Monte Carlo Simulation of Expected Outcomes With the AcrySof Lens
Background: To use a Monte Carlo simulation to predict postoperative results with the AcrySof Toric lens, evaluating the likelihood of over- or under-correction using various toric lens selection criteria.
Methods: Keratometric data were obtained from a large patient population with preoperative corneal astigmatism <= 2.50D (2,000 eyes). The probability distributions for toric marking accuracy, surgically induced astigmatism and lens rotation were estimated using available data. Anticipated residual astigmatism was calculated using a Monte Carlo simulation under two different lens selection scenarios.
Results: This simulation demonstrated that random errors in alignment, surgically induced astigmatism and lens rotation slightly reduced the overall effect of the toric lens. Residual astigmatism was statistically significantly higher under the simulation of surgery relative to an exact calculation (p < 0.05). The simulation also demonstrated that more aggressive lens selection criteria could produce clinically significant reductions in residual astigmatism in a high percentage of patients.
Conclusion: Monte Carlo simulation suggests that surgical variability and lens orientation/rotation variability may combine to produce small reductions in the correction achieved with the AcrySof Toric IOL. Adopting more aggressive lens selection criteria may yield significantly lower residual astigmatism values for many patients, with negligible overcorrections. Surgeons are encouraged to evaluate their AcrySof Toric outcomes to determine if they should modify their individual lens selection criteria, or their default surgically induced astigmatism value, to benefit their patients.
Continued advances in small-incision phacoemulsification have increased the stability and predictability of cataract surgery, reducing healing time and intraoperative/postoperative complications. For normal eyes, modern cataract surgery can typically provide a refractive correction that is often within 0.5D of the targeted spherical correction.
Advances in the ability to correct the refractive errors of cataract patients have also been made, particularly with regard to astigmatism. Corneal astigmatism is often reduced by the use of peripheral corneal relaxing incisions, but the introduction of toric intraocular lenses now provides an opportunity to more precisely reduce or eliminate a patient's astigmatism, particularly if consideration of the induced astigmatism from the surgical incision is included to calculate the expected postoperative corneal astigmatism.
The AcrySof Toric lens is used with a new method for correcting corneal astigmatism in pseudophakic eyes. This lens is implanted in conjunction with a Toric Lens Calculator, which uses surgeon-provided keratometry and surgically induced astigmatism data to select the most appropriate toric lens and calculate the optimal angle of placement (http://www.acrysoftoriccalculator.com, Alcon Laboratories, Inc.). Three different powers at the lens plane provide nominal corneal plane correction of 1.03D, 1.55D and 2.06D of astigmatism in a variety of spherical powers (Product Information, AcrySof Toric, Alcon Laboratories, Inc.).
The success of a toric lens hinges on accurate and stable correction. With regard to accuracy, the surgically induced astigmatism must be taken into account. The lens must be implanted in precise alignment with the required axis of correction; this is usually achieved using a corneal marker to intraoperatively identify the correct lens orientation. With regard to stability, the lens must maintain its intended orientation over time. This latter element is key to the success of the lens over the long term.
The challenge with regard to accurate and stable correction is that ophthalmic surgery is, by its very nature, a variable procedure. Surgically induced astigmatism, while it can be quite low on average, will vary from eye to eye and surgeon to surgeon, and can have a significant effect on outcomes. The process of marking the cornea to properly align the toric lens may also be less than perfect and the axis alignment achieved at the time of surgery may not be exact. Individual lenses may also rotate during the postoperative period. All of these rather small errors in alignment and stability will typically combine to reduce the potential effectiveness of any toric lens; the more these errors are brought under control, the lower the potential effect. In general, each 1 degree error in lens alignment will reduce the effectiveness of the astigmatism correction by 3.33%.
The current version of the Alcon toric calculator provides the surgeon a lens recommendation that is based on avoiding any overcorrection, as patients are believed more tolerant of low levels of astigmatism along their original axis than they are of astigmatism that is orthogonal to it. However, given the challenges in lens orientation and stability, it may be that this approach is too conservative to be practical. For instance, a patient with 1.03D of astigmatism is recognized as a candidate for the 1.03 diopter toric lens (T3). However, a patient with 0.80D may not be. This is because this patient would, in the case of exact correction, be left with 0.80D of astigmatism if a spherical lens were used but if the T3 lens were to be used this same patient might be left with 0.23D of astigmatism in an orthogonal meridian, which is deemed an undesirable result. The practical limitation here is that the analysis presumes a perfect correction. Because astigmatic correction is directional, a deviation from the desired orientation and stability of the toric lens will reduce the effect of the lens in the desired direction and introduce a corresponding change in net axis of astigmatism; the likelihood of an overcorrection of astigmatism is significantly lower when this occurs.
One might then argue that such a patient would have been better off with a T3 correction rather than the spherical correction recommended by the AcrySof toric IOL calculator.
The mathematical exercise in this paper was to assume that the factors involved in toric lens implantation are slightly variable, and to predict the likely effects of that variability using a Monte Carlo simulation with different lens selection criteria.
Abstract and Background
Abstract
Background: To use a Monte Carlo simulation to predict postoperative results with the AcrySof Toric lens, evaluating the likelihood of over- or under-correction using various toric lens selection criteria.
Methods: Keratometric data were obtained from a large patient population with preoperative corneal astigmatism <= 2.50D (2,000 eyes). The probability distributions for toric marking accuracy, surgically induced astigmatism and lens rotation were estimated using available data. Anticipated residual astigmatism was calculated using a Monte Carlo simulation under two different lens selection scenarios.
Results: This simulation demonstrated that random errors in alignment, surgically induced astigmatism and lens rotation slightly reduced the overall effect of the toric lens. Residual astigmatism was statistically significantly higher under the simulation of surgery relative to an exact calculation (p < 0.05). The simulation also demonstrated that more aggressive lens selection criteria could produce clinically significant reductions in residual astigmatism in a high percentage of patients.
Conclusion: Monte Carlo simulation suggests that surgical variability and lens orientation/rotation variability may combine to produce small reductions in the correction achieved with the AcrySof Toric IOL. Adopting more aggressive lens selection criteria may yield significantly lower residual astigmatism values for many patients, with negligible overcorrections. Surgeons are encouraged to evaluate their AcrySof Toric outcomes to determine if they should modify their individual lens selection criteria, or their default surgically induced astigmatism value, to benefit their patients.
Background
Continued advances in small-incision phacoemulsification have increased the stability and predictability of cataract surgery, reducing healing time and intraoperative/postoperative complications. For normal eyes, modern cataract surgery can typically provide a refractive correction that is often within 0.5D of the targeted spherical correction.
Advances in the ability to correct the refractive errors of cataract patients have also been made, particularly with regard to astigmatism. Corneal astigmatism is often reduced by the use of peripheral corneal relaxing incisions, but the introduction of toric intraocular lenses now provides an opportunity to more precisely reduce or eliminate a patient's astigmatism, particularly if consideration of the induced astigmatism from the surgical incision is included to calculate the expected postoperative corneal astigmatism.
The AcrySof Toric lens is used with a new method for correcting corneal astigmatism in pseudophakic eyes. This lens is implanted in conjunction with a Toric Lens Calculator, which uses surgeon-provided keratometry and surgically induced astigmatism data to select the most appropriate toric lens and calculate the optimal angle of placement (http://www.acrysoftoriccalculator.com, Alcon Laboratories, Inc.). Three different powers at the lens plane provide nominal corneal plane correction of 1.03D, 1.55D and 2.06D of astigmatism in a variety of spherical powers (Product Information, AcrySof Toric, Alcon Laboratories, Inc.).
The success of a toric lens hinges on accurate and stable correction. With regard to accuracy, the surgically induced astigmatism must be taken into account. The lens must be implanted in precise alignment with the required axis of correction; this is usually achieved using a corneal marker to intraoperatively identify the correct lens orientation. With regard to stability, the lens must maintain its intended orientation over time. This latter element is key to the success of the lens over the long term.
The challenge with regard to accurate and stable correction is that ophthalmic surgery is, by its very nature, a variable procedure. Surgically induced astigmatism, while it can be quite low on average, will vary from eye to eye and surgeon to surgeon, and can have a significant effect on outcomes. The process of marking the cornea to properly align the toric lens may also be less than perfect and the axis alignment achieved at the time of surgery may not be exact. Individual lenses may also rotate during the postoperative period. All of these rather small errors in alignment and stability will typically combine to reduce the potential effectiveness of any toric lens; the more these errors are brought under control, the lower the potential effect. In general, each 1 degree error in lens alignment will reduce the effectiveness of the astigmatism correction by 3.33%.
The current version of the Alcon toric calculator provides the surgeon a lens recommendation that is based on avoiding any overcorrection, as patients are believed more tolerant of low levels of astigmatism along their original axis than they are of astigmatism that is orthogonal to it. However, given the challenges in lens orientation and stability, it may be that this approach is too conservative to be practical. For instance, a patient with 1.03D of astigmatism is recognized as a candidate for the 1.03 diopter toric lens (T3). However, a patient with 0.80D may not be. This is because this patient would, in the case of exact correction, be left with 0.80D of astigmatism if a spherical lens were used but if the T3 lens were to be used this same patient might be left with 0.23D of astigmatism in an orthogonal meridian, which is deemed an undesirable result. The practical limitation here is that the analysis presumes a perfect correction. Because astigmatic correction is directional, a deviation from the desired orientation and stability of the toric lens will reduce the effect of the lens in the desired direction and introduce a corresponding change in net axis of astigmatism; the likelihood of an overcorrection of astigmatism is significantly lower when this occurs.
One might then argue that such a patient would have been better off with a T3 correction rather than the spherical correction recommended by the AcrySof toric IOL calculator.
The mathematical exercise in this paper was to assume that the factors involved in toric lens implantation are slightly variable, and to predict the likely effects of that variability using a Monte Carlo simulation with different lens selection criteria.
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