Leftover Astigmatism: The Missing Link Between Measured and Calculated Posterior Corneal Astigmatism

Area of Science:

• Ophthalmology
• Refractive Surgery
• Corneal Physiology

Background:

• Accurate prediction of refractive outcomes after cataract surgery is crucial for patient satisfaction.
• Astigmatism management is a key factor in achieving emmetropia.
• Existing methods for intraocular lens (IOL) power calculation often rely on anterior corneal astigmatism measurements.

Purpose of the Study:

• To quantify the total eye astigmatism not accounted for by anterior and posterior corneal astigmatism and IOL astigmatism.
• To assess correlations between this "leftover" astigmatism and potential contributing factors.

Main Methods:

• Vector subtraction was used to determine leftover astigmatism from total eye astigmatism (spectacle astigmatism) in pseudophakic eyes.
• The study analyzed 103 eyes with known IOL toricity.
• Correlations with candidate sources of leftover astigmatism were investigated.

Main Results:

• The mean leftover astigmatism was 0.71 ± 0.43 diopters.
• A significant correlation was found between leftover astigmatism and against-the-rule anterior corneal astigmatism (P < .001).

Conclusions:

• Leftover astigmatism represents a clinically substantial component of total eye astigmatism.
• Its inclusion in IOL cylinder power calculations may explain the success of refractive outcome-based adjustments.
• This finding suggests that adjustments based on refractive outcomes are more effective than those solely based on measured posterior corneal astigmatism.