Spotlight on the Corneal Back Surface Astigmatism: A Review

Area of Science:

• Ophthalmology
• Optometry
• Biomedical Engineering

Background:

• Corneal back surface astigmatism (CBSA) is increasingly recognized for its influence on refractive outcomes, especially after cataract surgery involving toric intraocular lenses.
• Despite its significance, skepticism exists regarding the accuracy and clinical utility of current CBSA assessment methods.

Purpose of the Study:

• To review the evidence for CBSA's contribution to refractive error.
• To critically evaluate the methodologies and limitations of current in vivo CBSA assessment systems.
• To highlight the need for improved calibration and practical procedures for CBSA measurement.

Main Methods:

• A comprehensive review of studies published over the past three decades focusing on CBSA.
• Analysis of existing automated systems for CBSA assessment and their underlying algorithms.
• Historical review of methods used to quantify posterior corneal curvature.

Main Results:

• Mean CBSA power reported in literature ranges from 0.18(±0.16)D to 1.04(±0.20)D.
• Significant discrepancies and poor agreement exist between current automated CBSA assessment systems.
• CBSA cannot be directly measured in vivo; current estimations rely on indirect parameters and algorithms with inherent potential for error.

Conclusions:

• Systematic errors in input data and algorithmic calculations contribute to the lack of agreement between CBSA measurement systems.
• While mean CBSA may be low, individual variations necessitate accurate assessment.
• There is a critical need for practical, independently calibratable procedures for clinicians to reliably estimate CBSA, reducing uncertainty and inter-instrument variability.