The lens should vault the entire cornea and limbus and land on the scleral conjunctiva, placing equal pressure around 360° of the sclera. ICD lenses are fitted using a 14-lens diagnostic fitting set, which varies in sagittal depth in 100 micron steps, for ease of fitting. The base curve of each lens is provided, but is essentially irrelevant to the fitting process as the lens does not need to align with the cornea, merely clear it.
Recent studies conducted at Pacific University College of Optometry in the US have demonstrated that the transition zone from the peripheral cornea to the limbus and onto the anterior sclera is in fact a straight line tangent, and not a concave angle (as is often assumed) (Figure 1). With this in mind, the ICD lens was designed using peripheral tangents (straight lines) rather than traditional curves. These are adjusted by modifying the angle of the tangents (steepening the angle to increase depth and flattening the angle to decrease depth).
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