A New Way To Evaluate Toric Lenses Misalignment : A Comparative Analysis Of Conventional Formulas With A Novel Mathematical Tool.

The Barrett Rx formula is more accurate than AstigmatismFix.com but is cumbersome to use due to the requirement of multiple data inputs. Both formulas are designed to calculate the new realignment axis and residual refraction.

However, they do not account for refractive surprise caused by other factors such as surgically induced astigmatism (SIA) and errors in posterior corneal astigmatism (PCA), which result from the difference between the preoperative PCA value  and the postoperative PCA value.

Additionally, neither formula can accurately determine the ideal toric power or assess whether the predicted or measured PCA value is more reliable for a given case.

Therefore, there is a need for a new formula that incorporates all these parameters.

Private practice, India.

AstigmatismFix.com and the Barrett Rx formula, were compared both theoretically and experimentally. The theoretical comparison was conducted using six non-toric IOL calculators, analyzing cases with varying axial length (AL) and anterior chamber depth (ACD).

For experimental validation, a lensometer was used.

A novel mathematical tool with a simplified and intuitive interface was evaluated for its ease of use and accuracy compared to the existing formulas.

The effects of SIA, errors in PCA, and their combined influence on refractive surprise and steep axis rotation were calculated separately.

Perfect toric power was calculateded based on post-op refraction for postoperatively and preoperatively also excluding SIA and error in PCA.

• The Barrett Rx formula was found to be more accurate as it is based on ELP ( effective lens position), while the Astigmatism.com is using fixed power of toric lens on toricity ratio.
• AstigmatismFix.com is easier to use.
• The newly developed mathematical formula demonstrated accuracy comparable to the Barrett Rx formula while being more user-friendly than AstigmatismFix.com
• Additionally, the study analyzed the individual contributions of various factors—including surgically induced astigmatism (SIA), errors in posterior corneal astigmatism (PCA), and their combined effects—to postoperative refractive surprise and axis deviation.

Errors in posterior corneal astigmatism (PCA), which result from the difference between the preoperative PCA value (used by toric calculators) and the postoperative PCA value (calculated based on postoperative refraction).

Postoperative refractive surprise after toric IOL implantation is influenced by misalignment of the toric IOL, SIA, error in in PCA  and inaccurate toric power recommended by toric calculators.

The precise cause of postoperative realignment axis deviation from the preoperatively recommended alignment, along with variations in the toric power of the IOL, was systematically analyzed.

 Surgeons can identify sources of error and refine biometry and surgical techniques to enhance accuracy and minimize refractive surprises.