Ocular parameters related to drug delivery in the canine and equine eye: aqueous and vitreous humor volume and scleral surface area and thickness

OBJECTIVE: To measure the ocular surface area, scleral thickness, and the aqueous and vitreous humor volumes in the canine and equine eye. Animals studied Fourteen canine and 16 equine cadaver eyes. PROCEDURE: Eyes were measured either fresh or following fixation in formalin. Ocular surface area was calculated using the fluid displacement method. Globes were hemisected and aqueous and vitreous humor were collected and quantitated. Scleral thickness was measured by digital caliper, by image projection, and by ultrasonic biomicroscopy (UBM). RESULTS: Mean +/- standard deviation (SD) scleral surface areas in canine and equine eyes were 12.87 +/- 2.24 and 57.23 +/- 5.63 cm2, respectively. Mean +/- SD aqueous humor volume was 0.77 +/- 0.24 in the dog and 3.04 +/- 1.27 mL in the horse. Mean vitreous humor volume was 1.7 +/- 0.86 mL for the canine eye and 26.15 +/- 4.87 mL for the equine eye. In canine and equine eyes, the sclera was thinnest at the ocular equator: 0.34 +/- 0.13 mm (canine) and 0.53 +/- 0.1 (equine). There were no significant differences between the direct caliper and projected thickness measurements or differences between measurements in the superior, inferior, nasal, and temporal quadrants of the eye. Scleral thickness in fresh tissue was greater than in fixed tissue at most sites. CONCLUSIONS: The UBM measurement method appeared to be most consistent and reproducible when compared to direct measurement techniques. Formalin fixation of the eyes was associated with significantly thinner scleral tissue than with fresh ocular tissue.