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Photorefractive Keratectomy Used to Be the Procedure of Choice Simply Because It Was the Only Show in Town. But, More Than A

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Photorefractive Keratectomy Used to Be the Procedure of Choice Simply Because It Was the Only Show in Town. But, More Than A OPTOMETRIC STUDY CENTER Corneal Refractive Surgery: Coming Full Circle Photorefractive keratectomy used to be the procedure of choice simply because it was the only show in town. But, more than a decade later, is there good reason to make it our top pick? By Richard B. Mangan, O.D. he concept of reshaping eyes included. Results early on with modern day microkeratomes uti- corneal tissue with a laser excimer laser PRK were excellent.3 lized during the stromal ablation without thermal collateral During this time, however, clini- revolution in the United States. Tdamage was first introduced cal trials were already underway Finally, we could offer patients a by Stephen L. Trokel, M.D., in in merging the pioneering work of bilateral, pain-free, convenient and 1983. Marguerite B. McDonald, Dr. Trokel and Dr. McDonald with immediately effective procedure. M.D., would later see excimer laser that of Drs. Barraquer and Ruiz. Stock volume soared, and laser technology through the human In 1949, ophthalmologist Jose centers began popping up all over clinical trials, with the first human Ignacio Barraquer, M.D., of Bogo- the place. eye being treated by her in 1989.1 ta, Columbia, theorized that the The result? Procedure volume Dr. Trokel’s vision of eliminating addition or subtraction of lamellar increased to more than 1,400,000 refractive errors via laser corneal tissue could modify the cornea’s cases in the year 2000, according refractive surgery came to fruition refractive power. Dr. Barraquer to Market Scope. And, through in October 1995 when the first conceptualized and developed a 2006, 24.6 million cases had been excimer laser received FDA approv- small handheld keratome, similar performed worldwide.4 LASIK had al in the United States for the treat- to a carpenter’s plane, which he quickly become the most common ment of mild to moderate myopia. used to resect layers of corneal tis- refractive procedure in the world.5 The laser used was the Summit SVS sue. Dr. Barraquer’s protégé, Luis During that time, newer genera- Apex (Summit Technologies, Inc.) Ruiz, M.D., would later expand tions of mechanical microkera- and the procedure was photore- on his ideas and develop the origi- tomes were being developed and fractive keratectomy (PRK). nal automated microkeratome. In femtosecond laser flap technology Despite the fact that we had no 1998, the FDA approved laser in was introduced (IntraLase Inc., significant long-term data to offer situ keratomileusis (LASIK). The 2001). More predictable flap cre- patients, roughly 70,000 PRK Chiron Automated Corneal Shaper ation had arrived and the excite- procedures were performed in the (ACS) and the Bausch + Lomb ment over LASIK was at an all-time United States in 1996.2 My own Hansatome were among the first high. While PRK was still used in 110 REVIEW OF OPTOMETRY NOVEMBER 15, 2010 cases of inadequate corneal thick- time, traditional ablation profiles trefoil and spherical aberration) ness or basement membrane dys- addressed second-order aberrations when compared to the LASIK- trophy, it became an afterthought only—notably myopia, hyperopia treated eyes. At six months post-op, at most. and astigmatism. However, stud- the root mean square (RMS, or sum In 2005, however, a panel of ies utilizing wavefront aberrometry total of higher-order aberrations) the world’s leading experts in laser showed that traditional PRK and for the PRK group was 0.45μm refractive surgery, cornea and LASIK procedures had a tendency to (+/- 0.13μm), representing a fac- ocular surface disease convened in increase higher-order aberrations.7,8 tor increase of 1.29. The LASIK Seattle during the American Society LASIK, especially, had been shown group had an RMS of 0.59μm of Cataract and Refractive Sur- to increase coma and spherical- (+/- 0.22μm), representing a factor gery (ASCRS) Summer Refractive like aberrations in larger myopic increase of 1.84. Congress.6 Moderated by Richard treatments.9 This provided some L. Lindstrom, M.D., this panel of explanation as to why patients with PRK Eliminates Concern Over experts gathered to discuss a noted postoperative 20/20 vision com- Flap-Related Complications resurgence in advanced surface plained about their quality of vision. While no refractive procedure is ablation techniques. The panel Since the introduction of wave- without the risk of intra- or postop- offered the following reasons to front-guided ablations, investigators erative complications, flap-related explain this renewed interest in sur- have shown that utilizing a wave- complications can be significant. face treatments: front-guided custom profile reduces With recent advancements in micro- • Fewer residual higher-order higher-order aberrations in eyes keratome technology, as well as the aberrations, as compared to that undergo LASIK.10 However, introduction of femtosecond flaps, LASIK. studies also show that in comparing intraoperative complications, such as • Concerns over LASIK flap- wavefront-guided PRK to wave- buttonhole flaps and free caps, are related complications. front-guided LASIK, PRK induces at an all-time low. Nonetheless, flap • Concerns over post-LASIK statistically fewer higher-order aber- complications still occur. ectasia. rations.11 A study out of Hong Kong • Concerns over LASIK’s effect Colleagues at the Moran Eye assessed the complication rate of on tear film stability. Center in Salt Lake City conducted a sub-Bowman’s keratomileusis (SBK) • Improved wound healing after prospective study comparing wave- in 3,009 eyes.13 The flaps were cre- PRK with mitomycin C (MMC). front-guided PRK and wavefront- ated with the IntraLase femtosecond In the following pages, we will guided LASIK; one of the outcome laser. Intraoperative complications look closely at these observations measures being higher-order aber- including flap tear, free cap, bubble from an evidence-based perspec- rations (HOAs).12 Some 104 eyes escape, and flap folds had occurred tive. received custom PRK, and 104 eyes at a complication rate of 0.33%. received custom LASIK. All were Postoperative flap-related complica- PRK May Be Better Suited for treated with Visx Star 4 IR Custom- tions occurred at a rate of 0.30% Wavefront Technology vue platform. and included diffuse lamellar kera- Wavefront-guided refractive Post-treatment wavefront analysis titis (DLK) and epithelial ingrowth. surgery received FDA approval showed that the PRK-treated eyes The aggregate peri-surgical flap in August 2002. Up until that had fewer residual HOAs (coma, complication rate in this series was Release Date: November 2010 treatments. This paper explores these and other reasons for the Expiration Date: November 30, 2013 resurgence in surface ablation techniques. Goal Statement: Refractive surgery has come a long way since its Faculty/Editorial Board: Richard Mangan, O.D. modest beginnings. When PRK was approved, clinicians reported Credit Statement: This course is COPE approved for 2 hours of CE excellent results. But, LASIK soon became the procedure of choice credit: COPE ID: 29708-RS. Check with your local state licensing board and has enjoyed it’s status ever since. Lately, however, there is to see if this counts toward your CE requirement for relicensure. renewed interest in surface treatments. For example, some clini- Joint-Sponsorship Statement: This continuing education course is cians note fewer higher order aberrations with these treatments, joint-sponsored by the Pennsylvania College of Optometry. compared to LASIK as well as some complications specific to LASIK Disclosure Statement: Dr. Mangan has no relationships to disclose. REVIEW OF OPTOMETRY NOVEMBER 15, 2010 111 OPTOMETRIC STUDY CENTER Courtesy: Scott Hauswirth, OD 1, 2. The pictures located above and at right are of the right and left eye of the same patient. Scans taken with Pentacam (Oculus). 0.63%. ed visual acuity.22,23 In comparison, a retrospective analysis of more than Several risk factors have been identified for post- 28,500 LASIK procedures that involved either the Chi- LASIK ectasia:24 ron Automated Corneal Shaper or Hansatome micro- • Thin cornea at baseline. keratomes determined an intraoperative complication • Thick corneal flap. rate of 0.302% and included partial flaps, buttonholes, • Low residual stromal bed (RSB). thin or irregular flaps, and free caps.14 • Excessive ablation. • Irregular corneal thickness. PRK Involves Less Risk of Corneal Ectasia • Diverse ablation rates. Iatrogenic corneal ectasia is a serious complication • Pre-existing keratoconus or forme fruste keratoconus that has been linked to LASIK; and in some cases has • High intraocular pressure (IOP). led to penetrating keratoplasty.15-17 The incidence of According to one recent study, abnormal topogra- corneal ectasia after LASIK has been estimated to be phy presents the greatest risk in the development of 0.2% to 0.66%.18,19 While ectasia can also occur with post-LASIK ectasia, followed (in order) by RSB thick- surface ablations, one group’s retrospective analysis of ness, age and preoperative corneal thickness.25 171 cases of ectasia determined that LASIK accounted Of greater concern to corneal refractive specialists is for 95.9% (n=164) of them.20 idiopathic ectasia. This is ectasia that develops despite Post-LASIK ectasia is most commonly seen within the absence of preoperative risk factors.26 four years of treatment21 and is characterized by cen- In 2003, investigators in Chicago reviewed 1,555 tral to inferior corneal thinning, steepening and irregu- potential cases of idiopathic post-LASIK ectasia found larity (figures 1 and 2). Compound myopic shifts in via refractive surgery-related internet bulletin boards. refractive error are common, as is loss of best-correct- Cases were considered idiopathic if the following 112 REVIEW OF OPTOMETRY NOVEMBER 15, 2010 criteria were met: (range 21μm to 125.4μm). • Calculated RSB greater than 250μm. • The mean calculated residual stromal bed was • Preoperative central pachymetry was not less than 299.5μm (range 254μm 373μm). 500μm. • Mean time to recognition of ectasia onset was 14.2 • Any and all K readings were less than 47.2D.
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