Roger, Roger. What’s our vector, Victor?
–Captain Clarence Oveur, Airplane!
The problem of astigmatism and its correction is a topic of renewed emphasis for surgeons, scientists, and industry. This interest is due partly to greater awareness and more refined descriptions of the astigmatic contributions of the posterior cornea (for example, Koch et al.1 and in this issue, Savini et al. [pages 1645–1653]). It is also being driven by the introduction of femtosecond laser–assisted keratotomy into the cataract surgery workflow and the increasing number of toric intraocular lens (IOL) options. Together, these changes have intensified the search for an answer to a very practical question: Which approach to treating astigmatism during cataract surgery will provide the most reliable and durable correction for my patient?
A vector is characterized by its magnitude and direction, and it is in these 2 properties of astigmatic power that toric IOLs and corneal relaxing incisions are most starkly divided. In toric IOL implantation, the intrinsic power of the IOL is a known entity and the major surgical variable is rotational alignment. Haptic design and deliberate surgical technique—including a properly sized continuous curvilinear capsulorhexis, careful removal of ophthalmic viscosurgical device, and avoidance of anterior chamber collapse after finalizing rotational position—are important factors for minimizing post-placement intracapsular motion.
In this issue, Miyake et al. (pages 1654–1660) present a large prospective study of long-term astigmatic outcomes and rotational stability of a commonly used toric IOL in 378 eyes. Using serial digital photographic analysis of anterior segment landmarks to assess absolute and relative rotational alignment, the authors documented a mean rotational misalignment of 4.1 degrees ± 3.0 (SD) at 2 years with an overall reduction in refractive astigmatism from 1.92 ± 1.45 diopters (D) to 0.67 ± 0.9 D. Of particular interest, the mean IOL rotation was greatest by postoperative day 1 (4.5 ± 4.9 degrees) and changed little in the subsequent 2 years. Six eyes demonstrated more than 20 degrees of rotation, and all these eyes had axial eye lengths greater than 25 mm, with-the-rule astigmatism, and early postoperative rotation. As pointed out in an editorial by Mamalis2 and discussed at the recent JCRS-sponsored symposium,A 42% of respondents to the annual American Society of Cataract and Refractive Surgery survey held the opinion that more than 5 degrees of rotational misalignment in toric IOL surgery is acceptable. However, even rotational errors below 5 degrees can result in significant loss of astigmatic effect. When we consider the standard deviations in the Miyake study and the finding that 25% of patients at 2 years had more than 5 degrees of misalignment, it is clear that misalignment is still an important source of postoperative residual astigmatism. In the absence of customized haptic designs for larger eyes with presumably larger capsules3 and the infeasibility of a perfectly standardized IOL insertion process, patients must be counseled and surgeons must be prepared to address refractive contingencies with spectacles or contact lenses, IOL repositioning or exchange, or keratorefractive surgery.
Corneal relaxing incisions present an entirely different intraoperative challenge with respect to astigmatic outcomes. Unlike toric IOLs, relaxing incisions maintain a fixed rotational alignment once placed. However, the magnitude of astigmatic effect is less predictable and standard nomograms are notoriously imprecise in eyes with altered corneal biomechanical states owing to previous corneal surgery, ectatic disease, or keratoplasty. Furthermore, corneal incision effects are more limited in range and take longer to stabilize, as confirmed in a rare 3-year follow-up study by Lim et al. (pages 1676–1681) of manual peripheral corneal relaxing incisions performed at the time of cataract surgery. In a randomized prospective contralateral eye study of multifocal toric IOLs versus nontoric multifocal IOLs with manual peripheral corneal incisions, Gangwani et al. (pages 1625–1632) demonstrated marginally lower residual refractive astigmatism in the toric IOL eyes but equivalent uncorrected and corrected distance visual acuity, contrast sensitivity, and subjective responses on visual function/patient satisfaction questionnaires.
In the future, the breakpoint for selecting lens-based or cornea-based astigmatic treatments will likely hinge on how effectively femtosecond laser technology is leveraged. Lens-based treatments may benefit from more predictable capsulorhexis geometry and centration, and optimization of capsulorhexis size may be useful for promoting rotational stability. With manual corneal astigmatic incisions, variability in achieved wound geometry (depth, length, arc, and position) adversely affects predictability; yet, it is also clear that creating the intended wound geometry with a laser is not enough to eliminate all the variables that contribute to refractive outcomes. The astigmatic effects of corneal relaxing incisions depend on the idiosyncrasies of the patient’s corneal geometry, biomechanical properties, and wound-healing characteristics, and treatment plans that capture predictive elements of this interindividual variability may be able to more effectively leverage the femtosecond laser’s surgical precision to the benefit of patients.
1. Koch DD, Jenkins RB, Weikert MP, Yeu E, Wang L. Correcting astigmatism with toric intraocular lenses: effect of posterior corneal astigmatism. J Cataract Refract Surg
2. Mamalis N. How are we doing? Not as well as we think [editorial]. J Cataract Refract Surg
3. Vass C, Menapace R, Schmetterer K, Findl O, Rainer G, Steineck I. Prediction of pseudophakic capsular bag diameter based on biometric variables. J Cataract Refract Surg
Other cited material
A. JCRS Symposium, “Improving Precision in Ophthalmic Surgery: Are We Doing Enough?” presented at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, Boston, Massachusetts, USA, April 2014