Phakic Intraocular Lenses

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Background of Refractive Error

Clinically significant refractive error, or ametropia, affects half of the general population in the United States. Significant ametropia includes hyperopia of 3.0 diopters (D) or greater, myopia of -1.0 D or less, and astigmatism with a cylinder of 1.0 D or greater[1]. Presence of ametropia and severity vary by gender, ethnicity, age, and anatomy. Correcting ametropia is important to accomplish in order to prevent complications such as accommodative esotropia or refractive amblyopia.

Extraocular interventions include glasses, contact lenses, and cornea-modifying surgery. Myopia can be corrected with glasses and contact lenses using a spherical concave or diverging lens with minus D, which shifts the focus of light rays from anterior to the retina, to the retina. Hyperopia can be corrected with a spherical convex or converging lens with positive D, which shifts the focus from posterior to the retina, to the retina. Astigmatism is corrected using a toric contact lens or cylindrical glass lens, which refocus light rays in two separate planes to compensate for an ovoid cornea.

Most forms of surgery for ametropia have the goal of reshaping the cornea using a high-energy excimer laser. The various techniques include photorefractive keratectomy (PRK), laser assisted subepithelial keratectomy (LASEK), and laser in situ keratomileusis (LASIK) along with its variations of sub-Bowman keratomileusis and epi-LASIK. Another surgery is intrastromal corneal rings (INTACS), which are implants that reshape the cornea.

Options for intraocular surgery include refractive lens exchange, which is essentially the same procedure as cataract surgery and phakic intraocular lens (pIOL) implants which will be the focus of discussion here. Bioptics combines laser surgery and for correction of high degrees of ametropia.

History

The first pIOLs were placed in the anterior chamber angle as early as 1953 by Dr. Strampelli [2]. These lenses were plagued with endothelial decompensation, angle fibrosis which led to subsequent glaucoma, and pupil distortion. Due to the significant complications experienced with these lenses nearly 30 years transpired before new lens designs began to emerge [3].

Advances in intraocular lens technology benefited the refractive surgeon when better materials and lens designs surfaced. Polymethylmethacrylate (PMMA) lens materials became prevalent as well as z-shaped haptics which would contact the angle structures less than prior lens designs. Acrylic became popular as well due to the successful use in cataract surgery intraocular lens replacement.

In 1977, Jan Worst introduced the iris-fixated “iris-claw” which was a biconcave design made from PMMA. This lens avoided many of the glaucoma and endothelial complications. [4].

Posterior chamber pIOLs came into existence in 1986 and were first developed by Dr. Fyodorov. The design later inspired the current model used by STAAR Surgical, the Visian implantable collamer lens (ICL). This lens is made of a trademark material known as “collamer,” which is a copolymer of hydroxyethyl methacrylate and porcine collagen [4].

Available pIOLs

Figure 1. Verisys lens in-situ.

Implantable pIOLs come in two broad varieties: anterior chamber  intraocular lenses (ACIOLs) and PCIOLs. ACIOLs can be further divided into angle-supported ACIOLs and iris-claw ACIOLs. In the United States, the only currently FDA-approved models are the iris-claw ACIOL Verisyse (Abbott Laboratories Inc, Abbott Park, IL, USA, Figure 1) and the PCIOL Visian ICL (Staar Surgical, Monrovia, CA, USA, Figures 2 & 3). The Verisyse lens is branded Artisan (Ophtec BV, The Netherlands) in Europe.

Figure 2. Visian ICL.
Figure 3. Successfully implanted ICL[5]. The white arrow indicates the ICL and the blue arrow indicates the crystalline lens. Redd, et al [Digital image] Retrieved from University of Iowa EyeRounds at http://webeye.ophth.uiowa.edu/eyeforum/cases/210-PhakicIOL.htm. Licensed under CC BY 3.0.

Newer models are in clinical trials in the United States and are already being used in Europe, having the CE mark of approval. The AcrySof (Alcon, Switzerland, Figure 4) is an angle-supported ACIOL that shows promising results at one year. The Artiflex is the foldable version of the Artisan iris-claw ACIOL and is available in Europe. The United States equivalent of the Artiflex is the Veriflex, which is currently in trials. A PCIOL currently in trials is the phakic refractive lens (PRL), which is a foldable lens that is suspended in aqueous in the poster chamber.

Two more angle-supported ACIOL models in trials in the EU and Russia are the ThinPhAc (ThinOpt-X, Medford Lakes, NJ, USA) and Vision Membrane (Vision Membrane Technologies, Inc., Carlsbad, CA, USA).  

Most lens brands are created in negative and positive D, and have toric forms available with and without correction of myopia or hyperopia. The Visian and Verisyse are both FDA-approved only for correction of myopia with or without astigmatism up to 2.5 D.[6]

Indications and Contraindications

Figure 4. AcrySof lens [7] by Pineda et al. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5139554/. Licensed under CC BY 3.0.
While pIOLs are available in prescriptions for ametropia ranging from mild to severe, implantation of pIOLs is typically recommended for more severe degrees of refractive error. The ideal candidates are >21 years old, have had stable refraction (change in vision <0.5 D) over at least one year, are poor candidates for excimer laser surgery, have poor tolerance of contact lenses or glasses, have irido-corneal angle >30 degrees, have central endothelial cell count >2300 cells/mm2 or >2500 if >21 years old and >2000 if >40 years old, and have a mesopic pupil size <5-6 mm. Candidates requiring careful consideration or contraindication for implantation are those with active anterior segment disease, recurrent or chronic uveitis, cataracts, previous ocular surgery, glaucoma or IOP >21mmHg, preexisting macular pathology, retinal disease, anomalous irises or pupils, or systemic diseases classically associated with poor postoperative healing (eg diabetes mellitus, connective tissue disease, autoimmune disease)[6].

Calculations

Lens companies typically provide software that will calculate the correct lens after entering patient measurements. Lens makers typically provide a nomogram that a provider can use to choose the proper lens strength based upon measurement of patient anatomy. ACIOL power is calculated using a Van der Hejde nomogram, which requires the patient’s refraction, keratometry, and anterior chamber depth (ACD). [8] PCIOL power is calculated using a Binkhorst nomogram which requires a patient’s spectacle plane refraction, corneal power, and ACD. PCIOLs may also require a measurement of the angle-to-angle distance for appropriate sizing of lens diameter. [8] Ultimately, providers enter patient measurements into software that uses formulas proprietary to the manufacturers and provides a phakic IOL power that is ideally within 0.5-1.0 D of emmetropia with as close to 20/20 vision as can be attained. Newer lenses may require other patient measurements and use alternate formulas and nomograms not mentioned above. 

There are several methods used to obtain the variables that are required to calculate pIOL power. Refraction can be performed by manifest refraction or autorefraction. ACD and angle-to-angle distance are best-obtained using optical coherence tomography (OCT), but can also be measured using ultrasound biomicroscopy (UBM) or Scheimpflug imaging. ACD is measured from the corneal apex to the anterior surface of the crystalline lens, and is use to calculate effective lens position (ELP) in PCIOLs by substracting the distance between the pIOL and the crystalline lens from the ACD. The ELP is typically 0.8mm in the Artisan/Verisyse lens.[9] White-to-white (WTW) measurement with an IOL-Master or calipers can be used to estimate angle-to-angle distance. Corneal power is calculated using keratometry or topography to measure the curvature of the cornea.[6][9]

Surgical Technique

Figure 5. Verisyse insertion.
Figure 6. Visian ICL insertion.

In general, peripheral iridotomies are strongly recommended to prevent pupillary block for both anterior and posterior pIOLs. Anterior chamber pIOLs require miotic drops to reduce pupils size. Main incisions are typically performed on the steep corneal axis to reduce surgically induced astigmatism. Details of surgical techniques differ slightly depending on the specific type of pIOLs being used.

The Verisyse/Artisan iris-claw ACIOL implantation (Figure 5) is typically done with retrobulbar or peribulbar anesthesia and induced miosis. As this lens must be centered over the iris, the cornea center should be marked to aid the surgeon’s placement. Toric lenses must be implanted carefully so as to avoid cyclotorsion of the lens over the pupil. A corneal or scleral incision is made along with two paracenteses for lens manipulation. OVD is injected into the AC followed by the pIOL, which is then fixated into the iris using an enclavation needle to hold the iris and an implantation forceps to depress the claw into the iris. Centration of the lens over the pupil is essential, and mild ovalization after the surgery is not uncommon due to the effect of the miotic agent. Intraoperative peripheral iridectomy or two preoperative neodymium:YAG (Nd:YAG) laser iridotomy should be done to prevent pupillary block. The incision is large enough to require 10-0 nylon sutures.

Verisyse implantation video (Vislisel J and Goins K. Verisyse phakic intraocular lens placement. Retrieved from https://vimeo.com/142181115. Licensed under CC BY 3.0.): https://vimeo.com/142181115

The foldable Veriflex/Artiflex iris-claw ACIOL allows for a smaller, watertight incision and is installed similar to the Verisyse/Artisan lens once it has been unfolded in the anterior chamber.

The success of Visian ICL implantation (Figure 6) begins with proper loading of the ICL into the injector. The cartridge is filled with OVD, loaded with the lens dome-up being careful not to rupture the footplates, and covered with a foam tip. Preoperative mydriasis is necessary. A 3.0mm incision is sufficient for injection, along with one or two paracenteses depending on surgeon preference. The pIOL is injected bevel-down after the AC is filled with OVD. A blunt spatula is used to press the haptics under the iris, miosis is pharmacologically induced, the OVD is extracted, and the AC is rehydrated. Intraoperative peripheral iridectomy or two preoperative Nd:YAG laser iridotomies should be performed to prevent pupillary block.

Visian ICL implantation video: https://youtu.be/LIsZz5y7QOY

The PRL is installed similar to the Visian ICL, with one difference being that two peripheral iridotomies are performed after implantation.[4][6][10]

The AcrySof foldable angle-supported ACIOL implantation requires a miotic pupil and topical anesthesia. An ophthalmic viscosurgical device (OVD) is injected into the anterior chamber, followed by the pIOL through a 2.6mm or 3.2mm incision, depending on the lens size. Intraoperative gonioscopy can be used to confirm placement of the haptics in the angle. The OVD is removed and the AC is then rehydrated. A 10-0 nylon suture may be used if the incision is >3.0 mm.

Clinical outcome

Phakic IOL surgery has been shown to give better vision, have higher patient satisfaction, and be safer than excimer laser surgery, and avoids the risk of corneal ectasia[11]. Phakic IOLs have better best spectacle-corrected visual acuity (BSCVA) and refractive predictability and stability compared with LASIK and PRK and have very high satisfaction rates [11]. Implantation of pIOLs forgoes the risks of corneal ectasia and retinal detachment associated with excimer laser surgery and refractive lens exchange, respectively. 

Comparative studies on pIOLs show conflicting results on outcomes of the different pIOLs in circulation. In FDA trials, Iris-fixated ACIOLs have been shown to have 20/40 uncorrected visual acuity (UCVA) or better in 84% of patients after three years and posterior chamber pIOLs have been shown to have an UCVA of 20/40 or better in 81% of patients after 3 years.[4] Artiflex/Veriflex and Visian ICL show better outcomes in highly myopic eyes than Artisan/Verisyse. Artisan/Verisyse lenses also have greater rates of higher order aberrations compared with other lenses.  One study showed a 31.7% higher rate of emmetropia in Artiflex/Veriflex lenses than Artisan/Verisyse lenses[12]. The foldable Artiflex/Veriflex lenses also heal faster postoperatively due to the smaller incision size.

Risks and Complications

Figure 7. Corneal decompensation ten years following Verisyse lens implantation [13] by Moshirfar et al. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3986296/. Licensed under CC BY 3.0.
Figure 8. Chronic inflammation despite ongoing anti-inflammatory treatment two months postoperatively in an iris claw ACIOL [14] by Taneri et al. Retrieved from https://www.hindawi.com/journals/joph/2014/923691/. Open access article. Licensed under CC BY 3.0.

There are concerns for inflammation, infection, and bleeding resulting from surgical trauma especially to the iris. Since some of these procedures entail larger corneal and scleral incisions, the risk of endophthalmitis and retinal detachment are notable risks. Elevated intraocular pressures from pupillary block and angle scarring are concerning due to insertion of a pIOL in or near the angle. Chronic intraocular pressure elevation can produce permanent vision loss. The surgical procedure can also induce endothelial cell damage and if the pIOL is in close proximity to the cornea, and chronic endothelial cell loss can lead to corneal decompensation (Figure 7). Finally, chronic inflammation or prosthetic-lens touch can induce cataract formation (Figure 8)[2].

The complication rate of pIOLs utilized today is dramatically lower than historical models due in large part to advances in imaging modalities used to properly size the lenses. Complications most often arise as a direct result of improper sizing of the lens, and vary depending on the type of lens used [15]. Explantation or reoperation have been necessary after implantation due to anterior subcapsular cataract development, unacceptable endothelial cell loss, pupil ovalization, lens subluxation or dislocation, refractive error, and pigment dispersion glaucoma.[16] [17]

While patients with pIOLs have had macular hemorrhage and retinal detachment, there has been no proof of causation between the pIOL and those morbidities since they occurred in the context of trauma. Endothelial cell loss can sometimes be confounded by the patient rubbing their eyes often after surgery.

The location of PCIOLs predisposes the crystalline lens and the iris to come into contact with the pIOL, leading to the specific complications of pigment dispersion with possible secondary glaucoma and anterior subcapsular cataracts. A PCIOL that is too long may cause angle-closure or pigmentary dispersion glaucoma, and a PCIOL that is too short may lead to anterior subcapsular cataracts.[6] Approximately one in five eyes will require cataract surgery and one in eight will require topical medication for ocular hypertension related to these complications of ICL implantation [18].

Angle-supported ACIOLs tend to cause chronic endothelial cell loss from the cornea, pupil ovalization, and nuclear cataract formation.[17] Iris-claw ACIOLs tend to encounter chronic endothelial cell loss and cataracts.[16]

Additional Resources

References

  1. Vitale, S., et al., Prevalence of refractive error in the United States, 1999-2004. Arch Ophthalmol, 2008. 126(8): p. 1111-9.
  2. 2.0 2.1 Chen, L.J., et al., Metaanalysis of cataract development after phakic intraocular lens surgery. J Cataract Refract Surg, 2008. 34(7): p. 1181-200.
  3. Praeger, D.L., A. Momose, and L.L. Muroff, Thirty-six month follow-up of a contemporary phakic intraocular lens for the surgical correction of myopia. Ann Ophthalmol, 1991. 23(1): p. 6-10.
  4. 4.0 4.1 4.2 4.3 Huang, D., et al., Phakic intraocular lens implantation for the correction of myopia: a report by the American Academy of Ophthalmology. Ophthalmology, 2009. 116(11): p. 2244-58.
  5. Redd T, V.J., Larson TL, Goins KM. Phakic intraocular lenses for high myopia. March 4, 2015; Available from: http://EyeRounds.org/cases/210-PhakicIOL.htm.
  6. 6.0 6.1 6.2 6.3 6.4 Guell, J.L., et al., Phakic intraocular lenses part 1: historical overview, current models, selection criteria, and surgical techniques. J Cataract Refract Surg, 2010. 36(11): p. 1976-93.
  7. Pineda, R., 2nd and T. Chauhan, Phakic Intraocular Lenses and their Special Indications. J Ophthalmic Vis Res, 2016. 11(4): p. 422-428.
  8. 8.0 8.1 Cataract and Refractive Surgery. Essentials in Ophthalmology, ed. T. Kohnen and D.D. Koch. 2005: Springer-Verlag Berlin Heidelberg.
  9. 9.0 9.1 Williams, G. and M. Muhtaseb Biometry Calculations for Phakic IOLs. Cataract & Refractive Surgery Today Europe, 2011.
  10. Hassaballa, M.A. and T.A. Macky, Phakic intraocular lenses outcomes and complications: Artisan vs Visian ICL. Eye (Lond), 2011. 25(10): p. 1365-70.
  11. 11.0 11.1 Barsam, A. and B.D. Allan, Excimer laser refractive surgery versus phakic intraocular lenses for the correction of moderate to high myopia. Cochrane Database Syst Rev, 2014(6): p. CD007679.
  12. Karimian, F., et al., Comparison of three phakic intraocular lenses for correction of myopia. J Ophthalmic Vis Res, 2014. 9(4): p. 427-33.
  13. Moshirfar, M., et al., Incidence rate and occurrence of visually significant cataract formation and corneal decompensation after implantation of Verisyse/Artisan phakic intraocular lens. Clin Ophthalmol, 2014. 8: p. 711-6.
  14. Taneri, S., S. Oehler, and C. Heinz, Inflammatory response in the anterior chamber after implantation of an angle-supported lens in phakic myopic eyes. J Ophthalmol, 2014. 2014: p. 923691.    
  15. Chang, D.H. and E.A. Davis, Phakic intraocular lenses. Curr Opin Ophthalmol, 2006. 17(1): p. 99-104.
  16. 16.0 16.1 Guell, J.L., et al., Five-year follow-up of 399 phakic Artisan-Verisyse implantation for myopia, hyperopia, and/or astigmatism. Ophthalmology, 2008. 115(6): p. 1002-12.
  17. 17.0 17.1 Alio, J.L., et al., Angle-supported anterior chamber phakic intraocular lens explantation causes and outcome. Ophthalmology, 2006. 113(12): p. 2213-20.
  18. Guber, I., et al., Clinical Outcomes and Cataract Formation Rates in Eyes 10 Years After Posterior Phakic Lens Implantation for Myopia. JAMA Ophthalmol, 2016.