George Jessen was the first to publish on OrthoK in his 1962 paper on Orthofocus Techniques that described the concept of using rigid PMMA to temporarily correct myopia, hyperopia and astigmatism.^{1 }The concepts Jessen introduced evolved to the OrthoK designs and approach to fitting used today.

It is the calculation of the base curve for an OrthoK lens described in this seminal paper that has lasted the test of time to become known as the Jessen Factor. To create an OrthoK effect a rigid lens is fit with a base curve that is flatter than the cornea to correct myopia and steeper than the cornea to correct myopia, with the degree of flattening or steepening based on the amount of refractive error that needs correcting.

Following Jessen Factors principals, the optimal OrthoK lens base curve to first fit when measured in dioptres is calculated as corneal curvature measured in dioptres plus the amount of refractive error that needs correcting. E.g. for a 43.00D (7.85mm) cornea and -3.00D refractive error, the Jessen Factor would predict the ideal base curve for the first fit lens to be 43.00D + -3.00D = 40.00D (8.44mm).

Nowadays, most OrthoK designs add a further ‘Compression Factor’ to the Jessen Factor result to account for the regression in refractive effect that occurs throughout the waking day in most wearers. Here a further -0.50D or -0.75D (varies according to manufacturer) is added to the targeted refractive error change. Using the same example as above and applying a Compression Factor of -0.50D would lead to a suggested first fit OrthoK base curve of 43.00D + -3.00D + -0.50D = 39.50D (8.55mm).

The Jessen Factor approach seems to work well in most cases, which probably explains why it has lasted the test of time. From reading this blog you now know, if you didn’t already, that it is also exceedingly simple to calculate just from knowing your patients baseline corneal curvature and refractive error.

#### References

- Jessen G. Orthofocus techniques.
*Contacto*1962;6:200-4

##### About Paul

Dr Paul Gifford is a co-founder of Eyefit, an information resource to assist contact lens practitioners in all modes of practice. Learn more about him here.