Intraocular Lens Power Determination Research Articles

Comparison of ocular biometric measurements and intraocular lens power calculation using different methods in eyes with implantable collamer lenses

This prospective cohort study included 80 healthy candidates for Implantable Collamer Lens (ICL) implantation who underwent biometric assessments with Scheimpflug imaging (the Pentacam-AXL) and swept-source optical coherence tomography (SS-OCT; the IOLMaster-700), both before and 3 months after surgery. The main outcome measures were mean keratometry, anterior chamber depth, axial length, and various intraocular lens (IOL) calculation formulas (Haigis, SRK/T, Hoffer Q, Holladay 1, Barrett Universal 2, and Olsen). The interchangeability of the devices was assessed by generating 95% limits of agreement (95% LoA) and associated Bland-Altman plots. The average age of the participants was 31.5 ± 5.4 years (22–43), with 58 (72.5%) being female. Among the cases analyzed, 11 (13.4%) had incorrect anterior lens surface segmentation using the IOLMaster-700, and 1 case (1.2%) had inappropriate segmentation using the Pentacam-AXL. Postoperative IOL power calculation resulted in readings that were, on average, 0.15 to 0.30 D higher compared to preoperative measurements. The 95% LoAs could differ by up to 0.85 D higher after surgery, indicating weak agreement between pre- and postoperative measurements. There was poor agreement between the IOLMaster-700 and Pentacam-AXL in IOL power calculation for eyes with post-ICL implantation, with a difference of more than 1 D in the 95% LoAs. In conclusion, Scheimpflug imaging was found to be less susceptible than the SS-OCT technique to segmentation errors of the anterior lens surface after ICL implantation. Neither device showed interchangeable results for pre- versus postoperative IOL power calculation. The determination of IOL power by the IOLMaster-700 versus Pentacam-AXL was not interchangeable in eyes with ICL implantation.

Comparison of Axis Determination With Different Toric Intraocular Lens Power Calculation Methods.

To compare the axis position of the measured total corneal astigmatism (TCA) with the axis of the anterior keratometry and the calculated axis position of different toric intraocular lens (IOL) calculators. A total of 163 astigmatic eyes of 163 patients were retrospectively analyzed. The axis of the actual TCA, measured with anterior segment optical coherence tomography, was compared to the anterior keratometric value (Group I) and three different methods of TCA calculation for toric IOL power determination: Abulafia-Koch regression formula (Group II), Barrett Toric Calculator V2.0 (Group III), and Barrett Toric Calculator V2.0 including measured posterior keratometric value (Group IV). Eyes were assigned to three subgroups: with-the-rule, against-the-rule, and oblique astigmatism. The mean deviation calculated from measured TCA was +0.56° (Group I), -0.32° (Group II), -0.37° (Group III), and -1.00° (Group IV). For with-the-rule astigmatism, the TCA axis agreed most with Group I (6.5% outliers > 5° deviation). For against-the-rule astigmatism, Group IV and Group II were closest to the measured TCA axis (1.5% and 3% outliers with > 5° deviation). The means of the calculated axis were similar to the measured TCA, but the proportion of outliers with an axis deviation of greater than 5° showed remarkable differences. Isolated anterior keratometric value measurements showed the fewest outliers in with-the-rule astigmatism. In against-the-rule astigmatism, Abulafia-Koch calculation should be used for axis determination. [J Refract Surg. 2021;37(9):642-647.].

Intraoperative Biometry versus Conventional Methods for Predicting Intraocular Lens Power: A Closer Look at Patients Undergoing Toric Lens Implantation for Astigmatic Correction

Purpose: To compare intraoperative refractive biometry to conventional methods for intraocular lens (IOL) power calculation in patients receiving toric IOLs. Setting: The New York Eye and Ear Infirmary of Mt. Sinai. Design: Retrospective Case Series. Methods: Separate cohorts of patients undergoing primary cataract extraction with toric IOL implantation were analyzed. In 52 patients, Optivue Refractive Analysis (ORA) was used to guide IOL power determination and axis adjustment. In a separate cohort of 52 patients, conventional methods were used to determine the astigmatic axis and calculate the IOL power. Prediction error (Actual spherical equivalent (SE)-Predicted SE) and median absolute error (MAE) were calculated in each group. The percentage of eyes within ± 0.50 diopters (D) and ± 1.00 D of the refractive target along with the residual cylinder and deviation from intended axis were compared. Results: The 52 patients in the ORA cohort achieved statistically significant better postoperative MAE than those in the conventional group (MAE 0.34 ± 0.29 (conventional) vs. 0.25 ± 0.22 (ORA), p=0.05). When ORA were used, patients were more likely to fall within 0.5D of the targeted refraction (87% vs. 79%). Residual astigmatism was less than 1D in 87% of the ORA group compared to 69% of the conventional group. In the ORA group, 29% of eyes ended up within 10 degrees of the intended axis compared to 12% of eyes in the conventional group. Conclusions: The current study shows that intraoperative biometry significantly improves refractive target outcomes for patients undergoing toric IOL implantation in comparison to conventional methodology.

Influence of ophthalmic viscosurgical devices on intraoperative aberrometry

To determine whether the presence of an ophthalmic viscosurgical device (OVD) in the anterior chamber influences intraoperative aberrometry and the suggested intraocular lens (IOL) power. Advanced Vision Care, Los Angeles, and Specialty Surgery Center, Beverly Hills, California, USA. Prospective interventional case series. Eyes scheduled for routine phacoemulsification and were divided into 6 equal groups, with each having 1 of 6 OVDs. After cataract removal, carefully controlled aberrometry was performed with the anterior chamber filled with balanced salt solution (BSS). Immediately thereafter, the BSS was replaced by 1 of the OVDs and the aberrometry repeated. The IOL power was selected from the BSS reading, and clinical manifest refraction was performed 3weeks after surgery. The mean absolute error (MAE) was determined and compared with the extrapolated refraction had the IOL power been selected from the aberrometry reading under OVD. The study comprised 120 eyes, 20 in each group. The IOL power determination was lower with OVD filling the chamber. For Discovisc and Amvisc Plus, the MAE determinations were statistically different because the suggested IOL power was approximately 0.50 diopter less than with a BSS fill. For the remaining OVDs (Amvisc, Healon, Healon GV, and Provisc), the MAE differences were insignificant. The strong correlation between differences in the index of refraction between BSS and specific OVDs appeared to be causal. Surgeons should be aware of the influence of OVDs on the accuracy of intraoperative aberrometry because specific agents can alter the optical results and suggested IOL power. Dr. Masket is a consultant to Wavetec Vision Systems, Inc., and Alcon Laboratories, Inc., and has a research grant from Alcon Laboratories, Inc. Dr. Fram has a research grant from Wavetec Vision Systems, Inc. Dr. Holladay is a consultant to Wavetec Vision Systems, Inc.

Pilot study on dry eye affecting the accuracy of intraocular lens power measurement in age-related cataract

Background The incidence of dry eye is gradually increasing, and the aging of population is one of factors. Researches showed that the change of tear film stability leads to shift of corneal refractive power, which probably affects the measurement of intraocular lens (IOL) power. The influence of dry eye on the calculation of IOL power is worth paying close attention. Objective This study was to investigate the influence of dry eye on the accuracy of IOL power calculation in age-related cataractous patients. Methods A non-randomized controlled clinical study was performed. Two hundred and sixteen eyes of 216 cataract patients were enrolled in Yanbian Eye Ear Nose and Throat Hospital from May 2013 to February 2014. The visual acuity, intraocular pressure, break up time of tear film (BUT), SchirmerⅠtest (SⅠt) and corneal fluorescein staining were examined in all the eyes, and then the patients were assigned to mild dry eye group (73 eyes), severe dry eye group (68 eyes) and non-dry eye control group (75 eyes). The axial length (AL) and anterior chamber depth (ACD) were measured using IOL Master for the calculation of IOL power and K value (corneal curvature). Phacoemulsification with IOL implantation was performed on all the eyes, and the refractive power was measured with KR 8100 automatic optometry 2 months after operation. The mean absolute refractive error (MARE) between actual refraction and predicted refraction was calculated, and percentage of eyes in different refractive error ranges was compared between the dry eye group and non-dry eye control group. The correlations of dry eye-related parameters with peroperative K value and refractive error were analyzed. This study was approved by the Ethics of Yanbian University, and written informed consent was obtained from each subject prior to entering the cohort. Results No significant differences were found in age, gender, AL, ACD and MARE among the mild dry eye group, severe dry eye group and non-dry eye control group (all at P>0.05), and significant differences in BUT, SⅠt and corneal fluorescein staining scores were found among these three groups (F=5.460, 4.521, 3.572, all at P<0.05). The K values were (43.59±1.39), (44.66±1.53) and (43.42± 1.32)D in the mild dry eye group, severe dry eye group and non-dry eye control group, and the K value in the severe dry eye group was significantly higher than that in the non-dry eye control group (P=0.012). The percentage of eyes with + 0.50-+ 0.75 D deviation was significantly elevated(28.8% versus 13.3%), and that with -0.50--0.75 D deviation was significantly declined (17.3% versus 28.0%)in the dry eye group compared with non-dry eye control group (χ2=4.513, P=0.032; χ2=4.236, P=0.037). A negative correlation was found between BUT and K value (r=-0.204, P=0.011). Conclusions Dry eye affects the accuracy of the determination of IOL power. Compared with the preoperative refraction predictive value, the postoperative refraction shift toward hyperopia. Key words: [Key words] Biometry; Dry eye; Cataract; Refractive error; Intraocular lens power

Effect of the equivalent refractive index on intraocular lens power prediction with ray tracing after myopic laser in situ keratomileusis

To determine the impact of the equivalent refractive index (ERI) on intraocular lens (IOL) power prediction for eyes with previous myopic laser in situ keratomileusis (LASIK) using custom ray tracing. AMO B.V., Groningen, the Netherlands, and the Department of Ophthalmology, Baylor College of Medicine, Houston, Texas, USA. Retrospective data analysis. The ERI was calculated individually from the post-LASIK total corneal power. Two methods to account for the posterior corneal surface were tested; that is, calculation from pre-LASIK data or from post-LASIK data only. Four IOL power predictions were generated using a computer-based ray-tracing technique, including individual ERI results from both calculation methods, a mean ERI over the whole population, and the ERI for normal patients. For each patient, IOL power results calculated from the four predictions as well as those obtained with the Haigis-L were compared with the optimum IOL power calculated after cataract surgery. The study evaluated 25 patients. The mean and range of ERI values determined using post-LASIK data were similar to those determined from pre-LASIK data. Introducing individual or an average ERI in the ray-tracing IOL power calculation procedure resulted in mean IOL power errors that were not significantly different from zero. The ray-tracing procedure that includes an average ERI gave a greater percentage of eyes with an IOL power prediction error within ±0.5 diopter than the Haigis-L (84% versus 52%). For IOL power determination in post-LASIK patients, custom ray tracing including a modified ERI was an accurate procedure that exceeded the current standards for normal eyes.

Comparison of Intraoperative Aberrometry, OCT-Based IOL Formula, Haigis-L, and Masket Formulae for IOL Power Calculation after Laser Vision Correction

To compare the accuracy of intraoperative aberrometry technology and the Fourier-domain optical coherence tomography (OCT)-based intraocular lens (IOL) formula for IOL power calculation in eyes undergoing cataract surgery after previous laser vision correction (LVC) compared with established methods. Retrospective consecutive case series. Patients undergoing cataract surgery with a history of LASIK or photorefractive keratectomy. The IOL power was estimated preoperatively using the IOLMaster 500 (Carl Zeiss Meditec, Dublin, CA) to calculate the Haigis-L and Masket regression formulae (when prior data were available), and the Optovue RTVue (Optovue Inc, Fremont, CA) spectral domain OCT was used to obtain the Fourier-domain OCT-based IOL formula. The Optiwave Refractive Analysis (ORA) System (WaveTec Vision Systems Inc, Aliso Viejo, CA) wavefront aberrometer measured aphakic refractive measurements intraoperatively and calculated the IOL power with a modified vergence formula. Comparative analysis was done for predictive accuracy of IOL power determination using 2 conventional methods and 2 new technologies: the Haigis-L formula, Masket regression formula, ORA intraoperative aberrometry, and Optovue RTVue Fourier-domain OCT-based IOL formula. Patients without historical data (N = 39) were compared using 3 methods (Haigis-L, ORA, and Optovue), and patients with historical data (N = 20) were compared using all methods (Masket regression formula, Haigis-L, ORA, and Optovue). Median absolute error (MedAE), mean absolute error (MAE), and percentage of eyes within ±0.25, ±0.50, ±0.75, and ±1.00 diopters (D) of refractive prediction error. A total of 39 eyes of 29 patients without historical data were analyzed separately from 20 eyes of 20 patients with historical data. In the group without historical data (N = 39), 49% of eyes were within ±0.25 D, 69% to 74% of eyes were within ±0.50 D, 87% to 97% of eyes were within ±0.75 D, and 92% to 97% of eyes were within ±1.00 D of targeted refractive IOL power prediction error. The MedAE was 0.26 D for Haigis-L, 0.29 D for ORA, and 0.28 D for Optovue. The MAE was 0.37 D for Haigis-L, 0.34 D for ORA, and 0.39 D for Optovue. In the group with historical data (N = 20), 35% to 70% of eyes were within ±0.25 D, 60% to 85% of eyes were within ±0.50 D, 80% to 95% of eyes were within ±0.75 D, and 90% to 95% of eyes were within ±1.00 D of targeted refractive IOL power prediction error. The MedAE was 0.21 D for the Masket regression formula, 0.22 D for the Haigis-L formula, 0.25 D for ORA, and 0.39 for Optovue. The MAE was 0.28 D for the Masket regression formula, 0.31 D for the Haigis-L formula, 0.37 D for ORA, and 0.44 D for Optovue. There was no statistically significant difference among the methods. Newer technology to estimate IOL power calculations in eyes after LVC shows promising results when compared with established methods.

特殊な角膜形状眼の眼内レンズ度数決定

特殊な角膜形状眼の眼内レンズ度数決定

Updated practical intraocular lens power calculation after refractive surgery

Since its introduction in the 1980s, more than 40 million people worldwide have undergone some form of kerato-refractive surgery. Many of these individuals are now candidates for cataract surgery and pose the challenge of attaining first-rate refractive outcomes in nonvirgin eyes. Numerous approaches have been developed to estimate intraocular lens (IOL) power in eyes postrefractive surgery. This review highlights the most practical, relevant options for accurate IOL power determination in these cases. With refined techniques and advances in instrumentation, more accurate assessments of true corneal power and thus, IOL power, are possible in postrefractive eyes. Optical coherence tomography and other corneal tomography instruments have markedly improved accuracy in this process. However, when expensive, modern equipments are not readily available, and online IOL calculators such as the American Society of Cataract and Refractive Surgery (ASCRS) calculator have become efficient, reliable options. Recent evidence confirms the accuracy of these online calculators. Emerging literature supports the use of methods that do not rely on prior refractive data in IOL power determination. Online IOL calculators provide user-friendly, efficient options that greatly facilitate accurate IOL power determination for cataract surgery in eyes that have undergone prior kerato-refractive surgery.

Intraocular lens power calculation after corneal refractive surgery

Corneal refractive procedures have become increasingly popular over the past decade, allowing patients to have excellent uncorrected visual acuity and spectacle independence. As these individuals mature, many will eventually undergo cataract surgery. With the advances in modern cataract surgery and lens implant technology, particularly presbyopic intraocular lens implants, patients and physicians have greater expectations regarding visual outcomes and independence from glasses after cataract surgery. Therefore, it is important to understand methods to accurately determine intraocular lens power calculation after keratorefractive procedures to avoid refractive surprises and patient dissatisfaction. In this review article, we provide an overview of intraocular lens power determination after corneal refractive surgery, highlighting sources of errors and potential methods to improve the accuracy of the lens power estimation. Newer methods to address errors in intraocular lens power calculations after keratorefractive surgery represent a paradigm shift from the previous gold standard of the clinical history method. Understanding the advantages and limitations of the various methods may be beneficial in obtaining more accurate estimations of the intraocular lens power after corneal refractive surgery, resulting in improved visual outcomes.

Scheimpflug imaging to determine intraocular lens power in vivo

To assess the feasibility of determining intraocular lens (IOL) power by measurement of the central optic thickness using clinically available Scheimpflug imaging (Pentacam, Oculus). King's College Hospital Ophthalmology Department, London, United Kingdom. Sixty-seven eyes were assessed 1 month after uneventful phacoemulsification with in-the-bag implantation of AcrySof MA60AC IOLs (Alcon). The correlation between IOL thickness measurement and IOL power was calculated. Repeatability of central optic thickness measurement was determined from 10 successive scans of 4 patients. Within-subject standard deviation increased with the subject mean. The coefficient of variability was 1.4%. Measured lens thickness was highly correlated with lens power (R(2) = 0.94, P<.001). Over the measured range, 95% confidence intervals varied between +/-0.83 diopters (D) and +/-0.92 D. Central IOL thickness measurements with the Pentacam Scheimpflug camera were highly repeatable and closely correlated with the known IOL power. The IOL power, calculated from a regression equation, is likely to be less than +/-1.00 D away from the actual power. Approximate in vivo IOL power determination is feasible with clinically available Scheimpflug imaging. This could be applied clinically in cases of unexplained postoperative refractive error.

Simple regression formula for intraocular lens power adjustment in eyes requiring cataract surgery after excimer laser photoablation

To develop a simple and accurate method for determining appropriate intraocular lens (IOL) power in cataract patients who had prior excimer laser photoablation for myopia or hyperopia, because laser vision corrective surgery interferes with traditional keratometry and corneal topography, rendering IOL power calculations inaccurate. Private Practice in Century City (Los Angeles), California, and free-standing outpatient surgery centers with institutional review boards. Based on the empiric experience of the senior author, an IOL power correction factor that was proportional to the prior laser photoablation was determined and applied to the IOL power calculated by the IOLMaster (Zeiss). It was necessary to add to the predicted IOL power in eyes with prior myopic laser ablation, whereas eyes having prior hyperopic laser vision correction required a reduction in the IOL power. The correction factor was applied to 30 eyes that required cataract surgery at some time after laser refractive surgery; 23 eyes had prior treatment for myopia, and the remaining 7 eyes had prior hyperopic laser ablation. A regression formula was generated from the IOL power correction factor that was used in the 30 eyes. Using the correction factor for 30 eyes, the mean deviation from the desired postcataract refractive outcome was -0.15 diopter (D) +/- 0.29 (SD); 28 of 30 eyes were within +/-0.5 D of the intended goal; the remaining 2 eyes were both -0.75 D from the desired optical result of cataract surgery. Fourteen of the 30 eyes were emmetropic. A simple IOL power corrective adjustment regression formula allowed accurate determination of IOL power after laser refractive photoablation surgery. The weakness of the current method is that knowledge of the amount of prior laser vision correction is necessary.

Intraocular lens power calculation after corneal refractive surgery.

Keratorefractive procedures designed to decrease refractive errors have gained enormous popularity among ophthalmologists and patients. As the post-refractive surgery patient population ages, visually significant cataracts will develop. With advances in techniques for cataract extraction and intraocular lens implantation, cataract surgery has evolved into a refractive surgical procedure as well as an operation to improve best corrected visual acuity. This raises expectations in terms of desired postoperative refractive status and uncorrected visual acuity. Although performing modern cataract surgery in post-refractive surgery eyes is technically no more complicated than operating on virgin eyes, the calculation of intraocular lens power for a desired refractive target can be challenging and complicated. This has become increasingly apparent as case reports of "refractive surprises" after cataract surgery appear in the literature more frequently. This paper reviews the current clinical experience with intraocular lens power determination after cataract surgery in post-keratorefractive patients, provides an overview of possible sources of error in intraocular lens power calculation in these patients, and analyzes methods to minimize intraocular lens power errors. The clinical and routine methods of intraocular lens power determination after keratorefractive surgery need to be modified to improve accuracy. Our knowledge of this subject is still evolving. Given the enormous impact of this problem on clinical practice, awareness of the shortcomings and suggested methods to improve accuracy can be valuable to clinicians.

Difficult lens power calculations.

Although cataract extraction seems to be feasible without major technical obstacles, the surgical technique has changed completely, and patients are no longer satisfied with good spectacle-corrected vision but anticipate complete visual rehabilitation after cataract surgery, without correction. To fulfill this desire, toric or accommodative intraocular lenses are of increasing popularity, and the intraocular lens power calculation after keratorefractive surgery has been improved. In this review article, we provide an overview of different mathematical strategies of calculating the intraocular lens power with standard formulas and with new algorithms, such as paraxial or numeric ray-tracing. These enhanced techniques may improve the validity of lens power calculation due to reduction of the prediction error, especially in cases with high or excessive corneal astigmatism and after refractive laser surgery. Furthermore, a new calculation scheme for the determination of bitoric eikonic intraocular lenses allows a distortion-free imaging in astigmatic eyes. The biometric determinants for the different formulas and calculation schemes are discussed in detail. In difficult cases, standard calculation schemes are overemployed and new mathematical algorithms are necessary to adequately address these problems. Ray-tracing algorithms and other complex mathematical computation schemes are of increasing interest and will more and more replace conventional calculation formulas for determination of intraocular lens power.

Analysis of intraocular lens power calculation in post-radial keratotomy eyes

Purpose To determine whether refractive complications can be prevented by applying the currently most accurate method of intraocular lens (IOL) power calculation in the post-radial keratotomy (RK) patient. Setting Department of Ophthalmology, University of California, Davis, Sacramento, and American Eye Institute, Cedars Sinai Medical Center, Los Angeles, California, USA. Methods Twenty-four eyes having cataract surgery after RK were studied retrospectively for the final postcataract refraction and for the target refraction used in selecting the IOL. Nine of the eyes were further studied for the keratometry (K) values obtained with different methods and for the theoretical postoperative refraction with an IOL aiming for plano or −1.50 diopters (D) based on the known flatter calculated K, axial length, power of the implanted IOL, and refraction after cataract surgery. Results Implantation of an IOL aiming for plano in the 24 post-RK eyes would have resulted in a hyperopic refraction in 83.4% cases. The choice of an IOL targeted at myopia reduced the frequency of hyperopia to 42.0% (24 cases). Selection of an IOL calculated with a flatter calculated K and aiming for plano decreased the frequency of hyperopia to 44.4%; however, aiming for −1.50 D still resulted in hyperopia in 44.4% of eyes (9 cases). Conclusions Unintentional hyperopia can be significantly decreased but not eliminated as a complication of post-RK cataract surgery. The accuracy of the IOL power determination can be improved if myopia is targeted as the postcataract surgery refractive error and the flatter calculated K is used in the IOL determination.

Determining the power of a thin toric intraocular lens in an astigmatic eye.

Astigmatism poses a number of special difficulties when attempting to achieve a desirable refractive outcome during cataract surgery and intraocular lens implantation. One of the most pertinent difficulties is that traditional Gaussian theoretical formulas are simply unable to handle general astigmatic optical systems, that is, systems in which the principal meridians of the refracting interfaces do not lie in two mutually orthogonal planes. This means that the traditional approach falls short when the cornea is treated as bitoric or when other toric optical devices are already present in the eye. In this paper, the modified step-along procedure is utilized in a demonstration of the derivation of the traditional Gaussian theoretical formulas for the determination of intraocular lens power in a stigmatic eye. Matrices are then used to derive a general formula for the determination of the power of a thin toric intraocular lens in an astigmatic eye. This formula is then modified to allow the user to determine the power of the secondary intraocular lens required to compensate for any residual refraction in a phakic or pseudophakic astigmatic eye. The formulas hold under all conditions, including the case when the cornea is treated as thick and bitoric and when other toric optical devices are already present in the eye. Worked examples are provided.

Intraocular Lens Calculation for Cataract Treated with Photorefractive Keratectomy Using Ray Tracing Method

Purpose: Conventional methods (such as the SRK-II formula) do not accurately calculate the power of the intraocular lens (IOL) after refractive surgery. Therefore, we compared a new formula including a ray tracing method to the conventional method for foldable IOL lens implantation. Method: Foldable IOLs (MA 60 BM) were implanted in 26 patients (32 eyes) using the phakoemulsification technique. The power of the IOL was measured preoperatively using the SRK-II formula in all cases. From the results of postoperative refractive errors of these cases, the power of IOL calculated by the ray tracing method was compared to the SRK-II formula. Cataract patients first treated with photorefractive keratectomy (PRK) received IOL implants using our ray tracing method and their postoperative refraction was measured. Results: The average postoperative refractive error was 1.32 D in SRK-II formula, 0.95 D in the ray tracing method with Ray 1 used and 0.89 D with Ray 2 used. Postoperative refraction of both eyes first treated with PRK was −1.00 D. Conclusion: The average postoperative refractive error was reduced in the ray tracing method using Olsen's predicted ACD (Ray 2) compared to SRK-II formula. This new tracing method appears to be useful for determination of IOL power and it may be applied for IOL calculation for cataract surgery after refractive surgery.

Intraocular lens power calculation after decentered photorefractive keratectomy

A 56-year-old patient who had pliotorefractive keratectomy (PRK) to correct highunilateral myopia developed a progressive nuclear cataract.. Phacoemulsification an and , intraocular lens (IOL) implantation were performed: However,: determination of IOL power using automated keratometry and computerized videokeratography was not successful il in this case of high axial myopia because of a decentered ablation zone, resulting in too-steep keratometric readings: Postoperative hyperopia could only be corrected by an IOL exchange. Because it may not be possible to determine the exact keratometric values for IOL calculation after PRK,: subtracting the change in refraction induced by PRK from the preoperative keratometric readings, might have been more accurate in this patient.

Corneal topography in cataract surgery.

Keratometry and corneal topography remain the most important means of evaluating induced corneal changes after surgery and have comparable sensitivities in the paracentral region of the cornea. However, keratometry gives no information about the peripheral cornea or about asymmetry of the cornea. Videokeratography should be performed after cataract surgery in cases in which best-corrected visual acuity is not adequate and there are no other obvious causes for poor vision to determine whether corneal irregularities are present. The recent literature on corneal topographic evaluation of induced astigmatism after cataract surgery suggests that in general, smaller, temporal incisions result in less astigmatism. Preoperatively, corneal topography can be used in the calculation of intraocular lens power as well as incision planning. Postoperatively, it can be used to detect tight sutures, torsion of the wound, internal wound gape, and irregular astigmatism, as well as to guide suture removal. In the future, corneal topography will become increasingly important in the determination of intraocular lens power in difficult cases such as patients undergoing combined cataract extraction and penetrating keratoplasty as well as patients with a history of radial keratotomy or photorefractive surgery.

The Determination of Lens Implant Power

This article reviews published formulas for intraocular lens power determination. In the author's opinion, lack of agreement as to which formula is the most accurate is because none of them reduce the fundamental (biological) problem of variation (scatter, or standard deviation) of results. Suggestions are made for wise use.