When your contact lenses do not sit in place the curvature of your lenses (BC or base curve) does not fit well with the curvature of your eye. This can cause the lens to stay in a decentered position on your eye which can happen with soft as well as with hard (RGP) contact lenses.
A slight decentration of contacts can oftentimes be observed. A smaller contact lens usually is more prone to decentration in comparison to a bigger soft lens. In addition to slight decentration, It is normal when contact lenses shift as little as a millimeter on the eye and then resettle after you blinked. But in most cases when the wearer notices the decentralized position of a contact lens, it is excessive and the curvature of the lens needs to be fitted a little tighter to fit the contact lens in a more centralized position on the eye.
Despite common belief, the curvature of the eye is not symmetric and therefore complete symmetry in the fitting oftentimes can not be achieved. When I talk about an acceptable decentration it should be in the range of a millimeter. Depending on the contact lens design worn sometimes a millimeter is too much.
This is highly dependent on the size of the pupil as well as if single vision or multifocal contact lenses need to be fitted. The form factor of the human sclera is rotationally asymmetric, and its shape varies considerably between subjects.
This is exactly why practitioners oftentimes observe the same contact lens on two different eyes to be decentralized in different positions. The shape of the eyelids and the pressure they put on the lens can also lead to a contact lens that does not stay in place.
How Do You Get Contact Lenses To Stay in Place?
With soft contact lenses, a minimum change of 0.3mm is required to produce a significant change in regard to the position of the contact lens. With RGP contact lenses the changes in base curve required are smaller and often get combined with a change in the total size of the diameter to change a contact lens to stay in place in a more centralized position.
With standard contact lenses sometimes a centralized position can not be achieved due to the limited changes to the parameters available. However, with custom contact lenses it is expected to have options that will lead to a good contact lens fit. In comparison to standard contact lenses with custom-fitted soft contact lenses for example the optician has more options not only to fit the lenses steeper or flatter but to choose different shaped peripheral designs of the contact lens.
This way the bearing of the contact lens can be better controlled and a more even position can be expected. Typically with higher prescriptions for example with RGP lenses the diameter of the contact lens needs to be manipulated a lot so a centralized fir can be achieved. Depending on the individual case in some cases the diameter needs to be made bigger and in others, it needs to be made smaller to produce a contact lens to stay in place better.
With a high riding or low riding lens, a lot of details matter to produce a great fit. One of these details is the chosen lens material that will increase or decrease the weight of the lens. Increasing the specific weight will help drop a superiorly riding lens while decreasing it will help to raise an inferiorly positioned lens. Of course, the thickness of a custom lens can also be manipulated with the same material.
In the picture above you can see the contact lens is not positioned correctly. We fixed this not by switching the lens in this case but rather provided other instructions on how to position the lens correctly in the center of the cornea. Now the customer is satisfied with the result and the contact lens stays in place. But at first, she rotated her eye too much before putting the lens on the eye.
Switching Another Contact Lens Design To Produce a More Better Centered Fit
When the whole contact lens design is switched to produce a lens that stays in place more the fitting rules change the base curve 0.3 mm to produce the desired outcome. Because when a contact lens in a certain base curve is ordered and the manufacturer is switched the same ordered base curve can lead to a tighter or flatter contact lens.
This sounds odd for inexperienced opticians but a change in material, as well as the lens design, will lead to another outcome. For example, a lot of contact lenses on the market produce little imprints on the white part of the eye which is obviously not ideal. Typically when you would optimize the fit you would make it flatter to make the fit of the contact lens less tight. But with daily contact lenses, you do not even have the options to change the lens BC to a flatter one.
In this example, an Acuvue Oasys 1-Day contact lens was fitted to the eye of my customer. The contact lens produced the markings I mentioned before that were only visible in combination with fluorescine. This lens had a very flat BC of 9.0mm. In order to optimize the fit, I changed the contact lens and chose the Miru Upside which has an 8.4mm BC. So it is a lot steeper. But this lens has a aspheric lens design and out of experience I can say I never saw any imprints with this lens.
The results speak for themself. The imprints/markings are gone although the BC of the new contact lens design is way steeper. See for yourself in the images below.
To be clear both lenses are great when they put on the correct eye but when it comes to optimizing a contact lens fit the optician needs to look at every case individually. There is not a single lens that works great all the time. So in this article I did not want to speak generally against standard lenses or against the Acuvue 1 day lenses. I just wanted to mention experience in fitting different contact lenses is king to improve contact lens fitting.
Because with experience of different types of lenses available one will not only focus on one or two parameters but see the lens as a product of multiple adjustment options that will lead to less shifting of the contacts and a more stable fit while having eye health as a priority in mind.
I wish you a great day.