The use of toric intraocular lenses (IOLs) during cataract surgery can reduce preexisting astigmatism and increase the likelihood of spectacle independence.1 Correct toric alignment of toric IOLs is essential to achieving the intended reduction in astigmatism. Toric misalignment is best defined as the difference between the desired implantation meridian and the final achieved position of a toric IOL. For each degree of toric IOL misalignment there is approximately a 3.3% loss of astigmatic correction; thus, at 30 degrees of misalignment, no astigmatic correction is achieved.2
Toric IOL misalignment is a consequence of intraoperative alignment error and postoperative rotation. Intraoperative misalignment, in turn, is a sum of 3 factors; that is, an error in determining the reference meridian, an error in marking the desired meridian of implantation, and misalignment of the IOL with the marked target meridian. Minimizing these factors improves the refractive outcomes.
Several methods are commonly used to mark the reference meridian; these include using the horizontal beam of a slitlamp or using a marker incorporating a bubble or a pendulum to assist in marking the horizontal meridian. Recently, a new method to improve the accuracy of reference marking was introduced. The method uses several applications using a smartphone built-in accelerometer and gyroscope. One of these applications is the toriCAM, which has been shown to improve the accuracy of determining the reference meridian.3 However, accurate marking of the target meridian in relation to the reference mark is still required to improve toric IOL alignment. Marking the desired implantation meridian from the reference meridian can be achieved with a toric marker or with an image-guided system.
In this study, we compared toric IOL misalignment using 2 toric markers; that is, the commonly used Mendez gauge (Storz Ophthalmics, Bausch & Lomb, Inc.) and the Barrett dual axis toric marker (Duckworth & Kent Ltd.).
Patients and methods
This retrospective cohort study was approved by the Ethics Committee, Sir Charles Gairdner Hospital, Perth, Australia. Toric IOL alignment was examined in 2 consecutive groups of patients operated on by the same surgeon (L.L.) using different toric markers at 2 public hospitals: Sir Charles Gairdner Hospital (Group 1) and Osborne Park Hospital (Group 2), both in Perth, Western Australia. In general, patients operated on at Osborne Park Hospital were healthier, while patients with complex systemic disease or patients expected to have a more challenging procedure were operated on at Sir Charles Gairdner Hospital. Both groups were included in the study during the same time period in 2018. No patient was excluded.
All eyes had cataract surgery with implantation of an AcrySof SN6A toric IOL (versions T2–T8, Alcon). The surgical techniques were identical in the 2 groups except for the type of toric marker that was used to mark the desired implantation meridian (Group 1: Barrett dual axis toric marker; Group 2: Mendez gauge). Although a purchase request for the Barrett marker was made, the marker was not available at Osborne Park Hospital at the time and a Mendez gauge was used instead at that site.
Patient Assessments and Intraocular Lens Power
Preoperative and postoperative examinations in both groups were performed at Sir Charles Gairdner Hospital by the same staff members using identical instruments. The examinations included uncorrected and spectacle-corrected visual acuities, slitlamp biomicroscopy, applanation tonometry, dilated fundus evaluation, corneal topography (Atlas, Carl Zeiss Meditec AG), and biometry using both the IOLMaster 500 system (Carl Zeiss Meditec AG) and Lenstar LS 900 system (Haag-Streit AG). The toric IOL power was calculated using the online Barrett toric calculator.A
Reference Meridian Marking
After the eye was anesthetized with preservative-free oxybuprocaine 0.4% eyedrops (Minims), patients were seated upright and instructed to fixate on a distant target with the other eye. Two freehand reference marks at an intended 0 degree and 180 degrees were applied to the limbus using a surgical marking pen. The toriCAM mobile phone application was downloaded from the App Store (Apple, Inc.) and used to determine the true meridian of the reference mark.
Target Axis Marking
The Barrett dual axis toric marker (Figure 1) has 2 dials. The outer dial aligns with the reference axis provided by the mobile phone application to correct for reference meridian alignment error. The inner dial is connected to the marking blades on the underside to mark the toric alignment axis.
The Mendez gauge (Figure 2) was placed at the limbus with 0-degree and 180-degree marks on the limbal reference marks. The target meridian was corrected according to the reference mark meridian by subtracting the reference misalignment angle measured using the mobile phone application. If the reference meridian was below 0 degree (ie, 91 to 179 degrees), it was regarded as a negative alignment error. For example, if the toric axis was 90 degrees and the reference meridian was measured at 5 degrees, the adjusted toric axis was 90 degrees − 5 degrees = 85 degrees. In another example, if the reference axis was 175 degrees, the adjusted toric axis was 90 degrees, −(−5 degrees) = 90 degrees +5 degrees = 95 degrees. The target axis was marked with a surgical marking pen.
Toric Intraocular Lens Implantation
All incisions were made temporally using a 2.4 mm keratome. The toric IOLs were inserted into the capsule under an ophthalmic viscosurgical device (sodium hyaluronate 1.0% [Healon]) and dialed to align with the toric marks. Coaxial irrigation/aspiration was used to remove all ophthalmic viscosurgical device from the capsule and the anterior chamber.
Postoperative Assessment of Alignment
The actual axis of the toric IOL was determined at 1 month. The pupils were pharmacologically dilated using tropicamide 1.0% eyedrops (Minims). A slitlamp beam was aligned with the toric IOL toric marks to determine the axis. The angle of the slitlamp beam was read off an axis measurement strip applied to the slitlamp as previously described.4 The examiner (L.L.) was masked to the intended implantation axis at the time of postoperative IOL axis measurement to avoid bias; however, this information was available to the reader on the same visit after documentation of the IOL axis. The toric IOL misalignment angle was defined as the difference between the planned implantation axis and the final toric IOL axis measured at 1 month. The manifest refraction was measured at 1 month by an orthoptist.
The data were analyzed using SAS software (SAS Institute), treating all patients’ eyes as independent observations. The Wilcoxon rank-sum test was used to compare the postoperative absolute error in toric IOL alignment between the 2 groups. This test was also used to compare the preoperative astigmatism and axial length between the 2 groups. The Fisher exact test was used to compare toric IOL toric power and the implantation axis between the 2 groups as follows: against-the-rule astigmatism (0 to 30 degrees and 150 to 180 degrees), with-the-rule astigmatism (60 to 120 degrees), and oblique astigmatism (30 to 60 degrees and 120 to 150 degrees). Postoperative astigmatism based on the manifest refraction and percentage of eyes with toric IOL rotational misalignment within ±5 degrees, ±10 degrees, ±15 degrees, ±20 degrees, and ±25 degrees were compared using logistic regression. Significance was set at the 5% level.
The study included 72 eyes of 56 patients. Group 1 (Barrett dual axis toric marker) comprised 36 eyes of 35 patients and Group 2 (Mendez guide), 36 eyes of 25 patients. Table 1 shows the baseline characteristics. The preoperative corneal astigmatism was similar in both groups (P = .75). More toric IOLs were implanted in eyes having with-the-rule astigmatism and less in the eyes with against-the-rule astigmatism in Group 1 than in Group 2; however, the difference was not significant (P = .505). The axial length was similar in the 2 groups (P = .88). The toric power of the implanted IOL was also similar in the 2 groups (P = .285). However, more T2 IOLs were used in Group 1 while T3 IOLs were more prevalently used in Group 2.
Table 2 shows the toric IOL lens alignment error data. The mean absolute alignment error (intended versus actual alignment at 1 month) was statistically significantly lower in Group 1 and than in Group 2 (P = .0015). Group 1 had significantly better alignment outcomes (ie, degrees within an intended axis) than Group 2 (within ±5 degrees, P = .0024; within 10 degrees: P = .0077). The percentage of eyes achieving a postoperative manifest refraction cylinder of 0.50 D or less was significantly higher in Group 1 than in Group 2 (P = .0445) (Table 3).
Proper toric IOL alignment is critical for achieving a desired postoperative refractive outcome. In this study, we compared the accuracy of 2 toric markers—the Barrett dual axis toric marker and the Mendez gauge—to mark the desired implantation meridian from the reference mark. The error in alignment in the first group was significantly lower, which resulted in a significantly higher percentage of eyes achieving a postoperative manifest refraction cylinder of 0.50 D or less.
Both a longer axial length and with-the-rule astigmatism have been shown to be associated with greater toric IOL rotation.5 These 2 factors were not significantly different between our 2 groups.
The type of toric marker used was the sole difference between our 2 groups, both of which were operated on by the same surgeon; thus, the type of marker used is the most likely explanation for the difference in toric alignment error. Accurate determination of the reference meridian using the toriCAM mobile phone application has been shown to be more accurate than freehand or slitlamp marking.3 The Barrett dual axis toric marker was designed to mark the target axis from the reference meridian by setting its outer dial to the reference meridian provided by the mobile phone application. Other markers can also be used in a similar fashion to adjust for the reference meridian provided by application. This can be done by aligning the toric marker so that the reference marks match the reference meridian measured by the toriCAM application. For example, if the reference mark is measured at 170 degrees, a Mendez gauge can be placed with the 170-degree scale mark aligned with the reference mark rather than with the usual 0- to 180-degree meridian. A second technique is subtracting the alignment error from the target axis, as discussed above. However, these cumbersome methods could be a potential source of error. Furthermore, the Mendez gauge we used had scale marks at 10-degree intervals, while the Barrett maker has 5-degree marks for more accurate results. The thin marking blades of the Barrett marker apply narrower marks than the ink dot used to mark the target axis with the Mendez gauge. Thinner marks make it easier to align the toric IOL more accurately. Similar narrow marks can be made using an axis marker in addition to the Mendez gauge; however, this can be more conveniently done using the Barrett dual axis marker alone.
A systematic review by Agresta et al.6 reported alignment errors for different toric IOLs that varied between 2.66 degrees and 8.9 degrees. In a more recent systematic review,1 the mean toric IOL alignment error ranged between 2.5 degrees and 7.67 degrees, with a mean value lower than 5 degrees in most studies. Toric IOL alignment depends on intraoperative alignment accuracy and postoperative rotational toric IOL stability. Inoue et al.7 evaluated intraoperative toric IOL positioning error and postoperative rotational error. They found that postoperative toric IOL rotation occurred mostly within the first hour and was responsible for the majority of toric IOL alignment error. The mean total misalignment at 1 year was 6.67 ± 7.36 degrees, of which 1.87 ± 2.11 degrees was attributed to placement and 4.80 ± 5.45 degrees to toric IOL rotation. Postoperative toric IOL rotation differs between toric IOL models, as seen in a large retrospective study comparing 2 commonly used toric IOLs, the AcrySof and the Tecnis (Johnson & Johnson Vision) in which the former showed significantly greater rotational stability.5
Intraoperative misalignment is a consequence of 3 sources; that is, an error in determining the reference meridian, an error in marking the desired meridian of implantation, and misalignment of the IOL with the target mark. Visser et al.8 found these sources explained toric IOL misalignment of 2.4 degrees, 3.3 degrees, and 2.6 degrees, respectively, summing up to approximately 4.9 degrees. Reference marking should be performed with the patient sitting upright to avoid cyclotorsion when assuming a supine position. This can be done using several methods, including freehand and slitlamp assisted, with a marking error of 3.55 degrees and 2.8 degrees, respectively.9 Gravity-based markers, such a bubble or pendulum makers, have been found to have a mean marking error of 2.42 degrees and 2.83 degrees, respectively.9 Another approach to reduce reference marking error is to mark the reference first, measure its meridian, and then adjust the implantation meridian accordingly. Using the toriCAM application to check the alignment axis in this manner has been shown to reduce reference marking error to only 1.28 degrees, which is up to 67%.3 We have minimized our intraoperative alignment error in this manner; thus, our intraoperative alignment error should predominantly be explained by target meridian marking error and inaccurate toric IOL alignment.
Using an image-guided system is another method to reduce reference and target marking errors. These systems can determine the reference meridian by tracking limbal and scleral vessels and project a target alignment axis line on the patient’s eye during surgery. A study comparing manual toric marking with image-guided marking systems10 found a toric IOL alignment error of 5.5 ± 3.3 degrees using manual marking with a bubble reference maker versus 3.6 ± 2.6 degrees using an image-guided system. This alignment error is comparable to our results using the Barrett marker with the toriCAM application for measuring the reference meridian. Previous results using this combination in 100 eyes presented in 2018B showed a toric alignment error of 3 ± 2 degrees. We believe that ours is the first published study to report toric alignment error and refractive outcomes using the toriCAM application. Clark11 reported 1-month toric refractive outcomes similar to ours using the Barrett marker and toriCAM application for reference marking and the same toric IOL; however, toric alignment error was not reported.
Several studies reporting low toric IOL alignment errors used different definitions and methodologies. Some reported marking system error but not actual toric IOL misalignment.9,12,13 Others used different methods for evaluating the alignment error. For example, a large study comparing AcrySof IOLs and Tecnis toric IOLs5 reported a mean alignment error of 2.72 degrees and 3.79 degrees, respectively. However, same-day or 1-day postoperative toric IOL rotation was measured. This might underestimate postoperative toric rotation.5 Other studies14,15 compared only postoperative error at different postoperative times without considering the intended implantation axis. A retrospective study by Montes de Oca et al.16 excluded patients with toric IOL rotation of 5 degrees or more. Another study by Webers et al.17 examined an image-guided system and found misalignment of 1.7 ± 1.5 degrees at 3 months; however, their methodology might underestimate alignment error. This was determined by comparing preoperative images and postoperative images taken by the image-guided system’s measurement unit, thus ignoring any error resulting in preoperative image misalignment.17 Our method compared the intended versus the actual toric IOL alignment at 1 month, which includes all intraoperative and postoperative errors.
Our study has limitations. First, in this retrospective study comparing 2 toric markers, we looked at total toric IOL misalignment at 1 month rather than comparing the accuracy of the target toric mark meridian itself. Our measured alignment error is a sum of intraoperative alignment accuracy and postoperative rotational toric IOL stability. Therefore, factors other than the toric marker used could have contributed to the difference between our 2 groups. One important factor is the accuracy of the reference meridian, which was minimized using the toriCAM application. Another factor is postoperative toric IOL rotation, which should be similar in 2 groups operated on by the same surgeon using the same technique and same toric IOL. Second, the difference in the refractive outcome between the groups is evidently related to a significant difference in toric IOL alignment. However, a nonsignificant imbalance between toric IOL power distribution in the 2 groups could account for some of the difference in the refractive outcome. There was a nonsignificant trend toward a higher proportion of T2 IOLs than T3 IOLs in the Barrett marker group. The higher the toric IOL toric power, the more its misalignment will affect the refractive outcome2; thus, for T2 toric IOLs, misalignment is less consequential. Third, postoperative toric IOL alignment measurement using a slitlamp has some inherent error that is mostly the result of head tilt and reading off the axis on the slitlamp measurement strip. We estimate these measurement errors to be 1 to 2 degrees. Fourth, one of the authors (G.B) is the developer of the toriCAM application and the Barrett dual axis toric marker and thus has a nonfinancial conflict of interest that could be a source of bias. However, the preoperative assessments, cataract operations, and follow-up visits were all performed by the other author (L.L.), limiting possible bias. Finally, during the postoperative examinations, the IOL axis was read and documented without knowing the intended IOL implantation axis to avoid bias toward minimizing the rotational error. This practice should also preclude bias toward a more favorable reading for 1 toric marker over the other. However, this approach might not be considered a fully masked reading because information regarding the intended axis was available to the reader on the same visit after documentation of the IOL axis. Considering these limitations, we encourage others to repeat our study in a prospective blinded design.
In conclusion, toric IOL misalignment compromises refractive outcomes. Proper reference and target meridian marking can improve intraoperative toric IOL alignment. In this retrospective study using the toriCAM application and marking the desired axis with the Barrett dual axis toric marker was more accurate, resulting in lower toric IOL misalignment and a better refractive outcome than the use of a Mendez gauge.
What Was Known
- Accurate marking of the reference meridian is crucial to achieving proper toric intraocular lens (IOL) alignment.
- The implantation meridian is then marked with a toric marker in relation to the reference mark.
What This Paper Adds
- The type of toric marker used for marking the implantation meridian affected postoperative toric IOL alignment and the refractive outcome.
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14. Nanavaty MA, Bedi KK, Ali S, Holmes M, Rajak S. Toric intraocular lenses versus peripheral corneal relaxing incisions for astigmatism between 0.75 and 2.5 diopters during cataract surgery. Am J Ophthalmol 2017;180:165-177.
15. Waltz KL, Featherstone K, Tsai L, Trentacost D. Clinical outcomes of TECNIS toric intraocular lens implantation after cataract removal in patients with corneal astigmatism. Ophthalmology 2015;122:39-47.
16. Montes de Oca I, Kim EJ, Wang L, Weikert MP, Khandelwal SS, Al-Mohtaseb Z, Koch DD. Accuracy of toric intraocular lens axis alignment using a 3-dimensional computer-guided visualization system. J Cataract Refract Surg 2016;42:550-555.
17. Webers VSC, Bauer NJC, Visser N, Berendschot TTJM, van den Biggelaar FJHM, Nuijts RMMA. Image-guided system versus manual marking for toric intraocular lens alignment in cataract surgery. J Cataract Refract Surg 2017;43:781-788.
Dr. Barrett is the developer of the toriCAM application and the Barrett dual axis toric marker. Neither author has a financial or proprietary interest in any material or method mentioned.
Other Cited Material
A. American Society of Cataract and Refractive Surgery. Barrett toric calculator. Available at: http://www.ascrs.org/barrett-toric-calculator
B. Barrett G, “Optimizing Toric IOL Outcomes: Intraoperative Alignment,” presented at the ASCRS•ASOA Annual Meeting, Washington, DC, USA, April 2018