Biometry and intraocular lens calculation in extreme myopia

Area of Science:

• Ophthalmology
• Biomedical Engineering
• Optics

Background:

• Extreme myopia, defined by axial lengths exceeding 30 mm, affects approximately 1.2% of the population.
• High myopia increases the risk of posterior staphyloma, complicating ocular biometry and intraocular lens (IOL) power calculations.
• Current IOL formulas exhibit different axial length behaviors, particularly around the transition from plus to minus IOL powers (approx. 31-32 mm).

Purpose of the Study:

• To investigate the specific challenges in IOL power calculation associated with the transition from plus to minus power IOLs in extreme myopia.
• To compare thick- and thin-lens approaches and analyze the role of IOL constants in this power range.

Main Methods:

• Model calculations were performed using the Alcon MA60MA lens, spanning a power range from +5 D to -5 D.
• A comparison between thick- and thin-lens theoretical approaches was conducted.
• An equation was derived to correlate lens geometry with the effective thin-lens position and the corresponding IOL constant.

Main Results:

• A dramatic change in lens geometry occurs at the transition point between plus and minus IOL powers.
• This geometric shift necessitates a corresponding adjustment in the required IOL constants.
• Failure to account for this effect can lead to refractive errors that increase with axial length, especially with minus-power IOLs.

Conclusions:

• Plus-power and minus-power intraocular lenses must be characterized by distinct sets of IOL constants for accurate refractive outcomes in extreme myopia.
• Accurate biometry and appropriate IOL selection are critical for managing patients with high myopia and preventing post-operative refractive errors.