Planning and Surgical Technique
The amount, axis, and regularity of the astigmatism should be measured accurately. First, corneal topography should be examined to determine the regularity and axis of astigmatism and to identify eyes with irregular astigmatism or ectatic disease. Keratometry should be used to confirm the corneal power axis and provide the primary data for corneal astigmatic power. The axis of astigmatism from the refraction should not be the sole source for axis or power determination, but should be considered in context with topographic and keratometric measurements.
Significant disagreement between measurements should prompt re-examination of the clinical data and may also suggest the effect of lenticular astigmatism or posterior corneal astigmatism. Posterior corneal astigmatism may vary widely from patient to patient, but may add 0.3–0.5 D of net against-the-rule astigmatic power in 80% of patients. While technology to accurately measure the posterior corneal astigmatism is evolving, surgeons may use regression formulas, such as the Baylor nomogram, or theoretical formulas, such as the Barrett toric IOL formula (available at www.ascrs.org/barrett-toric-calculator) to help compensate for the tendency of anterior corneal measurements to overestimate the with-the-rule corneal power and underestimate the against-the-rule corneal power. Intraoperative aberrometry may be useful in these cases.
The manufacturers of toric IOLs have online software available to aid in surgical planning. After the surgeon enters data such as keratometry measurements, axes, IOL spherical power generated by A-scan, average surgeon-induced astigmatism, and axis of astigmatism, these programs will generate the recommended power and model lens as well as orientation of the lens.
There are many ways that surgeons mark the cornea prior to surgery. The surgeon should establish and mark the vertical and/or horizontal meridians with the patient in an upright position to avoid potential misalignment resulting from torsional globe rotation, which sometimes occurs in the supine position. Intraoperative alignment systems are available. Cataract surgery with a wound that induces a predictable amount of astigmatism is necessary to achieve the intended benefit of a toric lens. All online toric IOL software requires input of the expected surgically induced astigmatism for lens power calculations.
After the IOL is injected into the capsular bag, the viscoelastic behind the IOL is aspirated and the IOL is rotated into position on the steep meridian. Some surgeons prefer to leave the toric IOL purposely underrotated by 10°–20° and then rotate it into position after all viscoelastic substance has been removed; others position the IOL in its planned orientation and then hold it in place with a variety of techniques while removing the viscoelastic material. If the IOL rotates beyond its appropriate position, it will need to be fully rotated around again, as the 1-piece IOLs tend not to rotate well against their haptics. This maneuver should be performed using irrigation or viscoelastic material to prevent capsule rupture during rotation.
Koch DD, Jenkins R, 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 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.