Pros and cons of wavefront-guided photorefractive keratectomy

KEYWORDS:

• Wavefront-guided
• photorefractive keratectomy

1. Introduction

Photorefractive keratectomy (PRK) is a widely used keratorefractive procedure for the correction of refractive error. Conventional PRK aims to treat lower order aberrations, sphere and cylinder, whereas wavefront-guided (WFG) PRK aims to treat both lower and higher order aberrations (HOAs). The currently available literature shows that WFG PRK is not only safe and predictable, but also has excellent visual outcomes and patient satisfaction [1–5]. Nonetheless, there are important considerations when counseling patients regarding refractive surgical options and expectations. Here we review the pros and cons of WFG PRK.

2. Pros of wavefront-guided PRK

2.1. Equivalent outcomes to WFG LASIK and other refractive platforms

WFG PRK has been shown in multiple studies to be equivalent to WFG LASIK [2,3]. When comparing WFG PRK to WFG LASIK or other laser based keratorefractive modalities (e.g., wavefront-optimized (WFO) ablations, small incision lenticule extraction (SMILE), etc) after the initial 3 months postoperatively, outcomes are generally equivalent [1]. Moshifar et al showed that WFO PRK and WFG PRK achieve equivalent results in all regards except slightly superior low contrast visual acuity WFG group as well as an increase in low contrast visual acuity relative to preoperative values [4]. Confirming this trend, Sia et al showed comparable visual results, subjective visual symptoms and patient satisfaction for WFG and WFO PRK, again with improved low contrast visual acuity in the WFG cohort [5]. He and Manche also showed equivalency when comparing WFO and WFG PRK [6]. Long the predominant form of refractive surgery performed in the United States military, a recent paper showed WFO and WFG PRK afforded excellent and comparable visual outcomes and performance on military specific visual tasks [7]. Recent technological advances to include a new high-resolution aberrometer have continued to improve results. Schallhorn et al published excellent early results, 3 months post-op, for high-resolution aberrometer based WFG PRK comparable or superior to other studies using lower resolution based aberrometers [8]. Although the direct comparisons of WFG PRK to some other modalities are few, direct comparisons of WFO PRK to WFG PRK and WFG PRK to WFG LASIK show equivalency and logic suggests that WFG PRK is generally equivalent to other keratorefractive options like WFG LASIK, WFO PRK and WFO LASIK and definitely superior to a conventional ablation profile. In sum, WFG PRK affords excellent refractive outcomes and does not represent a compromise in long term outcomes. In fact, with newer technology like the high-resolution wavefront aberrometer based ablations that have been shown to have superior low contrast visual acuity with LASIK, we expect similar results for WFG PRK to be forthcoming in future studies [9].

2.2. Potentially lower risk for post-refractive ectasia

Post-refractive ectasia is one of the most feared complications of laser vision correction, especially given its elective nature. Identifying patients at increased pre-operative risk has been, and continues to be, an area of active research. There are many risk factors that increase a patient’s risk: (1) preoperative topography, (2) residual stromal bed thickness, (3) age, (4) preoperative corneal thickness, (5) degree of myopia, (6) percent tissue altered > 40%, (7) family history of ectasia [10–13]. Yet, to date no single screening tool or metric has been able to accurately identify all patients at high risk for post-refractive ectasia. Mysterious cases without clear pre-operative risk factors continue to confound refractive surgeons. Surface ablations theoretically induce less structural change and are thought to have less risk of post-refractive ectasia but there are no validated screening tools or risk predictors for surface ablations. Sorkin et al evaluated all post-PRK ectasia cases reported in the literature to retrospectively identify risk factors and showed abnormal topography and pre-operative thin corneas to be the most significant risk factors [14]. As noted by these authors and others, a dedicated risk scoring system for PRK is needed given the paucity of data using the LASIK risk assessment tools for surface ablations. There is less known regarding true ectasia risk after PRK mostly due to the rarity of its occurrence. As noted by Santhiago et al, nearly all eyes with PRK-associated post-refractive ectasia have had abnormal topographies pre-operatively whereas some post-LASIK ectasia cases were normal pre-operatively [11]. Some surgeons will recommend PRK for patients with asymmetric but not highly concerning topographies or thinner corneas under the assumption that PRK is less of a biomechanical insult to the cornea.

The evidence for the safety of this approach is not well established in the literature. From a biomechanical standpoint the evidence in this regard is mixed: there are studies showing no significant difference in corneal biomechanical properties after either PRK or LASIK [15–17]. Others show more biomechanical change, but no clear clinical difference [18]. Although to our knowledge neither author has a had a patient develop post-refractive ectasia after PRK, we likely have a lower threshold than some surgeons for recommending PRK instead of LASIK or declining to treat for borderline cases. Our experience would support the body of clinical experience that suggests PRK is less of a biomechanical insult to the cornea.

2.3. No risk for flap-related complications

LASIK flap-related complications are thankfully infrequent. One example is flap dislocation with an incidence of 0.012% [19]. LASIK flaps made with femtosecond laser are likely more stable than microkeratome flaps and the risks of serious flap-related complications has decreased with this technology [20]. There is still some risk, even if minimal, and some patients’ profession or activities may warrant a surface ablation instead of assuming the additional lifetime risk of a flap-dislocation, martial artists are a classic example.

3. Cons of wavefront-guided PRK

3.1. Slower visual recovery and increased discomfort

Slower visual recovery time and increased discomfort are the primary drawbacks for PRK. As mentioned above, after the initial 3 months of healing PRK outcomes are equivalent to LASIK outcomes; yet, prior to that time point, both objective and subjective measures of vision are notably worse [2,21]. Our practice pattern, like most refractive surgeons, is to recommend the speedier visual recovery afforded by other modalities like LASIK and SMILE when possible and reserve PRK for those patients that may not be well qualified for LASIK or SMILE (i.e. thinner corneas) or have self selected for PRK surgery.

3.2. Risk of corneal haze and associated factors

The risks and complications for PRK generally align with those for LASIK. There are some PRK specific risks. A few of the most important ones center on mitigating the risk of corneal haze. Corneal haze after PRK is a potentially vision threatening complication that has been reported to occur in as many as 2-10% of cases. Consequently, there are multiple strategies to mitigate this risk, each with its own drawbacks. Mitomycin-C (MMC) has been used successfully for both conventional PRK, but more recently with WFG PRK and shown to be effective in reducing risk of haze [22,23]. MMC use is safe when using well established concentrations such as 0.01% or 0.02%, but complications have occurred with an increase to 0.04%. Both authors’ practice pattern is to confirm the concentration at the time of surgery for all cases where MMC will be used, and we use MMC for nearly all PRK cases.

Another strategy to mitigate risk of corneal haze is by modifying the steroid taper regimen. Some surgeons use more steroids with higher ocular penetrance and higher risk of steroid response, like dexamethasone and others use longer tapers of less potent steroid drops like fluorometholone. Busool et al in a retrospective case series showed an overall incidence of 2.97% of steroid response after PRK and others have shown rates up to 8% with modern laser technology [24,25]. Their study also confirmed higher rates of steroid response ocular hypertension in the subset of patients using dexamethasone rather than fluorometholone as well as the group on dexamethasone for a longer period. Perhaps most germane, they showed a statistically significant increased risk for steroid response ocular hypertension in those patients that developed corneal haze as these patients were treated with stronger steroids for a longer duration.

3.3. Increased cost for pre-operative evaluation and workflow

For WFG ablation profiles, whether PRK or LASIK, wavefront aberrometry data must be acquired. Typically, 4 to 6 scoptopic, undilated scans are obtained. Additionally, these scans are compared to the cycloplegic refraction and in some cases dilated, cycloplegic wavefront aberrometry scans are performed to ensure nonaccommodative wavefront scans have been obtained. Only scotopic non-pharmacologically dilated scans are used for treatment planning. When comparing the clinic workflow to a wavefront-optimized (WFO) workflow there is an additional time cost as WFO ablations do not need these. Additionally, there is the additional capital cost of the aberrometer.

In conclusion, WFG PRK expands the eligible patient population to those who may not qualify for other refractive modalities. It achieves excellent outcomes, equivalent to alternatives in the long term, with few major drawbacks. The primary complications or limitations are typically easily managed.

Declaration of interest

EE Manche is a consultant for Allergan, Avedro, Clarity Vision Technologies, Johnson & Johnson Vision, and Shire; had research sponsored by Alcon, Allergan, Avedro, Carl Zeiss Meditec, Johnson & Johnson Vision, Presbia; has equity in RxSight, Clarity Vision Technologies, VacuMed; a VacuMed patent. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This paper was not funded.