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Meister et al. BMC Veterinary Research (2018) EARCH ARTICLEOpen AccessIntraocular lens power calculation for theequine eyeUlrike Meister1* , Christiane Görig2, Christopher J. Murphy3, Hubertus Haan4, Bernhard Ohnesorge1†and Michael H. Boevé1†AbstractBackground: Phacoemulsification and intraocular lens (IOL) implantation during cataract surgery in horses occurwith increasing frequency. To reduce the postoperative refractive error it is necessary to determine the proper IOLpower. In the present study retinoscopy, keratometry and ultrasonographic biometry were performed on 98 healthyequine eyes from 49 horses. The refractive state, corneal curvature (keratometry) and the axial location of all opticalinterfaces (biometry) were measured. The influences of breed, height at the withers, gender and age on valuesobtained and the comparison between the left and right eye were evaluated statistically. Corresponding IOL powerwere calculated by use of Binkhorst and Retzlaff theoretical formulas.Results: Mean SD refractive state of the horses was 0.32 0.66 D. Averaged corneal curvature for Haflinger,Friesian, Pony, Shetland pony and Warmblood were 21.30 0.56 D, 20.02 0.60 D, 22.61 1.76 D, 23.77 0.94 Dand 20.76 0.88 D, respectively. The estimated postoperative anterior chamber depth (C) was calculated by theformula C anterior chamber depth (ACD)/0.73. This formula was determined by a different research group. C andaxial length of the globe averaged for Haflinger 9.30 0.54 mm and 39.43 1.26 mm, for Friesian 10.12 0.33 mmand 42.23 1.00 mm, for Pony 8.68 0.78 mm and 38.85 3.13 mm, for Shetland pony 8.71 0.81 mm and 37.21 1.50 mm and for Warmblood 9.39 0.51 mm and 40.65 1.30 mm. IOL power was calculated with the Binkhorstand Retzlaff theoretical formulas. Calculated IOL power for the several breeds ranged from 18.03 D to 19.55 D. Themean value across all horses was 18.73 D determined with Binkhorst formula and 18.54 D determined with Retzlaffformula.Conclusions: Mean result of this study is: an 18.5 D IOL seemed to be the most appropriate to achieve emmetropiaafter IOL implantation in horses. Cataract surgery without IOL implantation results in hyperopic and visual compromisedhorses. Retinoscopy, keratometry and ultrasonographic biometry should be performed on every affected horse andpostoperative visual outcome should be determined.Keywords: Retinoscopy, Keratometry, Ultrasonographic biometry, Intraocular lens, EquineBackgroundCataract has been reported to occur in horses with anestimated incidence of 5% to 7% of all horses with otherwise clinically normal eyes [1]. Cataract can impairvisual performance rendering the horse unfit for specificfunctions, results in devaluation of the individual andcan predispose the animal to self-injury [2–4]. Visualfunction can be restored through surgical extraction of* Correspondence: [email protected]†Equal contributors1Stiftung Tierärztliche Hochschule Hannover, Klinik für Pferde, Bünteweg 9,30559 Hannover, GermanyFull list of author information is available at the end of the articlethe cataractous lens. Reported methods include aspiration, intracapsular extraction, extracapsular extractionand phacoemulsification with aspiration. Without theimplantation of an intraocular lens following cataractextraction horses were left aphakic and were markedlyhyperopic [5–7]. Early reports suggested aphakic horsesto perform quite well and visual performance to be functionally acceptable [4, 5, 8]. However in other studiessignificant visual impairment in aphakic horses weredetected, which manifested as reduced night vision andreduced contrast sensitivity [3, 9]. In highly hyperopiceyes induced by aphakia, no object in space is accuratelyimaged on the retina with the degree of defocus The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication o/1.0/) applies to the data made available in this article, unless otherwise stated.

Meister et al. BMC Veterinary Research (2018) 14:123worsening as objects are moved closer to the horse’s eye.The mean refractive error in aphakic horses has beenreported as approximately 9.5 D [2–4]. In humans thisdegree of refractive error corresponds to a Snellen valueof 20/ 1200 with 20/200 acuity in the best performingeye of an individual being designated as legally blind [3,8]. It is reasonable to assume that induction of markedhyperopia associated with lens removal in the horsewould dramatically affect visual performance. Thedegree of defocus scales directly with a decrease in measured visual acuity in humans and dogs [10].While correction of the refractive error by implantation of an IOL during cataract surgery in humans anddogs is standard therapy, the number of horses, whichundergoes phacoemulsification and IOL implantation ismuch smaller [11–16]. To the author’s knowledge thereare currently three different foldable IOLs for horsesavailable. To assist the surgeon in achieving emmetropiain the patient after cataract surgery in humans, presurgical determination of required IOL power is performedwith a wide variety of IOLs of differing dioptric powerbeing commercially available. In veterinary patients biometry, keratometry and theoretical formulas have beenemployed to determine the ideal predicted power of IOLfor several species [11, 12, 17–19]. It is important tonote that many formulas used to predict required IOLpower in human patients are inappropriate for veterinary patients as they incorporate mathematical constants(eg the A constant in the SRK formula) that are not direct applicable to veterinary species. A few studies weredone to assess natural variation in the mentioned valuesin horses with healthy eyes [12, 17, 20, 21].The purpose of this study was to determine the degreeof refraction with retinoscopy, intraocular distances withA-mode (amplitude modulation) ultrasound with horseadjusted ultrasound velocities and corneal curvaturesusing a video-keratometer in healthy equine eyes. Furthermore the effect of breed, height at the withers, gender, age and differences between the left and right eyewas determined. The values were used to calculate theexpected IOL power with the Binkhorst and Retzlaff formulas and were compared with results of IOL implantations in reported studies.MethodsAnimals and ophthalmologic examinationThe study was approved by the ethics commissions of theUniversity of Veterinary Medicine Hannover, Foundation.98 healthy equine eyes from 49 horses were examined.The horses were private-owned and examined in a darkened stable. Informed consent was obtained from allowners. All horses underwent a dilated (tropicamide 0.5%)ophthalmic examination, including pupillary light reflex,dazzle reflex, menace response, slit-lamp biomicroscopy1Page 2 of 8and indirect ophthalmoscopy.2 Horses with ocular abnormalities were excluded from the study. Only manual restraint was used for all procedures. Following clinicalevaluation retinoscopy, keratometry and A-mode ultrasonography were conducted on both eyes of each horse.Prior to the examinations the horses received 1 drop oftropicamide 0.5% ophthalmic solution3 and directly infront of the ultrasonographic biometry 1 drop of lidocainophthalmic solution4 was instilled.RetinoscopyStreak retinoscopy was performed with a streak retinoscope5 and 2 skiascopy bars.6 Streak retinoscopywas performed about 30 min after the administrationof tropicamide ophthalmic solution3. The horizontaland vertical meridians were refracted at a workingdistance of 67 cm using the technique described in aprevious study [22]. Results of the horizontal andvertical measurements were averaged to calculate amean refractive error of each eye.KeratometryKeratometry was conducted by using a custom-madevideo-keratometer.The video-keratometer consisted of light-emitting diodes(LEDs) placed around a video camera, which recorded thereflected LED image from spherical surfaces. The sphericalsurfaces were either steel balls, for calibration purpose, orequine eyes producing Purkinje I images. The LEDs emitted in the near infrared, so invisible to the human andequine eye [3]. Switching on and off the LEDs failed toelicit a measurable response of the horse. The video camerawas sensible in the near infrared. The light of each LEDwas projected by a small collimator to the spherical surface,so that the origin of the light appeared to be at infinity. Asalso mentioned in an earlier study [23] this is mandatoryfor the virtual image to be located optically at the focuspoint of the aerial mirror. In this study two horizontal collimators and two vertical collimators were used, so the horizontal and vertical radius of curvature of the equine eyecould be measured separately. Each pair of collimators wasseparated 72 mm and arranged on a circle of 140 mmdiameter around a video camera. As the surface acts as anaerial mirror, the distance of two reflected light points inthe recording, called image height by opticians, was proportional by similar triangles to the radius of curvature of thesphere. The distance ring of the video camera was lockedduring all measurements and the apparatus was moved towards the horse eye (or steel ball) until the image wassharp. That was at approximately 300 mm distance fromthe collimator to the eye (or steel ball). As this methodguaranteed always the same distance in repeated measurements, no other distance measurement was necessary, sothat the measurement time was minimized for the horse.

Meister et al. BMC Veterinary Research (2018) 14:123By calibration with steel balls all constant factors likethe aforementioned distance, the magnification factor ofthe video camera lens and any magnification of the videosoftware are dumped in a single calibration value andneed not be known in detail.First of all the video-keratometer was calibrated by recording images of light reflected from steel balls ofknown radii of curvature. In the recordings the distanceof the two horizontal and the two vertical reflectionswere measured in pixels and two (one for the horizontalcase and one for the vertical case) calibration curveswere generated showing pixel distance over steel ball radius. The calibration curves were linear.Video-keratometry was performed on both eyes of everyhorse. The video-keratometer was moved to the horse’seye until the images were displayed sharp on the monitorof the video camera. The video camera recorded the twohorizontal and two vertical light points for a few minutes.After these measurements the recordings were transferredto a computer program for analysis.The distances of the two horizontal and the two vertical projected light points were measured in the video recordings and served, together with the calibration, todetermine the horizontal and vertical radius of curvatureand corneal astigmatism of the horse eye.Three measurements of the radii from each plane (horizontal and vertical) were conducted and averaged. Horizontal corneal curvature (K1) and vertical cornealcurvature (K2) was calculated by the eq. K [D] (N – 1/radius [m]). The effective corneal refractive index (N) wasset at N 1.336 [24]. Averaged corneal curvature (K) was2calculated by K 1þKin D.2IOL calculationThe detected data were used to calculate the predictedIOL power [D] by using the Binkhorst theoretical formula and the Retzlaff theoretical formula:Binkhorst theoretical formula:1336 ð4r LÞPe ¼ðL C Þð4r C ÞPe is the predicted IOL power in D, r is the averaged corneal radius in mm, L is the axial length (AxL) in mm and Cis the expected postoperative anterior chamber depth inmm and was calculated by the formula C ACD/0.73. Aprevious study [7] reported the mean preoperative-topostoperative ACD ratio in equine eyes to be 0.73.Retzlaff theoretical formula:Pe ¼NNK L C N K CPe is the predicted IOL power in D, N is the refractiveindex of aqueous and vitreous (1.336), L is the axialPage 3 of 8length (AxL) in m, C is the postoperative anterior chamber depth in m and K is the averaged corneal curvaturein D.A-mode ultrasonographyMeasurements of anterior chamber depth (ACD), crystalline lens thickness (LT), vitreous body length (VBL)and axial length (AxL) of the globe was obtained with anultrasonographic device7 with a 10 MHz A-scan probe.Ultrasound velocities were set at 1532 m/s for the anterior chamber and the vitreous body and 1641 m/s for thelens. To achieve more accuracy, values for lens thicknesswere corrected by the equine-specific conversion factorof 1.008 [25]. Five A-scan recordings of both eyes ofeach horse were obtained vertically along the optical axisand the values for each eye were averaged. To obtain optimal scans the manual freeze mode was used and theimages were frozen when all of the required echoes arepresent and of sufficient height.Statistical analysisAs no significant difference was detected for any measurements between left and right eyes (tested with WilcoxonMann-Whitney-U test), the data from both eyes were averaged for further statistical analyses. Values are reportedas mean SD. Statistical analyses were made with computerized statistical software.8 Normal distribution of datawas tested by using the Kolmogorov-Smirnov test. Correlations between age, height at the withers, refractive state,r, ACD, LT, VBL and AxL were evaluated by calculation ofSpearman correlation coefficients. The influence of genderand type of horse on refractive state, r and AxL were determined by using the Kruskal-Wallis test. WilcoxonMann-Whitney-U tests were used for comparisons between the horizontal and vertical refractive state and thehorizontal and vertical corneal radius in the same eye.Values of P 0.05 were considered significant.ResultsThe examined 49 horses contained 36 mares, 9 geldingsand 4 stallions. Ages ranged from 2 to 25 years (mean age 8.28 5.36 years). Breeds included Haflinger (n 10),Shetland pony (n 7), Warmblood (n 13), Friesian (n 8) and Pony (breed unspecified; n 11). Heights at thewithers ranged from 85 to 178 cm (mean height 145.06 27.26 cm).RetinoscopyRetinoscopy was performed on both eyes of the 49horses. Results of the horizontal and vertical measurements were averaged. The mean refractive state SD ofthe horses was 0.32 0.66 D. Emmetropia ( 1 D –1D) was present in 91 (92.8%) of 98 eyes. Ametropia occurred in 7 (7.2%) eyes, with 4 (4.1%) being myopic with

Meister et al. BMC Veterinary Research (2018) 14:123Page 4 of 8a maximum value of 5.0 D and 3 (3.1%) being hyperopic with a maximum value of 1.5 D. Anisometropia(refractive state of the left and right eye differing bymore than 0.5 D) occurred in 6 horses (12.2%). In 4 ofthe anisometropic horses hyperopia was present in oneeye, in one horse myopia was present in one eye and inone horse myopia was present in both eyes but of differing magnitudes. There were no significant differencesbetween the horizontal and vertical meridians (P 0.068). No significant influence was found for the breed ofhorse (P 0.074), gender (P 0.761), height at the withers (P 0.924), age (P 0.917), corneal radius (P 0.201)or globe axial length (P 0.503).KeratometryHorizontal and vertical corneal radii of curvature in thecentral corneal region were obtained for both eyes in all 49horses. The appropriate horizontal (K1), vertical (K2) andaveraged (K) corneal curvature were calculated and the corneal astigmatism was determined. The mean values SDfor the different types of horses are shown in Table 1.The horizontal corneal radius of curvature was significantly greater (P 0.001) than the vertical corneal radiusindicating the cornea to have an astigmatic curvature,being flatter horizontally than vertically. The differencesin horizontal and vertical meridian ranged between 0.34D and 0.95 D. There was no significant difference between the corneal curvature and the age (P 0.071) ofthe horses. Corneal curvature did correlate positively,however, with the height of the withers (P 0.001) andthe axial length of the globe (P 0.001). A significant influence also existed for the breed of horse (P 0.001)and the gender (P 0.027) on the corneal curvature.Ultrasonographic biometryBiometric values were obtained in both eyes of all horsesby use of A-mode ultrasonography. The expected postoperative anterior chamber depth (C) was calculatedusing the formula C ¼ ACD0:73 [7]. Mean values SD areprovided in Table 2.Significant influences were found between AxL andACD (P 0.001), LT (P 0.001), VBL (P 0.001), cornealcurvature (P 0.001), height at the withers (P 0.001),breed of horse (P 0.001) and age (P 0.008). The largerthe AxL the greater the ACD, LT and VBL and the flatter the corneal curvature became and the taller andolder the horses were the larger the AxL increased. Thegender (P 0.026) also influenced the AxL.IOL calculationsFor IOL calculation the Binkhorst theoretical formulaand the Retzlaff theoretical formula were used. The expected mean IOL dioptric power was calculated usingthe values for AxL, K and C. The calculated mean IOLdioptric powers for the several breeds of horses are provided in Table 3.DiscussionValues for corneal curvature, axial length of the globe andpostoperative anterior chamber depth of each horse are necessary to calculate the appropriate IOL power with theBinkhorst and Retzlaff theoretical formulas. While streakretinoscopy and ultrasound biometry are relatively simpleto perform and routinely used in equine cataract patients, itis rather difficult to determine keratometric data. In orderto verify the visual outcome after IOL implantation a postoperative evaluation via streak retinoscopy is necessary.Furthermore to establish the exact position of the IOL inthe eye and to determine the postoperative anterior champer depth ultrasound measurements after IOL implantationshould be performed [2, 26]. Moreover, these values couldbe used to improve the theoretical formulas with respect toequine cataract patients. In this study retinoscopy, videokeratometry and A-mode ultrasonography with horse adjusted ultrasound velocities were obtained on healthyequine eyes and the corresponding IOL powers were calculated using the Binkhorst and Retzlaff theoretical formulas.Investigators of 1 study [27] found only changes in therefractive state during the 20 min following the application of topical tropicamid 1%. Hence in the presentstudy streak retinoscopy was performed about 30 minafter the application of tropicamide 0.5% ophthalmic solution. As in another study [28] emmetropia was specified as 1.0 diopter. The mean refractive state SD of allhorses was 0.32 0.66 D. In the examined population92.8% of values for refractive state were within one diopter of emmetropia, 3.1% were hyperopic and 4.1%Table 1 Corneal radius, corneal curvature and corneal astigmatism are depicted depending on breedBreedhorizontal cornealradius (mm)vertical cornealradius (mm)averaged cornealradius (mm)K1 (D)K2 (D)Mean K (D)corneal astigmatism(D)Haflinger16.06 0.4715.53 0.4015.79 0.4220.94 0.6221.66 0.5521.30 0.560.72 0.36Friesian16.94 0.5016.66 0.4916.80 0.4819.86 0.6120.19 0.6220.02 0.600.34 0.29Pony15.16 1.1214.76 1.3414.96 1.2222.29 1.6122.94 1.9522.61 1.760.65 0.61Shetland pony14.46 0.6213.89 0.5714.18 0.5723.30 1.0024.25 0.9723.77 0.940.95 0.58Warmblood16.45 0.6615.99 0.7416.22 0.6920.46 0.8421.06 0.9620.76 0.880.60 0.35

Meister et al. BMC Veterinary Research (2018) 14:123Page 5 of 8Table 2 Anterior chamber depth, lens thickness, vitreous body length, axial length and calculated post operative anterior chamberdepth are depicted depending on breedBreedAnterior ChamberDepth (ACD) (mm)Lens ThicknessLT) (mm)Vitreous BodyLength (VBL) (mm)Axial Length(AxL) (mm)C (calculated postoperative ACD) (mm)Haflinger6.79 0.3910.91 0.2721.82 0.9439.43 1.269.30 0.54Friesian7.39 0.2411.70 0.3023.23 0.9742.23 1.0010.12 0.33Pony6.34 0.5811.43 0.8221.18 2.0738.85 3.138.68 0.78Shetland pony6.36 0.5811.50 0.5019.45 1.2237.21 1.508.71 0.81Warmblood6.85 0.3711.79 0.5022.10 0.9240.65 1.309.39 0.51were myopic. The results are congruous with another recent report [29]. Bracun found that 83.63% were emmetropic, 8.86% were hyperopic and 7.51% were myopic.However in another study [21] emmetropia was presentin only 48.7%, hyperopia in 24.1% and myopia in 27.2%of the horses examined, but emmetropia was strictly defined as a refraction of 0 D. In the present study anisometropia (refractive power of the two eyes differs morethan 0.5 D) occurred in 6 horses (12.2%).The values obtained for the averaged corneal curvature(see Table 1) correspond well with one previous reportemploying photokeratometry [21] but differ by 3.5-8.5 Dfrom two previously reported studies using photokeratometry and B-mode ultrasonography [12, 17]. Differencesin corneal curvature values may be influenced by the differing methods employed. Mouney and co-workers documented a significant difference in values obtained by Bmode ultrasonography vs. photokeratometry of approximately 1.2 D [12]. Other differences between the currentand previously reported studies include the state of sedation /anesthesia and presence of an auriculopalpebralnerve block [12, 21]. These differences highlight theneed to establish standardized instrumentation andmethods for performing keratometry.A significant difference was found between the horizontal and the vertical corneal curvature, which are expected and normal and do not result in differences inthe refractive error in the respective meridians. Cornealcurvature depended significantly on the breed of horse,the height of the withers and the axial length of theglobe. The taller the horse and the larger the axial lengthof the globe are, the flatter the corneal curvature. TheTable 3 IOL calculations using Binkhorst and Retzlaff theoreticalformulasBreedIOL dioptric power derived IOL dioptric power derivedusing Binkhorst formulausing Retzlaff formulaHaflinger19.55 D19.36 DFriesian18.20 D18.03 DPony18.27 D18.07 DShetland pony 19.03 D18.80 DWarmblood18.48 D18.66 Dage of the horses had no significant influence on the corneal curvature. In contrast to current findings, Grinnigerand co-workers found a positive relationship betweenage and corneal radius of curvature [21]. A possible reason for the difference between the present study and thisprevious report is the range of ages included in the studies. In the prior report the group of the youngest horseswas between 0 and 2.5 years old while the youngesthorse in the present study was 2 years. A similar correlation between age and radius of curvature has been reported for cats. A dramatic decrease in cornealcurvature in cats occurs between 9 weeks and 1 year ofage [30]. The age of the horse should be considered forIOL selection in very young horses. Townsed et al. [31]found a reduced dioptric strength of the juvenile corneaof horses and a shortened vitreous chamber depth.Hence, to achieve emmetropia in the juvenile eye anIOL with a dioptric strength greater than that requiredby an adult equine is necessary. However the grown uphorses is ametropic. Therefore, it is recommended touse IOL in young horses that will correct vision oncethe eye reaches its adult parameters.The present study found a significant correlation between gender and corneal curvature. Such a correlationwas not found in a prior study [21]. Results may have beeninfluenced by a much greater number of female (n 36)vs. male (n 13) horses being examined in this study.A-mode ultrasonography is deemed to be the most accurate method for ocular biometry [32] with B-mode considered inappropriate for ocular biometry [33]. In humancataract surgery, A-mode ultrasound is routinely used inpreoperative diagnostic and in postoperative monitoring[34]. Important parameters that can influence the accuracyof A-mode biometry are the values for ultrasound velocityused for determining exact distances. The ultrasound velocity in lens tissue differs considerably in different species.In dogs an underestimation of the lens thickness of 4%results when calculations of lens thickness are made on thebasis of ultrasound velocity in the human lens. Use ofinaccurate values for velocity can result in incorrectbiometric values, which will, in turn, result in incorrectprediction of optimal IOL power to achieve emmetropia[35]. In a previous study [25] species-specific ultrasound

Meister et al. BMC Veterinary Research (2018) 14:123velocity for equine aqueous humor, lens and vitreous bodywere determined. To obtain accurate A-mode biometricvalues it is recommended to use appropriate values forultrasound velocity [25]. In the current study, the A-modeinstrument utilized ultrasound velocities established for human patients. Hence, the values for lens thickness werecorrected by using a conversion factor of 1.008. Intraoculardistances (ACD, LT, VBL, AxL) determined in the presentstudy are congruous with prior reports [12, 17, 21, 36].Cataract surgery with subsequent IOL implantation following phacoemulsification is being performed with increasing frequency in horses [7]. It is a fact that IOLimplantation significantly reduces postoperative refractiveerror [12]. In human medicine several IOL power calculation formulas were developed. They could be classifiedinto theoretical formulas and regression formulas. Whilethe theoretical formulas only were influenced by the variables corneal curvature, axial length, postoperative anterior chamber depth and aqueous and vitreous refractiveindex, the regression formulas were derived by linear regression analysis of data sets and incorporate mathematical constants [37]. These constants depend on severalfactors and were not transferable to veterinary species. Tothe authors knowledge there are no regression formulasfor veterinary medicine available. Hence, the appropriateIOL power for horses was calculated with the Binkhorstand Retzlaff theoretical formulas, despite the fact, that regression formulas show more accuracy than theoreticalformulas in human patients [37, 38].Mean predicted IOL dioptric power for the varioustypes of horses was found to lie between 18.03 D and 19.55 D (see Table 3). Compared to two other studies [12, 17]which calculated an IOL power of 29.91 D (with Binkhorstformula) and 29 D (with Retzlaff formula) and 22 to 23 Dby the use of the Binkhorst formula values presented inthis study were considerably smaller. Essential factors,which influence the predicted IOL power, are cornealcurvature, axial length of the globe and predicted postoperative anterior chamber depth. In the present study,mean corneal curvature ranged over 20.02 – 23.77 D.These values differed from previous reports [12, 17] usingdifferent methods for obtaining keratometry results (16.46D, 16.5 D and 15.3D) by 3.5-8.5 D. Additionally the postoperative anterior chamber depth (C) influences the predicted IOL power. C was calculated by the formula C ACD/0.73, a value derived from a previous report [7]. Inanother study Mouney [12] used the measured ACD asthe predicted C (6.8 mm). Postoperative ultrasound measurements of the PACD and refractive data obtained viastreak retinoscopy are essential to determine actual position of the implanted IOL and to confirm the predictivepower of any theoretical calculations. Other investigators[7], who implanted 30- and 25- diopter IOLs, suggested a18-D IOL is the appropriate power to achieve emmetropiaPage 6 of 8in adult horses. This prediction correlates well with thepredicted IOL power in this study. Two previous in vivostudies [15, 16] implanted a 14 D IOL achieve emmetropiawithin 0.5 D. Currently, there are three foldable IOLs intwo powers ( 14 D and 18 D) commercially availablefor horses. Based on current findings and determinationsfrom previous studies [7, 15, 16, 26] the proper IOL powerfor use in horses lies between 14 D and 18.5 D. Additional studies using standardized methods that includepreoperative ultrasonographic biometry and keratometryas well as post-operative refraction are necessary to determine the appropriate IOL power and the long-term effectsof IOL implantation [7, 15, 16].ConclusionsAphakic horses have been documented to be markedlyhyperopic and visually compromised. They have reducednight vision and reduced contrast sensitivity [3, 9].Soundness is very important for the pleasure and performance horse. A visually compromised horse mayshow abnormal behavior and the rider faces an unpredictable, frightened animal [39]. Of the commerciallyavailable the IOL with 18 D appears to be the most appropriate to achieve emmetropia after IOL implantationin adult horses [7].Endnotes1Kowa-SL 15, Kowa Optimed Deutschland GmbH,Bendemannstr. 9, 40,210 Düsseldorf, Germany2Heine Omega 500, Heine Optotechnik, Herrsching,Germany3Mydriaticum Stulln, Pharma Stulln GmbH, Werksstr.3, 92,551 Stulln, Germany4Lidocaine, AST Farma B.V., Wilgenweg 7, 3421 TVOudewater, The Netherlands5Heine Beta 200, Heine Optotechnik, Herrsching,Germany6Oculus Optikgeräte, Wetzlar, Germany7 3I SYSTEM-ABD,InnovativeImagingInc.,Sacramento, Calif.8R Core Team (2016). R: A language and environmentfor statistical computing. R Foundation for StatisticalComputing, Vienna, Austria. URL https://www.R-project.org/AbbreviationsACD: Anterior chamber depth; A-mode: Amplitude modulation; AxL: Axiallength; B-mode: Brightness modulation; C: Postoperative anterior chamberdepth; cm: Centimeter;

University of Veterinary Medicine Hannover, Foundation. . so that the origin of the light appeared to be at infinity. As also mentioned in an earlier study [23]thisismandatory . software are dumped in a