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R. J. McMullen, Jr., J. H. Salmon, M. G. Davidson, B. C. Gilger; Refractive State of the Enucleated Equine Eye After Placement of a 30D Intraocular Lens. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3335.
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In a previous study we determined that a 30D intraocular lens implant should be appropriate to achieve emmetropia in adult horses. Pre-operative anterior chamber depth (ACD) plus 50% of the crystalline lens thickness (CLT) was used as the post-operative anterior chamber depth (PACD) in our calculations. Therefore, the purpose of this study was to evaluate the effect of a 30D intraocular lens implant on refraction in enucleated equine globes and to determine the post-operative ACD.
Extracapsular lens extraction and placement of a 30D equine IOL was performed on 6 enucleated equine eyes. Intraocular pressure was maintained at 22-25 mmHg with injections of saline through the optic nerve. Streak retinoscopy and A/B-Mode ultrasound was performed on each eye prior to, and immediately following, lens extraction and IOL implantation. A/B-Mode ultrasound was performed to obtain the following pre- and post-operative globe dimensions: ACD, CLT, and axial globe length (AxL). Streak retinoscopy was performed on 31 normal horses to provide information on the normal equine refractive state.
Pre-operative mean refractive error of -0.46D (SD +/- 1.03) was obtained from 6 eyes. Pre-operative globe dimensions: ACD: 7.12 mm (SD +/- 0.82), CLT: 11.32 mm (SD +/- 0.81), and AxL: 40.51 mm (SD +/- 1.26). Post-operative anterior chamber depth (PACD): 10.76 mm (SD +/- 1.16). None of the IOLs were positioned posterior to the pre-operative ACD + 50% CLT. The pre- to post-operative ACD ratio averaged 0.68 (SD +/- 0.59). The overall mean refractive error of adult horses in our clinic was 0.29D (SD +/- 0.77). Implantation of a 30D IOL led to an average overcorrection of 2.96D (SD +/- 0.84).
Normal horses are approximately emmetropic. Due to a more anterior PACD than that estimated in our previous work, a 30D IOL may overcorrect normal horses based on this in vitro study. An IOL with the refractive power of approximately 24 - 25.5D may be appropriate for the horse.
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