Over the last two decades, a notable evolution has occurred in customized laser ablations. The ablation profiles for conventional treatments do not consider the cornea’s asphericity and the eye’s higher-order aberrations. All conventional excimer laser profiles perform identical ablations in all patients with the same refractive error. Additionally, surgically induced corneal aberrations have been identified as contributing to decreased contrast sensitivity and issues such as glare, halos, and disturbances in night vision.

To achieve the best outcomes and enhance patient satisfaction, various “customized” ablation profiles have been introduced. These include wavefront-guided (WFG) and topography-guided (Topo-G) ablation, each offering a unique approach to addressing refractive errors. Topo-G ablation is distinct for its emphasis on topographical data to guide the laser reshaping of the cornea. The rationale behind Topo-G ablation is based on the cornea’s shape, and optical quality is intricately tied to the corneal topography. By taking into account the corneal surface’s unique characteristics, such as local irregularities and curvature variations, Topo-G ablation offers a tailored approach to corneal reshaping.

This review will explore the principles, clinical applications, outcomes, and comparative analyses associated with Topo-G excimer laser ablation. We will also delve into the importance of assessment and surgical planning clinical studies, technological advancements, and considerations that contribute to improving patient outcomes.

The main advantages and disadvantages of topo-guided treatments compared to wavefront-guided treatments can be listed as follows:

Studies comparing Topo-G with other ablation profiles for myopia and astigmatism have demonstrated comparable or superior clinical outcomes ( Table 1 ).

In a prospective randomized controlled contralateral eye study of 84 eyes, the Topo-G and WFO ablation profiles for myopia were compared. The six-month postoperative controls indicated that the Topo-G treatment group had statistically significantly higher UCVA. The postoperative spherical equivalent of the WFO group was -0,2 ± 0,07 D, whereas that of the Topo-G group was -0,016 ± 0,057 D. Additionally, the postoperative HOA of the Topo-G group was smaller than that of the WFO group, but the difference was not statistically significant. There was a decrease in most of the individual terms of HOAs in the Topo-G group, but it was only statistically significant in a vertical coma. The authors speculated that both ablation profiles provided good refractive results, but the Topo-G ablation induced fewer HOAs (25).

A prospective contralateral eye study from India compared Topo-G and WFO ablation in 60 eyes of 30 patients. Postoperative controls at six months showed similar visual gains in both groups, with less corneal asphericity change and lower aberrations in the Topo-G group (26).

Jain et al. compared Topo-G and WFO treatments in a 35-patient contralateral eye study and found that the Topo-G ablation group had better contrast sensitivity, less HOA induction, and less stromal ablation (4).

A recent meta-analysis compared Topo-G and WFO LASIK treatments to treat myopia with and without astigmatism. A total of 1168 eyes participating in 7 randomized controlled trials were included in the study. There were no statistically significant differences in the uncorrected distance visual acuity ratio. Topo-G LASIK exhibited more accurate postoperative refraction predictability and less surgically induced higher-order aberrations, spherical aberrations, and coma (27).

In another recent meta-analysis, the study compared Topo-G and wavefront-optimized treatment profiles. The analysis included a total of 1,425 eyes from 11 studies. No statistically significant differences were observed in the proportion of eyes achieving uncorrected distance visual acuity between Topo-G and WFO ablation procedures. After Topo-G ablation, a significantly higher proportion of patients’ eyes achieved postoperative refraction within ±0.5 diopter of the target refraction compared to those undergoing WFO ablation. The higher-order aberrations, spherical aberration, and coma were significantly lower in the Topo-G group (28).

Studies comparing SMILE and Topo-G treatments are also available in the literature. In a prospective randomized contralateral eye study by Kanellopoulos et al. in 2017, SMILE and Topo-G treatments were compared to treat myopia and myopic astigmatism. 44 eyes of 22 patients were included in the study. In the methodology of the study, the astigmatic axis and power in the Topo-G treatment group were modified according to the topographically measured astigmatic axis and power. Three months postoperatively, 86,4% of the Topo-G group and 68,2% of the SMILE group had UDVA of 20/20, and 59,1% and 31,8%, respectively, had UDVA of 20/16. The difference is statistically significant in favor of the Topo-G. Also, the preoperative CDVA vs postoperative UDVA difference indicated that 9,1% of the eyes in the Topo-G group and 4,5% of the eyes in the SMILE group gained two lines. The Topo-G group was statistically significantly more successful when both methods were compared regarding residual manifest spherical equivalent and residual refractive astigmatism. The authors speculated that Topo-G LASIK was superior to SMILE in all visual performance parameters studied, both subjective and objective (29).

In a study conducted by Yang et al. on 46 eyes, the clinical efficacy of Topo-G LASIK and SMILE treatment was compared. The study findings demonstrated that both treatments had similar clinical outcomes after three months. However, the Topo-G LASIK group exhibited better early postoperative UCVA and better contrast sensitivity in scotopic conditions (30).

Topography-based ablation aims to regularize corneal irregularities. The literature provides various cases, including ectatic corneal diseases, iatrogenic ectasias, minor or decentered optic zone treatment, post-keratoplasty cases, and diseases causing corneal irregularity.

Decentralized ablation is a rare complication of refractive laser surgery. Clinical complaints include visual distortion, astigmatism, halo, glare, monocular diplopia, and loss of visual acuity. The diagnosis is made by identifying the decentralized pattern of topography. Low decentration can negatively impact contrast visual acuity and lead to higher-order aberrations. On the other hand, decentration of more than 1.0 mm can significantly compromise visual performance (31). Nowadays, the incidence has decreased with the development of eye tracker and laser systems. In a case series of 11 eyes, Kymionis et al. reported that treatment of decentralized ablation with Topo-G ablation improved uncorrected and corrected visual acuity (32). In a retrospective case series, it was shown that topographically based treatment was effective and safe in the treatment of decentralization following refractive surgery complication (33).

A slight optic zone can result in complaints such as decreased visual function, halo, glare, and monocular diplopia, particularly in mesopic conditions. In the first generation of laser treatments, the optic zone was kept small to reduce stromal ablation, especially in high myopic treatments. If such complaints develop, reshaping the optic zone with a Topo-G laser to enlarge it can be helpful. Enlarging the optical zone guided by topography has been demonstrated to be more effective in improving corneal surface regularity and reducing patient symptoms (33, 34).

The combination of Topo-G laser and CXL can also be used to treat iatrogenic ectasias. Studies have shown that this treatment is effective and safe in halting the progression of ectasia and providing refractive and topographic stability, particularly in cases where it occurs after refractive surgeries such as LASIK (33, 35).

The Topo-G laser can also be used in cases of decentralization due to SMILE. An increase in visual acuity and a decrease in HOA were observed with Topo-G retreatment in a patient with visual complaints due to decentralization after SMILE surgery (36).

Corneal crosslinking therapy is a significant breakthrough in the treatment of keratoconus patients. This therapy has significantly reduced the need for corneal transplantation in such patients (37, 38). The increased strength of the collagen fibers in the cornea prevents disease progression and vision loss (39). The effectiveness of cross-linking treatment has led to the hypothesis that corneal irregularities can also be regularized in combination with excimer laser ablation. Research has demonstrated that irregular astigmatism can be reduced and best-corrected visual acuity (BCVA) can be improved with CXL + Topo-G PRK ablation (40–42).

Kanellopoulos was the first to report sequential CXL and final Topo-G ablation for the treatment of keratoconus in 2007 (43). After the success in this case and the demonstration of stability in a follow-up period of up to 30 months, successful results have been reported by the same author and others from all over the world, and different techniques have been described (44–47).

Although many studies describe different techniques, there still needs to be generally accepted methods. There is still controversy about whether treatment should be performed sequentially or simultaneously. Both approaches have advantages and disadvantages. In recent years, the simultaneous approach has been used more frequently. The advantages and disadvantages of simultaneous treatment can be listed as follows (3, 48, 49):

The disadvantages of simultaneous treatment are that corneal flattening and refractive changes may occur after CXL. In addition, CXL may cause excessive haze formation in some patients. Rarely there are patients who show progression despite CXL. Sequential treatment may be considered in these patients.

CXL treatment is recommended for all patients diagnosed with progressive keratoconus, regardless of age. In addition, it is reported that patients under the age of 35 may undergo CXL if the risk of progression is assessed at the time of initial diagnosis. The minimum corneal thickness recommended for patients undergoing combined CXL and Topo-G PRK is 450 microns (33, 46, 48, 49). The maximum ablation depth is recommended to be 50 microns. This value is arbitrary and based on the clinical experience of the authors (48). The minimum intraoperative stromal bed after maximum correction is recommended to be 350 microns. Ideal candidates for TG-PRK + CXL have a dioptric difference of less than 10 D across their cornea, calculated within a 6mm optical zone by measuring the difference between the flattest and steepest areas (47).

The localization of the cone is also effective in the success of treatment. One study compared two groups of patients who underwent Topo-G PRK and CXL for cone location and found that visual success was superior in the group in which the cone was located in the central 2 mm zone (50).

The primary objective is to achieve corneal regularization rather than refractive correction. Therefore, setting the optic zone at 5.5 mm is recommended to minimize tissue ablation. The aim is to correct a maximum of 70% of the spherical and astigmatic errors found. It is recommended that this correction is at most 50 microns (33, 44, 48).

This treatment is commonly used for younger patients with progressive keratoconus. It may also be used for stable keratoconus patients who cannot tolerate contact lenses. Older keratoconus patients who require cataract surgery may be preferred for corneal regularization before surgery. The procedure is not recommended for patients with corneal scarring, a history of herpes, or a high dioptric difference of 10D in the cornea due to excessive ablation. Additionally, it is not suitable for very thin corneas or advanced diseases (47).

It is essential to obtain sufficient high-quality topographical scans before treatment. Once at least six similar scans have been taken, treatment planning begins. The first step is to consider the ablation profile to be used to correct higher-order aberrations without considering refractive correction. To minimize stromal ablation, refractive correction is planned as an under-correction ( Figure 1 ).

In the original Athens protocol, after insertion of the lid speculum, the epithelium is exposed to 20% alcohol for 20 seconds and peeled with a sponge. After recognition by the eye tracker and cyclotorsion compensation, excimer laser ablation is performed aligned to the corneal apex. After 0.02% mitomycin solution was applied to the ablated tissue for 20 seconds, it was washed with 10 ml cold BSS solution; then, the stroma was saturated with 0.1% riboflavin solution every 2 minutes for 10 minutes. CXL is then applied using a UVA beam with a wavelength of 370 nm and an irradiance of 3 mW/cm (2) at a distance of 2.5 cm for 30 minutes (48). Day 45 patient appearence with keratoconus after combined topo-G and CXL treatment ( Figure 2 ).

Several studies showed that, in the keratoconic cornea, the epithelium is thicker over the depressed stroma and thinner over the cone, so topography-guided ablation, applied to the stroma, can correct only the irregularity that the epithelium has not compensated (51, 52). Manual epithelial removal allows debridement of variable epithelial thickness with resultant underlying contours that vary from preoperative epithelium on measurements.

For these reasons, some authors have suggested using PTK mode during the epithelial debridement stage to eliminate the compensatory effect of the epithelium. This ensures a uniform tissue ablation in the selected optical zone, resulting in underlying tissue similar to the preoperative topography (49, 53).

In 2019, the 10-year results of 144 eyes treated with the Athens protocol were published. These results show that patients’ refractive and topographic improvement was maintained over ten years. Ectasia stabilization was achieved in 94.4% of patients. Progressive hyperopic shift was observed in 3.5% of patients (54).

A review published in 2020 analyzed the literature on combined excimer laser and CXL treatment for keratoconus, with a total of 479 eyes included in the review. The literature suggests that CDVA, UDVA, and HOA in mild to moderate keratoconus patients improved with combined treatment without compromising the biomechanical stability of the cornea, but longer follow-up studies are needed (42).

SI: Writing – original draft, Writing – review & editing. CU: Writing – original draft, Writing – review & editing.