William Shakespeare once wrote "Never be the first by which the new is tried ? nor yet the last to lay the old aside." Refractive surgeons are sometimes faced with patients who have had LASIK surgery and need or would benefit from enhancement surgery to complete the planned refractive change for their vision correction surgery, but do not have enough tissue available under the flap.1 This could be a result of having had limited tissue available from the onset,2 or they may have had a relatively thicker flap created than anticipated during the first surgery. We report our technique of excimer laser ablation on the back surface of the flap in patients who did not have adequate stromal tissue available for an enhancement ablation to be performed following LASIK.
PATIENTS AND METHODS
Preoperatively, all patients underwent an ophthalmic examination that included wavefront and corneal topography measurement with the NIDEK OPD- Scan (NIDEK, Gamagori, Japan) and the LaserSight AstraMax 3D topographer (LaserSight Technologies Ine, Winter Park, FIa), contrast sensitivity/functional acuity assessment and glare testing with the CSV-1000 (VectorVision, Greensville, Ohio), visual acuity with manifest refraction (and cycloplegic refraction in select cases), Javal keratometry, slit-lamp microscopy of the anterior segment, central and peripheral retinal fundus examination, tear film assessment (and Schirmer testing in select cases), Goldman tonometry, biometry/A-scan, and pachymetry with Tomey AL-2000 ultrasound (Tomey Corp, Nagoya, Japan).
The original LASIK procedure for eligible patients was performed using a Moria M2 microkeratome with the 130 head (Moria, Antony, France), creating a superior hinge. Ultrasonic pachymetry was performed on all patients after flap reflection to assess the amount of stromal bed available for ablation. The appropriate excimer laser ablation was then performed with a NIDEK EC-5000 CX with software version 1.24. The flap was replaced onto the stromal bed and the interface was irrigated with balanced salt solution (BSS; Alcon, Ft Worth, Tex). The flap thickness was calculated from the preoperative pachymetry minus the intraoperative stromal pachymetry. The remaining stromal bed after ablation was calculated from the predicted amount of stroma to be ablated by the excimer laser.
Patients who needed enhancement following previous LASIK who had inadequate stromal tissue available had part or all of the ablation performed on the back of the hinged flap. Patients were considered for enhancement at 3 months after LASIK. All candidates for enhancement underwent an ophthalmic examination that included uncorrected visual acuity (UCVA), manifest refraction and best spectacle-corrected visual acuity (BSCVA), keratometry, contrast sensitivity and glare assessment, topography using the NIDEK OPD-Scan and AstraMax, slit-lamp microscopy, funduscopy, applanation intraocular pressure, and ultrasonic pachymetry.
The enhancement surgical procedure was performed in two stages. Stage 1 involved stromal laser application of the cylindrical component to the stromal bed. Stage 2 involved treating the spherical component on the undersurface of the flap. In each case, prior to laser treatment, ultrasonic pachymetry was performed on the stromal bed to determine residual stromal thickness with a goal of leaving at least 250 pm after laser retreatment.
Prior to surgery, a standard eye preparation and peri-orbital skin cleansing was performed followed by instillation of topical anesthetic drops. Circular marks were placed centrally over the pupil entrance, and the optical axis was marked with a Slade spatula. Alignment marks were placed on the peripheral cornea. The flap was lifted and reflected onto the sclera or hard contact lens (Fig 1). The stromal bed is ablated as per plan for the cylindrical correction (Stage 1).
For Stage 2, a hard contact lens was positioned under the flap and the flap was reflected superiorly over the lens while the patient looked inferi orly. The laser was focused and centered on the visual axis mark and the spherical ablation was applied to the back of the flap, while controlling the position and smoothness of the flap. Subsequently, the flap was folded back onto the stromal bed. Meticulous interface irrigation, with attention to the area of stroma that may have touched other surfaces, was performed. The alignment of the stromal bed to the flap edge marks was verified for proper repositioning.
Figure 1. The patient is instructed to look downward and the flap is then reflected out on a specially designed rigid contact lens. The excimer laser is focused and centered on the marked optical axis while a Merocel spear helps control the position and smoothness of the reflected flap. (Reprinted with permission from Kim J-H, Joo M-J. Ablation of flap and bed can be effective in high myopes. Ocular Surgery News Europe/ AsiaPacific. 2004;15:20-22. Copyright ? 2004. SLACK Incorporated.)
A 22-year-old man who underwent LASIK and an enhancement with ablation of the underside of the flap had preoperative manifest refraction of -3.75 ?3.75 × 12° with BSCVA of 20/15 in the right eye and manifest refraction of -3.75 -4.25 × 165° with BSCVA of 20/15 in the left eye. Ultrasonic pachymetry was 534 µm in the right eye and 529 µm in the left eye. The patient underwent primary LASIK with a measured flap thickness of 172 µm in the right eye and 167 µm in the left eye. The optical and transition zones used for excimer laser ablation were 6 and 8 mm, respectively, with removal of 81 µm in the right eye and 85 µm in the left eye.
Three months after LASIK, manifest refraction and BSCVA in the right eye were -1.25 -0.75 × 35° and 20/15 with UCVA of 20/60, and -1.00 -0.75 v 150° and 20/15 with UCVA of 20/50 in the left eye. At 3 months postoperatively, the patient underwent an enhancement procedure that involved ablation of the underside of the flap. Ultrasonic bed pachymetry was 266 pm in both eyes. For the astigmatic correction, 10.6 µm was removed from the stromal bed in the right eye and 10.3 µm from the left eye. The spherical ablation was delivered to the back of the flap with 12.9 µm of tissue removed from the right eye and 9.3 µm from the left eye. The total calculated residual bed was 255 µm in both eyes.
Postoperatively, mild Sands of the Sahara syndrome was seen in both eyes, which was successfully treated with topical steroids. Manifest refraction at 3 months postoperatively was -0.25 -0.25 × 80° in the right and left eyes with BSCVA and UCVA of 20/12 in both eyes. Both eyes gained one line of BSCVA after excimer laser ablation of the underside of the flap. Figure 2 shows the OPD-Scan analysis of both eyes 3 months after enhancement.
Figure 2. Case 1. Postoperative NIDEK OPD-Scan corneal topography and wavefront measurements of the A) right and B) left eyes after LASIK with ablation of the underside of the flap.
A 37-year-old woman underwent LASIK, a primary enhancement, and second enhancement with ablation of the underside of the flap in the right eye. Preoperative manifest refraction was -6.75 -2.25 × 170° with BSCVA of 20/20. Preoperative pachymetry was 555 pm. The patient underwent primary LASIK with a measured flap thickness of 212 µm. The optical and transition zones for the laser ablation were 5 and 7 mm, respectively, with removal of 80.1 µm. The calculated residual bed was 263 µpm.
Three months after LASIK manifest refraction in the right eye was -0.75 -1.00 × 165° yielding 20/15 with UCVA of 20/25. At 3 months postoperatively, the patient underwent an enhancement procedure that did not involve ablation of the underside of the flap. The flap was relifted. Optical and transition zones of 5.5 and 7.5 mm were used, respectively. In the right eye, 14.6 pm of the stromal bed was removed. The calculated residual bed thickness after enhancement was 248 pm. Three months after enhancement, manifest refraction was -0.75 with 20/20 BSCVA and 20/25 UCVA.
Four months after the first enhancement procedure, the patient underwent a second enhancement procedure that required ablation of the underside of the flap due to reduced stromal tissue thickness. A spherical ablation removed 14 pm of tissue from the underside of the flap using a 6-mm optical zone and an 8-mm transition zone. Four months after this enhancement, manifest refraction was piano -0.50 × 165° with BSCVA of 20/20 and UCVA of 20/20. Figure 3 shows the OPD-Scan analysis of the right eye 4 months postoperatively.
Multiple factors require attention when performing excimer ablation on the back of the flap. The first of these is the need for accurate pachymetry in the stromal bed at the time of enhancement. Fortunately, we have found during routine enhancement procedures that the anticipated corneal stromal thickness available correlates closely with the actual stromal pachymetry. In situations where the stromal tissue is inadequate to perform the necessary enhancement in the bed, treatment on the undersurface of the flap is a viable option. Thin flap LASIK reduces the need for the ablation to be done on the back of the flap, as more stroma is available for the primary treatment as well as possible enhancement.3 We have switched to using Moria's 90-pm head and have not needed to perform any enhancements on the back of the flap, as adequate stromal tissue is usually available for ablation. Very thin flaps require careful handling so as to reduce the risk of buttonhole formation.
The choice of speculum type is the second factor to pay attention to with retreatments on the back of the flap. It is helpful to have an aspirating lid speculum during LASIK, so as to eliminate excess tears and fornix debris from gaining access to the stromal bed or undersurface of the flap, causing an irregular laser ablation. Meticulous irrigation under the flap is also required to ensure that debris is eliminated to decrease the risk of diffuse lamellar keratitis.
The third factor of importance in this procedure is careful attention to the flap edges during reapproximation and adequate lubrication to minimize the risk of epithelial ingrowth. The quicker and more accurately the flap edge is sealed, the lower the chance of epithelial ingrowth. Frequent postoperative lubrication with artificial tears and gels is essential in the healing process and helps the epithelium in its recovery from the procedure. Preservative-free products have advantages. Punctal plugs should be used, either short-term collagen or longer-term silicone, as they possess advantages to assist in healing of the flap edge and minimizing discomfort.
Excimer laser ablation performed on the back of the LASIK flap is a valuable technique with which to preserve available stromal tissue in patients who do not have adequate stromal tissue to accommodate an enhancement procedure following primary LASIK. Care needs to be taken with the centration of the treatment on the back of the flap so as to reduce the chances of decentration of the treatment ablation.
Figure 3. Case 2. Four months postoperative NIDEK OPD-Scan corneal topography and wavefront measurements of the right eye after LASIK enhancement with ablation of the underside of the flap.
1. Maldonado MJ. Undersurface ablation of the flap for laser in situ keratomileusis retreatment Ophthalmology. 2002;109:1453-1464.
2. Kim JH, Joo MJ. Ablation of flap and bed can be effective in high myopes. Ocular Surgery News Europe/ Asia-Pacific. 2004;15:20-22.
3. Dougherty PJ. Thin flap LASIK. Clinical & Surgical Journal of Ophthalmology. 2003;l/21:326-328,330.