Opposite Clear Corneal Incisions for Astigmatism
With progressive advancements in phacoemulsification, significant attention is now focused on attaining good refractive outcomes after cataract surgery. The efforts made toward achieving accurate biometric analyses and the increasing importance of astigmatic outcomes are testimony to this desire.
The incidence of clinically significant preoperative keratometric astigmatism is considerable. In the Blue Mountain Eye Study, for example, approximately 25% of patients had at least 1.50 D of keratometric astigmatism.1 In terms of defocus equivalent, only 75% of patients were within 0.75 D of intended correction postoperatively compared with the currently achieved biometric spherical outcome of more than 90%. Thus, in the absence of any surgical correction of astigmatism, a significant proportion of patients may be rendered spectacle dependent irrespective of the biometric accuracy.
In this regard, it would seem of little value to further refine biometric outcomes unless some method of astigmatic correction is routinely undertaken. Herein, we describe our technique of astigmatic correction, the opposite clear corneal incision, which may be used during the implantation of premium IOLs as well as standard lenses.
As we continue to use accommodating or multifocal IOLs with more frequency, the need to address astigmatism at the time of cataract surgery gains importance. Management of astigmatism is required in order for these premium IOLs to offer the greatest benefit to the patient.
CONSIDERATIONS: ASTIGMATIC CORRECTION
Ideally, preoperative assessment includes topography and measurement of corneal thickness. There are numerous techniques to correct keratometric astigmatism at the time of cataract surgery, the simplest being to place the main corneal incision along the steep corneal meridian. However, this has limited merit when performed alone because it does not achieve notable correction. Other methods of astigmatic correction, including limbal relaxing incisions and astigmatic keratotomy, yield good results. Despite the established efficacy of these procedures, uptake and use appear to be limited, primarily because safe and predictable execution requires additional instrumentation, training, and time. Toric IOLs and corneal laser ablation are good management options; however, availability and cost also limit their use.
OPPOSITE CLEAR CORNEAL INCISIONS
Lever and Dahan2 modified the standard approach to the clear corneal incision, adding an identical incision on the opposite side (180° away). They described their technique as an opposite clear corneal incision (OCCI; Figure 1). In a series of 33 eyes, mean keratometric astigmatism changed from 2.80 D preoperatively to 0.75 D postoperatively. A more recent randomized trial compared single on-axis incisions to paired incisions in 40 eyes. In this study, there was a 1.66 D reduction in astigmatism with OCCI versus 0.80 D with single incisions.3 Previously, an unrandomized study in 66 eyes found a 1.30 D reduction with OCCI but only a 0.40 D reduction with a single incision.4 Other studies found similar reductions.5
ROUTINE PRACTICE
In our practice, we routinely apply OCCIs to address preoperative keratometric astigmatism. Therefore, the number of additional preoperative assessments and interventions is limited. Our preoperative assessment of cataract surgery patients includes biometry and keratometry using the IOLMaster (Carl Zeiss Meditec AG, Jena, Germany). The correspondence of keratometric astigmatism values with manifest refractive astigmatism is used as a secondary check; formal topography is used only in the presence of any marked discrepancy. We use OCCI in patients with greater than 1.50 D preoperative keratometric astigmatism, subject to discussion and appropriate consent.
Our surgical technique for OCCI consists of a 2.6-mm stab clear corneal incision with a steel microkeratome. We use a single sideport incision for phacoemulsification. Following removal of lens matter with phaco chop and automated aspiration, the main incision is slightly enlarged to allow IOL insertion.
Subsequent to bag reformation with an ophthalmic viscosurgical device—but prior to IOL insertion—a second clear corneal incision is made with a new 2.6-mm microkeratome approximately 180° from the main incision. This second incision is not extended, nor is it subject to any further manipulation. It is not used during phacoemulsification. This intervention has negligible additional time or cost implications.
We typically implant a one-piece hydrophobic acrylic IOL (AcrySof SA60AT; Alcon Laboratories, Inc., Fort Worth, Texas) in the bag. Routine postoperative antibiotics and steroid eye drops are prescribed; follow-up is performed 5 to 6 weeks postoperatively.
CURRENT STUDY
Prior to our use of paired OCCIs, we preferred superior incisions for patients with less than 1.50 D of keratometric astigmatism and a single incision on the steep meridian for those with more than 1.50 D. Following the introduction of the OCCI technique, we assessed the keratometric, refractive, and visual outcomes in a small group of patients.
A prospective OCCI intervention group was compared with two retrospective control groups. Eyes with at least 1.50 D keratometric astigmatism had paired full-thickness OCCIs (intervention group, n=11 patients, 15 eyes). Of the eyes that had a single incision on the steep meridian, a random sample formed the control group 1 (n=7). Sixteen eyes with at least 1.50 D preoperative keratometric astigmatism underwent superior corneal incisions and formed control group 2. We compared groups for the change in keratometric and refractive astigmatism and for visual acuity. All patients in the control group and six patients in the OCCI group were available for follow-up. Visual acuity, keratometry with the IOLMaster, and autorefraction were measured. Subjective refraction was available for only five patients in the OCCI group. Changes from baseline were analyzed using both simple arithmetic and vector analysis.
STUDY RESULTS
The mean decrease in keratometric astigmatism in eyes that underwent the OCCI procedure was 1.32 D, compared with 0.19 D in eyes that underwent a single on-axis incision and a 0.40 D increase with a superior incision (Figure 2). This was the most clinically relevant finding in our study.
We also determined the vector values of induced astigmatism (Figure 3). Interestingly, in this small group there was significantly less difference between groups 1 and 2 compared with the OCCI group (1.42 D in the steep-axis group vs 1.53 D in the OCCI group). This suggests that the enhanced effect of performing an OCCI may be due to a predictable effect in the alignment of the axis. Previous work suggesting a significant astigmatic rotation (ie, torque effect) with single corneal incisions supports our findings.6
Preoperative preparation did not differ between the groups; patients were not marked preoperatively, making a surgical bias in accurate alignment unlikely. The large angle of error in the single-incision group seems unlikely to be a surgical error, again suggesting that the OCCI procedure may inherently result in a more predictable axis of effect, rather than just a larger effect size.
In the OCCI group, UCVA was also better than in the control groups. The mean logMAR visual acuity was 0.05 in the OCCI group, 0.24 in group 1 and 0.2 in group 2. In terms of subjective spectacle refraction, the OCCI patients showed a mean decrease of 1.12 D of astigmatic correction (Figure 4). In contrast, groups 1 and 2 showed an increase—although small—of 0.45 D and 0.86 D, respectively.
CONCLUSION
In summary, OCCIs are a simple means for addressing preexisting astigmatism during cataract surgery. They require no additional expertise, instrumentation, time, or cost. As such, the technique has potential for widespread use during cataract surgery, providing that clear corneal incisions of significant length continue to be used for cataract surgery.7 As the use of multifocal and accommodating IOLs increases worldwide, we believe that OCCIs will also become more popular. Higher patient expectations have in part driven this trend toward the increasing importance of refractive outcomes after cataract surgery. This trend will undoubtedly continue.
Mohammed Muhtaseb, FRCOphth, is a Consultant Ophthalmologist, Department of Ophthalmology, the Singleton Hospital, Swansea, Wales, UK. Dr. Muhtaseb did not provide financial disclosure information. He may be reached at +44 1792 205666; mohammedmuhtaseb@yahoo.co.uk.
Achyut Mukherjee, MRCOphth, is a Specialist Registrar, Department of Ophthalmology, the Singleton Hospital, Swansea, Wales, UK. Dr. Mukherjee did not provide financial disclosure information. He may be reached at +44 1792 205666.
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