Comparing Eyeglasses and Contact Lenses

1. In General

Vision correction is a chronic issue worldwide. About 168.5 million residents in the United states use either eyeglasses or contact lenses in order to amend ocular refractive problems. That Is over half of the people living in America only (Cheng et al. 2016). For nearly anyone who needs correction of their vision and does not prefer wearing eye glasses, then contact lenses are a suitable option. Eyeglasses have some advantages over contact lenses in that they are easy to wear decreasing the risk of one getting eye infection since there’s little contact with the eye as in in the case of wearing contact lenses. Another advantage id the ability of photochromic lenses to adjust the amount of light that can get into the eye. These lenses can act as sunglasses when exposed to light and clear up when indoors. This is a technology that most if not all contact lenses lack.

Having said that, contact lenses also has some advantages over glasses. Their ability to be placed directly on the eye prevents obstruction of a persons’ peripheral view. Being less bulky enables people with active lifestyles who have eye problems to participate freely in sports. Their ability to avert being affected with weather conditions such as rain and fog also makes them more preferable over spectacles. Usage of contact lenses has risen drastically in past years, with over a third of the people with ocular refractive issues using contact lenses on a daily basis. Contact lenses can be described as small prescribed lenses worn in direct contact with the human eye. They are developed to rectify refractive flaws and sustain good ocular health (Van der Worp et al. 2014).

Currently there exist two kinds of contact lenses in the market; soft contact lenses and rigid gas permeable contact lenses (RGPs). Soft contact lens is developed from soft, flexible and oxygen permeable material which allows the cornea to be well nourished and avoid irritability and eye dryness. People easily adjust to these soft lenses and are more comfortable in them as compared to the rigid gas permeable ones. The latest materials used in the making of soft contact lenses are silicon and hydrogel. Silicone based hydrogel is more permeable to oxygen to ensure a healthy cornea is maintained. Rigid gas permeable contact lenses on the other hand are long lasting and resist buildup of deposits in the eye. They also give the wearer a crisper and clearer vision. Rigid gas lenses are also cheaper over the life of the lens because they are durable than their counterpart, the soft contact lenses. They are not easy to tear and are easy to handle. On the downside, they have less comfort and it takes a couple of weeks to adapt to them as compared to soft contact lenses (Heiting, 2017)

In addition to pupil size and SA, the clarity of crystalline lens can also impact success with lenses that are multifocal in nature. There is a significant advantage that multifocal intraocular lenses have over contact lens types of corrections in that clear optics is provided when the crystalline lens is removed. It is critical to check the patients’ media clarity before fitting as this can influence on vision with lenses that are multifocal and may partially count for the dissimilarities in success (Madrid-costa et al., 2015).

2. HISTORY OVERVIEW

There is a lengthy and complex history in the development of contact lenses, this begins with development in Germany, which moved to Europe then the United States before it became international.

Leonardo da Vinci, in 1958, laid out and illustrated the various forms of contact lenses but was not actually able to come up with an actual lens or device. In the following years, other scientists outlined and proposed contact lenses in fundamental forms. F.E Muller a glassblower from Germany developed the first recorded equipment that could cover the eye and be considerably tolerated in 1887. A Hungarian Physician named Joseph Dallos, in 1929, modified the art of extracting molds from human eyes in order to develop lenses that could suit the ocular curvature (Codina & Efron, 2004).

Years later, in 1936, a New York optometrist known as William Feinbloom invented contact lens using plastic material. This was the first invention that was purely American-made. Williams’ invention initiated the utilization of plastic lenses which revolutionized how contact lenses were made then. He later came up with scleral lenses that were a combination of plastic and glass. These lenses were somewhat lighter compared to the preceding glass blown contact lenses. Kevin Tuohy from California was the first person to develop a modern-day gas permeable (GP) contact lens in the year 1948. The GP was fully made out of plastic and were named ‘corneal contact lenses’ due to the fact that they had a small diameter and covered only the cornea as opposed to the earlier contact lenses.

These corneal lenses were made out of polymethyl methacrylate (PMMA); a plastic material that was completely non porous. The PMMA lenses could not allow any gas to pass through and were fitted in such a way that they moved with every blink a person made. By doing so, tears carrying oxygen could be ‘injected’ under the lens to maintain a healthy cornea. A polymacon was later developed by a Czechoslovakian; Otto Wichteria, in 1960. The polymacon, a soft plastic that absorbs water (hydrophilic), was utilized for the invention of the first soft lens. Later on, Bausch & Lomb Inc. introduced the soft contact lens technology to the world. This saw the revolutionization of the contact lens industry in 1987, thanks to Vistakon, Inc., Johnson Johnson Company who made the Acuvue discardable soft lenses available in the market. These disposable soft lenses improved ocular comfort, reduced complications of the cornea and were very convenient. Up to date, the finest soft lenses worldwide are all disposable in nature (Lamb & Bowden, 2018).

3. MULTIFOCAL SOFT CONTACT LENS DESIGN

Multifocal contact lenses are designed using various materials; they are either manufactured as soft lenses or rigid gas permeable contact lenses, and others however come in as hybrid or half and half contact lenses. The lenses can be worn as disposables, which mean one has the comfort of discarding those (lenses) at specific times such as on a daily, weekly or monthly basis. Majority of the companies which manufacture multifocal soft contact lenses offer contact lenses contrived from silicone hydrogel material, which allows a good amount of oxygen penetration to the cornea as compared to the other conventional soft lenses. This ensures great comfort while wearing them and are mainly designed to allow one to wear them for longer periods of time (Talu et al., 2011).

Some brands available for contact lenses made out of the multifocal silicone hydrogel include: Acvue Oasys, Bausch + Lomb Ultra which are great for presbyopia, Biofinity multifocal, Air Optix Aqua Multifocal, for presbyopia among others. The design model of these multifocal soft contact lenses is categorized into two: The simultaneous vision design and the segmented designs. We shall first have a look at the simultaneous vision models which are contact lenses with distinct sections of the lens specifically for near, distant and additionally transitional viewing (Berntsen & Kramer, 2013). The contact lens wearer uses the lens area that provides the clearest vision depending on the distance of the object in view. The simultaneous vision designs are classified into aspheric and concentric designs as shown in figure 1 and 2 below. The other structure of multifocal lenses, the segmented design, is a rigid gas permeable multifocal lenses which is designed just like the trifocal and bifocal lenses. The top and center sections of the lens have the suitable strength to view objects that are distant whereas the lower section of the lens has been powerfully magnified for viewing objects that are nearer. Synonyms of the segmented designs are translating or alternating designs and is illustrated in figure 3 below (Heiting, 2017)

Zonal aspheric designs: Biofinity multifocal and reusable proclear are types of aspheric lenses that present in particularly four add powers with Centre Distant (CD) and Centre near (CN) options. In this situation, the CD is generally customized for the predominant eye whereas the CN lens to the non-inferior eye. These lenses are not similar to other types of designs since the optics has not been enhanced for age as the reading inclusion advances (Cheng et al., 2016).

For the preclear multilocular, the CN lens has about a 2mm focal zone that is spherical, followed by a transitional zone of about 1mm in which the power of the lens then shifts to distance prescription. Then there is the distance zone that has a spherical exterior. The center distant lens contains a medial aspherical zone of about 3mm, a steep transitional zone with the peripheral optics of the lens bearing an aspheric close by prescription. Regardless of power, both the center distant and center near contain a set optical area (Neadle et al., 2011).

On-eye effect: The design of the lens can’t be viewed by isolating the other ocular optics. The comparative powered multifocal lens contoured to eyes with the similar optical prescription and pupil size may not yield a similar vision (Lin et al., 2014).

As indicated by Kim et al. (2017), the quality of image of model eyes with immense positive spherical deviation was higher with a center near (CN) multifocal though in this case, the intensity of focus was brought down. Essentially, when wearing CN lenses, eyes that have greater positive spherical aberration (SA) are highly likely to have improved sharpness for moderate/nearer vision, yet low multifocal effect.

4. MEASURING OPTICAL POWER CHANGE (MODELS)

The diopter is also known as the lens power. The focal length is normally measured in millimeters, for instance, 25mm, 60 mm, 8.8mm and 72mm. The lens power/diopter is described as below:

This simply means that when 1000 is divided the focal length (mm) of a lens, it yields the diopter of that lens. For instance, a lens od 25mm has a diopter of 1000/25=40, consequently, a lens of 60mm has a diopter of 1000/60= 16.7. It should be noted that the diopter d of any lens is displayed as +d, hence, 25mm and 60mm have, respectively, diopters +40 and +16.7. Also, if you have the diopter value of a lens, you can easily determine the focal length. According to the above formula, the focal length is calculated as below:

If the diopter of a lens is +4, then the focal length is 1000/4=250mm, for the lens that has a diopter of +10, the focal length is 1000/10= 100mm.

This empowers simple assurance of a lens from the focal length and the focal length of the close-up lens from the diopter power provided (Kim et al., 2017). Converging lenses have positive optical strength while diverging lenses possesses negative power. Immersion of a lens in any refractive medium changes both the focal length and optical power.

At the point when a soft contact lens is fitted into the eye, the interaction of the eyelid and the surface pressure of tears impact the dimensions of the lens, consequently distorting the disposed refractive power. These adjustments in the optical intensity of soft contact lenses during the process of fitting have been detailed by different scientists for many years (Berntsen & Kramer, 2013). Notwithstanding, it is hard to precisely foresee the operation of clinical soft lens fitting. ‘Supplemental power’ is the degree to which the optical power is altered during the fitting process. Supplemental power is attributed to a combination of influences from the alteration of the shape and tear film thickness (Abbas et al., 2019)

An investigation carried out by Abbas et al., (2019) depicted that rounded contact lenses that have high negative and positive solutions showed greater maximum and minimum power changes on fitting when compared to minor prescriptions or remedies. This could have been due to the fact that high powered lenses, whether positive or negative, are thicker and need increasingly extreme front surface structural design through which light can be refracted and the optical pathway shortened. Along these lines, their increasingly extreme geometries could be profoundly influenced by conforming to the cornea. Power changes that are this high are very important in clinical practice. It is usually challenging to fit contact lenses in these patients as it consumes a lot of time and replacing the lens with potential incorrect degrees because of power changes within it incurs a higher cost (Madrid-Costa et al., 2015).

The human eye has an optical system that is made up of the lens and the cornea. The shape of the cornea has an influence on the optical system. Whereby a spherically shaped cornea will have a spherical aberration that is positive. Luckily, the cornea of the eye has a prolate elliptical shape, meaning the periphery is flat- therefore reduces the SA by creating a self-correcting mechanism on the eye. The lenticular and corneal aberrations compensate for one another partially through optical coupling by the internal optics of the eye. This is a purely natural correction. The aftermath is that, in young people, the eyes have a higher order aberration of the entire eye which are the less than the summation of their individual parts, they cancel out each other and result in a robust ocular system. As one advances in age, the abberations of the internal eye rise significantly due to the changes in the crystalline lens. As a matter of fact, there is the crystalline lens induces ten times greater abberations with time as compared to the cornea. Since there is constant increase in the SA which becomes more positive in the eye that is ageing due to lens changes, the depth of the field is increased. If there was an increase in the spherical aberration alone, there would have been sophisticated adaptation of the eye, however, there are surges in other unwanted aberrations along with the SA increase (Kim et al., 2017).

5. IDEAL MATERIAL FOR MULTIFOCAL SOFT CONTACT LENS

There is considerable negligence in the choice of a suitable material when prescribing multifocal contact lens. As a matter of fact, material is significant as design particularly in the case of presbyopia eyes. This is because age reduces tear stability and environmental factors like the prolonged use of computers highly affect the ageing eye. Selecting a material that sustains a stable tear film, reduces dryness and discomfort and provides a persistent vision is of great importance. (Cheng et al., 2016).

The design of the lens cannot be contemplated separately from the optics of the eye. The similar powered multifocal soft contact lens fitted to eyes that have similar size of pupil and optical prescription most likely will not yield the same vision (Berntsen, & Kramer, 2013).

A polymeric material that is suitable should be available in the manufacture of contact lenses. This ensures the opening of incredible opportunities right from the different scope of polymers to the equation of elements within a given formula (Berntsen & Kramer, 2013). In addition to this, there could be deliberations on the various types of polymerization systems to develop a similar polymer. An example is radical versus reactant polymerizations and subsidiaries in which the states of polymerization: vessel utilized, temperature, type of initiator among others: can be changed a twin polymer however with various properties. Also, the material ought to be appropriate for the phases of manufacturing which are combination, review and packing stages (Codina & Efron, 2004).

According to Kang et al. (2013), silicone hydrogel lenses are a great target for coatings that are highly biocompatible and can be encapsulated within or on the surface of the lenses through chemical techniques. The experiments carried out in the study depicted that the lens performance was enhanced, especially after the application of anti-biofouling materials. Materials that are both hydrophilic and anti-biofouling in nature are in effect continually applied to silicone hydrogel contact lens, which presses emphasis on the renovations in which silicone-based hydrogel lens materials are experiencing. There are two benefits of anti-biofouling materials and silicone hygrogel: (1) The hydrophilic modifications/ grafts are the cause to anti-biofouling characteritics and (2) due to their hydrophilic nature they increase the wetting characteristic of the lenses, an impact additionally critical to the HEMA-based lenses. However, it has a greater influence on the silicone lenses, since they have intrinsic wettability downsides and market size. Constant modifications in these materials will lead to constant market supremacy for the silicone based hygrogel contact lenses.

There are various scientific parameters that influence the material of contact lenses and the end product should account for comfort and wear time. These two characterizes often depend on the material, but also factor in the process of manufacturing such as plasma treatment (Cheng et al., 2016).

The main advantage of silicon based hydrogel lenses over the other available contact lenses such as the standard hydrogel lens is that they are absorbent and highly porous thus allowing high oxygen permeability to reach the sclera and cornea. Their oxygen permeability is up to five times more than that of the standard hydrogel lens. Inability of oxygen to reach the cornea might cause an increase exposure to eye infections, blurred vision and decreased visual clarity, a condition referred to as Hypoxia. Silicon hydrogels have an improved comfort level making them perfect for use as extended wear lenses which are recommended for both day and night wear without the constant need of removing them (Van der Worp.,et al 2014). This can either be weekly or monthly depending on one’s prescription. Extended wear lenses are also preferred for people with busy lives, persons suffering from dry eyes and people with active schedules as they minimize the hassle of a cleaning routine.

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REFERENCES

Cheng, X., Xu, J., Chehab, K., Exford, J., & Brennan, N. (2016). Soft contact lenses with positive spherical aberration for myopia control. Optometry and Vision Science, 93(4), 353-366.

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Kang, P., Fan, Y., Oh, K., Trac, K., Zhang, F. and Swarbrick, H.A., 2013. The effect of multifocal soft contact lenses on peripheral refraction. Optometry and Vision Science, 90(7), pp.658-666.

Ţălu, Ş., Ţălu, M., Giovanzana, S. and Shah, R.D., 2011. A brief history of contact lenses.

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Lin, C.H., Yeh, Y.H., Lin, W.C. and Yang, M.C., 2014. Novel silicone hydrogel based on PDMS and PEGMA for contact lens application. Colloids and Surfaces B: Biointerfaces, 123, pp.986-994.

Kim, E., Bakaraju, R.C. and Ehrmann, K., 2017. Power profiles of commercial multifocal soft contact lenses. Optometry and Vision Science, 94(2), p.183.

Abass, A., Stuart, S., Lopes, B. T., Zhou, D., Geraghty, B., Wu, R., ... & Leca, R. (2019). Simulated optical performance of soft contact lenses on the eye. PloS one, 14(5).

Madrid-Costa, D., Ruiz-Alcocer, J., García-Lázaro, S., Ferrer-Blasco, T., & Montés-Micó, R. (2015). Optical power distribution of refractive and aspheric multifocal contact lenses: effect of pupil size. Contact Lens and Anterior Eye, 38(5), 317-321.

Berntsen, D. A., & Kramer, C. E. (2013). Peripheral defocus with spherical and multifocal soft contact lenses. Optometry and vision science: official publication of the American Academy of Optometry, 90(11), 1215.