An IOL may be placed at cataract removal, or later as a second operation. The lens is implanted inside the eye to take over the work of the cloudy natural lens. Most patients receive it during the cataract operation itself. When that is not possible, the surgeon can implant one as a secondary surgery months or years afterward. Either way, the eye must be measured first.
A-scan ultrasonography supplies the axial length, the front-to-back length of the eye. That number is needed to calculate IOL power. Small measuring errors cause large power errors. About 0.30 mm of axial length error equals about 1 diopter of miscalculation, and a diopter is the unit of lens power. So the scan is repeated until the readings are accurate and consistent. The same measurement is needed whether the implant goes in today or years from now. An eye that has already had cataract surgery still needs its own fresh reading before a secondary implant. The technician records which eye was scanned and compares the two eyes, because a large difference usually means a poor scan. The surgeon then feeds the length, the keratometry readings, and the patient's goals into an IOL power formula. A good measurement is what lets the patient see well without thick glasses.
Remember that the implant is not locked to the day of the cataract operation. It can go in then or later, and both routes start with an accurate A-scan. Measuring the eye for lens power sits at the center of the diagnostic ultrasound content area of the COA exam.
Incorrect Answers
- A. The cloudy lens is removed before the implant goes in. The natural lens and the implant do not share the same space in the eye.
- B. A corneal transplant replaces cloudy corneal tissue and is a separate operation. It is not a step required before an IOL can be implanted.
- D. There is no one-week window for implantation. A secondary lens may be placed months or even years after the cataract was taken out.
Testing Pearls
- An IOL may be implanted at cataract extraction or later as a secondary surgery.
- A-scan axial length is the chief measurement used to choose IOL power.
- About 0.30 mm of length error shifts the calculation by about 1 diopter.
- Every eye needs its own measurement, even one that has had surgery before.
- Repeat the scan: accurate and consistent readings are the goal before surgery.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 10, Adjunctive Tests and Procedures.
