Intraocular Lens Power Calculations in Refractive Lens Exchange
High patient expectations for excellent uncorrected visual acuity (UCVA; also called uncorrected distance visual acuity, UDVA) after RLE make accurate IOL power determination crucial. However, IOL power formulas are less accurate at higher levels of myopia and hyperopia. In addition, in high myopia, a posterior staphyloma can make the axial length measurements less reliable. Careful fundus examination and B-scan ultrasound imaging can identify the position and extent of staphylomas. The subject of IOL power determination is covered in greater detail in BCSC Section 3, Clinical Optics, and Section 11, Lens and Cataract.
In the case of a patient with high hyperopia, biometry may suggest an IOL power beyond what is commercially available. The upper limit of commercially available IOL power is now +40.00 D. A special-order IOL of a higher power may be available or may be designed, but acquiring or designing such a lens usually requires the approval of the institutional review board at the hospital or surgical center, which delays the surgery. Another option is to use a “piggyback” IOL system, in which 2 posterior chamber IOLs are inserted. One IOL is placed in the capsular bag, and the other is placed in the ciliary sulcus. When piggyback IOLs are used, the combined power may need to be increased +1.50 to +2.00 D to compensate for the posterior shift of the posterior IOL. One serious complication of piggyback IOLs is the potential for developing an interlenticular opaque membrane. These membranes cannot be mechanically removed or cleared with the Nd:YAG laser; the IOLs must be removed. Interlenticular membranes have occurred most commonly between 2 acrylic IOLs, especially when both IOLs are placed in the capsular bag. When piggyback lenses are used, they should be of different materials, ideally with one IOL placed in the bag and the other in the sulcus. Piggyback IOLs may also shallow the anterior chamber and increase the risk of iris chafing, especially in smaller eyes.
Hill WE, Byrne SF. Complex axial length measurements and unusual IOL power calculations. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2004, module 9.
Shammas HJ. IOL power calculation in patients with prior corneal refractive surgery. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2013, module 6.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.