Indian Journal of Ophthalmology

ARTICLES
Year
: 1975  |  Volume : 23  |  Issue : 1  |  Page : 6--11

Study of corneal power and diameter in simple refractive error


D Ganguli, IS Roy, SK Biswas, M Sengupta 
 Department of Ophthalmology, University College of Medicine, Calcutta University, Calcutta-12, India

Correspondence Address:
D Ganguli
Department of Ophthalmology, University College of Medicine, Calcutta University, Calcutta-12
India




How to cite this article:
Ganguli D, Roy I S, Biswas S K, Sengupta M. Study of corneal power and diameter in simple refractive error.Indian J Ophthalmol 1975;23:6-11


How to cite this URL:
Ganguli D, Roy I S, Biswas S K, Sengupta M. Study of corneal power and diameter in simple refractive error. Indian J Ophthalmol [serial online] 1975 [cited 2021 Sep 22 ];23:6-11
Available from: https://www.ijo.in/text.asp?1975/23/1/6/31331


Full Text

In the mechanism of image formation on the retina, cornea and the lens act as strong convex lens. The converging power of the normal eye is slightly less than +60D and contribution of cornea in an average subject is about +43D. [5] It is logical to think that cornea can influence the various ametropic conditions of the eye by means of its curvature.

By measuring the anterior corneal curvature and its dioptric value thereof, it is possible to assess its dioptric contribution in various refra­ctive states.

This study was undertaken to investigate the (a) average corneal power, (b) average corneal diameter, (c) correlation of corneal power with age groups, (d) correlation of corneal power with degree of refractive error, (e) correlation of corneal diameter with refrac­tive error and (f) distribution of corneal power in different refractive states.

 Materials and Methods



Selection of the cases

A total of 300 cases, comprising 600 eyes, were stud­ied including 100 emmetropic cases as control in the Eye Infirmary, Medical college Hospitals, Calcutta. Follow­ ing conditions were excluded from the study: 1. Deve­lopmental or acquired ocular abnormalities, diseases of the eyeball and adnexa, 2. restricted ocular movements, 3. opacities of the cornea and the lens, 4. high or irregular astigmatism, and 5. variations of astigmatic pattern of retinoscopy from keratometry by more than one diopter in each meridian to avoid the influence of abnormal astigmatism from the posterior corneal surface and the lens.

Diagnosis of the cases

Emmetropia, myopia and hypermetropia were diag­nosed on retinoscopy findings after full cycloplegic mydriasis.

Emmetropia between + 1.00 and +1.75D.

Myopia from +0.75D to the negative side.

Hypermetropia from +2.OOD to the positive side.

Instruments

Keratometer, Javal & Schiotz design, manufactured by Haag-Streit (Modern Type, Model-1953) was used to study the anterior corneal surface.

Self-recording P.D. ruler of Hamblin was used to measure the corneal diameter.

Methods

1. Retinoscopy was done under full cycloplegic mydriasis.

2. Slit-Lamp examination was done on the day of retinoscopy under mydriasis to examine the anterior segment.

3. Post-mydriatic test (P.M.T.) was done on the 5th day of retinoscopy.

4. Corneal diameter was measured by the vertical wires of the P.D. ruler. Wires were closely approxi­mated to the corneal margins after closing the left eye of the examiner and the corneal diameter was noted from the mm. scale of the ruler. Horizontal and vertical meridians were recorded separately.

5. Keratometric study was done on the day of P.M.T. in a dynamic condition of the ciliary muscles. Horizontal axis was measured first, followed by vertical axis and studied separately. Cases with axis of 135° and 45° degrees, 135° degree was considered as hori­zontal.

Observations

1. Average corneal power in dioptres in emmetropia and ametropia. [Table 1].

2. Average corneal diameter in mm. in emmetropia and ametropia. [Table 2].

3. Correlation of corneal power with age groups in emmetropia [Figure 1] in myopia [Figure 2] and in hyper­metropia [Figure 3]

(Only vertical axis is shown in the graphs).

4. Correlation of corneal power with degree of re­fractive error in myopia [Figure 4] and in hypermetropia [Figure 5]

No definite opinion can be formed from the study of these graphs and correlation coefficients were studied. [Table 3]

From the study of corneal power [Table 1] it is observed that myopic corneas are more curved and hypermetropic corneas are more;, flat in comparison to emmetropic corneas; so, expectation of a positive cor­relation in myopia and negative correlation in hypermetropia is quite logical. But correlation co­efficient study is practically reverse. To find out any other factor which might influence the correlation co-efficient study, relationship of refractive error with different age groups was studied, in myopia [Figure 6] and in hypermetropia [Figure 7].

The refractive error and age group relation ship is modifying the study on corneal power and refractive error to a marked extent, and significantly positive relation in hypermetropia may be as a result of this spurious relationship.

From [Table 2] it appears that practically no signi­ficant variations of corneal diameter occur with or without refractive error in both the sexes.

6. Distribution of corneal power in emmetropia and ametropia. [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13]

 Discussion



In this study the observation was made from 10 years of age and according to Sorsby et a1 [12] maximum growth of the eyeball and flattening of the cornea occurs within 14 years in emmet­ropic eyes. Probably due to this reason sharp fall of corneal power is not observed in the earlier age groups in emmetropic cases.

Corneal power of higher dioptric value, particularly in females, are observed in myopic cases in comparison to emmetropic corneas. Gardiner [6] in his study on myopic corneas in children found +44.418D in females and +43.605D in males, a difference of 0.813D. In this study sex-difference is more marked (1.51D) in combined age groups. Gardiner's [6] figure on myopic corneal power ranges from 39.50 to +47D in males and from 41.00 to +49.OOD in females (children), surprisingly similar to this study. Sorsby et at [13] observed the range varying from +39.2 to 49.9D in both sexes combined.

In hypermetropia average corneal power is less than emmetropic corneas and such a wide range of corneal power is not yet been reported by any observer. Sorsby et ales observed the range from ±39.00 to 47.6D in both sexes combined.

Onuma [9] stated that refractive changes with age do not seem to be due ..o changes of the corneal power, but, in this cross-sectional study a definite lowering of corneal power is observed with increase of age in ametropic cases.

The figures on corneal diameter compare favourably with the statement of Duke-Elders as 11.70 mm. horizontally and 10.60mm. verti­cally in males, but, the difference between the meridians are less in this study. The corneal diameter is also slightly less in females as quoted by Duke-Elder [4] .

Gardiners [6] observed no correlation of the corneal power with the degree of myopia in children, but Cooper et al [2] claimed a general sweep towards the high degree of myopia with increase in the refractive power of the cornea. In this study, inspite of negative `age and refractive error' relationship, no significant correlation is observed.

In hypermetropia `age and refractive error' relationship is strongly negative and majority cases belong to the younger age groups. So expectation of a negative relation i.e., corneal power is less in high degree of hypermetropia is not beyond expectation after eliminating the abnormal influence of age in the study.

 Conclusions



This study leads to the following conclu­sions

1. Average corneal power in emmetropia

Emmetropic males --43.57 + .085D.

Emmetropic females --44.13 + 0.1202D.

Corneal power is more in females than males whether the eyes are emmetropic, myopic or hypermetropic, but more marked in female myopes.

Trend of the average corneal power is more in myopia and less in hypermetropia in com­parison to emmetropic corneas.

2. Average corneal diameter in

Emmetropic males 11.45 mm. horizontally

10.86 min. vertically

Corneal diameter is slightly less in the females.

Variations of horizontal diameter from the vertical one is to the extent of 0.50 nun. on an average.

3. Dioptric power of the cornea remaining more or less uniform in different age groups in emmetropic conditions, but, the relation is negative in ametropic conditions.

4. No relationship exists between corneal power and increase of refractive error.

5. No relationship exists between corneal diameter and ametropic conditions.

6. Distribution of corneal power in

Emmetropia +40.50 to +48.00D

Myopia +39.50 to + 49.OOD

Hypermetropia +37.50 to +51.00D

 Summary



In the mechanism of image formation on the retina, cornea takes the leading role by acting as a strong convex lens and may influence the various ametropic conditions by its flexibi­lity and various external and internal influences. Corneal power and diameter was studied in 300 unselected Indians with normal eyeball and adnexa and conclusions are drawn regarding corneal power, corneal diameter, relation of corneal power with age and refractive error and distribution of corneal power, etc.

 Acknowledgements



We are grateful to Sri Suhash Biswas, M. Sc., Dr. Amitabha Basu, M. Sc., Ph.D. and Dr. Ranjit Chakra­vorty, M.Sc., Ph.D., Vice-Dean of the Indian Statistical Institute for their help in this study.[15]

References

1Basu, A., 1960 Proc. All Ind. Ophthal Soc., 23, 94.
2 Cooper, S.N. and Docrat, Y.A., 1951 Proc. All Ind. Ophthal Soc., 12, 27.
3Donders, F.C., 1858, v. Graefes Arch. Opthal, 4 (1), 301 Anomalies of Accommodation and Refraction of the Eye, Lond. (1864). Quoted by Duke-Elder in 5.
4 Duke-Elder, S., 1961, System of Opthal, mology., 2, 92 Henry Kimpton, London.
5 Duke-Elder, S., 1970, System of Ophthal., alway, 5, 240 Henry Kimmpton, London,
6 Gardiner, P.A. 1962, Brit. J. Ophthal., 46, 138.
7 von. Helmholtz, H., 1856, Hb. d. Physiol. Optik, 1st ed., Leipzig quoted by Duke-Elder in 5.
8Javal and Schiolz, 1881, Ann. Oculist (Paris), 86, 5 quated by Duke-Elder in 5.
9 Onuma, M. 1965, Acta Soc. Opthal Jap., 69,2024
10Scheiner, C., 1619. Oculus hoe est: fundamen­tum, Opticum Oeniponti, 13 (1619), quoted by Duke-Elder in 5.
11Sorsby, A., Davey, J.B., Sheridan, M. and Tanner, J.M., 1951, "Emmetropia and its Aber­rations" Spec. Rep. Series No. 293, Lond.
12 Sorsby, A., Benjamin, B., and Sheridan, M., Spec. Rep. Series No. 301, London.
13 Sorsby, A., Leary, G.A., and Richards, M.J., 1962, Vision Res., 2, 309.
14Steiger, A., 1913, Die Entsteheeng d. mens­chlichen Auges, Berlin, quoted by Duke-Elder in 5.
15 Tron, E.J., 1934-1935, von Graefes Arch. Ophthal., 132, 182 (1934); 133, 211 (1935), quoted by Duke-Elder in 5.