Excimer Laser Correction of Astigmatism: Principles and Clinical Results

Fig. 10.1
Against the rule simple myopic astigmatism ablation profile. The laser treatment flattens the steep meridian


Fig. 10.2
Against the rule compound myopic astigmatism ablation profile. The laser treatment flattens the steep meridian, in addition to a spherical central myopic ablation, which takes into account the hyperopic shift of the cylindrical correction

In terms of technology, astigmatism was first treated with broad beam excimer lasers, through an aperture created by a mobile set of computer-controlled parallel blades. The astigmatic correction was controlled by the width and the speed of the blade movement, which results in a selective flattening of the meridian perpendicular to the long axis of the slit [3, 8, 9].

Second-generation lasers used a large scanning slit, which provided smoother ablations and larger optical and transition zones [10].

Third-generation platforms incorporated small-beam flying spots of 1 mm or less, combined with active eye trackers and sophisticated algorithms to precisely shape the cornea [11, 12].

Current modern laser platforms offer two approaches to correct spherocylindrical lower-order aberrations and also higher-order aberrations: wavefront-guided (WFG) and wavefront-optimized (WFO) ablations. These treatments do not obey the Munnerlyn formula but are based, for WFG ablations, on preoperative wavefront measurements of the whole eye, in order to decrease induced levels of higher-order aberrations (HOAs). In parallel, WFO ablations rely on population nomograms and add aspheric characteristics to balance the induction of spherical aberration. These WFG or WFO treatments use high-speed pulses placed pseudorandomly to correct sphere and cylinder simultaneously and ablate a customized toric volume. These optimizations theoretically diminish the induced wavefront aberration and improve the efficacy of the astigmatic correction.

10.2.2 Hyperopic Astigmatism

Hyperopia and hyperopic astigmatism are less frequent in refractive patient population. Their excimer laser treatments became available in the mid-1990s thanks to new ablation profiles, focused on the midperiphery of the cornea [13].

Simple astigmatic hyperopic laser treatment results in a steepening of the flattest meridian with a midperipheral selective ablation (Fig. 10.3 and Video 10.1). Compound hyperopic photoablation includes an additional annular spherical treatment to correct the spherical part of the refraction (Fig. 10.4 and Video 10.1). As opposed to myopic treatment, hyperopic photoablation is located at the periphery of the cornea, with a 6.5–9 mm optical zone, in order to decrease the risk of regression and improve the postoperative quality of vision [1416]. Theoretically, several ablation profiles can be applied depending on how we consider the sign of the spherical and cylindrical component of the refraction. However, with current laser platforms, simple and compound hyperopic astigmatic treatments tend to privilege a combination of positive cylinder and sphere notation, which implies a peripheral ablation and provides the best pattern to preserve the central cornea thickness with optimized stromal ablation depth [17].


Fig. 10.3
Against the rule simple hyperopic astigmatism ablation profile. The laser treatment steepens the flat meridian, with a peripheral ablation


Fig. 10.4
Against the rule compound hyperopic astigmatism ablation profile. The laser treatment steepens the flat meridian, in addition to a spherical peripheral hyperopic ablation, which takes into account the myopic shift of the cylindrical correction

10.2.3 Mixed Astigmatism

The treatment of mixed astigmatism has been a significant challenge to refractive surgeons for quite some time. Incisional techniques, such as AK, were considered an adequate option to reduce astigmatism. In particular, they were used to correct mild and moderate degrees of astigmatism. However, they are reserved for patients with a spherical equivalent close to zero as it is known that arcuate incisions flatten the steep meridian with an associated flattening effect 90° away, on the flat axis, with a negligible coupling effect, leaving the spherical equivalent of the patient almost unchanged.

Excimer laser treatments for mixed astigmatism were developed at the end of the 1990s. This technology changed the surgical approach of mixed astigmatism, widening the pool of candidates and improving the refractive outcome. Mixed astigmatism ablation profiles are now based on a combination of previously detailed principles, a flattening of the steepest meridian and a steepening of the flattest one [18, 19].

Initially, first ablation patterns involved the use of a toric ablation to flatten the steep meridian, in combination with a hyperopic ablation to compensate for the residual hyperopic shift [20]. Other options included a steepening of the flat meridian combined with a myopic spherical ablation [8].

Subsequently, another ablation pattern, named bitoric LASIK, consisted of flattening the steep meridian doing a cylindrical ablation, in combination with a paracentral ablation over the flat meridian to steepen it. Its rationale was to provide the largest optical zone to prevent visual symptoms and to correct the hyperopic shift induced by the toric ablation over the steep meridian. Additionally, it compensated for the need of a spherical ablation to correct the residual myopia after the toric ablation of the flat meridian [21].

Current laser platforms optimize the treatment to reduce stromal depth ablation and provide an improved quality of vision. The ablation profile prioritizes peripheral hyperopic treatment combined with minimal central myopic treatment [10] (Fig. 10.5).


Fig. 10.5
Against the rule mixed astigmatism ablation profile. The laser treatment steepens the flat meridian, in addition to a spherical peripheral myopic ablation, which takes into account the myopic shift of the cylindrical correction

10.3 Patient Selection

Astigmatic excimer laser surgery can be performed on refractive and cataract patients. Refractive patients must be older than 18 years of age and with a stable refraction for at least 1 year before surgery. Exclusion criteria for surgery are pregnancy and nursing, any active ocular or systemic disease affecting corneal healing, and keratoconus suspect. A minimal estimated residual posterior stromal bed of 250 μm is required to allow for LASIK technique and preserve corneal architecture integrity. Today, a more conservative 300 μm posterior stromal bed thickness limit has become more widely accepted. In cataract patients, photoastigmatic laser surgery is a valuable option when the postoperative refractive result needs to be fine-tuned. Incisional techniques can be used with success, with limited additional cost, when the residual refractive error is limited and the spherical equivalent is close to zero. For significant spherical or astigmatic refractive errors, LASIK can be performed 3 months after the surgery and provides excellent results. However, this technique has a certain cost, which implies a proper patient counseling preoperatively to guarantee patient satisfaction. Ideally, the astigmatism needs to be regular and symmetrical. In terms of magnitude, myopic and hyperopic astigmatisms can be corrected up to 6 D of cylinder. The latest laser platforms provide fast photoablation, thanks to high ablation rate, optimized ablation profiles with large optical and blend zones, and efficient rotational eye tracking systems able to compensate for cyclotorsional movements between seated and supine position and during the photoablation as well. Advances in excimer laser technology have significantly reduced the risk of postoperative haze after PRK. Surface ablation remains a primary indication in patients with thin corneas or asymmetrical astigmatisms, whenever LASIK is susceptible to expose the patient to a potential postoperative ectasia. The use of MMC during surface excimer laser ablation has proven to be safe and beneficial in terms of haze prevention, particularly for treatments superior to −4 to −6 D of myopia, astigmatism greater than 1.25 D, or ablation depth superior to 50–75 μm. The most important effect of MMC in preventing haze is inhibition of keratocyte proliferation [22]. Today, published studies agree on a standard concentration of 0.02%, with a brief exposition time of 10 s. The application of MMC must be confined to the central part of the cornea, and prevent any contact with the limbus and conjunctiva, to avoid reepithelialization delay (Video 10.1). A nomogram adjustment is recommended to avoid overcorrection, up to 8%, as the use of MMC minimizes the postoperative hyperopic regression during the reduced wound healing process [23].

10.4 Clinical Results

Over the last 25 years, excimer laser treatment of astigmatism has become the technique of reference with excellent outcomes in terms of safety and predictability. Clinical results have certainly improved with technological evolutions. Constant improvements in laser platform software and hardware, coupled with surgical technique evolutions, such as PARK and then LASIK, have enabled more reliable photoastigmatic treaments and faster visual recovery.

10.4.1 Myopic Astigmatism

Surface photoastigmatic keratectomy has demonstrated a high degree of predictability and stability for low and high levels of myopic astigmatism. Published studies showed a mean astigmatic correction ranging between 70 and 90% depending on the magnitude of the preoperative cylinder. In terms of axis shift, more than 90% of the eyes were within 10° of the preoperative and intended axis. Uncorrected visual acuity (UCVA) reached 20/40 and 20/20 in 84 and 49% of the cases, respectively [24, 25]. However, visual results of photoastigmatic keratectomy can be influenced by several factors such as laser technology, magnitude of astigmatism, degree of combined myopia, and wound healing. Some authors found that PARK was better in patients with a preoperative cylinder comprised between 1 and 2.50 D and with myopia inferior to 10 D [26]. Other authors found PARK less effective to correct low degree of astigmatism inferior to 1 D, with a mean reduction of cylinder of 47% compared to the 70–80% mean reduction in high and moderate astigmatisms, respectively [27]. Interestingly, recent studies have showed that PARK and LASIK procedures for astigmatism >3 D provided comparable results in terms of efficacy, safety, and predictability. Vector analysis revealed only a slightly better significant correction index of LASIK over PARK [28].

Over the last decade, surface ablation gave way to LASIK, which became the primary surgical option for most refractive surgeons worldwide, and is still considered the reference. Today, most platforms are equipped with efficient three-dimensional eye tracking systems, particularly indicated in astigmatic patients. We know that the preciseness of the axis treatment is crucial as 1° shift implies 3% of undercorrection of the cylinder. Studies have shown that cyclotorsion between seated and supine position may occur in a small percentage of our patients and also with large amplitude during the photoablation in some cases [29]. Delivery systems may differ, and proprietary algorithms allow for each laser manufacturer to offer sophisticated ablation profiles and propose customized algorithms, such as wavefront-guided (WFG) or wavefront-optimized (WFO) treatments, tissue-saving, or topography-guided ablations. All platforms strive to optimize efficacy, safety, and predictability of the refractive outcome, which implies the most reliable correction of lower-order spherocylindrical refractive errors. However, such conventional treatments may induce postoperative visual complaints, such as halos and glare linked to increased higher-order aberrations [30]. Specific ablation patterns have been developed to counteract these side effects. First, WFG algorithms were proposed to generate a custom ablation based on preoperative aberrometry measurements in order to decrease induced higher-order aberrations [3133]. Wavefront-guided treatments require a concordance between subjective refraction and wavefront refraction, with a less than 0.75 D of difference for the sphere and the cylinder. Subsequently, WFO patterns have been developed to create an aberration-neutral aspheric ablation profile based on a population nomogram [34]. In personal retrospective study, performed on 88 consecutive eyes, we evaluated the efficacy and predictability of compound myopic astigmatic LASIK, with a WFO high-speed laser platform. Mean preoperative spherical equivalent was −3.95 ± 1.10 D, ranging from −1.00 to −8.00 D, and mean cylinder was −0.98 ± 0.65 D, ranging from −0.75 to 2.75 D. At 6 months, the mean residual astigmatism was −0.25 ± 0.20 D. Ninety-five percent of the patients were 20/25 or better, and 80% reached 20/20 or better. Eighty-five percent of the eyes were within ±0.50 D of emmetropia and 98% within ±1.00 D. Ninety percent of the eyes had 0.50 or less of residual astigmatism, and all the eyes had 1.00 D or less (Graph 10.1a, b).


Fig. 10.7
(a) Manifest astigmatism at 6 months. (b) Efficacy at 6 months (UCDVA uncorrected distance visual acuity)

Comparative studies of astigmatic outcomes between WFG and WFO LASIK with the same laser platform showed similar astigmatic results. In fact, surgically induced astigmatism, difference vector, magnitude of error, correction index, flattening effect, and index of success were not statistically different with both ablation profiles. The angle of error was less important in the WFG treatment group [35]. Comparative studies with different laser platform showed satisfactory outcomes in terms of UDVA, CDVA, spherical correction, and preservation of higher-order aberrations, with some differences for the cylinder correction [36].

In terms of astigmatic correction, clinical results may depend on the magnitude of attempted correction. Several studies have demonstrated that low preoperative cylinder of < 0.75 D tended to be overcorrected when combined with low, moderate, and high myopia. Vector analysis indices, such as correction index, magnitude of error, index of success, and flattening index, suggested significant overcorrection of the cylinder correction. This overcorrection was independent of the degree of spherical ametropia and the preoperative cylinder axes. The authors of these studies recommend caution about treating full refractive cylinders of <0.75 D using wavefront-optimized LASIK [37, 38]. Other studies, performed to evaluate the effectiveness of LASIK to correct moderate to high degrees of astigmatism, superior to 2 D, with an aberration-neutral ablation profile, showed very good visual, optical, and refractive results. Mean decrease of astigmatism was 93%, with 72% of eyes within ±0.50 D of intended correction and 92% within ±1.00 D. Uncorrected postoperative visual acuity was 20/25 or better in 96% of the patients [39]. Other studies evaluated the refractive results in patients with high cylinder, > to 3 D, using a fast repetition rate excimer laser with optimized aspheric profiles and cyclotorsion control. Six-month results showed UDVA of 20/40 or better in 94% of the cases and 20/20 or better in 61%. Eighty-seven percent of the patients were within ±0.50 D and 97% within 1.00 D of emmetropia. Sixty-seven percent of the patients had 0.50 D or less of residual astigmatism and 93% 1.00 D or less. Vector analysis showed a slight undercorrection of the astigmatic component of the refraction [40].

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Feb 4, 2018 | Posted by in OPHTHALMOLOGY | Comments Off on Excimer Laser Correction of Astigmatism: Principles and Clinical Results

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