Depending on how much you have already read or what advice you have received, you may be under the impression that you should assess the lens fit before proceeding to overnight wear. While I agree with the potential benefits this offers, in practice, I have found mixed success over what would have been achieved from just using the predicted best lens suggested by the lookup table, slide rule or computer software for the lens design in question.
At the start of your Orthok fitting career, I suggest that you have enough on your plate already. Asking you to become skilled at the visual assessment of fit in the open eye only adds unnecessary complication. If you have taken care in capturing and averaging your corneal topography images before extracting the measurements required for first fit lens selection, jumping straight into starting overnight trials is highly likely to achieve a pretty good outcome.
If you are using an empirical design this is of course not an option anyway. Indeed, the reported reliability of first lens success from many empirical lens designs points towards accurate corneal topography measures as being the most useful factor in deciding the first lens to fit.
Open eye assessment
Having just said that you shouldn’t attempt to modify lens selection by assessing the fit in the open eye, this doesn’t stop you taking a look to start familiarizing yourself with the procedure and the appearance of different types of lens fit. To start building this experience, once the lenses are safely inserted instill some fluorescein and take a look, either with a Burton lamp or slit-lamp using blue light enhanced by viewing through a yellow Wratten filter.
What you should see in a good fitting lens is a trapped annulus of fluorescein enhanced trapped tear film surrounding the aligned corneal apex and itself surrounded by a dark band where the landing zone of the lens aligns with the cornea towards the periphery as shown in the image below. The edge lift of the lens will show a thin band of fluorescein enhanced tear film. The lens should move with the blink but center quickly shortly after the blink, though don’t be surprised if the lens is moving around, especially if the eye is tearing excessively in response to the lens.
A shallow (flat) fitting lens will tend to decenter either superiorly to create a Smiley Face topography difference map appearance at follow up or inferiorly to create a Frowny Face. In both these cases, the fluorescein pattern will be like the normal appearance shown above except maybe having a thicker band of edge lift. Deep (steep) fitting lenses remain well centered but will lift off the eye typically resulting in a narrower band of edge lift and/or reducing apical bearing indicated by a feint fluorescein glow within the central optic zone.
All of this is difficult to tell apart as a beginner leading to my advice that you just observe and take notes of the appearance, even better capture video if you have this option, and refer back to these notes once you know the fit outcome from topography difference map assessment at follow up. This will help you make the associations between what ultimately turned out to be a well or poorly fitted lens and the open eye fluorescein pattern appearance that you observed at initial fit so that in time you can start predicting fit from the open eye assessment alone.
Closed eye assessment
OrthoK lenses work surprisingly fast to the point that within 15-minutes of close eyewear it is often possible to ascertain fit outcome from corneal topography difference map observation. To make this happen, get your patient to keep their eyes closed for at least 15-minutes, remove the lens(es) and then quickly capture topography images to use for the difference map comparison.
The induced corneal topography changes from this short period of wear will be subtler than from overnight wear, making them harder to assess. For this reason, if you are starting out I discourage you from adopting this approach, purely from inexperience in difference map pattern recognition. Once you have a few successful fits under your belt, however, short-term wear offers a way to potentially accelerate the overall fit process through more quickly reaching the lens specifications that achieves the best-fit outcome from initial overnight wear.