Pre-LASIK Considerations Following PK

Several short-term studies have shown that performing LASIK after penetrating keratoplasty (PK) can be an effective and safe procedure.1-8 However, LASIK is not always the panacea one may conceive. As a corneal surgeon who has been performing LASIK on PK-treated eyes since 1997, I will present 12 reasons why LASIK may not always be a reliable refractive surgical alternative for visual rehabilitation after PK.

1. Studies Do Not Reflect Typical Patients
Most of the studies today investigating the safety and efficacy of LASIK after PK have been performed on younger patients (mean age: 37 to 53 years) who have a preoperative diagnosis of keratoconus (60% to 90%). The majority of these eyes have been keratoconic; very few have had other diagnoses. These studies have failed to examine the safety and efficacy of LASIK in older patients diagnosed with pseudophakic corneal edema and aphakic corneal edema. The patient samples are not a true reflection of typical cornea transplant patients in a representative corneal practice.

2. Astigmatism Limits Predictability
Astigmatism is the main culprit in poor refractive predictability after LASIK. The incidence of high astigmatism following keratoplasty varies from 10% to 27%. Many studies have shown that LASIK after PK is relatively more effective in treating spherical myopic refractive error than cylindrical correction.1-8 The safety and efficacy of LASIK after PK has been studied in several articles, and, based on existing data, 50% to 80% of eyes still had high degrees of astigmatism, not decisively better than those obtained with incisional surgery.1-8 I routinely perform relaxing incisions, with or without compression sutures, for managing postkeratoplasty astigmatism in the range of 6.0 D and higher prior to executing any LASIK procedure.

3. Nonorthogonal and Irregular Astigmatism
Nonorthogonal and irregular astigmatism are important limiting factors to the successful outcome of LASIK after PK, more so than regular astigmatism (Figure 1). Studies have shown that, following PK, approximately 8% to 20% of eyes have significant irregular astigmatism that cannot be satisfactorily managed with glasses or contact lenses.9-17 Oftentimes, this level of astigmatism has no symmetrical integrity; there is a component of irregularity, which compromises the possibility of regular LASIK ablation. Truly symmetrical, regular astigmatism is a rare condition after PK, and irregular astigmatism cannot be corrected with existing technology. With the advent of customized corneal ablation, surgeons may hope to treat such corneal surfaces and facilitate more predictable results for LASIK after PK.

4. Potential Visual Loss
Visual loss post-LASIK is reported more frequently in eyes that have undergone PK. Loss of best potential vision is a major concern when the surgeon performs refractive surgery in the central cornea. Traditional LASIK has been associated with a loss of two or more lines of Snellen acuity in up to 8% of the cases18 when there was a level of myopia greater than 8.0 D and hyperopia greater than 3.0 D. Snellen loss seems to be more frequent in post-PK LASIK patients.4 One study has shown that 29% of eyes can lose one line of BSCVA after LASIK.19

5. Unknown Parameters
There is no established time interval between the initial PK and LASIK procedure. In the literature, investigators recommend performing LASIK approximately 3 to 6 months after suture removal, and 12 to 18 months after the corneal graft procedure (Figure 2).1-8 I believe other factors must be examined in addition to the time interval between procedures. The graft must attain a low activity of wound healing and remodeling. The stability of refraction, corneal topography, underlying initial corneal pathology, endothelial integrity, graft versus host size disparity, donor and host age, and duration of topical steroid application are some of the less quantifiable factors that can influence the appropriate surgical time frame for undergoing LASIK. The best time to surgically correct postkeratoplasty refractive error should be identified on an individual basis.

6. Higher Enhancement Rates
The higher rate of enhancement for LASIK after PK stems from an unpredicted healing pattern related to the mechanical forces involved in interfacing the donor tissue with the host tissue, which remain to be studied and evaluated. In one study,20 the incidence of retreatment has been as high as 50%. The LASIK flap heals more aggressively over the donor/host junction and causes difficulty in lifting the flap for enhancement and increases the possibility of epithelial ingrowth. Its incidence after retreatment has been noted to occur in as many as 14% of cases (Figure 3). Additional surgical intervention can increase the risk of corneal rejection and compromise the graft's longevity.

7. Post-LASIK neuropathic nature of PK-treated eyes
Superficial punctate keratopathy can become a major source of visual fluctuation. Most graft eyes are neurotrophic with suboptimal corneal sensation. In order to reduce the neurotrophic component of the cornea, the surgeon should consider preserving the nasal nerve plexus. Eyes diagnosed preoperatively with herpetic keratitis, or a neurotrophic condition with borderline limbal cells, have a relative or absolute contraindication for LASIK.

8. High Degree of Regression
It has been noted that some patients after PK have a very reactive epithelium (Figure 4).21 Many of these corneas exhibit epithelial hyperplasia after PK to reduce the amount of irregular astigmatism. After LASIK, a number of these eyes may again have epithelial hyperplasia, which can cause a high degree of regression.

9. Intraoperative Flap Complications
A review of the literature reveals a higher rate of flap complications with LASIK on a grafted cornea.5,7,19 Intraoperative paracentral perforation and dislocation of the flap postoperatively have been reported (Figure 5).5 A buttonhole flap is also more frequently seen in grafted corneas than in those without grafts.7 Another noted intraoperative complication is a loss of suction and an incomplete flap. The high prevalence of atopic disease in eyes with PK for keratoconus appears to result in more rapid conjunctival chemosis, which is particularly obvious if more than one attempt is required to achieve suction. DLK after LASIK in grafted eyes has been observed due to preoperative collagen vascular disease and severe host corneal neovascularization (personal observation). The microkeratome may induce or aggravate the preexisting vascularization and increase the risk of graft rejection.

10. Unknown Biomechanical Forces
Unknown biomechanical forces involved in lamellar flap creation over the donor/host interface will increase the unpredictable nature of the LASIK procedure. The lamellar flap may influence the postoperative corneal topography. Hypothetically, the corneal flap may retract toward the hinge and create asymmetry in the postoperative topography. Studies have shown that the donor/host wound biomechanics are influenced by the flap cut that occurs in LASIK after PK.22-23 The donor/host size disparity has a mechanical role as well. The biomechanics of flap formation may be very different in patients with bullous keratopathy versus patients with keratoconus. Keratoconic eyes are usually thinner and generally have a smaller disparity in donor/host size versus the traditional 0.25-mm disparity for eyes with bullous keratopathy.4 This donor/host disparity will create different forces at the graft/host junction, which may result in very different outcomes for eyes with different preoperative diagnoses.

11. Graft Endothelial Integrity
PK leads to continued endothelial cell loss. Therefore, the surgeon should always be concerned with the long-term longevity of a corneal graft after additional incisional or laser ablative procedures. Few studies did not show active endothelial cell loss during LASIK.24-25 However, corneal grafts have a continuous endothelial cell loss. Ten years posttransplant, the majority of grafts have endothelial cell densities of less than 1,000/mm2, which may cause flap complications at a later date. A preoperative endothelial cell count is very important. Eyes with cell counts less than 1,500/mm2 should not undergo LASIK.

12. Poor Predictability in Corneal Grafts
The preoperative corneal curvature can vary in graft patients. The curvature may be as high as 50.0 D and as low as 32.0 D. Many corneal grafts can have flat and steep meridians concomitantly. As a result, the regular PRK nomogram that has been used for LASIK treatment on these patients will have unpredictable results. This may be an explanation for the variable refractive outcomes with higher enhancement rates after LASIK in PK-treated eyes. In addition, the new 7- to 9-mm limbus of a grafted cornea is much closer to the area treated with the excimer laser and to the edge of the corneal flap. This may influence the outcome because of stress forces different from those in a normal limbus.

Conclusion
We hope that, by better refinement of original transplantation surgeries and using meticulous surgical techniques, we can improve and obviate the need for further intervention of postkeratoplasty astigmatism. I believe that the LASIK procedure has proven to be safe and effective for reducing refractive error after PK. I also feel that graft thickness, endothelial cell integrity, and keratometry are important parameters that must be ascertained to improve the predictability and safety of this procedure in corneal graft eyes. Surgeons should also be aware of the possibility of BSCVA loss after this procedure. In addition, they must consider a higher level of interface epithelial ingrowth, intraoperative flap complication, and unpredictable results for refractive astigmatism, with a less favorable outcome than the spherical component.

Just because a patient has a refractive error, it does not mean that he is a candidate for receiving laser treatment. Many patients may require a relaxing incision or wound revision to reduce the amount of astigmatism, or spherical equivalent, prior to LASIK. It is important to understand the tissue distribution and mechanical forces involved at the donor/host interface prior to proceeding with an uncalculated LASIK procedure. Surgical intervention should be considered only when optical methods fail to provide adequate visual rehabilitation.

Majid Moshirfar, MD, FACS, is Associate Professor of Ophthalmology and the Director of Corneal Refractive Surgery Services at the University of Utah, Department of Ophthalmology, John A. Moran Eye Center, in Salt Lake City, Utah. Dr. Moshirfar may be reached at (801) 585-3937; Majid.Moshirfar@hsc.utah.edu.
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