Size does matter: what is the corneo‐limbal diameter?

Abstract

This review surveys available literature for corneal and limbal dimensions. With modern scleral lenses, these measures have become central to determining the overall size of a lens for satisfactory fitting. In general, published values are not based on a definition of what is being measured. In addition, the most widely accepted average corneal diameter measurement, 11.7 × 10.6-mm, emanates from one source published more than 50-years ago. This value was not based on a measurement but appears to be the cumulative impression of measurements from seven studies conducted between 67 and 127-years ago. Furthermore, in most instances, if at all stated, quoted measures are based on horizontal visible iris diameter, providing limited acknowledgement of limbal width and its inclusion as part of the corneal diameter. The corneo‐scleral sulcus from one side to the other has been measured, giving a larger diameter, but may include at least part of the limbus. More objective measurements are possible with modern ophthalmic instrumentation but the lack of structural definition and low magnification resolution with these techniques raises concerns with the accuracy of the results. Measurement of the horizontal visible iris diameter does not include the limbal width, which means that the horizontal visible iris diameter is an underestimate of the true corneo‐limbal diameter. This review concludes that the width of the limbus has been neither structurally defined nor accurately measured and that there is a need for the development of new protocols for determining the dimensions of the average cornea and limbus. It is predicted that more accurate measures will indicate that to vault across cornea with limbus and provide excellent comfort, the average cornea will need a lens to have a diameter of 16.0-mm or larger.

Key words:

• corneal diameter
• corneo‐limbal diameter
• horizontal visible iris diameter
• limbal width
• scleral lens

ACKNOWLEDGEMENTS

We are grateful to Professor Alan Burns for his valuable input to the manuscript and the light micrograph accompanying the text, and Ms Kimberley Thompson for her expert assistance in preparing the other images for this article. We wish to also gratefully acknowledge Mr Richard Pearson, Tonbridge, Kent, UK for his insightful knowledge of the historical literature presented here.