Technical spec.
The material used is a high oxygen permeable fluorocarbon silicone acrylate copolymer
specially formulated to suit the production of RGP scleral lenses. Scleral lens
practice is not a part of mainstream contact lens work, but there are many useful
and surprising applications.
Glossary
This glossary is intended as a broad guide to the following terms and is not a definitive
item.
Scleral
As in Scleral lens, a contact lens large enough to bear on the sclera, fits underneath
both the top and bottom lids of the eye.
Sclera
The white of the eye
RGP (rigid gas permeable)
The material from which some contact lenses is made (both corneal and scleral).
The lens is hard, but allows oxygen exchange, hence rigid and gas permeable.
Keratoconus
An eye condition in which the cornea becomes thin and protrusive, causing distorted
vision, which can only be optically corrected by the use of rigid contact lenses.
High Myopes
Severely short sighted
Aphakes
People who have had their natural lens surgically removed from their eye.
Enhanced Oxygen Flux
Oxygen can easily pass through this material.
Fenestrated Designs
Old style scleral lenses, made in PMMA, with small holes drilled through the material
to allow a supply of fresh tears for the cornea.
Axial Profile
A cross-section of the eye in any direction.
Pre-corneal liquid reservoir
The fluid which fills the space between the eye and any rigid contact lens when
it is being worn.
Corneal lens tolerance
The ability of a person to wear rigid lenses in their eye.
High Oxygen Permeable Fluorocarbon silicone acrylate copolymer
This describes the chemical for the material used in rigid gas permeable contact
lenses.
Estimate
Our estimate of numbers of patients wearing our scleral lenses is based
on:
• A very conservative percentage of the total number of RGP lenses manufactured,
allowing for lenses lost, broken or rejected
• Allowance for replacement and refitted
lenses
Quality
We are assessed and registered under the requirements of Directive 93/42/EEC Annex
VI (CE marking).
Advantages of scleral lenses
The large size and scleral bearing surface, while commonly perceived as being disadvantages,
should be considered positive benefits.
• As scleral lenses bear on the sclera,
they do not depend on precise alignment to the corneal surface so even highly irregular
corneal topography can be fitted with some kind of scleral lens.
• They are almost
never dislodged because they fit under the eyelids.
• They are dimensionally stable,
robust and not subject to much deterioration.
• Insertion and removal is easier
for clumsy patients, eg elderly aphakics who do not have an implant for some reason.
• Lid sensation is minimal and foreign bodies under lens are rare.
• Powers in excess
of ± 40.00 D are possible.
Disadvantages of scleral lenses
• Even if skilfully fitted, the full corneal coverage considerably reduces oxygen
available to the cornea for PMMA sclerals.
• They are labour intensive to produce
compared to most other lens types.
• Their physical size can intimidate some patients.
• Some patients are conscious of the feeling of bulk.
• The scleral zone substance
may cause a pseudo-proptosis appearance during wear.
• Fenestrations in scleral
lenses admit air bubbles to the pre-corneal reservoir causing visual disturbances
and localised dehydration.
• Fenestrations also cause settling back on the globe
with consequent tightness of the fit, especially at the limbus.
Indications
There are only two - an inadequate result with other lens types or when the unique
advantages of scleral lenses can be applied. Any eye condition for which contact
lens correction is an option can be considered for scleral lenses if the wearer
is experiencing problems with conventional lens forms.
Irregular or abnormal corneal topography
• High astigmatism
• Keratoconus or other primary corneal ectasia
• Corneal transplant
•
Traumatized eye
• Post-refractive surgery
High refractive errors
•
High powers leading to centration difficulties with high-power corneal lenses.
• Intolerance
to corneal or hydrogel lens wear in myopia or hypermetropia
• Significant non-pathological
corneal astigmatism
Iris encapsulation
•
Intractable diplopia.
• Cosmetic shells.
• Unsightly blind eyes.
• Aniridia.
• Microphthalmos.
Therapeutic or protective applications
•
Corneal hydration in serious dry eye conditions such as Stevens Johnson syndrome
and cicatricising pemphigoid
• Potential for corneal healing
• Prevention of tear film
evaporation with poor lid closure or lid absence
• Corneal protection against trichiasis
or lid margin keratinisation
• Preventing mucus filaments adhering to the cornea
• Ptosis
The main role of scleral lenses is to augment the options when contact lenses are
clinically necessary for satisfactory vision or as a therapeutic appliance. However,
there is also a role across the whole range of contact lens usage.
Recreational or occupational applications
•
Intermittent use where short-term adaptation may be easier than with corneal lenses
• Contact sports
• Water sports
• Work in dusty environments
Rigid Gas Permeable Lenses RGP
materials enabled a major shift away from both traditional preformed fitting methods
and from impression methods as mainstream scleral lens clinical practice. Corneal
swelling studies and clinical experience gained so far, indicates that sealed scleral
lens designs provide sufficient oxygenation to alleviate significant corneal hypoxia.
This crucial development has transformed scleral lens practice. Air bubbles are
largely excluded from the pre-corneal fluid reservoir, and its positive pressure
is retained, thereby significantly reducing settling back and enabling controlled
corneal clearance. Complex and unpredictable fitting processes are now only necessary
in a minority of cases encountered. Virtually any corneal topography can be easily
fitted with sealed RGP scleral lenses, provided that the sclera is regular enough
to be fitted with a preformed design. If this is not the case, an impression may
have to be taken, from which a PMMA scleral lens can be made in the traditional
way, or the final fitted PMMA scleral lens can be duplicated in an RGP material.
However, it should be pointed out that this duplication exercise is a cumbersome
manufacturing exercise compared to any other scleral lens manufacturing process.