IT IS NEITHER GOOD nor bad luck. A successful first fit comes from understanding and utilizing the technology available to modern clinicians. A solid understanding of shape, size, and elevation metrics leads to greater first-fit success—reducing the number of revisions, improving patient satisfaction, and decreasing chair time, thereby strengthening profitability.

Eccentricity as a Predictor of Success

Eccentricity (e-value) describes how fast or slow the cornea flattens from the central cornea to the peripheral cornea. An early orthokeratology (ortho-k) study suggested that corneas that have an e-value of 0.50 or higher tend to achieve greater changes in refractive power with lens wear (Mountford, 1997). Although later studies have found that many factors contribute to the success of ortho-k lenses for each individual, considering the e-value of a cornea could help when assessing candidacy for successful ortho-k fitting and should not be overlooked during the initial patient evaluation.

Clinical Pearl: Utilizing e-value to predict the level of difficulty aids clinicians in setting expectations, selecting suitable lens designs, and quoting fees appropriately.

Understand the Corneal Size Relationship

Attention to visual iris diameter (VID) is critical. The overall diameter of a cornea determines the sagittal depth of the lens required for corneal reshaping. A general rule of thumb is that a 1-mm change in VID equals approximately 100 µm of difference in sagittal height (Mountford and Ruston, 2004). Larger corneas usually require a deeper sagittal height, which helps predict centration. An undersized sagittal height is a common reason for inferior decentration. Many clinicians choose lens designs that have overall diameters ranging from 90% to 97% of the corneal diameter, depending on their individual measurement technique.

Clinical Pearl: Measure VID on the oblique meridian (45° and 135°) to avoid overestimating corneal size, as many corneas are ovoid in shape (Carney et al, 1997).

The Value of Elevation Maps 

Last but not least, careful attention to elevation predicts fit success. Elevation maps show corneal height relative to the best-fit sphere—critical for identifying pressure zones. Positive inferior elevation suggests an inferiorly positioned corneal apex, which often correlates with inferior decentration of the lens (Chen et al, 2017). Early identification of asymmetry in corneal elevation helps clinicians choose toric or asymmetric designs, reducing the need for revisions.

Clinical Pearl: Corneal elevation differences greater than 25 µm between the horizontal and vertical meridians along an 8-mm chord generally require toric peripheral curves (Figure 1).

Conclusion

To elevate your first-fit success rate, become intimately acquainted with the data points and maps in your topographer. This includes axial, tangential, and elevation maps, as well as tools for identifying eccentricity and measuring VID. Lean on design consultants at your preferred labs. Ask questions—get to know the lens design and how it interacts with your topographer. It’s a win-win for both patients and clinicians.

References

1. Mountford J. An analysis of the changes in corneal shape and refractive error induced by accelerated orthokeratology.  Int Contact Lens Clin 1997;24:128-143.

2. Mountford J, Ruston D. Orthokeratology: Principles and Practice. Oxford: Butterworth-Heinemann; 2004.

3. Carney LG, Mainstone JC, Henderson BA. Corneal topography and morphology of the normal human cornea. Invest Ophthalmol Vis Sci. 1997;38(12):2917-2923.

4. Chen Z, Xue F, Zhou J, Qu X, Zhou X; Shanghai Orthokeratology and Study (SOS) Group. Prediction of orthokeratology lens decentration with corneal elevation. Optom Vis Sci. 2017;94(9):903-907. doi: 10.1097/OPX.0000000000001109