Simulated Keratometry Versus Total Corneal Power by Ray Tracing: A Comparison in Prediction Accuracy of Intraocular Lens Power.

PURPOSE: To investigate the relationship between corneal powers measured by simulated keratometry (Sim-K) and ray tracing and understand whether their difference influences intraocular lens (IOL) power calculation. METHODS: In a first sample of healthy eyes, corneal curvature was measured using a rotating Scheimpflug camera (Sirius, CSO). Sim-K was obtained from anterior corneal curvature using the 1.3375 keratometric index. Ray tracing was performed through both corneal surfaces to calculate the total corneal power (TCP). The difference between Sim-K and TCP was correlated with various parameters. In a second sample of patients undergoing cataract surgery, IOL power was subsequently calculated using both Sim-K and TCP. RESULTS: In the first sample (114 eyes), Sim-K (43.64 ± 1.44 D) was higher than TCP (43.07 ± 1.41 D, P < 0.0001); the difference ranged between 0.07 and 1.95 D and correlated with the anterior/posterior (A/P) ratio (r = 0.7292, P < 0.0001), which ranged between 1.10 and 1.30. In the second sample (107 eyes), the A/P ratio influenced the outcomes of the Holladay 1 and SRK/T, but not Haigis and Hoffer Q formulas. However, using TCP, which takes the A/P ratio into account, did not improve the prediction error of any formula. CONCLUSIONS: Sim-K provides a higher corneal power compared with TCP. This difference is not constant but depends on the A/P ratio and can influence the refractive outcome of IOL power calculation by theoretical formulas. However, TCP values do not improve the results of these formulas, as they were developed for keratometric values such as Sim-K.

Optimized keratometry and total corneal astigmatism for toric intraocular lens calculation.

PURPOSE: To compare keratometric astigmatism (KA) and different modalities of measuring total corneal astigmatism (TCA) for toric intraocular lens (IOL) calculation and optimize corneal measurements to eliminate the residual refractive astigmatism. SETTING: G.B. Bietti Foundation IRCCS, Rome, Italy. DESIGN: Prospective case series. METHODS: Patients who had a toric IOL were enrolled. Preoperatively, a Scheimpflug camera (Pentacam HR) was used to measure TCA through ray tracing. Different combinations of measurements at a 3.0 mm diameter, centered on the pupil or the corneal vertex and performed along a ring or within it, were compared. Keratometric astigmatism was measured using the same Scheimpflug camera and a corneal topographer (Keratron). Astigmatism was analyzed with Næser's polar value method. The optimized preoperative corneal astigmatism was back-calculated from the postoperative refractive astigmatism. RESULTS: The study comprised 62 patients (64 eyes). With both devices, KA produced an overcorrection of with-the-rule (WTR) astigmatism by 0.6 diopter (D) and an undercorrection of against-the-rule (ATR) astigmatism by 0.3 D. The lowest meridional error in refractive astigmatism was achieved by the TCA pupil/zone measurement in WTR eyes (0.27 D overcorrection) and the TCA apex/zone measurement in ATR eyes (0.07 D undercorrection). In the whole sample, no measurement allowed more than 43.75% of eyes to yield an absolute error in astigmatism magnitude lower than 0.5 D. Optimized astigmatism values increased the percentage of eyes with this error up to 57.81%, with no difference compared with the Barrett calculator and the Abulafia-Koch calculator. CONCLUSION: Compared with KA, TCA improved calculations for toric IOLs; however, optimization of corneal astigmatism measurements led to more accurate results.

Corneal Asphericity and IOL Power Calculation in Eyes With Aspherical IOLs.

PURPOSE: Given that a previous study found that corneal asphericity influences the refractive outcome of intraocular lens (IOL) power calculation by means of thin-lens formulas in eyes with spherical IOLs, the authors aimed to verify whether such influence can also be observed in eyes with aspherical IOLs. METHODS: In this retrospective comparative case series, IOL power was calculated with the Haigis, Hoffer Q, Holladay 1, and SRK/T formulas in two groups of eyes undergoing phacoemulsification and implantation of an aspherical IOL (Acrysof SN60WF; Alcon Laboratories, Inc., Fort Worth, TX). For each formula, the refractive prediction error was calculated once the constants had been optimized. Biometric data were obtained by partial coherence interferometry in one group and immersion ultrasound biometry and corneal topography in the other. Corneal asphericity was assessed by two different corneal topographers in the optical biometry group (Magellan; Nidek, Gamagori, Japan) and ultrasound biometry group (Keratron, Optikon 2000 Spa, Rome, Italy). RESULTS: The mean Q-value was -0.12 ± 0.08 in the optical biometry group (n = 51) and -0.22 ± 0.14 in the ultrasound biometry group (n = 79). In both groups, linear regression disclosed a statistically significant correlation between the Q-value and the prediction error (the only exception being the SRK/T formula). More negative Q-values were correlated to a positive prediction error, indicating a myopic outcome for prolate corneas. However, the correlation coefficients were lower than those previously reported for spherical IOLs. CONCLUSIONS: Corneal asphericity also influences the refractive outcomes of IOL power calculation by thinlens formulas when aspherical IOLs are implanted, although this influence is exerted to a lesser degree compared to spherical IOLs. [J Refract Surg. 2017;33(7):476-481.].

Correction: Accuracy of optical biometry combined with Placido disc corneal topography for intraocular lens power calculation.

[This corrects the article DOI: 10.1371/journal.pone.0172634.].

Accuracy of optical biometry combined with Placido disc corneal topography for intraocular lens power calculation.

PURPOSE: To investigate the accuracy of a new optical biometer for intraocular lens (IOL) power calculation in eyes undergoing cataract surgery. METHODS: Consecutive eyes of patients undergoing cataract surgery with the same IOL model were enrolled in a prospective cohort study. Axial length (AL) and corneal power were measured with an optical biometer based on optical low-coherence interferometry and Placido-disc corneal topography. IOL power was calculated with the Hoffer Q, Holladay 1 and SRK/T formulas. For each formula the lens constant was optimized in retrospect in order to achieve a mean prediction error (PE) of zero (difference between the predicted and the postoperative refraction). Median absolute error (MedAE) and percentage of eyes with PE ±0.50 D were calculated. RESULTS: Seventy-four eyes of 74 cataract patients were enrolled. The MedAE was 0.25 D with all formulas. A PE within ±0.50 D was obtained in 89.04% of cases with the Hoffer Q and SRK/T formulas, and in 87.67% of cases with the Holladay 1 formula. CONCLUSIONS: The optical biometer investigated in the present study provides accurate measurements for IOL power calculation.

Total Corneal Astigmatism Measurements: Agreement Between 2 Rotating Scheimpflug Cameras.

PURPOSE: To investigate agreement between rotating Scheimpflug camera (Pentacam HR, Oculus) and rotating Scheimpflug camera combined with Placido disc corneal topography (Sirius, CSO) in measuring total corneal astigmatism (TCA). METHODS: In this observational study, all patients undergoing cataract surgery with preoperative measurement of TCA by both devices and a validated corneal topographer (Keratron, Optikon 2000) were retrospectively evaluated. Astigmatism analysis was performed with and without vector analysis separately in eyes with with-the-rule, against-the-rule, and oblique astigmatism. Vector analysis was performed using the Næser polar system. RESULTS: In 130 eyes of 130 subjects, nonvectorial analysis revealed that the mean TCA values obtained with the Sirius were higher than the corresponding values given by the Pentacam HR in all subgroups, although the difference was statistically significant only in eyes with against-the-rule astigmatism (P = 0.0009). This finding was confirmed by vector analysis. A TCA magnitude difference greater than 0.5 diopters was observed in 20.8% of cases, and a TCA axis difference greater than 10 degrees was observed in 45.4% of cases. Axis differences dropped to 18.5% when only eyes with astigmatism >0.75 diopters were analyzed and 3 measurements were averaged. The mean difference in the meridional and torsional power of TCA was close to zero in all subgroups, but with relatively large standard deviations (approximately 0.5 D). CONCLUSIONS: Agreement between both devices in measuring TCA is only moderate with respect to both magnitude and axis orientation.

Hemorrhagic Occlusive Retinal Vasculitis After First Eye Cataract Surgery Without Subsequent Second Eye Involvement.

A case of monocular postoperative hemorrhagic occlusive retinal vasculitis (HORV) after uncomplicated bilateral cataract surgery is described. HORV is a severe syndrome that leads to painless visual loss after uncomplicated cataract surgery. The same surgical procedure was adopted in both eyes except for the use of intracameral vancomycin, which was injected only in the eye that developed HORV. Diffuse retinal ischemia with vascular sheathing and intraretinal hemorrhages were detected during the fourth postoperative day. Despite treatment, the patient developed severe neovascular glaucoma. This case supports the causative role of vancomycin in the pathogenesis of HORV and suggests avoiding it for chemoprophylaxis. [Ophthalmic Surg Lasers Imaging Retina. 2016;47:764-766.].

Estimating the Preoperative Corneal Power With Scheimpflug Imaging in Eyes That Have Undergone Myopic LASIK.

PURPOSE: To investigate whether the Estimated Pre-Refractive SimK accurately predicts the preoperative corneal power (K) in eyes with previous myopic femtosecond laser-assisted LASIK (FS-LASIK) and whether a better prediction can be developed. METHODS: Preoperatively and 3 months after FS-LASIK, patients underwent anterior segment imaging with the Pentacam HR rotating Scheimpflug camera (Oculus Optikgeräte, Wetzlar, Germany). Posterior and anterior corneal curvature and power and asphericity (Q-value at 8 mm) were recorded. Multiple linear regression was used to assess the relationship among variables. RESULTS: One hundred four eyes of 104 patients were enrolled. The Estimated Pre-Refractive SimK (43.19 ± 1.63 diopters [D]) was lower (P = .0014) than the mean preoperative K (43.44 ± 1.49 D). The difference was within ±0.50 and ±1.00 D in 53.8% and 86.5% of eyes, respectively. Multiple linear regression detected a statistically significant relationship (r(2) = 0.8497, P < .0001) among the investigated parameters: preoperative anterior corneal radius = 1.156 + 1.012 * postoperative posterior corneal radius - 0.3800 * postoperative Q-value. When this equation was used to predict the preoperative K, the difference between the actual preoperative K and the predicted value (43.41 ± 1.38 D) was not statistically significant. The rate of eyes with a difference between the predicted and actual preoperative K within ±0.50 and ±1.00 D increased to 61.5% and 93.3%, respectively. CONCLUSIONS: The corneal power before FS-LASIK can be accurately predicted from the postoperative corneal measurements of a rotating Scheimpflug camera. The multiple regression formula described may be more accurate than the Estimated Pre-Refractive SimK available on the Pentacam. [J Refract Surg. 2016;32(5):332-336.].

Agreement Between Predicted and Measured Ablation Depth After Femtosecond Laser-Assisted LASIK for Myopia.

PURPOSE: To investigate agreement between the predicted ablation depth calculated by the EX500 excimer laser (Wavelight Laser Technologie AG, Erlangen, Germany) and the measured ablation depth in eyes that have undergone femtosecond laser-assisted LASIK (FS-LASIK) for myopia. METHODS: Corneal thickness was measured with a rotating Scheimpflug camera preoperatively and 3 months postoperatively and the difference between these values was defined as the measured ablation depth. The difference between the predicted and the measured ablation depth was defined as the difference in ablation depth (ΔAD). RESULTS: In 85 eyes of 85 patients, no statistically significant difference was detected between the mean predicted ablation depth (66.33 ± 24.15 µm) and the measured ablation depth at the thinnest corneal location (67.04 ± 30.94 µm), the corneal apex (67.52 ± 31.22 µm), or the pupil center (67.73 ± 31.48 µm). Bland-Altman plots revealed moderate agreement for measurements at the thinnest point (95% limits of agreement [LoA]: -25.13 to 23.70 µm), corneal apex (95% LoA: -24.70 to 22.33 µm), and pupil center (95% LoA: -25.30 to 22.51 µm), with a proportional bias between the average ablation depth and ΔAD. The predicted ablation depth was overestimated in eyes with lower correction and underestimated in eyes with higher correction. CONCLUSIONS: Moderate agreement between the predicted and measured ablation depth warrants caution when planning myopic FS-LASIK and calculating the residual bed thickness and percent tissue altered. When higher amounts of correction are planned, the laser software may underestimate the predicted ablation depth.

Influence of the effective lens position, as predicted by axial length and keratometry, on the near add power of multifocal intraocular lenses.

PURPOSE: To calculate the near focal distance of different multifocal intraocular lenses (IOLs) as a function of the 2 parameters that are measured before cataract surgery; that is, axial length (AL) and refractive corneal power (keratometry [K]). SETTING: GB Bietti Foundation IRCCS, Rome, Italy. DESIGN: Noninterventional theoretical study. METHODS: The IOL power for emmetropia was first calculated in an eye model with the AL ranging from 20 to 30 mm and K from 38 to 48 diopters (D). Then, the predicted myopic refraction for any given IOL add power (from +1.5 to +4.0 D) was calculated, and from this value the near focal distance was obtained. Calculations were also performed for the average eye (K = 43.81 D; AL = 23.65 mm). RESULTS: The near focal distance increased with increasing values of K and AL for each near power add. The near focal distance ranged between 53 cm and 72 cm (21 inches and 28 inches) for a multifocal IOL with +2.50 D, between 44 cm and 60 cm (17 inches and 24 inches) for a multifocal IOL with +3.00 D add, and between 33 cm and 44 cm (13 inches and 18 inches) for a multifocal IOL with +4.00 D add. In the average eye, the near focal distance ranges between 36 cm (near add power = 4.00 D) and 99 cm (near add power = 1.5 D). CONCLUSIONS: Longer eyes with steeper corneas showed the longest near focal distance and could experience more difficulties in focusing near objects after surgery. The opposite was true for short hyperopic eyes. FINANCIAL DISCLOSURE: Dr. Hoffer receives licensing fees for the commercial use of the registered trademark Hoffer from all biometry manufacturers using the Hoffer Q formula to ensure that it is programmed correctly and book royalties from Slack, Inc., for the textbook IOL Power. None of the authors has a financial or proprietary interest in any material or method mentioned.

Objective Monitoring of Corneal Backward Light Scattering After Femtosecond Laser-assisted LASIK.

PURPOSE: To investigate the changes in corneal backward light scattering, as measured by a rotating Scheimpflug camera with automated corneal densitometry software, in eyes treated with femtosecond laser-assisted LASIK (FS-LASIK). METHODS: The cornea was examined preoperatively and postoperatively at 1 day, 1 week, and 1, 3, and 6 months in 23 patients who underwent myopic FS-LASIK. Local analysis of corneal backscatter was performed on four concentric radial zones across a 12-mm diameter (0 to 2, 2 to 6, 6 to 10, and 10 to 12 mm) and at a different corneal depth (anterior 120 µm, central and posterior 60 µm). RESULTS: A statistically significant increase in corneal backward light scattering (P < .0001) was detected within the central 10 mm of the anterior cornea. The increase in corneal densitometry was gradually reversed over 6 months. The difference compared to preoperative values was no longer statistically significant at 3 and 6 months after surgery in the central cornea, whereas it remained significant in the mid-peripheral annulus (ranging from 6 to 10 mm), where the flap edge was located. CONCLUSIONS: FS-LASIK is followed by an increase in corneal backward light scattering during the early postoperative period that returns to baseline by 3 months. Whereas the increase in corneal densitometry at the flap edge location can be related to a scarring reaction, the explanation for such an increase in the central anterior cornea remains speculative.

Intraocular lens power calculation after myopic excimer laser surgery: Selecting the best method using available clinical data.

PURPOSE: To compare the results of methods to calculate intraocular lens (IOL) power after myopic excimer laser surgery. SETTING: G.B. Bietti Foundation-IRCCS, Rome, Italy. DESIGN: Prospective interventional case series. METHODS: Eyes were classified into 4 groups: Group 1 (preoperative keratometry available, refractive change known), Group 2 (preoperative keratometry available, refractive change uncertain), Group 3 (preoperative keratometry unavailable, refractive change known), and Group 4 (preoperative keratometry unavailable, refractive change unknown). The IOL power was calculated by 19 methods. The median absolute error in refraction prediction and the percentage of eyes with a refraction prediction error within ±0.50 diopter (D) were calculated. RESULTS: In Group 1 (n = 30), the Savini, Seitz/Speicher/Savini, and Masket methods provided the lowest median absolute error (0.29 D, 0.35 D, and 0.34 D, respectively), with more than 70% of eyes within ±0.50 D of the predicted refraction. In Group 2 (n = 16), the Seitz/Speicher method achieved the best result (median absolute error 0.37 D), with 75% of eyes within ±0.50 D of the predicted refraction. In Group 3 (n = 18), the Masket method provided the lowest median absolute error (0.24 D), with 72.2% of eyes within ±0.50 D of the predicted refraction. In Group 4 (n = 6), the Shammas no-history method had the lowest median absolute error (0.31 D), with 83% of eyes within ±0.50 D of the predicted refraction. CONCLUSION: Intraocular lens power can be accurately calculated in post-laser surgery eyes when the preoperative corneal power and refractive change are known and when they are not. FINANCIAL DISCLOSURE: Dr. Hoffer receives book royalties for IOL Power and formula royalties from all manufacturers using the Hoffer Q formula to ensure it is programmed correctly. No other author has a financial or proprietary interest in any material or method mentioned.

Biomechanical changes in the human cornea after transepithelial corneal crosslinking using iontophoresis.

PURPOSE: To evaluate the corneal response to variable intraocular pressure (IOP) in human eye globes after ultraviolet-A (UVA) transepithelial corneal crosslinking using iontophoresis. SETTING: Fondazione G.B. Bietti IRCCS, Rome, Italy. DESIGN: Experimental study. METHODS: Four human donor eye globes were treated with transepithelial crosslinking using iontophoresis and rapid UVA corneal irradiation, and 4 globes had standard crosslinking. Inflation experiments were performed on the globes before and after crosslinking. Topographic maps of the anterior and posterior cornea were acquired using Scheimpflug topography. Images were obtained using a mechanical regimen to analyze corneal strain in response to cyclic stress. Corneal shape changes were analyzed as a function of IOP, and corneal stress-strain curves were generated. RESULTS: Before crosslinking, instillation of hypotonic riboflavin-5-phosphate sodium 0.1% solution using iontophoresis increased corneal thickness by 5% and instillation of dextran-enriched riboflavin 0.1% solution decreased corneal thickness by 13%. Five minutes after treatment, both crosslinking procedures reduced corneal thickness by 2%. Young's modulus (E) of the anterior cornea increased by a mean of 1.8 times (from 1.6 to 2.9 MPa) and 1.9 times (from 1.3 to 2.5 MPa) after transepithelial crosslinking using iontophoresis and standard crosslinking, respectively. The E value of the posterior cornea also increased after both procedures (mean 1.7 times versus 3.1 times). CONCLUSIONS: Transepithelial crosslinking using iontophoresis increased the biomechanical strength of human corneal tissue in inflation testing of donor eye globes. The effect on corneal stiffness was almost comparable to that of standard crosslinking. FINANCIAL DISCLOSURE: No author has a financial or proprietary interest in any material or method mentioned.

Influence of posterior corneal astigmatism on total corneal astigmatism in eyes with moderate to high astigmatism.

PURPOSE: To evaluate the influence of posterior corneal astigmatism on total corneal astigmatism in patients with 1.00 diopter (D) or more of corneal astigmatism. SETTING: Private practice, Bologna, Italy. DESIGN: Prospective case series. METHODS: Corneal astigmatism was measured using a Scheimpflug camera combined with a corneal topographer (Sirius). Keratometric astigmatism, anterior corneal astigmatism, posterior corneal astigmatism, and total corneal astigmatism were evaluated. Vector analysis was performed according to the Næser method. RESULTS: One hundred fifty-seven eyes were enrolled. Keratometric astigmatism was with the rule (WTR), against the rule (ATR), and oblique in 84.0%, 11.5%, and 4.5% of eyes, respectively. Posterior corneal astigmatism exceeded 0.50 D and 1.00 D in 55.4% of eyes and 5.7% of eyes, respectively. The mean posterior corneal astigmatism was 0.54 D, inclined 91 degrees in relation to the steeper anterior corneal meridian. The steepest meridian was vertically aligned in 93.0% of cases. Compared with total corneal astigmatism, keratometric astigmatism overestimated WTR astigmatism by a mean of 0.22 D ± 0.32 (SD), underestimated ATR astigmatism by 0.21 ± 0.26 D, and overestimated oblique astigmatism by 0.13 ± 0.37 D. In the whole sample, a difference in astigmatism magnitude of 0.50 D or more was detected between keratometric astigmatism and total corneal astigmatism in 16.6% of cases and the difference in the location of the steep meridian was greater than 10 degrees in 3.8% of cases. CONCLUSION: In patients who are candidates for surgical correction of astigmatism, measuring only the anterior corneal curvature can lead to inaccurate evaluation of the total corneal astigmatism. FINANCIAL DISCLOSURE: Mr. Versaci and Mr. Vestri are employees of Costruzione Strumenti Oftalmici Srl. No author has a financial or proprietary interest in any material or method mentioned.

Influence of intraocular lens haptic design on refractive error.

PURPOSE: To assess the influence of intraocular (IOL) haptic design on the refraction prediction error in patients having cataract surgery. SETTING: Private practice. DESIGN: Comparative case series. METHODS: Corneal power and axial length were measured with the same devices in eyes with a 3-piece Acrysof IOL and eyes with a 1-piece Acrysof IOL and were entered into the Haigis, Hoffer Q, Holladay 1, and SRK/T formulas. The median absolute error and mean absolute error in refraction prediction (ie, difference between expected refraction and actual refraction) were assessed 1 month postoperatively. RESULTS: The study evaluated 110 eyes with the 3-piece IOL and 84 eyes with the 1-piece IOL. With all formulas, the median absolute error was lower with the 3-piece IOL. It ranged between 0.15 diopter (D) (Haigis and Holladay 1) and 0.19 D (SRK/T) with the 3-piece IOL and between 0.23 D (Haigis) and 0.30 D (SRK/T) with the 1-piece IOL. With all formulas, a higher percentage of eyes with the 3-piece IOL were within ±0.25 D and ±0.50 D of the target refraction. CONCLUSIONS: Three-piece IOLs may yield better refractive outcomes than 1-piece IOLs. A possible reason is that once the early forward IOL shift previously observed with the 3-piece design occurs because of the haptic-compression force decay typical of these IOLs, the rigid haptics of 3-piece IOLs still exert more pressure against the capsular bag than the haptics of 1-piece IOLs. Therefore, 3-piece IOLs may better resist subsequent capsule contraction and provide a more predictable effective lens position. FINANCIAL DISCLOSURE: Dr. Hoffer receives royalties for his book IOL Power, Slack, Inc., and formula royalties from all manufacturers using the Hoffer Q formula. No other author has a financial or proprietary interest in any material or method mentioned.