6 Toric IOL Calculations
6.1 Introduction
Advances in cataract surgery have dramatically improved patients’ outcomes and expectations, increasing the need for predictable and accurate results. One key factor that can influence visual outcomes is astigmatism. Thirty percent of cataract patients have more than 0.75 diopters (D) of corneal astigmatism, 22% have more than 1.5 D, and 8% have more than 2 D. 1 Toric intraocular lenses (IOLs) correct corneal astigmatism at the time of cataract surgery and are a predictable treatment, in general. 2 , 3 , 4 Toric IOLs are currently available in both monofocal and multifocal versions. Monofocal toric IOLs can be used to correct the patient’s vision for distance, near, or both (the latter being attained with monovision),s. Literatur , 3 whereas multifocal toric IOLs aim at providing good uncorrected vision at distance and either intermediate or near, depending on the built-in add. 4 The two major challenges in achieving satisfactory toric IOL outcomes are choosing the proper level of toric correction and accurately aligning the toric IOL in the patient’s eye.
6.1.1 Important Factors in Toric IOL Power Calculations
The accuracy of toric IOL power calculations depends on several factors. Obtaining reliable measurements of anterior corneal astigmatism is critical, and two additional factors should also be considered—the posterior corneal astigmatism and the surgically induced astigmatism (SIA).
6.2 Corneal Astigmatism Measurements
Toric IOLs are used to compensate for total corneal astigmatism, which derives from the combination of the anterior and posterior corneal astigmatism. Obtaining accurate measurements is an essential step in toric IOL planning. Various methods are available, including manual keratometry, automated keratometry, corneal topography, slit-scanning tomography, optical coherence tomography, and Scheimpflug imaging.s. Literatur The first three methods measure the anterior corneal surface only. Using a standardized corneal refractive index, most commonly 1.3375, these methods assume a fixed anterior-posterior corneal curvature ratio to calculate total corneal power and astigmatism. On the other hand, slit-scanning technology, optical coherence tomography, and Scheimpflug imaging directly measure the anterior and posterior corneal surfaces. Therefore, total corneal power and astigmatism can be calculated based on the measured anterior and posterior corneal data.
No single method has emerged as the best for measuring corneal astigmatism. Previous studies have shown that manual keratometry, automated keratometry, simulated keratometry of Placido-based corneal topography, and simulated keratometry of Scheimpflug imaging provide similar values for anterior corneal curvature, 6 , 7 , 8 although significant disagreement in location of the steep and flat meridians has been reported. 7 It has also been shown that corneal astigmatism measurements of automated, manual, and simulated keratometry may differ significantly from those derived from total corneal power and equivalent keratometry. 8 , 9 Although automated, manual, and simulated keratometry are based on anterior corneal surface measurements only, total corneal power and equivalent keratometry are based on the combination of anterior and posterior corneal curvature measurements. Total corneal power is calculated by tracking the path of incident light through the anterior and posterior corneal surfaces via ray tracing, which uses Snell’s law and the true refractive indices (1.376 for cornea and 1.336 for aqueous) to calculate the anterior and posterior corneal powers. 10 Interestingly, some authors defend the combined use of keratometry techniques when choosing IOL power and meridian of alignment in order to optimize outcomes. The combination of measurements obtained with a manual keratometer and any automatic keratometer has been shown to increase the accuracy of toric IOL calculations.s. Literatur Therefore, it is recommended to perform two or more measurements and reconcile any disagreements.
6.2.1 Posterior Corneal Astigmatism
Recent studiess. Literatur , 13 have demonstrated the importance of considering the posterior corneal surface when determining total corneal astigmatism and in planning the astigmatism correction strategy. The posterior cornea acts as a negatively powered lens. It generally has a vertically oriented steep meridian, which remains vertical as age increases. The anterior corneal steep meridian is typically oriented vertically in younger individuals, but it shifts toward a horizontal orientation as patients age. Thus, in general, posterior corneal astigmatism partially compensates for anterior corneal astigmatism in young adults and increases total corneal astigmatism in older individuals. 12
If the patient’s cataract is felt not to interfere with the accuracy of their manifest refraction, the patient’s refractive astigmatism may provide some clues to the contribution of the posterior cornea. For example, if the patient has with-the-rule (WTR) astigmatism, the corneal astigmatism measured from the anterior surface may be greater than what is measured in their refraction due to the potential against-the-rule (ATR) astigmatism in the posterior cornea. Conversely, in patients with ATR anterior corneal astigmatism, the manifest refraction may demonstrate a greater magnitude of astigmatism than is found with anterior corneal measurements.
The mean magnitude of posterior corneal astigmatism is approximately − 0.3 D. Although manual keratometry, automated keratometry, and corneal topography measure the anterior corneal surface and use a standardized corneal refractive index to account for the posterior corneal astigmatism, Koch et al 12 have shown that one cannot accurately predict the posterior corneal astigmatism based only on anterior corneal measurements. They found maximal values of posterior corneal astigmatism of more than 0.8 D in corneas that had WTR astigmatism on the anterior corneal surface and over 0.5 D in corneas that had ATR corneal astigmatism on the anterior corneal surface. The correlation between anterior and posterior corneal astigmatism was moderate when the steep anterior meridian was aligned vertically, weak when it was oriented obliquely, and absent when it was aligned horizontally. 12
A second study by Koch et als. Literatur found a 0.5 to 0.6 D overestimation of total WTR astigmatism in eyes that had WTR astigmatism on the anterior corneal surface and a 0.2 to 0.3 D underestimation of total ATR astigmatism in eyes that had ATR corneal astigmatism on the anterior corneal surface. Posterior corneal astigmatism can be measured directly using tomographic devices, such as the dual Scheimpflug analyzer, although the accuracy in individual cases is still uncertain. Alternatively, one can account for posterior corneal astigmatism using nomograms, such as the Baylor toric IOL nomogram (Table 6-1).
Effective IOL cylinder power at corneal plane (D) | WTR (D) | ATR (D) |
0 | ≤ 1.69 (PCRI if > 1) | < 0.39 |
1 | 1.7–2.19 | 0.4a–0.79 |
1.5 | 2.2–2.69 | 0.8–1.29 |
2 | 2.7–3.19 | 1.3–1.79 |
2.5 | 3.2–3.69 | 1.8–2.29 |
3 | 3.7–4.19 | 2.3–2.79 |
3.5 | 4.2–4.69 | 2.8–3.29 |
4 | 4.7–5.19 | 3.3–3.79 |
Abbreviations: IOL, intraocular lens; WTR, with-the-rule astigmatism; ATR, against-the-rule astigmatism; D, diopter; PCRI, peripheral corneal relaxing incision. Note: Postoperative target: up to 0.4 D WTR astigmatism. Note: If an SN6AT2 is available, consider implanting it in WTR astigmatism of 1.4–1.69 D, and in ATR of 0.3–0.49 D (in this latter case, T3 would be implanted in astigmatism ranging from 0.5 to 0.79 D). aValues in the table are the vector sum of the anterior corneal and surgically induced astigmatism. | ||
6.3 Step-by-Step Use of the Baylor Toric IOL Nomogram
6.3.1 Example 1
Patient with preoperative ATR anterior corneal astigmatism of 1.16 D @ 10°.
We’ve found that our surgically induced astigmatism results in ~ 0.30 D of flattening with a 2.2 mm temporal clear corneal incision.
The predicted postoperative anterior corneal astigmatism would therefore be 0.86 D (Table 6-1, row 3, column 3).
An implant with 1.5 D of toricity at the corneal plane is chosen to account for the equivalent ATR astigmatism of the posterior cornea and to target a small amount of postoperative WTR astigmatism.
At the 3-week postoperative visit, this patient had a visual acuity of 20/20 with a manifest refraction of − 0.25 D sphere.
6.3.2 Example 2
Patient with preoperative WTR anterior corneal astigmatism of 2.49 D @ 95°.
The predicted postoperative anterior corneal astigmatism would be ~ 2.79 D after factoring in the 0.3 D of SIA from the temporal clear corneal incision (Table 6-1, row 4, column 2).
An implant with 2 D of toricity at the corneal plane is chosen to account for the equivalent ATR astigmatism of the posterior cornea and to target a small amount of postoperative WTR astigmatism.
On the 3-week postoperative refraction, this patient had a visual acuity of 20/20 with a refraction of – 0.25 + 0.25 × 90.
6.4 Surgically Induced Astigmatism
When calculating the power of toric IOLs, it is also important to consider the SIA, which results from flattening in the meridian of the incision and steepening 90° away (coupling effect). The amount of SIA depends on several factors, including the size, shape, and location of the incision; the suture used; and the biomechanical response of the patient’s corneal tissue. As an incision is placed further from the optical axis, its astigmatic effect decreases. Thus a longer scleral tunnel incision can induce as much SIA as a shorter clear corneal incision. 14 With regard to the shape of the scleral tunnel incision, various authors have investigated the SIA associated with straight, curved (parallel to the limbus), reverse curved (frown incision), and V-shaped incisions. However, none of these shapes have been clearly shown to induce less astigmatism. 15 Conversely, previous papers have shown that incision location plays an important role in the amount of SIA. Nasal, superonasal, and superior incisions induce more astigmatism than temporal and superotemporal incisions. 16 , 17 Surgeons should calculate their own SIA, which can be done using standard astigmatic vector analysis. 18 One website that can be used for this purpose is http://www.doctor-hill.com/.s. Literatur
6.4.1 Calculating Toric IOL Power
The optimal IOL toricity can be determined with a calculation program provided by the IOL manufacturer, methods described in the literature, 20 or nomograms, such as the Baylor toric IOL nomogram (Table 6-1). The cylindrical power of the toric IOL should be chosen based on the total corneal astigmatism, taking into consideration anterior corneal astigmatism, posterior corneal astigmatism, and SIA. It is also important to consider the impacts of the sphero-equivalent IOL power and effective lens position on the toric IOL’s effective cylinder power at the corneal plane. Effective toricity of the IOL diminishes with increasing anterior chamber depth and lower IOL spherical power. The Holladay IOL Consultant Program and the Tecnis Toric Calculator (Abbott Laboratories, Inc.) 21 take this into account in their calculation of toric IOL power. Furthermore, when choosing IOL toricity, it may be desirable to leave patients with a slight amount of WTR astigmatism, due to the normal tendency for astigmatism to drift ATR with advancing age. This may prolong the period of time in which the patient’s corneal astigmatism is optimally compensated by the toric IOL. 12 , 22
A promising new technology that aids in toric IOL power calculations is intraoperative wavefront aberrometry. The Optiwave Refractive Analysis (ORA; WaveTec Vision Systems, Inc.) system is an example. It is directly attached to the operating microscope and connected to a monitor. After cataract extraction, it measures the patient’s aphakic refraction, which is used to calculate the toric IOL power and alignment meridian (Fig. 6.1). After lens implantation, real-time refractive measurements can be used to align the toric IOL to minimize residual astigmatism. Another intraoperative aberrometer that has recently been developed is the Holos (Clarity Medical Systems). It also provides a continuous video readout of the refractive data during surgery, which can be used to guide toric IOL power calculations. Unfortunately, intraoperative wavefront aberrometry may still be influenced by several factors, such as eyelid speculum pressure, intraocular pressure, and corneal hydration. Therefore, care must be taken to minimize these confounding variables. However, it is a very promising technology that can play an important role in toric IOL calculation and alignment.
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