September 2018

RESIDENTS

EyeWorld journal club
Review of “Refractive outcomes after limbal relaxing incision or femtosecond laser-assisted astigmatic keratotomy in the management of corneal astigmatism at the time of cataract surgery”


by Mary Kate Russell, MD, Alexander Knezevic, MD, Lauren May, MD, and Dmitry Pyatetsky, MD


Dmitry Pyatetsky, MD,
ophthalmology residency program director, Northwestern University Feinberg School of Medicine


From left: Alex Knezevic, MD, Dmitry Pyatetsky, MD, Mary Kate Russell, MD, and Lauren May, MD
Source: Northwestern University

Given the cost differences, the paucity of studies comparing manual and femtosecond laser astigmatic keratotomy is surprising. I invited the Northwestern residents to review this comparison study that appears in the August issue of JCRS.

—David F. Chang, MD,
EyeWorld journal club editor


Astigmatism is a common component of refractive error. A study of 24,000 preoperative cataract patients showed that 30% of patients had >1 D of cylinder on biometry, with the average being 0.98 D.1 Patients increasingly expect spectacle independence after cataract surgery. Thus, the management of astigmatism during cataract surgery is of utmost importance. Today, many surgeons push for <0.5 D, as this is considered the threshold of success in studies of astigmatic correction.2,3
Common treatments for astigmatism include manual limbal relaxing incisions (LRIs), femtosecond laser astigmatic keratotomies (FS-AK), and toric IOLs. Roberts et al.’s study “Refractive outcomes after limbal relaxing incision or femtosecond laser-assisted astigmatic keratotomy in the management of corneal astigmatism at the time of cataract surgery” investigates the correction of mild to moderate astigmatism in conjunction with standard cataract surgery.
Limbal relaxing incisions have been performed for nearly 20 years. The typical procedure involves making a single or paired 600-micron (within 50 microns of full thickness) incision along the steep axis, parallel to the limbus.4
FS-AK is a newer addition to this genre of procedures, and typically involves arcuate incisions at 95% depth along the steep meridian of the midperipheral cornea or as in Roberts et al.’s study, non-penetrating intrastromal arcuate corneal laser cuts.5
To our knowledge, a direct comparison of outcomes between manual LRIs and the use of FS-AK in routine cataract surgery has not been published. In Roberts et al.’s study, each procedure was performed in conjunction with cataract surgery (phacoemulsification with Infiniti vs. LenSx femtosecond laser-assisted cataract surgery, Alcon, Fort Worth, Texas) at a single surgery center. Four hundred patients with >0.9 D of cylinder were randomized. Paired LRIs were done prior to cataract surgery with a 600-micron guarded diamond knife. Donnenfeld’s nomogram was used for LRI calculations, aiming to correct 100% of the astigmatism. All of the FS-AK cases used the Day et al. nomogram,6 which is intended to correct up to 70%, but for calculation simplicity, Roberts et al.’s study had a goal of 100% correction.
The authors recorded postoperative visual acuity, refraction, and corneal topography (Pentacam, Oculus, Wetzlar, Germany). Postoperative astigmatism was compared to the preoperative measurements using the Alpins method. The Alpins method is commonly used in refractive surgery, comparing the ideal outcome, target-induced astigmatism (TIA), to the actual outcome, surgical-induced astigmatism (SIA). The difference between these values is the difference vector (DV). Each value has a magnitude and an axis.7 They also reported an “Index of Success,” which is the ratio of DV to TIA.
Both procedures undercorrected the astigmatism on average (SIA < TIA). Manual LRI corrected significantly less cylinder than FS-AK (p=0.02): 48% of the measured astigmatism versus 73%, respectively. When using a goal of <0.5 D of residual cylinder, 20% of the manual LRI group met the goal versus 44% of the FS-AK group. When the goal was set at <1 D postoperative cylinder, 44% of manual LRI cases were successful compared to 74% of FS-AK cases. Despite multiple parameters revealing statistically significant differences favoring FS-AK, the “Index of Success,” which factors in the preoperative criteria, was not statistically significant between the two groups (p=0.07). In addition, both groups achieved similar visual acuity outcomes with approximately 60% of patients in each group not needing additional refractive correction. Neither group had a misalignment of treatment as indicated by the arithmetic mean angle of error. This means that alignment accuracy was similar between the manual Mendez ring and the femtosecond laser, which can adjust for torsion.
The authors undertook a worthwhile goal of comparing manual LRIs to non-penetrating FS-AKs for astigmatism correction. This randomized study was well-powered, and though there were subtle differences in the baseline groups (namely preoperative visual acuity and axial length), we do not think this detracted from the significance of the results. The inclusion and exclusion criteria were appropriate and the results are likely predictive of the general population (with the caveat of intraoperative variables, e.g., surgeon performance on manual steps). The authors did a good job recognizing these limitations in their discussion.
An interesting finding in this study is that the alignment accuracy was similar between a manual Mendez-style ring and the femtosecond laser. However, there is an unreliable step in both manual LRI and FS-AK, which is the initial marking of the 180-degree axis. The authors did not detail how they did so in this paper, though traditionally it is manually marked by the surgeon while the patient is sitting up and fixating on a distant target. Application of digital markerless alignment systems would make this a more reliable step and possibly further improve outcomes.
An important caveat in this study is that only 1 month of follow-up was recorded. It would be beneficial to compare these two entities over a longer period of time to ensure regression does not occur. The authors address this issue, but are confident in their results because prior data shows stability of both manual LRI8 and FS-AK9 out to 2 and 3 years postop, respectively. Our own literature review similarly did not find evidence of regression of these procedures on longer-term follow-up.10 Nonetheless, it would be prudent to directly compare the changes between these two procedures over time. The authors of this study plan to publish their 12-month outcomes. There has not been a published cost analysis comparing manual limbal relaxing incisions to the use of the femtosecond laser, which also may have a significant influence on the procedure that the surgeon and patient choose. The authors plan to submit this data in the future.
Lastly, the authors discussed that the manual LRI group and the FS-AK group had similar “need for spectacle correction” after surgery. Many surgeons would consider this the most important outcome when it comes to patient satisfaction. Prior studies cite that ocular symptoms manifest at 0.75 D of astigmatism.11 We think it would be interesting to design a study measuring visual data other than distance acuity, e.g., situations with bright lights in a dim setting, to see if there is a difference in patient function due to glare or ghosting of images. One could also compare quality of life in relation to spectacle independence (as performed in a study comparing toric IOLs to peripheral corneal incisions).12
This study is an important contribution to the literature on astigmatic correction alongside cataract surgery. We look forward to seeing the 12-month results as well as the cost analysis that Roberts et al. plan to publish.

References

1. Hoffmann PC, Hutz WW. Analysis of biometry and prevalence data for corneal astigmatism in 23,239 eyes. J Cataract Refract Surg. 2010;36:1479–85.
2. Razmjoo H, et al. Toric intraocular lens for astigmatism correction in cataract patients. Adv Biomed Res. 2017;6:123.
3. Young G, et al. Prevalence of astigmatism in relation to soft contact lens fitting. Eye Contact Lens. 2011;37:20–5.
4. Muller-Jensen K, et al. Limbal relaxing incisions to correct astigmatism in clear corneal cataract surgery. J Refract Surg. 1999;15:586–9.
5. Ruckl T, et al. Femtosecond laser-assisted intrastromal arcuate keratotomy to reduce corneal astigmatism. J Cataract Refract Surg. 2013;39:528–38.
6. Day AC, et al. Nonpenetrating femtosecond laser intrastromal astigmatic keratotomy in eyes having cataract surgery. J Cataract Refract Surg. 2016;42:102–9.
7. Alpins NA. A new method of analyzing vectors for changes in astigmatism. J Cataract Refract Surg. 1993;19:524–33.
8. Lim R, et al. Long-term stability of keratometric astigmatism after limbal relaxing incisions. J Cataract Refract Surg. 2014;40:1676–81.
9. Chan TC, et al. Corneal astigmatism and aberrations after combined femtosecond-assisted phacoemulsification and arcuate keratotomy: two-year results. Am J Ophthalmol. 2016;170:83–90.
10. Arraes JC, et al. [Limbal relaxing incisions during cataract surgery: one-year follow-up]. Arq Bras Oftalmol. 2006;69:361–4.
11. Nichamin LD. Astigmatism control. Ophthalmol Clin North Am. 2006;19:485–93.
12. Mingo-Botin D, et al. Comparison of toric intraocular lenses and peripheral corneal relaxing incisions to treat astigmatism during cataract surgery. J Cataract Refract Surg. 2010;36:1700–8.

Contact information

Pyatetsky
: d-pyatetsky@northwestern.edu

Review of “Refractive outcomes after limbal relaxing incision or femtosecond laser-assisted astigmatic keratotomy in the management of corneal astigmatism at the time of cataract surgery” Review of “Refractive outcomes after limbal relaxing incision or femtosecond laser-assisted astigmatic
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