SUMMARY
Contact lenses are a fantastic medical device. The tempting idea of correcting refraction directly on the Ocular Surface has a long history and dates back at least to the times of Leonardo da Vinci around 1500. Contact Lenses can correct refractive errors directly on the cornea and therefore, they have some principal optical advantages compared to glasses and certainly provide greater ´freedom´ for the user in a lot of sporting, outdoor and society activities.
Contact lenses are sitting in the middle of the ocular surface unit - in fact they are swimming in the tear film … with pros and cons. Although fitting principles usually try to avoid too much physical ´touch´, contact lenses are still, inevitably, in contact with the highly sensitive cornea and the equally sensitive conjunctiva of the lid margin ... as well as with conjunctiva on the eye ball and on back side of the lids.
There are many different types of contact lenses that vary mainly in their size and material. Soft hydrogel lenses, that are typically slightly larger than the cornea, require a very short adaptation period be the wearer and thus have by far the widest market share. Rigid gas permeable lenses, typically smaller than the cornea, are much less wide-spread because they need a certain adaptation time but may have some principal advantages for long term wear. Specialty contact lenses are generally rare. These are e.g. Large Scleral Contact Lenses, that are an exciting medical tool for chronic corneal pathologies including severe Dry Eye Disease and ´Orthokeratology´(´OK´-Lenses) that are typically worn over-night and ´correct´ the outer shape of the cornea for a day-time without the lens on the eye.
Many issues in contact lens wear are nowadays greatly improved or completely overcome. This applies to e.g. thickness of the material, low oxygen permeability, and imperfect lens geometry. HYGIENE, on the contact lens in the storage containers, is still an important issue, particular in soft contact lenses, in extended/ overnight wear and in unexperienced wearers. Disposable contact lenses are reducing the issue of insufficient hygiene to some extent because a potentially contaminated lens is always quickly replaced by a new one. Still, Contact Lens Discomfort (CLD), which refers to the fact that the contact lens is still ´noticed´ on the eye as an uncomfortable sensation, remains an issue for further optimization of Contact Lenses.
With the exiting technological advancements, that have become available in terms of the miniaturization of electronic devices and biosensors, Contact Lenses are gaining amazing new medical applications.
Contact lenses can correct refractive power directly in the location where most of the natural refraction occurs - on the Cornea
Since contact lenses are sitting in the middle of the ocular functional unit they offer connections to many tissues at the ocular surface and to the tear film and they have remained of continuing interest to us. Members of the Ocular Surface Center Berlin (OSCB) have started their scientific work with contact lens studies.
Members of the Ocular Surface Center Berlin (OSCB) have served as members in the TFOS International Workshop on Contact Lens Discomfort (CLD). This initiative of the international scientific non-profit organization Tear Film and Ocular Surface Society (TFOS) has delivered the respective TFOS CLD REPORT in 2013 that found enormous world-wide interest.
The tempting idea of correcting refraction directly on the eye has a long history
Contact lenses are a fantastic medical device dating back at least to ideas of Leonardo da Vinci, around the year 1500, who is said to have developed such an idea when he submerged his head and eyes in a glass bowl filled with water and noticed the change of the refractive power of his eyes.
Contact lenses have still needed a few more centuries until they were practically usable and in fact widely used and have eventually achieved the present grade of professionalism. Contact lenses would seem an ideal tool because they can correct inappropriate refractive power in the place where most of the eye´s refraction is naturally generated - directly on the cornea.
Contact Lenses can correct refractive errors directly on the cornea - with many optical and other advantages
Contact Lenses are great tool for the correction of refractive errors because they act directly where the main refraction occurs, i.e. on the cornea
... and contact lenses neither have a margin nor frame like glasses - Thereby contact lenses generally allow a more natural vision impression than glasses.
Some specific optical advantages of Contact Lenses are:
Contact lenses allow a larger field of vision compared to spectacles.
´Higher diopters´ can be corrected without thick ´bottle-bottom´ glasses that may lead to optical distractions in the periphery and have cosmetic disadvantages
Particularly complicated irregular astigmatism can be corrected easier with Contact Lenses because they can ´iron-out´ smaller inequalities of the corneal surface
All cases where the necessary ´strength´ of an optical correction varies greatly for both eyes an optical correction by contact lenses is generally preferable because the difference of the image size in both fellow eyes is smaller and the vision is thus better and less irritating.
`long-sightedness´ (termed ´presbyopia´) that inevitably occurs with advancing age can be corrected with contact lenses using different optical principles.
Contact Lenses certainly provide greater ´freedom´ to the user as compared to glasses
In outdoor and sports activities contact lenses certainly offer, apart from the wider field of view, some further advantages such as more ´freedom´, less risk of wounding that may potentially occur due to the material of the frame and of the optical glasses themselves.
Contact Lenses (here a soft corneo-scleral lens) are sitting in the middle of the Ocular Surface Unit and interact with all components - from the tear film to the tissue - with pros and cons
At the same time, Contact Lenses are sitting in the middle of the ocular surface unit.
Therefore, they interact with all surrounding tissues and with the tear film – which has its pros and cons.
Contact lenses have to swim in the tear film.
In fact, they split the tear film into two parts - one on top and one underneath the contact lens. Contact lenses thus have higher demands for the volume and quality of the tear film.
Contact lenses are swimming in the tear film
Conventional soft contact lenses and their movement on the eye ball during a blink are shown in this schematic animation. The contact lens fitting is probably a bit ´loose´ here but eventually depends on the subjective wearing comfort of the individual and on the visual acuity.
Contact lenses are typically swimming in the tear film and should thus glide over the tissue. Therefore contact lenses generally have increased demands for the volume and the quality of the tear film.
Although fitting principles usually try to avoid too much physical ´touch´ they are still, inevitably, in contact - just as their name indicates - with the tissue.
They touch the highly sensitive cornea and the equally sensitive conjunctiva of the lid margin - as well as the conjunctiva of the eye ball and on back side of the lids. The latter is in distinct proximity to the underlying Meibomian glands. and contact lenses therefore appear to have some negative impact on these eye lid glands.
Contact lenses and their interaction with the ocular surface are among the starting points of the scientific work of members of the Ocular Surface Center Berlin (OSCB). We were, for example, able to identify the first conclusive evidence that soft contact lens wear leads to alterations of the conjunctival epithelial cells that are known as squamous metaplasia together with its frequency and time course of development in contact lens wearers.
There are many different types of Contact Lenses
Soft/ flexible contact lenses (top) are the most popular type and they are somewhat larger than the rigid/ gas permeable contact lenses (bottom)
There are many different types of contact lenses that vary mainly in their size and material properties - Size and material are often combined.
Today the most popular type of contact lenses world-wide are soft hydro-gel corneo-scleral contact lenses (as the one in the previous figure). They are termed corneo-scleral because they are slightly larger than the cornea and reach onto the corneal margin (limbus).
They are soft by material and thus require almost no adaptation time for the user to get accommodated to a contact lens on the eye. Nowadays they also have a highly oxygen-permeability which removes some potential corneal issues of blocking oxygen and nutrition supply for the cornea. This is important because the cornea has no own blood vessels and thus depends on a supply by diffusion from the tears and the inside of the eye.
The small rigid gas-permeable (RGP) contact lenses were the first ones with high gas-permeability. They require a certain adaptation time of some days for the wearer but they have overall superior long-term wearing advantages, less problems with deposition of materials on the lens and typically lower rates of infection. In times where the wearer is used to ´immediate´ comfort the market share of this lens type is still relatively low.
These two lens types (soft and rigid-gas-permeable) together have a market share of well over 90%. Other specialty lenses, even though they have great potential for specific applications, are ´niche products´. Specialty contact lenses such as Ortho-Keratology (´OK´ lenses) and large therapeutic Scleral Contact Lenses will be covered on separate pages of the ´Deeper Insight´ ... further ´down´.
Contact lens hygiene is still an important issue
Contact lens HYGIENE has always been and is still an important issue in Contact Lens Wear. This is due to a number of reasons:
Contact lenses are inevitably in contact to microbes, mainly bacteria.
This comes from the inserting fingers of the wearer - even when they are cleaned with water, they may not be quite ´clean´ enough, depending on the local water quality.
Microbes are also on the normal ocular surface, where they might increase in numbers under certain conditions and they typically bind and stick to a contact lens surface.
Contact lens containers are therefore another and probably even more importance source of bacterial contamination. They should always be investigated in case of clinically relevant bacterial infections or simply be replaced by a new one when any doubts about such issues come up.
The manipulation of the ocular surface, but also contact lens wear itself, has the inherent risk of causing micro-trauma to the surface that might pave the way for the entrance of microbes, that are normally excluded by a healthy and thus tight epithelium.
The use of ´Disposable´ Contact Lenses that are thrown away after use and are replaced at the next occasion by a perfectly new and clear contact lens, removes some critical hygiene issues in contact lens wear..
Contact Lens Discomfort (CLD) remains an issue for the optimization of Contact Lenses
The technical improvement of material properties and curvature design has greatly improved the contact lens tolerance on the eye in the past decades.
Many issues that can lead to uncomfortable wear of contact lenses such as thickness of the material, low oxygen permeability, and imperfect lens geometry are nowadays considerably improved or even completely overcome.
In addition, disposable lenses have now greatly reduced the risk of serious ocular infections due to an insufficient hygiene in lens storage containers.
Still, in principle, a contact lens remains a foreign body on the eye and its interaction with the ocular surface has to be closely monitored. Therefore not every lens with every characteristics is seamlessly tolerated by every wearer and may thus induce sensations of Contact Lens Discomfort (CLD) which often leads to discontinuation of use. This was the topic of the first TFOS workshop on contact lenses with a respective report in 2013.
Contact lenses are gaining amazing new medical applications
Many tempting ideas for the FUTURE of Contact Lenses (FROM: lifeboat.com blog 2016 03 fish-and-insects-guide-design-for-future-contact-lenses - Posted by Klaus Baldauf)
With the advent of technically refined material properties contact lenses appear to see new applications and indications. Applications means that contact lenses can be equipped e.g. with biosensors for the measurement of tear parameters, e.g. of glucose levels in diabetic patients. New indications means, e.g that the Large Scleral Contact Lenses are now seeing a revival as high tech curvature designs and novel material properties have given them an amazing potential as medical tools for many ocular surface conditions, including severe dry eye disease. They can postpone or even replace surgical treatments in a number of acute and chronic ocular surface diseases.