Setting Limits for PRK and LASIK

Photorefractive keratectomy (PRK) and laser-assisted in-situ keratomileusis (LASIK) are two of the oldest and most common refractive surgeries. But just because surgeons can use them doesn’t mean they should, especially when other treatment options exist, said Robert Edward T Ang MD.

Dr Ang pointed out excimer laser treatment range recom­mendations vary considerably by manufacturer, from up to -12.00 D to -16.00 D for myopia and +3.00 D to as much as +9.00 D for hyperopia. For astigmatism, recommendations range from -3.00 D to -7.00 D.

“But of course, like my car, I don’t drive 300 km per hour even if the speedometer will allow me to,” he observed. “We have our own comfort levels and go from there, even though the lasers can do this much treatment.”

Corneal anatomy and biomechanics, published literature, personal experience, and alternative treatments should all be considered when setting limits for PRK and LASIK, he added.

Thickness and topography

Dr Ang stressed the importance of calculating the residual stromal bed, achieved by deducting the flap thickness and the amount of tissue to remove from the preoperative total corneal thickness.

“Most systems have planning software,” he noted.

The literature suggests leaving at least 250 microns of residual stroma, but many surgeons who have been burned by ectasia opt for a thicker remaining cornea.

So, is there a maximum amount of tissue that can be ablated with PRK or LASIK? Higher treatments are associated with decreased outcome accuracy, regression, induction of higher-order aberrations, ectasia, and corneal scarring, Dr Ang noted. “The higher you go, the more risk you take on.” He sets boundaries and limits to avoid complications such as ectasia and postoperative haze and to aid in screening, coun­selling, and considering treatment alternatives.

Other factors that affect limits include topography and tomography. Compromised preoperative biomechanics will cause corneal decompensation, leading to ectasia. Dr Ang recommended using as many screening metrics as possible, including irregular and asymmetric bowtie topographies and Pentacam and Corvis corneal indices, such as BAD, TBI, and CBI.

“It’s not only about corneal thickness,” he explained. “Suspicious irregularities are as important, or maybe even more so.”

That goes for all forms of corneal refractive surgery, including PRK and SMILE, he added. “I actively look to disqualify from LASIK. It is a proactive approach.”

Consider options

But a disqualification for LASIK isn’t the end of the road, Dr Ang said. For suspicious but not abnormal or forme fruste corneas, he considers PRK or implantable collamer lenses (ICL). As for refraction, he does LASIK up to -5.00 D myopia with up to -2.00 D cylinder and hyperopia up to +3.00 D. For PRK, he will go to -7.00 D myopia with up to -2.00 D cylinder and does not do hyperopic PRK.

Dr Ang will not perform LASIK on corneas under 500 microns, regardless of topography or refractive error. He does no tissue ablation above 120 microns and leaves at least 300 microns of residual stromal bed for LASIK and 350 microns for PRK.

Limits, boundaries, and risk tolerance are based on screening data and personal judgment, with limits reducing with experience, Dr Ang concluded. “I know this lowers your volume of patients, but you sleep better at night.”

Dr Ang spoke at the 2024 ESCRS Congress in Barcelona.

Robert Edward T Ang MD is a senior consultant and head of cornea and refractive surgery at the Asian Eye Institute, Makati City, Philippines. angbobby@hotmail.com