Fitting Contact Lenses Post-LASIK
Fitting Contact Lenses Post-LASIK
Using the full spectrum of contact lens options will help optimize difficult post-surgical fits.
By Jessica O. Yu, OD
Dr. Yu is a graduate of the SUNY College of Optometry, where she also completed a Fellowship in Cornea and Contact Lens. During this time, Dr. Yu trained with a leading ophthalmologist in Manhattan specializing in laser vision correction. Dr. Yu specializes in post-LASIK cases as well as other specialty lens fittings. She is currently in private practice in Connecticut.
Laser-Assisted in-Situ Keratomileusis (LASIK), one of the most popular refractive surgery procedures performed today, has made rapid advancements and evolutions since its inception in the early 1990s. During the procedure a microkeratome is used to create an epithelial flap before excimer laser ablation of stromal tissue, proportionate to the amount of refractive error correction.
Prior to the evolution of IntraLase LASIK, the risk and complications associated with laser vision correction were more prominent, with corneal ectasia being the most potentially devastating complication that could occur. With an incidence of 0.2 percent, one in every 2,000 to 10,000 individuals could develop ectasia anywhere from months to years after the surgery (Rad et al, 2004). Traditionally, ectasia was reported in cases involving high myopia (>8.00D), forme fruste keratoconus, thinner corneas, irregular astigmatism, and a residual stromal bed of less than 250μm. Corneal ectasia is regarded as an alteration in the integrity of the cornea, with a progressive inferior steepening (Randleman et al, 2003). This is often accompanied by an increase in myopia and irregular astigmatism, a decrease in uncorrected visual acuity (UCVA), and a loss of best-corrected visual acuity (BCVA). An irregular scissoring reflex may be apparent upon retinoscopy.
Biomechanics of Ectasia
Researchers and clinicians have long sought after the mechanism behind corneal ectasia. A well-supported theory deemed that the changes and variations made to the depth and diameter of the flap during laser vision correction modified the biomechanical stability of the cornea. With excessive tissue removal, the residual cornea is not strong enough to resist the stresses and strains from the intraocular pressure (IOP). In a sense, this effect of LASIK on susceptible eyes was thought to be very similar to keratoconus (Figure 1), as decreasing the load-bearing capacity of the cornea led to a progressive shape change in response to the IOP.
Figure 1. Post-LASIK topography map showing the similarity between post-LASIK ectasia and keratoconus.
This reduction in corneal biomechanical strength seems to be the essential element in the chain of events leading to keratectasia after LASIK. The cornea does not have a uniform biomechanical strength, as the anterior lamellae are smaller, more densely packed, and demonstrate an interweave pattern. On the other hand, the posterior lamellae are larger, more loosely packed, and do not interweave. Hence, the cornea intrinsically has weak and strong zones. The anterior stroma confers more biomechanical strength to the cornea than the posterior stroma does, and it is the anterior stroma that is weakened by flap generation and tissue ablation in LASIK (Hafezi et al, 2007).
In accordance with this theory of corneal biomechanics alteration, traditional risk factors for ectasia have been a thin baseline cornea, thick corneal flap, irregular corneal thickness, high IOP, and pre-existing forme fruste keratoconus (Tabbara and Kotb, 2006). Excessive ablation, especially in cases of higher myopia in which more tissue is removed, was a notorious caution sign because of the risk of leaving a thinner residual stromal bed. Recent findings that have delved more into the nature of ectasia, however, have shown resultant ectasia even in cases with low refractive error. Moreover, ectasia has also occurred in the absence of any "recognized risk factors" (Binder, 2007).
The advent of so many newer screening technologies and advanced surgical techniques available for laser vision correction has diminished the risk of corneal ectasia. Yet, in cases in which keratectasia still occurs, the visual and emotional consequences can be devastating for the patients. In such instances, the application of GP lenses and other specialty contact lenses can be beneficial as a form of visual rehabilitation.
Managing Post-LASIK Patients
A successful surgery can be characterized by three aspects: quantity of vision, quality of vision, and patient satisfaction. The quantity of vision is defined by the postsurgical measured VA, whether at one day, one week, or one year. The quality of vision is characterized by how the patient sees subjectively, including the frequency of halos, glare, ghosting, or shadowing. The level of post-operative satisfaction directly correlates to the patient�s initial expectations, which can be diminished or augmented by prior research and by education from the surgeon. In a survey of patients who were dissatisfied, the most common subjective complaints were blurred distance vision, glare or night-vision disturbances, and dry eyes. The most common complications noted included over-correction (30.4 percent), irregular astigmatism (29.8 percent), dry eyes (29.8 percent), and corneal haze (16.7 percent) (Jabbur et al, 2004) (Figure 2).
Figure 2. The most common subjective complaints and complications of dissatisfied post-LASIK patients.
Managing post-LASIK patients can be challenging from several vantage points. The population typically presents with complaints of decreased BCVA or UCVA, often without improvement in vision with either glasses or soft contact lenses. Conventional GPs may cause pain as corneal sensitivity has been altered, and impaired contrast sensitivity may be present (Choi et al, 2004).
More importantly, this patient base is generally dissatisfied with the results of the surgery and often presents with a range of emotions, varying from nonacceptance to reluctant compliance with a contact lens fitting. Nonetheless, once patient amenability is achieved, contact lenses offer a safe, reversible, and non-invasive approach to improving vision in an irregular or ectatic cornea.
Varying Corneal Presentations
To efficiently utilize the spectrum of contact lenses available, it is crucial to fully understand the anatomy and physiology of post-LASIK corneas. Assuming a three- to six-month period for postoperative stability in refraction and topography findings, you can manage simple refractive error shifts, under- or over-corrections, and unexpected problems related to presbyopia with a spectacle correction or with spherical or toric soft contact lenses.
Due to the nature of the ablation, the central zone in hyperopes is more elevated than the periphery. In this way, the cornea topographically simulates keratoconus, and hence, a similar fitting approach can be utilized. On the other hand, in myopes, because the steeper central portion is ablated to reduce myopia, the resulting cornea is flatter centrally. The tear film meniscus tends to accumulate there, which facilitates the fitting process. However, as the tear film takes on a convex shape, it also creates a refractive power that you must take into consideration with the overrefraction (Alio et al, 2002). It is not unusual for the power of the contact lens to be higher than that of the spectacle correction in these cases.
In contrast, multifocal corneas, corneas that exhibit central islands, and irregular astigmatism pose a distinct challenge to the fitting process. The postoperative oblate shape and typically irregular anterior surface often result in contact lens instabilities, while multifocal corneas have different planes of focus. This forces patients to create adaptive turning or tilting head movements to see more clearly through a specific part of the cornea. The false apices of abnormal central islands lead to inadequate centration, which may be a factor to consider when choosing an initial lens design.
Irregular astigmatism, another common finding among post-surgical corneas, occurs secondary to irregularities in corneal curvature and can occur with or without a defined pattern. In post-LASIK corneas, irregular astigmatism can be induced by a flap displacement or tear, epithelial in-growth at the interface, abnormal scarring after an infection, and decentered ablations. As the principal meridians are not 90 degrees apart, a scissoring reflex is apparent upon retinoscopy. Not only does BCVA decrease by at least two lines, but the subjective quality of vision is also very poor (Alio et al, 2002). Patients complain of monocular diplopia, polyopia, distortion of vision, and glare and halos. Correction with spectacles is difficult secondary to these visual symptoms that result from the surface irregularities.
Topography frequently serves as the most useful tool in imaging irregular corneas and enabling you to more accurately choose an initial lens with an appropriate base curve, diameter, and design. Fluorescein patterns can greatly assist in making any fine adjustments to create the optimal fit. Perform over-refraction once the fit is acceptable, with some allowance for improvement in the BCVA once the final lens is on the eye.
Soft Contact Lenses
In a post-LASIK cornea in which simple or compound myopic or hyperopic refractive error exists, soft spherical or toric lenses may be sufficient to improve vision. If there is a small degree of astigmatism, an aspheric lens (such as CooperVision's Frequency 55 Aspheric, Biomedics XC, or Avaira; Bausch & Lomb's PureVision) may mask certain aberrations and provide improved subjective vision. One disadvantage to soft torics is that they often exhibit atypical rotations on oblate corneas, and stabilization is difficult. Furthermore, soft torics do not offer maximal correction for high degrees of corneal astigmatism. Overall, because of the non-prolate cornea, a low-modulus lens may drape over the cornea more readily, and a higher-modulus may result in lens fluting. Similarly, you should avoid lowmodulus/ higher-oxygen transmissibility (Dk/t) materials with GPs, as they may introduce unwanted flexure.
In a number of post-LASIK cases for which soft contact lenses offer limited visual benefits, GP lenses come to play an important role in achieving optimal quality and quantity of vision. The wide array of GPs available includes standard spherical, keratoconic, custom multi-curve, and reverse geometry lenses.
Consider large-diameter GP lenses for post-LASIK cases in which patients have decentered ablations, leaving residual uncorrected myopia. Such patients are often left viewing through the edge of the ablation and complain of monocular diplopia, ghosting, and loss of BCVA, especially in scotopic conditions. Symptoms also tend to be exaggerated with larger pupils. Lenses ranging anywhere from 10.8mm to 12.5mm in diameter will vault over the cornea and mask many central irregularities. In this way, BCVA improves through the smooth, uniform refractive surface of the rigid lens. This approach may also be applicable in highly astigmatic corneas and tilted grafts. In choosing an appropriate diameter, a total diameter 0.2mm smaller than that of the cornea should provide adequate lens movement and tear exchange.
Specific designs of intralimbal and other types of large-diameter lenses vary with each manufacturing company. For instance, intralimbal and corneo-scleral designs can range up to 13.0mm in diameter, whereas semi-scleral and mini-scleral designs can range from 13.0mm to 18.0mm in diameter.
In more severe and extreme cases of corneal irregularities, scleral lenses offer more stable centration. Supported almost entirely by the sclera, surface irregularities are neutralized by the much larger diameter. However, because scleral lenses must be made thicker for lens stability, higher Dk/t materials must be used to reduce the risk of corneal edema.
Reverse Geometry Lenses
Reverse geometry (RG) lenses are indicated in corneas that have a significant difference between the central and peripheral zones (Figure 3). Oblate corneas that are fit with regular design lenses, in which the central zone of the lens is steeper than the periphery, frequently result in central or midperipheral bubbles and peripheral binding. The central zone of an RG lens is flatter than the surrounding peripheral curves, allowing a more contoured alignment with an oblate cornea. An ideal fit demonstrates slight central clearance, midperipheral alignment, and good overall centration (Figure 4). Figure 5 shows a less-than-ideal fit, with the RG lens riding too low.
Figure 3. This post-LASIK patient is a good candidate for a reverse geometry lens design.
Figure 4. An ideal reverse geometry fluorescein pattern.
Figure 5. This reverse geometry lens is riding too low.
Even after attaining an adequate initial contact lens fit, however, follow-up care after several hours' wear is crucial because the lens-to-cornea interface can change due to molding effects. Potential problems include dryness, itchiness, scratchy sensations, and limbal injection secondary to lens binding in a snug fit. This lack of tear exchange and discomfort precludes prolonged lens wear throughout the course of the day. In addition to molding effects, also check for other possible issues with RG lenses such as dimple veiling (Figure 6).
Figure 6. Dimple veiling with reverse geometry lens.
Lens Dynamics manufactures RG lenses with preset reverse curve-to-lift ratios. The Series A/B/C designation allows for adjustments to achieve a nearalignment fit, with minimal central vaulting and adequate tear exchange. With a diameter of 11.2mm, the lens provides good comfort. In some cases, however, the large diameter negatively affects movement and large air bubbles may be easily trapped.
Bridge II (RK Bridge, Conforma) Design RG lenses can also be custom-designed with specific curve changes and widths. The slightly smaller 10.0mm diameter facilitates better tear exchange and less bubble entrapment, though the real advantage is the ability to customize specific curve radii. Similar to other RG contact lens designs, the complex aspheric posterior surface aims to accommodate changing corneal curvatures as the periphery flattens.
In many post-LASIK cases, patients experience difficulty adapting or re-adapting to GP lens wear. Between alterations in corneal sensitivity and greater overall difficulty adapting to rigid lenses, motivation often wanes due to initial or continued discomfort. In turn, wearing time decreases to a minimum and becomes sporadic. Hybrid contact lenses present an alternative solution to comfort issues. Hybrid designs from SynergEyes have a GP center and a soft skirt of 27-percent water composition. The crisp vision of the GP center parallels that of regular GPs, while the soft skirt provides for greater comfort and, in turn, longer wearing time.
SynergEyes represents a second generation of hybrid lenses, improving on its forerunner Softperm (CIBA) by incorporating a higher-Dk GP material, allowing for more oxygen transmissibility and decreasing risks of neovascularization. The center GP material, Paragon HDS (Paragon Vision Sciences), has a Dk of 100. Unlike Softperm lenses, SynergEyes offers the option of several skirt curvatures to provide greater tear exchange. In the past, Softperm lenses tended to tighten excessively on the eye, resulting in inadequate or no movement.
There are five lines of SynergEyes hybrid lenses: SynergEyes A, SynergEyes Multifocal, SynergEyes KC, SynergEyes PS, and SynergEyes ClearKone. All sets have a lens diameter of 14.5mm. Depending on the severity and location of corneal irregularity or ectasia, the SynergEyes A or KC set could be utilized in addition to the PS set. SynergEyes A, with a BC range of 7.10mm to 8.00mm, is designated for normal corneas with up to 6.00D of astigmatism. The KC design accordingly has a steeper base curve range (5.7mm to 7.1mm), with a flat, medium, or steep skirt option. The PS set has a flatter base curve range (7.6mm to 9.0mm) and varying degrees of lift, which is more appropriate for corneas that have undergone refractive surgery or penetrating keratoplasty. Figure 7 shows a post-surgical corneal topography for which a SynergEyes lens or a largerdiameter GP lens would work best. The Synergeyes lens fit must be evaluated with high-molecularweight fluorescein, with attention to areas of touch and the presence of any bubbles. The location and shape of the bubbles would designate areas of necessary adjustment.
Figure 7. A larger-diameter or a hybrid lens would work best for this variation of a post-surgical cornea.
For highly-irregular corneas in which larger-diameter lenses or RG lenses do not provide adequate stabilization, several companies have incorporated strategies that allow you to modify specific quadrants of the lens in the design. Rose K (Menicon/ Blanchard Contact Lens) lenses are now available with the option of Asymmetric Corneal Technology (ACT), including in the Post-Graft and Irregular Cornea designs. The inferior quadrant can be steepened independent of base curve and overall edge lift-off. Not only can this be applicable in keratoconic corneas, but the ability to steepen the inferior portion could be infinitely useful in keratectasia, which occurs most commonly in that area (Figure 8). In a similar vein, Lens Dynamics now offers Quadrant Specific Technology, or Quad Sym, in which the back surface of the lens can be altered to match irregularities of the paracentral and peripheral cornea. TruForm Optics also incorporates Quad Technology into its QuadraKone lens. New lathes and software analysis have vastly improved GP technology and allow for designs to better serve the postsurgical population.
Figure 8. Topography showing severe inferior steepening post-LASIK.
Larger-diameter and reverse geometry lenses have been applied with much success in post-surgical corneas not only in the United States, but also in Europe and Asia. In Spain, 29 post-surgical eyes were examined comparing BCVA using spectacles versus contact lenses. Overall, 79 percent gained two lines in BCVA with the contact lenses while 14 percent gained one line (Alio et al, 2002). The most success in vision improvement was noted with GP lenses. In Korea, six post-surgical eyes had BCVA ranging from 20/100 to 20/1000. BCVA of 20/30 or better was obtained in all eyes with the use of multi-curve lenses or RG lenses (Choi, Kim, and Lee, 2004). All patients reported good subjective vision and comfort.
The ever-growing popularity of refractive surgery has been augmented by immense advances in laser and screening technologies. Yet, the prospect of superior vision is bundled with the minimal but potential risk of debilitating consequences. Newer technologies have reduced, but not eliminated, the possibility of complications. Despite much research, the true etiology of keratectasia is still unknown, though it remains one of the most devastating post-LASIK complications.
As a solution, the spectrum of current specialty contact lenses can provide visual rehabilitative means for symptomatic patients, with caution about stressing an already compromised cornea. Post-refractive surgery corneas, with their multiple diverse presentations, present unique challenges to contact lens fittings. CLS
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #168.
Contact Lens Spectrum, Issue: November 2009