is an associate professor of ophthalmology at the Cullen Eye Institute, Department of Ophthalmology, Baylor College of Medicine. Her areas of research include various aspects in cataract surgery, refractive surgery, diagnostic devices, optics, and wavefront technology and its use in refractive and cataract surgery.
is an associate professor at the Cullen Eye Institute, Baylor College of Medicine, where he specializes in corneal, cataract, and refractive surgery. His research interests include biomedical optics, anterior segment imaging, intraocular lens technology, and wavefront applications in cataract and refractive surgery. He is the residency program director at the Cullen Eye Institute, Baylor College of Medicine, and the medical director of the Lion’s Eye Bank of Texas.
is professor and the Allen, Mosbacher, and Law Chair in ophthalmology at the Cullen Eye Institute, Baylor College of Medicine, where he specializes in cataract and refractive surgery. His research interests include optics of cataract and refractive surgery, intraocular lens technology, anterior segment imaging, and surgical techniques in cataract and refractive surgery. He is Editor Emeritus of the Journal of Cataract and Refractive Surgery and past president of the American Ophthalmological Society, American Society of Cataract and Refractive Surgery, and International Intraocular Implant Club.
8.1 Introduction
Astigmatism is a key factor to consider when planning cataract surgery, once postsurgical residual astigmatism can compromise visual acuity. It has been estimated that 30 % of cataract patients have more than 0.75 diopters (D) of corneal astigmatism, that 22 % have more than 1.50 D, and that 8 % have more than 2.00 D [1, 2]. There are several methods to surgically treat corneal astigmatism, including adjustment of wound size and location, corneal relaxing incisions, opposite clear corneal cataract incisions, laser refractive surgery, and toric intraocular lens (IOL) implantation. Toric IOLs correct corneal astigmatism at the time of cataract surgery and are a predictable and permanent treatment [3].
Accurate measurement of corneal astigmatism is mandatory for choosing toric IOL power and planning optimal alignment. Various measuring methods are available, such as manual keratometry (manual K), automated keratometry, reflection methods of corneal topography, slit-scanning technology, optical coherence tomography, and Scheimpflug imaging. The first three methods measure the anterior corneal surface only. They assume a fixed posterior/anterior corneal curvature ratio to calculate total corneal power and astigmatism using a standardized corneal refractive index, most commonly 1.3375. Conversely, slit-scanning technology, optical coherence tomography, and Scheimpflug imaging measure the anterior and posterior corneal surfaces. Therefore, they provide total corneal power and astigmatism based on the measured anterior and posterior corneal data.
Manual and automated keratometry and corneal topography have been traditionally used to assess astigmatism for planning cataract surgery, since it has been assumed that the posterior cornea contributes with negligible amounts of astigmatism to the total corneal astigmatism. However, studies with toric IOLs have shown significant residual astigmatism after surgery [4, 5] and documented that postoperative anterior corneal astigmatism is not the only factor determining the amount of residual refractive astigmatism [6]. In addition, studies using a range of devices have investigated the posterior cornea and have shown that posterior corneal astigmatism ranges from 0.26 D to 0.78 D [7–10]. Thus, two recent studies were conducted to assess the importance of posterior corneal astigmatism in planning cataract surgery, and these are the studies that will be analyzed and discussed in this chapter [11, 12]. The first study used a combined Placido and dual Scheimpflug analyzer to investigate the contribution of posterior corneal astigmatism to total corneal astigmatism and the accuracy in estimating total corneal astigmatism from measurements of the anterior corneal surface [11]. The second study used five devices to evaluate the clinical impact of posterior corneal astigmatism on outcomes of cataract surgery with toric IOLs and provided a nomogram to guide the selection of the appropriate toric IOL power, factoring in posterior corneal astigmatism [12].
8.2 Methods
An institutional review board approval was obtained for both studies, and they followed the tenets of the Declaration of Helsinki.
8.2.1 Study 1 [11]
This retrospective study included consecutive patients who were screened for cataract or refractive surgery and had corneal measurements made using the dual Scheimpflug analyzer (Galilei, Ziemer Ophthalmic Systems AG, Port, Switzerland) between January 2008 and March 2011 at the Cullen Eye Institute, Baylor College of Medicine. Exclusion criteria were a history of previous ocular surgery or trauma, any ocular diseases that might affect the cornea or good fixation, contact lens wear within 2 weeks of the measurement, and image quality below “good quality.”
The Galilei combines dual Scheimpflug cameras and a Placido disk to assess both the anterior and posterior corneal surfaces. It derives the anterior corneal measurements from the combination of the Placido and Scheimpflug data and the posterior corneal measurements from the Scheimpflug data.
From the four corneal astigmatic values (CA) that were investigated in the study, we will assess two in this chapter:
CAant: Corneal astigmatism from the anterior corneal surface, which derives from the CA from simulated keratometry (SimK) [CASimK over the 1.0- to 4.0-mm zone. It is based only on anterior corneal measurement]. The CAant is calculated by multiplying the CASimK by (1.376–1.0)/(1.3375–1.0), assuming that the refractive index of the air is 1.0, the refractive index of the cornea is 1.376, and the standardized corneal refractive index is 1.3375. The CAant meridian is the steep SimK meridian.
CApost: Corneal astigmatism from the posterior corneal surface over the 1.0- to 4.0-mm zone, which is calculated with the indices of refraction of the cornea (1.376) and the aqueous humor (1.336), assuming that the rays approach the posterior corneal surface parallel to each other.
8.2.1.1 Data Analysis
We calculated the (1) mean magnitude, standard deviation (SD), and range of CAant and CApost, (2) the percentage of eyes with corneal astigmatism magnitudes up to 0.25 D, 0.50 D, 0.75 D, and 1.00 D, and (3) the correlation of magnitude and alignment of astigmatism on the anterior and posterior corneal surfaces. The eyes were subdivided based on the patients’ age at the time of the Galilei exam. To assess the changes in location of the steep meridian over time, the percentages of eyes with the steep meridian aligned vertically (60–120°), obliquely (30–60° or 120–150°), and horizontally (0–30° or 150–180°) on the anterior and posterior corneal surfaces were calculated for each age group. Chi-square test was used to compare the proportion data between age groups, and a Bonferroni correction was used for multiple comparisons. SPSS for Windows software (version 15.0, SPSS, Inc.) was used for statistical analysis. A P value less than 0.05 was considered statistically significant.
8.2.2 Study 2 [12]
This prospective study enrolled patients of the Cullen Eye Institute, Baylor College of Medicine, from July 2011 to September 2012. To be included, patients were required to have AcrySof toric IOL implantation without postoperative decentration/tilt under the slit lamp examination and good-quality preoperative and 3-week postoperative scans of the following devices: IOLMaster (Carl Zeiss Meditec AG, Jena, Germany), Lenstar (Haag-Streit, Koeniz, Switzerland), Atlas Corneal Topographer (Carl Zeiss Meditec AG, Jena, Germany), manual K (Bausch and Lomb, Rochester, New York, USA), and Galilei. Exclusion criteria were a history of previous ocular surgery or trauma, any ocular diseases that might affect the cornea or good fixation, contact lens wear within 2 weeks of the measurement, and poor image quality with each device.
Also, subjects with oblique corneal astigmatism (steep corneal meridian at 30–60° or 120–150°) measured by the IOLMaster were excluded, due to the small number of eyes.
8.2.2.1 Corneal Astigmatism Measurements
The following five devices were used to measure corneal astigmatism in this study:
1.
IOLMaster: Measures automated keratometry (K) based on a hexagonal array of 6 points reflected off the surface of the cornea at a diameter of approximately 2.3 mm, depending on the corneal curvature.
2.
Lenstar: Keratometry is calculated from an array of 32 light reflections projected off the anterior corneal surface. These lights are arranged in two rings at diameters of approximately 1.65 mm and 2.3 mm, depending on the corneal curvature.
3.
Atlas: The Atlas is a Placido-disk-based corneal topographer and provides SimK values along the steepest and flattest meridians at the 3-mm annular zone.
4.
Manual K: This is the conventional method for measuring corneal power at a diameter of approximately 3 mm, depending on the corneal curvature.
5.
Galilei: Calculates the total corneal power (TCP) by tracking the path of incident light rays through the anterior and posterior corneal surface using ray-tracing method and Snell’s law.
The IOLMaster, Lenstar, Atlas, and manual K measure anterior corneal curvature only, and their astigmatism values are the differences between the anterior corneal steep K and flat K. The Galilei provides a TCP astigmatism value, which is the difference between the steep TCP and flat TCP at the 1.0- to 4.0-mm central zone.
Biometry was done using the IOLMaster and Lenstar. The Holladay 1 formula was used for toric IOL power calculation. Selection of the toric lens power and alignment was determined by the surgeons based on all data available and on Study 1 [11]. The axis of the toric IOL alignment was recorded at the time of surgery and at the slit lamp exam in the 3-week postoperative visit. Manifest refraction was performed 3 weeks after surgery.
8.2.2.2 Data Analysis
Based on the anterior corneal steep meridian measured by the IOLMaster, the eyes were divided into two groups: (1) with-the-rule (WTR) group with corneal steep meridian between 60 and 120° and (2) against-the-rule (ATR) group with corneal steep meridian between 0–30° and 150–180°.
Vector analysis was used in all calculations [13]. To account for the impact of IOL power and anticipated effective lens position, the effective toric power of the IOL at the corneal plane was calculated using the Holladay 2 Consultant program. The assumed “actual” corneal astigmatism was calculated as the difference between the postoperative manifest refraction corrected to the corneal plane and the effective toric power. The corneal astigmatism prediction error for each device, or the deviation from actual corneal astigmatism, was obtained by subtracting the “actual” corneal astigmatism from the corneal astigmatism measured by each device.
Analysis of aggregate corneal astigmatism prediction errors was performed. Using pre- and postoperative corneal astigmatism measurements, both pre- and postoperative corneal astigmatism prediction errors were assessed for each device. The corneal astigmatism prediction errors were further analyzed as follows: (1) WTR/ATR prediction errors, i.e., the magnitudes of errors along the 90- and 180-degree meridians, with negative values indicating WTR prediction errors, and positive values indicating ATR errors and (2) oblique prediction errors, in which positive values indicate oblique astigmatism prediction errors along 45° and negative values along 135°.
8.2.2.3 Statistical Analysis
We aimed at detecting corneal astigmatism prediction error of >0.2 D. To achieve a significance level of 5 % and a test power of 80 %, a minimum sample size of 32 eyes was required.
To assess whether the prediction errors were WTR/ATR or oblique, a one sample t-test was performed to evaluate if the mean vector component values were significantly different from zero. Bonferroni correction was used for multiple comparisons. SPSS for Windows software (version 15.0, SPSS, Inc.) was used for statistical analysis. A P value less than 0.05 was considered statistically significant.
8.3 Results
8.3.1 Study 1 [11]
This first study included 715 eyes of 435 patients, with a mean age of 55 ± 20 years (range, 20–89 years). When subdivided by age, 101 (14.1 %) eyes were from patients with 20–29 years, 104 (14.5 %) eyes were from patients with 30–39 years, 101 (14.1 %) eyes were from patients with 40–49 years, 101 (14.1 %) eyes were from patients with 50–59 years, 101 (14.1 %) eyes were from patients with 60–69 years, 105 (14.7 %) eyes were from patients with 70–79 years, and 102 (14.3 %) eyes were from patients with 80–89 years.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
