When fitting lenses empirically (i.e. without using trial lenses) an important and perhaps obvious question to ask yourself is “does this look like what I would expect to see if I put the lens on the eye?” or equivalently “does this simulation look realistic?”.
Making this judgement - and deciding what to do if it doesn’t look realistic - can really take your fitting to the next level. This all relates to the tilt functionality in EyeSpace, which is a common topic for our support staff. So if you're ready to delve a bit deeper into EyeSpace then read on below.
Fundamentally, it’s just a picture, a visualisation, a guess at what you should expect to see once you put a contact lens on the eye.
Most of the time it’s a very good guess, especially when the topographical data is decent (see our article on good topographical capture). But still, as a practitioner who is learning how to fit lenses empirically, an important skill is being able to look at a NaFl simulation and decide for yourself whether the simulation looks realistic.
Note that we're not too concerned about the fitting characteristics of the lenses here, and most of these topography maps are artificially generated in order to best illustrate a point.
So what makes a realistic simulation? Well, intuitively you already know most of this. Let’s look at an extreme example:
This is a cross-section of a contact lens on a cornea. As the markings show we’re looking at the 0-180 meridian. The blue is the contact lens, the purple is the cornea. Does this look realistic? Not really.
Intuitively you know that the lens isn’t going to sit like this. It’s not going to sit up like a see-saw, it is going to tilt (rotate) so that it drops down and bears evenly on the cornea. Like this:
This is the same lens, just tilted to show a more realistic simulation.
Note: Tilting a lens doesn’t change the parameters of the lens. It just shows a more realistic simulation of how that lens looks on the eye.
Most of the time EyeSpace will calculate the tilt correctly, but it is your job as a practitioner to decide if it looks realistic. EyeSpace allows you to tilt the lens if it doesn’t look quite right. Let’s review the EyeSpace simulation screen:
Let’s break this down. The bottom half contains the NaFl picture, which is what you are hoping we’ll see when you put the lens on the eye:
But what is that blue ring? That is just part of the colour scale, and it indicates where the contact lens is bearing on the cornea. Fluorescein isn’t visible at less than 15-20 microns thickness, so black doesn’t necessarily mean the lens is bearing on the cornea, but blue does. Here we have a nice even bearing in all 360 degrees.
The cross section is an important part of the simulation in EyeSpace, and allows you to gain more insight into the fit than the top-down view alone:
The top part of this we’ve seen already. It just shows the cornea and the contact lens (scale in mm). Below that is a blown up view of the tear layer profile (scale in µm). Notice the hints of blue at +/-4 mm which indicates where the contact lens is bearing on the cornea.
Remember the cross section is showing a particular meridian as indicated by the little numbers (0-180 in this example). You can change which meridian we show in the cross section by clicking anywhere in the bottom NaFl picture - it will draw a cross section through the meridian in which you click.
Let’s look at an unrealistic simulation and see if we can tilt the lens to correct it:
This example isn’t as extreme as the one above, but we can still see that the lens isn’t bearing evenly on the cornea; the blue at 3 o'clock indicates bearing there, but that’s the only place we see touch. So we need to tilt the lens to produce a realistic simulation.
In EyeSpace you can tilt a lens in four independent meridians by clicking any of the four little circles (with plus symbols) surrounding the lens. Think of it as pushing the lens in that direction to get it to tilt.
We want to push the button at 9 o'clock to tilt the lens in that direction. Doing so changes the simulation to this:
We now have even bearing in 360 meridians in the NaFl picture, and the 0-180 cross section shows this also.
We’ve already said that tilting the lens doesn’t change the lens parameters, so why bother? Why do we care about a realistic simulation?
Well, because you may want to change the lens parameters after tilting the lens.
A prime example of this is when you’re fitting Forge Ortho-K lenses. You typically want between 15-20 µm of central clearance to avoid central staining. If the initial simulation doesn’t show even bearing on the cornea you may need to tilt the lens, but tilting might cause the lens to drop down (decrease the cTFT), which means you will need to increase the sagittal height of the lens in order to get sufficient central tear film thickness.
Look again at the above example. It’s not an Ortho-K lens but it still illustrates this point nicely. Notice the cTFT reading in the top right corner of the simulation? That’s the central tear film thickness. This has decreased from 21.9 µm to 13.9 µm after tilting the lens.
Of course, clinical significance is another matter. If you’re only adjusting the lens by a small amount then you may find this is not clinically significant. This is where practice and experience will help to develop your skills.
The above examples are simplified - it’s not often you will get perfectly even bearing in all 360 meridians as no cornea is perfectly spherical or perfectly toric. The example below is more typical:
There’s nothing wrong with this. Most of the time you’re looking for the lens to be balanced on either side of the cornea with two or three zones of touch.
Hopefully, this has helped to introduce you to tilt in EyeSpace. With practice, you will develop your skills to quickly determine whether a simulation looks realistic or not.
For more information on how to empirically design specialty lenses please read these articles:
Note: If you need help from our support staff then you can always hit the review and order button and we’ll check it over for you first. You can check what we did and then learn that way.