LASIK After Penetrating Keratoplasty

perienced surgeons, residual refractive errors, including high amounts of astigmatism, are common after PK. Most studies report average postkeratoplasty astigmatism of 4 to 5 diopters (D) (2). Kirkness and associates reported that 20% of patients required refractive surgery for astigmatism after successful PK (3).

B. EVALUATION OF ASTIGMATISM

Postkeratoplasty astigmatism is measured after final stabilization of the cornea. This requires an interval of 4 to 6 months after removal of all sutures. Using a common methodology allows investigators to compare results across different studies. The advantages of standard office keratometry include widespread availability and familiarity by clinicians, as well as ease of calibration. The main disadvantage is limitation of measurement to the central anterior corneal surface, encompassing an area with a diameter of approximately 3 mm. Central keratometry does not evaluate the status of the peripheral graft surfaces. Unlike naturally occurring orthogonal astigmatism, peripheral curvatures and cylinder after keratoplasty may not correlate well with central measurements. Computerized videokeratography, or topographic analysis, projects a Placido disk image onto the anterior corneal surface and interprets the reflected pattern. Topography provides a representation of the surfaces of peripheral graft and recipient cornea, allowing estimation of degree and direction of vector forces acting upon the transplanted tissue. Current programs permit comparison of serial images and provide subtraction analysis of interim changes, which is useful as sutures are removed and postoperative corneal healing and remodeling occur. Analysis of posterior corneal surfaces is now possible by technology that interprets optical sections by a scanning slit beam.

C. MANAGEMENT OF POSTKERATOPLASTY ASTIGMATISM

1. Refractive aids

Spectacles are helpful in some postkeratoplasty patients, but their use is limited by image disparity caused by anisometropia. Tilt and optical aberrations from spectacle correction of high amounts of astigmatism may be difficult to adapt to, especially in older patients. Finally, spectacles cannot correct irregular astigmatism. Rigid gas permeable lenses create less object minification or magnification than spectacles and thereby reduce anisometropia. Additionally, they provide a smooth anterior surface and effectively reduce both regular and irregular astigmatism. Postkeratoplasty astigmatism as high as 17 D has been managed with rigid gas permeable lenses (4). Contact lens fitting in the postkeratoplasty patient may be difficult and time-consuming. Altered corneal topography at the graft–host interface may complicate lens wear. Lens wear may stimulate corneal neovascularization, with increased potential for graft rejection. Wearing rigid lenses in a setting of high myopia and astigmatism is difficult; in a study of postkeratoplasty patients, 13 percent of patients discontinued rigid gas permeable lenses owing to intolerance (4).

2. Incisional Surgical Procedures

a. Radial Keratotomy

Radial keratotomy (RK) has been used to reduce postkeratoplasty myopia, allowing the wearing of spectacles (5). RK is not effective for reducing postkeratoplasty astigmatism.

Potential problems with RK after PK include permanent structural weakening of the cornea, induction of irregular astigmatism, and neovascular ingrowth along the radial cuts. The use of RK for correction of myopia has decreased with the introduction of the excimer laser.

b. Corneal Relaxing Incisions

Troutman described the basic concepts of corneal relaxing incisions (6). Incisions placed in the cornea cause relaxation of tissue, effectively adding tissue to the area incised. Transverse incisions placed in the steep meridian cause reduction of astigmatism. Factors influencing the magnitude of cylinder reduction include distance of the cut from the optical center and the length and depth of the incision. Relaxing incisions in general do not change the spherical power of the cornea; thus they may reduce astigmatism but not myopia. Arcuate incisions provide a longer chord length than linear incisions. Relaxing incisions may be combined with compression sutures of polypropylene placed through the graft–host interface in the flat meridian, allowing some titration of effect by postoperative suture adjustment or removal. Average keratometric reduction of cylinder of 7.95 D has been reported (7). Arffa reported a 77% reduction in mean astigmatism in six patients treated with relaxing incisions and compression sutures, with a minimum time of 8 months after penetrating keratoplasty (8). Complications of relaxing incisions include corneal perforation, infection, graft rejection, and overor undercorrection (Figure 23.1).

3. Excimer laser

Precise ablation of a desired amount of corneal tissue is possible using the excimer laser at a wavelength of 193 nm. The excimer laser has the ability surgically to correct myopia, hyperopia, and astigmatism, depending on the laser system and profile chosen. Correction of myopia and myopic astigmatism is possible using an expanding slit diaphragm, as in the VISX laser delivery systems. Simultaneous treatment of myopia and cylinder is possible by expanding the circular and slit diaphragms concurrently; however, with this method the amount of astigmatism corrected cannot exceed the spherical component. Sequential expansion of the circular and slit diaphragms allows correction of more astigmatism than myopia. An undesired hyperopic shift of the spherical equivalent is prevented by limiting the width of the slit to 80% of the length (9).

Scanning laser technology uses a smaller beam for photoablation. Astigmatism is corrected by additional treatment in a specific meridian. Both myopic and hyperopic astigmatism can be corrected. However, because of the smaller beam size, more time is required for treatment, and a reliable eye tracking mechanism is necessary for precise treatment.

An ablatable mask allows transference of the anterior curvature of the mask onto the anterior surface of the cornea, theoretically allowing a precise ablation of any profile, including myopia, myopic astigmatism, hyperopia, hyperopic astigmatism, and irregular astigmatism (9). Several laser systems use variations of this principle to achieve a desired refractive effect. Nonorthogonal, or irregular, astigmatism is often present after PK. Simple geometric profiles will not reduce this surface irregularity. Gibralter and Trokel have described a customized ablation using circles of varying sizes to create a more regular optical surface (10). Linkage of a scanning laser to a topographic map holds promise for individualized treatment of irregular corneas.

A

B

PRK photorefractive keratectomy; Int PK minimal interval after previous penetrating keratoplasty in months before PRK; F/U minimal follow-up in months; preop myopia mean spherical equivalent of myopia in diopters before PRK; postop myopia mean spherical equivalent of myopia in diopters after PRK; preop cylmean cylinder in diopters before PRK; postop cyl mean cylinder in diopters after PRK; PARK photoastigmatic refractive keratectomy; mo months; D diopters; NA not available. [Color in original.]

4. Photorefractive Keratectomy (PRK) After PK

Treatment of the anterior corneal surface by PRK has been employed for the correction of myopia and myopic astigmatism after penetrating keratoplasty. Results of published studies are listed in Table 1. John and colleagues treated three PK patients for myopia, with initial improvement in all (11). Amm and associates used an Aesculap Meditec MEL excimer laser with a rotating mask system for correction of postkeratoplasty cylinder in 16 eyes. The mean preoperative cylinder of 5.7 D was reduced by 2.8 D with 6 months follow-up (12). Campos and associates performed toric ablations on 12 patients with disabling astigmatism after PK. Mean reduction of astigmatism was 38% with a minimum follow-up of 6 months (13). Nordan and associates reported a mean reduction in cylinder of 45% in five PK patients followed for at least 6 months (14). Tuunanen and associates studied ten eyes of nine patients treated 4 months or later after suture removal. The preoperative astigmatism ranged from 3.5 to 11.25 D (mean 5.98); net corneal astigmatism was reduced 48% with a minimum follow-up of 12 months (15). In a series by Lazzaro and colleagues, an average

Figure 23.1 Corneal topography of a 35-year-old man who presented 5 years status after keratoplasty and 3 years status after astigmatic keratectomy with high astigmatism in the operated left eye. His visual acuity was 20/30 in the unoperated right eye and he counted fingers at 5 feet in the left eye. Manifest refraction OD was plano-2.00 95 and OS 4.50–8.00 167. Patient underwent a LASIK flap cut with Hansatome microkeratome initially without any laser treatment. On postoperative day 7, his manifest refraction OS had stabilized to the preoperative measurement of 4.50–8.75 155. Subsequently, the flap was lifted and the patient underwent partial treatment of the refractive error. The laser was programmed with 1.8–3.75 170 and his 2 weeks postoperative refraction improved to 4.25–3.00 155. Three months later the flap was lifted and the patient was given the rest of the treatment as the laser was programmed with 3.00–3.75 155. Two months later, the patient’s manifest refraction had stabilized to 0.25–2.50 47. The pre-LASIK corneal topography is shown in A and the final corneal topography is shown in B.

reduction of astigmatism by 48% was found in seven eyes after treatment with a minimum follow-up of 12 months (16).

Problems were encountered with PRK after previous PK. John et al. reported substantial regression in three of three patients, with significant haze in one (11). Campos et al. found a mean shift in axis of 58 degrees, with astigmatic overcorrections in four patients (13). Significant haze and regression were encountered in some cases. Tuunanen et al. noted regression as late as 7 to 8 months postoperatively; in their series four patients developed significant haze and scarring (15). Lazzaro et al. reported that one eye developed dense haze with scar formation (16).

Single case reports raise the possibility of increased risk of graft rejection after surface ablation with an excimer laser. An endothelial rejection occurred 2 weeks after phototherapeutic keratectomy for recurrent lattice corneal dystrophy in a six-year-old graft (17); in another case endothelial rejection was diagnosed 5 days after PRK for myopia and astigmatism in a three-year-old graft, despite postoperative use of 0.1% fluomethalone (18). These cases suggest a temporal association of an immune response to surface ablation by an excimer laser. Possible catalysts include mechanical epithelial debridement, ultraviolet radiation emission by the excimer laser, or postoperative epithelial defect.

D. LASER-ASSISTED IN-SITU KERATOMILEUSIS (LASIK)

1. Introduction

LASIK offers refractive ablation under a hinged flap of epithelium, Bowman’s layer, and anterior stromal tissue. Replacement of this flap after the procedure lessens postoperative pain and reduces wound remodeling, haze, and regression. The association of potentially severe haze and regression with PRK after PK has led to increased interest in LASIK as the preferred refractive method for correction of postkeratoplasty astigmatism and myopia. In all instances, surgery was performed in patients who were intolerant of both spectacle and contact lens wear.

A Chiron Vision Automated Corneal Shaper microkeratome was used in all published reports to date. This microkeratome makes about an 8.5 mm flap with a nasal hinge, depending on the corneal curvature. In one series an attempt was made to avoid initiating a microkeratome cut at the graft–host interface (19). Kritzinger and Probst described a sequenced LASIK technique in which the suction ring is applied and the flap created. However, the flap is left undisturbed for 2 to 3 weeks. Preoperative assessment is repeated; the refraction changes in about half of eyes, especially the amount and orientation of cylinder. Using the new parameters, the flap is lifted and treatment performed. Kritzinger and Probst speculate that the dynamic forces acting upon a postkeratoplasty donor cornea are altered by the LASIK cut (20). Standard LASIK nomograms were utilized in these studies. A variation of this technique with sequential laser correction has also proven successful (Fig 23.1).

2. Results of Published Studies

Published studies of LASIK after penetrating keratoplasty are listed in Table 2. Arenas and Maglione performed LASIK on four eyes from 1.7 to 9 years after PK with a Technolas Keracor 116 excimer laser to correct myopia and astigmatism. Mean spherical equivalent decreased from 10.75 to 2.37 D after surgery with a mean follow-up of 7 months. However, mean astigmatism increased after surgery from 2.87 to 3.50 D (21). Parisi and associates described LASIK in a case 2 years after PK. A Chiron Vision Keracor 117

F/U (min) minimal follow up in months; Int PK (min) minimal interval in months between penetrating keratoplasty and LASIK; Int PK (mean) mean interval in months between penetrating keratoplasty and LASIK; Preop myop mean spherical equivalent of preoperative myopia in diopters; Postop myop mean spherical equivalent of postoperative myopia in diopters; Preop cyl mean preoperative cylinder in diopters; Postop cyl postoperative cylinder in diopters; TIA target induced astigmatism (attempted reduction of astigmatism); SIA surgically induced astigmatism; ACS automated corneal shaper (Chiron Vision); mo months; D diopters; NA not available.

excimer laser was used. Refraction changed from 7.00-1.25 60 to 1.00-0.75 80 after 11 months of follow-up (22).

Forseta and colleagues performed LASIK on 22 eyes with a VISX 20/20 excimer laser with a mean interval after PK of 5.3 years. Spherical equivalent decreased from 4.55 to 0.67 D after LASIK with at least 6 months follow-up. There was no statistically significant difference between 1-month and 6-month values. Mean preoperative cylinder decreased from 4.24 to 1.79 D after surgery (23). In a series by Donnenfeld and associates, 23 eyes underwent LASIK with a VISX Star excimer laser with a mean time of 44 months after PK and 35.3 months after suture removal. The mean spherical equivalent was reduced from 7.58 preoperatively to 0.79 D (22 eyes) at 3 months postoperatively. Mean cylinder decreased from 3.64 preoperatively to 1.64 D (22 eyes) at 3 months (19).

Koay and colleagues evaluated eight eyes that received LASIK with a Technolas 217 excimer laser after a mean 71 months after PK and several months after suture removal. The mean spherical equivalent dropped from-6.79 preoperatively to 1.93 D at 6 months fol- low-up. The mean cylinder changed from 6.79 to 1.93 D at 6 months follow-up (24).

Spadea and associates reported four eyes that underwent LASIK after a mean interval of 24.75 months after PK. In all cases a VISX 20/20 B excimer laser was used. The mean spherical equivalent changed from 9.31 preoperatively to 1.13 D at 3 months postoperatively (25).

Webber and associates reported a series of 26 eyes undergoing LASIK after PK with a Summit Apex Plus excimer laser. Fourteen eyes with higher degrees of cylinder also received arcuate cuts in the stromal bed with a guarded diamond knife set at 360 m. The mean interval between PK and LASIK was 8 years and 11 months. Sutures had been removed at least 10 months before LASIK. The mean preoperative spherical equivalent of5.20 D was reduced to 0.24 at 1 month. The mean preoperative cylinder of 8.67 D was reduced to 2.48 D at 1 month. At 1 month the mean surgically induced astigmatism (SIA)

was 7.04 D for the entire group. SIA was 8.86 D for the group with arcuate incisions, and 4.86 D for the group without incisions (26).

3. Complications

Complications were rare in the reported series. A major concern in LASIK after PK is wound dehiscence from the high intraocular pressure achieved during application of suction. Internal ocular pressure is documented to be over 65 mm Hg with the Barraquer tonometer; however, pneumotonometry often reveals pressures over 100 mm Hg. For this reason, wound healing of the graft–host interface was allowed for several years before LASIK in most of the previous studies. In general, LASIK after PK should be performed by experienced surgeons to minimize total time of suction.

Abnormality or loss of endothelial cells as a consequence of elevated intraocular pressure is another potential complication of LASIK. Several studies suggest that short-term loss of endothelium is minimal after LASIK in corneas without previous surgery. Fresh cadaver globes were treated for up to 25 D of myopia with LASIK; after organ culture for 1 week the endothelial cells were stained and evaluated with fluorescence microscopy. There were no significant differences between treated eyes and untreated control globes (27). Specular microscopy of the corneal endothelium after LASIK has revealed no significant differences between preoperative values and postoperative intervals of 3 and 6 months (28). Previous ocular surgery changes the pace of endothelial dropout. Corneal transplantation leads to accelerated endothelial cell loss for at least 5 years. Bourne reported that mean endothelial cell density declined at a rate of 7.8% per year from 3 to 5 years after PK, compared with approximately 0.5% per year in the same time interval in unoperated normal corneas. The mean cell loss at 5 years after keratoplasty was 59% compared with preoperative values (29). The long-term response of the endothelium to superimposed LASIK on a previous keratoplasty is unknown; however, no significant differences were noted in preoperative and postoperative central endothelial cell counts of 22 eyes undergoing LASIK after PK with a minimal followup of 6 months (23).

A long interval between PK and LASIK also allows reestablishment of corneal sensation. Theoretically, there is less loss of sensation with LASIK than with PRK, because the hinge area contains intact superficial nerve fibers. Sterile ulceration and intractable dry eye have not been reported in this setting.

Endothelial rejection of a donor graft after LASIK is a cause of concern and may arise either from flap creation or handling, or from the ultraviolet radiation of excimer laser treatment. However, none has been reported during the study interval of these series. Possibly the maintenance of an epithelial cap diminishes immunostimula tion by cytokines produced from a wounded corneal surface. Postoperative use of topical steroids may also exert a protective effect.

E. SUMMARY

Despite modern instrumentation and techniques to minimize tissue disparity and unequal suturing, ametropia, including high astigmatism, remains a common problem after penetrating keratoplasty. Anisometropia leads to spectacle intolerance. Rigid contact lenses are very beneficial in the postkeratoplasty setting, but satisfactory wear is not possible in all patients. Incisional refractive techniques have been utilized with some successes. Arcuate cuts to reduce cylinder are useful, and may be combined with excimer laser surgery for

higher amounts of astigmatism. The excimer laser is a tool capable of precise tissue modeling. PRK has demonstrated significant reductions in myopia and astigmatism, but unacceptable haze and regression reduce its effectiveness. LASIK has also proven useful in reducing postkeratoplasty myopia and astigmatism, with few complications. Prerequisites for LASIK after penetrating keratoplasty include adequate healing and wound strength of the graft–host interface, corneal and refractive stabilization after removal of all sutures, and, in principle, spectacle and contact lens intolerance. The long-term effects of LASIK on the corneal endothelium and graft longevity remain to be determined, but early results are promising. LASIK provides the corneal surgeon with a means of rehabilitating this group of patients with clear grafts and disabling refractive consequences.

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