- Email: [email protected]
May consultation #2
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CONSULTATION SECTION
Figure 2. Intraocular lens power calculation sheet with the SRK/T formula (PCI biometry).
OTHER CITED MATERIAL A. Hill W, Wang L, Koch DD. IOL calculator for eyes that have undergone LASIK/PRK/RK. Available at: http://iolcalc.org/. Accessed March 2, 2012
- Intraocular lens power calculation after refractive surgery has become a challenging problem for most clinicians. The inability to correctly measure the true corneal power after laser photoablation is the primary problem. The preoperative and postoperative information for this patient illustrates the aforementioned issue. The changes in the keratometry (K) readings and spherical equivalent (SE) after PRK are 2.64 D and C4.87 D, respectively. The data were reanalyzed using the ASCRS postrefractive surgery IOL calculator.A Formulas using perioperative historical data give higher IOL powers. The clinical history, Feiz-Mannis, and corneal bypass methods output 14.77 D, 14.92 D, and 14.68 D, respectively. We do not have the 6-month postoperative K readings and have to assume that they are the same as the current K values. We are also missing standard topographic data, such as the Atlas numerical map (Carl Zeiss Meditec AG) and the Pentacam equivalent K reading for the central 4.5 mm, as described by
Holladay et al.1 The remaining methods result in lower IOL powers; the Shammas, Haigis-L, and modified Masket formulas yield 14.61 D, 13.89 D, and 13.48 D, respectively. The ASCRS mean value recommends using an Acrysof SN60WF IOL (Alcon Laboratories, Inc.) with a power of 14.17 D (range 13.02 to 14.92 D). We have found that using a greater number of formulas leads to a more accurate IOL selection. OcularMD2 is another post-refractive IOL calculator available online that includes an additional subset of formulas, such as the Aramberri double-K, Latkany flat-K, and Latkany average-K methods. DeMill et al.2 found that combining the OcularMD and ASCRS data yielded more accurate outcomes. For this patient, the Ocular MD SRK/T and Haigis mean is 14.18 D (range 12.37 to 16.00 D) and essentially equals the ASCRS recommendation. Both Holladay and Haigis believe that the clinical history method (CHM) provides the most reliable estimate; thus, we would aim closer to 14.77 D and choose an Acrysof SN60WF 14.50 D IOL in this case.1,3 A conservative approach should always err on the side of myopia, and picking the same IOL with a 15.00 D power is also reasonable.
J CATARACT REFRACT SURG - VOL 38, MAY 2012
CONSULTATION SECTION
Other considerations include a surgeon's individual correction factors. Wound placement, capsulorhexis size, and induced astigmatism can affect the final refractive outcome. Also, given the very oblate profile of this cornea, we believe that an IOL with a more negative corneal asphericity would improve contrast sensitivity.4 We did not use the IOL power recommendation from the intraoperative wavefront aberrometry mentioned in the case. Although it is promising technology, Stringham et al.5 found that operating room variables as basic as speculum type can induce different anatomic changes that can confound the biometry. Majid Moshirfar, MD Gene Kim, MD Salt Lake City, Utah, USA REFERENCES 1. Holladay JT, Hill WE, Steinmueller A. Corneal power measurements using Scheimpflug imaging in eyes with prior corneal refractive surgery. J Refract Surg 2009; 25:862–868 2. DeMill DL, Moshirfar M, Neuffer MC, Hsu M, Sikder S. A comparison of the American Society of Cataract and Refractive Surgery post-myopic LASIK/PRK intraocular lens (IOL) calculator and the Ocular MD IOL calculator. Clin Ophthalmol 2011; 5:1409– 1414. Available at: http://www.dovepress.com/getfile.php? fileIDZ11070. Accessed March 2, 2012 3. Haigis W. Intraocular lens calculation after refractive surgery for myopia: Haigis-L formula. J Cataract Refract Surg 2008; 34:1658–1663 4. Nochez Y, Majzoub S, Pisella P-J. Effect of residual ocular spherical aberration on objective and subjective quality of vision in pseudophakic eyes. J Cataract Refract Surg 2011; 37:1076–1081 5. Stringham J, Pettey J, Olson RJ. Evaluation of variables affecting intraoperative aberrometry. J Cataract Refract Surg 2012; 38:470–474
OTHER CITED MATERIAL A. Hill W, Wang L, Koch DD. IOL calculator for eyes that have undergone LASIK/PRK/RK. Available at: http://iolcalc.org/. Accessed March 2, 2011
- The number of patients who have had keratorefractive surgery is increasing every year, which in turn increases the number of post-refractive surgery patients who will require cataract surgery. New-generation IOLs and biometry calculation methods are raising surgeon and patient expectations of refractive accuracy. A common goal today is to minimize refractive errors and reduce dependence on spectacles. It is well documented that predicting refractive outcomes after cataract surgery is more difficult in eyes with previous keratorefractive surgery than in virgin eyes. There are numerous formulas to calculate which intraocular lens (IOL) power to use in such patients. One is the Haigis-L, which is the correlation between the incorrectly measured corneal radii and the effective
917
corneal power after refractive surgery. However, with this formula, an error of nonlinear relation is present. This type of error is similar to the error of the earlier generation of IOL calculation formulas, such as the SRK/T. Online IOL calculators facilitate selection of IOL power for use in cataract surgery in patients with previous refractive surgery. The ASCRS calculator1,A uses 11 formulas, including the modified Masket and Haigis-L, and produces 1 mean IOL power. So, rather than using 1 individual formula, such as the SRK/T or Haigis-L, I would consider using ASCRS calculator mean IOL power in this case. Another technology for further refining the refractive outcomes of cataract surgery and refractive lens exchange is the intraoperative wavefront aberrometer platform mentioned in the case. The device measures the sphere, cylinder, and axis during surgery and is thought to have the potential to increase the precision of optical biometry. It can be used in aphakic eyes to verify the power of the intended IOL before implantation to avoid the need for future lens exchange. It has been reported that more than 90% of patients with a history of laser in situ keratomileusis (LASIK) evaluated with the intraoperative wavefront aberrometer platform during cataract surgery showed a difference of less than 1.00 D between the predicted IOL power and the 1-month postoperative refraction.B However, several factors influence overall aphakic readings, including intraocular pressure, corneal hydration, tightness of lid speculum, and where the patient is fixating. General operative technique is important in controlling these variables for achieving consistency of data and expected results. The incision can also affect the readings. If stromal hydration is required to close the incision, corneal curvature may be altered, which may affect the overall astigmatic power and axis readings. Intraocular pressure that is too low may result in a soft eye with an irregular cornea, which would cause erratic readings. A pressure too high may cause central corneal steepening, which may affect the accuracy of the wavefront reading. In conclusion, none of the calculation formulas and instruments mentioned above is perfect. I believe that using the ASCRS IOL calculator mean IOL power and then verifying this prediction intraoperatively by intraoperative wavefront aberrometry would result in the best IOL power for this patient. Kemal Ozulken, MD Miami, Florida, USA
REFERENCE 1. Wang L, Hill WE, Koch DD. Evaluation of intraocular lens power prediction methods using the American Society of Cataract and
J CATARACT REFRACT SURG - VOL 38, MAY 2012
CONSULTATION SECTION
Figure 2. Intraocular lens power calculation sheet with the SRK/T formula (PCI biometry).
OTHER CITED MATERIAL A. Hill W, Wang L, Koch DD. IOL calculator for eyes that have undergone LASIK/PRK/RK. Available at: http://iolcalc.org/. Accessed March 2, 2012
- Intraocular lens power calculation after refractive surgery has become a challenging problem for most clinicians. The inability to correctly measure the true corneal power after laser photoablation is the primary problem. The preoperative and postoperative information for this patient illustrates the aforementioned issue. The changes in the keratometry (K) readings and spherical equivalent (SE) after PRK are 2.64 D and C4.87 D, respectively. The data were reanalyzed using the ASCRS postrefractive surgery IOL calculator.A Formulas using perioperative historical data give higher IOL powers. The clinical history, Feiz-Mannis, and corneal bypass methods output 14.77 D, 14.92 D, and 14.68 D, respectively. We do not have the 6-month postoperative K readings and have to assume that they are the same as the current K values. We are also missing standard topographic data, such as the Atlas numerical map (Carl Zeiss Meditec AG) and the Pentacam equivalent K reading for the central 4.5 mm, as described by
Holladay et al.1 The remaining methods result in lower IOL powers; the Shammas, Haigis-L, and modified Masket formulas yield 14.61 D, 13.89 D, and 13.48 D, respectively. The ASCRS mean value recommends using an Acrysof SN60WF IOL (Alcon Laboratories, Inc.) with a power of 14.17 D (range 13.02 to 14.92 D). We have found that using a greater number of formulas leads to a more accurate IOL selection. OcularMD2 is another post-refractive IOL calculator available online that includes an additional subset of formulas, such as the Aramberri double-K, Latkany flat-K, and Latkany average-K methods. DeMill et al.2 found that combining the OcularMD and ASCRS data yielded more accurate outcomes. For this patient, the Ocular MD SRK/T and Haigis mean is 14.18 D (range 12.37 to 16.00 D) and essentially equals the ASCRS recommendation. Both Holladay and Haigis believe that the clinical history method (CHM) provides the most reliable estimate; thus, we would aim closer to 14.77 D and choose an Acrysof SN60WF 14.50 D IOL in this case.1,3 A conservative approach should always err on the side of myopia, and picking the same IOL with a 15.00 D power is also reasonable.
J CATARACT REFRACT SURG - VOL 38, MAY 2012
CONSULTATION SECTION
Other considerations include a surgeon's individual correction factors. Wound placement, capsulorhexis size, and induced astigmatism can affect the final refractive outcome. Also, given the very oblate profile of this cornea, we believe that an IOL with a more negative corneal asphericity would improve contrast sensitivity.4 We did not use the IOL power recommendation from the intraoperative wavefront aberrometry mentioned in the case. Although it is promising technology, Stringham et al.5 found that operating room variables as basic as speculum type can induce different anatomic changes that can confound the biometry. Majid Moshirfar, MD Gene Kim, MD Salt Lake City, Utah, USA REFERENCES 1. Holladay JT, Hill WE, Steinmueller A. Corneal power measurements using Scheimpflug imaging in eyes with prior corneal refractive surgery. J Refract Surg 2009; 25:862–868 2. DeMill DL, Moshirfar M, Neuffer MC, Hsu M, Sikder S. A comparison of the American Society of Cataract and Refractive Surgery post-myopic LASIK/PRK intraocular lens (IOL) calculator and the Ocular MD IOL calculator. Clin Ophthalmol 2011; 5:1409– 1414. Available at: http://www.dovepress.com/getfile.php? fileIDZ11070. Accessed March 2, 2012 3. Haigis W. Intraocular lens calculation after refractive surgery for myopia: Haigis-L formula. J Cataract Refract Surg 2008; 34:1658–1663 4. Nochez Y, Majzoub S, Pisella P-J. Effect of residual ocular spherical aberration on objective and subjective quality of vision in pseudophakic eyes. J Cataract Refract Surg 2011; 37:1076–1081 5. Stringham J, Pettey J, Olson RJ. Evaluation of variables affecting intraoperative aberrometry. J Cataract Refract Surg 2012; 38:470–474
OTHER CITED MATERIAL A. Hill W, Wang L, Koch DD. IOL calculator for eyes that have undergone LASIK/PRK/RK. Available at: http://iolcalc.org/. Accessed March 2, 2011
- The number of patients who have had keratorefractive surgery is increasing every year, which in turn increases the number of post-refractive surgery patients who will require cataract surgery. New-generation IOLs and biometry calculation methods are raising surgeon and patient expectations of refractive accuracy. A common goal today is to minimize refractive errors and reduce dependence on spectacles. It is well documented that predicting refractive outcomes after cataract surgery is more difficult in eyes with previous keratorefractive surgery than in virgin eyes. There are numerous formulas to calculate which intraocular lens (IOL) power to use in such patients. One is the Haigis-L, which is the correlation between the incorrectly measured corneal radii and the effective
917
corneal power after refractive surgery. However, with this formula, an error of nonlinear relation is present. This type of error is similar to the error of the earlier generation of IOL calculation formulas, such as the SRK/T. Online IOL calculators facilitate selection of IOL power for use in cataract surgery in patients with previous refractive surgery. The ASCRS calculator1,A uses 11 formulas, including the modified Masket and Haigis-L, and produces 1 mean IOL power. So, rather than using 1 individual formula, such as the SRK/T or Haigis-L, I would consider using ASCRS calculator mean IOL power in this case. Another technology for further refining the refractive outcomes of cataract surgery and refractive lens exchange is the intraoperative wavefront aberrometer platform mentioned in the case. The device measures the sphere, cylinder, and axis during surgery and is thought to have the potential to increase the precision of optical biometry. It can be used in aphakic eyes to verify the power of the intended IOL before implantation to avoid the need for future lens exchange. It has been reported that more than 90% of patients with a history of laser in situ keratomileusis (LASIK) evaluated with the intraoperative wavefront aberrometer platform during cataract surgery showed a difference of less than 1.00 D between the predicted IOL power and the 1-month postoperative refraction.B However, several factors influence overall aphakic readings, including intraocular pressure, corneal hydration, tightness of lid speculum, and where the patient is fixating. General operative technique is important in controlling these variables for achieving consistency of data and expected results. The incision can also affect the readings. If stromal hydration is required to close the incision, corneal curvature may be altered, which may affect the overall astigmatic power and axis readings. Intraocular pressure that is too low may result in a soft eye with an irregular cornea, which would cause erratic readings. A pressure too high may cause central corneal steepening, which may affect the accuracy of the wavefront reading. In conclusion, none of the calculation formulas and instruments mentioned above is perfect. I believe that using the ASCRS IOL calculator mean IOL power and then verifying this prediction intraoperatively by intraoperative wavefront aberrometry would result in the best IOL power for this patient. Kemal Ozulken, MD Miami, Florida, USA
REFERENCE 1. Wang L, Hill WE, Koch DD. Evaluation of intraocular lens power prediction methods using the American Society of Cataract and
J CATARACT REFRACT SURG - VOL 38, MAY 2012