Sagittal (sag) height describes the height of the cornea at a given point. If you imagined resting a completely flat surface onto the front of the eye there would be no gap between the flat surface and the cornea at the apex, as this is where it is resting on the eye. Move 2mm away from the apex though and there will be a gap, move a bit further, say 3mm from the apex, and the gap will be bigger still. If we were to measure this gap we would call it the corneas sagittal height, or sag, at that location. The term ‘chord’ is given to the distance from the center of the cornea to the sag measurement location indicate as 'S1 chord' and 'S2 chord' in the next image.
The same principles apply to a contact lens, except here we are now interested in the back surface of the lens as it is this surface that is going to be in contact with the cornea. The image below gives a graphical representation of measuring sag height of the lens back surface. To measure the sag height at a chord of 2mm we need to measure out 2mm from the center of the back surface, then draw a line that is perpendicular to the central axis of the lens. The distance from the center of the back surface and where this perpendicular line crosses the central axis is the sag height. Look closer at the image below and you will appreciate that the sag height will be similarly greater for larger compared to smaller chords as described in the previous paragraph when measuring corneal sag heights.
Remembering back to the previous section it is the sag of the peripheral zone that is our primary concern, as this is where we want the lens to nicely align with the cornea while ensuring that the back center of the lens gently caresses the apex of the cornea. Sag based fitting makes this simple as all that is required is to measure the sag of the cornea at the location where the peripheral zone is designed to land on the cornea and then match the sag of the lens at the same chord to the measured sag of the cornea. The lens design achieves this match by taking into consideration the calculated corneal clearance at the edge of the flat fitting optic zone and then altering the steepness of the reverse curve to compensate and bring the lens back into the required sag height at the peripheral landing zone. Steepen the reverse curve and the sag height increases. Conversely flattening the reverse curve will cause the sag height to reduce. Get all of this right and the lens will fit nicely as shown in the next image.
The point of this ‘reasonably’ short introduction is purely that you understand OK is all about sag-based fitting, so you need to make this your mindset and forget about curve-based fitting that you were taught for rigid lens fitting. Well don’t forget them, just don’t apply these rules to OK lens fitting. In this world, it is all about sags. The good news is that corneal sag height can be easily measured from corneal topography, and proprietary OK lens designs do the heavy lifting of matching sag height on your behalf.