Identifying candidates for non-corneal refractive surgery
Today, the surgical correction of refractive error is most commonly performed on the cornea. LASIK surgery is the most popular form of refractive surgery in the U.S. with more than 16.5 million procedures performed to date.1 For many of our patients, LASIK may be the best surgical option, but sometimes removing tissue from the cornea may not be the best surgical option.
Table 1. PIOL Verisyse vs. Visian
Today, the surgical correction of refractive error is most commonly performed on the cornea. LASIK surgery is the most popular form of refractive surgery in the U.S. with more than 16.5 million procedures performed to date.1 For many of our patients, LASIK may be the best surgical option, but sometimes removing tissue from the cornea may not be the best surgical option. The risks of potential biomechanical destabilization of the cornea, exacerbation or prolonged dry eye, and reduced quality of vision can make some patients a better candidate for intraocular surgery. Let’s discuss some clinical findings that are contraindications for laser vision correction.2
Figure 1. Corneal dystrophies and corneal degenerations can often be diagnosed with topography and tomography. Irregular corneas
Cornea dystrophies and corneal degenerations can often be diagnosed with topography and tomography (Figure 1). Patients with pre-operative signs of keratoconus or other types of corneal ectasia, such as pellucid marginal degeneration, who undergo laser vision correction may experience corneal destabilization.3 Progressive corneal thinning may cause irregular astigmatism and potential loss of best-corrected vision.
Thin corneas
Average cornea thickness is about 540 µm, but most surgeons limit their laser vision correction to patients with central corneal thickness of 475-500 µm due to concern of corneal destabilization.
While the FDA has recommended a minimal residual stromal bed (RSB) of 250 µm, it has been well established that RSB alone is not a good predictor of ectasia risk.4 Significant variability in biomechanical strength of an individual cornea, actual LASIK flap thickness, and IOP may account for poor predictability of RSB.5,6 Today, most surgeons use 275-300 µm as a minimal RSB to reduce the risk of post-LASIK ectasia.
High myopia
The higher the level of myopic treatment, the greater the laser ablation depth. Most modern lasers remove 15-18 µm per diopter of treatment. While the structural integrity of the cornea is multifactorial, the more tissue removed, the greater the risk of corneal destabilization.
Higher-order aberrations
LASIK flap creation with mechanical microkeratomes and femtosecond lasers induce higher-order aberrations that can decrease a patient’s quality of vision. While better than old conventional treatments, modern wavefront-guided and wavefront-optimized corneal ablation patterns can induce unwanted aberrations that reduce quality of vision predominantly under scotopic viewing conditions.7,8
Figure 2. The Verisyse PIOL is an anterior chamber PIOL and attached (enclavation) to the anterior iris surface with PMMA clips.Dry eye
Dry eye following laser vision correction is not fully understood and likely multifactorial. The most significant cause is likely alteration of corneal nerves during flap creation and excimer ablation.9 Goblet cell destruction and alteration of cornea shape may also play a role in post-operative dry eye symptoms.
Phakic IOLs
Phakic IOLs (PIOL) are approved in the U.S. for the correction of moderate to high myopia. Approved in September 2004, the first PIOL approved in the U.S. was the Verisyse (AMO, Inc.). Previously known as the Artisan PIOL, the U.S. version of the Verisyse PIOL is not foldable, requiring at large (5.0 mm) corneal incision. The Verisyse PIOL is an anterior chamber PIOL is attached (enclavation) to the anterior iris surface by with PMMA clips (Figure 2).
Figure 3. The Visian ICL is a posterior chamber PIOL, placed behind the iris and in front of the crystalline lens.
Approved in December 2005, the Visian Implantable Collamer Lens (ICL) (Staar Surgical) is a posterior chamber PIOL, placed behind the iris and in front of the crystalline lens (Figure 3). The Visian ICL is made of a soft biocompatible collagen copolymer which allows the lens to be folded and inserted into the eye though a small (3.0mm) incision, inducing less astigmatism and allowing for more rapid healing.
PIOL patient selection
To be considered for a PIOL, the patient must be a non-presbyopic, moderate to severe myopes, from 21 to 45 years of age with anterior chamber depth > 3.0mm, and an age-dependent endothelial cell count > 3,875 - 1,800 cells/mm2 (Table 1). All patients must have a stable Rx (less than 0.50 D change x 12 months).
U.S. Army clinical studies of Visian ICL show similar or better efficacy when compared to LASIK with 96% of patients achieving UCVA of 20/20 or better. Safety was established with 33% of the 135 eyes gained 1 or more lines of best-corrected visual acuity. The Visian ICL was also extremely predictable with more than 90% with 0.5D of target.10 Additional studies show that the Visian ICL is an effective alternative to LASIK and PRK for myopic patients as low as -3D.11ODT
Coming next month: Phakic IOLs complications and perioperative care
References
1. Market Scope LLC. Q4â2013. Refractive Quarterly Survey. Available at: https://www.practice-scope.com/pages/refractive. Accessed: 04/18/2014.
2. Randleman JB, Russell B, Ward MA, et al. Risk factors and prognosis for corneal ectasia after LASIK. Ophthalmology. 2003 Feb;110(2):267-75.
3. Schmitt-Bernard CF, Lesage C, Arnaud B. Keratectasia induced by laser in situ keratomileusis in keratoconus.J Refract Surg. 2000 May-Jun;16(3):368-70.
4. Ou RJ, Shaw EL, Glasgow BJ. Keratectasia after laser in situ keratomileusis (LASIK): evaluation of the calculated residual stromal bed thickness.Am J Ophthalmol. 2002 Nov;134(5):771-3.
5. Binder PS. Ectasia after laser in situ keratomileusis. J Cataract Refract Surg. 2003 Dec;29(12):2419-29.
6. Reinstein DZ, Srivannaboon S, Archer TJ, et al. Probability model of the inaccuracy of residual stromal thickness prediction to reduce the risk of ectasia after LASIK part II: quantifying population risk. J Refract Surg. 2006 Nov;22(9):861-70.
7. Smadja D, Santhiago MR, Mello GR, et al. Corneal higher order aberrations after myopic wavefront-optimized ablation. J Refract Surg. 2013 Jan;29(1):42-8.
8. Igarashi A1, Kamiya K, Shimizu K, et al. Visual performance after implantable collamer lens implantation and wavefront-guided laser in situ keratomileusis for high myopia. Am J Ophthalmol. 2009 Jul;148(1):164-70.
9. Chao C, Golebiowski B, Stapleton F. The role of corneal innervation in LASIK-induced neuropathic dry eye.Ocul Surf. 2014 Jan;12(1):32-45.
10. Parkhurst GD, Psolka M, Kezirian GM. Phakic intraocular lens implantation in United States military warfighters: a retrospective analysis of early clinical outcomes of the Visian ICL. J Refract Surg. 2011 Jul;27(7):473-81.
11. Sanders DR. Matched population comparison of the Visian Implantable Collamer Lens and standard LASIK for myopia of -3.00 to -7.88 diopters. J Refract Surg. 2007 Jun;23(6):537-53.
12. Pérez-Vives C, Dominguez-Vicent A, García-Lázaro S, et al. Optical and visual quality comparison of implantable Collamer lens and laser in situ keratomileusis for myopia using an adaptive optics visual simulator. Eur J Ophthalmol. 2012 Jul 30:0.
13. Barnes S. Is the ICL ready for service in the US Army. Hawaiian Eye Meeting 2010.
