Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001
.pdf
Chapter 18
neal reaction to the procedure differs.4 A number of patients with residual myopia after RK exist and need additional treatment. Correcting residual myopia can be done by spectacles, contact lenses, or reoperations either by redeepening, or extending the RK incisions, or performing additional RK incisions. However, although there are many nomograms for performing radial keratotomy,5,6 nomograms for enhancing refractive procedures do not exist. Reoperations cannot be based on the same calculations used in the initial surgery because the predictable effect of adding incisions and reducing optical zones is lower than in primary nomograms.7 An increase in the incidence of microperforations is observed with RK enhancement procedures.8 Overcorrection is another serious complication that has been reported in several studies using RK as an enhancement procedure.5,9 While young patients may be able to accommodate to compensate for an overcorrection now, in time the patient will be complaining of severe and early presbyopia.
Photorefractive keratectomy is another form of treatment, but today there is a major concern about using it after RK. Several complications have been reported, including different degrees of haze and regression due to keratocyte activation, dehiscence of the RK incisions during scraping of the epithelium, and signifícant decrease in best spectacle corrected visual acuity (BSCVA) due to surface irregularity and subepithelial scarring.10,11 Azar, et al in 1998 advised against using PRK to correct residual myopia after RK in patients with high amounts of pre-RK and residual post-RK myopia.4 LASIK is more likely to provide an accurate result with early and long-term stability without the risk of haze (figura 18-1).
Hyperopia After RK
Progressive hyperopia is a common complication following RK. It may result from lack of preoperative cycloplegic refraction, extending the radial incisions to the limbus, multiple RK enhancement procedures, redeepening procedures, extended contact lens wearing after RK, and possibly postoperative ocular rubbing.12
Treatment of hyperopia after RK is a complicated problem.13 Hexagonal keratotomy was used,
Figure 18-1: LASIK over RK.
but the results were not predictable. Grene lasso sutures are more predictable but still not highly efficient. Thermal keratoplasty shows variable degrees of regression; thus, it is not reliable in treating hyperopic shift after RK. Hyperopic PRK has a high incidence of postoperative haze and disappointing results.
Hyperopic LASIK is a promising technique in the management of these cases, especially with new reliable software for treating hyperopia.
The Cornea After RK
RK incisions can be seen for a long time after surgery. These incisions never completely heal. Epithelial ingrowth may be found in the RK incisions. Patients wearing contact lenses may have deep vascularization especially in deep incisions. Flat corneas are more common in patients with overcorrections.
LASIK AFTER RK
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
LASIK seems to be an attractive alternative to correct residual myopia and hyperopic shift after RK. However, due to the fact that the cornea underwent previous RK surgery, it requires special handling both preoperatively and postoperatively to get the best results, to avoid any refractive surprise, and to decrease the possibility of developing haze.
216 SECTION III
Preoperative Considerations
LASIK should only be attempted after 1 year post-RK, with a stable refraction for at least the last 6 months, and a corneal topographic pattern stable for two consecutive examinations in a 1 month interval. Timing is very important especially with patients showing regression after RK. Overactive healing is responsible for this regression, and its effect may continue after LASIK if it is performed too early.
Patients wearing contact lenses should discontinue use for at least 15 days before evaluation. Soft contact lenses should be discontinued for 15 days before LASIK, and both hard and gas-permeable contact lenses should be discontinued for at least 1 month before LASIK. In patients with blood vessels in the incisions, more time is needed to allow for blood vessel regression.
If the patient has irregular astigmatism, or if the astigmatic value is larger than the spherical value, topographic linked excimer laser ablation (topolink) is preferred. A classic LASIK procedure will produce unpredictable results.
Contraindications
Post-RK corneas are unstable corneas that may cause unpredictable results, thus great care is needed while dealing with these corneas. LASIK should not be performed if one or more of the following items exist.
•Epithelial ingrowth: epithelial inclusions in the RK incisions is a serious problem and LASIK should be avoided in these cases, as the epithelium may pass under the flap, causing flap melting.
•Macroperforation: LASIK should not be attempted in any case with prior macroperforations.
•Deep vascularization: may he found in the deep incisions and is more common in patients who wear contact lenses.
•Flat cornea: LASIK on flat corneas may cause a free cap, which will make it difficult to achieve good
LASIK AFTER PREVIOUS CORNEAL SURGERY
results, as the diameter of the cut will not be large enough to perform hyperopic ablation.
• Unstable refraction: this should be excluded before any attempt to perform LASIK; the patient may end up with an unpredictable and untreatable refractive condition.
lntraoperative Considerations
In post-RK corneas, we are cutting across RK incisions, and it is well-documented that these incisions never completely heal. Our main concern is to prevent opening of the incisions while creating the flap. As long as the RK incisions are well-healed without epithelial ingrowth at the time of surgery, a safe regular cut can be performed with a 160 m blade; however, it is better to use a thicker depth blade. A 180 m or 200 m blade is safer, but the corneal thickness and amount of ablation determine this. lt is important to always keep the corneal epithelium wet during the cut, this will serve as a lubricant and facilitate the pass of the microkeratome.
Lifting the flap should he done very carefully with a wide spatula, while avoiding forceps to grab the edge of the flap. This will protect the incisions from splitting apart. Perfect fixation is needed. lf it is not maintained, eccentric correction and unpredictable astigmatism could result. When replacing the flap, good apposition is mandatory; this will prevent migration of the epithelium, especially if an incision in the flap is opened. During the procedure, avoid traumatizing the epithelium to avoid any epithelial scraping, which may cause keratocyte activation and increase in corneal haze.
Applying a contact lens after surgery is not necessary unless there is opening in one or more of the RK incisions in the flap.
If we are treating hyperopic shift, we attempt to obtain a large flap at least 9.5 mm, by using a large suction ring to avoid hinge syndrome. Fixation is usually more difficult with these patients. For better results, we give the patient extra training in fixation.
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
LASIK AND BEYOND LASIK 217
Chapter 18
Figure 18-2: Pilot study on patients with LASIK over RK, depending on they were undercorrected or hipercorrected. BSCVA and UCVA before and after LASIK
Results (Pilot Study)
Ten myopic patients had previous RK. After 1 year, six patients had significant undercorrection, and the other four patients developed overcorrection. LASIK was done to correct the residual refractive defects in the 10 patients.
Patients with undercorrection showed the following results 3 months after LASIK: the mean spherical equivalent changed from -2.50 D ± 2.47 (-6.25 to -0.50) to 0.12 D ± 0.26 (-0.25 to +0.50). Mean BSCVA improved from 0.75 ± 0.24 (0.5 to 1.0) to 0.83 ± 0.25 (0.5 to 1.0), and mean uncorrected visual acuity (UCVA) significantly improved from 0.33 ± 0.22 (0.1 to 1.0) to 0.80 ± 0.24 (0.5 to 1.0).
Patients with overcorrection showed the following results 3 months after LASIK: the mean spherical equivalent changed from 1.87 D ± 0.66 (1.00 to 2.50) to -0.25 D ± 0.50 (-0.50 to 0.50). Mean BSCVA improved from 0.72 (0.6 to 0.8) to 0.80 (0.7 to 0.9), and mean UCVA improved from 0.52 (0.4 to 0.6) to 0.70 (0.6 to 0.8). Treating both undercorrection and overcorrection with LASIK following RK is almost equally safe, effective, and highly predictable. There were no major intraoperative or postoperative complications (Figure 18-2).
Conclusions
Although LASIK for the correction of residual refractive errors after RK seems to be a promising and safe procedure, great care should be taken with the flap during the entire procedure to avoid possible complications.
LASIK AFTER AK
As the treatment of spherical refractive errors, myopia, and hyperopia evolved, treatment of astigmatism has lagged behind. The incidence of clinically significant astigmatism varies between 7.5% to 75%.14 However, an astigmatic refractive error of more than 2.0 D is less common, between 3% and 15%. 15 Astigmatism corrected by spectacles may cause distortion due to the meridional magnification.16 Contact lenses may alleviate this problem, but not all patients can tolerate them. Here, the discussion is limited to the surgical correction of naturally occurring mixed astigmatism more than - 3.0 D.
The general goal of incisional or ablative astigmatic surgery is to reduce the magnitude of astig-
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
218 SECTION III
matism by flattening the cornea at its steepest meridian, steepening the cornea at its flattest meridian, or a combination. Any corneal incision flattens the comea adjacent to it and at the meridian perpendicular to the cut. AK is a common method to correct astigmatism and is a very powerful tool in reducing astigmatism. It flattens the steep cylinder axis and, at the same time, steepens the flat axis, a process known as coupling.17 The coupling ratio (flattening/ steepening ratio) depends on the location, length, and depth of the incision. In patients with large amounts of astigmatism, AK can be used to significantly lessen the astigmatism however it is important to consider the patient’s refractive error and how astigmatismreducing surgery will affect the spherical equivalent.
The benefits of AK are greater in patients with myopic astigmatism. Transverse incisions in the cornea cause flattening in the meridian of the incision and steepening of the meridian 90° away. Arcuate or curvilinear incisions have heen reported as more effective than straight transverse incisions.18,19 The distance between the AK and the center of the pupil is an important factor as well. The smaller the distance, the smaller the optical zone, with a higher incidence of irregular astigmatism near the pupil result in poor visual quality, especially in low-light conditions.
McDonnell and colleagues were the first to report the success of toric sculpting of the cornea with an excimer laser to correct regular corneal astigmatism.20 This initial success encouraged refractive surgeons to use the same principle to correct astigmatism. The current approach to correct astigmatism by excimer laser, involves a nonradially symmetrical ablation of the corneal tissue, with greater ablation in the steep axis and minimal or no ablation in the flat axis.21 Recently, the Technolas, LaserSight, Autonomous, and Nidek lasers have been used to correct astigmatic errors in which the scanning beam moves along the axis of astigmatism and differentially ablates the cornea.
The Cornea After AK
Following uncomplicated AK, the anatomical structure of the cornea does not show significant alteration, both in the superficial layers and the deep
LASIK AFTER PREVIOUS CORNEAL SURGERY
stroma. Scars from the previous arcuate keratotomy are usually seen at a 7.0 mm optic zone. There is no need to avoid cutting through them with the microkeratome. A LASIK procedure can be carried out without special intraoperative precautions (Figure 18-3).
Figure 18-3: LASIK over AK
Performing LASIK After AK
The predictability of AK is of great concern for refractive surgeons, undercorrection, overcorrection, and change in the axis are complications that must be dealt with and corrected. Undercorrection is more common and better tolerated than overcorrection. In spite of several medical and surgical options used to manage these conditions, results are not predictable, but with LASIK we are getting more predictable and stable results.
Coupling effect and hyperopic shift in the spherical equivalent are commonly seen after AK. For small amounts of myopia in association with astigmatism, the astigmatic surgery may be all the patient needs. However, astigmatism associated with high myopic or hyperopic spherical error will need a second approach in an attempt to achieve emmetropia. Radial keratotomy, photorefractive keratotomy, or LASIK can treat coupling and residual refractive defects. LASIK has proved itself as the most predictable and reliable procedure in dealing with most refractive errors.
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
LASIK AND BEYOND LASIK 219
Chapter 18
Preoperative Considerations
Allowing the refraction to stabilize is just as important in astigmatic surgery as in any other form of refractive surgery. We wait about 3 months after AK to perform LASIK; this period suffices in achieving a stable refraction. Usually in astigmatic correction with or without spherical error, treatment is based on the manifest refraction for axis correction; however, if the refractive axis differs from the topographic axis by more than 10°, we prefer to use the topographic axis. Patients who wear contact lenses should stop use of soft for 3 days and hard contact lenses for 2 weeks before getting manifest refraction results. Patients with binocular spectacle-corrected astigmatism are showing adaptation to the meridional magnification induced by their spectacles. Surgical correction of their astigmatism may result in torsional diplopia, and readaptation may take months. We discuss this problem with astigmatic patients before any surgery to correct astigmatic errors is attempted.
With irregular astigmatism, normal LASIK treatment is contraindicated. It may worsen the condition, and it is advisable to treat these cases with the topography-assisted lasers.
lntraoperative Considerations
In cases of previous AK, LASIK can be carried out as normal. The procedure has proven to he very safe following AK. All LASIK steps can be carried out as usual, as long as the procedure does not take place before 3 months following AK. Centration should be very precise and accurate to avoid decentration, which is usually more common with astigmatic treatment. Corneal topography should be our guide to achieve the best possible centration.
Results (Pilot Study)
Ten patients with mixed astigmatism underwent AK. The mean spherical equivalent after AK was +0.57 D ± 2.8 (-1.5 to +6.0). The mean astigmatic value after AK was -1.50 D ± 0.60 (-0.5 to -2.5). Mean BSCVA was 0.76 ± 0. 15 (0.4 to 0.9), and mean UCVA was 0. 51 ± 0.16 (0.3 to 0.8). All patients underwent LASIK surgery in an attempt to
correct the residual refractive error. In all cases, the procedure was carried out at least 3 months after AK. One month after LASIK, mean spherical equivalent was +0.87 D ± 0.5 (0.0 to +2), mean BSCVA was 0.76 ± 0.16 (0.4 to 1.0), and mean UCVA significantly improved to 0.73 ± 0.14 (0.4 to 0.9). Three months after LASIK, mean spherical equivalent became +0.60 D ± 0.31 (0.25 to 1.25), mean BSCVA improved to 0.79 ± 0.17 (0.4 to 1.0), and mean UCVA was 0.74 ± 0.18 (0.4 to 1.0). The cylinder’s vectorcorrected change was 1.61 D ± 0.71. LASIK after AK proved to he safe, highly efficient, and predictable. There were no adverse events during LASIK and no major complications were reported during or after the procedure.
Conclusions
Patients with a residual refractive defect after AK can benefit from LASIK. For the best results, LASIK should he done 3 months after AK. There were no problems with the cut, and with handling the flap. Surgeons are advised to treat the cornea as a virgin one (Figure 18-3 and 18-4).
LASIK AFTER PRK
Since the introduction of excimer laser PRK,22 there has heen a steady increase in the number of PRK procedures performed worldwide.23 The most frequent complications after PRK are regression, haze, central islands, decentered ablations, as well as other less frequently seen complications.24 An estimated 10% to 20% of patients require a repeat PRK procedure for significant regression. Regression is caused by the corneal wound healing response, which may differ from one patient to another and results in various refractive outcomes and incidence of complications.
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
The Cornea After PRK
It is now well documented that the cornea demonstrates specific acute and delayed responses to excimer laser ablation. Epithelial wounds usually heal over a period of months following PRK. The epithelium first slides to cover the defect initially it
220 SECTION III
is thinner than normal, but later hyperplasia takes place, and the number of cells becomes greater than normal. Epithelial hyperplasia may be responsible for postoperative regression. The basement membrane, which is removed during PRK, usually regenerates with focal discontinuities and duplication. Normal epithelial attachment completes are regenerated within weeks to months after surgery.25 Stromal changes continue for months or even years after PRK. After closure of the epithelial defect, keratocytes begin transformation into activated fibroblasts and migrate into the treated region, so that the subepithelial 10 to 15 microns become hypercellular. These activated keratocytes synthesize new collagen and extracellular matrix, which may contribute to corneal haze that is observed postoperatively. The new collagen lacks the organized lamellar arrangement characteristics of corneal stromal collagen fibers.26 Proteoglycans, including keratan sulfate and hyaluronic acid, are produced in response to the injury. The produced hyaluronic acid may change the water balance and thus create disruptions in the lamellar arrangement.26 Depending on the depth of ablation, Bowman’s layer may be partially or completely excised during the procedure.25
Performing LASIK After PRK
PRK retreatment for significant regression will significantly reduce residual myopia. However, the risk of further regression, haze and loss of visual acuity exists.27 In addition, treating residual myopia by PRK is less successful than primary PRK.30 LASIK has been used primarily to treat moderate to high myopia because of its superiority over PRK for this range of refractive error.28 Many surgeons are now advocating the use of LASIK rather than PRK for lower levels of myopia, because LASIK preserves Bowman’s layer, decreases the amount of disruption of keratocytes and anterior stromal collagen, and avoids the large epithelial defect seen with surface PRK.29 Because LASIK causes less regression and haze, we studied the results of LASIK in treating residual rnyopia after primary PRK.
LASIK AFTER PREVIOUS CORNEAL SURGERY
Preoperative Considerations
It is clear that regression and haze are the most common complications after PRK. These complications will determine, to a great extent, the outcome of treating these patients with LASIK.
•R egression: the amount of regression after PRK is related to the amount of myopic correction attempted. The deeper the ablation, the more frequently regression occurs. Regression may continue over months, thus a stable refraction is important to prevent further regression after LASIK. An interval of 1 year is usually enough to achieve a stable refraction. This should be documented by repeated refraction and corneal topography at least twice within 1 month before any attempt to perform LASIK.
•H aze: the grade of haze present after PRK can affect the outcome of LASIK. The incidence of regression after LASIK is higher in corneas with grade 2 haze or more. In patients with grade 2 or more corneal haze, our target should be overcorrection to compensate for expected postoperative regression. In patients with minimal to no corneal haze, our target is emmetropia, as regression is less likely to occur. For example, in a patient with manifest refraction of -4.0 D with corneal haze grade 2, the LASIK surgical plan should be -5.0 D. The immediate postoperative overcorrection will be compensated by the expected regression.
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Intraoperative Considerations
The cut is a critical step in performing LASIK after PRK. The flap should be as thick as
possible —not less than 160 m— the thicker the Help ? better. With a thin flap, we may encounter two prob-
lems:
•First, the microkeratome blade will pass through a peripherally normal clear cornea, and then through a more tough area due to the previous PRK treatment. This will affect the smoothness of the cut in the corneal stroma, resulting in an irregular surface. With a thicker blade, we can avoid this problem by passing beneath the previous PRK treatment area.
LASIK AND BEYOND LASIK 221
Chapter 18
•S econd, after PRK, Bowman’s membrane may be partially or completely removed, thus the flap will be more liable to wrinkles due to lack of Bowman’s membrane support. This can be compensated by creating a thicker flap. Patients with a keratometric value less than 40 D are more likely to have a free cap and require care in creating the flap.
Postoperative Treatment
Patients who undergo LASIK after PRK should be managed with the same regimen used after PRK, using extensive steroids for a long period. Although after a regular LASIK procedure prolonged steroid therapy is not necessary, we found it very effective in decreasing the amount and incidence of haze in LASIK after PRK (Figures 18-3 and 18-4).
Results after LASIK
Mean UCVA significantly improved to 0.4
± 0.29 (0.2 to 0.8) at 1 month, 0.6 ± 0.26 (0.2 to 0.9) at 3 months, and 0.6 ± 0.18 (0.2 to 1.0) at 6 months. One month after LASIK, BSCVA was 0.5 ± 0.31 (0.2 to 0.9); at 3 months, BSCVA was 0.7 ± 0.22 (0.2 to 1.0); and at 6 months, BSCVA was 0.7 ± 0.17 (0.4 to 0. 1). In 78% and 85% of eyes, UCVA was better than 0.5 at 3 and 6 months respectively. Only one eye lost more than two lines of BSCVA after LASIK; this was related to severe haze that developed following an intraoperative flap complication in which the flap was cut into two halves. At the end of fol- low-up, 98% of the patients where within ± 1.0 D of intended refraction and 77% were within ± 0.5 D.
Conclusions
LASIK seems to be a good alternative to correct post-PRK regression; the procedure is safe, effective, and highly predictable. The curve of visual improvement after LASIK seems to follow that of PRK (decrease in immediate postoperative visual acuity, followed by an improvement after the first month). This could be related to the significant amount of haze observed in this group of patients immediately after LASIK, therefore aggressive and prolonged use of topical corticosteroids is necessary (Figure 18-5). The microkeratome cut is more difficult after PRK than with virgin corneas. The flap has to be as thick as possible to avoid the increased risk of developing wrinkles and an irregular surface.
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Figure 18-4: Corneal haze over RK, PRK and LASIK.
Help ?
Results (Pilot Study)
Thirty patients with regression after PRK were treated by LASIK. The procedure was performed at least 12 months after PRK. The mean preLASIK spherical equivalent was -3.65 ± 1.9 (-1.75 to -6.0), mean pre-LASIK BSCVA was 0.7 ± 0.23
(0.4 to 1.0), and rnean UCVA was 0.24 ± 0.41
Figure 18-5: Haze in LASIK over PRK
(0.1 to 0.6).
222 SECTION III
LASIK AFTER LTK
When hyperopic errors are corrected by corneal refractive surgery, the goal is to steepen the central cornea in an amount proporcional to the hyperopic error to be corrected.
With recent advances in laser technology, LTK was studied for the correction of hyperopia. Erbium, C02, and holmiurn (Ho): YAG lasers were investigated as potencial candidates for this procedure. The C02 (10.6 mm) LTK was studied by Peyman, et al, and resulted in superficial retraction of the corneal collagen, as well as early regression of the refractive effect.31 Yr-erbium- glass laser spots (1.54 mm) resulted in extensive penetration and tissue necrosis.32
Ho: YAG laser (2.06 microns) LTK was then used for the correction of hyperopia. Ho: YAG LTK changes the anterior corneal curvature by using the infrared laser energy heat generated in the cornea to change the anterior corneal curvature.33 The corneal collagen shrinks by 30% to 45% of its original length at temperatures ranging from 58ºC to 60ºC. Higher temperatures cause tissue necrosis and relaxation.34 Stromal haze at the treatment site extends from 50% to 70% of the corneal thickness.35
LTK flattens the periphery and thus steepens the central area. The results from Koch3,5 indicate that this could be a promising technology to correct low to moderate hyperopic refractive error. Alió, et al 37 recommend that algorithms to improve final results should include an initial calculated overcorrection adjusted to variables that influence regression, such as age and corneal thickness. However, in spite of all these refinements, regression of effect has been a major limitation to the potential refractive outcome of LTK. Regression is variable and may even be total. It was found to be mainly a biophysical mechanism,38 which proved difficult or impossible in most cases to be solved with LTK retreatment.
The Cornea After LTK
After LTK, the opacities in each treatment spot (average diameter is 0.7 mm) decrease with time. After 2 months, they can be observed only under the slit lamp. Although the degree of opacity decreases
LASIK AFTER PREVIOUS CORNEAL SURGERY
over time, it is usually present for a long period after LTK. The density and depth of haze are related to the pulse energy. Up to 1 year after LTK, the mean central corneal thickness was slightly thinner than the preoperative value. However, after 2 years, the mean central corneal thickness was almost identical to the preoperative value. From our observations, it seems that the cornea remains unstable for a long time after LTK treatment, especially with unsuccessful treatment. The corneas in these patients tend to return to their original preoperative topographic status, with a multifocal irregular corneal surface.
Performing LASIK After LTK
Many patients previously treated with LTK are seeking an altemative surgical treatment for the correction of their residual refractive error. LASIK may offer a good alternative for these corrections. With LASIK, it is possible to ablate the corneal periphery by stromal photorefractive ablation and prevent strong epithelial regression with the overlying flap.39 With virgin hyperopic corneas, LASIK proved to be very efficient, safe, and predictable. However, laser energy is expected to have its own effect at the level of the previous LTK spots, and this may significantly influence its effect on the correction achieved, stability of the refractive results, and corneal wound healing. Thus, performing LASIK on corneas with previous LTK treatment requires special care in both preoperative evaluation and intraoperative precautions to achieve the best possible results.
The only contraindication for this procedure is the presence of a dense corneal opacity that interferes with vision; but even in cases developing irregular astigmatism, topography-linked excimer laser ablation can be used.
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
Preoperative Considerations
As, regression is the main complication after LTK and may continue over a variable duration, LASIK should be postponed until regression has stopped. This might take up to 1 year or even more. Factors affecting regression are:
LASIK AND BEYOND LASIK 223
Chapter 18
Figure 18-6: LASIK over LTK. |
Figure 18-7: LASIK over LTK |
•Age. Greater regression is seen in young adults with relatively elastic stromal tissue and Bowman’s membrane, thus complete refractive stability is essential before LASIK.
•H igh pre-LTK hyperopia invites greater and prolonged regression. We need to have at least three consecutive stable corneal topographies over 3 months before performing LASIK.
It is important to distinguish between undercorrection and regression; undercorrection is present in the immediate postoperative period, while regression occurs during the course of healing. However, a longer interval between the two operations allows us to perfonn more accurate surgery and avoid future complications. With our patients, we wait at least 1 year after LTK to perform LASIK. Patients with pre-LTK high degrees of hyperopia should wait up to 18 to 24 months, as they usually show more regression. In general, no LASIK attempt should be considered unless we have a stable refraction and corneal topography for 2 consecutive months. Corneal topography is important to assess the size and shape of the optic zone and to plan the new surgery.
Biomicroscopic examination is important in assessing the sites, degree, and extension of stromal scars, usually seen at the LTK treatment sites, to plan the LASIK cut (Figures 18-6 & 18-7).
lntraoperative Considerations
Centration is always essential. Decentration is more common in hyperopic patients and will be
more accurate and easier if corneal topography is used to assess centration.
The LASIK cut should be performed away from the LTK corneal spots, otherwise the cornea will show dense ring-shaped haze.40 Although this does not influence the immediate visual result, the long term stability of the achieved refractive results are still unknown.
A large flap is always preferable to allow perfect peripheral corneal ablation; the flap should be 8.5 mm or more.
Results (Pilot Study)
Twenty-three eyes with significant regression following noncontact LTK treatment underwent LASIK in an attempt to correct their refractive error. LASIK was performed at least 18 months after the LTK treatment. The pre-LASIK mean spherical equivalent changed from +3.14 D ± 1.82 (+0.50 to +6.50) to +0.52 D ± 1.71 (-2.75 to +3.75) 6 months after LASIK. There was a significant change in refraction between the preoperative and postoperative spherical equivalent values at 1, 3, and 6 months (p < 0.05). There was a minor insignificant change between the pre-LASIK mean BSCVA (0.74 ± 0.15, range: 0.4 to 1.0) and the post-LASIK mean BSCVA at 6 months (0.74 ± 0.18, range: 0.4 to 1.0). Three patients lost one line of BSCVA, and two patients lost more than one line of BSCVA. Six months after LASIK, UCVA significantly improved from a mean value of 0.36 + 0.16 (0.1 to 0.7) to 0.61 ± 0.25 (0.2
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Help ?
224 SECTION III
to 1.0). Although the procedure seems to be safe, it was not as effective. Six patients (26%) showed no change in preoperative BSCVA from the postoperative UCVA. Three patients (13%) gained one or more Snellen lines, five patients (21%) lost one Snellen line, and nine patients (39%) lost two or more Snellen lines. Seventeen patients (73 %) were within ± 1 D of intended hyperopic correction. Four patients (27%) had regression during the period of 6 months alter LASIK. Regression ranged from 0.75 D to 3.75 D; we believe this regression was a continuation of the regression taking place after LTK.
Conclusions
Hyperopic LASIK is a good alternative for the correction of residual refractive errors after holmium LTK. Efficacy and predictability are inferior to that of virgin hyperopic corneas that undergo LASIK, but the procedure seems to be equally safe. We should keep in mind that these corneas are unstable, and a completely stable corneal topographic map is very important to decrease the incidence of further regression after LASIK. To avoid the development of severe haze after LASIK, we perform the LASIK cut away from the previous LTK spots.
LASIK AFTER PKP
PKP is a procedure frequently performed worldwide, with more than 34,000 yearly in the United States.40 Most of these cases are left with refractive errors both spherically and cylindrically that may cause variable degrees of anisometropia. Irregular astigmatism is frequently found after PKP, leading to significant limitation in visual performance.
The visual result after PKP is influenced by biological and refractive factors. The biological quality of the donor tissue and episodes of graft rejection affect the transparency of the graft. Despite the corneal graft being optically clear, a high astigmatism average of 4 to 6 D,42 and irregular astigmatism associated with the spherical error explains why these patients are unable to reach BCVA with spectacles or contact lenses.
LASIK AFTER PREVIOUS CORNEAL SURGERY
In summary, the main aspects involved in the optical and refractive outcome of this surgical procedure include the following:
Biological factors:
•Quality of the donor cornea
•D ifference in thickness between the donor and recipient corneas
•Wound healing
•Underlying corneal disease
Surgical factors:
•G raft-recipient disparity
•Wound dehiscence
•Wound configuration
•E ccentric trephination of donor or host cornea
•P revious astigmatism of donor cornea
•P revious anterior segment surgery (PKP, phacoemulsification, RK)
•Time of suture removal and suturing technique are the most important of all. The double-run- ning 10-0 nylon sutures or the combined interrupted and continuous sutures can minimize irregular post-keratoplasty astigmatism when compared with interrupted sutures.43,44
In addition to the wound healing process, there are different responses according to the age of the patient. The younger the patient, the stronger and faster the wound healing. Wound integrity is determined by the amount of whitening and scarring at the PKP wound, especially if associated with vascular invasion.45
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
Corneal Refractive Surgery
Several techniques are available to correct refractive errors after PKP:
•Incisional surgery: effective but unpredictable
due to the different quality of the donor cornea, |
Help ? |
wound healing, and tensional forces of the cor- |
|
nea generated by the wound healing structure. |
|
•PRK: produces problems due to laser interaction with wound healing, increasing the risk of haze. Moreover, this can lead to a major risk of graft rejection due to the removal of the epithelium and Bowman’s membrane.
LASIK AND BEYOND LASIK 225