Efficacy Analysis Of Applying Corrected Corneal Power In Intraocular Lens Power Calculation After Myopic Lasik Surgery

BackgroundLaser in situ keratomileusis(LASIK) is the most widely performed refractive surgery and an increasing number of myopic patients accept LASIK surgery.With time,an increasing number of patients who have undergone LASIK will develop symptomatic cataracts(complicated cataract or age-related cataract) requiring surgery. Cataract extraction microsurgery and IOL implant techniques are sophisticated.But underestimation of the required IOL power with resultant hyperopia and not yet obtaining anticipated therapeutic effect is common in patients undergoing cataract extraction after myopic corneal refractive surgery.These postoperative IOL power errors in IOL power calculation for patients after myopic corneal refractive surgery can be attributed primarily to four factors:1. The cornea power measured by Standard keratometers is inacuurate for patients after myopic LASIK surgery.Most of conventional Standard keratometry(keratometry and videokeratometry) methods which are based on Gullstrand's model eye are no longer as accurate for keratometric measurements after myopic LASIK surgery because of inaccuracies in anterior corneal curvature measurements and calculation after remodeling of the anterior surface by the excimer laser. Using the fixed keratometric refractive index(1.3375) entered into the formula (K=1.3375-1/r)and assuming a consistent rate of curvature(7.7/6.8)to calculate the central corneal power are not valid for the changed relationship between the anterior and posterior corneal surfaces.The measured cornea power with conventional keratometry or corneal topography is steeper than the actual value and this results in the underestimation of the required IOL power,causing significant hyperopic error after surgery.2. Effective lens position(ELP) prediction of the IOL implanted is inaccurate by the third or fourth generation IOL power calculation formula in IOL power calculation for patients after myopic LASIK surgery.ELP is the distance between the anterior surface of cornea and the plane of the IOL implant.Another problem with the commonly used two variable prediction formulas(e.g. Holladay I formula) is that they rely on the axial length and the central corneal power to predict the postoperative position of the IOL implanted.These formulas assume that the cornea becomes more flat and anterior chamber depth(ACD) becomes shallower after refractive surgery. So it is thought that ELP moves forward and is closer to the cornea but actual ELP does not change after refractive surgery. Application of this ELP value in IOL power calculation would produce postoperative hyperopic errors and ELP error of1mm would result in about postoperative refractive error of1.5D-2D.3. Measurement of axis length is inaccurate for eyes after myopic LASIK surgery.LASIK surgery ablated some portion of central zone cornea tissue(about100～200μ m thickness) and this make axial length shorten and change the ratio of the anterior and posterior corneal surfaces.Error of intraocular lens power is significantly correlated to the actually corrected amount of refraction errors by LASIK. Axial length was commonly measured ultrasonically with the immersion technique and even experienced operators can not avoid measurement error of0.14～0.28mm because of pressure on the cornea surface.Anatomy axial length(the distance between the vertex of cornea and the topmost of posterior scleral staphyloma) rather than the optical axial length(the distance between the vertex of cornea and the macular foveal) was usually obtained because of the existence of posterior scleral staphyloma especially in high myopia.And it is difficult to measure the optical axial length because posterior scleral staphyloma make normally posterior pole of eyeball deformed. 4. The cornea's refractive index for calculating cornea power is inaccurate for patients after myopic LASIK surgery.Most keratometry units are based on Gullstrand's model eye rather than on actual measurements.The cornea's refractive index is assumed to be1.3375and keratometry assumes a consistent rate of curvature(7.7/6.8).These assumptions remain valid for unaltered corneas but fail to account for the changes induced by LASIK because refractive surgery ablated some portion of central zone cornea tissue for which created flattened anterior surface of cornea and altered relationship between the powers of the front and back corneal surfaces.How to obtain the accurate IOL power for patients who have undergone myopic LASIK surgery continues to an increasingly important issue in cataract refraction surgery domain.Calculating IOL power for these post-LASIK patients has been the subject of a significant amount of research including many authors over the past decade.IOL power calculation methods described in these studies for eyes that have undergone myopic LASIK can be categorized according to whether or not they require knowledge of data acquired before LASIK was performed.Those that depend upon pre-LASIK data and the specific values that are needed include the clinical history method,the Aramberri double-K method,Shammas'refractive-derived method,the Hamed method,the Feiz-Mannis method, the Latkany method, the Wake Forest method,the Masket method, AS biometry technique and others.Methods that do not require knowledge of any of the pre-LASIK data include contact lens overrefraction method, the Maloney/Wang method, Shammas'clinically-derived method, Mackool method,Haigis-L formula exclusivly used for IOL power calculation after myopic LASIK/PRK/LASEK surgery,directly measurement using the Pentacam system and others besides Cheng method and Geggel method more recently published.Calculating IOL power after refractive surgery presents a significant challenge for cataract surgeons.In eyes with previous myopic LASIK surgery,the most common source of error in predicting the required IOL power is the overestimation of corneal curvature using the current keratometry.IOL power calculation produces postoperative hyperopic errors in many of these patients.In above two classes of methods, a great amount of attention has been given to obtaining accurate keratometry readings after myopic LASIK to precisely determine corneal power. Many methods have been proposed to improve the accuracy of estimating corneal power in eyes that have undergone myopic LASIK.Because most of these studies were the retrospective nature and include the relatively small number of patients,a prospective study on a large group of patients is needed to validate these findings.It is often the case that these different IOL power calculation methods used to calculate IOL power for the same eye often give widely differing suggested IOL power and this can place the clinician in a therapeutic effect dilemma.So unwanted expense and distress for some of these patients may be caused after they accept IOL implantation surgery.LASIK has been widely performed in the past twenty years.The climax of developing symptomatic cataracts that requires IOL implantation surgery will appear in the soon coming decades. This information of pre-LASIK will become increasingly difficult to obtain for calculation before cataract surgery many years after their original LASIK surgery. It will be frequent to see myopic post-LASIK patients requiring cataract surgery but without pre-LASIK data. The reliance on pre-refractive surgery information should be considered a shortcoming of any technique requiring pre-LASIK data.Many researchers believe that new techniques for Calculating IOL power after refractive surgery should focus specifically on IOL power calculations that do not require preoperative information.It is important for chinese patients who have undergone LASIK to gain high quality vision if our domestic researchers can summarize reliable and appropriate IOL power calculation formulae base on the structure feature of Chinese patient's eyeball.In our theoretical and prospective study, an attempt is made to derive a new regression formula for correcting the postoperative corneal power of eyes after myopic LASIK surgery and validate its efficacy.Our expectation in this article is to investigate an effective IOL power calculation method independent of preoperative data and provide more practical value for myopic post-LASIK patients requiring cataract surgery but without pre-LASIK data. Objective1. To derive a new regression formula for correcting postoperative corneal power that may be independent of prerefractive surgery data in IOL power calculation after myopic LAS IK surgery.2. To validate the efficacy of the above new regression formula for correcting postoperative comeal power in IOL power calculation for eyes that have previously undergone LAS IK surgery.MethodsThe preoperative and postoperative data of the193eyes from97patients who had undergone myopic LASIK surgery in the excimer laser treatment center of Yantai Yuhuangding Hospital from September1,2009, to February28,2010, were retrospectively analyzed in the first part of the study. Mean preoperative spherical equivalent(SE) was-5.17diopters(D)(standard deviation:±2.02D,Range:-1.25～-10.75D). Another set preoperative and postoperative data of67myopic patients(132eyes) who accepted myopic LASIK surgery in the excimer laser treatment center of Yantai Yuhuangding Hospital from May1,2010, to October31,2010, were included in the second part of the study. Mean preoperative spherical equivalent(SE) was-5.43diopters(D)(standard deviation:±1.94D,Range:-1.5～-11.00D).Preoperatively, all patients had completely preoperative assessment and were free from pathology other than refractive error.Examinations before and3months after LASIK surgery included uncorrected visual acuity, best spectacle-corrected visual acuity, manifest and cycloplegic refraction, spherical equivalent refraction. Best corrected visual acuity of all eyes included in this study before and after LASIK surgery were≥1.0.Follow-up time was≥3months and refraction were stable.Measurement data(including visual axis,keratometry,corneal radius,anterior chamber depth) before and3months after LASIK surgery for each patient obtained from direct measurement by the IOLMaster(SW Option A+,5.4.3.0002version software,Carl Zeiss Meditec AG,Jena,Germany). The keratometric refractive index of IOLMaster is1.3375.Routine LASIK procedure was performed by using Moria M2microkeratome system(Moria S.A. France) to create a superiorly hinged lamellar flap. The Allegretto Wave Excimer Laser (WaveLight Laser Technologie AG, Erlangen, Germany) with the standard aspheric algorithm was used to perform the coneal ablation.There were no complications during the study period in all patients.0.5%Levofloxacin eyedrops and0.1%Fluorometholone eyedrops were prescribed to patients after LASIK surgery.The clinical history method which is regarded as gold standard for IOL calculation after corneal refractive surgery was used to calculate the actual postoperative corneal power in the first part of the study.On the basis of above data,an attempt was made to establish a regression model between the actual postoperative corneal power calculated by clinical history method(Kchm) and postoperative mean corneal power(post-Km)directly measured by the IOLMaster.Axial length (AL), anterior chamber depth (ACD), and keratometry (including corneal radius)measurements before and3months after LASIK surgery for each patient obtained from direct measurement by the IOLMaster in the second part of this study. IOL power calculation with the spherical equivalent(SE) as the target refraction by the IOLMaster was done in this set of67post-LASIK patients(132eyes)before and3months after LASIK surgery.IOL power was calculated with four fomula(SRK-T,Holladay,Haigis,Hoffer-Q) using the postoperative corneal power corrected by the regression fomula established in the first part of this study.In addition, IOL power was calculated with Haigis-L formula which is exclusivly used for IOL power calculation for patients without preoperative data myopic refractive corneal surgery in IOLmaster. Accepting the "equivalent intraocular lens" principle conception,postoperative IOL refraction difference(versus preoperative equivalent IOL) was compared in above five IOL power calculation formula(SRK-T,Holladay,Haigis,Hoffer-Q, Haigis-L) by assuming the implantation of Alcon SA60AT,AlconSN60WF,AMO Tecnis Z9000. Efficacy comparison in different IOL type each with exclusive parameters was analysed between above five formula for three selected IOL types.Results1. Establishment of a new regression formula for correcting postoperative corneal power in intraocular lens power calculation after myopic LASIK surgery.The Pearson correlation test showed the statistically positive correlation between the Kchm and the post-Km(r=0.957).The regression model(Kchm=1.033X post-Km—2.192) was established between the Kchm and the post-Km in this study. Statistical analysis results demonstrated excellent model fitting effect(the model R square was0.916) and significance test for this regression model and its coefficient showed P＜0.01.2. Efficacy analysis of this above new regression formula for correcting corneal power in intraocular lens power calculation after myopic LASIK surgery.In the second part of this study, postoperative corneal power was corrected by this above regression formula(Kchm=1.033X post-Km—2.192)then IOL power was calculated with SRK-T,Haigis,Holladay I,Hoffer-Q formula by using this corrected postoperative corneal power for patients3months after LASIK surgery.Comparing above four IOL power calculation results for three selected IOL types,IOL power calculation with Holladay I or SRK-T formula using this above corrected postoperative corneal power were more accurate than that with Haigis or Hoffer-Q formula within±0.5D or±1.0D of postoperative refraction difference versus preoperative equiverlent IOL;IOL power calculation with Holladay I,SRK-T or Haigis formula using corrected postoperative corneal power presented almost equal accuracy and were more accurate than that with Hoffer-Q formula within±2.0D of postoperative refraction difference versus preoperative equiverlent IOL.For patients3months after myopic LASIK surgery,IOL power calculation with Holladay I or SRK-T formula using corrected postoperative corneal power were more accurate than that with Haigis-L formula for three selected IOL types within±0.5D or±1.0D of postoperative refraction difference versus preoperative equivalent IOL.In addition, results of IOL power calculation with Haigis formula by entering conected postoperative corneal power was compared with that calculated by Haigis-L formula for patients3months after LASIK surgery.Difference of IOL power calculation results calculated by the two methods were more than eighty-seven percent within+0.5D and more than ninety-five percent within±1.0D for three selected IOL types.Conclusion1. The regression (Kchm=1.033×post-Km—2.192) in IOL power calculation after myopic LASIK surgery was established in this study.This regression is independent of prerefractive surgery data and it can be directly used for correcting postoperative corneal power in intraocular lens power calculation after myopic LASIK surgery.2. To obtain the actually required IOL for eyes that have previously undergone LASIK surgery,the first selection is IOL power calculated with Holladay I or SRK-T formula using corrected postoperative corneal power.3. For eyes that have previously undergone LASIK surgery,IOL power calculation with Holladay I or SRK-T formula using corrected postoperative corneal power were more accurate than that with Haigis-L formula exclusivly used for IOL power calculation for patients without preoperative data after myopic refractive corneal surgery in IOLmaster.4. For eyes that have previously undergone LASIK surgery,IOL power calculation with Haigis formula using corrected postoperative corneal power presented almost the same efficacy as that with Haigis-L formula exclusivly used for IOL power calculation independent of preoperative data after myopic refractive corneal surgery in IOLmaster.