice is left to the surgeon. The entire results can be printed on a single page and entered in the patient’s chart. Calculations can also be performed through the American Society of Cataract and Refractive Surgery website, but it lacks the Hoffer Q formula in all calculations, especially needed in short eyes.
Perhaps in the future there will be a more satisfactory method of measuring true corneal power by use of topography and advanced measuring techniques. At present, the ideal method for use with post–refractive surgery patients has yet to be determined.
Hoffer KJ. The Hoffer/Savini LASIK IOL Power Tool. Available at www.iolpowerclub.org/post-surgical-iol-calc. Accessed June 3, 2013.
Koch DD, Liu JF, Hyde LL, Rock RL, Emery JM. Refractive complications of cataract surgery after radial keratotomy. Am J Ophthalmol. 1989;108(6):676–682.
Intraocular Lens Power in Corneal Transplant Eyes
It is very difficult to predict the ultimate power of the cornea after the eye has undergone penetrating keratoplasty. Thus, in 1987 Hoffer recommended that the surgeon wait for the corneal transplant to heal completely before implanting an IOL. The current safety of intraocular surgery allows for such a double-procedure approach in all but the rarest cases. Geggel has proven the validity of this approach by showing that posttransplant eyes have better uncorrected visual acuity (68% with 20/40 or better) and that the range of IOL power error decreases from 10 D to 5 D (95% within ±2.00 D).
If simultaneous IOL implantation and corneal transplant are necessary, surgeons may use either the K reading of the fellow eye or the average postoperative K value of a previous series of transplants, but these approaches are fraught with error. When there is corneal scarring in an eye but no need for a corneal graft, it might be best to use the corneal power of the other eye or even a power that is commensurate with the eye’s AL and refractive error.
Geggel HS. Intraocular lens implantation after penetrating keratoplasty. Improved unaided visual acuity, astigmatism, and safety in patients with combined corneal disease and cataract. Ophthalmology. 1990;97(11):1460–1467.
Hoffer KJ. Triple procedure for intraocular lens exchange. Arch Ophthalmol. 1987;105(5):609–610.
Silicone Oil Eyes
Ophthalmologists considering IOL implantation in eyes filled with silicone oil encounter 2 major problems. The first is obtaining an accurate AL measurement with the ultrasonic biometer. Recall that this instrument measures the transit time of the ultrasound pulse and, using estimated ultrasound velocities through the various ocular media, calculates the distance. This concept must be taken into consideration when velocities differ from the norm, for example, when silicone oil fills the posterior segment (980 m/s for silicone oil vs 1532 m/s for vitreous). Use of the IOLMaster to measure AL solves this problem somewhat. It is recommended that retinal surgeons perform an optical or immersion AL measurement before silicone oil placement, but doing so is not common practice. The second problem is that the oil filling the vitreous cavity acts like a negative lens power in the eye when a biconvex IOL is implanted. This problem must be counteracted by an increase in IOL power of 3–5 D.
Pediatric Eyes
Several issues make IOL power selection for children much more complex than that for adults. The first challenge is obtaining accurate AL and corneal measurements, which is usually performed when the child is under general anesthesia. The second is that, because shorter AL causes greater IOL power errors, the small size of a child’s eye compounds power calculation errors, particularly if the child is very young. The third problem is selecting an appropriate target IOL power, one that will not only provide adequate visual acuity to prevent amblyopia but also allow adequate vision due to the large myopic shift that occurs after maturity.
A possible solution to the third problem is to implant 2 (or more) IOLs simultaneously: one IOL with the predicted adult emmetropic power placed posteriorly and the other (or others) with the power that provides childhood emmetropia placed anterior to the first lens. When the patient reaches adulthood, the obsolete IOL(s) can be removed (sequentially). Alternatively, corneal refractive surgery may be used to treat myopia that develops in adulthood. Most recent studies have shown that the best modern formulas do not perform as accurately for children’s eyes as they do for adults.
Hoffer KJ, Aramberri J, Haigis W, Norrby S, Olsen T, Shammas HJ; IOL Power Club Executive Committee. The final frontier: pediatric intraocular lens power. Am J Ophthalmol. 2012;154(1):1–2.e1.
Image Magnification
Image magnification of as much as 20%–35% is the major disadvantage of aphakic spectacles. Contact lenses magnify images by only 7%–12%, whereas IOLs magnify images by 4% or less. An IOL implanted in the posterior chamber produces less image magnification than does an IOL in the anterior chamber. The issue of magnification is further complicated by the correction of residual postsurgical refractive errors. A Galilean telescope effect is created when spectacles are worn over pseudophakic eyes. Clinically, each diopter of spectacle overcorrection at a vertex of 12 mm causes a 2% magnification or minification (for plus or minus lenses, respectively). Thus, a pseudophakic patient with a posterior chamber IOL and a residual refractive error of –1 D would have 2% magnification from the IOL and 2% minification from the spectacle lens, resulting in little change in image size.
Aniseikonia is defined as a difference in image size between the 2 eyes and can cause disturbances in stereopsis. Generally, a person can tolerate spherical aniseikonia of 5%–8%. In
clinical practice, aniseikonia is rarely a significant problem; however, it should be considered in patients with unexplained vision symptoms.
Lens-Related Vision Disturbances
The presence of IOLs may cause numerous optical phenomena. Various light-related visual phenomena encountered by pseudophakic (and phakic) patients are termed dysphotopsias. These phenomena are divided into positive and negative dysphotopsias. Positive dysphotopsias are characterized by brightness, streaks, and rays emanating from a central point source of light, sometimes with a diffuse, hazy glare. Negative dysphotopsias are characterized by subjective darkness or shadowing. Such optical phenomena may be related to light reflection and refraction along the edges of the IOL. High-index acrylic lenses with square or truncated edges produce a more intense edge glare (Fig 5-11A). These phenomena may also be due to internal re-reflection within the IOL itself; such re-reflection is more likely to occur with materials that have a higher IR, such as acrylic (Fig 5-11B). With a less steeply curved anterior surface, the lens may be more likely to have internal reflections that are directed toward the fovea and are therefore more distracting (Fig 5-11C, D).