Optical quality is a fundamental criterion for evaluating current methods of refractive correction, as well as newer, proposed laser ablation profiles and IOLs. Although LASIK remains the method of choice for correcting medium and low ametropias, the procedure's limitations have prompted surgeons and manufacturers to seek alternate treatment options for myopia greater than 10.00 D, as well as hyperopia and astigmatism higher than 5.00 D.
In particular, phakic IOLs are gaining increasing support. Although they are not a recent development, there have been recent innovations and improvements in phakic lens technology. This article examines the treatment possibilities of these lenses.
ANTERIOR CHAMBER IOLsIris-Fixated Lenses
When evaluating an IOL, it is important to assess the biomechanical changes it induces. A derivate of the Worst lens, the Artisan lens (OPHTEC, Gronigen, The Netherlands; pending FDA approval, this IOL will be marketed in the US by Advanced Medical Optics, Inc. [Santa Ana, CA], as the Verisyse) features claw-like haptics that fix in the iris stroma. This PMMA IOL has a 5-mm optical zone with a concave posterior surface and anterior meniscus in the case of negative powers. The lens is located 0.5 mm from the crystalline lens in the central region. The inclusion/exclusion criteria for patients are strict enough to guarantee that the external meniscus of the lens is located a minimum of 1.5 mm from the corneal endothelium (Figure 1A). This distance seems to prevent the endothelial cell loss that frequently occurred earlier with this lens.1-3
Iris fixation risks endothelial cell loss due to surgical trauma, and claw compression of the iris stroma could cause iris atrophy, in addition to increasing the incidence of decentration and lens tilting (both key factors in achieving quality vision) (Figure 1B). Nevertheless, the mechanical rubbing of the lens against the iris does not appear to be a negative factor; no incidences of pigment scattering or increased rates of glaucoma have been reported. Neither has the proximity of the Artisan lens to the crystalline lens resulted in reports of associated cataracts thus far.
In our experience, this lens type most effectively maintains a 0.5-mm distance from the crystalline lens. In many cases, phakic IOL power calculations are based on either the Van Der Heidge or Holladay formulas. With both, one assumption is a constant 0.5-mm distance between the IOL and the crystalline lens. This distance is not achieved or maintained with all phakic lenses, so there may be residual ametropia (Figure 1C and D).
A new design of the Artisan lens uses an acrylic material, which allows the IOL to be folded.Angle-Fixated Lenses
Analyzing anterior chamber phakic lenses entails differentiating between those using four- and three-point fixation styles. Similar to the Artisan lens, the Vivarte lens (CIBA Vision, Duluth, GA) features a 5.5-mm optic, a concave posterior surface, and a negative anterior meniscus (Figure 2A). New, customized designs of this IOL achieve angulations between the optic and the haptic zone of approximately 12º and 14º to guarantee a minimum distance of 1.5 mm between the IOL and the endothelium.
An anterior chamber phakic IOL designed with four supporting points has greater stability than a tripod-fixation lens. Nevertheless, the former type of lens concentrates all the pressure caused by the haptics in the main axis of the lens, a situation that induces atrophy in the iris root and thereby leads to pupil ovalization (Figure 2B). Although the strength distribution of the tripod-type IOL reduces the incidence of pupil ovalization, unfortunately, this sort of lens is less stable and frequently rotates in the anterior chamber.
New customized designs of tripod-type IOLs have been manufactured in an attempt to achieve a balance between positioning stability and the pressure under the supporting zones at the angle. The white-to-white value that clinicians currently use is inaccurate. Of interest is research on a system to determine the precise internal dimensions of the anterior chamber angle. For instance, ultrasound biomicroscopy (eg, the Artemis from Ultralink [St. Petersburg, FL]) and the anterior segment optical coherence tomography being developed by Carl Zeiss Meditec Inc. (Dublin, CA) show promise.
Also intriguing is work supported by IOLTECH Laboratories (La Rochelle, France) on a tripod-type lens designed with an intermediate zone that has the flexibility of a spring and is located in the superior haptic. The idea is to avoid unnecessary tension over the anterior chamber angle. Selective polymerization of the lens material makes this effect possible by helping to release the pressure of the haptic without weakening the lens.
POSTERIOR CHAMBER PHAKIC IOLsThe ICL and the PRL
The potential complication of posterior chamber phakic lenses relates to their location in a narrow region between the iris and the crystalline lens. The most popular posterior chamber phakic IOLs are the ICL (STAAR Surgical Company, Monrovia, CA) and the PRL (CIBA Vision), but with four fixation points in the case of the ICL. The significant difference between these lenses is their material. The ICL is an acrylic lens (HEMA) with collamer, whereas the PRL is made of silicone. The former is hydrophilic, and the latter is hydrophobic.
Because the ICL is fixated in the ciliary sulcus, the surgeon must measure the eye's ciliary diameter. An overestimation will result in incorrect fixation and insufficient vaulting, which increases the risk of the ICL's touching the crystalline lens. Underestimating the ciliary diameter will cause the surgeon to fixate the lens in the sulcus and produce excessive vaulting of the central zone that will push both the optic and the iris forward (Figure 2C and D).
The silicone material allows the PRL to “float” on the aqueous humor without touching the anterior surface of the lens.4 This property grants the lens various theoretical advantages. First, calculating the sulcus-to-sulcus diameter is unnecessary because this lens does not require fixation. The convexity of its posterior surface centers the IOL over the crystalline lens, and the hydrophobic properties of the silicone material guarantee this separation. A unique size could be used for most patients.
Like other investigators, we found that the PRL did not behave as the manufacturers reported. Our personal experience with ultrasound biomicroscopy showed the PRL leaning over the zonules on one side, while the other side of the lens was located in the sulcus and its central zone could touch the crystalline lens, especially during accommodation (Figure 2E). The proximity of the PRL to the crystalline lens is the principal factor in this lens' most significant associated complication: the induction of cataracts.
Although, as reported in a published series of patients,5 this incidence of cataract formation is low, every case of cataract formation showed characteristic opacification in the central, anterior region of the crystalline lens, denoting an anterior subcapsular cataract. This complication results from mechanical trauma caused by the rubbing and a metabolic effect due to stagnant aqueous humor trapped under the lens in the central region.6,7
General Drawbacks
There are no current reports of pigment scattering or major incidences of pigmentary glaucoma after the implantation of posterior chamber phakic IOLs, but the proximity of these lenses to the pupil and further movement of the iris have caused complications such as ocular hypertension, pupillary block, and Urrets Zavalia syndrome with iris atrophy and paralytic mydriasis.8 Additionally, when there is excessive vaulting or the implant is too small and close to the crystalline lens, the IOL power is also incorrect.
Size and Material
Changes in lens materials have made phakic IOLs more flexible and enabled them to fit through progressively smaller corneal incisions. Although the incidence of surgical trauma and the length of postoperative recovery have thereby decreased, alterations in lens material can diminish the optical quality. Optical materials may be evaluated in terms of the Abbe V-number (or reciprocal relative dispersion; a measure of chromatic dispersion as by a prism), refractive index, and modulation transfer function (a measure of the IOL's efficiency across the range of spatial frequencies) (Figure 3). Although laboratory studies are not always identical to clinical conditions, these analyses can assist us when choosing a phakic lens.
Bioptics
Bioptics or adjustable refractive surgery attempts to correct residual refractive error after implantation of a phakic lens. The current trend is to select the phakic IOL with the highest refractive power and a large optic and then to treat any residual refractive error via LASIK. Choosing a large optic helps to avoid halos.
In addition to the potential complications of corneal surgery, centering the corneal treatment with both an implanted IOL and the optical axis of the eye can be difficult. Imperfect centration negatively affects visual quality by producing aberrations (essentially coma) that are particularly uncomfortable for patients. As a result, we do not favor this technique.
GEOMETRY
Regarding positive lenses, the optimal geometry is convex-to-concave with a posterior face of -6.00 D and an anterior face of variable optical power. For negative lenses, the posterior concave face is fixed at +6.00 D, and the anterior convex face varies its power depending on the biometry of the lens. Nowadays, mathematical formulas may be used to calculate the power and adjust the IOL's anterior and posterior faces. This practice decreases the occurrence of several postoperative phenomena, including spherical aberrations and coma.
LENS POSITION
Theoretically, introducing a new lens into an optical system changes its principal planes depending on the geometric characteristics and position of the lens. If the lens is located near the principal planes of the optical system, it induces few changes in the dimensions of retinal images, and phenomena such as lens tilting, IOL decentration, and the induction of higher-order aberrations have a minimally negative bearing on visual quality. By contrast, the effect of these phenomena becomes progressively more severe when the lens is located far from the principal planes of the optical system (eg, as occurs with corneal surgery, contact lenses, or spectacle correction).
Because phakic lenses rest within the eye, 0.5 mm from the crystalline lens, and close to the principal planes, patients achieve highly satisfactory visual acuity, and, in some cases, gain some lines of visual acuity compared with their preoperative BCVA (Figure 4).
CONCLUSION
With careful patient selection, we find that the benefits of phakic IOL implantation outweigh the potential drawbacks. A review of the recent literature supports our opinion that these lenses provide patients with excellent optical quality.
Elena Millá, MD, PhD, practices at the Department of Ophthalmology at the Institut Universitari Dexeus in Barcelona, Spain. She holds no financial interest in any products mentioned herein. Dr. Millá may be reached at +34 93 227 47 47.
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