Introduction
The resurgence of lamellar procedures in corneal surgery over the past several years has come not only in the guise of laser in situ keratomileusis (LASIK) for the correction of refractive error but also as a solution to corneal pathology, such as scarring and ectasia. Theoretical advantages of partial-thickness corneal transplantation (lamellar keratoplasty [LK]) over full-thickness corneal transplantation (penetrating keratoplasty [PK]) include avoiding intraocular surgery and its incumbent risks (including bleeding and infection) and the specter of allograft rejection. With the increased use of lamellar techniques, we have learned much about their limitations, complications, and drawbacks.
Although the excimer laser’s primary use has been correction of refractive error using either lamellar (LASIK) or surface ablation (photorefractive keratectomy [PRK]), it may also be used to treat corneal pathology via phototherapeutic keratectomy (PTK). First investigated in 1988, this technique has been used to treat a variety of conditions, including to smooth irregularities, treat recurrent erosions, or remove superficial opacities, such as scars and deposits. PTK can offer an alternative to other surgical techniques, such as superficial keratectomy, lamellar keratoplasty, or penetrating keratoplasty. Advantages of PTK include precise control of corneal ablation, provision of a smooth base for corneal reepithelialization, ease of use, ability to repeat treatment, and relatively fast visual recovery. Disadvantages of PTK include possible hyperopic shift, scarring, and postoperative discomfort. PTK may be contraindicated in patients with deep stromal pathology or active inflammation or infection. This chapter will outline novel applications of PTK in the treatment of interface opacities and irregularities within the cornea following lamellar surgery.
Excimer Laser Advantages and Safety
The excimer laser uses a high-energy, ultraviolet, 193-nm argon fluoride beam (ArF) to precisely ablate corneal tissue with submicron accuracy, without causing significant injury to nonablated tissue. The depth and shape of excimer laser ablative photodecomposition can be accurately controlled, and ablation of tissue is not affected by corneal opacity. Reepithelialization and wound healing begin shortly after surgery and are associated with a small degree of tissue reorganization. In contrast, incisions made with diamond and steel blades produce relatively irregular and more diffuse tissue damage. This is also in contrast to the 248-nm krypton excimer laser, which produces irregular and scattered areas of tissue damage.
The far UV laser radiation is thought to be within the limits of safety for the human eye. Adjacent tissues undergo minimal distortion and suffer no apparent thermal damage after 193-nm excimer laser PTK. At a histologic level, there is a clear boundary between the treated and untreated area, and the stromal lamellae show no evidence of distant disorganization. Endothelial cell loss can occur after PTK when the remaining stromal thickness is 40 µm or less. It is not clear, however, whether similar endothelial loss would occur with equally deep mechanical lamellar dissection.
In lamellar surgery, the excimer laser has been used to prepare the recipient bed and to create donor tissue of more regular diameter and thickness in order to get more congruent surfaces. The excimer laser has also been used for the treatment of postoperative interface haze and optical irregularity. In this case, the donor graft is reflected away from the host bed, and both the posterior surface of the graft and the anterior surface of the host bed (100 µm residual tissue) are polished using the excimer laser. Only one case has been reported; this patient had an improvement of best corrected visual acuity (BCVA) from 20/100 to 20/22 with no measurable loss of endothelial cells at 9-month follow-up. Again, long-term follow-up is not available.
Another technique has been reported for thin corneas with irregular astigmatism after repeated unsuccessful LASIK or due to keratoconus in which a donor stromal button graft modulated by excimer laser was positioned inside a host stromal pocket as part of an LK. This “sandwich” technique allows additional excimer laser ablation. Although there has been visual acuity (VA) improvement after a follow-up of 14 months, more trials are needed to evaluate this technique.
Phototherapeutic Keratectomy and Intralamellar PTK Indications
PTK is used for the treatment of pathology of the anterior cornea, including diseases that affect corneal transparency and induce corneal surface irregularity. Treatment depth is limited by the thickness of the residual untreated stroma, which should be at least 250 µ. Conditions that have been treated include corneal opacities, corneal dystrophies, and recurrent corneal erosions. Corneal surface irregularities including Salzman nodular dystrophy and irregularities after refractive surgery have also been treated. Of 271 consecutive PTK cases at 17 VISX US centers reviewed by Sanders, 55% of patients had corneal scars or leukomas, 39% had corneal dystrophies, and 5% had corneal surface irregularities.
In general, eligible patients should be free of active inflammation or infection, including keratoconjunctivitis, uncontrolled uveitis, and severe blepharitis. PTK has been used to treat microbial keratitis, including infectious crystalline keratopathy, but its use is very limited because of the risk of spreading of micro-organisms during treatment. Significant dry eye, lagophthalmos, systemic immunosuppression, and collagen vascular disease are also contraindications under many protocols. A neurotrophic cornea such as that caused by herpes simplex virus (HSV) infection may also be a contraindication to PTK. Corneal surface irregularity resulting from endothelial disease is a contraindication to treatment. Hyperopia may be a relative contraindication because PTK causes corneal flattening that may induce a hyperopic shift.
For intralamellar PTK (IL-PTK), the decision to perform the procedure should be based on a series of factors. If the primary indication for lamellar surgery is anterior stromal scarring, the ability to complete the initial lamellar dissection at the proper depth is hindered by poor depth determination at the operating scope, technical difficulty of lamellar dissection, or scar depth that varies across the lesion. In such a case, especially when a residual scar is noted intraoperatively, the use of IL-PTK might be warranted. The residual scar could be safely and rapidly removed by laser.
A second, but related, intraoperative indication for IL-PTK could be a notably irregular dissection and/or a poorly fitting donor lamella. By smoothing the donor and recipient lamellae, problems with interface irregularity could be anticipated and dealt with before they become a problem. If IL-PTK can improve BCVA through its polishing effect, it could become a standard part of LK. Given the additional time and expense required for IL-PTK, it will most likely be used when BCVA following LK is significantly less than the presumed visual potential and when interface abnormalities are noted after surgery.
Surgical Planning and Technique
Preoperative Evaluation
Preoperative evaluation includes a complete eye examination with dilation. Visual acuity and preoperative refraction should be measured. Visual potential can be assessed using a pinhole, hard contact lens, and potential acuity meter. Pupil size, corneal sensation, and corneal thickness, measured via ultrasound pachymetry, are performed. Corneal topography is a useful means of assessing the contribution of corneal pathology to surface irregularity. Once a patient is recumbent beneath the laser, it may be difficult to accurately assess depth of pathology. Therefore careful preoperative slit lamp biomicroscopy is important to determine the degree of corneal involvement. If extensive corneal deposits make visualization of the corneal slit difficult, other devices can be used to assess corneal involvement. Optical pachymetry ( Fig. 20.1 ) and optical coherence tomography have been used to evaluate patients with corneal scarring and dystrophies. Rapuano used ultrasound biomicroscopy (UBM) to examine patients with anterior stromal corneal dystrophies before and after PTK treatment. In his study of 20 eyes, he found that UBM was not useful, as it did not measure the depth of pathology accurately. Hard contact lens overrefraction is valuable in distinguishing between blurred vision resulting from scarring and opacification vs corneal surface irregularities ( Fig. 20.2 ).
Preoperative Preparation
Before each treatment, the laser is calibrated according to the guidelines of each laser manufacturer. A standard treatment is ablated into a calibration plate made of polymethyl methacrylate (PMMA) test block or other material, depending on the laser used. Nitrogen gas flow, previously used during the PMMA calibration in some lasers, is rarely used today. The appropriate corneal ablation rate is determined using nomograms and entered into the laser computer program. The patient is carefully positioned beneath the laser. Attention is paid to patient comfort, and the head should be stable and level. The skin surface is sterilized, and a lid speculum is placed. Before treatment, the plane of the corneal surface is determined by focusing the microscope at high magnification while the patient looks at the fixation light. If a treatment centered on the entrance pupil is planned, the eye-tracking mechanism of the laser should be engaged.
Laser Treatment and General Surgical Techniques
Each PTK treatment must be customized to the individual patient. The duration and pattern of ablation are guided by the depth and location of corneal pathology. Considerations must be made regarding centration and ablation zone size, manual vs laser removal of the epithelium, the use of masking agents, transition zones, and smoothing techniques.
The location of corneal pathology guides centration of PTK treatment. Ideally, laser treatments should be centered over the entrance pupil because decentration can lead to the induction of astigmatism and higher-order aberrations. Diffuse corneal lesions can be treated with a large-diameter ablation centered over the pupil. Eye-tracking mechanisms can help ensure centration. If the cornea contains only a few lesions, each spot is treated and treatment diameter is adjusted for the size of each lesion. Multiple small lesions pose a greater risk of irregular astigmatism, especially when lesions are paracentral or peripheral. For peripheral or paracentral lesions, irregular astigmatism may be minimized by performing manual superficial keratectomy followed by PTK treatment. Additionally, masking agents, described later, can be used to smooth the ablated surface.
Corneal lesions may induce surface irregularity by affecting the corneal epithelium or the underlying stroma. The smoothness of the ablated cornea can be improved by choosing the appropriate technique for removal of the corneal epithelium—either manual debridement or laser ablation. Prior to epithelial removal, it is important to assess the smoothness of the epithelial surface and anterior stromal surface. If the epithelium is a major cause of surface irregularity and the anterior stromal surface is judged to be smooth, the epithelium should be removed manually with a blade. Ablation then proceeds, starting with the smoother surface of the anterior stroma. If the anterior stromal surface is thought to be irregular, the epithelium is ablated with the laser. The epithelium helps mask the irregularity of the underlying stromal surface, resulting in a smoother stromal surface contour following PTK.
Masking agents or surface modulators are fluids that are applied to the cornea following epithelial removal to help smooth the ocular surface. Agents that have been used include 1% hydroxy-methylcellulose, 0.5% tetracaine, or Tears Naturale II. The viscous fluid is applied to the irregular corneal surface before ablation and fills the valleys, exposing the peaks to the excimer laser ( Fig. 20.3 ). The masking agent can be reapplied as needed between laser pulses. Highly viscous fluids (2% hydroxymethylcellulose or Healon) are not appropriate because they do not mask surface irregularities uniformly, only partially covering the peaks and valleys. Low-viscosity fluids tend to expose both the peaks and valleys. Another surface modulating agent, BioMask, has been studied by Kremer and colleagues. BioMask is a collagen that is applied as a liquid to the surface of the cornea, forms under a rigid gas-permeable contact lens, and then is ablated. The corneal epithelium also acts as a masking agent. Preoperative evaluation showing greater smoothness of the epithelium than the epithelial–stromal interface should alert the surgeon to consider performing transepithelial PTK instead of epithelial scraping.
A transition zone is usually created during stromal ablation. It is intended to allow smooth and uniform reepithelialization over the ablation bed. This procedure is referred to as a standard taper ablation and may reduce the induction of halos and hyperopia that can be seen after PTK. Sher et al. used a “smoothing” technique in their early cases, in which the eye was moved in a circular manner under the laser beam. A similar “polish technique” was used in the Summit excimer laser clinical trials. The surgeon moved the patient’s head in a controlled circular manner under the laser beam to “polish” the corneal surface. Stark et al. have described a “modified taper” technique, in which the surgeon attempts to decrease central flattening by moving the eye under the laser in a circular fashion and treating the circumference of the ablation zone with a 20-µm-deep, 2-mm-diameter spot size. This edge modification creates a ring-shaped ablation pattern at the periphery of the PTK to reduce the hyperopic shift that is often seen after PTK ( Fig. 20.4 ). Approximation of the amount of final hyperopic error without the circumferential treatment may also be used to add a hyperopic PRK treatment at the end of the procedure.
IL-PTK: Surgical Technique ( Fig. 20.5 )
The first stage of the IL-PTK should be gaining access to the intracorneal lamellae that require treatment. If the IL-PTK is performed at the time of surgery, access is available following dissection of the donor tissue and recipient bed. If the IL-PTK procedure is performed postoperatively, the donor tissue must be at least partially removed. Residual sutures should be removed, but a single suture may be left in place to serve as a hinge during reflection of the donor tissue away from the stromal bed. The hinge facilitates accurate reapproximation of the lamellae at the end of the procedure. In any event, marking the flap edge is recommended in case the flap is inadvertently completely displaced. Unless the IL-PTK is to be performed in the immediate postoperative period, gentle, blunt dissection may be necessary at the graft interface in order to adequately reflect the graft away from the recipient bed. Once access to the lamellae has been achieved, the recipient bed and posterior surface of the graft should be carefully dried and inspected for obvious irregularities, fibrosis, or scarring.
In our technique, the ablation is done in two phases. First, a limited ablation is performed at the optical center of the cornea. Then, the ablation is slowly moved centrifugally to allow treatment of a larger zone, while minimizing the hyperopic shift. The posterior surface of the graft can be similarly treated by applying the laser first centrally with slow movement to the periphery. Smoothing of both surfaces is likely to result in the best fit. When intraoperative IL-PTK is performed during deep lamellar keratoplasty, more extensive IL-PTK of the posterior surface of the donor tissue can be performed in lieu of true lamellar dissection with a blade.
Following the primary IL-PTK treatment, further application of the excimer laser to the residual bed using a masking agent can improve the smoothness of the interface. In this step, balanced salt solution (BSS) is applied to the central corneal bed and laser pulses applied until the bed appears dry. This procedure may be repeated several times. By ablating the cornea with the masking agent in place, slightly elevated regions of the cornea are exposed to the laser energy first, as the fluid is removed by the laser. The “ridges” of tissue are therefore ablated without affecting the “valleys.”
Once the lasering is complete, the stroma should be rinsed and dried to remove debris prior to reapproximating the treated lamellae. After proper alignment of the graft and bed, the flap is sutured into place with either interrupted or continuous 10-0 nylon sutures ( Fig. 20.6 ).
Elevated Central Corneal Nodules ( )
Treatment of elevated corneal opacities located in the central optical zone is difficult, even with the use of surface modulators. Some surgeons suggest using a blade to excise the elevated lesion prior to PTK ( Figs. 20.7 and 20.8 ). We present a case report illustrating a surgical technique utilizing epithelial debridement to create a depression around the lesion, followed by application of surface modulators to fill the annular furrow around the lesion before laser treatment. This technique may result in removal of the elevated central corneal opacity and improve postoperative visual acuity.
We used the technique described in Fig. 20.7 to treat an 87-year-old white woman with a central Salzmann nodule and irregular astigmatism in the right eye. She had undergone cataract extraction and posterior chamber lens implantation 9 years earlier and also had received LK for a Salzmann nodular degeneration of the right cornea. At initial presentation, her BCVA was 20/400 (+1.25 +3.00 × 145 correction) in the right eye and 20/20 in the left eye. Hard contact lenses improved her visual acuity to 20/50 OD. On slit lamp examination, the cornea had linear elevated subepithelial opacities 1 to 2 mm from the visual axis ( Fig. 20.9 ). Superficial punctate keratitis was noted around the opacities. Corneal topography showed localized +5 diopters (D) steepening over the opacities. Fundus examination showed age-related macular degeneration and mild epiretinal membrane. Intraocular pressure (IOP) and other ocular findings were within normal limits. PTK was performed as described earlier using ArF excimer (fluence: 160 mJ/cm 2 ; repetition rate: 5 Hz; epithelial ablation rate: 0.24 µm/pulse; stromal ablation rate: 0.27 µm/pulse) followed by photoastigmatic keratectomy (cylindrical correction). After PTK, the corneal surface appeared smooth. The central elevated opacity noted preoperatively disappeared ( Fig. 20.10 ). Anterior stromal haze was barely detectable throughout the 12-month follow-up period.
In Salzmann Nodular Degeneration (SND), mid-peripheral corneal elevations often form an asymmetric pool of tear film, which can lead to optical corneal plana, resulting in a pronounced hyperopic shift and irregular astigmatism. In such scenarios, combination of excimer laser PTK with mechanical pannus removal is reported to give good outcomes by normalizing the corneal curvature and by inducing myopic shift. Overall, PTK is reported to be safe and effective in treating elevated corneal nodules in pediatric and adult patients alike.
Multiple Surface Irregularities
Multiple surface irregularities can be treated with the aid of surface modulators or “masking” agents. The epithelium is first removed either manually or with the laser. A masking fluid is then applied to the ocular surface, followed by laser application. This process is repeated as needed to smooth the ocular surface ( Figs. 20.11–20.13 ). In addition to creating a smooth corneal surface, masking agents may reduce the amount of induced hyperopia.
Corneal Dystrophies ( and )
Corneal dystrophies have been traditionally treated with lamellar and penetrating keratoplasty. Following grafting, the primary pathology can eventually recur. PTK has become a useful alternative that may help patients delay or avoid keratoplasty. Patients with superficial corneal lesions, as in epithelial (Meesmann; Fig. 20.14 ) basement membrane dystrophies ( Figs. 20.15 and 20.16 ) rarely require PTK. Patients with Reis-Buckler dystrophy respond well to PTK ( Fig. 20.17 ).
Patients with recurrent granular or lattice dystrophy in a graft have relatively superficial lesions. The success rate in these cases is very high and is similar to that for primary Reis-Buckler dystrophy, in which the deposits are limited to the Bowman layer ( Fig. 20.18 ). Most patients achieve a relatively smooth ablation bed by treating through the epithelium. For patients with granular dystrophy, the aim is to ablate most of the areas of diffuse haze between the granular deposits and not necessarily all the granular hyaline deposits ( Figs. 20.19–20.23 ). A similar approach is used for the management of lattice dystrophy ( Fig. 20.24 ) and macular dystrophy ( Fig. 20.25 ). The healing of the corneal epithelium is typically delayed after PTK treatment of lattice dystrophy compared to others. Therefore extra care must be taken for these patients by providing adequate counseling and close follow-up until complete wound healing to prevent scarring, ulceration, and infection. Medications (such as autologous serum drops and hyaluronic acid drops), simultaneous lateral tarsorrhaphy, or amniotic membrane patching may be helpful for these cases. For patients with granular corneal dystrophy type 2 and cataracts, PTK treatment done in the PRK mode following cataract surgery is reported to be effective.
If corneal dystrophy recurs following primary treatment, PTK can be repeated ( Fig. 20.26 ). Some studies have reported the efficacy of mitomycin C (MMC) in terms of recurrences and visual outcomes when used in conjunction with PTK in treating corneal dystrophies. However, another recent study reported severe recurrences with the use of MMC over a longer follow-up of at least 3 years. Another study reported success in using femtosecond laser-assisted anterior lamellar keratoplasty (FALK) postkeratoplasty for four cases of recurrent granular corneal dystrophy. Corneal grafting can also be performed successfully following PTK.
Recurrent Corneal Erosions ( and )
Patients with recurrent corneal erosions will often respond to manual epithelial debridement or anterior stromal puncture. PTK is usually reserved for cases that are recalcitrant to more conservative measures or cases in which recurrences involve the central visual axis (see Fig. 20.16 ). Epithelial debridement is performed prior to laser application with a dry Weck-cell sponge. A wet Weck-cell sponge is used to sweep any residual deposits. Treatment depth with PTK is relatively minimal (5–10 µm) and is usually limited to the Bowman layer. Long-term follow-up (34–68 months) of shallow ablation PTK than typically considered (4.6 µm) is also reported to be successful after single treatment in 84.6% cases. Additionally, PTK with low frequency and low energy of laser pulses ensures fast and durable epithelial closure in most patients with corneal map-dot-fingerprint dystrophy while preventing recurrent cornea erosions and increasing visual acuity. Significant postoperative hyperopic shift is not observed, and corneal wound healing is less prolonged. In most cases, the use of surface modulators before laser ablation is not necessary. Combination of PTK with PRK can also prove to be effective in resolution of symptoms in patients who have symptomatic epithelial basement membrane disorders along with myopia or myopic astigmatism.
Corneal Scars
Treatment of corneal scars limited to the superficial stroma (see Figs. 20.11 and 20.12 ) can produce significant improvement of visual function. Visual improvement with deeper postinfectious and posttraumatic scars is less likely to occur. The scar may ablate at a different rate than the adjacent normal stroma, which may not benefit from laser ablation. This and the presence of calcified or cartilaginous tissue may result in postoperative irregular astigmatism ( Fig. 20.27 ). Long-standing posttraumatic superficial stromal scars may be resistant to ablation. This can be minimized by using surface modulating agents or a “smoothing” technique. Gentle rotation of the head under the laser beam blends the edges of the irregularities. By maintaining the corneal surface meticulously clear of debris and cellular remnants, further irregularities may be avoided. At times, the scars and keloids on the cornea can be very large such that laser ablation can take too long. In these cases, manual resection prior to excimer laser use for surface smoothening can both eliminate irritation and produce cosmetically a acceptable result. The surface can also be rendered smooth enough for the placement of a prosthesis or contact lens. Band keratopathy has also been treated with PTK, although EDTA is still the standard means of treatment.
Infectious Keratitis
PTK has been attempted for the treatment of infectious keratitis cases that include fungal, viral, bacterial, and parasitic origins, but is discouraged because of the possibility of spread of infectious agents during and following treatment. Involvement of the stroma in most microorganism infections extends deeper than the clinically observable lesion. As the tissue penetration depth of 193-nm radiation is no more than 1 mm, deep stromal infiltration may limit the effectiveness of treatment of infectious keratitis with the excimer laser. Reactivation of latent HSV has been reported following excimer PTK. Overall, though, early PTK intervention can prove advantageous in treating unresponsive acanthamoeba keratitis because of the direct removal of the amoebic cysts. Sclerotic scatter illumination technique can provide better visualization of the opacity and help prevent excessive ablation in cases of diffuse keratitis. PTK combined with other techniques, such as flap amputation and collagen cross-linking, is also reported in cases of severe intractable post-LASIK mycobacterial infection with corneal melt.
Refractive Surgery Complications
PTK is a valuable tool for the treatment of surface irregularities following refractive surgery (see Figs. 20.13 and 20.28 ). Ablation of the epithelium with the excimer laser results in fluorescence in the blue wavelength that can be visualized under low illumination. With excimer laser ablation of the corneal stroma, fluorescence is lost. This characteristic has been used to help manage patients with central islands or decentration following PRK and flap striae following LASIK. Rachid et al. have described a technique for the management of central islands and decentrations in which transepithelial PTK is followed by PRK and repeat PTK. Transepithelial PTK set at 50 µ is performed first to expose the island or decentered area. PRK is applied to treat the residual refractive error, and PTK is repeated to eliminate any residual epithelium, confirmed by absence of epithelial fluorescence ( Fig. 20.28 ). Steinert et al. have described a technique for the treatment of flap striae after LASIK. Transepithelial ablation is performed until epithelial fluorescence begins to disappear between the striae. A masking agent is then applied with PTK to smooth the surface.
Prismatic Photokeratectomy
We have studied the use of a modified PTK technique, prismatic photokeratectomy (PPK) in patients with small-angle strabismic deviations and diplopia. Diplopia results when the visual axes are misaligned with simultaneous images falling on the fovea of one eye and a nonfoveal point in the other eye. The same object is seen at two different locations in subjective space. In physiologic diplopia, objects outside Panum’s area fall on noncorresponding points.
The angle of deviation, PD, can be calculated from the equation
PD=360(N−1)π⋅hOZ,
orPD=42⋅hOZ
orh=PD⋅OZ42.
Figs. 20.29 and 20.30 show a schematic of PPK used to correct binocular diplopia. PPK makes the cornea resemble a curved prism with spherical sides. The prismatic correction is proportional to the depth of ablation and inversely proportional to the diameter of the ablation zone. Theoretical analysis has been performed on the optics of prisms with spherical sides and the relationship of the prismatic correction to the diameter and maximal depth of laser ablation. Variable degrees of small-angle prismatic correction can be obtained. For example, a deep PPK treatment of 240 µm maximal depth and 5 mm diameter ablation would induce 2.5 prism diopters (D). The healing process that follows the stromal prismatic ablation may alter the final effect depending on how well the epithelial surface conforms to the ablation bed.
Preoperative Evaluation
Preoperative evaluation includes a complete eye examination with dilation. Visual acuity and preoperative refraction should be measured. Visual potential can be assessed using a pinhole, hard contact lens, and potential acuity meter. Pupil size, corneal sensation, and corneal thickness, measured via ultrasound pachymetry, are performed. Corneal topography is a useful means of assessing the contribution of corneal pathology to surface irregularity. Once a patient is recumbent beneath the laser, it may be difficult to accurately assess depth of pathology. Therefore careful preoperative slit lamp biomicroscopy is important to determine the degree of corneal involvement. If extensive corneal deposits make visualization of the corneal slit difficult, other devices can be used to assess corneal involvement. Optical pachymetry ( Fig. 20.1 ) and optical coherence tomography have been used to evaluate patients with corneal scarring and dystrophies. Rapuano used ultrasound biomicroscopy (UBM) to examine patients with anterior stromal corneal dystrophies before and after PTK treatment. In his study of 20 eyes, he found that UBM was not useful, as it did not measure the depth of pathology accurately. Hard contact lens overrefraction is valuable in distinguishing between blurred vision resulting from scarring and opacification vs corneal surface irregularities ( Fig. 20.2 ).
Preoperative Preparation
Before each treatment, the laser is calibrated according to the guidelines of each laser manufacturer. A standard treatment is ablated into a calibration plate made of polymethyl methacrylate (PMMA) test block or other material, depending on the laser used. Nitrogen gas flow, previously used during the PMMA calibration in some lasers, is rarely used today. The appropriate corneal ablation rate is determined using nomograms and entered into the laser computer program. The patient is carefully positioned beneath the laser. Attention is paid to patient comfort, and the head should be stable and level. The skin surface is sterilized, and a lid speculum is placed. Before treatment, the plane of the corneal surface is determined by focusing the microscope at high magnification while the patient looks at the fixation light. If a treatment centered on the entrance pupil is planned, the eye-tracking mechanism of the laser should be engaged.
Laser Treatment and General Surgical Techniques
Each PTK treatment must be customized to the individual patient. The duration and pattern of ablation are guided by the depth and location of corneal pathology. Considerations must be made regarding centration and ablation zone size, manual vs laser removal of the epithelium, the use of masking agents, transition zones, and smoothing techniques.
The location of corneal pathology guides centration of PTK treatment. Ideally, laser treatments should be centered over the entrance pupil because decentration can lead to the induction of astigmatism and higher-order aberrations. Diffuse corneal lesions can be treated with a large-diameter ablation centered over the pupil. Eye-tracking mechanisms can help ensure centration. If the cornea contains only a few lesions, each spot is treated and treatment diameter is adjusted for the size of each lesion. Multiple small lesions pose a greater risk of irregular astigmatism, especially when lesions are paracentral or peripheral. For peripheral or paracentral lesions, irregular astigmatism may be minimized by performing manual superficial keratectomy followed by PTK treatment. Additionally, masking agents, described later, can be used to smooth the ablated surface.
Corneal lesions may induce surface irregularity by affecting the corneal epithelium or the underlying stroma. The smoothness of the ablated cornea can be improved by choosing the appropriate technique for removal of the corneal epithelium—either manual debridement or laser ablation. Prior to epithelial removal, it is important to assess the smoothness of the epithelial surface and anterior stromal surface. If the epithelium is a major cause of surface irregularity and the anterior stromal surface is judged to be smooth, the epithelium should be removed manually with a blade. Ablation then proceeds, starting with the smoother surface of the anterior stroma. If the anterior stromal surface is thought to be irregular, the epithelium is ablated with the laser. The epithelium helps mask the irregularity of the underlying stromal surface, resulting in a smoother stromal surface contour following PTK.
Masking agents or surface modulators are fluids that are applied to the cornea following epithelial removal to help smooth the ocular surface. Agents that have been used include 1% hydroxy-methylcellulose, 0.5% tetracaine, or Tears Naturale II. The viscous fluid is applied to the irregular corneal surface before ablation and fills the valleys, exposing the peaks to the excimer laser ( Fig. 20.3 ). The masking agent can be reapplied as needed between laser pulses. Highly viscous fluids (2% hydroxymethylcellulose or Healon) are not appropriate because they do not mask surface irregularities uniformly, only partially covering the peaks and valleys. Low-viscosity fluids tend to expose both the peaks and valleys. Another surface modulating agent, BioMask, has been studied by Kremer and colleagues. BioMask is a collagen that is applied as a liquid to the surface of the cornea, forms under a rigid gas-permeable contact lens, and then is ablated. The corneal epithelium also acts as a masking agent. Preoperative evaluation showing greater smoothness of the epithelium than the epithelial–stromal interface should alert the surgeon to consider performing transepithelial PTK instead of epithelial scraping.
A transition zone is usually created during stromal ablation. It is intended to allow smooth and uniform reepithelialization over the ablation bed. This procedure is referred to as a standard taper ablation and may reduce the induction of halos and hyperopia that can be seen after PTK. Sher et al. used a “smoothing” technique in their early cases, in which the eye was moved in a circular manner under the laser beam. A similar “polish technique” was used in the Summit excimer laser clinical trials. The surgeon moved the patient’s head in a controlled circular manner under the laser beam to “polish” the corneal surface. Stark et al. have described a “modified taper” technique, in which the surgeon attempts to decrease central flattening by moving the eye under the laser in a circular fashion and treating the circumference of the ablation zone with a 20-µm-deep, 2-mm-diameter spot size. This edge modification creates a ring-shaped ablation pattern at the periphery of the PTK to reduce the hyperopic shift that is often seen after PTK ( Fig. 20.4 ). Approximation of the amount of final hyperopic error without the circumferential treatment may also be used to add a hyperopic PRK treatment at the end of the procedure.
IL-PTK: Surgical Technique ( Fig. 20.5 )
The first stage of the IL-PTK should be gaining access to the intracorneal lamellae that require treatment. If the IL-PTK is performed at the time of surgery, access is available following dissection of the donor tissue and recipient bed. If the IL-PTK procedure is performed postoperatively, the donor tissue must be at least partially removed. Residual sutures should be removed, but a single suture may be left in place to serve as a hinge during reflection of the donor tissue away from the stromal bed. The hinge facilitates accurate reapproximation of the lamellae at the end of the procedure. In any event, marking the flap edge is recommended in case the flap is inadvertently completely displaced. Unless the IL-PTK is to be performed in the immediate postoperative period, gentle, blunt dissection may be necessary at the graft interface in order to adequately reflect the graft away from the recipient bed. Once access to the lamellae has been achieved, the recipient bed and posterior surface of the graft should be carefully dried and inspected for obvious irregularities, fibrosis, or scarring.
In our technique, the ablation is done in two phases. First, a limited ablation is performed at the optical center of the cornea. Then, the ablation is slowly moved centrifugally to allow treatment of a larger zone, while minimizing the hyperopic shift. The posterior surface of the graft can be similarly treated by applying the laser first centrally with slow movement to the periphery. Smoothing of both surfaces is likely to result in the best fit. When intraoperative IL-PTK is performed during deep lamellar keratoplasty, more extensive IL-PTK of the posterior surface of the donor tissue can be performed in lieu of true lamellar dissection with a blade.
Following the primary IL-PTK treatment, further application of the excimer laser to the residual bed using a masking agent can improve the smoothness of the interface. In this step, balanced salt solution (BSS) is applied to the central corneal bed and laser pulses applied until the bed appears dry. This procedure may be repeated several times. By ablating the cornea with the masking agent in place, slightly elevated regions of the cornea are exposed to the laser energy first, as the fluid is removed by the laser. The “ridges” of tissue are therefore ablated without affecting the “valleys.”
Once the lasering is complete, the stroma should be rinsed and dried to remove debris prior to reapproximating the treated lamellae. After proper alignment of the graft and bed, the flap is sutured into place with either interrupted or continuous 10-0 nylon sutures ( Fig. 20.6 ).
Elevated Central Corneal Nodules ( )
Treatment of elevated corneal opacities located in the central optical zone is difficult, even with the use of surface modulators. Some surgeons suggest using a blade to excise the elevated lesion prior to PTK ( Figs. 20.7 and 20.8 ). We present a case report illustrating a surgical technique utilizing epithelial debridement to create a depression around the lesion, followed by application of surface modulators to fill the annular furrow around the lesion before laser treatment. This technique may result in removal of the elevated central corneal opacity and improve postoperative visual acuity.
We used the technique described in Fig. 20.7 to treat an 87-year-old white woman with a central Salzmann nodule and irregular astigmatism in the right eye. She had undergone cataract extraction and posterior chamber lens implantation 9 years earlier and also had received LK for a Salzmann nodular degeneration of the right cornea. At initial presentation, her BCVA was 20/400 (+1.25 +3.00 × 145 correction) in the right eye and 20/20 in the left eye. Hard contact lenses improved her visual acuity to 20/50 OD. On slit lamp examination, the cornea had linear elevated subepithelial opacities 1 to 2 mm from the visual axis ( Fig. 20.9 ). Superficial punctate keratitis was noted around the opacities. Corneal topography showed localized +5 diopters (D) steepening over the opacities. Fundus examination showed age-related macular degeneration and mild epiretinal membrane. Intraocular pressure (IOP) and other ocular findings were within normal limits. PTK was performed as described earlier using ArF excimer (fluence: 160 mJ/cm 2 ; repetition rate: 5 Hz; epithelial ablation rate: 0.24 µm/pulse; stromal ablation rate: 0.27 µm/pulse) followed by photoastigmatic keratectomy (cylindrical correction). After PTK, the corneal surface appeared smooth. The central elevated opacity noted preoperatively disappeared ( Fig. 20.10 ). Anterior stromal haze was barely detectable throughout the 12-month follow-up period.
