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Relationship Between Corneal Radius of Curvature and Age in Rabbits
Br. vet.
J.
(1969), 125, 560
RELATIONSHIP BETvVEEN CORNEAL RADIUS OF CURVATURE AND AGE IN RABBITS By N.
ROTH
*
Jules Stein Eye Institute, University of California at Los Angeles
SUMMARY
Corneal radii of curvature were measured in a litter of four Dutch rabbits over an interval of time that was long enough to provide information on the relationship between corneal radius and age. The period of maximum corneal growth rate was found to end at the age of about 150 days. Results of this preliminary study suggest that, for a given strain of rabbit, corneal growth information may permit' approximate standardization of corneal curvatures and other eye dimensions among rabbits used in various types of eye research. INTRODUCTION
Rabbits have been subjects for various kinds of vision research ranging from electrophysiological to pharmacological studies. In such research, it is often necessary to keep the cornea in good condition over a relatively long p eriod of time, while the rabbit is anaesthetized and the lids retracted. A convenient way to keep the cornea from drying out and, at the same time, improve the optical quality of the eye is to use a corneal contact lens (Roth, 1965). The contact lens must be well fitted, otherwise the cornea may be damaged and rendered unsuitable for experimentation. Because a good fit is required between contact lens and cornea, it would be advantageous to standardize the corneal dimensions of the experimental animals so that only a small range of lens sizes would be needed. In addition, corneal standardization would increase the probability that other dimensions of the eye, such as axial length and pupil size, would be similar. For the purpose of standardization, it would be useful to determine, for a particular strain of animal, the relationship between cornea!" radius of curvature and age. This report presents the results of a preliminary step in the desired direction wherein the author measured the corneal radii offour rabbits for the purpose of plotting the corneal growth curve. MATERIALS AND METHODS
The experimental animals consisted of four Dutch rabbits from a litter born in the vivarium at UCLA. • Permanent address: Pacific University College of Optometry, Forest Grove, Ore. 97 116.
THE CORNEA AND AGE IN RABBITS
Corneal measurements, obtained with a Bausch and Lomb keratometer, were begun at the age of 54 days. This keratometer gives readings in units of refractive power (dioptres) that must be converted to radii in millimeters by taking into account the assumed refractive index of corneal tissue for which the instrument is calibrated. The conversion from power to radius can be made either by referring to a table supplied by the manufacturer or by performing a calculation based on a mean refractive index for corneal tissue of 1'3375 (Emsley 1952). The latter method of conversion was used in this study because of the need to extend the range of the keratometer, as explained below. When measurements were first begun, corneal radii were too short to be measured directly by the keratometer. To extend its range, an accessory lens (+ 1 '25 D) was placed in front of the keratometer aperture. Power readings obtained with the accessory lens were corrected by adding a constant (5.8 D) found by reference to a steel bearing ball of known radius. Adequate immobilization of the animal was achieved by placing it into an adjustable restraining box that limited body movements and allowed the head to protrude through a neck opening. The keratometer was aligned in front of the rabbit's left eye and a total of ten corneal measurements were taken at each session, five at 90° and five at 180°. The right eye was not included in this study. RESULTS
The two sets of measurements-five at 90 ° and five at 180°-were averaged to obtain a mean radius for each rabbit on a given day, without regard for any possible corneal astigmatism. An additional calculation provided the mean radius for the entire litter on a given day. There were, of course, individual differences in mean radius among the litter mates. The primary purpose of this
, 7·0
6'8
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E
•• •
~ 6'6
'0 0
0:
•
"0
~ 6·4
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• 6'2
6·0
• •
••
Fig. I. Corneal growth in four rabbit siblings. Each data point represents the mean of ten corneal radius measurements per animal on a given day. The straight line was fitted by the method of least squares.
BRITISH VETERINARY JOURNAL,
125, I I
study, however, was to obtain an evaluation of the mean growth curve rather than of the distribution of radii at a given age-an evaluation that would have required a greater number of animals. The mean growth rate from the age of 54 to 124 days was about 0 ·08 mm. per week (Fig. I). Thereafter, illness claimed one of them, and measurements were continued on only three of the original four rabbits. A plot of the mean corneal radius for the surviving three rabbits to the age of 520 days indicates that the corneal growth rate diminished noticeably at the age of approximately ISO days (Fig. 2). Beyond this age, the average growth rate, estimated by fitting a least-squares straight line, diminished to about 0.015 mm. per week.
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6·5
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100
200
300 Age (days)
Fig. 2. Corneal growth in three of the four rabbits in Fig. 1. Data points are mean values for all three animals on a given day. The least-squares straight line was calculated for the age interval from 150 to 520 days.
DISCUSSION
Results of this study suggest that corneal growth information can be useful in experiments that require limitation of the variability of corneal curvatures in the experimental animals. The author has, in fact, used the information presented here as a basis for ordering animals by approximate age (Roth, 1965). Corneas of individual animals must be measured with a keratometer or similar instrument-a relatively simple procedure requiring little practice. An inventory of ten corneal contact lenses* ranging from 6'75 to 8'23 mm. in ocular radius was more than adequate to provide well-fitted lenses to all animals falling in the age range from 200 to 350 days. ACKNOWLEDGMENT
The author is grateful to Mr Robert Petrus for assistance with experimental measurements and with processing of data.
* Supplied through the courtesy of Plastic Contact Lens Co., Los Angeles, California.
THE CORNEA AND AGE IN RABBITS REFERENCES
EMSLEY, H. H. (1952). Visual Optics, 5th edn, Vol. I, p. 323. London: Hatton Press. ROTH, N. (1965). Br. J. Physiol. Opt., 22, 84.
(Accepted for publication 15 April 1969)
Relation entre Ie rayon de courbure de la cornee et l'age chez les lapins (Roth) ResUD1e. Les rayons de courbure de la cornee furent mesures dans une portee de quatre lapins hollandais, pendant une periode suffisamment longue pout fournir des renseignements sur la relation qui existe entre Ie rayon corneen et Page. On decouvrit que la periode de croissance maximum de la cornee se terminait a Page d'environ I50jours. D'apres les resultats de cette etude preliminaire, il semble que, pout une variete donnee de lapins, des informations sur la croissance corneenne puissent permettre une uniformisation approximative des courbures de la cornee et d'autres dimensions de l'oeil chez les lapins utilises pour des types de recherche oculaire divers. Verhaltnis zwischen Kornea-Kriinunungsradius und Alter bei Kaninchen (Roth) ZusaDlDlenfassung. Kornea-Krtimmungsradien wurden bei einem Wurf von vier holliindischen Kaninchen tiber einen Zeitraum gemessen, der lang genung war, Informationen tiber das Verhiiltnis zwischen dem Kornearadius und dem Alter zu liefern. Es wurde gefunden, daB die Periode der maximalen Kornea-wachstumsziffer im Alter von ungefahr 150 Tagen endet. Die Ergebnisse dieser vorliiufigen Untersuchung weisen daraufhin, daB Korneawachstumsinformation bei einem gegebenen Kaninchenstamm eine ungeHihre Standardisierung der Korneakrtimmungen und anderer Augendimensionen unter den bei verschiedenen Augenforschungsarten erlauben kann. Relacion entre el radio de curvatura de la cornea y edad en los conejos (Roth) Resulllen. Los radios de curvatura de la cornea se midieron en una lechigada de cuatro conejos holandeses sobre un intervalo de tiempo que fue 10 suficiente para obtenerse informacion sobre la relacion entre el radio de la cornea y la edad. El periodo de la razon maxima de crecimiento de la cornea, de conformidad con la investigacion, finaliza a la edad de 150 dras aproximadamente. Los resultados de este estudio preliminar sugieren que, para una raza dada de conejo, la informacion de crecimiento de la cornea puede dermitir normalizacion aproximada de las curvatures de la cornea y otras dimensiones del ojo entre los conejos empleadas en diferentes tipos de investigacion del ojo.
J.
(1969), 125, 560
RELATIONSHIP BETvVEEN CORNEAL RADIUS OF CURVATURE AND AGE IN RABBITS By N.
ROTH
*
Jules Stein Eye Institute, University of California at Los Angeles
SUMMARY
Corneal radii of curvature were measured in a litter of four Dutch rabbits over an interval of time that was long enough to provide information on the relationship between corneal radius and age. The period of maximum corneal growth rate was found to end at the age of about 150 days. Results of this preliminary study suggest that, for a given strain of rabbit, corneal growth information may permit' approximate standardization of corneal curvatures and other eye dimensions among rabbits used in various types of eye research. INTRODUCTION
Rabbits have been subjects for various kinds of vision research ranging from electrophysiological to pharmacological studies. In such research, it is often necessary to keep the cornea in good condition over a relatively long p eriod of time, while the rabbit is anaesthetized and the lids retracted. A convenient way to keep the cornea from drying out and, at the same time, improve the optical quality of the eye is to use a corneal contact lens (Roth, 1965). The contact lens must be well fitted, otherwise the cornea may be damaged and rendered unsuitable for experimentation. Because a good fit is required between contact lens and cornea, it would be advantageous to standardize the corneal dimensions of the experimental animals so that only a small range of lens sizes would be needed. In addition, corneal standardization would increase the probability that other dimensions of the eye, such as axial length and pupil size, would be similar. For the purpose of standardization, it would be useful to determine, for a particular strain of animal, the relationship between cornea!" radius of curvature and age. This report presents the results of a preliminary step in the desired direction wherein the author measured the corneal radii offour rabbits for the purpose of plotting the corneal growth curve. MATERIALS AND METHODS
The experimental animals consisted of four Dutch rabbits from a litter born in the vivarium at UCLA. • Permanent address: Pacific University College of Optometry, Forest Grove, Ore. 97 116.
THE CORNEA AND AGE IN RABBITS
Corneal measurements, obtained with a Bausch and Lomb keratometer, were begun at the age of 54 days. This keratometer gives readings in units of refractive power (dioptres) that must be converted to radii in millimeters by taking into account the assumed refractive index of corneal tissue for which the instrument is calibrated. The conversion from power to radius can be made either by referring to a table supplied by the manufacturer or by performing a calculation based on a mean refractive index for corneal tissue of 1'3375 (Emsley 1952). The latter method of conversion was used in this study because of the need to extend the range of the keratometer, as explained below. When measurements were first begun, corneal radii were too short to be measured directly by the keratometer. To extend its range, an accessory lens (+ 1 '25 D) was placed in front of the keratometer aperture. Power readings obtained with the accessory lens were corrected by adding a constant (5.8 D) found by reference to a steel bearing ball of known radius. Adequate immobilization of the animal was achieved by placing it into an adjustable restraining box that limited body movements and allowed the head to protrude through a neck opening. The keratometer was aligned in front of the rabbit's left eye and a total of ten corneal measurements were taken at each session, five at 90° and five at 180°. The right eye was not included in this study. RESULTS
The two sets of measurements-five at 90 ° and five at 180°-were averaged to obtain a mean radius for each rabbit on a given day, without regard for any possible corneal astigmatism. An additional calculation provided the mean radius for the entire litter on a given day. There were, of course, individual differences in mean radius among the litter mates. The primary purpose of this
, 7·0
6'8
••
E
E
•• •
~ 6'6
'0 0
0:
•
"0
~ 6·4
(; (J
• 6'2
6·0
• •
••
Fig. I. Corneal growth in four rabbit siblings. Each data point represents the mean of ten corneal radius measurements per animal on a given day. The straight line was fitted by the method of least squares.
BRITISH VETERINARY JOURNAL,
125, I I
study, however, was to obtain an evaluation of the mean growth curve rather than of the distribution of radii at a given age-an evaluation that would have required a greater number of animals. The mean growth rate from the age of 54 to 124 days was about 0 ·08 mm. per week (Fig. I). Thereafter, illness claimed one of them, and measurements were continued on only three of the original four rabbits. A plot of the mean corneal radius for the surviving three rabbits to the age of 520 days indicates that the corneal growth rate diminished noticeably at the age of approximately ISO days (Fig. 2). Beyond this age, the average growth rate, estimated by fitting a least-squares straight line, diminished to about 0.015 mm. per week.
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.,.,
"0
••
~c: (;
u
6·5
••
•
•• ••
100
200
300 Age (days)
Fig. 2. Corneal growth in three of the four rabbits in Fig. 1. Data points are mean values for all three animals on a given day. The least-squares straight line was calculated for the age interval from 150 to 520 days.
DISCUSSION
Results of this study suggest that corneal growth information can be useful in experiments that require limitation of the variability of corneal curvatures in the experimental animals. The author has, in fact, used the information presented here as a basis for ordering animals by approximate age (Roth, 1965). Corneas of individual animals must be measured with a keratometer or similar instrument-a relatively simple procedure requiring little practice. An inventory of ten corneal contact lenses* ranging from 6'75 to 8'23 mm. in ocular radius was more than adequate to provide well-fitted lenses to all animals falling in the age range from 200 to 350 days. ACKNOWLEDGMENT
The author is grateful to Mr Robert Petrus for assistance with experimental measurements and with processing of data.
* Supplied through the courtesy of Plastic Contact Lens Co., Los Angeles, California.
THE CORNEA AND AGE IN RABBITS REFERENCES
EMSLEY, H. H. (1952). Visual Optics, 5th edn, Vol. I, p. 323. London: Hatton Press. ROTH, N. (1965). Br. J. Physiol. Opt., 22, 84.
(Accepted for publication 15 April 1969)
Relation entre Ie rayon de courbure de la cornee et l'age chez les lapins (Roth) ResUD1e. Les rayons de courbure de la cornee furent mesures dans une portee de quatre lapins hollandais, pendant une periode suffisamment longue pout fournir des renseignements sur la relation qui existe entre Ie rayon corneen et Page. On decouvrit que la periode de croissance maximum de la cornee se terminait a Page d'environ I50jours. D'apres les resultats de cette etude preliminaire, il semble que, pout une variete donnee de lapins, des informations sur la croissance corneenne puissent permettre une uniformisation approximative des courbures de la cornee et d'autres dimensions de l'oeil chez les lapins utilises pour des types de recherche oculaire divers. Verhaltnis zwischen Kornea-Kriinunungsradius und Alter bei Kaninchen (Roth) ZusaDlDlenfassung. Kornea-Krtimmungsradien wurden bei einem Wurf von vier holliindischen Kaninchen tiber einen Zeitraum gemessen, der lang genung war, Informationen tiber das Verhiiltnis zwischen dem Kornearadius und dem Alter zu liefern. Es wurde gefunden, daB die Periode der maximalen Kornea-wachstumsziffer im Alter von ungefahr 150 Tagen endet. Die Ergebnisse dieser vorliiufigen Untersuchung weisen daraufhin, daB Korneawachstumsinformation bei einem gegebenen Kaninchenstamm eine ungeHihre Standardisierung der Korneakrtimmungen und anderer Augendimensionen unter den bei verschiedenen Augenforschungsarten erlauben kann. Relacion entre el radio de curvatura de la cornea y edad en los conejos (Roth) Resulllen. Los radios de curvatura de la cornea se midieron en una lechigada de cuatro conejos holandeses sobre un intervalo de tiempo que fue 10 suficiente para obtenerse informacion sobre la relacion entre el radio de la cornea y la edad. El periodo de la razon maxima de crecimiento de la cornea, de conformidad con la investigacion, finaliza a la edad de 150 dras aproximadamente. Los resultados de este estudio preliminar sugieren que, para una raza dada de conejo, la informacion de crecimiento de la cornea puede dermitir normalizacion aproximada de las curvatures de la cornea y otras dimensiones del ojo entre los conejos empleadas en diferentes tipos de investigacion del ojo.