ASCRS News: EyeWorld Journal Club
December 2022

by James Tian, MD, Esteban Peralta, MD, Katherine Peters, MD, Sri Meghana Konda, MD, Cason Robbins, MD, C. Ellis Wisely MD, and Pratap Challa, MD

Accurate IOL calculation in eyes with a history of refractive surgery is challenging but important, especially given the high patient expectations in this population.^1,2 Alongside intraoperative aberrometry and ray tracing technology, multiple formulas have been developed to account for the change in refractive and anatomic assumptions after ablative refractive surgery with no single formula demonstrating proven superior efficacy.³ This has led to the development of an ASCRS web-based tool that facilitates the simultaneous calculation of multiple formulas and generates an average recommended IOL power of the included methods.⁴ A review by the American Academy of Ophthalmology (AAO) of 11 studies found that the ASCRS calculator average provided the lowest residual refractive errors among all methods.⁵

However, downsides of using the web-based ASCRS calculator are the time and potential transcription error when manually transcribing data. Meanwhile, the Barrett True-K has been shown to be one of the most accurate methods for post-refractive eyes.^6,7 It is integrated into the IOLMaster 700 (Carl Zeiss Meditec) and Lenstar 900 (Haag-Streit), which eliminates the need for manual data entry into the web-based ASCRS calculator. In order to test whether manual transcription into the ASCRS calculator is necessary, the authors sought to compare the accuracy of the Barrett True-K biometer-embedded formula alone versus the other no-history formulas used in the ASCRS online calculator in a series of post-myopic and post-hyperopic ablation eyes.

Methods

The authors did a retrospective chart review of all cases of cataract surgery performed in patients with a history of myopic or hyperopic laser ablation performed in a single calendar year at their institutions. A variety of lenses and surgeons were included. Patients were excluded if they had a history of RK, incomplete data, corrected distance VA of worse than 20/40, or any intraoperative or postoperative complications that would skew final refractive data. 

All patients underwent preoperative biometry with the IOLMaster 500 or 700 as well as Atlas topography (Carl Zeiss Meditec) and Scheimpflug tomography. All data was entered into the ASCRS online calculator without historical data. The ASCRS calculator for post-myopic ablation outputs the recommended IOL power for the Barrett True-K No-History, Wang-Koch-Maloney, Shammas, Haigis-L, and Potvin-Hill Pentacam formulas, as well as an average IOL power from these formulas. The calculator uses the same process for post-hyperopic ablation patients for the Barrett True-K No-History, Haigis-L, and Shammas formulas.

The authors were able to calculate IOL prediction error by setting the actual postop refraction to the target refraction in the Barrett True-K and ASCRS calculators. Those calculators were used to calculate the predicted IOL power, and the difference between the predicted IOL power and the actual implanted IOL power was the IOL prediction error. The IOL prediction error was multiplied by 0.7 to estimate the refractive error at the spectacle plane since the ASCRS calculator does not provide an expected refraction for a given lens recommendation.

Results

In the post-myopic ablation group of 96 eyes of 81 patients, the mean IOL prediction errors were analyzed by a one-sample t-test for each formula to assess whether the error was statistically different from 0 (plano). The Barrett True-K, ASCRS calculator average, and Haigis-L did not have an IOL prediction error statistically different from 0, while the other calculators did. In the post-hyperopic group of 47 eyes from 34 patients, similarly, the Barrett True-K and ASCRS calculator average were not statistically different from 0. 

The mean refractive prediction error as well as the mean and median absolute refractive prediction errors were also analyzed. There was a statistically significant difference in the variances between the absolute refractive errors of all calculators, and post-hoc paired t-tests showed that the Barrett True-K also had the lowest standard deviation and residual absolute refractive error compared to all other formulas for both myopic and hyperopic ablations.

In addition, the Barrett True-K achieved the highest proportion of eyes with postop refraction within 0.25 D (44.8%) and within 0.50 D (71.9%), though this difference was not statistically different from the ASCRS mean proportion (38.3% for within 0.25 D and 65.6% within 0.50 D). 

Outlier analysis was also performed for eyes with greater than 1.00 D of IOL prediction error. In 12 post-myopic eyes and 6 post-hyperopic eyes where the IOL prediction error of the Barrett True-K was greater than 1.00 D, no other formula outperformed the Barrett True-K, and no outliers were detected in the biometry measurements. 

Discussion

This study demonstrated the advantages of the Barrett True-K formula in eyes for both post-myopic and post-hyperopic ablation. The authors demonstrated how the Barrett True-K formula produced the highest proportion of eyes within 0.25 D and 0.50 D of the target refraction, lowest mean and absolute refractive error, and lowest standard deviation of those errors. In addition, for 12 post-myopic and 6 post-hyperopic outliers, the Barrett True-K still outperformed all other formulas. 

This finding is significant because the Barrett True-K formula is embedded into the IOLMaster and Lenstar, improving convenience and eliminating the possibility for transcription error when transferring data into the online ASCRS calculator. The accuracy of the Barrett True-K formula for a refractive outcome within 0.50 D of the target is approaching the accuracy reported in the literature for eyes even without a history of refractive surgery. 

It should be noted that the cataract surgeries included in the study included different IOL manufacturers and types, as well as different surgeons. This is both a weakness and a strength because the results may then be applied to a wider group of surgeons and IOL types. 

During our Journal Club discussion, residents pointed out that additional strengths include the large number of post-hyperopic ablation eyes in the study, 47 eyes from 34 patients, when much of the literature focuses on patients with post-myopic ablations. All patients had both Atlas and Pentacam (Oculus) imaging, thus allowing for more formulas to be used and averaged (only the Galilei [Ziemer] and OCT-based formulas were not used). 

We noted that by forgoing the ASCRS calculator, we lose the range of recommended IOL powers in favor of a single IOL power generated by the Barrett True-K. While there is no way to test the benefit of having a range of values, it may be reassuring to see a range to decrease the risk that any one formula would create an outlier. An outlier analysis was performed on 12 myopic and 6 hyperopic outliers; however, this outlier sample size may be insufficient to reassure against seeing a range of calculators. 

We also noted the that the study included the Barrett True-K in the ASCRS average to which the Barrett True-K was compared with. For future directions, we are interested in the results of a similar analysis in post-RK eyes as well as a comparison with the Barrett True-K Total Keratometry. 

Conclusion

The authors demonstrated that for eyes with previous myopic or hyperopic laser refractive surgery, the Barrett True-K formula built into the biometer was equal or non-inferior to the multiple method approach using the ASCRS online calculator. The advantages of using the Barrett True-K formula built into the biometer include elimination of transcription error risk and reduced time and personnel needed. 

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IOL power calculations after LASIK or PRK: Barrett True-K biometer-only calculation strategy yields equivalent outcomes as a multiple formula approach

Ferguson TJ, et al. J Cataract Refract Surg. 2022;48:784–789 

• Purpose: To compare the accuracy of intraocular lens (IOL) power calculations performed using the biometer-embedded Barrett True-K formula versus a multiple formula approach using the ASCRS post-refractive calculator in eyes with prior myopic or hyperopic refractive surgery. 
• Setting: Cleveland Clinic Cole Eye Institute, Cleveland, Ohio  
• Design: Retrospective, consecutive case series
• Methods: Patients who underwent cataract surgery with a history of corneal refractive surgery were included. For each formula, the IOL prediction error (PE) and refractive PE was calculated. Main outcome measures included mean absolute error (MAE) as well as the percentage of eyes within ±0.25, ±0.50 and ±1.00 diopters (D). 
• Results: A total of 96 post-myopic eyes and 47 post-hyperopic eyes were analyzed. In the post-myopic cohort, the Barrett True-K had the lowest MAE (0.36 D) followed by the Haigis-L (0.41 D). The Barrett-True K had a significantly higher percentage (44.8%) of eyes within ±0.25 D in comparison to the Haigis-L (34.4%), which had the second highest percentage (P<.01). In the post-hyperopic cohort, the Barrett True-K had the lowest MAE (0.41 D) followed by the ASCRS–mean (0.46). The Barrett True-K and ASCRS–mean had the highest percentage of eyes within ±0.25 D (42.6% vs 38.3%, P=.16).  
• Conclusions: The Barrett-True K formula built into the biometer performed equivalently to a multiple formula approach on the ASCRS online calculator in both post-myopic and post-hyperopic eyes. This approach reduces the potential for transcription error from data entry for lens power calculation approaches that require manual data entry.

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References

1. Wang L, Koch DD. Intraocular lens power calculations in eyes with previous corneal refractive surgery: review and expert opinion. Ophthalmology. 2021;128:e121–e131. 
2. Randleman JB, et al. Intraocular lens power calculations after laser in situ keratomileusis. Cornea. 2002;21:751–755. 
3. Wen D, et al. Network meta-analysis of no-history methods to calculate intraocular lens power in eyes with previous myopic laser refractive surgery. J Refract Surg. 2020;36:481–490. 
4. Wang L, et al. Evaluation of intraocular lens power prediction methods using the American Society of Cataract and Refractive Surgeons post-keratorefractive intraocular lens power calculator. J Cataract Refract Surg. 2010;36:1466–1473. 
5. Pantanelli SM, et al. Intraocular lens power calculation in eyes with previous excimer laser surgery for myopia: a report by the American Academy of Ophthalmology. Ophthalmology. 2021;128:781–792. 
6. Vrijman V, et al. Evaluation of different IOL calculation formulas of the ASCRS calculator in eyes after corneal refractive laser surgery for myopia with multifocal IOL implantation. J Refract Surg. 2019;35:54–59. 
7. Abulafia A, et al. Accuracy of the Barrett True-K formula for intraocular lens power prediction after laser in situ keratomileusis or photorefractive keratectomy for myopia. J Cataract Refract Surg. 2016;42:363–369. 

Contact 

Challa: pratap.challa@duke.edu