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Is Phorcides more likely to give better vision than treating the manifest refraction?

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

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Journal of Cataract & Refractive Surgery: October 2020 - Volume 46 - Issue 10 - p 1451-1452
doi: 10.1097/j.jcrs.0000000000000386
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We read with interest the article by Lobanoff et al., “Clinical outcomes after topography-guided LASIK: comparing results based on a new topography analysis algorithm with those based on manifest refraction.”1

We noted that, in the manifest group, there was a trend to better postoperative spherical equivalent refraction (SEQ) accuracy within ±0.25 diopter (D) (84.9% in manifest group vs 81.4% in Phorcides Analytic Engine group [Phorcides LLC]), even if not statistically significant. Although postoperative mean refractive astigmatism accuracy was similar overall (0.15 D ± 0.33 manifest vs. 0.16 D ± 0.32 Phorcides), it was statistically inferior in the eyes in the Phorcides group matched to the U.S. Food and Drug Administration topography-guided customized ablation treatment study criteria (0.15 D ± 0.22 manifest vs. 0.20 D ± 0.23 Phorcides; P = 0.01).

Surprisingly, the lesser postoperative SEQ and refractive astigmatism accuracy results with Phorcides were accompanied with significantly better postoperative 20/16 uncorrected distance visual acuity (UDVA) (41.3% manifest vs. 62.5% Phorcides group) yet identical 20/20 UDVA. In all of our published outcomes studies, visual efficacy corresponded with refractive accuracy.2–4

The authors stated that the lower 20/16 rate in the manifest group “may be due to changes in the preoperative to postoperative cylinder axis,” but they did not conduct an astigmatism vector analysis or include the JCRS/Journal of Refractive Surgery standard target-induced astigmatism vs surgically induced astigmatism graph for verification. They comment that “the retrospective data here do not lend themselves to such detailed analysis.”1 An astigmatism vector analysis only requires refractive astigmatism data,5 which were available in that study. It would reveal whether the postoperative astigmatism was overcorrected at a new axis. Although this might explain inferior 20/16 outcomes in the manifest group, it would also highlight that eyes in the manifest group were subjected to an imprecise nomogram that could be improved.

The methods state “nomogram adjustments were permitted in both groups.”1 There is no detail whether these nomograms were discretionary, used by all surgeons, applied with the same rules, or identical for each group. A lack of nomogram standardization would directly impact the outcomes.

Considerably more patients were comanaged in the manifest vs the Phorcides groups (54% vs 35%; P < .01). This study design, where not all data were collected in a similar way, introduced inherent observer and confirmation biases due to lack of rigorous controls and standards on vision measurements between surgeons and numerous comanagers and a greater number of nonblinded surgeons collecting vision data in the Phorcides group. These shortcomings might explain the worse 20/16 vision outcomes in the manifest group.

We matched 3449 eyes to this study's inclusion/exclusion criteria and found that 65% of manifest-treated eyes achieved a postoperative UDVA of 20/16 or better (Figure 1). With well-calibrated nomograms, multiple accurate refractions, and standardized vision measurements, treating on the manifest refraction leads to visual outcomes that are better than those of the Phorcides group in this study.

Figure 1.
Figure 1.:
Percentage of eyes with postoperative cylinder ≤0.50 D, percentage of eyes achieving a postoperative CDVA of 20/16 or better, and percentage of eyes achieving a postoperative UDVA of 20/16 or better in the manifest and Phorcides groups of the study by Lobanoff et al., and in a large consecutive standardized series of 3449 eyes matched to the inclusion/exclusion criteria of the Lobanoff et al. study and treated on the manifest refraction using advanced big data nomograms, and a mean of up to 3 manifest refractions prior to nomogram calculations (CDVA = corrected distance viual acuity; UDVA = uncorrected distance viual acuity).

In summary, the conclusion “Phorcides increased the likelihood of 20/16 UDVA relative to using manifest” is questionable considering: the undefined criteria in choosing between manifest and Phorcides treatment, the clinical inconsistencies between visual acuity and refractive accuracy, the omission of vector analyses, the observation bias introduced from nonblinded surgeons assessing vision, and the confirmation bias of unmatched study groups with a larger number of comanaged patients in the manifest group with variability in vision measurement standards.


1. Lobanoff M, Stonecipher K, Tooma T, Wexler S, Potvin R. Clinical outcomes after topography-guided LASIK: comparing results based on a new topography analysis algorithm to those based on the manifest refraction. J Cataract Refract Surg 2020;46:814–819
2. Wallerstein A, Gauvin M, Cohen M. Effect of anterior corneal higher-order aberration ablation depth on primary topography-guided LASIK outcomes. J Refract Surg 2019;35:754–762
3. Wallerstein A, Gauvin M, Qi SR, Bashour M, Cohen M. Primary topography-guided LASIK: treating manifest refractive astigmatism versus topography-measured anterior corneal astigmatism. J Refract Surg 2019;35:15–23
4. 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
5. Alpins N. Practical Astigmatism Planning and Analysis. Thorofare, NJ: SLACK Incorporated; 2017
Copyright © 2020 Published by Wolters Kluwer on behalf of ASCRS and ESCRS