March 2015




Intrastromal arcuate incisions

by David A. Salz, MD


In the study, capsulotomy, lens fragmentation, and intrastromal astigmatic keratotomy were performed with the Catalys femtosecond laser. Source: David A. Salz, MD

Study shows that using the femtosecond laser to correct astigmatism intrastromally offers greater precision than manual techniques

The majority of patients presenting for cataract surgery have at least some corneal astigmatism, and 41% of them have visually significant astigmatism ≥0.75 D.1 To achieve optimal distance vision, we want to correct that astigmatism at the time of cataract surgery, either with a toric IOL, limbal relaxing incisions (LRI), or astigmatic keratotomy (AK). LRIs and AK have been particularly useful to correct astigmatism in patients implanted with multifocal IOLs. However, with manual techniques it is difficult to perform these in a reliable manner. Not only does one sometimes get an over- or undercorrection, but it is also possible to induce irregular astigmatism. Patients sometimes experience foreign body sensation or a worsening of dry eye from the incisions. Rarely, there are serious complications such as an infection or corneal perforation.

The femtosecond laser offers the opportunity to increase the precision of the incision depth, length, angular position, and optical zone, with better reproducibility than manual techniques, although admittedly at greater expense. The ability to perform intrastromal arcuate keratotomy (ISAK), rather than penetrating incisions, with the laser minimizes the risk of infection and patient discomfort. The limited reports to date of intrastromal astigmatism correction suggest that it produces less effect than penetrating correction, at least with current nomograms.

Recently, we undertook a study to evaluate the safety and efficacy of ISAK and to gather data for improvement of our nomogram. Advanced technologies, including toric and multifocal IOLs and femtosecond lasers in cataract surgery, are an important part of the residency training program at Wills Eye Hospital in Philadelphia. This year, all of our graduating residents will leave the program certified to perform laser-assisted cataract surgery (LACS).

ISAK study

Our goal was to enroll 50 subjects in this prospective, non-randomized study. We enrolled subjects with preoperative astigmatism of 0.5 D to 3.0 D undergoing routine cataract surgery, excluding those with irregular astigmatism, corneal pathology, and any operative or postoperative complications that would severely limit vision.

Preoperatively, a slit lamp examination and dilated fundoscopic exam were performed. Astigmatism was measured preoperatively and at 1 and 6 months with manual keratometry, IOLMaster (Carl Zeiss Meditec, Jena, Germany), and corneal topography. Refraction was also performed at 1 and 6 months post cataract extraction.

All subjects had capsulotomy, lens fragmentation, and ISAK performed with the Catalys femtosecond laser (Abbott Medical Optics, Abbott Park, Ill.), followed by cataract extraction and implantation of a monofocal or multifocal IOL. The laser was set to create the intrastromal incisions at a depth of 20% below the epithelium to 20% above the endothelium. The arc length was calculated using the Donnenfeld LRI nomogram. Assuming that non-penetrating incisions would have less effect, we compensated by moving the optical zone more centrally, to 8.0 mm. Interim results are available for 20 subjects with a mean age of 69.4 years (range 3582), who have been followed out to at least 1 month, with about half reaching 6 months. Enrollment is continuing.

Clinical results

Even without knowing exactly how to adjust the penetrating LRI nomogram, the results with ISAK thus far have been excellent. Mean preoperative corneal astigmatism in the first 20 eyes was 1.43 D (range 0.25 to 3.00 D) by manual keratometry and 1.47 D (range 0.59 to 2.67 D) on the IOLMaster. One month postoperatively, using manual keratometry, the residual refractive astigmatism was 0.74 D (range 0.25 D to 1.50 D), for a 0.67 D mean change in astigmatism. Using IOLMaster keratometry, the residual astigmatism was 0.71 D at 1 month and 0.60 D at 6 months. This was a statistically significant reduction of preoperative astigmatism. There were no adverse events.

The targeted surgically induced astigmatism (SIA) vector was 1.42 D. Vector analysis demonstrates that the actual SIA vector was 1.72 D, yielding a correction index (CI) of 1.16 overall. Ten of 20 eyes had a CI >1.0, 6 of 20 eyes had a CI of exactly 1.0, and 4 of 20 eyes had a CI <1.0. Qualitatively, ISAK is superior to the manual incisions we have made in the past. The intrastromal incisions are geometrically perfect and are created exactly as they are programmed. ISAK is less invasive than penetrating incisions, so we would expect it to be safer, minimizing the risk of infection and increasing patient comfort. High quality, 3D imaging is critical to the safety of femtosecond laser ISAK because accurate identification of the epithelium and endothelium helps prevent an accidental penetration of either the anterior or posterior cornea. The surgeon should take care to observe there is no shift in the identified structures prior to treatment. It is not yet clear where the ideal optical zone is for ISAK. Other surgeons have taken a variety of approaches, adjusting their LRI nomograms by a certain percentage and/or moving the optical zone to between 9.0 mm and 7.0 mm. More work remains to develop ISAK-specific nomograms, but once we can appropriately adjust existing nomograms for intrastromal incisions, we should be able to replicate or surpass the efficacy of manual LRIs with this technique, with enhanced safety and patient comfort.


1. Ferrer-Blasco T, Monts-Mic R, Peixoto- de-Matos SC, et al. Prevalence of corneal astigmatism before cataract surgery. J Cataract Refract Surg. 2009;35(1):705.

Editors note: Dr. Salz is a fellow at the New England Eye Center at Tufts Medical Center, Boston. Dr. Salz has no financial interests related to this article.

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