PK remains a safe and successful technique that reverses blindness caused by corneal opacities. Following successful PK, corneal astigmatism is the most common cause of suboptimal uncorrected vision when the corneal graft is clear. Based on many studies, 15% to 31% of patients who undergo PK may develop greater than 5.0 D of astigmatism postoperatively.¹ Additionally, postoperative corneal astigmatism is common, causing significant visual impairment.²

In most instances, the graft-host interface is completely healed one-year post-surgery. (Individual factors that may affect healing include age, diabetes, collagen vascular disease, and length of topical steroid use.³) The complete stabilization of the corneal surface occurs 3 to 4 months following suture removal. Therefore, it is generally recommended to initiate astigmatism management 3 to 4 months after the surgery.

That said, a complete comprehensive ophthalmic exam should be initiated before managing post-PK corneal astigmatism to eliminate any non-corneal cause for poor vision. Furthermore, understanding uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BsCVA), and best contact lens-corrected visual acuity (BcCVA) are all important in deciding which corrective method to utilize. Other key factors, in addition to manifest refraction, include corneal topography, keratometry, corneal pachymetry, and cycloplegic refraction in pediatric patients (Figure 1).

Here, I discuss the techniques to manage and reduce post-PK astigmatism. They are in order of more invasive/higher risk, and within that, starting with the most common.

Suture Adjustment and Removal

Adjustments can be made to titrate the tension of sutures with running sutures or the selective removal of sutures when using an interrupted suture pattern, which ultimately reduces corneal astigmatism.⁴ The combination of refraction, keratometry, and corneal topography can aid in identifying the steep and flat axis of astigmatism. Additionally, the identification of tight sutures via slit lamp can help determine which sutures to remove. (A small increase in infection risk or rejection occurs from suture removal, so topical antibiotic and steroid use should be adjusted accordingly.)

In removing interrupted sutures, remove 1 suture on either side of the steep axis at a time, leaving the neighboring sutures in place to promote wound integrity. The neighboring sutures can be removed after intervals of 6 weeks at a rate of 1 to 2 sutures per follow-up visit until the desired astigmatism correction is achieved.

When adjusting the running sutures, the adjustments should be made within 2 months after the surgery when it is believed the graft-host interface is stabilized.

The hybrid running and interrupted suture technique allows for the removal of multiple interrupted sutures at a time and earlier than with the other techniques (Figure 2). The benefit includes less concern for jeopardizing the wound integrity, given the remaining running sutures.

When astigmatism is minimal in any of these techniques, the sutures can be left in place until they break, loosen or need to be removed in preparation for cataract surgery.

Spectacle and Contact Lens Correction

Spectacles can be prescribed for refractions tolerated by the patient. (Some patients can tolerate up to 6 D, while others can tolerate up to 3 D, and then in some cases aniseikonia can occur.)

When astigmatism is more significant, contact lenses, such as rigid gas permeable (RGP), hybrid or scleral, are a superior option to achieve BcCVA. To determine which lens to utilize, the overall shape of the cornea should be considered.

RGPs are usually ideal, providing a good fit, and improved and correctable vision. A caveat: An irregular curvature either at the graft-host interface or centrally can lead to RGP decentration, providing a poor fit. In these instances, piggyback or hybrid lenses may fit more successfully and correct poor vision. Scleral contact lenses provide excellent corrected visual acuity, while potentially avoiding corneal surface trauma and decreasing the incidence of ocular inflammation.

Regardless of lens choice, all post-PK patients need to be followed for signs of trauma to the corneal graft and/or neovascularization. In some cases, it may be problematic to fit the graft with a lens, or the patient may not be able to tolerate a lens. In these instances, the other techniques should be explored (Figure 3).

Corneal Relaxing Incisions

Incisional keratectomy treats 3.0 D to 9.0 D of corneal astigmatism. The incisions are typically made based on corneal topography, with 90% corneal depth within the corneal graft or at the graft-interface with arcs ranging from 45° to 90°. If a sufficient amount of correction is not achieved with relaxing incisions, compression sutures can be placed 90° away from the steep axis to achieve overcorrection in the flat axis and, thereby, decrease astigmatism in the steep axis.

Corneal relaxing incisions can be paired with cataract extraction, and toric and phakic IOL implantation when all intended sutures have been removed, or once suture management is complete. To determine the intended IOL power, the potential for a 1.5 D myopic shift induced by incision keratectomy should be considered.³ Arcuate keratotomy utilizing femtosecond laser is effective in decreasing PK astigmatism.⁵ Risks for wound dehiscence, infection, or perforation should be discussed with the patient prior to incisional keratectomy.

Wedge Resection

Wedge removal of corneal tissue in both the host and donor in the steep axis of astigmatism, followed by closure with 6 to 8 tight sutures, is commonly used when corneal astigmatism is greater than 10 D.³ There are various nomograms available to guide the size of excised tissue and proximity to the central cornea. Hugo, et al provides an example of such a nomogram.⁶ On average, 0.05 mm to 0.1 mm of tissue is excised for every 1 D of astigmatism.³

The post-surgery results can be unpredictable due to the variability in the depth and the exact amount of tissue excised. (The use of the femtosecond laser has been proposed to help improve the precision of the corneal wedge recesection.⁷ A post-procedural myopic shift can also occur secondary to an increase in the corneal curvature from the tissue excised. Finally, postoperative complications, such as corneal perforation, can occur.⁸

Laser Refractive Surgery

Both LASIK and PRK can treat spherical and astigmatic refractive errors following PK. PRK may be advantageous for thinner corneas, as there are no concerns with flap-related issues, and the risk for post-PRK haze can be significantly reduced to some degree with mitomycin C. On the other hand, PRK in post-PK patients is associated with greater unpredictability in treatment outcomes, and is less effective for higher levels of astigmatism typically seen in PK eyes. Topographic-guided treatments can be useful in patients who have irregular astigmatism.

LASIK after PK is associated with quick recovery, low amounts of regression, and greater predictability when compared to PRK. Depending on the graft diameter, special considerations should be made regarding flap hinge placement and diameter. Potential flap-related complications in post-PK eyes are suction loss, incomplete flap, buttonholes, and epithelial ingrowth. The femtosecond laser allows for more accurate, thinner and uniform flaps. Separating the flap creation and the ablation treatment by a few weeks can lead to better outcomes by incorporating the astigmatic changes induced when creating the LASIK flap. On the other hand, increased postop haze and epithelial ingrowth can occur from separating the procedures.

The risk for post-laser rejection induced by laser treatment exists for both PRK and LASIK. Therefore, these patients should be followed closely (i.e., 6 to 12 months) after the completion of either procedure. Contraindications for laser refractive surgery are a history of previous graft rejection, herpes simplex virus-related keratitis, or other inflammatory conditions.

Intrastromal Corneal Ring Segments (ICRS)

These improve post-PK astigmatism and reduce low levels of myopia, leading to an improved UCVA. Intraoperative complications, such as graft dehiscence, can occur during tunnel dissection. Also, post-op inflammation can lead to the rejection and vascularization of the ICRS channels.

IOL Placement

This can reduce the spherical correction of post-PK patients. A toric IOL can treat or lower moderate-to-high amounts of regular astigmatism up to about 4.0 D. (A staged approach is recommended with the completion of suture removal/adjustment before proceeding with the cataract extraction for more accurate IOL measurements.)

When lens opacities are not present, phakic IOLs can treat both spherical and toric refractive errors. The incision size and risk for endothelial cell loss vary according to phakic IOL used, so IOL selection should be deferred to the surgeon.

The risks of IOL placement are infection and inflammation. In this case, graft-induced rejection is also a risk, so adjustments to the use of topical steroid and antibiotic drops should be made accordingly.

Repeat Keratoplasty

Corneal graft re-grafting should be considered as a last resort when the graft is clear, but the patient shows extremely high astigmatism not correctable by the means described above, and not correctable with a scleral lens. When performing the re-grafting, a larger-sized trephination and graft, and a delay in the suture removal may improve the visual outcome by decreasing the central corneal power and ultimately lowering corneal astigmatism. (The aforementioned techniques are utilized to manage successful re-grafting.)

Options Abound

The perfect algorithm to manage post-PK astigmatism does not exist. However, multiple techniques can be tailored to the patient’s specific clinical and logistical needs. In cases of higher astigmatism, combining two or more techniques can lower post-PK astigmatism and ultimately improve uncorrected visual acuity. CP

References:

1. Asena L, Altinors DD. Visual Rehabilitation After Penetrating Keratoplasty. Exp Clin Transplant. 2016;14(Suppl 3):130-134.
2. Riddle HK, Parker DA, Price FW. Management of postkeratoplasty astigmatism. Review Curr Opin Ophthalmol. ;9(4):15-28.
3. Feizi S, Zare M. Current Approaches for Management of Postpenetrating Keratoplasty Astigmatism. J Ophthalmol. 2011:708736.
4. Mader TH, Yuan R, Lynn MJ, Stulting RD, Wilson LA, Waring GO. Changes in keratometric astigmatism after suture removal more than one year after penetrating keratoplasty. Ophthalmology. 1993 Jan;100(1):119-26; discussion 127.
5. Nubile M, Carpineto P, Lanzini M, et al. Femtosecond laser arcuate keratotomy for the correction of high astigmatism after keratoplasty. Ophthalmology. 2009 Jun;116(6):1083-92.
6. Hugo M, Donnenfeld ED, Arentsen JJ. Corneal wedge resection for high astigmatism following penetrating keratoplasty. Ophthalmic Surg. 1987;18:650–3.
7. Han SB, Liu YC, Mohamed-Noriega K, Mehta JS. Application of Femtosecond Laser in Anterior Segment Surgery. J Ophthalmol. 2020; 2020: 8263408. Published online 2020 Apr 10.
8. Lindstrom RL. Surgical correction of postoperative astigmatism. Indian Journal of Ophthalmology. 1990; 38(3):114-123.

Dr. BEHSHAD is an assistant professor of Ophthalmology and Cataract, Cornea & External Diseases Chief at Emory Eye Center at Emory St. Joseph’s Hospital in Atlanta. His interests include corneal diseases, transplantation, and surgery (PKP, DSEK, DMEK, DALK), and severe ocular surface disease. Dr. Behshad has been the director of the Clarkston Community Health Center’s Eye Clinic, a free clinic for uninsured and underinsured patients in Clarkston, Ga. Disclosures: None relevant to this topic.