In this article, you will learn about the differences between scleral lenses and (hard or corneal) gas permeable lenses. You will learn when it makes more sense to choose what kind of lens. At first, let us start with a list of differences:
- Scleral lenses have bearing on the white part of the eye while RGP lenses only have bearing on the cornea
- Scleral lenses have a thicker fluid reservoir compared to gas permeable lenses
- Scleral lenses are more complex in the fitting process compared to gas permeable lenses
- Scleral lenses have practically no movements on the eye compared to RGP lenses
- RGP lenses are easier to lose than scleral lenses
Those are the very basic characteristics you can recognize right from the start by looking at how scleral lenses are fitted vs RGP lenses. But when does it make sense to chose one of the lens designs. In most cases, scleral lenses become an option when the try with the simpler normal and smaller corneal gas permeable lenses was unsuccessful.
The types of persons fitted with both lenses may have similar problems. It could be a degenerative corneal disease, dry eyes, or irregular astigmatism. But when the simpler solutions the RGPs do not work your eye care professional still has an option for you. This is when the consulting goes into sclerals.
Scleral Lenses Have Bearing on the White Part of the Eye While Rgp Lenses Only Have Bearing on the Cornea
RGP lenses are much smaller than scleral lenses. A bigger RGP lens has diameter of 11mm. A smaller scleral lens for example has a diameter of 14mm. With the smaller size the RGP lens just sits on your cornea which leads to some benefits later discussed in this article. Due to the sheer size of a scleral lens the scleral lens touches not the cornea but the white part of your eye. Actually there is always a fluid reservoir between the scleral lens and your cornea.
Scleral Lenses Have a Thicker Fluid Reservoir Compared to Gas Permeable Lenses
The fluid reservoir found between the scleral lens and the cornea is a lot bigger compared to standard RGP lenses. This is great for people with dry eyes where scleral lenses really can be an efficient tool to reduce the stress of dry spots on your eye. But the fluid reservoir is enclosed by the shape of the lens and the your eye. This leads to a situation where the fluid does reservoir does not get exchanged with fresh fluid.
For example the tear exchange beneath a RGP contact lens provides an ongoing fluid replenishment between the ocular surface and the lens. Due to the smaller surface of the RGP where a smaller part of the eye is covered fresh eyelid comes under the lens with every blink. This exchange is considerably lower during the wear of scleral lenses compared with rigid lenses. As a result, the accumulation of tear film debris and metabolic by-products between the cornea and a soft contact lens increases, potentially leading to complications.
It does not have to be this way but for example the term mid day fogging usually comes up with people wearing sclerals and not RGPs. This term describes the buildup of metabolic byproducts that could lead to less visual acuity due to cloudy vision. The fluid reservoir gets hazy. When the wearer takes of the sclerals cleans them refills them with new saline the vision will get back to clear again.
The experienced optician has ways to optimize mid day fogging. But depending on the case the cornea did not function optimally in the first place. Especially when specialty lenses are needed. So in some cases the bigger scleral lenses will lead to such an experience.
Scleral Lenses Are More Complex in the Fitting Process Compared to Gas Permeable Lenses
The fit of corneal gas permeable lenses can be evaluated faster than with scleral lenses. Even though in more complex cases with keratoconus the optician or optometrist can see very fast if the lens will work our for the customer or where it needs to be optimized.
With scleral lenses however the time until the evaluation is complete lasts longer and requires more check up attpointments. This is due to the differences in where the contact lenses land on the eye. When the RGP lens just sits on the cornea minimal change is expected in the next few hours in comparison to the first 30 minutes.
With scleral lenses, it is totally normal for the fit of the lens to change because the lenses compress the tissue of the conjunctiva a little and sink in a little. This reduces the distance between the back surface of the scleral lens and the apex of the cornea.
This is step is totally normal and the optician needs to observe if the expected values have been met usually for the first check up after four hours. Out of experience fitting scleral lenses I can tell the tissue on the eye is very different from person to person. That means there is greater variation in which how the lens could settle in comparison to corneal lenses.
Scleral Lenses Have Practically No Movements on the Eye Compared to RGP Lenses
The bigger a contact lens is the less movement it has on the eye after every blink and while performing eye movements. This is why scleral lenses basically have no movements and therefore produce practically no foreign body sensation. That is great for comfort but also leads to a very stable vision even under extreme conditions like playing volleyball. Compared to this RGP lenses constantly move on the cornea. Looking at traffic lights or at the volleyball in our example will make the foreign body sensation bigger.
The eye lids oftentimes get a little irritate by the edge of a corneal lens when they do not cover the edge of the lens right from the beginning. With a smaller RGP lens the probability is also a lot higher for a foreign body to get under the contact lens. This usually produces a strong foreign body sensation in combination with a tearing eye which less likely with scleral lenses.
RGP Lenses Are Easier To Lose Than Scleral Lenses
The percentaged amount of lens surface covered by your eyelids is a lot smaller with normal gas permeable contacts in comparison to scleral lenses. When the eyelids cover a smaller part of the lenses they can easily fall out. Something as simple as a pull on the side of the eyelids could lead to a loss of one of the contacts. This will not happen with scleral lenses. This is why they also often get recommended when the wearer is more active and performs in sports.
This was my information on the differences of RGP lenses and scleral lenses. I hope you found the information you were looking for about the lenses.
I wish you a great day.
