Refractive error of canine cataract patients following implantation with three types of intraocular lenses

Mihal Kaminsky Eye Care for Animals, Pasadena, CA

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 DVM, DACVO
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Allison Hoffman Eye Care for Animals, Pasadena, CA

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 DVM, DACVO
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Ron Ofri Koret School of Veterinary Medicine, Hebrew University of Jerusalem, Israel

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 DVM, PhD, DECVO
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Kricket Konrade Eye Care for Animals, Pasadena, CA

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 DVM, MS, DACVO
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Liat Gantz Department of Optometry and Vision Science, Hadassah Academic College Jerusalem, Israel

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 BOptom, PhD, FAAO

Abstract

OBJECTIVE

To evaluate refractive state outcomes following phacoemulsification and implantation of 3 different intraocular lenses (IOLs).

ANIMALS

A prospective, randomized, controlled study was conducted on 43 client-owned dogs undergoing phacoemulsification with IOL implantation.

METHODS

Eyes were randomized to receive either an-vision Fo-X (n = 26), an-vision MD8 (18), or I-MED I-LENS (24) IOL. Refraction was measured 1 week, 1 month, and 3 months postoperatively using streak retinoscopy by 2 examiners masked to each other’s results.

RESULTS

Postoperative refractive outcomes were highly correlated and not significantly different between 2 examiners for all time points (r = 0.97, 0.98, and 1.00; P = .76, .94, and .98, respectively). One week postoperatively, the refractive errors (mean ± SD) for Fo-X, MD8, and I-LENS were –0.14 ± 2.02 diopters (D), 0.97 ± 2.01 D, and 0.15 ± 2.55 D, respectively. One month postoperatively, the refractive errors were 0.35 ± 2.04 D, 0.06 ± 2.41 D, and –0.82 ± 2.20 D, respectively. Three months postoperatively, the refractive errors were –0.16 ± 2.67 D, 1.60 ± 2.99 D, and 0.59 ± 1.51 D, respectively. There were no significant differences in refractive error outcomes between Fo-X, MD8, and I-LENS at 1 week, 1 month, and 3 months postoperatively (P = .16; F (df=2,66)- = 1.89). However, the Fo-X was the only IOL to yield nearly emmetropic outcomes (±0.50 D) at all 3 time points.

CLINICAL RELEVANCE

The postoperative refractive states of dogs were not statistically different when comparing 3 types of IOLs at 3 postoperative time points, though the Fo-X was the only IOL to yield nearly emmetropic outcomes at all 3 time points.

Abstract

OBJECTIVE

To evaluate refractive state outcomes following phacoemulsification and implantation of 3 different intraocular lenses (IOLs).

ANIMALS

A prospective, randomized, controlled study was conducted on 43 client-owned dogs undergoing phacoemulsification with IOL implantation.

METHODS

Eyes were randomized to receive either an-vision Fo-X (n = 26), an-vision MD8 (18), or I-MED I-LENS (24) IOL. Refraction was measured 1 week, 1 month, and 3 months postoperatively using streak retinoscopy by 2 examiners masked to each other’s results.

RESULTS

Postoperative refractive outcomes were highly correlated and not significantly different between 2 examiners for all time points (r = 0.97, 0.98, and 1.00; P = .76, .94, and .98, respectively). One week postoperatively, the refractive errors (mean ± SD) for Fo-X, MD8, and I-LENS were –0.14 ± 2.02 diopters (D), 0.97 ± 2.01 D, and 0.15 ± 2.55 D, respectively. One month postoperatively, the refractive errors were 0.35 ± 2.04 D, 0.06 ± 2.41 D, and –0.82 ± 2.20 D, respectively. Three months postoperatively, the refractive errors were –0.16 ± 2.67 D, 1.60 ± 2.99 D, and 0.59 ± 1.51 D, respectively. There were no significant differences in refractive error outcomes between Fo-X, MD8, and I-LENS at 1 week, 1 month, and 3 months postoperatively (P = .16; F (df=2,66)- = 1.89). However, the Fo-X was the only IOL to yield nearly emmetropic outcomes (±0.50 D) at all 3 time points.

CLINICAL RELEVANCE

The postoperative refractive states of dogs were not statistically different when comparing 3 types of IOLs at 3 postoperative time points, though the Fo-X was the only IOL to yield nearly emmetropic outcomes at all 3 time points.

Introduction

For over 30 years, veterinary ophthalmologists have been implanting intraocular lenses (IOLs) following cataract extraction to achieve emmetropia and correct hyperopia. The optimal intraocular lens power necessary to achieve approximate emmetropia in canines is 40.0 to 41.5 diopters (D).1,2 However, the refractive state varies among canine breeds and ages, with one-third of dogs presenting with ametropia > ±0.5 D. For example, breeds such as German Shepherd Dogs, Rottweilers, Miniature Schnauzers, Collies, and Toy Poodles are myopic, and in some breeds the degree of myopia increases with age.3,4 Breeds with hyperopic refractive states include Australian Shepherds, Alaskan Malamutes, and Bouvier des Flandres.3 A recent study5 from Brazil evaluated the refractive states of police and guide dogs, and police dogs were slightly myopic and guide dogs were emmetropic. In many cases, this is due to breed differences in the refractive power of the lens, but differences among breeds in intraocular dimensions can also serve as important factors that influence the refractive state.6 Labrador Retrievers were shown to have an increased length of the vitreous chamber and a thin crystalline lens, which contributes to myopia as reported in humans in 1 study.7 Ametropia can have a significant effect on a dog’s everyday activity and behavior. In Labrador Retrievers, optical defocusing inducing mild myopia has been shown to negatively affect visual acuity observed with behavior-based vision testing.8

Cataract surgery with IOL implantation provides a 1-time, golden opportunity to correct preexisting ametropia in the patient by implanting an IOL that will result in postoperative emmetropia. In human patients undergoing cataract surgery, preoperative keratometry and biometry measurements are used to calculate the power of the IOL needed to restore emmetropia, and the patient is implanted with the appropriate IOL to achieve this result. In veterinary ophthalmology, however, the challenge lies in lack of commercially available intraocular lenses with varying optical power to correct ametropia of canine patients. The novel Fo-X (an-vision Inc) IOL was designed to overcome the lack of lenticular accommodation and restore most myopic and hyperopic canines to emmetropia by allegedly having an enhanced depth of focus of ±1.5 D. Theoretically, this would result in significant improvement of postoperative refractive error of many canine patients.

The purpose of this study was to compare the postoperative refractive error among dogs implanted with 3 types of IOLs using streak retinoscopy. An additional aim of the study was to evaluate the reliability of the Binkhorst and Retzlaff theoretical IOL power formulas and compare their results to the power of implanted IOLs. We hypothesized that, postoperatively, patients implanted with the novel Fo-X IOL would be closer to emmetropia compared to patients implanted with the MD8 (an-vision Inc) and I-LENS (I-MED Animal Health) IOLs.

Methods

Patients and preoperative workup

This was a prospective study that included client-owned dogs of various breeds, sex, and ages that underwent phacoemulsification with IOL implantation between January 2021 and January 2022. The study was approved by the Eye Care for Animals Research Committee. Clients completed a clinical study consent form prior to enrollment into the study. Dogs with unilateral or bilateral immature, mature, hypermature, and/or intumescent cataracts were included in the study. Dogs were excluded from the study for the following reasons: if postoperative refraction was precluded due to corneal disease, anterior/posterior uveitis, if undergoing vitreoretinal surgery, or if postoperative complications such as retinal detachment, hyphema, glaucoma, or complicated corneal ulcerations occurred.

All patients underwent a complete ophthalmic examination preoperatively including Schirmer tear test-1, fluorescein staining (Altafluor benox; Altaire Pharmaceuticals Inc), rebound tonometry (TonoVet; iCare), slit lamp biomicroscopy (SL-17; Kowa Ltd), and, whenever possible, binocular indirect ophthalmoscopy (Vantage Plus; Keeler Ltd) using a 28-D indirect lens (Volk Optical Inc). Surgical candidacy was determined by preoperative normal flash electroretinography (Retinographics), B-mode ultrasonography (12.5-MHz probe; Capistrano Labs Inc), and gonioscopy using a Koeppe goniolens (Ocular Instruments Inc).

Keratometry, biometry, and IOL calculation

To measure corneal curvature, the mean keratometry (R1/R2) values of the minor (R1) and major (R2) meridians were measured using an automatic handheld keratometer (HandyRef-K; Nidek Inc) preoperatively and 1 month postoperatively. This instrument has previously been shown to have good reliability in dogs.9 Biometry values were obtained by measuring the axial length (AxL) and anterior chamber depth (ACD) preoperatively using amplitude modulation and bright-scan ultrasonography with a 12.5-MHz transducer (Figure 1). In 36 randomly selected eyes, biometry data were also collected 1 month postoperatively. At each session, 2 readings of the AxL and ACD were measured and averaged.

Figure 1
Figure 1

Representative image of biometry measurements using combined amplitude modulation (blue trace) and bright-scan ultrasonography. In this patient, the axial length is 19.55 mm and the anterior chamber depth (ACD) is 2.89 mm.

Citation: Journal of the American Veterinary Medical Association 262, 1; 10.2460/javma.23.08.0445

The keratometry and biometry values were used to calculate the theoretical IOL power using both the Binkhorst and Retzlaff formulas. These formulas were chosen because A-constants that are applied in other regression formulas used in human medicine are inapplicable and have never been calculated in veterinary patients.10 The mean and SD values were compared between the 2 formulas and between the implanted IOL power and the theoretical IOL power. Correlation between the theoretical IOL power, implanted IOL, and the refractive error outcome at the 3-month follow-up was also evaluated.

IOL calculation

The Binkhorst theoretical formula is as follows:

article image

Pe = Predicted IOL power (D). r = Mean corneal curvature (mm). L = AxL (mm). C = Expected postoperative ACD (mm) calculated by C = ACD/0.73.

The Retzlaff theoretical formula is as follows:

article image

Pe = Predicted IOL power (D). N = Refractive index of aqueous and vitreous (1.336). L = AxL (mm). C = Postoperative ACD in meters (m). K = Mean corneal curvature (D).

Surgery and IOL implantation

A board-certified veterinary ophthalmologist (AH or KK) or veterinary ophthalmology resident (MK) performed bilateral or unilateral phacoemulsification surgery with IOL implantation using the DORC EVA Phaco-Vitrectomy System (Dutch Ophthalmic). General anesthesia was maintained with isoflurane inhalant, and rocuronium (rocuronium bromide, 50 mg/mL; XGen) was used as a neuromuscular blockade. Pharmacological mydriasis was initiated with 1% tropicamide (Akorn) preoperatively, and intraoperative intracameral preservative-free epinephrine 1:10,000 (epinephrine, 1 mg/mL; VetOne) was injected to maintain intraoperative mydriasis. A limbal-based clear corneal 2-step entry was performed with a 2.8-mm keratome. A continuous curvilinear capsulorrhexis with a unimanual phacoemulsification technique was performed. The anterior chamber was maintained with a cohesive viscoelastic agent and removed with irrigation/aspiration at corneal closure. Following lens extraction, the corneal incision was enlarged 1 mm with the keratome to accommodate the foldable lens introducer. Eyes were implanted with 1 of 3 IOLs: Fo-X, MD8, and I-LENS using a randomization table (https://www.randomizer.org/). The randomization table was designed in a manner that a patient undergoing bilateral phacoemulsification surgery would not receive the same IOL in both eyes. The corneal incision was closed with a 8-0 polyglactin 910 suture in a double-sawtooth pattern (Vicryl; Ethicon Inc). At the conclusion of surgery, subconjunctival injections of gentamicin (gentamicin sulfate, 100 mg/mL; VetOne) and betamethasone (BetaVet, 6 mg/mL; American Regent Inc) were administered. Our standardized postoperative medication protocol included topical nonsteroidal (diclofenac or ketorolac) and steroidal (difluprednate or prednisolone acetate) anti-inflammatories, carbonic anhydrase inhibitors (dorzolamide or brinzolamide) with or without timolol 0.5%, and an antibiotic/anti-inflammatory combination (bacitracin/neomycin/polymyxin or neomycin/polymyxin/betamethasone). Systemic postoperative medications included antibiotics (amoxicillin–clavulanic acid or cefovecin SC) and nonsteroidal anti-inflammatory (carprofen or meloxicam) or glucocorticoid (prednisone/prednisolone). The protocol for postoperative ocular hypertension was instillation of latanoprost and aqueous paracentesis if postoperative ocular hypertension persisted up to the time of discharge.

Retinoscopy

Streak retinoscopy was performed 1 week, 1 month, and 3 months post–IOL implantation to compare refractive error outcome between the Fo-X, MD8, and I-LENS. Streak retinoscopy was performed using a handheld retinoscope (Welch Allyn) and a set of Luneau lens bars. Dogs were refracted in a dimmed room, at a working distance of 50 cm, along the vertical meridian. Cycloplegia was not performed routinely for streak retinoscopy in this study, as it does not significantly affect the refractive error outcome.11 The refractive error was calculated by subtracting the working distance power of +2.0 D from gross refraction to obtain the net refraction. Emmetropia was defined as ±0.5 D, myopia < –0.5 D, and hyperopia > +0.5 D.3 Each dog was refracted by 2 examiners. The order of eyes refracted and the order of retinoscopists were randomized. The retinoscopists were masked to each other’s results, but they were not masked for IOL type because of the distinctive shapes of the IOLs used in this study.

Statistical analysis

Normally distributed variables were compared between time points using a 1-way repeated-measures ANOVA. The Kolmogorov-Smirnov test of normality was used to evaluate the refraction values between the 2 examiners for each IOL type. Normally distributed outcome measures of the 2 examiners were analyzed using Pearson correlation and Student t tests. Refractive error outcomes for 1-week, 1-month, and 3-month time points were compared between the IOL types using 1-way ANOVA for independent measures. The pre- and postoperative keratometry values were compared using Student t tests.

Results

Refractive outcome

The study included 43 client-owned dogs (74 eyes) of various breeds (mean ± SD age, 9.7 ± 3.04 years; range, 2 to 15 years). Small-breed (< 10 kg; n = 30) and female spayed dogs (19) were overrepresented. There were no significant differences in the mean ages of dogs implanted with the 3 IOLs, 10.2 ± 3.4, 9.4 ± 2.7, and 9.1 ± 2.8 years for Fo-X, MD8, and I-LENS, respectively. A total of 30 of 74 (41%) eyes were implanted with Fo-X, 19 of 74 (26%) eyes with MD8, and 25 of 74 (33%) eyes with I-LENS IOLs. Postoperatively, 16 of 30, 29 of 30, and 20 of 30 eyes with Fo-X IOLs presented for their 1-week, 1-month, and 3-month follow-ups for streak retinoscopy, respectively. For MD8 IOLs, 9 of 19, 17 of 19, and 12 of 19 eyes presented at their 1-week, 1-month, and 3-month follow-ups for streak retinoscopy, respectively. Lastly, 18 of 25, 22 of 25, and 18 of 25 eyes with I-LENS IOLs presented at their 1-week, 1-month, and 3-month follow-ups for streak retinoscopy, respectively.

The refractive outcomes for each IOL type were normally distributed (Ks = 0.22; P = .34). Postoperative refractive outcomes were highly correlated and not significantly different between 2 examiners at the 1-week, 1-month, and 3-month recheck time points (r = 0.97, 0.98, and 1.00, respectively; P = .76, .94, and .98, respectively). Therefore, the refraction results of the 2 examiners were averaged and the mean refraction values were used in the analyses. The refractive error outcomes for each IOL type were compared using 1-way ANOVA for independent measures resulting in no statistically significant differences (F(df=2,66) = 1.89; P = .16). Thus, a pairwise Student t test was not performed.

One week postoperatively, the mean refractive errors for Fo-X, MD8, and I-LENS were 0.14 ± 2.02 D, 0.97 ± 2.01 D, and 0.15 ± 2.55 D, respectively (Figure 2). One month postoperatively, the mean refractive errors were 0.35 ± 2.04 D, 0.06 ± 2.41 D, and –0.82 ± 2.20 D, respectively. Three months postoperatively, the mean refractive errors were –0.16 ± 2.67 D, 1.60 ± 2.99 D, and 0.59 ± 1.51 D, respectively. There were no significant differences in refractive error outcomes between Fo-X, MD8, and I-LENS at the 3 time points (P = .16; F(df=2,66)- = 1.89). The Fo-X IOL was the only IOL that yielded emmetropic mean refractive outcomes (±0.50 D) at all 3 time points.

Figure 2
Figure 2

The mean ± SD refractive errors of dog eyes implanted with Fo-X (n = 30), MD8 (19), and I-LENS (25) IOLs 1 week, 1 month, and 3 months postoperatively.

Citation: Journal of the American Veterinary Medical Association 262, 1; 10.2460/javma.23.08.0445

The Fo-X IOL yielded emmetropic outcomes in 4 of 16 eyes at the 1-week, 13 of 29 eyes at the 1-month, and 4 of 20 eyes at the 3-month follow-ups. The MD8 IOL yielded emmetropic outcomes in 2 of 9 eyes at the 1-week, 3 of 17 eyes at the 1-month, and 5 of 12 eyes at the 3-month follow-ups. The I-LENS IOL yielded emmetropic outcomes in 6 of 18 eyes at the 1-week, 4 of 23 eyes at the 1-month, and 4 of 18 eyes at the 3-month follow-ups. Interestingly, eyes implanted with MD8 IOL were closest to emmetropia at 1 month postoperatively (0.06 ± 2.41 D; range, –3.00 to +6.00 D), while eyes implanted with Fo-X were closest to emmetropia 1 week (0.14 ± 2.02 D; range, –3.00 to +4.00 D) and 3 months (–0.16 ± 2.67 D; range, –7.00 to +6.00 D) postoperatively. The mean refraction outcomes for all IOLs combined resulted in emmetropia at 1 week (0.22 ± 2.25 D; range, –3.00 to +6.00 D), 1 month (–0.11 ± 2.22 D; range, –5.00 to +8.00D), and 3 months (0.11 ± 2.51 D; range, –7.00 to +7.00 D) postoperatively.

Biometry and theoretical IOL calculation

Pre- and postoperative R1 and R2 values were normally distributed and therefore compared using a t test. There was no significant difference between preoperative (P = .19) and postoperative keratometry (P = .32) values among the 3 different types of IOLs (Figure 3).

Figure 3
Figure 3

Mean ± SD R1 and R2 keratometry values (D) of all study eyes measured preoperatively and 1 month postoperatively.

Citation: Journal of the American Veterinary Medical Association 262, 1; 10.2460/javma.23.08.0445

Preoperatively, mean ± SD AxL and ACD were 19.28 ± 1.46 mm and 2.70 ± 0.63 mm, respectively. Ultrasound was also performed 1 month postoperatively in 36 randomly chosen eyes, including 11 eyes implanted with Fo-X, 10 eyes implanted with MD-8, and 15 eyes implanted with I-LENS. There were no significant differences in the ACDs of the 3 IOL types based on a Kruskal-Wallis test comparing the 3 cohorts (F(df=2.36) = 4.33, P = .11; Figure 4).

Figure 4
Figure 4

Mean ± SD ACD measured 1 month postoperatively in the 3 cohorts. There were no significant differences in the ACD of the 3 IOL types.

Citation: Journal of the American Veterinary Medical Association 262, 1; 10.2460/javma.23.08.0445

The Retzlaff and Binkhorst formulas were used to calculate the theoretical IOL power using the preoperative biometry measurements for all dogs included in the study. The mean ± SD theoretical IOL powers were 40.67 ± 6.35 D and 40.13 ± 6.36 D for the Binkhorst and Retzlaff formulas, respectively. The difference between the implanted IOL power and the theoretical IOL power was < 1 D for the Binkhorst formula and 1.25 D for the Retzlaff formula, demonstrating no significant differences between the 2 formulas (P > .05). There was no significant difference between the theoretical IOL power predicted by the 2 formulas and the postoperative refractive error at the 3-month follow-up, demonstrating that both formulas are reliable in canine patients (P > .05).

Discussion

The present study reports the refractive outcomes following implantation of 3 common commercially available IOLs in canine ophthalmology. All 3 IOLs produced adequate postoperative refractive outcomes at various time points, although the Fo-X IOL was superior, as it was the only IOL that yielded mean emmetropic refractive values at all 3 time points, thus supporting our study hypothesis.

Even though the Fo-X IOL showed the most consistent postoperative emmetropic outcomes, the refractors experienced a challenge in performing streak retinoscopy through the optic with multiple rings of this IOL. The refractionists also encountered “break-phenomena” more often with the Fo-X compared to the other IOLs. This phenomenon is seen in patients with a higher amount of astigmatism and is observed when the retinal reflex does not align with the retinoscopic streak.12 The authors hypothesized this phenomenon was encountered because of the multiring design optic of the Fo-X IOL.

A study by Gift et al13 compared the 10- to 12-week postoperative refractive error of dogs implanted with 3 other IOLs. They report that the Polymethylmethacrylate (PMMA) (I-lens; I-med Pharma Inc) IOL was the closest to emmetropia compared to hydrophilic (Dioptrix) and hydrophobic (Acrysof natural single-piece acrylic foldable IOL; Alcon Laboratories Inc) acrylic IOLs. Furthermore, both acrylic IOLs resulted in slightly hyperopic refractive outcomes. By contrast, our study evaluated only hydrophilic acrylic IOLs and we refracted patients at 3 different time points postoperatively. We found no significant differences in refractive error outcomes between the IOLs that we evaluated. Despite this lack of significant differences, ametropia was present at different time points with the I-LENS and MD8, and the only IOL that had a mean refractive error of emmetropia at all 3 time points was the Fo-X. The mean refractive outcome for the MD8 IOL was hyperopia at 1 week (0.97 ± 2.01 D) and 3 months (1.60 ± 2.99 D) postoperatively. However, the MD8 also yielded the most mean refractive emmetropic outcome (0.06 ± 2.41 D) at 1 month compared to the other IOLs in the study. Implantation of the I-LENS resulted in a mean refractive error of myopia (–0.82 ± 2.20 D) at 1 month postoperatively and a mean refractive error of low hyperopia (0.59 ± 1.51 D) at 3 months postoperatively.

The slight differences in refractive outcomes of the IOLs at different time points postoperatively can be attributed to IOL shifting in the capsular bag, which would affect the refractive error. A refraction study in adult rabbits calculated the size and dioptric power to bring rabbits to emmetropia with a capsular tension ring and retinoscopy. After multiple recalculations using a proprietary predicative formula, the study concluded that a +58 D and 13-mm IOL achieved emmetropia (0 D) in rabbits. Other IOLs in the study included a +41 D and +49 D and resulted in hyperopia (+8 D and +6 to 8 D, respectively).14 A capsular tension ring was not utilized in our study patients but may have prevented any possible IOL shifting within the capsular bag. In addition, capsule contraction syndrome is a common postoperative complication in humans.1518 This syndrome is characterized by capsular fibrosis causing shrinkage and contraction of the capsule, thereby leading to IOL decentration.1518 In addition to reduced vision due to capsular opacification, capsule contraction syndrome also causes a hyperopic shift in humans after implantation of hydrophilic acrylic IOLs.15,17 Future studies with postoperative anterior chamber and vitreous depth measurements are indicated to determine whether (as suspected) a similar process occurs in dogs. In addition, preoperative lens instability, capsulorhexis size, and axial globe length could also be contributing factors to the variation in refractive errors.

To the best of our knowledge, this was the first study to apply the automated handheld keratometer to calculate the theoretical IOL equations in dogs undergoing cataract surgery. Other studies applied the Binkhorst and/or Retzlaff formulas in dogs, cats, horses, raptors, a lion, and a tiger and concluded they were reliable to predict the required IOL dioptric power.2,10,1923 The keratometry and biometry (AxL and ACD) values must be applied to the theoretical formulas that predict the IOL power needed to bring the patient to emmetropia, which have been used in a variety of species.2,10,14,1923 Our study demonstrated that there was neither a significant difference between the power of the 3 implanted IOLs and the theoretical IOL power, nor any significant difference when comparing the postoperative refractive error at 3 months postoperatively and theoretical IOL power. Therefore, both formulas can be used in ametropic dogs requiring custom IOLs.

A few limitations were encountered in the present study. We had an uneven distribution of IOLs in the analysis due to a variety of factors. First, a number of patients had persistent anterior uveitis 1 week and 1 month postoperatively, which hindered retinoscopic measurements. Second, a few patients missed their postoperative follow-ups, leading to further uneven distribution of IOLs at the 3 time points. Furthermore, our refractive results may have been affected by other complications including capsular contraction, haptic luxation, and IOL decentration. Haptic luxation and IOL decentration were all due to either preoperative or intraoperative lens capsule ruptures. The degrees of the lens capsule ruptures were not recorded for all patients, and we did not correlate these complications with the refractive error. In addition, our outcomes may have been affected by other covariants that were not analyzed, including vitreal degeneration, age, gender, and corneal endothelial decompensation. Furthermore, the majority of the dogs in our study were small and medium-sized breeds, and we did not investigate the correlation between IOL predictive power, refraction outcomes, and dog breed. Finally, dogs were refracted in just 1 meridian. This was mostly due to the fact that the study was conducted in a private practice, and retinoscopy of a second meridian by 2 investigators would have been quite time-consuming, especially for the postoperative readings, which were sometimes challenging due to ocular media haze and the uncooperative nature of patients. For similar reasons, postoperative keratometry was performed only at 1 month. We have previously published refractive studies in which retinoscopy was performed in just 1 meridian,26 a decision justified by previous studies reporting a very low prevalence of astigmatism in dogs. Kubai et al3 reported astigmatism of just 1% in 1,440 dogs of various breeds, the largest published cohort of dogs to be refracted, and a prevalence of 1.33% in 226 English Springer Spaniels.27 Indeed, as stated, the break phenomenon, which is usually associated with astigmatism, was encountered mostly in Fo-X implanted eyes and was probably due to the rings of this IOL. All of these variants can be evaluated in future studies with a larger sample size.

In conclusion, the present study found mean refractive errors < 1 D in 8 of 9 postoperative outcomes (3 time points for 3 IOLs), with the Fo-X IOL yielding emmetropic outcomes at the 3 postoperative time points. We also demonstrated that the automated handheld keratometer can be used to measure corneal curvature meridians in pre- and postoperative canine cataract surgery patients. Finally, we discovered that both the Retzlaff and Binkhorst theoretical IOL power formulas were accurate and reliable in dogs. These findings provide the groundwork for the future use of customizable IOLs in ametropic canine patients undergoing cataract surgery.

Acknowledgments

None reported.

Disclosures

The authors have nothing to disclose. No AI-assisted technologies were used in the generation of this manuscript.

Funding

The authors have nothing to disclose.

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    Sanchez RF, Becker R, Dawson C, Escanilla N, Lam R. Calculation of posterior chamber intraocular lens (IOL) size and dioptric power for use in pet rabbits undergoing phacoemulsification. Vet Ophthalmol. 2017;20(3):242249. doi:10.1111/vop.12405

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    Feng KM, Chang YH, Liang CM, Pao SI. Anterior capsular contraction syndrome with hyperopic shift. Am J Ophthalmol Case Rep. 2022;25:101328. doi:10.1016/j.ajoc.2022.101328

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    Scorolli L, Martini E, Scalinci SZ, Scorolli LG, Meduri R. Capsule contraction after continuous curvilinear capsulorhexis. J Cataract Refract Surg. 1996;22(9):12451246. doi:10.1016/s0886-3350(96)80077-0

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    Kim TG, Moon SW. Hyperopic shift caused by capsule contraction syndrome after microincision foldable intraocular Lens implantation: case series. BMC Ophthalmol. 2019;19(1):116. doi:10.1186/s12886-019-1117-y

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    Davison JA. Capsule contraction syndrome. J Cataract Refract Surg. 1993;19(5):582589. doi:10.1016/s0886-3350(13)80004-1

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    McMullen RJ Jr, Gilger BC. Keratometry, biometry and prediction of intraocular lens power in the equine eye. Vet Ophthalmol. 2006;9(5):357360. doi:10.1111/j.1463-5224.2006.00493.x

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  • 20.

    Viñas M, D’Anna N, Guandalini A, et al. Bilateral phacoemulsification and intraocular lens implantation in a young African lion (Panthera leo). Can Vet J. 2019;60(2):186192.

    • Search Google Scholar
    • Export Citation
  • 21.

    Kuhn SE, Hendrix DVH, Jones MP, Ward DA, Baine KH, Franklin SR. Biometry, keratometry, and calculation of intraocular lens power for the bald eagle (Haliaeetus leucocephalus). Vet Ophthalmol. 2015;18 suppl 1:106112. doi:10.1111/vop.12120

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    Gilger BC, Davidson MG, Howard PB. Keratometry, ultrasonic biometry, and prediction of intraocular lens power in the feline eye. Am J Vet Res. 1998;59(2):131134.

    • Search Google Scholar
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  • 23.

    Owens CD, Michau TM, Boorstein J, Wynn ER, McMullen RJ. Keratometry, biometry, and prediction of intraocular lens power in adult tigers (Panthera tigris). Am J Vet Res. 2021;83(2):140146. doi:10.2460/ajvr.21.04.0060

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    Hoffman A, Wolfer J, Occelli L, et al. Refractive state following retinal reattachment and silicone oil tamponade in dogs. Am J Vet Res. 2012;73(8):12991304. doi:10.2460/ajvr.73.8.1299

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  • 27.

    Kubai MA, Labelle AL, Hamor RE, Mutti DO, Famula TR, Murphy CJ. Heritability of lenticular myopia in English Springer Spaniels. Invest Ophthalmol Vis Sci. 2013;54(12):73247328. doi:10.1167/iovs.12-10993

    • Search Google Scholar
    • Export Citation
  • Figure 1

    Representative image of biometry measurements using combined amplitude modulation (blue trace) and bright-scan ultrasonography. In this patient, the axial length is 19.55 mm and the anterior chamber depth (ACD) is 2.89 mm.

  • Figure 2

    The mean ± SD refractive errors of dog eyes implanted with Fo-X (n = 30), MD8 (19), and I-LENS (25) IOLs 1 week, 1 month, and 3 months postoperatively.

  • Figure 3

    Mean ± SD R1 and R2 keratometry values (D) of all study eyes measured preoperatively and 1 month postoperatively.

  • Figure 4

    Mean ± SD ACD measured 1 month postoperatively in the 3 cohorts. There were no significant differences in the ACD of the 3 IOL types.

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  • 14.

    Sanchez RF, Becker R, Dawson C, Escanilla N, Lam R. Calculation of posterior chamber intraocular lens (IOL) size and dioptric power for use in pet rabbits undergoing phacoemulsification. Vet Ophthalmol. 2017;20(3):242249. doi:10.1111/vop.12405

    • Search Google Scholar
    • Export Citation
  • 15.

    Feng KM, Chang YH, Liang CM, Pao SI. Anterior capsular contraction syndrome with hyperopic shift. Am J Ophthalmol Case Rep. 2022;25:101328. doi:10.1016/j.ajoc.2022.101328

    • Search Google Scholar
    • Export Citation
  • 16.

    Scorolli L, Martini E, Scalinci SZ, Scorolli LG, Meduri R. Capsule contraction after continuous curvilinear capsulorhexis. J Cataract Refract Surg. 1996;22(9):12451246. doi:10.1016/s0886-3350(96)80077-0

    • Search Google Scholar
    • Export Citation
  • 17.

    Kim TG, Moon SW. Hyperopic shift caused by capsule contraction syndrome after microincision foldable intraocular Lens implantation: case series. BMC Ophthalmol. 2019;19(1):116. doi:10.1186/s12886-019-1117-y

    • Search Google Scholar
    • Export Citation
  • 18.

    Davison JA. Capsule contraction syndrome. J Cataract Refract Surg. 1993;19(5):582589. doi:10.1016/s0886-3350(13)80004-1

  • 19.

    McMullen RJ Jr, Gilger BC. Keratometry, biometry and prediction of intraocular lens power in the equine eye. Vet Ophthalmol. 2006;9(5):357360. doi:10.1111/j.1463-5224.2006.00493.x

    • Search Google Scholar
    • Export Citation
  • 20.

    Viñas M, D’Anna N, Guandalini A, et al. Bilateral phacoemulsification and intraocular lens implantation in a young African lion (Panthera leo). Can Vet J. 2019;60(2):186192.

    • Search Google Scholar
    • Export Citation
  • 21.

    Kuhn SE, Hendrix DVH, Jones MP, Ward DA, Baine KH, Franklin SR. Biometry, keratometry, and calculation of intraocular lens power for the bald eagle (Haliaeetus leucocephalus). Vet Ophthalmol. 2015;18 suppl 1:106112. doi:10.1111/vop.12120

    • Search Google Scholar
    • Export Citation
  • 22.

    Gilger BC, Davidson MG, Howard PB. Keratometry, ultrasonic biometry, and prediction of intraocular lens power in the feline eye. Am J Vet Res. 1998;59(2):131134.

    • Search Google Scholar
    • Export Citation
  • 23.

    Owens CD, Michau TM, Boorstein J, Wynn ER, McMullen RJ. Keratometry, biometry, and prediction of intraocular lens power in adult tigers (Panthera tigris). Am J Vet Res. 2021;83(2):140146. doi:10.2460/ajvr.21.04.0060

    • Search Google Scholar
    • Export Citation
  • 26.

    Hoffman A, Wolfer J, Occelli L, et al. Refractive state following retinal reattachment and silicone oil tamponade in dogs. Am J Vet Res. 2012;73(8):12991304. doi:10.2460/ajvr.73.8.1299

    • Search Google Scholar
    • Export Citation
  • 27.

    Kubai MA, Labelle AL, Hamor RE, Mutti DO, Famula TR, Murphy CJ. Heritability of lenticular myopia in English Springer Spaniels. Invest Ophthalmol Vis Sci. 2013;54(12):73247328. doi:10.1167/iovs.12-10993

    • Search Google Scholar
    • Export Citation

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