This single-arm study provided one intervention: use of the dual-zone autokeratometry feature of the Lenstar LS 900® biometer as the basis for toric IOL selection. This met the current standard of care in the study clinic so there was no additional risk to patients. Institutional Review Board approval was applied for and obtained before patients were enrolled (Southwest Independent Institutional Review Board, Inc., Fort Worth, TX). Patients signed an informed consent form permitting use of their de-identified data.
The Lenstar LS 900® uses 32 measuring points arranged in 2 concentric rings (outer 2.3
mm, inner 1.65
mm) of 16 measuring points each. Each displayed keratometry measurement is a composite of the mean of 4 measurements, totaling 128 measuring points. With the recommended 5 scans, the keratometry is therefore calculated on the basis of 640 measuring points. Once the data are captured, the spherical equivalent radius is calculated for each individual measuring point. The keratometric calculation considers the best-fit ellipsoid built by the reflected points to determine the radii of the circumscribed ellipsoid. Results are then expressed in dioptric or millimeter notation (Lenstar LS 900® Biometer, Instruction Manual, Section 4.3.2. Haag-Streit AG, Koeniz, Switzerland). In contrast, manual keratometry measurement generally consists of a single reading of the alignment of 2 perpendicular mires at a diameter of approximately 3.2
During the measurement process the software checks every image for the quality of the marker point representation and the reproducibility of the points. Plain text messages advise the user how to improve the measurement quality e.g. if eye lashes or low eye lids distort or even block markers, then the user is indicated to encourage the patient to open up the eyes wide. Additionally to these individual checks, all repetitive measurements are tested for their plausibility as compared to the other measurements on the same as well as on the adjacent eye. Anomalous measurements are flagged for review. As a final quality control the standard deviation of the repetitive measurements is displayed to the user. The standard deviation of the calculated angle of astigmatism is one measure of interest in this study.
A total recruitment of 50 eyes was planned for the study, with an AcrySof® Toric IOL (Alcon, Fort Worth, TX), implanted in each eye that was enrolled. These lenses correct from 1.5D to 6.0D of astigmatism at the IOL plane, corresponding to 1.03D to 4.11D at the corneal plane for a nominal eye. Surgical planning for the astigmatism power of the lens was completed using the online toric calculator provided for that purpose (http://www.acrysoftoriccalculator.com
). The previously calculated surgically induced astigmatism (SIA) for the surgeon was used along with the keratometry readings from the Lenstar LS 900®. The SIA calculation was based on previous surgeries with the same incision size and orientation, to reduce potential variability related to these variables. Operative data included the IOL implanted and the axis at which it was implanted, along with the surgeon’s SIA and the incision angle for each surgery.
All study eyes were operated on by the same surgeon (KGG).
Just prior to surgery, the axis of astigmatism (as determined from the toric calculator) was marked directly by the surgeon. The first 21 eyes were manually marked with waterproof ink at the slitlamp, whereas the last 29 eyes were marked with waterproof ink using a handheld pendulum marking instrument (G-33776 by Geuder AG, Heidelberg, Germany).
Standard phaco surgery was performed in all study eyes. The main incision (2.2
mm width) was placed at the 12 o’clock position, with two side port incisions (1
mm width) placed at the 10 and 2 o’clock positions. All incisions were clear corneal at a 45° to 60° angle. A corneal ring of 5
mm was used as a guide to obtain a standardized capsulorhexis size and location centered around the visual axis. A 5
mm capsulorhexis size was chosen to obtain a 360° contact between the anterior capsule and the anterior surface of the IOL, to ensure maximal stability of the IOL inside the capsular bag.
After implantation of the AcrySof IQ Toric IOL in the capsular bag, the lens was rotated and aligned to the ink marks and finally pushed back to the posterior capsule to ensure proper and durable alignment and “fixation”. After instillation of intracameral antibiotics, stromal hydration of all incisions ended the procedure.
The primary end point was the post-operative refractive data at 3
months. Secondary end points were the post-operative keratometry and visual acuity. Data collection included pre-operative keratometry, along with keratometry, visual acuity and refractive data at 1
month and 3
Clinical data were tabulated and de-identified on case report forms, along with printouts from the AcrySof Toric Calculator and the Lenstar LS 900® biometer. Preliminary data checking and analysis was performed using Access database software (Microsoft Corp.). The preliminary analysis included calculating corneal astigmatism from keratometry values. Statistical analyses were performed using the Statistica data-analysis software system (version 9.1, Statsoft, Inc.).
The primary measure of interest was the distribution of residual astigmatism (magnitude and axis) for the patient population. Levels of uncorrected and best-corrected visual acuity were also of interest. A secondary measure of interest was whether the standard deviations of the keratometric astigmatism or keratometric axis were correlated to higher variability in refractive outcomes. For this last measure, statistical testing with appropriate parametric and non-parametric tests was conducted using a significant p value of<0.05.