Laser-assisted refractive cataract surgery using femtosecond lasers has become the most recent disruptive technology to reach the cataract surgeon. The femtosecond lasers currently being brought to market are indicated for anterior capsulotomy, lens fragmentation and partial- and full-thickness corneal cuts, with applications for refractive keratectomy and surgical incisions. Femtosecond lasers have a unique ability to create small, discreet photodisruption of tissue with minimal collateral effects.
Improved anterior capsulotomy precision
Laser capsulotomy brings precision and reproducibility to the process of creating a capsular opening. This enables very precise cutting of ocular tissues, including the cornea, lens capsule and crystalline lens itself. (See Figure 1.)
Figure 1. SEM of the effect of a single laser pulse on a crystalline lens fiber.
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Studies have shown that the diameter with laser capsulotomy is significantly closer to the intended diameter than with manual continuous curvilinear capsulorhexis (CCC). Differences in the deviation from the intended diameter reported between published papers reflect differences in the way the outcomes were measured. Such differences can arise when researchers attempt to measure the capsular opening and an arbitrary correction has to be made for anterior chamber depth and the refractive index of the aqueous humor or balanced salt solution (BSS) present at the time of measurement, or while measuring the removed capsular button, which may have undergone changes due to variations in hydration or mechanical changes from the cutting of the capsule fibers.
Ramon Naranjo-Tackman, MD, and colleagues compared the deviation from intended capsulotomy diameter in buttons removed from cataract patients with a laser system versus a manual CCC.1 As shown in Table 1, the laser capsulotomies were significantly closer to the intended diameter. Figure 2 shows a typical button retrieved after laser capsulotomy, illustrating the regular spherical shape obtained.
Table 1. Capsulotomy data by group. Differences are significant (P = 0.03).1
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Figure 2. This typical capsule button created by femtosecond laser capsulotomy demonstrates regularity of shape. (Courtesy of LensAR, Inc.)
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Another recently published study reported on a similar series, although the analysis of button diameter was based on the mean of just four diameters across the button.2 The researchers reported a mean deviation from intended diameter of 29 μm for laser capsulotomy compared with 337 μm for manual capsulorhexis.
Importance of the capsulotomy
The capsulotomy and its relationship to the IOL implanted in the bag have a significant influence on the final resting position of the implant. The centration of the capsulotomy influences the centration of the IOL, which makes proper placement of the capsulotomy key.
Some systems offer the option of centering the capsulotomy over the pupil's center, which is typically where a manual CCC is placed, or over the optical axis of the crystalline lens. This latter option will place the optical axis of the IOL in the same position as that of the crystalline lens being removed (assuming centration of the lens within the bag) and is least likely to cause avoidable induced optical aberrations.
The size of the capsulotomy may influence the progression of posterior capsule opacification (PCO). Current practice requires the capsulotomy to be in contact with the optic of the IOL around its circumference. However, significant variations in the extent of this contact or areas where there is no contact may cause the lens to decenter and to tilt as the capsule contracts after surgery. This will influence the postoperative refractive outcome, which has become increasingly important, especially with premium IOLs. Two studies have reported improved overlap of the capsulotomy edge on the optic of the IOL and less horizontal decentration of the IOL when laser capsulotomy was used.3,4
It has been proposed that the consistency of laser capsulotomy may increase the consistency of effective lens position (ELP) and hence the ability to hit the target postoperative refractive result. Early data from two studies lend support to this idea.5,6 In a study of 44 cases undergoing laser capsulotomy and 62 cases undergoing manual CCC, a significantly higher proportion produced the intended refractive outcome at six months by a factor of four. Table 2 shows the results.5
Table 2. Percentage of cases achieving the required refractive outcome.5
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Lens fragmentation
Fragmentation of the lens prior to cataract surgery is intended to reduce or eliminate the need for ultrasound energy during nuclear disassembly. In the ideal situation, the nucleus would simply be removed by aspiration. However, harder nuclei might still require some ultrasound emulsification. In the quest for optimal outcomes, different cutting patterns and algorithms (shot placement, energy and pulse repetition frequency) may all influence the efficiency of the fragmentation.
Research conducted by Louis D. Nichamin, MD, and Harvey S. Uy, MD, reveals some of the patterns evaluated during an early phase of the development of lens fragmentation (see Figure 3).7 They found that different patterns had varying levels of effectiveness depending on the hardness of the cataract being treated. Overall, the pie pattern proved the most effective over the range of cataract grades from 1 to 5+.
Figure 3. Lens fragmentation patterns evaluated. The pie pattern was the most effective over the range of cataract grades.7
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LensAR has presented data on the effectiveness of phaco-fragmentation obtained by comparing the total ultrasound energy required for nuclear disassembly following laser lens fragmentation with that required during conventional ultrasound phacoemulsification surgery. The data was from a single surgeon who used the Alcon Infiniti System with the Ozil handpiece for all cases. The results submitted to the FDA as part of the successful 510(k) application are shown in Tables 3 and 4.8 The reduction in the use of ultrasound energy may lead to other benefits, such as less corneal edema, faster visual recovery and a lower rate of endothelial cell loss.
*Grade 4+ includes all higher graded cataracts.
Table 3. Analysis of mean (SD) cumulative dissipated energy (CDE) as a function of preoperative nuclear cataract grade.8
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Table 4. Number of subjects and statistical significance by cataract grade.8
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Dr. Uy and I will present data at the Academy's 2011 Annual Meeting in Orlando demonstrating that endothelial cell density changes from baseline are less following laser lens fragmentation compared with conventional ultrasound phacoemulsification.9 Table 5 summarizes these results.
Table 5. Changes in endothelial cell density between baseline and three months after surgery.9
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Corneal incisions
The application of femtosecond lasers to corneal surgical incisions is less well documented in the literature. In an early study with cadaver eyes, Samuel Masket, MD, and colleagues demonstrated the ability of a single-plane laser to construct self-sealing incisions that remained competent in the presence of significant IOP elevation or indentation.10
Daniel Palanker, MD, and colleagues have reported that three-plane laser incisions were self-sealing and watertight at physiological IOPs.11 It is not clear whether this applies to the incision immediately after its creation or at the end of surgery following the use of the phaco hand-piece and IOL insertion. Figures 4a and 4b show OCT images of the wound one day after surgery.
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Figure 4a and 4b. OCT images of incisions as they appear one day after surgery. (Courtesy of LensAR, Inc.)
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The effectiveness of laser limbal relaxing incisions or astigmatic keratotomy has yet to be established in the literature, although the precision of the laser at creating incisions of the precise length and depth required suggests that the procedure should be more reproducible and reliable than manual methods.
References
- Tackman RN, Villar Kuri J, Nichamin LD, Edwards K. Anterior capsulotomy with an ultrashort-pulse laser. J Cataract Refract Surg. 2011;37(5):819-824.
- Friedman NJ, Palanker DV, Schuele G, Andersen D, Marcellino G, Seibel BS, Batlle J, Feliz R, Talamo JH, Blumenkranz MS, Culbertson WW. Femtosecond laser capsulotomy. J Cataract Refract Surg. 2011;37(7):1189-1198.
- Kránitz K, Takacs A, Miháltz K, Kovács I, Knorz MC, Nagy ZZ. Femtosecond laser capsulotomy and manual continuous curvilinear capsulorrhexis parameters and their effects on intraocular lens centration. J Refract Surg. 2011;27(8):558-563.
- Nagy ZZ, Kránitz K, Takacs AI, Miháltz K, Kovács I, Knorz MC. Comparison of intraocular lens decentration parameters after femtosecond and manual capsulotomies. J Refract Surg. 2011;27(8):564-569.
- Uy H, Hill WE, Edwards K. Refractive results after laser anterior capsulotomy. Invest Ophthalmol Vis Sci. 2011;52:5695.
- Hill WE, Uy H. Effective lens position following laser anterior capsulotomy. Paper scheduled to be presented at: Annual Meeting of the American Academy of Ophthalmology; October 2011; Orlando, FL.
- Nichamin LD, Uy H. Choice of fragmentation algorithm impacts the reduction in CDE during cataract surgery. Paper presented at: Annual Meeting of the American Academy of Ophthalmology; Oct. 17, 2010; Chicago, IL.
- LensAR, Inc. 510(k) Number K102727. For: LensAR Laser System for Anterior Capsulotomy. Orlando, FL. Received by FDA Sept. 22, 2010.
- Packer M, Uy H. Endothelial changes after laser phaco-fragmentation. Paper scheduled to be presented at: Annual Meeting of the American Academy of Ophthalmology; October 2011; Orlando, FL.
- Masket S, Sarayba M, Ignacio T, Fram N. Femtosecond laser-assisted cataract incisions: Architectural stability and reproducibility. J Cataract Refract Surg. 2010;36(6):1048-1049.
- Palanker DV, Blumenkranz MS, Andersen D, Wiltberger M, Marcellino G, Gooding P, Angeley D, Schuele G, Woodley B, Simoneau M, Friedman NJ, Seibel B, Batlle J, Feliz R, Talamo J, Culbertson W. Femtosecond laser-assisted cataract surgery with integrated optical coherence tomography. Sci Translat Med. 2010;2(58):58ra85.