Toric Intraocular Lenses

Excerpt

Ocular astigmatism accounts for 13% of the total refractive errors encountered in patients. The first treatment of astigmatism dates back to 1825, when George Airy used cylindrical lenses to correct his own refractive error. Uncorrected ocular astigmatism results in a blurred image and significant deterioration of visual acuity.

The cause of astigmatism is still not known; several etiologies, including genetics, the pressure of eyelids over the globe, tension due to extraocular muscles, and visual feedback mechanisms, have been attributed. Total ocular astigmatism is considered to be a sum of corneal astigmatism which is contributed by the corneal surface changes, and internal or residual astigmatism, which is contributed by the irregularities of the crystalline lens. The anterior and posterior corneal curvatures contribute to the total corneal astigmatism.

Initially, the estimation of corneal astigmatism was based on the measurement of anterior corneal curvature by keratometry and videokeratography. The calculation of the refractive power was based on empirical estimation of the posterior corneal surface. With advanced imaging technologies like Scheimpflug imaging, Purkinje images-based technologies, and optical coherence tomography, the measurement of the posterior corneal surface has helped us aim for better refractive outcomes in cataract surgery. Posterior corneal astigmatism was evaluated to range from -0.26 to -0. 78 D.

Modern-day cataract surgery has emerged as a refractive procedure that aims to eliminate spherical and cylindrical power and achieve spectacle independence. The prevalence of corneal astigmatism >1.00D is 40%, greater than 1.50D was 20% in patients, and >2. 00 D was found in 8% of patients with corneal astigmatism undergoing cataract surgery.

The various methods of correcting corneal astigmatism during cataract surgery include toric intraocular lenses (IOL), the placement of the clear corneal phacoemulsification incision on the steeper corneal axis, paired opposite clear corneal incisions over the steeper meridian, and limbal relaxing incisions over the steeper meridians.

Toric IOLs are considered the most predictable way of correcting corneal astigmatism. Shimizu et al introduced toric IOLs 1992 as a rigid 3-piece polymethylmethacrylate lens. The toric IOLs have undergone multiple improvements since then to address stability and postoperative alignment issues.