CATARACT SURGERY BONUS FEATURE The Promise of Precise LRIs With the Femtosecond Laser Femtosecond LRIs have the potential to increase the accuracy and predictability of astigmatism correction at the time of cataract surgery or refractive lens exchange. BY MARK PACKER, MD, FACS, CPI n astigmatically neutral postoperative result is a major goal of modern small-incision phacoemul- sification and refractive lens surgery. Techniques Laser-mediated LRIs should make and techologies including carefully planned inci- outcomes more predictable and Asion locations, intraoperative limbal relaxing incisions reliable for all surgeons. (LRIs),1-8 and toric lenses for higher levels of preoperative astigmatism have allowed surgeons to achieve this goal.9-11 LRIs are widely performed for low dioptric corrections.12 They preserve the optical qualities of the cornea, do not introduced as a more precise way to achieve incisional cause significant patient discomfort, and are sufficiently correction of astigmatism, including high degrees of effective for correcting lower levels of astigmatism. The astigmatism in postkeratoplasty eyes.16-18 Abbey et al limbal location of these incisions results in a consistent 1:1 demonstrated correction of about 2.50 to 3.00 D of natu- coupling ratio13 that causes little change in spherical rally occurring refractive astigmatism with femtosecond equivalent and eliminates the need to change implant laser astigmatic keratotomy.19 power. Gills1 and Osher14 were early advocates of these Common sense suggests that automating the incisional incisions. technology and thus eliminating the variability in per- Successfully placed LRIs reduce cylinder without an formance that is an attribute of manual incision construc- overcorrection or axis shift. Determining the exact loca- tion will lead to greater reproducibility in results. tion of the cylinder, however, is often challenging. The potential for femtosecond LRIs to place the pho- Keratometry, refraction, and corneal topography may todisruptive cutting effect at the right orientation and to not correlate. Of these, corneal topography has been the make cuts of the correct length and depth to create the most helpful and is most often used to guide the surgical desired refractive effect should lead to greater consistency plan and to evaluate the postoperative result. In addition of outcomes. The laser’s ability to make sub-Bowman’s to diagnostic methodology, incision construction tech- incisions may have benefits in limiting the effect of healing, nique has also varied among surgeons.15 Peripheral which can be responsible for regression of the effect. Sub- corneal pachymetry, incision depth, and optical zone size Bowman’s incisions may also improve patient comfort (distance from the visual axis or corneal apex to the inci- postoperatively. Overall, laser-mediated LRIs should make sions) appear to play important roles in effectiveness. outcomes more predictable and reliable for all surgeons. Variability in results is likely multifactorial, but a consis- tent technique is the minimum requirement for begin- UNDERSTANDING THE INCISION EFFECT ning to understand the variation in response. To develop LRIs with femtosecond technology, it is necessary to model the cornea so that changes induced BENEFITS OF AUTOMATION by the incisions can be predicted and the most effective Recently, femtosecond laser keratotomy has been treatment algorithm selected. The model should be able 18 CATARACT & REFRACTIVE SURGERY TODAY EUROPE JANUARY 2011 CATARACT SURGERY BONUS FEATURE Figure 1. Finite element analysis model of the cornea with Figure 2. Model of incision effect with respect to IOP. five layers. to predict the published outcomes of current surgical methods so that laser treatments can be compared. Changes in topography should be predictable and con- firmed on animal and human cadaver eye models. Finally, clinical testing should confirm the outcomes predicted from the model. Nichamin and coworkers developed a corneal model to study the effects of LRIs (Figure 1). The model includes five layers with different material properties: Bowman’s mem- brane (one layer), stroma (three layers), and Descemet’s membrane (one layer). These reflect the mechanical prop- erties of the cornea at different depths. An incision profile Figure 3. Regression of relaxing incision effect stratified by age. created for use with the corneal model was well defined in terms of arc length, depth and profile, and optic zone The regression of corneal power after incisional surgery diameter. To assess the stress distribution of the incisions, a has been studied for radial keratotomy; there is a tenden- nominal IOP must be selected. Outcomes will vary cy for greater regression with younger age. Initial surgery depending on the IOP selected (Figure 2). must therefore overcompensate the intended correction In using this model to study the effects of LRIs, we to allow for regression; the amount of compensation will assessed the effects of age, stiffness, and various incision be smaller for older patients. At 3 months, the model pre- lengths and optic zones. Figure 3 shows the results in dicts 39% regression for a 90° arc length at age 25 but only simulated 25- and 75-year-old corneas. The change in 25% regression at age 45. For shorter arc lengths the dif- refractive cylinder increases with the arc length of the ference is greater: 65% for a 30° arc in a 25-year-old and incision and is greater in the younger cornea than the 42% in a 45-year-old. When the finite element analysis older one. model data prediction is compared with the Nichamin nomogram for with-the-rule astigmatism, there is close TAKE-HOME MESSAGE correlation for both younger and older corneas.20 • Automating incisional technology and eliminating CORNEAL RESPONSE variability in performance may lead to greater reproducibility. The use of the femtosecond laser to create LRIs intro- • LRIs created with the femtosecond laser may mitigate the duces the possibility to create intrastromal incisions, effects of healing on regression. which may mitigate the effects of healing on regression. • Finite element analysis modeling and ex-vivo studies In our corneal model, as expected, the change in refrac- suggest that femtosecond LRIs produce results equivalent to tive power is reduced compared with surface incisions, those of manual LRIs. but the results can be enhanced by using multiple inci- sional arcs and differing incision heights (ie, how much of JANUARY 2011 CATARACT & REFRACTIVE SURGERY TODAY EUROPE 19 CATARACT SURGERY BONUS FEATURE eyes (16%) went on to LASIK enhancement. In a group of 30 eyes of patients who were measured with the device, one eye (3%) went on to LASIK enhancement. Eight of the 30 eyes (27%) had received intraoperative LRIs during the primary procedure based on aberrometry findings. The groups had similar pre- and postoperative characteristics. CONCLUSION The drive toward greater accuracy and predictability in the outcomes of our incisional surgery for astigma- tism represents a tremendous benefit for our patients because they will be able to achieve their desired results with a single procedure. Ultimately, our abilities to titrate and enhance precise LRIs may prove superior to outcomes achieved with toric IOLs for most degrees of astigmatic correction. ■ Figure 4. The intraoperative aberrometry: device is attached to microscope, and the display screen is behind the surgeon. Mark Packer, MD, FACS, CPI, is a Clinical Associate Professor at the Casey Eye Institute, the intrastromal tissue is ablated).20 Department of Ophthalmology, Oregon Health Ex-vivo work has demonstrated the feasibility of these & Science University, and is in private practice procedures on animal eyes, in which corneal power can at Drs. Fine, Hoffman & Packer, LLC, Eugene, be verified using instruments such as the Keratron Scout Oregon. He states that he is a consultant to LensAR, Inc., (Optikon 2000 Industrie, Rome). Finite element analysis and to WaveTec Vision Systems, Inc. Dr. Packer may be modeling has confirmed the corneal response to LRIs. reached at tel: +1 541 6872110; e-mail: Younger corneas showed a greater response to incisional [email protected]. surgery but also regressed more postoperatively.20 1.Gills JP.Treating astigmatism at the time of cataract surgery.Curr Opin Ophthalmol.2002;13(1):2-6. The model and early ex-vivo studies suggest that fem- 2.Kohnen T,Koch DD.Methods to control astigmatism at the time of cataract surgery.Curr Opin Ophthalmol.1996;7:75-80. tosecond LRIs produce results equivalent to those of 3.Nichamin LD. Astigmatism management at the time of cataract surgery.In:Krachmer JH,Mannis MJ,Holland EJ,eds. Cornea.2nd edition. St.Louis:Mosby;2004:7-11. manual LRIs. The effect of sub-Bowman’s incisions have 4.Nichamin LD.Astigmatism control.Ophthalmol Clin N Am.2006;19(4):485-493. been demonstrated in animals. 5.Amesbury EC,Miller KM.Correction of astigmatism at the time of cataract surgery.Curr Opin Ophthalmol. 2009;20(1):19-24. 6.Müller-Jensen K,Fischer P,Siepe U.Limbal relaxing incisions to correct astigmatism in clear corneal cataract surgery.J INTRAOPERATIVE ABERROMETRY Refract Surg.1999;15(5):586-589. 7.Budak D,Friedman NJ,Koch DD.Limbal relaxing incisions with cataract surgery. J Cataract Refract Surg.1998;24:503- Modeling and automation may reduce variation in 508. technique, but they do not take into account variation in 8.Kaufmann C,Peter J,Ooi K,et al.Limbal relaxing incisions versus on-axis incisions to reduce corneal astigmatism at the time of cataract surgery.J Cataract Refract Surg.2005;31(12):2261-2265. response. A recent advance in astigmatism management 9.Horn JD.Status of toric intraocular lenses.Curr Opin Ophthalmol.2007;18(1):58-61. may prove complementary to the femtosecond laser in 10 Mendicute J,Irigoyen C,Aramberri J,et al.Foldable toric intraocular lens for astigmatism correction in cataract patients.J Cataract Refract Surg.2008;34(4):601-607. this regard. ORange (WaveTec Vision, Aliso Viejo, 11.Bauer NJ,de Vries NE,Webers CA,et al.Astigmatism management in cataract surgery with the AcrySof toric intraoc- California) is an intraoperative aberrometer based on ular lens.J Cataract Refract Surg.2008;34(9):1483-1488.