A supplementary IOL inserted between the cornea and the crystalline lens is the most effective surgical method to correct high refractive errors.1,2 A phakic IOL may enhance to perfection visual performance in all light conditions. Outstanding results, even in extreme conditions, including high astigmatism following keratoplasty and ectatic corneal disorders (ie, keratoconus, marginal pellucid degeneration, iatrogenic keratectasia), are possible if the IOL is naturally stable or stabilized with corneal collagen crosslinking or intrastromal ring segments.3,4
The dangers of an open-eye procedure may be controlled with state-of-the-art care and a safe surgical environment. Feared late implant-related complications mainly occur because of static or dynamic less-than-ideal clearances with internal structures; in clear terms, late complications are the result of human errors from selecting an unwelcoming intraocular environment or the wrong phakic IOL.
Thorough preoperative evaluation of the anterior segment, carried out with specialized high-resolution biometry, and customized implants are necessary to avoid surgical complications.5 In the past 10 years, we have learned more about the long-term behavior of phakic IOLs. Therefore, it seems worthwhile to reassess the classic safety guidelines, which are now proven inadequate.
GUIDELINE NO. 1
Size the lens implant based on the white-to-white (W-to-W) distance. I presented this concept for the first time in January 1998, at the Second European Society of Cataract and Refractive Surgeons Refractive Meeting in Munich, Germany. Although at that time I was a lone voice screaming in the desert, IOL surgeons worldwide no longer debate the necessity to accurately measure intraocular distances before implanting a phakic IOL. We must, however, update what measurement we use because W-to-W is no longer safe.
Surgeons now agree that sizing the lens by adding 0.5 mm to the patient's horizontal corneal diameter is too empirical to ensure a safe long-term cohabitation between the IOL and the eye. Beyond the lack of reliability of corneal limbus landmarks, findings from in vivo ultrasound and anatomic observations on cadaver eyes6 have shown no proportional correlation between external and internal dimensions. As a consequence, we know that outside measurements cannot predict internal ocular anatomy; therefore, a calculation such as W-to-W does not size phakic IOLs accurately.7 Surgeons are finally asking what other eye measurements and imaging technologies should take the place of W-to-W calculations.
Update. Use optical devices or proprietary software to accurately size the lens implant. Widely used optical devices, like slit-scanning systems with or without a rotating Scheimpflug camera—including the Orbscan (Bausch & Lomb, Rochester, New York), Precisio (Ligi Tecnologie Medicali, SpA, Taranto, Italy), Pentacam (Oculus Optikger‰te GmbH, Wetzlar, Germany), or Galilei (Ziemer, Port, Switzerland)—as well as infrared light optical coherence tomography (Visante OCT; Carl Zeiss Meditec AG, Jena, Germany) permit high-resolution, cross-sectional imaging of the anterior segment. Reproducibility of measurements is excellent.
These optical devices use an interference profile of reflections from the cornea, iris, and crystalline lens. Unfortunately, they do not visualize the retroirideal spaces. The statistical correlation between angle and sulcus diameters is also poor,6 rendering the devices useless for sizing posterior chamber IOLs, like the Visian ICL (Implantable Collamer Lens; STAAR Surgical, Monrovia, California), whose haptics rest on the ciliary sulcus recesses.
Accurate imaging of this hidden anatomical site can only be obtained with ultrasound devices that use very high frequency (VHF) waves (35–50 MHz). Available devices include the Artemis 2 (Ultralink LLC, Saint Petersburg, Florida), VuMax (Sonomed, Inc., Lake Success, New York), and I3ABD (Ellex Medical Lasers Ltd., Adelaide, Australia).
We use proprietary software, the Lovisolo Custom Phakic IOL Sizer, to predict postoperative vault height and clearance between the corneal endothelium, iris, and crystalline lens.8 The Lovisolo Custom Phakic IOL Sizer also uses VHF ultrasonographic images; the latest version of the software includes new measurements for ICLs, including: (1) sclerociliary processes angle, (2) horizontal and oblique 45° dimensions of the ciliary sulcus, as measured under dim light conditions, (3) the crystalline lens rise on the iris plane, (4) iridocorneal angle, (5) specific features, including vault at rest, central and peripheral optic thickness, balanced saline solution- or NaCl-labelled ICL, of the chosen lens implant for each power and overall length; (6) behavior under compression as predicted by finite element analysis, given the elasticity of the material, and (7) the age/life expectancy of the patient, where an average annual reduction of the anterior chamber depth (0.015 mm) is calculated to predict the anatomic relationships.
The ICL sizer also predicts the postoperative reduction of angle parameters that influence angle-closure glaucoma, as classically provided by Pavlin and Foster.9 A series of 394 eyes implanted with the ICL (Version 4), whose overall length was selected with this software, was followed for 9 years (mean follow-up, 39 months). No iatrogenic cataract, pigmentary dispersion, or angle-closure glaucoma were noted. The mean central vault height was 386 mm (standard deviation, ±113 mm; minimum vault, 189 µm). An expected versus achieved vault height (range, ±150 mm) was obtained with 95% confidence interval.
A control group of 237 eyes underwent ICL implantation according to patients' W-to-W measurements. Mean central vault height (406 mm) was not statistically significant; however, the standard deviation was highly significant (±667 mm). The minimum vault was 0 mm, and the incidence of size-related complications, including angle-closure glaucoma, cataract, and clinically significant pigmentary dispersion, was approximately 8%. An expected versus achieved vault height (range, ±730 mm) was obtained with 95% confidence interval.7
GUIDELINE NO. 2
Do not use a phakic IOL if the anterior chamber depth (ACD) is less than 2.8 mm. This old rule must be updated. It was artificially derived from studies that used eyes with naturally occurring angle-closure glaucoma. We recently implanted an ICL in 51 eyes that had an ACD less than 2.8 mm. Our safety ratio was excellent; however, a small group of eyes with an ACD greater than 3 mm were contraindicated for phakic IOL surgery.
Typically, central ACD is obtained with a conventional A-scan ultrasound biometry. Although still a preclusive medicolegal reference point, a single measurement is not a reliable predictor for the risk of angle-closure glaucoma. It shows no precise correlation between the shape of the anterior segment and the width or occludability of the iridocorneal angle. Similar ACDs often show a different anterior segment shape, angle aperture, iris configuration, and original asymmetry.6
Update. For surgical anatomy, the only rule is that there is no rule. Thorough examination of the eye's anatomy with VHF ultrasonography and software showing the statistical risk of developing angle-closure glaucoma may help the surgeon to perform safe implantations. Vault predictability (ie, proper sizing) is crucial; the limit of the angle that risks pupillary block in pristine eyes is approximately 15°.
Ideally sized, 500-µm vaulted myopic ICLs reduce the iridocorneal angle by an average of 25% (from approximately 42° to 28°). Theoretically, they do not require peripheral iridectomies; however, to avoid angle-closure glaucoma with a 1-mm vaulted ICL, a peripheral iridectomy is required because it reduces the angle aperture by 55%.
GUIDELINE NO. 3
Do not implant phakic IOLs in patients younger than 21 years of age. When phakic IOLs were introduced, excess caution in children was wise.
Update. Use caution when implanting a phakic IOL in a patient younger than 21 years of age. Minority age is no longer an absolute exclusion factor for surgery because we now have long-term follow-up suggesting that phakic IOLs may be safe to use in younger patients in certain circumstances. Phakic IOLs implanted in patients with high anisometropia, contact lens-intolerance, and functional strabismus have been shown to be a safe and effective option in pediatric applications.10,11
GUIDELINE NO. 4
Do not use a phakic IOL if the endothelial cell count is less than 2,000 to 2,500 cells/mm² (age dependent). Enough evidence quantifies that intraoperative trauma to the endothelial corneal layer may be minimized in less than 10% of patients. In many cases—as a consequence of cell centripetal migration and enhanced metabolism after ceasing to wear contact lenses—endothelial cell counts increase and the morphological indices improve 1 year postoperatively.5 In the long term, cell counts stabilize with physiological loss (0.5–0.6% per year) because the protective iris barrier prevents mechanical chafing.
Update. Estimate the endothelial cell loss. Rather than require a minimum cell density, we should strive instead for endothelial cell loss that is no greater than the expected physiological loss over time. Thus, we may implant a phakic IOL in eyes with relatively poor endothelial cell counts, including chronic contact lens wearers, if we feel that their endothelial cell loss will not affect the final outcome. Moreover, we may treat eyes with early-stage, stable keratoconus; astigmatism after penetrating or lamellar keratoplasty; ammetropia after keratorefractive surgery; or overcorrected radial keratotomy.
GUIDELINE NOs. 5 and 6
Eyes with corneal disorders or previous corneal surgery are contraindicated. It is advisable to continue to adhere to these two original guidelines.
GUIDELINE NO. 7
Operate only on eyes that cannot undergo excimer laser surgery. This guideline is under debate, considering the comparable safety ratios and the superior quality of vision achieved with phakic IOL surgery.1,2,12
Update. It is too early to suggest a new guideline in this area because there is not enough evidence to indicate whether excimer laser or phakic IOL is better.
CONCLUSION
Since the introduction of the phakic IOL, surgeons have recommended guidelines for its use. As we learn more about the long-term effects of this category of IOLs, it is imperative that we continually revisit and update established guidelines.
Carlo F. Lovisolo, MD, practices in the Department of Ophthalmology and Visual Sciences, San Raffaele Hospital and QuattroElle Eye Center, Milan, Italy. Dr. Lovisolo states that he has no financial interest in the products or companies mentioned. He may be reached at carol.lovisolo@quattroelle.org.
