Comment on: Clinical and refractive outcomes after topography-guided refractive surgery planned using Phorcides surgery planning software : Journal of Cataract & Refractive Surgery

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Comment on: Clinical and refractive outcomes after topography-guided refractive surgery planned using Phorcides surgery planning software

Wallerstein, Avi MD, FRCSC; Gauvin, Mathieu BEng, PhD

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Journal of Cataract & Refractive Surgery: December 2022 - Volume 48 - Issue 12 - p 1478-1479
doi: 10.1097/j.jcrs.0000000000001068
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We read “Clinical and refractive outcomes after topography-guided refractive surgery planned using Phorcides surgery planning software.”1 The authors claim, “When there are no HOAs in the eye's optical system, lower order cylinder measured by the manifest refraction will be identical to the astigmatism measured by the topolyzer.”1 This statement promotes the refuted notion that differences between anterior corneal astigmatism (ACA) and manifest refractive astigmatism (RA) are entirely caused by higher-order aberrations (HOAs). A large study found identical amounts of preoperative coma in eyes with small vs large RA to ACA discrepancies.2 If HOAs were causing this discrepancy, then greater HOAs would have been found in large discrepancy eyes. Posterior corneal astigmatism is the main cause.

The authors continue with “Localized elevations of the corneal topography are outlined by [Phorcides], and their effect on low-order astigmatism is calculated.”1 They assert that in virgin healthy corneas, anterior HOAs predictably affect refraction. Virgin corneas differ from traumatically, surgically, or biomechanically induced irregular corneas where newly created HOAs may affect refraction. The central corneal optical zone corresponding to a 3 to 5 mm pupil used to measure refraction has negligible HOAs. This area with minimal irregularity cannot induce a meaningful refractive effect that can then be predicted. This explains why no relationship was found between topolyzer-measured coma and the amount of discrepancy between RA and ACA in a study of 37 000 virgin eyes.3 Although letters to the editor for previous Phorcides studies addressed these issues, the authors omit to discuss studies that contradict their claims.4,5

The authors' study purpose was “to confirm earlier retrospective observations with the first prospective multicenter clinical trial.” The registered clinical trial (NCT04495829) states the main outcomes measure as being “residual refractive cylinder,” yet the authors do not compare it with earlier Phorcides studies. Their supplementary comparative table shows their EX500 laser outcomes vs the 400 Hz wavelight laser U.S. Food and Drug Administration (FDA) trial. This comparison is made with an older technology with a slower repetition rate, without cyclo-torsional tracking, using earlier software, and without updated nomograms. The TCAT FDA study exclusively included eyes with small RA to ACA discrepancy, which have been shown to have greater preoperative cylinder—introducing another comparison bias.6 These factors make this historical data comparison of limited use.

Although the current prospective study reported exceptionally good outcomes, previous comparative and back-calculated Phorcides studies reported significantly inferior refractive and visual outcomes, revealing a lack of reproducibility (Table 1).7-11 The current prospective study reported 20/15 uncorrected distance visual acuity in 89% of eyes, compared with significantly lower rates of 63%, 60%, and 48% in previous studies (Table 1).1,7,8,11

Table 1. - Percentage of eyes: with postoperative cylinder ≤ 0.25 D and ≤ 0.50 D, achieving a postoperative CDVA of 20/15 or better, and a postoperative gain in CDVA lines in previous Phorcides studiesa
Study Type Postop Cyl ≤ 0.25 D (%) Postop Cyl ≤ 0.50 D (%) Average postop Cyl (D) 20/15 b UDVA postop (%) CDVA line gain (%)
Stulting et al. 1 Single arm—prospective 99 100 NA 89 46
Brunson et al. 7 Comparative—retrospective 96 100 0.03 ± 0.11 60 29
Lobanoff et al. 8 Comparative—retrospective 82 94 0.16 ± 0.32 63 43
Stulting et al. 10 Back-calculated—retrospective 36 69 0.39 ± 0.28 NA NA
Motwani et al. 9 Back-calculated—retrospective 11 c 51 c 0.55 ± 0.28 c NA NA
Stonecipher et al. 12 Contralateral—prospective NA NA NA 48 NA
NA = not available
aThe current study reports a residual refractive astigmatism ≤ 0.25 D in 99% of eyes, when previous Phorcides (comparative and back-calculated) retrospective studies reported lower rates of 96%, 82%, 36% and 11%. The average postoperative residual cylinder was also irreplicable. In the current study 20/15 UDVA is reported in 89% of eyes, with previous Phorcides studies having rates of 63%, 60% and 48%. These previous 20/15 rates are inferior to topography-guided studies that treated on the manifest refractive astigmatism, such as the 65% reported in 3449 eyes by Wallerstein and Gauvin (2020), or the 81% reported in 1027 eyes by Stonecipher at ASCRS (2020). References and details of the reanalysis will be provided on requests.
bWhen 20/15 was not available 20/16 was used.
cReanalyzed using the same back-calculations that were used in Stulting et al. (2020).

In summary, the conclusion “Phorcides can be used to optimize visual outcomes for the correction of myopia and myopic astigmatism” remains unsubstantiated considering the clinical inconsistencies in outcomes between this trial and previous Phorcides studies, the omission of peer-reviewed literature with contradictory findings, and a nondisclosed potential financial bias introduced from a trial sponsor managed by Dr. Lobanoff (American Corneal Consultants).


1. Stulting RD, Lobanoff M, Mann PMI, Wexler S, Stonecipher K, Potvin R. Clinical and refractive outcomes after topography-guided refractive surgery planned using Phorcides surgery planning software. J Cataract Refract Surg 2022;48:1010–1015
2. Wallerstein A, Gauvin M, Qi SR, Cohen M. Effect of the vectorial difference between manifest refractive astigmatism and anterior corneal astigmatism on topography-guided LASIK outcomes. J Refract Surg 2020;36:449–458
3. Wallerstein A, Gauvin M, McCammon K, Cohen M. Topography-guided excimer treatment planning: contribution of anterior corneal coma to ocular residual astigmatism. J Cataract Refract Surg 2019;45:878–880
4. Wallerstein A, Gauvin M. Disagreement between theoretical and actual Phorcides outcomes: is Phorcides inferior to treating on the manifest refraction? [Letter]. Clin Ophthalmol 2020;14:3829–3830
5. Wallerstein A, Gauvin M. Is Phorcides more likely to give better vision than treating the manifest refraction? J Cataract Refract Surg 2020;46:1451–1452
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