Association of Optometric Contact Lens Educators

Contact Lens Fitting for Patients After Refractive Surgery

INTRODUCTION: 
While the patient undergoing refractive surgery does not typically contemplate the subsequent possibility of spectacle or eyeglass wear, the reality is that when complications occur, the use of contact lenses is often necessary and preferred to restore acuity and/or visual efficiency.  Prior to contact lens fitting, there needs to be stable corneal physiology as well as refractive stability.  For incisional surgery such as RK or AK, there should be a minimum interval of 3 to 4 months prior to commencement of contact lens fitting.  For LASIK, the interval should be 6 months, and for PRK 8-12 months.  This is necessary to allow corneal healing and facilitation of more adherent bonding of the flap to the underlying stroma  (in LASIK) as well as to prevent the contact lens itself from triggering corneal haze formation and refractive regression (with PRK).  Waiting the appropriate time interval also allows the practitioner to establish a refractive baseline with which to judge acuity measurements and evaluate lens fitting baseline measures.  Although patients requiring contact lens treatment after refractive surgery are anxious to have visual acuity restored, many have had multiple procedures (euphemistically termed "enhancements") and as such have corneas that need even more time to stabilize.  Contact lens fitting is often more appreciated when such intervals are observed.  The patient will often notice a dramatic improvement in visual performance with contact lenses compared to their final post-surgical acuity with attendant surgical complications such as irregular astigmatism, corneal haze or scarring, lipid deposition, or decentered ablation zones.

REFRACTIVE SURGERY COMPLICATIONS: 
RK incisions that heal minimally will result in greater myopic reduction or increased overcorrection leading to hyperopia.  Studies by Waring et al 1 looking at PERK Study results 10 years out indicated that 43% of RK patients experienced a 1D. or greater shift toward hyperopia.  Visual fluctuations in the first months after incisional placement have been common (incidence between 2-60%) and with refractive shifts as high as 1.5D.  This is typically caused by individual variation in wound healing, but is also influenced by the number and depth of incisions.  In the post-RK eye, special attention must be paid to the radial or arcuate incisions, with careful assessment of wound gape, epithelial plugs within the incisions, as well as neovascularization along the incision lines. 

With PRK, "central islands" have typically been a significant complication.  These topographic abnormalities are associated with broad beam excimer lasers and represent centrally elevated or steeper corneal zones of 1-3 D. higher than the surrounding paracentral cornea. They can be associated with glare, monocular diplopia, image ghosting and reduced acuity.  Although most central islands after PRK will dissipate after several months, some persistent cases will require surgical retreatment with a PTK-like ablation to reduce the elevated 1-3mm zone to the level of the surrounding ablation.

Soft lenses may be used with post-PRK or LASIK  patients in cases where the patient is undercorrected, whether from persistent central island formation,  undercorrection of refractive error, or regression.  Decentration of ablation of 1.0mm or more can produce clinically significant symptoms, including glare, haloes, and/or starburst effects, worse in dim illumination.  Although surgical retreatment for decentration complications is sometimes attempted, either by ablating adjacent tissue or by an increase in the treatment zone, additional surgery may often compound and worsen the condition.  Contact lens treatment may be the patient’s best option in these cases, to enhance visual acuity and to reduce or eliminate ghosting and irregular images, but will require the use of RGP lenses of large diameter and with large optic zones. 

               PRK Central Island

Although the topic of LASIK complications is beyond the scope of this section, it is important to realize that visual performance may be reduced in any situation where there is undercorrection, decentered ablation, persistent central island formation, haze, flap wrinkling or striae, or epithelial ingrowth.  In general, any complication that causes topographic irregularity can affect both refractive error correction and ultimately visual acuity.  Because irregular corneas may not be amenable to further surgical treatment, the value of rehabilitative contact lens fitting becomes self-evident.

LENS FITTING GUIDELINES

SOFT LENSES:
Soft lenses after RK have been met with varying degrees of success, and sometimes catastrophe.  Thin, disposable lenses often produce a better visual result in that they drape more easily over the new flattened corneal contour, whereas the use of thicker conventional lenses may lead to vaulting and ultimate buckling over the flattened central cornea resulting in visual fluctuation.  The use of higher water content lenses is preferred to reduce hypoxia and chances of vascularization.  Extended wear itself it contraindicated with hydrogel polymers, as the hypoxia inherent with this modality is likely to trigger vascularization along the incision lines.  This risk is much greater in cases of older surgical techniques when deep incisions were made starting at the limbus.  Newer RK incisional techniques in which incisions avoid the limbal plexus have greatly reduced the risks of vascularization with daily wear hydrogel use.  In general, the risks typically seen in extended wear in nonsurgical eyes are exaggerated in eyes with incisional surgery and should be avoided.  However, the advent of new silicone polymers for extended wear hold promise for these patients, as the risks of hypoxic complications should be greatly minimized.

Soft lens fitting after PRK or LASIK generally employs the use of flatter base curves, to conform to the flatter corneal profile, as well as to avoid a tight-fitting lens. If a satisfactory fitting relationship cannot be obtained, with adequate lens movement, then hydrogel use should not be undertaken.  Somewhat larger lenses may need to be employed if centration cannot be adequately achieved with typical lens diameters.

Paragon Vision Sciences developed the first soft contact lens approved specifically for refractive correction after corneal refractive procedures such as RK.  The Flexlens or Harrison post-refractive surgery lens has a reverse curve geometry, with the peripheral radius of curvature steeper than the base curve.  In order to achieve optical stability, the lens is made fairly thick (0.28mm at ?1.00D) in order to reduce lens flexure during the blink.  The periphery of the lens is thin in order to maximize oxygen transmission to the peripheral and limbal regions.  Trial fitting is utilized rather than any use of formulas based on central keratometric values.  The 8.7 base curve is often used as the initial diagnostic test lens.  The practitioner will evaluate centration and movement, using a steeper lens if edge lift is present or a flatter lens if the lens is tight using the manual pushup test.  Over-refraction is performed in the standard manner, and the practitioner has the option of 45% or 55% water content polymers, although the 55% is generally preferred. 
Although many practitioners prefer the use of RGP lenses for the post-RK patient, soft lenses can be utilized with success in a number of cases with careful assessment, and the judicious use of materials and lens designs.2

RGP LENSES: 
For RK patients requiring postsurgical contact lenses, RGP lenses remain the most versatile and optically helpful modality.  As many of these patients suffer from irregular astigmatism, the benefits of RGP lenses in restoring near normal levels of presurgical visual acuity are paramount.  Although both hydrogel and RGP lens options should be presented to the patient, it is important to emphasize that the visual benefits of RGP lenses far outweigh any temporary concerns of initial adaptive lens comfort issues.

 SPHERICAL DESIGNS:  Because of the highly altered corneal topography after RK, in which the typically prolate shape (positive e-value in which the cornea is steeper centrally and flattens aspherically ) of the cornea becomes oblate or inverted, centration almost always requires the use of a large diameter lens, typically in the 9.8 to 11.0 mm range.  The clinician should recognize that it will be impossible to align the post-surgical central cornea and provide a lens that will remain on the eye.  Thus, fitting the midperipheral cornea utilizing either pre-surgical K readings (if these are available) or taking post-surgical midperipheral K readings (using fixation dots placed on the keratometer, or utilizing corneal topography data) is recommended.  The goal is to produce a reasonably well-centered lens that demonstrates an alignment fluorescein pattern throughout the midperiphery, with central apical pooling remaining (reflecting the flattened central corneal curvature).

Patients undergoing PRK and who require RGP lenses for irregular astigmatism or for visual disturbances secondary to decentered optic zones can also be fitted utilizing the above spherical lens guidelines.  A study by Astin et al 3 of 10 patients fit after PRK with RGP lenses found the optimal results when most patients were fit with a base curve 0.1mm steeper than the mean K reading, and when diameters of 9.2 - 10.0mm were used.  Of interest, however, is that about 50% of these patients ultimately were refit from RGP lenses into soft lenses, despite the fact that the RGP lenses restored visual acuity to normal levels.  Thus, it appears that RGP lens comfort issues are still a primary consideration in the long-term management of these patients.  As such, the introduction of "reverse geometry" RGP lenses has been crucial to making RGP lenses the modality of choice for refractive surgery patients.

"REVERSE GEOMETRY" LENSES:  Because refractive surgical procedures for myopia create an oblate corneal profile, these post-surgical eyes will do much better with lens designs that are better at approximating this new shape.  "Reverse geometry" or "plateau" lens designs have secondary and mid-peripheral curves that are steeper than the base curve.  These lenses are typically made with large diameters (9.5mm and greater) and with small optic zones (typically in the 6.0 ? 6.5mm range).  Secondary curves may be 2-4 diopters steeper than the lens base curve radius.  Examples of such lenses are:  The OK  TM   series lenses (Contex Inc., Sherman Oaks, CA), the Menicon Plateau (Menicon lenses are now supplied through ConCise Contact Lenses of San Leandro, CA), the RK Bridge Lens (Conforma Laboratories, Norfolk, VA), and the RK Splint (Lexington Lenses).

While some of these designs were developed for non-surgical applications, such as orthokeratology, their design is ideal for patients who have undergone myopic refractive surgery and who require post-surgical contact lens therapy.  However, in this capacity, the goal of the fitter is to utilize the diagnostic lenses to achieve a lens-cornea relationship showing more parallel fit of the midperiphery with obvious central fluorescein pooling. The use of "simulated fluorescein patterns" utilizing corneal topography with contact lens design software can further assist the clinician who may not have a large array of diagnostic lenses at his or her disposal.

For hyperopic LASIK patients who have residual irregular astigmatism with or without undercorrection, we are looking at a group of patients who still have a prolate shape to their corneas, but who have steeper than normal central curvatures.  These eyes can usually be successfully treated with RGP lenses utilizing steeper spherical base curve values.  Smaller lenses can be employed for these patients, and sometimes the use of steeper?fitting keratoconic lens designs, such as the Rose K ™ (Lens Dynamics Inc.) may be helpful
.

 ALTERNATIVE DESIGNS:  A special len design may be helpful in treating patients with irregular astigmatism or with decentered optic zones.  The MacroLens ™ (C&H Contact Lenses, Dallas Tx) was approved in 1999 and is a semiscleral RGP lens, with diameters ranging from 13.9mm to 15.0mm, with base curves from 6.60mm to 8.88mm, and is made with Boston EO (Dk 58).  As with any very large lens, comfort is often greatly enhanced, often achieving levels rivaling soft lenses, as well as excellent lens centration. 4  Irregular astigmatism is essentially eliminated with the stable optics of the RGP material.  Due to the large size, however, the lens is fenestrated near the limbal region to prevent lens binding and to promote tear exchange.  The peripheral lens design is variable, as the lens can be provided with a variety of peripheral curvatures, e-values, and widths.  The recommended starting design uses an e-value of 0.8 although a tighter peripheral fit with a 0.6 e-value or a looser fit with a 1.0 e-value are available.  This design is useful not only for refractive surgery patients, but also for a wide variety of corneal conditions in which conventional RGP lenses cannot be successfully used (keratoconus, pellucid marginal degeneration, post-penetrating keratoplasty, etc.)
 

KEY REFERENCES:

1. Waring GO, Lynn MJ, McDonnell PJ, et al:  Results of the Prospective Evaluation of Radial Keratotomy (PERK) study 10 years after surgery.  Arch Ophthalmol 112:1298, 1994.
2. Gubman, DT.  Complications after corneal refractive surgery.  p.399. In: Silbert JA (ed.): Anterior Segment Complications of Contact Lens Wear (2nd ed). Butterworth-Heinemann, Boston, 2000. 
3. Astin CL, Gartry DS, McG-Steele AD: Contact lens fitting after photorefractive keratectomy.  Br J Opthalmol 80:597. 1996.
4. Caroline PJ, Andre MP: Fitting after refractive surgery: Is there hope?  Contact Lens Spectrum 14:56, 1999.

1. Ackley KD, Caroline PJ, Davis LJ: Retrospective evaluation of rigid gas permeable contact lenses on radial keratotomy patients.  Optom Vis Sci (suppl.)70:32, 1993.
2. Aquavella JV, Shovlin JP, Pascucci SE, et al: How contact lenses fit into refractive surgery. Rev. Ophthalmol 1:36, 1994.
3. Bennett ES, Gans L: Corneal topography in pre- and post-surgical contact lens fitting. p.1  In Harris MG (ed): Contact Lenses in Pre- and Post-surgical Contact Lens Fitting. Mosby, St. Louis, 1998.
4. Klyce SD: Role of corneal topography in keratorefractive surgery. p.2039.  In Krachmer JH, Mannis MJ, Holland EJ (eds):Cornea. Vol. III: Surgery of the Cornea and Conjunctiva. Mosby, St. Louis, 1997.
5. Koffler BH, Smith VM, Clements DC.  Achieving additional myopic correction in under-corrected radial keratotomy eyes using the Lexington RK Splint design.  CLAO J  25:21, 1999.
6. Lee AM, Kastl PR: Rigid gas permeable contact lens fitting after radial keratotomy.  CLAO J  24:33, 1998.
7. Machat JJ, Slade SG, Probst LE (eds): The Art of LASIK, 2nd ed.. SLACK, Inc., Thorofare, NJ, 1999.
8. Shovlin JP, Kame RT, et al: How to fit an irregular cornea. Rev Optom 124:88, 1987.