Spherotoric bag-in-the-lens intraocular lens: Power calculation and predictive misalignment nomogram

PurposeTo propose a method of calculating the power of the 1-sided posterior chamber toric bag-in-the-lens (BIL) intraocular lens (IOL) and propose a misalignment nomogram to calculate the postoperative rotational misalignment or predict the effect of preoperative existing irregular corneal astigmatism.SettingAntwerp University Hospital, Department of Ophthalmology, Antwerp, Belgium.DesignCohort study.MethodsThe new IOL calculation formula uses the steepest corneal meridian and flattest corneal meridian separately (regular spherical IOL formula) followed by a customized A-constant approach based on the changes in the IOL principal plane depending on the spherical and cylindrical powers (thickness) of the IOL. The calculation of the remaining astigmatism (power and axis) in cases of postoperative rotational misalignment resulted in a nomogram that can also be used to predict the degree of tolerance for irregular corneal astigmatism correction at the lenticular plane. The calculation is performed using a worksheet.ResultsBecause 10 degrees of misalignment would result in 35% refractive inaccuracy, it is the maximum acceptable corneal astigmatic irregularity for correction at the lenticular plane.ConclusionsCalculation of spherocylindrical power is specific to each toric IOL. Because the surgeon must fully understand the optical properties of the toric IOL that is going to be implanted, a comprehensive outline of a new calculation method specific to the toric BIL IOL is proposed. Primary rotational misalignment of the toric BIL IOL can be fine tuned postoperatively.Financial disclosureDrs. Gobin and Mathysen have no financial or proprietary interest in any material or method mentioned. Additional disclosures are found in the footnotes.