Better IOL Position, Better Vision After Cataract Surgery
Filkorn T, Kovacs I, Takacs A, Horvath E, Knorz MC, Nagy ZZ
J Refract Surg. 2012;28:540-544
Many patients are disappointed after cataract surgery because the visual outcomes do not meet their preoperative expectations. The primary reason for this shortfall has been residual refractive error. Recent studies have shown that 30%-50% of eyes have more than 0.50 diopters of postoperative ametropia, resulting in correspondingly reduced uncorrected visual acuities of 20/40 or less, 45% of the time. These outcomes fall short of what surgeons and patients have come to expect from excimer laser keratorefractive surgery, after which 20/20 vision is achieved in 90% of eyes within a month after surgery. These amazing visual outcomes following refractive surgery (eg, LASIK, PRK) have fostered the same high level of expectations after cataract surgery.
Marketing of "premium" intraocular lens (IOL) cataract surgery by both surgeons and the IOL industry has also raised expectations, while at the same time optimal unaided visual results with premium IOLs require a higher level of precise IOL power selection than does nonpremium IOL surgery.
Although many factors are cumulatively responsible, the most important component has been the unpredictability of the final resting place of the IOL in the eye -- both front-to-back (the effective lens position [ELP]) and side-to-side (IOL centration). A small 0.1-mm anterior-posterior deviation of the IOL from its predicted ELP can cause as much as 0.10 diopter of deviation from the intended outcome. Lateral "decentration" of an IOL can lead to aberrations and astigmatism, resulting in reduced visual acuity and contrast sensitivity. One of the mediators of the final IOL position and the ELP is believed to be the combination of the size, shape, and position of the anterior capsulotomy. Small anterior capsulotomy openings displace the IOL (and the ELP) position relatively posteriorly and larger openings allow the lens to move forward, with a hyperopic effect and a myopic effect, respectively. Irregular and/or decentered capsulotomy openings induce suboptimal early IOL centration as well as late lateral displacement of the IOL through progressive asymmetric capsular contraction.
Anterior capsulotomies performed with the femtosecond laser have been shown to be cut precisely at the diameter, circularity, and position, as programmed into the laser. This shape, size, and position of the capsulotomy seem to be more precise than those that are manually created; this difference is maintained well into the postoperative period. Advocates of femtosecond laser-assisted refractive cataract surgery have theorized that the laser's ability to predictably create the capsulotomy would similarly result in a more predictable ELP and IOL centration, with greater stability of the refraction over time.
This study by Filkorn and colleagues sets out to test the theory that a predictable and uniform anterior capsulotomy made during cataract surgery with the LenSx femtosecond laser (Alcon, Fort Worth, Texas) has better refractive and visual outcomes than the traditional manual continuous curvilinear capsulorrhexis technique. One surgeon (ZZ Nagy) did all the procedures. In 77 eyes of 77 patients, the LenSx femtosecond cataract laser was used to make incisions (main and side port incisions) as well as 4.5-mm anterior capsulotomies centered on the dilated pupil. The laser was also used to prechop the nucleus into 4 quadrants to facilitate extraction.
In a separate group of 57 eyes of 57 patients, a conventional continuous curvilinear capsulotomy of an attempted diameter of 4.5 mm was made manually. The main clear corneal incision in both groups was 2.75 mm at 120 degrees, and the side port incision was 1.2 mm at 60 degrees for both the laser incision-created cases and the manual blade incision cases. A standard chop phacoemulsification procedure was used. The IOL power calculation was estimated with the Lenstar LS 900 (Haag-Streit AG, Koeniz, Switzerland) using the appropriate third-generation IOL formula depending on axial length. Six models of IOLs were used.
Comparison of the results between the 2 groups at 6-12 weeks showed that more eyes were within ±0.25 diopters of emmetropia in the laser group (42%) than in the manual group (28%). The laser group had 99% within ± 1.00 diopter, whereas the conventional (manual) group had 88% ± 1.00 diopter. The average absolute error for the 2 groups was correspondingly better for the laser group (0.38 ± 0.28D) vs the manual group (0.50 ± 0.38D). These differences were statistically significant and probably resulted from a more predictable size and shape of the anterior capsulotomy in the laser group than in the manual group, although the difference in capsulotomy size was not measured or assessed in this study.
Previous studies have reported on the precision of shape and diameter of the anterior capsulotomies created with the femtosecond laser compared with manually made capsulotomies. This precision would be expected to reduce the variation in the effective lens position in the axial dimension as well as improve IOL centration in the coronal plane. In addition, a perfectly round anterior capsulotomy would be expected to have less IOL tilt and more regular and predictable fibrosis of the anterior capsular rim. The cumulative effect should be less residual refractive error and better uncorrected vision.
Astigmatism was not addressed in this study, but the ability of the femtosecond laser to create very precise corneal relaxing incisions would probably result in better uncorrected postsurgical vision. Also, with 360-degree overlap of the anterior capsule over the edge of the IOL optic, less posterior capsule fibrosis would be expected in the laser cases than in the manual cases.
This study, as well as others sure to follow, should validate the utility of the femtosecond laser in optimizing visual results and the surgical experience for patients. With very predictable capsulotomy dimensions, and position, correspondingly greater predictability of the "surgeon factor" should further refine IOL power selection.
Abstract
Comparison of IOL Power Calculation and Refractive Outcome After Laser Refractive Cataract Surgery With a Femtosecond Laser Versus Conventional Phacoemulsification
Filkorn T, Kovacs I, Takacs A, Horvath E, Knorz MC, Nagy ZZ
J Refract Surg. 2012;28:540-544
Why Are Patients Dissatisfied After Cataract Surgery?
Many patients are disappointed after cataract surgery because the visual outcomes do not meet their preoperative expectations. The primary reason for this shortfall has been residual refractive error. Recent studies have shown that 30%-50% of eyes have more than 0.50 diopters of postoperative ametropia, resulting in correspondingly reduced uncorrected visual acuities of 20/40 or less, 45% of the time. These outcomes fall short of what surgeons and patients have come to expect from excimer laser keratorefractive surgery, after which 20/20 vision is achieved in 90% of eyes within a month after surgery. These amazing visual outcomes following refractive surgery (eg, LASIK, PRK) have fostered the same high level of expectations after cataract surgery.
Marketing of "premium" intraocular lens (IOL) cataract surgery by both surgeons and the IOL industry has also raised expectations, while at the same time optimal unaided visual results with premium IOLs require a higher level of precise IOL power selection than does nonpremium IOL surgery.
Although many factors are cumulatively responsible, the most important component has been the unpredictability of the final resting place of the IOL in the eye -- both front-to-back (the effective lens position [ELP]) and side-to-side (IOL centration). A small 0.1-mm anterior-posterior deviation of the IOL from its predicted ELP can cause as much as 0.10 diopter of deviation from the intended outcome. Lateral "decentration" of an IOL can lead to aberrations and astigmatism, resulting in reduced visual acuity and contrast sensitivity. One of the mediators of the final IOL position and the ELP is believed to be the combination of the size, shape, and position of the anterior capsulotomy. Small anterior capsulotomy openings displace the IOL (and the ELP) position relatively posteriorly and larger openings allow the lens to move forward, with a hyperopic effect and a myopic effect, respectively. Irregular and/or decentered capsulotomy openings induce suboptimal early IOL centration as well as late lateral displacement of the IOL through progressive asymmetric capsular contraction.
Anterior capsulotomies performed with the femtosecond laser have been shown to be cut precisely at the diameter, circularity, and position, as programmed into the laser. This shape, size, and position of the capsulotomy seem to be more precise than those that are manually created; this difference is maintained well into the postoperative period. Advocates of femtosecond laser-assisted refractive cataract surgery have theorized that the laser's ability to predictably create the capsulotomy would similarly result in a more predictable ELP and IOL centration, with greater stability of the refraction over time.
Study Summary
This study by Filkorn and colleagues sets out to test the theory that a predictable and uniform anterior capsulotomy made during cataract surgery with the LenSx femtosecond laser (Alcon, Fort Worth, Texas) has better refractive and visual outcomes than the traditional manual continuous curvilinear capsulorrhexis technique. One surgeon (ZZ Nagy) did all the procedures. In 77 eyes of 77 patients, the LenSx femtosecond cataract laser was used to make incisions (main and side port incisions) as well as 4.5-mm anterior capsulotomies centered on the dilated pupil. The laser was also used to prechop the nucleus into 4 quadrants to facilitate extraction.
In a separate group of 57 eyes of 57 patients, a conventional continuous curvilinear capsulotomy of an attempted diameter of 4.5 mm was made manually. The main clear corneal incision in both groups was 2.75 mm at 120 degrees, and the side port incision was 1.2 mm at 60 degrees for both the laser incision-created cases and the manual blade incision cases. A standard chop phacoemulsification procedure was used. The IOL power calculation was estimated with the Lenstar LS 900 (Haag-Streit AG, Koeniz, Switzerland) using the appropriate third-generation IOL formula depending on axial length. Six models of IOLs were used.
Comparison of the results between the 2 groups at 6-12 weeks showed that more eyes were within ±0.25 diopters of emmetropia in the laser group (42%) than in the manual group (28%). The laser group had 99% within ± 1.00 diopter, whereas the conventional (manual) group had 88% ± 1.00 diopter. The average absolute error for the 2 groups was correspondingly better for the laser group (0.38 ± 0.28D) vs the manual group (0.50 ± 0.38D). These differences were statistically significant and probably resulted from a more predictable size and shape of the anterior capsulotomy in the laser group than in the manual group, although the difference in capsulotomy size was not measured or assessed in this study.
Viewpoint
Previous studies have reported on the precision of shape and diameter of the anterior capsulotomies created with the femtosecond laser compared with manually made capsulotomies. This precision would be expected to reduce the variation in the effective lens position in the axial dimension as well as improve IOL centration in the coronal plane. In addition, a perfectly round anterior capsulotomy would be expected to have less IOL tilt and more regular and predictable fibrosis of the anterior capsular rim. The cumulative effect should be less residual refractive error and better uncorrected vision.
Astigmatism was not addressed in this study, but the ability of the femtosecond laser to create very precise corneal relaxing incisions would probably result in better uncorrected postsurgical vision. Also, with 360-degree overlap of the anterior capsule over the edge of the IOL optic, less posterior capsule fibrosis would be expected in the laser cases than in the manual cases.
This study, as well as others sure to follow, should validate the utility of the femtosecond laser in optimizing visual results and the surgical experience for patients. With very predictable capsulotomy dimensions, and position, correspondingly greater predictability of the "surgeon factor" should further refine IOL power selection.
Abstract
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