Major developments in cataract surgery have contributed to significant improvement in visual outcomes. One development is the improvement in intraocular lens (IOL) design as well as the availability of premium IOLs. Another is a shift to smaller corneal incisions for IOL implantation. As a result, better quality of vision after cataract surgery has been obtained in recent years. Therefore, the management of preexisting corneal astigmatism has become clinically more important.1
Surgical options for correcting preoperative astigmatism include the size and position of the main incision,2 astigmatic keratotomy,3 limbal relaxing incisions,4 and implantation of toric IOLs. The toric IOL was devised by Shimizu et al.5 in 1994 and has been widely used in clinical settings. A major problem of the technique is postoperative rotation of the toric IOL.6–8
The purpose of this study was to prospectively assess the long-term visual and refractive outcomes and rotational stability of toric IOL implantation.
Patients and methods
This prospective observational study included consecutive patients with cataract and preexisting regular corneal astigmatism who had implantation of an Acrysof IQ toric SN6AT IOL (Alcon Laboratories, Inc.) at Kitasato University Hospital between August 2009 and July 2012. The toric IOL was approved for clinical use in Japan in August 2009.
Inclusion criteria included cataract and preoperative regular corneal astigmatism. Exclusion criteria included preoperative irregular corneal astigmatism, a history of glaucoma or retinal detachment, corneal disease, previous corneal or intraocular surgery, an abnormal iris, pupil deformation, macular degeneration or retinopathy, neuro-ophthalmic disease, and a history of ocular inflammation.9,10
The uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), refractive cylinder, and corneal astigmatism were recorded preoperatively and 1 day, 1 week, 3 months, and 1 and 2 years postoperatively. Intraocular lens misalignment and IOL rotation were measured at all postoperative visits. Minus cylinder notation was used in this study.
Preoperatively, corneal astigmatism was determined with an autokeratometer (ARK-700A, RKT-7700, Nidek Co. Ltd.) and corneal topography evaluated (Atlas, Carl Zeiss Meditec AG). Preoperative corneal astigmatism and axial length (AL) were measured with an optical biometer (IOLMaster, Carl Zeiss Meditec AG). Manual keratometry was not used to determine IOL orientation. Calculation of IOL power, axis placement of the IOL, and the appropriate IOL model was performed using a program available from the IOL manufacturer.A
Digital anterior segment photographs were taken to determine the alignment of the toric IOL axis postoperatively. A blood vessel of the bulbar conjunctiva, the pigment of the bulbar conjunctiva, or the iris pattern in the photographs was selected as a reference point. The axis of IOL rotation was measured by comparing the follow-up photographs with the photographs taken immediately postoperatively. In all cases, the photographs were confirmed to be of sufficient quality to allow determination of the amount of IOL rotation.
For accurate astigmatic correction, it is important to control for potential ocular cyclotorsion caused by changes in the patient’s position. The mean ocular cyclotorsion has been reported to be 2.59 ± 1.91 (SD) degrees, which was not negligible in a clinical setting.11 In this study, preoperative marking was performed by the same examiner (T.M.). Using the axis-registration method, horizontal marks were made on the cornea and conjunctiva using an axis marker (AE-2748, American Surgical Instruments Corp.) under a slitlamp with the patient seated. The marking on the conjunctiva was used to identify the steepest meridian on the corneal topographic image (TMS-5, Tomey Corp.), and the marking on the cornea was used to identify the steepest meridian during the toric IOL implantation. The marking on the conjunctiva was easily identified on the topographic image, allowing confirmation of the graphic relation between the mark and the steepest meridian.12
The surgery was performed by 1 of 2 experienced surgeons (K.S., K.K.).With the patient supine on the surgical table, the corneal limbus of a direction fixing the axis of the toric IOL was marked with a 30-gauge needle with the aid of the premarked reference point. The 30-gauge needle was used to allow easy measurement of IOL misalignment immediately after surgery. Next, phacoemulsification was performed through a 2.65 mm temporal corneal incision and the toric IOL inserted in the capsular bag. After the ophthalmic viscosurgical device (OVD) was removed, the surgeon rotated the toric IOL to align with the reference marks on the cornea. No sutures were used to close the wound.
Vector analysis was performed using the Alpins method.6,7,13,14 Terms used to describe the change in refractive cylinder were as follows:
- Target induced astigmatism vector (TIA), which is the astigmatic change (by magnitude and axis) the surgery was intended to induce.
- Surgically induced astigmatism vector (SIA), which is the amount and axis of astigmatic change the surgery actually induced.
- Astigmatism correction index, which is calculated by determining the ratio of the SIA to the TIA by dividing the SIA by the TIA. The preferable astigmatism correction index is 1.0. It is greater than 1.0 if overcorrection occurs and less than 1.0 if undercorrection occurs.
- Angle of error, which is the angle described by the vectors of the achieved correction (SIA) versus the intended correction (TIA). The angle of error is positive if the achieved correction is counterclockwise to its intended axis and negative if the achieved correction is clockwise to its intended axis.
- Difference vector, which is the induced astigmatic change (by magnitude and axis) needed for the initial surgery to achieve its intended target. The difference vector is an absolute measure of success, and the preferable value is zero.
- Flattening effect, which is the amount of astigmatism reduction achieved by the effective proportion of the SIA at the intended meridian (flattening effect = SIA Cos2 × angle of error).
- Flattening index, which is calculated by dividing the flattening effect by the TIA. The preferable value is 1.0.
- Index of success, which is calculated by dividing the difference vector by the TIA. The index of success is a relative measure with a preferable value of zero.
The means and standard deviation were calculated using Excel 2010 software (Microsoft Crop.). All statistical analyses were performed using Ekuseru-Toukei 2010 software (Social Survey Research Information Co., Ltd.). One-way analysis of variance (ANOVA) was used for the analysis of the time course of changes and with the Dunnett test for multiple comparisons. A probability value less than 0.05 was considered statistically significant.
The study enrolled 378 eyes of 302 consecutive patients, 135 men and 167 women. The mean patient age at the time of surgery was 63.4 ± 16.9 years. The number of eyes at each postoperative examination was 378 at 1 week, 322 at 3 months, 175 at 1 year, and 73 at 2 years. No eyes were lost during the follow-up period in this series.
Table 1 shows the mean preoperative and postoperative UDVA, CDVA, refractive cylinder, and corneal astigmatism. The CDVA 3 months after surgery was 20/25 or better in 305 eyes (94.7%). Seventeen eyes (5.3%) had a CDVA of worse than 20/25 as a result of residual astigmatism, glaucoma, macular atrophy, or amblyopia. The variance in the data between preoperatively and all postoperative timepoints was statistically significant (P<.001, ANOVA). Multiple comparisons showed statistically significant differences between preoperative measurements (UDVA, CDVA, and refractive cylinder) and measurements 3 months, 1 year, and 2 years after surgery (all P<.001).
Table 2 shows results of the refractive astigmatism analysis using the Alpins method. The mean correction index was 1.02 ± 0.36, suggesting that there was no tendency toward undercorrection or overcorrection of refractive astigmatism postoperatively. Figure 1 shows the vectorial display of the difference vector of the refractive astigmatism 3 months after surgery.
Table 3 shows the mean IOL misalignment postoperatively. The mean IOL misalignment was 4 to 5 degrees at all follow-up examinations. Table 4 shows the mean IOL rotation postoperatively. The IOL rotation was 4.5 ± 4.9 degrees within 1 day after surgery but decreased to 1 to 2 degrees thereafter. Six (1.6%) of 378 eyes had IOL rotation of more than 20 degrees; Table 5 shows the characteristics of these cases. In all 6 cases, the AL was 25.0 mm or longer and the corneal astigmatism was with the rule (WTR). The IOL rotation occurred within 1 day after surgery in 4 cases, from 1 day to 1 week in 1 case, and from 1 week to 1 month in 1 case. Thereafter, there was no significant IOL rotation in any case. Figures 2 to 5 show the anterior segment photographs before and after IOL rotation in cases 2, 4, 5, and 6.
Correction of astigmatism in cataract surgery is an important issue. We previously studied the distribution of corneal astigmatism in 12 428 eyes that had cataract surgery at our hospital.15 Of these eyes, 36.3% had more than 1.0 D of corneal astigmatism, 8.0% had more than 2.0 D, and 2.4% had more than 3.0 D. Our results indicate that implantation of the Acrysof IQ toric SN6AT IOL is an effective option to correct preexisting astigmatism during cataract surgery and that it remains effective over the long term.
One significant problem with toric IOLs is postoperative misalignment. In our study, the mean IOL misalignment was 4 to 5 degrees 2 years postoperatively. Overall, the toric IOL in our study showed good rotational stability over the long term; however, 6 patients had IOL rotation of more than 20 degrees. In all 6 cases, the AL was 25.0 mm or longer, the corneal astigmatism was WTR, and the IOL rotated relatively soon after surgery. If axis misalignment reaches 30 degrees, the preoperative astigmatism magnitude is not reduced. Instead, the astigmatism meridian is shifted 30 degrees in the cyclical direction of the treatment. In this case, IOL repositioning has to be considered. If the misalignment on the angle of error (absolute) reaches 20 degrees, the loss of effect on the flattening index is 25%.16
In our study, cases 1 and 2 were followed without repositioning the IOL because the refractive cylinder ranged from −0.75 to −0.50 D postoperatively. In cases 3 and 4, in which the IOL had not rotated more than 5 degrees, the IOL was repositioned 2 weeks postoperatively. We assume the IOL did not rotate again, possibly because the capsular bag had begun to shrink. At the time of IOL repositioning, a cannula with a balanced salt solution and without OVD was used. Case 5 had an AL of 31.0 mm; in this case, the IOL repositioning was performed immediately after surgery at the patient’s request. By the next day, however, the IOL had rotated 47 degrees. The IOL repositioning was performed again 2 weeks after cataract surgery, after which the IOL remained stable. In case 6, the IOL had not rotated within 1 week postoperatively; however, the patient developed acute visual loss 10 days postoperatively. At the time of the 1-month examination, the IOL had rotated 95 degrees. After the IOL was repositioned, the patient’s UDVA improved from 20/100 to 20/16 and the refractive cylinder decreased significantly (from −4.0 to 0.0 D). The IOL did not rotate thereafter.
There are several factors in toric IOL rotation, including intraoperative failure resulting from incomplete removal of OVD and insufficient extension of the IOL haptics. In addition, because the scale of the preoperative measurement by topographic imaging and the scale of the intraoperative measurement using the toric marker are at 5-degree intervals, slight IOL misalignment is clinically unavoidable. The mean IOL rotation of 4.5 ± 4.9 degrees within 1 day postoperatively may have resulted from preoperative and intraoperative measurement errors and from postoperative IOL rotation. Furthermore, in the first 3 months after surgery, capsular bag shrinkage can cause IOL rotation.8 In our study, the IOL rotation in all cases that had capsular bag shrinkage was less than 10 degrees.
An appropriately sized continuous curvilinear capsulorhexis, stable fixation of the IOL in the capsular bag, and complete removal of OVD are essential to prevent IOL rotation.17 In our study, the eye in case 5 had a second, large IOL rotation after the IOL had been repositioned immediately after surgery. The eye in case 6 had large IOL rotation 10 days after surgery. One report suggests that 1 cause of early postoperative IOL rotation is related to the capsular bag size; that is, a longer AL is correlated with a larger capsular bag.9 In a study by Chang,18 7 of 700 eyes that had Acrysof IQ toric IOL implantation had IOL rotation of more than 15 degrees off axis and all 7 eyes required surgical repositioning of the IOL. Table 6 shows the characteristics of these cases of IOL rotation. Minor trauma was involved in 1 case. In 5 cases, the IOL rotated within 1 week of surgery. Similarly, in 5 cases, the AL was 25.0 mm or longer and the capsular bag diameter measured during surgery was larger than average. A longer AL may be associated with a greater capsule width.19 Accordingly, we assume that eyes with a longer AL are likely to have a larger capsular bag, which in some cases may lead to implantation of the incorrect IOL size. We believe this may be the cause of the IOL rotation in our study. Moreover, the power of the implanted IOLs was less than 16.5 D; thus, rotational instability may be associated with lower-powered IOLs, which have a thinner optic. All 6 cases of significant rotation in our study had WTR astigmatism, and the IOL haptics were fixated in the vertical direction. We found no significant IOL rotation in eyes with a long AL and ATR astigmatism. An IOL fixated in a vertical direction may rotate more easily.
In conclusion, the results in our study indicate that implantation of the Acrysof IQ toric SN6AT IOL effectively corrects preexisting corneal astigmatism in cataract surgery and has overall good rotational stability over the long term. Significant IOL rotation may occur in the early postoperative period in some eyes with a relatively long AL and WTR astigmatism.
What Was Known
- Implantation of toric IOLs in cataract surgery is effective in the correction of preexisting corneal astigmatism.
What This Paper Adds
- Significant toric IOL rotation may occur in the early postoperative period in eyes with a relatively long AL in which the IOL is oriented vertically.
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Other Cited Material
A. Alcon, Inc. AcrySof®
Toric IOL Web Based Calculators. Available at: www.acrysoftoriccalculator.com
. Accessed June 7, 2014