14 Calculations IOL Power
CLINICAL VARIABLES Patient Needs and Desires
E. There are other miscellaneous causes for IOL power errors that can be just as serious as those mentioned above
A rare manufacturer labeling error can be very serious and very difficult to pick up before the patient is discharged from the facility. If the OR nurse hands the surgeon the wrong IOL power during the surgery this may not be easily recognized in time to correct the error.
Lastly, transcription mistakes can cause some of the largest errors seen.
Prevention of Common Errors
• Use immersion A-scan and/or IOLMaster to measure the AL.
• Suspect a staphyloma in eyes >25 mm: Use IOLMaster and/or Shammas A/B-scan technique.
• Use CALF method: measure eye using 1532 m/sec and add +0.32 mm to the result to correct for any error in sound velocity.
• Employ a well-trained, experienced technician.
• Regularly calibrate manual keratometers.
• Carefully evaluate the IOLMaster scan for reliability.
• Keep CL out for 2 weeks prior to keratometry (at least in one eye.)
• Silicone oil eyes need IOLMaster if possible or ultrasound AL times 0.71.
• Use the Hoffer® Q formula in eyes <22 mm and in post-refractive surgery eyes.
• Use the Holladay® 1 formula in eyes 24.5-26 mm in length
• Use the SRK/T formula in eyes longer than 26 mm.
• Never use the SRK Regression formulas (SRK I or II)
• Personalize your ELP factors in the formulas.
• Surgeon should personally select the IOL power for the individual patient.
• Prepare a sheet with all IOL powers that may be needed and place it on the wall and also on the microscope in the OR for the surgeon and OR nurse to verify the correct IOL power.
• Be sure to set the IR to 1.3375 in the setup screen of the IOLMaster.
• Use the Clinical History & Contact Lens Methods [have PMMA CLs in the clinic] for post-refractive surgery corneas and use the lowest (highest for hyperopes).
• Consider the Shammas “No History” Formula: K
= 1.14 × KPO – 6.8 or the Maloney or Koch Corneal topography methods.
• Use the Aramberri Double K: Calculate the ELP using the preoperative K and the IOL power using the PO K.
• Consider using the Masket Method.
• Consider using the Haigis formula.
• Consider delaying the IOL implantation until the cornea has healed after a penetrating keratoplasty rather than performing a “triple procedure.”
Suggestions for Diagnosing and Treating IOL Power Surprises
• Make it a routine to perform a manifest refraction on PO day #1 so as to discover the problem early enough
to take the patient back to the OR and correct the problem in the first 48 hours. The patient is imme-diately pleased and medico-legal actions are completely eliminated.
• Consider the use of a piggyback IOL or phakic IOL if the eye has healed beautifully and removal of the errant IOL would be more traumatic to the eye. For myopic error use 1 times the error and for hyperopic errors use 1.5 times the error (or the Shammas Formula).
• Consider a minimal 4-incision RK if repeat intraocular surgery is not possible.
• Measure the power of a removed IOL using the McReynolds Analyzer [William McReynolds 217-222-6656] or ask a manufacturer to be present in the OR to do it.
CONCLUSION
Simple steps and attention to detail can be very useful in preventing IOL power errors and recent advances in IOL power range availability has made this problem more easily corrected. Since performing the first American ultrasound IOL power calculation48 in 1974, the past 30 years have seen great improvement in the accuracy of postoperative refractive prediction. Future improvements may someday eliminate the problems we have left.
REFERENCES
1. Ossoinig KC. Standardized echography: Basic principles, clinical applications, and results. Int Ophthalmol Clin 1979;
19(4):127.
2. Shammas HJF. A comparison of immersion and contact techniques for axial length measurements. Am Intra-Ocular Implant Soc J 1984;10:444-47.
3. Schelenz J, Kammann J. Comparison of contact and immersion techniques for axial measurement and implant power calculation. J Cataract and Refract Surg 1989;15:
425-28.
4. Hoffer KJ. Ultrasound speeds for axial length measurement.
J Cataract Refract Surg 1994;20:554-62.
5. Encyclopedia of Polymer Science and Engineering. New York, Wiley and Sons, 1989;1:147-49.
6. Holladay JT. Standardizing constants for ultrasonic biometry, keratometry, and intraocular lens power calculation. J Cataract Refract Surg 1997;23:1356-70.
7. Hoffer KJ. Ultrasound axial length measurement in biphakic eyes. J Cataract Refract Surg 2003;29:961-65.
8. Holladay JT, Prager TC. Accurate ultrasonic biometry in pseudophakia. Amer J Ophthalmol 1989;107:189-90.
9. Holladay JT. IOL calculations following radial keratotomy surgery. Refract Corneal Surg 1989;5:36A.
10. Hoffer KJ. IOL power calculation in RK eyes. Phaco &
Foldables 1994;7(3):6.
11. Hoffer KJ. Calculation of intraocular lens power in post-radial keratotomy eyes. Ophthalmic Practice (Canada) 1994;
12(5):242-43.
IOL Power Calculations
91
12. Hoffer KJ. Ways to calculate IOL power in RK eyes. Refractive Surgery Update (Thornton) Ocular Surg News 1995;
13(10):86.
13. Hoffer KJ. Intraocular lens power calculation for eyes after refractive keratotomy. J Refract Surg 1995;11:490-93.
14. Hoffer KJ. How to do cataract surgery after RK. Review of Ophthalmol 1996;20:117-20.
15. Hoffer KJ. Intraocular lens power calculation for eyes after refractive keratotomy. Consultation Section, Ann Ophthalmol 1996;28(2):67-68.
16. Ridley F. Development in contact lens theory. Trans Ophthalmol Soc UK 1948;68:385-401.
17. Soper JW, Goffman J. Contact lens fitting by retinoscopy.
Contact Lenses, Stratton Intercontinental Medical Book Corp:
Soper JW (Ed): New York, NY, 1974;99.
18. Aramberri J. Double K method for IOL power calculation in post-refractive surgery eyes. Personal communication.
19. Shammas HJ. A formula to predict effective corneal power in post-refractive surgery eyes using only the postoperative corneal power measurement. Presented at the American Society of Cataract and Refractive Surgery Symposium, April 12, 2003, San Francisco, CA.
20. Hoffer KJ. Triple procedure for intraocular lens exchange.
Arch Ophthalmol 1987;105:609.
21. Geggel HS. Intraocular lens implantation after penetrating keratoplasty: Improved unaided visual acuity, astigmatism, and safety in patients with combined corneal disease and cataract. Ophthalmol 1990;97:1460-67.
22. Cua IY, Qazi MA, Lee SF, Pepose JS. Intraocular lens calculations in patients with corneal scarring and irregular astigmatism. J Cataract Refract Surg 2003;29:1352-57.
23. Fyodorov SN, Kolonko AI. Estimation of optical power of the intraocular lens. Vestnik Oftalmologic (Moscow) 1967;
4:27.
24. Colenbrander MC. Calculation of the power of an iris-clip lens for distance vision. Brit J Ophthalmol 1973;57:735-40.
25. Hoffer KJ. Intraocular lens calculation: The problem of the short eye. Ophthalmic Surg 1981;12:269-72.
26. Binkhorst RD. The optical design of intraocular lens implants.
Ophthalmic Surg 1975;6(3):17-31.
27. Gills JP. Regression formula. (editorial) Amer Intra-Ocular Implant Soc J 1978;4:163.
28. Gills JP. Minimizing postoperative refractive error. Contact and Intraocular Lens Med J 1980;6:56-59.
29. Retzlaff J. A new intraocular lens calculation formula. Am Intra-Ocular Implant Soc J 1980;6:148.
30. Sanders DR, Kraff MC. Improvement of intraocular lens power calculation: Regression formula. Am Intra-Ocular Implant Soc J 1980;6:263.
31. Sanders DR, Retzlaff J, Kraff MC, et al. Comparison of the accuracy of the Binkhorst, Colenbrander and SRK implant power prediction formulas. Am Intra-Ocular Implant Soc J 1981;7:337-40.
32. Hoffer KJ. Biometry of the posterior capsule: A new formula for anterior chamber depth of posterior chamber lenses.
(Chapter 21) Current Concepts in Cataract Surgery (Eighth Congress), Emery JC, Jacobson AC (Editors), Appleton-Century Crofts: New York, 1983;56-62.
33. Hoffer KJ. The effect of axial length on posterior chamber lenses and posterior capsule position. Current Concepts in Ophthalmic Surg 1984;1:20-22.
34. Binkhorst RD. Biometric A-scan ultrasonography and intraocular lens power calculation. In Emery JE, (ed.): Current Concepts in Cataract Surgery: Selected Proceedings of the Fifth Biennial Cataract Surgical Congress, St. Louis, MO, Mosby CV, 1987;175-82.
35. Sanders DR, Retzlaff J, Kraff MC. Comparison of the SRK II formula and the other second generation formulas. J Cataract Refract Surg 1988;14:136-41.
36. Holladay JT, Prager TC, Chandler TY, Musgrove KH. A three-part system for refining intraocular lens power calculations.
J Cataract Refract Surg 1988;14:17-24.
37. Retzlaff J, Sanders DR, Kraff MC. Development of the SRK/
T intraocular lens implant power calculation formula. J Cataract Refract Surg 1990;16:333-340 ERRATA: 1990;
16:528
38. Hoffer KJ. The Hoffer Q formula: a comparison of theoretic and regression formulas. J Cataract Refract Surg 1993;
19:700-712. ERRATA: 1994;20:677:2007;33:2-3.
39. Olsen T, Oleson H, Thim K, Corydon L. Prediction of postoperative intraocular lens chamber depth. J Cataract Refract Surg 1990;16:587-90.
40. Holladay JT. Refractive power calculation for intraocular lenses in the phakic eye. Amer J Ophthalmol 1993;116:63-66.
41. Hoffer KJ. Clinical results using the Holladay 2 intraocular lens power formula. J Cataract Refract Surg 2000;26:
1233-37.
42. Retzlaff J. Calculating the surgeon’s personal A-constant. In Retzlaff J, Sanders DR, Kraff MC (eds.): Lens Implant Power Calculation Manual, ed. 3. Thorofare, NJ, Slack Inc., 1990;
12-13.
43. Hoffer KJ. Selection of lens power for implantation in infants and children. Am Intra-Ocular Implant Soc J 1975;1(2):49.
44. Gordon RA, Donzis PB. Refractive development of the human eye. Arch Ophthalmol 1985;103:785-89.
45. Plager DA, Kipfer H, Sprunger DT, Sondhi N, Neely EN.
Refractive change in pediatric pseudophakia: 6-year follow-up. J Cataract Refract Surg 2002;28:810-15.
46. Hoffer KJ. Lens power calculation for multifocal IOLs, (Chapter 17) Current Concepts of Multifocal Intraocular Lenses, Andrew Maxwell MD and Lee T. Nordan, MD (Editors), Slack, Inc, Thorofare, New Jersey, 1991;193-208.
47. Hoffer KJ. Early lens exchange for power calculation error. J Cataract Refract Surg 1995;21:486-87.
48. Hoffer KJ. In: The history of IOL power calculation in North America, The History of Modern Cataract Surgery, ML Kwitko and CD Kelman (Editors), Kuglen Publications, The Hague, Netherlands, 1998;193-208.