Journal of Refractive Surgery

Original Article 

Accuracy of Intraoperative Aberrometry, Barrett True-K With and Without Posterior Cornea Measurements, Shammas-PL, and Haigis-L Formulas After Myopic Refractive Surgery

Karen L. Christopher, MD; Jennifer L. Patnaik, PhD; D. Claire Miller, MS; Anne M. Lynch, MB, MSPH; Michael J. Taravella, MD; Richard S. Davidson, MD

Abstract

PURPOSE:

To assess the accuracy of intraoperative aberrometry, the Barrett True-K No History (Barrett TKNH), Barrett TKNH with posterior corneal measurements (Barrett TKNH with PC), Shammas-PL, and Haigis-L formulas in patients with cataract who had prior myopic refractive surgery.

METHODS:

This was a retrospective consecutive case series of patients with prior myopic refractive surgery undergoing cataract extraction. Mean absolute error (MAE) and median absolute error (MedAE) of refraction prediction were compared for each formula. Interactions of each biometry measurement were modeled for each formula to evaluate those with the most significant impact on refraction prediction.

RESULTS:

One hundred sixteen eyes of 79 patients were analyzed. MAE was 0.40 ± 0.33 diopters (D) for intraoperative aberrometry and 0.42 ± 0.31 D for the Barrett TKNH, 0.38 ± 0.30 D for the Barrett TKNH with PC, 0.47 ± 0.38 D for the Shammas-PL, and 0.56 ± 0.39 D for the Haigis-L formulas. Comparisons between formulas were significant for Barrett TKNH versus Barrett TKNH with PC formulas (P = .046), Barrett TKNH with PC versus Shammas-PL formulas (P = .023), and for all comparisons with the Haigis-L formula (P < .001), and not significant for all other comparisons (P > .05). Eyes were within ±0.50 D of prediction 73%, 72%, 69%, 62%, and 52% of the time for intraoperative aberrometry, the Barrett TKNH with PC, Barrett TKNH, Shammas-PL, and Haigis-L formulas, respectively. Corneal asphericity (Q value) was significantly associated with prediction error for all five methods. Changes in anterior chamber depth had a significant impact on Shammas-PL prediction errors.

CONCLUSIONS:

Newer technology using information from the posterior cornea modestly improved outcomes when compared to established methods for intraocular lens selection in eyes that had previous laser refractive surgery for myopia.

[J Refract Surg. 2021;37(1):60–68.]

Abstract

PURPOSE:

To assess the accuracy of intraoperative aberrometry, the Barrett True-K No History (Barrett TKNH), Barrett TKNH with posterior corneal measurements (Barrett TKNH with PC), Shammas-PL, and Haigis-L formulas in patients with cataract who had prior myopic refractive surgery.

METHODS:

This was a retrospective consecutive case series of patients with prior myopic refractive surgery undergoing cataract extraction. Mean absolute error (MAE) and median absolute error (MedAE) of refraction prediction were compared for each formula. Interactions of each biometry measurement were modeled for each formula to evaluate those with the most significant impact on refraction prediction.

RESULTS:

One hundred sixteen eyes of 79 patients were analyzed. MAE was 0.40 ± 0.33 diopters (D) for intraoperative aberrometry and 0.42 ± 0.31 D for the Barrett TKNH, 0.38 ± 0.30 D for the Barrett TKNH with PC, 0.47 ± 0.38 D for the Shammas-PL, and 0.56 ± 0.39 D for the Haigis-L formulas. Comparisons between formulas were significant for Barrett TKNH versus Barrett TKNH with PC formulas (P = .046), Barrett TKNH with PC versus Shammas-PL formulas (P = .023), and for all comparisons with the Haigis-L formula (P < .001), and not significant for all other comparisons (P > .05). Eyes were within ±0.50 D of prediction 73%, 72%, 69%, 62%, and 52% of the time for intraoperative aberrometry, the Barrett TKNH with PC, Barrett TKNH, Shammas-PL, and Haigis-L formulas, respectively. Corneal asphericity (Q value) was significantly associated with prediction error for all five methods. Changes in anterior chamber depth had a significant impact on Shammas-PL prediction errors.

CONCLUSIONS:

Newer technology using information from the posterior cornea modestly improved outcomes when compared to established methods for intraocular lens selection in eyes that had previous laser refractive surgery for myopia.

[J Refract Surg. 2021;37(1):60–68.]

Cataract surgery can be challenging in eyes with a history of laser refractive surgery. These patients tend to be more visually demanding because of their desire to return to spectacle independence following cataract surgery. However, eyes with previous refractive surgery are at a higher risk of having a refractive surprise following cataract surgery because of alterations in the ratio of anterior to posterior corneal curvature, difficulty in accurately measuring corneal curvature, and errors in estimating effective lens position in many formulas using post-refractive surgery keratometry.1–6 Multiple formulas, including the Haigis-L, Shammas-PL, and Barrett True-K, have been developed to improve the accuracy of intraocular lens (IOL) selection in eyes with previous refractive surgery; however, to date no formula has emerged as the clear preference.5,7–10 However, recent publications have suggested that the Barrett True-K formula (and the Barrett True-K No History [Barrett TKNH] formula for those eyes without data from before refractive surgery available)11 may be one of the most accurate formulas for eyes with a history of myopic laser refractive surgery.12–14

Use of posterior corneal measurements and total corneal power has also modernized lens calculation methods, especially in the subgroup of eyes with prior myopic laser refractive surgery.15 These data refine outcomes by improving on assumptions about the ratio of anterior to posterior corneal curvature and its effect on the index of refraction. Flattening of the anterior cornea while leaving the posterior corneal curvature unchanged, as is the case in myopic laser refractive surgery, overestimates the total corneal power because the standard keratometric index of refraction of 1.3375 can no longer be applied.16 No study has evaluated measured posterior corneal values using the Barrett True-K or Barrett TKNH formulas. Recent updates to the Barrett True-K formula have made this a possibility now that there is an additional option of inputting measured posterior corneal values from either the IOLMaster 700 (Carl Zeiss Meditec) total keratometry or Pentacam (Oculus Optikgeräte GmbH).

Conversely, intraoperative aberrometry has been used to aid in lens selection for patients after refractive surgery. The ORA system (Alcon Laboratories, Inc) can be used to obtain intraoperative Talbot-Moiré interferometry measurements following removal of the cataractous lens in the operating room and, in combination with preoperative biometry measurements, predicts an IOL to be implanted for a chosen refractive target.17 Several studies have shown that intraoperative aberrometry has comparable18 or improved19,20 refractive outcomes for patients after myopic refractive surgery compared with many other conventional formulas. We conducted this study to: (1) address gaps in knowledge regarding the comparison of the Barrett TKNH formula and intraoperative aberrometry, (2) evaluate outcomes of the Barrett TKNH formula with the addition of posterior corneal measurements in comparison to other available formulas, and (3) assess biometry values that may be predictive of one method being more accurate than another.

Patients and Methods

The study was conducted in compliance with the Declaration of Helsinki and was reviewed and approved by the Colorado Multiple Institutional Review Board.

We retrospectively extracted data from all patients with a history of myopic laser refractive surgery who underwent cataract surgery by two surgeons at our institution (MT and RD) between March 6, 2018 and September 24, 2019. Preoperative measurements were completed using the IOLMaster 700 (software version 1.80.10.61129). Using the Barrett TKNH formula,11 an IOL was chosen using the preoperative IOLMaster 700 biometry measurements.

Surgery was performed using the standard phacoemulsification technique either with or without the use of a femtosecond laser depending on surgeon preference. Intraoperative aphakic lens measurements were collected using the ORA device following instillation of a cohesive viscoelastic and use of a Barraquer tonometer to achieve intraocular pressure of approximately 21 mm Hg. The IOL selected for implantation was based on the surgeon's best prediction using information from both the Barrett TKNH formula and intraoperative aberrometry.

Variables collected from the IOLMaster 700 included keratometry, axial length, anterior chamber depth (ACD), and lens thickness. Data collected from the Pentacam (software version 1.21r43) included radius of curvature of the posterior cornea, central corneal thickness at the pachymetry apex, and corneal asphericity (Q value) of the central 6 mm of the front of the cornea.

Retrospective data on the intraoperative aberrometry–predicted refraction for the actual lens implanted were collected using the measurements from each surgery. The Barrett TKNH–predicted refraction for the actual lens implanted was also collected retrospectively using the Barrett TKNH formula V2.0, and Barrett TKNH with PC predictions for actual lens implanted were calculated using the Barrett True-K Formula V2.0 using posterior corneal measurements from the Pentacam. The refraction predicted by the Shammas-PL21,22 and Haigis-L23 formulas was calculated after programming the respective formulas into Excel software (Microsoft Corporation) and confirming accuracy of the results with a random set of eyes using each respective validated formula on the post-refractive surgery calculator on the American Society of Cataract and Refractive Surgery website ( https://iolcalc.ascrs.org/).

For the Barrett TKNH, Barrett TKNH with PC, Shammas-PL, and Haigis-L calculations, optimized lens constants from the User Group for Laser Interference Biometry (ULIB) were used.24

Postoperative refractions were performed by trained ophthalmic technicians. Spherical equivalent of the postoperative refraction was collected at the last visit available between 3 weeks and 4 months following cataract surgery and is reported in diopters (D). Refractive prediction error (RPE) was calculated by subtracting the predicted refraction from the actual refractive error for the lens implanted (RPE = actual refraction – predicted refraction). Mean absolute error (MAE) and median absolute error (MedAE) were reported both before and after zeroing out of the arithmetic mean error, as suggested by Wang et al.25

Eyes with incomplete data (lack of completed intraoperative aberrometry measurements or lack of follow-up for postoperative refraction) were excluded, as were eyes with corrected distance visual acuity (CDVA) worse than 20/40 postoperatively and eyes that had radial keratotomy in addition to laser refractive surgery.

Statistical analysis was performed using SAS software version 9.4 (SAS Institute, Inc). Continuous variables were described by numeric and absolute means with standard deviations and medians and ranges. One-sample t tests were used to assess whether RPEs from each formula were significantly different than zero. Comparisons between the groups were performed using linear modeling with generalized estimating equations to account for intrasubject correlation of some patients having 2 eyes included in the analysis. Various biometry values were graphed against the prediction error for each formula. The outcome of the numeric prediction error was modeled with type of formula and predictors of ACD and Q value to evaluate the significance of these interactions. A P value of .05 was considered statistically significant.

Results

Three patients were excluded for final CDVA worse than 20/40. Six additional patients were excluded due to lack of postoperative refractive data. This left 116 eyes from 79 patients, whose demographic and preoperative data are shown in Table 1. The average axial length was 25.9 ± 1.4 mm and the average keratometry value was 40.6 ± 2.1 mm, reflective of eyes with prior myopic laser ablations. Approximately half of the cases (51.7% or 60 of 116) were performed using a femtosecond laser, and the remainder using manual incisions and standard phacoemulsification technique. Implanted lenses included TECNIS monofocal (ZCB00 or PCB00, 74 lenses), monofocal toric (ZCTxxx, 5 lenses), and extended depth of focus (ZXR00 17 lenses and ZXTxxx toric, 3 lenses) (Johnson & Johnson), as well as Alcon monofocal (SN-60WF, 11 lenses) and monofocal toric (SA6ATx, 6 lenses) lenses (Alcon Laboratories, Inc).

Characteristics of Patient Eyes

Table 1:

Characteristics of Patient Eyes

The lens that the surgeon had chosen preoperatively using the Barrett TKNH formula was implanted 62.1% (72 of 116) of the time, either because intraoperative aberrometry and the Barrett TKNH formula agreed or because the intraoperative aberrometry measurements were taken but the decision was made to go with the Barrett TKNH prediction instead. Of the 44 times where intraoperative aberrometry and the Barrett TKNH formula disagreed on the lens to be implanted, 22 times the Barrett TKNH formula gave the better prediction and 22 times intraoperative aberrometry gave the better prediction.

Postoperatively, all patients had excellent corrected vision with a mean CDVA of 20/20 (0.02 ± 0.08 logMAR).

Using unadjusted data, the mean RPE was 0.03 ± 0.52, 0.002 ± 0.52, 0.09 ± 0.48, −0.30 ± 0.53, and −0.48 ± 0.48 D for intraoperative aberrometry, Barrett TKNH, Barrett TKNH with PC, Shammas-PL, and Haigis-L formulas, respectively. The mean RPE for the Barrett TKNH and Barrett TKNH formulas and intraoperative aberrometry were not significantly different than zero (P = .962, .054, and .463), but the mean RPEs for the Shammas-PL and Haigis-L formulas were (P < .001 for both). All formulas except the Haigis-L had a MedAE of 0.35 D; the MedAE for the Haigis-L formula was 0.49 D. MAE was 0.40 ± 0.33 D for intraoperative aberrometry, 0.42 ± 0.31 D for the Barrett TKNH formula, 0.38 ± 0.30 D for the Barrett TKNH with PC formula, 0.47 ± 0.38 D for the Shammas-PL formula, and 0.56 ± 0.39 D for the Haigis-L formula (Table 2, Figure 1). There was no statistically significant difference in absolute prediction errors between intra-operative aberrometry and the Barrett TKNH, Barrett TKNH with PC, or Shammas-PL formulas (P = .28, .56, and .06 respectively). However, the Haigis-L formula did have a larger absolute prediction error than all other formulas (P = .0007, .0001, and .0003, and P < .001 for Barrett TKNH, Barrett TKN with PC, intra-operative aberrometry, and Shammas-PL, respectively) (Table 3). Additionally, the Barrett TKNH with PC formula had a smaller absolute prediction error than either the Barrett TKNH or Shammas-PL formula (P = .046 and .023, respectively) (Table 3). Intraoperative aberrometry and the Barrett TKNH with PC formula achieved predictions within ±0.25 D of actual 40.0% of the time, followed by the Shammas-PL formula with 37.0%, Barrett TKNH formula with 36.2%, and Haigis-L formula with 26.7%. Intraoperative aberrometry was within ±0.50 D of prediction 73.3% of the time, followed by the Barrett TKNH with PC formula at 71.6%, Barrett TKNH formula at 69.0%, Shammas-PL formula at 62.1%, and Haigis-L formula at 51.7%. The Barrett TKNH with PC formula produced the most results within ±1.00 D of prediction, with 94.8% (versus 91.4%, 90.5%, 90.0%, and 86.2% for Barrett TKNH, intraoperative aberrometry, Shammas-PL, and Haigis-L, respectively) (Figure 2).

RPE, MAE, and MedAE Produced by Each Formula/Technique

Table 2:

RPE, MAE, and MedAE Produced by Each Formula/Technique

Refraction prediction error for each formula. Barrett TKNH = Barrett True-K No History; Barrett TKNH with PC = Barrett TKNH with posterior corneal measurements; IA = intraoperative aberrometry; D = diopters

Figure 1.

Refraction prediction error for each formula. Barrett TKNH = Barrett True-K No History; Barrett TKNH with PC = Barrett TKNH with posterior corneal measurements; IA = intraoperative aberrometry; D = diopters

Significance of Difference Between Each Formulaa

Table 3:

Significance of Difference Between Each Formula

Distribution of prediction errors by formula. Barrett TKNH = Barrett True-K No History; Barrett TKNH with PC = Barrett TKNH with posterior corneal measurements; IA = intraoperative aberrometry

Figure 2.

Distribution of prediction errors by formula. Barrett TKNH = Barrett True-K No History; Barrett TKNH with PC = Barrett TKNH with posterior corneal measurements; IA = intraoperative aberrometry

The Barrett TKNH with PC formula produced the most accurate result in 30.2% (35 of 116) of eyes, followed by intraoperative aberrometry in 22.4% (26 of 116). In 2 of 116 eyes (1.7%), all five methods did poorly with a prediction error greater than 1.00 D. All five prediction methods were within ±0.50 D of predicted 27.6% of the time (32 of 116).

Additional analyses were performed after zeroing out of the arithmetic mean error, as suggested by Wang et al25 as a proxy for optimization of the lens constants for each formula. After adjusting the mean RPE to zero for each method, the MedAE was 0.34, 0.35, 0.32, 0.38, and 0.34 D for intraoperative aberrometry, the Barrett TKNH, Barrett TKNH with PC, Shammas-PL, and Haigis-L formulas, respectively, and MAE was 0.39 ± 0.34, 0.42 ± 0.31, 0.38 ± 0.29, 0.41 ± 0.33, and 0.39 ± 0.28 D, respectively (Table 2). Again, there was a significant difference between the Barrett TKNH and Barrett TKNH with PC formulas (P = .016), but no other comparison was significantly different (Table 3).

Charts were generated for each biometry value to evaluate its impact on each method's prediction (Figure 3). All five methods were significantly affected by corneal asphericity (Q value), with a linear relationship noted between increasing asphericity and a more hyperopic outcome (P < .0001, P = .016, P = .001, P = .032, and P = .041 for Barrett TKNH, Barrett TKNH with PC, intraoperative aberrometry, Shammas-PL, and Haigis-L, respectively). The Shammas-PL formula also had a significant interaction with ACD, with greater ACD resulting in more hyperopic outcomes (P = .009, vs P = .162 for Barrett TKNH, P = .056 for Barrett TKNH with PC, P = .261 for intraoperative aberrometry, and P = .193 for Haigis-L) (Figure 3). None of the other variables collected appeared to have a significant clinical effect on accurate outcomes (average keratometry, axial length, white-to-white value, Q value of the central 6 mm of the posterior cornea, and posterior corneal curvature) (all graphs not shown).

Models of prediction error by formula for various biometry values. Barrett TKNH = Barrett True-K No History; Barrett TKNH with PC = Barrett TKNH with posterior corneal measurements; IA = intraoperative aberrometry; D = diopters

Figure 3.

Models of prediction error by formula for various biometry values. Barrett TKNH = Barrett True-K No History; Barrett TKNH with PC = Barrett TKNH with posterior corneal measurements; IA = intraoperative aberrometry; D = diopters

Discussion

For eyes with prior myopic laser refractive surgery, use of Barrett TKNH with PC measurements produced more accurate refractive outcome following cataract surgery when compared to Barrett TKNH without PC measurements using both the raw data and data adjusted for RPE to be equal to zero, although the magnitude of difference was small. Using the unadjusted data, the Barrett TKNH with PC formula also proved to be more accurate than the Shammas-PL and Haigis-L formulas. When compared to intraoperative aberrometry, the Barrett TKNH with PC formula also produced slightly better results, but this comparison did not reach statistical significance. Corneal asphericity had a significant impact on RPE, with greater asphericity tending to result in a more hyperopic outcome.

The Barrett True-K formula has non-inferior or superior results when compared to many of the other formulas used for eyes with previous refractive surgery.12–14 Benefits of this formula include that it is freely available online, can be used with standard biometry values, and can be further improved if data from prior to refractive surgery are input.12 Although the methodology remains unpublished, the Barrett True-K formula uses ocular biometry measurements to calculate modified keratometry values using a modified Double-K method and assumptions about eyes with prior laser refractive surgery. Consistent with our results, prior studies of the Barrett TKNH formula in eyes with prior myopic laser in situ keratomileusis/photorefractive keratectomy (LASIK/PRK) have reported median absolute errors of 0.33 to 0.42 D12–15 and 52.8% to 63.3%12,13,15 of eyes within ±0.50 D of prediction.

The ratio of anterior to posterior corneal curvature changes after myopic laser refractive surgery; therefore, the standard keratometric index of refraction cannot be accurately used in these eyes.16 Optical coherence tomography–based IOL calculation formulas use total corneal power and have demonstrated promising results for eyes with previous refractive surgery using RTVue (Optovue, Inc).13,26 Similar favorable results have been observed with Scheimpflug-Placido disk tomographer–based formulas using the Galilei G6 (Ziemer Ophthalmic Systems AG),27 Scheimpflug-based formulas using the Pentacam,28 and swept-source optical coherence tomography–derived total keratometry using the IOLMaster 700 and the Haigis formula.15 Our center has access to the Pentacam for preoperative cataract evaluation, which is one reason it was chosen to obtain posterior corneal measurements in this study. The IOLMaster 700 also has the capability to measure the posterior cornea; however, the software necessary for these measurements was not available at our center for many of the eyes measured and therefore it was not used in the current study. Larger multicenter studies of the various methods of posterior corneal measurement and formulas using their respective information are warranted.

Intraoperative aberrometry has also been shown to improve refractive outcomes in these eyes when compared to many of the older online formulas.19,20 The drawbacks of this method include the cost to have access to the device in the surgery center and the time it takes to complete the measurements intraoperatively. Prior studies of intraoperative aberrometry in eyes with prior myopic LASIK/PRK have reported a MedAE of 0.29 to 0.35 D18,19 and 67% of eyes with ±0.50 D of prediction,19 similar to the results in this study of 0.35 D and 73.3%. The impact on intraoperative aberrometry of artificially low intraocular pressure readings by applanation tonometry in thinner corneas with previous refractive surgery has not been well-established, and its potential effect on refractive predictions should also be considered.

Increased corneal asphericity (Q value of the front 6 mm of the cornea) would be more common in eyes with a history of larger myopic ablations, and those with high asphericity were more likely to deviate from the mean error in the hyperopic direction for all five methods in this study. Surgeons may consider this when choosing a lens for eyes with a high amount of ablation or a large Q value of the front 6 mm of the cornea. The Shammas-PL formula was particularly sensitive to changes in ACD and should be interpreted with caution in those eyes with atypical ACD measurements.

In 2009, the Royal College of Ophthalmologists suggested benchmarks for cataract surgery refractive outcomes that included a standard of 85% of patients achieving a final spherical equivalent within ±1.00 D of predicted and 55% within ±0.50 D of predicted.29 Although all five methods in this study, with the exception of the Haigis-L formula for predictions within ±0.50 D of predicted, met those benchmarks in this study, increasing accuracy of biometry measurements and continued improvements of formulas make it likely that standards like this will and should become even tighter in the future. In particular, for the laser refractive surgery population with refractive surgery results within ±0.50 D of target 90.9% of the time,30 those standards may not meet their expectations.

Several methods for studying IOL formulas have been proposed.25,31 There are arguments to primarily compare MAE versus MedAE to avoid ignorance of significant outliers32; therefore, we chose to present our data both ways. Similarly, there are reasons to perform analysis before (representing real-world scenarios) or after (representing optimal formula performance) lens constant optimization, but because both analyses are of interest we have also presented both.33 Before adjustment, the Barrett TKNH and Barrett TKNH with PC formulas and intraoperative aberrometry all had a mean prediction error not significantly different than zero, meaning that all lens constants were relatively well-optimized in this subgroup of patients. However, the Haigis-L and Shammas-PL formulas did have mean prediction errors of −0.48 and −0.30 D, respectively, which were significantly different than zero (P < .0001 for both), indicating that the optimized lens constants from the ULIB were not well-optimized for this subset of eyes with regard to these formulas.

Another limitation of our study is the use of multiple IOL brands and types. Similar analyses were performed after removal of extended depth of focus and multifocal lenses, as well as limiting to just one brand of lenses. Overall numerical outcomes were similar, but due to smaller numbers the comparisons no longer reached statistical significance. A power analysis was not performed prior to data collection because all available data from our institution were used and post hoc power analyses have not been found to be useful.

By limiting our results to patients with final CDVA better than 20/40 and refractions done in the 3-week to 4-month postoperative period, we increase the likelihood of stable and reliable final postoperative refractive outcome. However, refractions collected later in that period may have shifted some from early refractions secondary to capsule fibrosis. Results of all formulas and intraoperative aberrometry are dependent on accurate preoperative biometry values. All biometry in this study was measured by an experienced technician, but this represents a source of error that may decrease formula accuracy. Also, the surgeons in this study had used intraoperative aberrometry in their practices for several years and hundreds of eyes prior to this study, but these results with intraoperative aberrometry should not be generalized to those without standardized methods using this technology.

This study has several strengths, including being the first to use posterior corneal measurements in the Barrett TKNH formula to improve accuracy in these eyes with previous refractive surgery where the posterior cornea is critical in estimating total refractive power of the cornea. Also, it is the first to compare intraoperative aberrometry to the Barrett True-K formula in eyes that have had previous myopic laser refractive surgery and looking at biometry variables that are prognostic of prediction error for the various formulas. We are also presenting data that have been analyzed in several ways, including both before and after lens constant optimization.

Similar studies regarding eyes with prior hyperopic laser refractive surgery and radial keratotomy are warranted. Additionally, our patients did not have data from prior to refractive surgery available, so it is possible that use of these historical data could have improved the refractive accuracy even further.12

Our results indicate that for eyes with prior myopic laser refractive surgery, intraoperative aberrometry, Barrett TKNH, Barrett TKNH with PC, Shammas-PL, and Haigis-L formulas all provide predictions well within the benchmark standards for cataract surgery. However, the Barrett TKNH with PC formula did prove to be slightly more accurate than the Barrett TKNH without posterior corneal measurements, Shammas-PL, and Haigis-L formulas. Comparisons to intraoperative aberrometry were not statistically significant. The Shammas-PL formula should be used with caution in eyes with extremes in ACD because the prediction error is sensitive to this, and special care should be taken in patients with high corneal asphericity due to more hyperopic predictions than average in this group.

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Characteristics of Patient Eyes

CharacteristicMean ± SDMedian (Range)
Patients (n = 79)
  Age (years)65.7 ± 8.566.8 (41.4 to 84.9)
  Male gender, n (%)42 (53.2%)
Eyes (n = 116)
  Axial length (mm)25.9 ± 1.425.7 (23.3 to 29.8)
  K steep (D)41.00 ± 2.1041.00 (35.90 to 46.00)
  K flat (D)40.20 ± 2.1040.10 (35.30 to 45.50)
  Anterior chamber depth (mm)3.45 ± 0.443.54 (2.22 to 4.32)
  Preoperative astigmatism (D)0.83 ± 0.470.76 (0.00 to 2.33)
  IOL power implanted (D)20.00 ± 2.3020.00 (11.50 to 24.00)

RPE, MAE, and MedAE Produced by Each Formula/Technique

FormulaWithout Adjusting Mean RPE to Zero (D)After Adjusting Mean RPE to Zero (D)


Mean RPE ± SDRangeMAE ± SDMedAEMean RPE ± SDRangeMAE ± SDMedAE
Barrett TKNH0.002 ± 0.52−1.18 to 1.350.42 ± 0.310.350.00 ± 0.52−1.18 to 1.340.42 ± 0.310.35
Barrett TKNH with PC0.09 ± 0.48−1.14 to 1.120.38 ± 0.300.350.00 ± 0.48−1.23 to 1.040.38 ± 0.290.32
IA0.03 ± 0.52−1.60 to 1.240.40 ± 0.330.350.00 ± 0.52−1.64 to 1.210.39 ± 0.340.34
Shammas-PL−0.30 ± 0.53−1.70 to 0.990.47 ± 0.380.350.00 ± 0.53−1.40 to 1.290.41 ± 0.330.38
Haigis-L−0.48 ± 0.48−1.73 to 0.660.56 ± 0.390.490.00 ± 0.48−1.24 to 1.150.39 ± 0.280.34

Significance of Difference Between Each Formulaa

FormulaBarrett TKNHBarrett TKNH with PCIAShammas-PLHaigis-L
Barrett TKNH.016.215.807.179
Barrett TKNH with PC.046.499.091.427
IA.284.559.394.995
Shammas-PL.124.023.0560.278
Haigis-L.0007.0001.0003< .0001
Authors

From the Department of Ophthalmology, University of Colorado School of Medicine, Aurora, Colorado.

Dr. Taravella reports personal fees from Johnson & Johnson Vision and VISX, outside the scope of this work, and Dr. Davidson reports personal fees from Carl Zeiss Meditec AG, outside the scope of this work. The remaining authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (AML, MJT, RSD); data collection (KLC, DCM, AML, MJT); analysis and interpretation of data (KLC, JLP, AML); writing the manuscript (KLC); critical revision of the manuscript (JLP, DCM, AML, MJT, RSD); statistical expertise (JLP); supervision (AML, MJT)

Correspondence: Karen L. Christopher, MD, 1675 Aurora Court, MS 731, Aurora, CO 80045. Email: karen.christopher@cuanschutz.edu

Received: May 03, 2020
Accepted: October 30, 2020

10.3928/1081597X-20201030-02

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