Effective Lens Position
ELP (sometimes called estimated lens position) is an estimate of the distance behind the cornea at which the intraocular lens will be located. This factor was previously called the anterior chamber depth (ACD). In older formulas, ACD was a constant value and was incorporated into the “A constant” value for a lens. The A constant was specified by the lens manufacturer and was a theoretical value that related the lens power to AL and keratometry. Formulas have become more advanced, however, and the complexity of estimating the ELP has increased. The estimate is now generally based on the relationship between the AL and central corneal power (K) readings. In patients with prior myopic keratorefractive surgery, the flatter anterior cornea surface can lead to errors in ELP, causing hyperopic refractive surprises.
The intended anatomic location of the lens within the eye also has an impact on the lens power selected. As the lens moves forward from in-the-bag placement to sulcus placement to anterior chamber placement, the distance from the retina increases, and the power required for the implant decreases. In general, an anterior chamber IOL (ACIOL) will be about 3.00 D lower in power than a planned in-the-bag posterior chamber IOL (PCIOL). This measurement will be different for eyes at the extremes of AL (ie, those with high myopia or high hyperopia). When shifting a PCIOL from the capsular bag to the sulcus, an adjustment can be made relative to the AL and projected in-the-bag IOL power (Table 7-1). If the lens is placed in the sulcus and optic capture is performed, no adjustment needs to be made, because the optic is then effectively in the capsular bag.
Table 7-1 I OL Power Adjustment for Sulcus Placement
Koch DD, Jenkins RB, Weikert MP, Yeu E, Wang L. Correcting astigmatism with toric intraocular lenses: effect of posterior corneal astigmatism. J Cataract Refract Surg. 2013; 39(12):1803–1809.
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.