Refractive unpredictability after penetrating keratoplasty (PKP) is extremely common owing to the inherent imprecision of the operation. Most series document a mean postoperative astigmatism of 4.00–5.00 D. In many cases, these refractive errors are not amenable to spectacle correction, and 10%–30% of patients require rigid gas-permeable contact lens correction to achieve good vision after PKP. However, contact lens fitting may not be successful in this patient population due to abnormal corneal curvature or the patient’s inability to tolerate or manipulate a contact lens.
Surgical alternatives for the correction of post-PKP astigmatism include astigmatic keratotomy, compression sutures, and wedge resections. In a series of 201 corneal transplants for keratoconus, 18% of patients required refractive surgery to correct the astigmatism. Although these procedures can significantly decrease corneal cylinder and are highly effective, they have minimal effect on spherical equivalent. In addition, they can be unpredictable and may destabilize the graft–host wound.
Phakic eyes with significant corneal astigmatism after suture removal could undergo crystalline lens replacement with a toric intraocular lens if the astigmatism is regular centrally and has stabilized. Patients with significant anisometropia after PKP surgery may be candidates for intraocular lens (IOL) exchange or piggyback IOL implantation (see Chapter 8). These alternatives require another intraocular procedure, which increases the risk of endothelial decompensation, glaucoma, and cystoid macular edema and may incite graft rejection.
Given the successful use of the excimer laser in treating myopia and astigmatism, PRK has been studied and used to treat post-PKP refractive errors. PRK has the disadvantages associated with epithelial removal in a corneal transplant and may result in corneal haze when high refractive errors are treated. With the use of prophylactic topical mitomycin C, PRK has become a more acceptable treatment option for refractive errors after PKP. Although the refractive results are often good, PRK in patients who had past PKP is generally less predictable and less effective than it is for naturally occurring astigmatism and myopia.
LASIK after PKP is subject to the same patient-selection constraints as conventional LASIK. Without extenuating circumstances, patients with monocular vision or patients with limited vision potential in the fellow eye usually are poor candidates. In addition, patients with a wound-healing disorder, significant dry eye syndrome, or a collagen-vascular disease should be offered other options. Finally, patients should have realistic expectations for their rehabilitation after post-PKP LASIK. The goal of LASIK following PKP is to return the patient to spectacle-corrected binocularity or to enable the patient to wear contact lenses successfully, as the accuracy of the procedure is less predictable than that of conventional LASIK. Also, note that there are no FDA-approved procedures at this time to treat irregular astigmatism. Preoperative evaluation of the post-PKP patient who is considering refractive surgery should include the original indications for the PKP. It has been the experience of many surgeons that patients with low endothelial cell counts may be at increased risk of flap dislocation after LASIK because of impairment of the endothelial cell pump function.
Optimal timing of refractive surgery after PKP is controversial. All sutures should be removed, and the refraction should be stable. To avoid wound dehiscence, many surgeons wait at least 1 year after PKP, and an additional 4 months after all sutures are removed, before performing the refractive surgery. An interval of at least 18–24 months after PKP provides sufficient wound healing in most cases. No matter how much time has elapsed since the PKP surgery, the entirety of the graft–host wound should be carefully inspected to identify areas of variability in coaptation of the graft–host junction. Complications that can occur with a LASIK procedure include a small but significant risk of keratoplasty wound dehiscence during application of the vacuum ring used to create the LASIK flap, or during PRK or astigmatic keratotomy procedures.
Refraction and corneal topography should be stable, as documented by 2 consecutive readings on separate visits at least 1 month apart. Areas of stromal thinning should be confirmed to avoid exacerbation or, in extreme cases, perforation during LASIK flap creation. Refractive surgery should be avoided if the corneal graft shows evidence of inflammation, diffuse vascularization, ectasia, inadequate healing of the graft–host interface, refractive instability, or if there are signs of rejection or endothelial decompensation.
Because eye alignment under the laser is crucial for accurate treatment of astigmatism, some surgeons mark the vertical or horizontal axis of the cornea at the slit lamp before placing the patient under the laser. If the corneal curvature is very steep, cutting a thicker flap during the microkeratome pass may decrease the risk of buttonhole formation. PRK should also be considered in steep corneas to avoid flap complications.
Another potential problem specific to post-PKP LASIK is that the creation of a lamellar flap may itself cause a change in the amount and axis of the astigmatism. Therefore, some surgeons perform LASIK in 2 stages. In the first stage, the flap is cut and laid back down. Several weeks later, after the curvature and refraction have stabilized, the second stage is performed, where the flap is lifted and laser ablation is applied. Some reports describe minimal refractive changes after flap creation, and some surgeons prefer to perform LASIK in 1 stage to avoid increasing the potential for the complications associated with performing 2 separate procedures, including infection, graft rejection, and epithelial ingrowth. Flap retraction and necrosis have been reported in patients undergoing LASIK after keratoplasty.
The mean percentage reduction of astigmatism after LASIK following PKP ranges from 54.0% to 87.9%. Although most series report improvement in uncorrected visual acuity (UCVA; also called uncorrected distance visual acuity, UDVA), up to 42.9% of patients require enhancement because of cylindrical undercorrection. In addition, up to 35% of patients in some series have lost 1 line of BCVA. Corneal graft rejection has been described after PRK; thus, higher and more prolonged dosing with topical corticosteroids should be prescribed for post-PKP refractive surgery patients to decrease this risk.
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Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.