For many years, it has been recognized that there may be a difference between refractive astigmatism and anterior corneal astigmatism. Historically, this difference was mainly blamed on lenticular astigmatism, which it has not been possible to measure with conventional methods. Recently, thanks to new corneal measuring devices, several studies have shown that posterior corneal astigmatism can significantly influence total corneal astigmatism, and, therefore, measurement of posterior corneal astigmatism can help to improve the accuracy of toric IOL power selection in certain cases. The patient described in Getting to Know the Patient is one such case.Calculations for the toric IOL (Lentis Tplus; Oculentis) to be implanted in this patient were performed using keratometry readings obtained with the IOLMaster. The IOL implanted in the patient’s right eye was a model LS-313 with spherical equivalent of 15.50 D and cylinder of 3.00 D. With this lens, predicted residual astigmatism was 0.01 X 93º. The lens for the patient’s left eye was again a model LS-313 with 15.50 D spherical equivalent and cylinder of 4.50 D. Predicted residual astigmatism for this eye was 0.06 X 84º.

Both surgeries and the immediate postoperative periods were completed with normal results and no complications. The IOLs appeared to be perfectly aligned on mydriasis. During follow-up, however, the patient expressed discontent with his visual acuity without correction, especially for distance vision (Table 3).

The patient underwent femtosecond LASIK in his right eye to achieve emmetropia and meet the initially proposed objective of spectacle independence at all distances. During the post-LASIK follow-up, the patient affirmed that he now performs most of his activities of daily life without any spectacle correction. Refraction and visual acuities 1 month after LASIK are shown in Table 4.

FACTORS THAT CONTRIBUTE TO
A REFRACTIVE SURPRISE

The development of toric IOLs has improved refractive results in cataract surgery for patients with significant astigmatism.¹ However, residual astigmatism after implantation of toric IOLs is not always predictable,² and unsatisfactory UCVA can be a postoperative outcome. Several factors may contribute to an astigmatic refractive surprise, including imprecise corneal measurements, miscalculation of the toric IOL power, error in the preoperative marking of the cornea, misalignment of the IOL, and the effect of the incision.^3-6 In the case presented here, one factor that might have reduced residual astigmatism was adjustment of the IOL toricity based on the patient’s posterior corneal astigmatism.

The surfaces of the anterior and posterior cornea each contribute to the total corneal astigmatism. Studies have indicated that posterior corneal astigmatism contributes significantly to total corneal astigmatism⁷ and that its effect may vary from -0.26 to -0.78 D.^7-14

Posterior corneal astigmatism in most patients is relatively stable, and almost 80% is vertical, or with-the-rule.7 This may contribute toward overcorrection of with-the-rule and undercorrection of against-the-rule astigmatisms, if posterior corneal astigmatism is not taken into account.

In our case presented here, we saw a significant difference in refractive and keratometric astigmatisms, which could have meant a greater influence of posterior corneal astigmatism on the total astigmatism. After observing this refractive surprise, we performed topography with the Cassini (i-Optics) to measure the astigmatism of the posterior corneal face (Figure 2). This confirmed our suspicion that the patient had a greater degree of posterior astigmatism than is usual.

A study of error in residual astigmatism prediction after implantation of toric IOLs did not find a significant difference whether the calculation of toricity was done based on total corneal power (as determined by ray-tracing Scheimpflug topography) or with the values of anterior automated keratometry.¹⁴ However, particularly in our case, it is evident that the overcorrection of astigmatism could have been reduced if the calculation had been done using the astigmatic value provided by the Cassini topographer.

Since the Baylor nomogram was published by Koch et al,⁷ multiple formulas have been proposed to reduce residual astigmatism after implantation of toric IOLs, and there is currently a lack of clear consensus on the best formula to use. It has recently been proposed that lower residual astigmatism can be achieved by using vector analysis, which is performed by integrating the posterior corneal astigmatism as measured by Scheimpflug camera with the anterior corneal astigmatism as measured by low-coherence optical reflectometry.¹⁵

LESSONS LEARNED

Surgeons may be able to minimize residual astigmatism after implantation of toric IOLs by taking posterior corneal astigmatism into consideration, especially in eyes in which there is a remarkable difference between the refractive astigmatism and the keratometric astigmatism. Had we done so in the case presented here, a retreatment procedure may likely have been avoidable.

Daniel Elies, MD
• Cornea and Refractive Surgery Specialist, Instituto de Microcirugia Ocular, Barcelona, Spain
• Associate Professor of Ophthalmology, ESASO University, Lugano, Italy
• elies@imo.es
• Financial interest: None acknowledged

Jose L. Güell, MD, PhD
• Director, Cornea and Refractive Surgery Unit, Instituto de Microcirugía Ocular, Barcelona, Spain
• Associate Professor of Ophthalmology, Universitat Autonoma de
• guell@imo.es
• Financial disclosure: Consultant (Ophtec, Alcon, Carl Zeiss Meditec, Thea Laboratories, Orca Surgical), Partial owner (Calhoun Vision, Visiometrics)

Pablo Laso, MD
• Fellow, Cornea and Refractive Surgery Unit, Instituto de Microcirugía Ocular, Barcelona, Spain
• pablo.laso.u@gmail.com
• Financial interest: None acknowledged

Mercé Morral, MD, PhD
• Cornea and Refractive Surgery Specialist, Instituto de Microcirugia Ocular, Barcelona, Spain
• morral@imo.es
• Financial interest: None acknowledged

Fernando Santander, MD, FEBO
• Fellow, Cornea and Refractive Surgery Unit, Instituto de Microcirugia Ocular, Barcelona, Spain
• fsantandert@gmail.com
• Financial interest: None acknowledged